Co-reporter:Chi Zhang, Joshua Jasensky, and Zhan Chen
Analytical Chemistry 2015 Volume 87(Issue 16) pp:8157
Publication Date(Web):July 15, 2015
DOI:10.1021/acs.analchem.5b00641
We developed a multireflection data collection method in order to improve the signal-to-noise ratio (SNR) and sensitivity of sum frequency generation (SFG) spectroscopy, which we refer to as multireflection SFG, or MRSFG for short. To achieve MRSFG, a collinear laser beam propagation geometry was adopted and trapezoidal Dove prisms were used as sample substrates. An in-depth discussion on the signal and SNR in MRSFG was performed. We showed experimentally, with “m” total internal reflections in a Dove prism, MRSFG signal is ∼m times that of conventional SFG; SNR of the SFG signal-to-background is improved by a factor of >m1/2 and
Co-reporter:Xiaodong Zhang, Fu-Gen Wu, Peidang Liu, Hong-Yin Wang, Ning Gu, Zhan Chen
Journal of Colloid and Interface Science 2015 Volume 455() pp:6-15
Publication Date(Web):1 October 2015
DOI:10.1016/j.jcis.2015.05.029
The problem of stability hinders the practical applications of nanomaterials. In this research, an innovative and simple synthetic method was developed for preparing ultrastable and multifunctional gold nanoclusters (Au NCs). HS–C11–EG6–X is a class of molecules consisting of four components: a mercapto group (–SH), an alkyl chain (C11), a short chain of polyethylene glycols (EG6) and a functional group (X, X = OH, COOH, NH2, GRGD, etc). The present work demonstrated the importance of using HS–C11–EG6–X to prepare Au NCs with excellent properties and the role each component in this molecule played for synthesizing Au NCs. Au NCs with tunable surface functionalities were successfully synthesized and characterized. It was found that Au NC precursors had a fluorescent quantum yield of 0.4%; in contrast, after capping with HS–C11–EG6–X, the quantum yield significantly increased to 1.3–2.6%. The HS–C11–EG6–X capped Au NCs exhibited superior stability under various solution conditions (including extreme pH, high salt concentration, phosphate buffered saline and cell medium) for at least 6 months, even after conjugation with anticancer drug doxorubicin. Besides, we have also demonstrated that other commonly employed thiol-containing ligands failed to prepare stable fluorescent Au NCs. Moreover, the Au NCs showed negligible toxicity to A549 lung cancer cells up to 100 μM, and the application of the ultrastable Au NCs for anticancer drug delivery has also been demonstrated.
Co-reporter:Xiaoxian Zhang, John N. Myers, Qinghuang Lin, Jeffery D. Bielefeld and Zhan Chen
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 39) pp:26130-26139
Publication Date(Web):08 Sep 2015
DOI:10.1039/C5CP03649F
Fully understanding the effect and the molecular mechanisms of plasma damage and silylation repair on low dielectric constant (low-k) materials is essential to the design of low-k dielectrics with defined properties and the integration of low-k dielectrics into advanced interconnects of modern electronics. Here, analytical techniques including sum frequency generation vibrational spectroscopy (SFG), Fourier transform infrared spectroscopy (FTIR), contact angle goniometry (CA) and X-ray photoelectron spectroscopy (XPS) have been employed to provide a comprehensive characterization of the surface and bulk structure changes of poly(methyl)silsesquioxane (PMSQ) low-k thin films before and after O2 plasma treatment and silylation repair. O2 plasma treatment altered drastically both the molecular structures and water structures at the surfaces of the PMSQ film while no bulk structural change was detected. For example, ∼34% Si–CH3 groups were removed from the PMSQ surface, and the Si–CH3 groups at the film surface tilted toward the surface after the O2 plasma treatment. The oxidation by the O2 plasma made the PMSQ film surface more hydrophilic and thus enhanced the water adsorption at the film surface. Both strongly and weakly hydrogen bonded water were detected at the plasma-damaged film surface during exposure to water with the former being the dominate component. It is postulated that this enhancement of both chemisorbed and physisorbed water after the O2 plasma treatment leads to the degradation of low-k properties and reliability. The degradation of the PMSQ low-k film can be recovered by repairing the plasma-damaged surface using a silylation reaction. The silylation method, however, cannot fully recover the plasma induced damage at the PMSQ film surface as evidenced by the existence of hydrophilic groups, including C–O/CO and residual Si–OH groups. This work provides a molecular level picture on the surface structural changes of low-k materials after plasma treatment and the subsequent silylation repair.
Co-reporter:Peipei Hu, Xiaoxian Zhang, Chi Zhang and Zhan Chen
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 15) pp:9873-9884
Publication Date(Web):05 Mar 2015
DOI:10.1039/C5CP00477B
The interactions between nanoparticles (NPs) and cells are of huge interest because NPs have been extensively researched for biomedical applications. For the cellular entry of NPs, it remains unclear how the cell membrane molecules respond to the exposure of NPs due to a lack of appropriate surface/interface-sensitive techniques to study NP–cell membrane interactions in situ in real time. In this study, sum frequency generation (SFG) vibrational spectroscopy was employed to examine the interactions between lipid bilayers (serving as model mammalian cell membranes) and Au NPs of four different sizes with the same mass, or the same NP number, or the same NP surface area. It was found that lipid flip-flop was induced by Au NPs of all four sizes. Interestingly, the lipid flip-flop rate was found to increase as the Au NP size increased with respect to the same particle number or the same NP surface area. However, the induced lipid flip-flop rate was the same for Au NPs with different sizes with the same mass, which was interpreted by the same “effective surface contact area” between Au NPs and the model cell membrane. We believe that this study provided the first direct observation of the lipid flip-flop induced by the interactions between Au NPs and the model mammalian cell membrane.
Co-reporter:Xiaoxian Zhang, Yaoxin Li, Jeanne M. Hankett and Zhan Chen
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 6) pp:4472-4482
Publication Date(Web):2015/01/02
DOI:10.1039/C4CP05287K
Tributyl acetyl citrate (TBAC), a widely-used “green” plasticizer, has been extensively applied in products for daily use. In this paper, a variety of analytical tools including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), contact angle goniometry (CA), and Fourier transform infrared spectroscopy (FTIR) were applied together to investigate the molecular structures of TBAC plasticized poly(vinyl chloride) (PVC) and the migration behavior of TBAC from PVC–TBAC mixtures into water. We comprehensively examine the effects of oxygen and argon plasma treatments on the surface structures of PVC–TBAC thin films containing various bulk percentages of plasticizers and the leaching behavior of TBAC into water. It was found that TBAC is a relatively stable PVC plasticizer compared to traditional non-covalent plasticizers but is also surface active. Oxygen plasma treatment increased the hydrophilicity of TBAC–PVC surfaces, but did not enhance TBAC leaching. However, argon plasma treatment greatly enhanced the leaching of TBAC molecules from PVC plastics to water. Based on our observations, we believe that oxygen plasma treatment could be applied to TBAC plasticized PVC products to enhance surface hydrophilicity for improving the biocompatibility and antibacterial properties of PVC products. The structural information obtained in this study will ultimately facilitate a molecular level understanding of plasticized polymers, aiding in the design of PVC materials with improved properties.
Co-reporter:Nathan W. Ulrich, John N. Myers and Zhan Chen
RSC Advances 2015 vol. 5(Issue 128) pp:105622-105631
Publication Date(Web):02 Dec 2015
DOI:10.1039/C5RA24332G
Buried interfacial structures containing epoxy underfills are incredibly important in the microelectronics industry and their structures determine the interfacial adhesion properties and ultimately their lifetime. Delamination at such interfaces leads to premature failure of microelectronic devices. In this work, the intrinsically surface sensitive technique, sum frequency generation (SFG) vibrational spectroscopy, was utilized to investigate the molecular structure of buried epoxy interfaces before and after accelerated stress testing in order to relate the molecular-level structural changes to the macroscopic adhesion strength and determine what effect silane adhesion promoters have on polymer/epoxy systems. Strongly hydrogen bonded water was detected at hydrophilic epoxy interfaces and this was correlated with a large decrease in adhesion strength. The addition of a small amount of adhesion promoters drastically improved the adhesion strength following accelerated stress testing at a relatively hydrophilic polymer/epoxy interface, and they were also capable of preventing interfacial water. A hydrophobic polymer/epoxy interface was also studied and silane adhesion promoters were found to improve the adhesion strength following stress testing of the hydrophobic interface as well. This research demonstrates that molecular structural studies of buried epoxy interfaces during hygrothermal aging using SFG vibrational spectroscopy can greatly contribute to the overall understanding of moisture-induced failure mechanisms of organic adhesives found in microelectronic packaging.
Co-reporter:Fu-Gen Wu;Xiaodong Zhang;Siqi Kai;Mengyi Zhang;Hong-Yin Wang;John N. Myers;Yuxiang Weng;Peidang Liu;Ning Gu
Advanced Materials Interfaces 2015 Volume 2( Issue 16) pp:
Publication Date(Web):
DOI:10.1002/admi.201500360
Fluorescent silicon nanoparticles (SiNPs) have shown potential applications in bioimaging/biolabelling, sensing, and nanomedicine/cancer therapy due to their superior properties such as excellent photostability, low cytotoxicity, and versatile surface modification capability. Here, a simple, high-yield, and one-pot method is developed to prepare superbright, water-soluble, and amine-functionalized SiNPs with photoluminescence quantum yield (PLQY) comparable to fluorescent II–VI semiconductor quantum dots (QDs) but with much lower cytotoxicity. By introducing a commercially available amine-containing silane molecule, N-[3-(trimethoxysilyl)propyl]ethylenediamine (DAMO), water-soluble SiNPs are prepared with PLQY of 82.4% via a microwave-assisted method. To the best of our knowledge, this is the highest PLQY value ever reported for water-soluble fluorescent SiNPs. The silicon element in our SiNPs is mainly four-valent silicon and thus these SiNPs may also be termed as oxidized silicon nanospheres or silica nanodots. We have also demonstrated the importance of the silane structure (e.g., a suitable amine content) on the photoluminescence property of the prepared SiNPs. As revealed by the time-resolved photoluminescence technique, the highest PLQY value of DAMO SiNPs is correlated with their monoexponential decay with a relatively long fluorescence lifetime. In addition, the potential use of these SiNPs has also been demonstrated for fluorescent patterning/printing and ion sensing (including Cu2+ and Hg2+).
Co-reporter:Lauren Soblosky, Ayyalusamy Ramamoorthy, Zhan Chen
Chemistry and Physics of Lipids 2015 Volume 187() pp:20-33
Publication Date(Web):April 2015
DOI:10.1016/j.chemphyslip.2015.02.003
•Peptide–bilayer interaction is different on simple and complex lipid bilayers.•The differences in interactions within the two systems vary depending on the peptide.•Use of more complex bilayers could provide some specific interaction dynamics.Supported lipid bilayers are used as a convenient model cell membrane system to study biologically important molecule–lipid interactions in situ. However, the lipid bilayer models are often simple and the acquired results with these models may not provide all pertinent information related to a real cell membrane. In this work, we use sum frequency generation (SFG) vibrational spectroscopy to study molecular-level interactions between the antimicrobial peptides (AMPs) MSI-594, ovispirin-1 G18, magainin 2 and a simple 1,2-dipalmitoyl-d62-sn-glycero-3-phosphoglycerol (dDPPG)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) bilayer. We compared such interactions to those between the AMPs and a more complex dDPPG/Escherichia coli (E. coli) polar lipid extract bilayer. We show that to fully understand more complex aspects of peptide–bilayer interaction, such as interaction kinetics, a heterogeneous lipid composition is required, such as the E. coli polar lipid extract. The discrepancy in peptide–bilayer interaction is likely due in part to the difference in bilayer charge between the two systems since highly negative charged lipids can promote more favorable electrostatic interactions between the peptide and lipid bilayer. Results presented in this paper indicate that more complex model bilayers are needed to accurately analyze peptide–cell membrane interactions and demonstrates the importance of using an appropriate lipid composition to study AMP interaction properties.
Co-reporter:John N. Myers, Zhan Chen
Chinese Chemical Letters 2015 Volume 26(Issue 4) pp:449-454
Publication Date(Web):April 2015
DOI:10.1016/j.cclet.2015.01.016
Polyimides are widely used as chip passivation layers and organic substrates in microelectronic packaging. Plasma treatment has been used to enhance the interfacial properties of polyimides, but its molecular mechanism is not clear. In this research, the effects of polyimide surface plasma treatment on the molecular structures at corresponding polyimide/air and buried polyimide/epoxy interfaces were investigated in situ using sum frequency generation (SFG) vibrational spectroscopy. SFG results show that the polyimide backbone molecular structure was different at polyimide/air and polyimide/epoxy interfaces before and after plasma treatment. The different molecular structures at each interface indicate that structural reordering of the polyimide backbone occurred as a result of plasma treatment and contact with the epoxy adhesive. Furthermore, quantitative orientation analysis indicated that plasma treatment of polyimide surfaces altered the twist angle of the polyimide backbone at corresponding buried polyimide/epoxy interfaces. These SFG results indicate that plasma treatment of polymer surfaces can alter the molecular structure at corresponding polymer/air and buried polymer interfaces.The molecular structure at polyimide/air and buried polyimide/epoxy interfaces was nondestructively characterized in situ using sum frequency generation (SFG) vibrational spectroscopy. Our studies indicate that plasma treatment of polymer surfaces can alter the molecular structure at corresponding polymer/air and buried polymer interfaces.
Co-reporter:Lei Shen, Nathan W. Ulrich, Charlene M. Mello, Zhan Chen
Chemical Physics Letters 2015 Volume 619() pp:247-255
Publication Date(Web):5 January 2015
DOI:10.1016/j.cplett.2014.10.035
Abstract
Surface immobilized peptides/proteins have important applications such as antimicrobial coating and biosensing. We report a study of such peptides/proteins using sum frequency generation vibrational spectroscopy and ATR-FTIR. Immobilization on surfaces via physical adsorption and chemical coupling revealed that structures of chemically immobilized peptides are determined by immobilization sites, chemical environments, and substrate surfaces. In addition, controlling enzyme orientation by engineering the surface immobilization site demonstrated that structures can be well-correlated to measured chemical activity. This research facilitates the development of immobilized peptides/proteins with improved activities by optimizing their surface orientation and structure.
Co-reporter:Yaoxin Li
The Journal of Physical Chemistry C 2015 Volume 119(Issue 13) pp:7146-7155
Publication Date(Web):March 11, 2015
DOI:10.1021/jp5125487
Antimicrobial peptides, because of their unique structural and chemical properties, hold a promising future for the development of a new class of bacterial-resistant antibiotics, effective antimicrobial coatings, and high performance biosensors. To understand the structure/function relationship of surface-bound peptides as they relate to such applications, sum frequency generation (SFG) vibrational spectroscopy, coarse grained molecular dynamics simulations, and antimicrobial activity tests were used to characterize both surface peptide structural information and peptide activity. Results from MSI-78, an antimicrobial peptide, chemically immobilized via the N- (nMSI-78) or C-terminus (MSI-78n), demonstrate that the attachment site influences the structure and behavior of surface-bound peptides. Although both immobilized peptides adopt an α-helical structure in aqueous buffer, nMSI-78 stands up and MSI-78n lies down on the surface, as indicated by both SFG and MD simulations. Antimicrobial activity tests indicated that peptides that stand up interact with bacterial cells much quicker than peptides that lie down. We believe that this study provides fundamental insights into how to rationally engineer peptides and substrate surfaces to produce optimized abiotic/biotic interfaces for antimicrobial applications and beyond.
Co-reporter:Chuan Leng
The Journal of Physical Chemistry C 2015 Volume 119(Issue 16) pp:8775-8780
Publication Date(Web):April 1, 2015
DOI:10.1021/acs.jpcc.5b01649
Understanding the surface hydration of nonfouling materials such as zwitterionic polymers and poly(ethylene glycol) (PEG) aids in the design of new and effective nonfouling materials. Sum frequency generation (SFG) vibrational spectroscopy is a powerful technique used to probe water structures at solid/liquid interfaces. However, SFG signals of H2O consisting of symmetric and asymmetric stretches and Fermi resonance overlap heavily, which complicates the interpretation of the spectra and leads to controversy. In this work, isotopically diluted water was used instead of H2O to study the surface hydration of three zwitterionic polymers and a PEG coating. Because the water signal contains only an O–H stretch, strongly and weakly hydrogen-bonded water structures were easily distinguished. SFG results showed a majority of strongly hydrogen-bonded water molecules at the nonfouling polymer surfaces. For comparison, the water spectra at the surfaces of poly(methyl methacrylate) and poly(ethylene terephthalate) suggested significant amounts of weakly hydrogen-bonded water. The effects of pH on the surface hydration of the nonfouling polymers were also investigated. The materials respond to pH differently because of their different structural formulas. The flip of the water molecules at a carboxybetaine polymer surface in response to pH was also observed.
Co-reporter:Qiuming Wang
The Journal of Physical Chemistry C 2015 Volume 119(Issue 39) pp:22542-22551
Publication Date(Web):September 3, 2015
DOI:10.1021/acs.jpcc.5b06882
Surfaces immobilized with biological molecules such as peptides and proteins are widely used in many important applications including biosensors, medical devices, and food packaging. It was found that the structures of surface-immobilized peptides control their surface properties. In this study, we investigated interfacial behaviors of antimicrobial peptide cecropin P1 (CP1) immobilized onto a maleimide-terminated self-assembled monolayer (Mal SAM) and a mixed SAM (Mal-OH SAM, a mixture of maleimide-terminated SAM and hydroxyl-terminated SAM) surface via C-terminus modified cysteine (CP1c). The surface coverage, secondary structure, orientation, and antimicrobial activity of immobilized CP1c were investigated using surface plasmon resonance (SPR), circular dichroism (CD), sum frequency generation (SFG) vibrational spectroscopy, dynamic contact antimicrobial assay, and coarse grained molecular dynamics (MD) simulations. It was found that the surface coverages of CP1c on the Mal SAM and the mixed Mal-OH SAM are similar. CP1c on Mal SAM possessed a dominant helical structure with a single orientation of ∼32° versus surface normal. CP1c on Mal-OH mixed SAM surface also possessed a dominant helical structure but with multiple orientations. MD simulation results can be correlated to the experimental data: the simulation results indicate a narrow distribution of orientations of CP1c immobilized on Mal SAM, but multiple orientations are sampled on the more hydrophilic Mal-OH SAM. Even though the surface orientations of CP1c immobilized on the two SAM surfaces are different, activity against both Gram-negative and Gram-positive bacteria (Escherichia coli and Staphylococcus aureus) exhibited similar results for CP1c immobilized on both SAM surfaces. We believe that this is because the antimicrobial activity of the surface-immobilized peptides is mainly affected by the electrostatic interactions between the strong basic N-terminal residues and the negatively charged bacteria cell wall/cell membrane.
Co-reporter:John N. Myers
The Journal of Physical Chemistry C 2015 Volume 119(Issue 39) pp:22514-22525
Publication Date(Web):September 3, 2015
DOI:10.1021/acs.jpcc.5b06725
The effects of oxygen plasma treatment on molecular structures at low-k organosilicate (SiCOH) film surfaces and buried interfaces were investigated using sum frequency generation (SFG) vibrational spectroscopy and Raman spectroscopy. SFG and Raman spectra were acquired from pristine and plasma-treated low-k SiCOH films to characterize the interfacial and bulk molecular structures of the films before and after plasma treatment. Two SiCOH films with similar molecular structures but different porosities were investigated to correlate the effects of plasma treatment to the porosity of low-k films. SFG spectra indicated that the surface molecular structure of dense SiCOH films was more resistant to plasma damage than the surface molecular structure of porous SiCOH films. Furthermore, the ratio of SFG peak intensities before and after plasma treatment enabled quantification of methyl depletion at the surface of SiCOH films after plasma treatment. The molecular structure at buried Si/SiCOH interfaces was characterized by simulating SFG spectra from pristine and plasma-treated SiCOH films. SFG spectra from plasma-treated low-k films were simulated by adjusting the methyl orientation and number density parameters at the low-k/air interface to match experimental results. Simulated SFG spectra indicated that methyl groups at the buried interface of both dense and porous SiCOH films were oriented with a high tilt angle before and after plasma treatment. The developed methodology is general and can be extended to characterize the effects of many different plasma treatments, wet chemical treatments, and surface repair treatments on the interfacial molecular structures of many polymer or organic films.
Co-reporter:John N. Myers, Xiaoxian Zhang, Jeff Bielefeld, Qinghuang Lin, and Zhan Chen
The Journal of Physical Chemistry B 2015 Volume 119(Issue 4) pp:1736-1746
Publication Date(Web):January 5, 2015
DOI:10.1021/jp510205u
As low-k dielectric/copper interconnects continue to scale down in size, the interfaces of low-k dielectric materials will increasingly determine the structure and properties of the materials. We report an in situ nondestructive characterization method to characterize the molecular structure at the surface and buried interface of silicon-supported low-k dielectric thin films using interface sensitive infrared-visible sum frequency generation vibrational spectroscopy (SFG). Film thickness-dependent reflected SFG signals were observed, which were explained by multiple reflections of the input and SFG beams within the low-k film. The effect of multiple reflections on the SFG signal was determined by incorporating thin-film interference into the local field factors at the low-k/air and Si/low-k interfaces. Simulated thickness-dependent SFG spectra were then used to deduce the relative contributions of the low-k/air and low-k/Si interfaces to the detected SFG signal. The nonlinear susceptibilities at each interface, which are directly related to the interfacial molecular structure, were then deduced from the isolated interfacial contributions to the detected SFG signal. The method developed here is general and demonstrates that SFG measurements can be integrated into other modern analytical and microfabrication methods that utilize silicon-based substrates. Therefore, the molecular structure at the surface and buried interface of thin polymer or organic films deposited on silicon substrates can be measured in the same experimental geometry used to measure many optical, electrical, and mechanical properties.
Co-reporter:Minyu Xiao, Xiaoxian Zhang, Zachary J. Bryan, Joshua Jasensky, Anne J. McNeil, and Zhan Chen
Langmuir 2015 Volume 31(Issue 18) pp:5050-5056
Publication Date(Web):April 15, 2015
DOI:10.1021/la5048722
Enhancement of charge transport in organic polymer semiconductors is a crucial step in developing optimized devices. A variety of sample preparation conditions, such as film fabrication method, solvent species, and annealing, were found to influence the hole mobility of organic polymers. Despite the fact that many factors can influence their performance, it is believed that polymer surface ordering plays a key role in determining organic polymer function. Here, sum frequency generation (SFG) vibrational spectroscopy was used to nondestructively map the surface/interfacial ordering of poly(3-hexylthiophene) (P3HT) films prepared using different solvents; we believe that solvent interactions determine the degree of surface/interfacial ordering. Both X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM) were used to supplement SFG to systematically study bulk crystallinity and surface morphology. We conclude that SFG is a powerful tool to elucidate the surface/interfacial structural information on polymer semiconducting films. We demonstrate that the solvent composition used to prepare P3HT thin films influences the resulting film surface morphology, surface/interfacial ordering, and bulk crystallinity.
Co-reporter:Chuan Leng, Hilda G. Buss, Rachel A. Segalman, and Zhan Chen
Langmuir 2015 Volume 31(Issue 34) pp:9306-9311
Publication Date(Web):August 5, 2015
DOI:10.1021/acs.langmuir.5b01440
Amphiphilic polypeptoids can be designed with specific sequences of hydrophilic and hydrophobic units, which determine their surface properties for antifouling/fouling release purposes. Although the sequence-dependent surface structures of polypeptoids have been extensively investigated, e.g., with X-ray spectroscopy, their molecular structures under the aqueous conditions relevant to marine fouling have not been studied. In this work, we applied sum frequency generation (SFG) vibrational spectroscopy to study the surface structures and hydration of a series of amphiphilic polypeptoid coatings with different sequences in air and water. SFG spectra, in agreement with X-ray spectroscopy studies, revealed that the surface coverage of the hydrophilic N-(2-methoxyethyl)glycine (Nme) units in air is affected by both the number and position of the hydrophobic N-(heptafluorobutyl)glycine (NF) units in the peptoid chain and is negatively correlated with the surface concentration of the fluorine element. Our ability to probe the SFG signals of water molecules at the peptoid surface provides new information on the hydrated film properties. From these SFG signals and the time evolution of water contact angles on the polymers, we see that the hydrated film properties are also dependent upon the peptoid sequence. This work indicates that the surface presence of the Nme groups and the ability of the polymers to order and strongly hydrogen bond with interfacial water molecules determine their antifouling properties, whereas the surface restructuring rate upon contact with water affects their fouling release behaviors.
Co-reporter:Xiaoxian Zhang, John N. Myers, Jeffery D. Bielefeld, Qinghuang Lin, and Zhan Chen
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 21) pp:18951
Publication Date(Web):October 14, 2014
DOI:10.1021/am504833v
Water adsorption in porous low-k dielectrics has become a significant challenge for both back-end-of-line integration and reliability. A simple method is proposed here to achieve in situ observation of water structure and water-induced structure changes at the poly(methyl silsesquioxane) (PMSQ) surface and the PMSQ/solid buried interface at the molecular level by combining sum frequency generation (SFG) vibrational spectroscopic and Fourier transform infrared (FTIR) spectroscopic studies. First, in situ SFG investigations of water uptake were performed to provide direct evidence that water diffuses predominantly along the PMSQ/solid interface rather than through the bulk. Furthermore, SFG experiments were conducted at the PMSQ/water interface to simulate water behavior at the pore inner surfaces for porous low-k materials. Water molecules were found to form strong hydrogen bonds at the PMSQ surface, while weak hydrogen bonding was observed in the bulk. However, both strongly and weakly hydrogen bonded water components were detected at the PMSQ/SiO2 buried interface. This suggests that the water structures at PMSQ/solid buried interfaces are also affected by the nature of solid substrate. Moreover, the orientation of the Si-CH3 groups at the buried interface was permanently changed by water adsorption, which might due to low flexibility of Si-CH3 groups at the buried interface. In brief, this study provides direct evidence that water molecules tend to strongly bond (chemisorbed) with low-k dielectric at pore inner surfaces and at the low-k/solid interface of porous low-k dielectrics. Therefore, water components at the surfaces, rather than the bulk, are likely more responsible for chemisorbed water related degradation of the interconnection layer. Although the method developed here was based on a model system study, we believe it should be applicable to a wide variety of low-k materials.Keywords: interface; low-k material; molecular structure; sum frequency generation; vibrational spectroscopy; water component
Co-reporter:John N. Myers, Chi Zhang, Chunyan Chen, Zhan Chen
Journal of Colloid and Interface Science 2014 Volume 423() pp:60-66
Publication Date(Web):1 June 2014
DOI:10.1016/j.jcis.2014.02.027
•Molecular structures of spin coated polymer thin films elucidated using surface sensitive spectroscopy.•Different solvents used in spin-coating resulted in varied molecular surface structures.•Aromaticity of the solvent mediated surface phenyl group orientation on the surface.Controlling the surface molecular structure of spin cast polymer films is important for the rational design of surface properties. However, the relationship between spin casting parameters and film surface molecular structure is poorly understood. We report that the surface molecular structure of spin cast homopolymers which contain phenyl groups is influenced by the solvent aromaticity, investigated by a nonlinear optical spectroscopy, sum frequency generation (SFG) vibrational spectroscopy. When phenyl groups were located in a linear polymer backbone, spin casting with aromatic solvents enhanced the phenyl SFG signal relative to when a non-aromatic solvent was used which suggests that the aromatic solvent induced the surface phenyl groups to be more ordered and/or to lie more perpendicular to the film surface. In addition, when alkyl structures were believed to be present at the solvent/air interface, alkyl structures were observed at the film/air interface which suggests that molecular structure at the solvent/air interface was carried to the film surface. The effects of solvent aromaticity on phenyl ordering at spin cast film surfaces can be explained by different molecular structures of polymer chains at solvent/air interfaces, preferential solvation of functional groups during evaporation, and re-orientation of bulky side groups at the polymer film/air interface.
Co-reporter:Jeanne M. Hankett, Xiaolin Lu, Yuwei Liu, Emily Seeley and Zhan Chen
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 37) pp:20097-20106
Publication Date(Web):08 Aug 2014
DOI:10.1039/C4CP03206C
Upon water contact, phthalate-plasticized poly(vinyl chloride) (PVC) surfaces are highly unstable because the plasticizer molecules are not covalently bound to the polymer network. As a result, it is difficult to predict how the surface polymer chains and plasticizers may interact with water without directly probing the plastic/water interface in situ. We successfully studied the molecular surface restructuring of 10 wt% and 25 wt% bis 2-ethylhexyl phthalate (DEHP)-plasticized and pure PVC films (deposited on solid substrates) in situ due to water contact using sum frequency generation (SFG) vibrational spectroscopy. SFG spectral signals from both the top and the bottom of the plastic film were obtained simultaneously, so a thin-film model spectral analysis was applied to separately identify the molecular changes of plastics at the surface and the plastic/substrate interface in water. It was found that in water both the structures of the plastic surface and the buried plastic/substrate interface changed. After removing the samples from the water and exposing them to air again, the surface structures did not completely recover. Further SFG experiments confirmed that small amounts of DEHP were transferred into the water. The leached DEHP molecules could reorder and permanently transfer to new surfaces through water contact. Our studies indicate that small amounts of phthalates can transfer from surface to surface through water contact in an overall scope of minutes. This study yields vital new information on the molecular surface structures of DEHP plasticized PVC in water, and the transfer behaviors and environmental fate of plasticizers in polymers.
Co-reporter:Jeanne M. Hankett;Alexer Welle;Joerg Lahann
Journal of Applied Polymer Science 2014 Volume 131( Issue 16) pp:
Publication Date(Web):
DOI:10.1002/app.40649
ABSTRACT
Millions of tons of plasticized poly(vinyl chloride) (PVC) materials are disposed every year. A biologically sustainable and green method for removal of toxic plasticizers from polymer systems after disposal is highly desired since plasticizers can leach out into the environment over decades. Here we compare the surface and bulk structural changes of DEHP-plasticized PVC after two treatments intended to degrade bis-2-ethylhexyl phthalate (DEHP) in PVC plastic: short wave (254 nm) UV with and without the addition of 35 wt % H2O2. Sum frequency generation vibrational spectroscopy (SFG) reveals the addition of aqueous H2O2 decreases CH3 signals on the surface of the films up to 8 h, due to increased molecular disorder and the removal of alkyl chains. Secondary ion mass spectrometry demonstrates that the degradation of DEHP after 8 h of reaction is similar with and without the use of H2O2. However, FTIR results reveal that the introduction of H2O2 reduces bulk DEHP degradation and leads to competing radical chain scission reactions with PVC. Therefore, simple short wave UV exposure may be an effective means to degrade DEHP within and on PVC plastic and the addition of H2O2 is only beneficial if additional degradation of PVC is needed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40649.
Co-reporter:Fu-Gen Wu ; Pei Yang ; Chi Zhang ; Xiaofeng Han ; Minghu Song
The Journal of Physical Chemistry C 2014 Volume 118(Issue 31) pp:17538-17548
Publication Date(Web):July 14, 2014
DOI:10.1021/jp503038m
Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy were applied to study interactions between an antipsychotic agent, chlorpromazine (CPZ), and model cell membranes consisting of either distearoylphosphatidylcholine (DSPC) or dipalmitoylphosphatidylglycerol (DPPG). The PC and PG lipids represent the zwitterionic and anionic components of the cell membranes, respectively. For an isotopically asymmetric bilayer composed of a deuterated lipid leaflet and a hydrogenated lipid leaflet, the time-dependent SFG signals from the lipids revealed that CPZ can significantly accelerate the flip-flop process of the neutral DSPC bilayer and such an acceleration effect is more pronounced at higher CPZ concentrations. While for the negatively charged DPPG bilayer, it was found that CPZ molecules can immediately bind to and disrupt the outer lipid leaflet and then gradually reduce the ordering of the inner lipid leaflet. A higher CPZ concentration in the subphase leads to a faster disordering effect on the inner leaflet. The association of CPZ to the lipid membranes can be verified by the change in the SFG spectra of the OH stretching vibration of the interfacial water molecules. ATR-FTIR results revealed that addition of CPZ to the subphase did not exert significant effect on the dDSPC/dDSPC bilayer, especially at low CPZ concentrations (<2 mM). It was found that CPZ can cause gel-to-fluid phase transition of the dDPPG/dDPPG bilayer at CPZ concentrations below 2 mM, and higher CPZ concentrations can lead to dissolution of the bilayer. This work demonstrated that SFG (along with ATR-FTIR) is a powerful in situ and label-free technique that can be used to study various aspects of the drug–membrane interactions at the molecular level.
Co-reporter:Bolin Li ; Xiaolin Lu ; Xiaofeng Han ; Fu-Gen Wu ; John N. Myers
The Journal of Physical Chemistry C 2014 Volume 118(Issue 49) pp:28631-28639
Publication Date(Web):November 20, 2014
DOI:10.1021/jp509272k
During the model membrane formation process from a lipid monolayer of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (sodium salt) (DPPG) in air to a lipid bilayer of DPPG/deuterated-DPPG (dDPPG) on water, the intensity of sum frequency generation (SFG) vibrational signals from the DPPG molecules increased by ∼34 times. The increased signal intensity could be caused by inherently different molecular ordering of lipid molecules in the monolayer/bilayer or by optical effects induced by different contacting mediums (prism in air and prism contacting water) (see Figure 2). We resolve the two possibilities by analyzing tilt angles of the methyl groups at DPPG hydrophobic ends for the monolayer/bilayer which reflect the molecular ordering information and by evaluating the Fresnel coefficients which reflect the contacting medium-induced optical effect. DPPG molecules were more ordered after transformation from a lipid monolayer in air to a lipid bilayer on water, which induced a slight signal increase. The augmentation of the Fresnel coefficients was well-correlated to the enhancement of the SFG resonant signal. Therefore, this case study is applicable to similar interfacial systems probed by the SFG spectroscopy to deduce the relative contributions of interfacial molecular ordering and interfacial Fresnel coefficients to the detected SFG signal.
Co-reporter:Xiaofeng Han, Yuwei Liu, Fu-Gen Wu, Joshua Jansensky, Taehoon Kim, Zunliang Wang, Charles L. Brooks III, Jianfeng Wu, Chuanwu Xi, Charlene M. Mello, and Zhan Chen
The Journal of Physical Chemistry B 2014 Volume 118(Issue 11) pp:2904-2912
Publication Date(Web):February 20, 2014
DOI:10.1021/jp4122003
Molecular structures such as conformation and orientation are crucial in determining the activity of peptides immobilized to solid supports. In this study, sum frequency generation (SFG) vibrational spectroscopy was applied to investigate such structures of peptides immobilized on self-assembled monolayers (SAMs). Here cysteine-modified antimicrobial peptide cecropin P1 (CP1) was chemically immobilized onto SAM with a maleimide terminal group. Two important characteristics, length of the poly(ethylene glycol) (PEG) segment in the SAM and location of the cysteine residue in the peptide, were examined using SFG spectroscopy to determine the effect of each on surface immobilization as well as peptide secondary structure and its orientation in the immobilized state. Results have shown that while each length of PEG chain studied promotes chemical immobilization of the target peptide and prevents nonspecific adsorption, CP1 immobilized on long-chain (PEG2k) maleimide SAMs shows random coil structure in water, whereas CP1 demonstrates α-helical structure when immobilized on short-chain (with four ethylene glycol units - (EG4)) maleimide SAMs. Placement of the cysteine residue at the C-terminus promotes the formation of α-helical structure of CP1 with a single orientation when tethered to EG4 maleimide SAM surfaces. In contrast, immobilization via the N-terminal cysteine of CP1 results in a random coil or lying-down helical structure. The bacteria capturing/killing capability was tested, showing that the surface-immobilized CP1 molecules via C- and N- terminal cysteine exhibit only slight difference, even though they have different secondary structures and orientations.
Co-reporter:Zunliang Wang, Xiaofeng Han, Nongyue He, Zhan Chen, and Charles L. Brooks III
The Journal of Physical Chemistry B 2014 Volume 118(Issue 21) pp:5670-5680
Publication Date(Web):May 6, 2014
DOI:10.1021/jp5023482
Biosensors using peptides or proteins chemically immobilized on surfaces have many advantages such as better sensitivity, improved stability, and longer shelf life compared to those prepared using physically adsorbed biomolecules. Chemical immobilization can better control the interfacial conformation and orientation of peptides and proteins, leading to better activity of these biomolecules. In this research, molecular dynamics (MD) simulations were employed to systematically investigate the structure and dynamics of surface-tethered antimicrobial peptide cecropin P1 (CP1) modified with a cysteine residue at the C- (CP1c) or N-terminus (cCP1). Such CP1c and cCP1 molecules were chemically immobilized onto a silane-EG4-maleimide self-assembled monolayer (SAM) surface by forming a thio-ether bond between the cysteine group in CP1c or cCP1 and the surface maleimide group. The simulation results showed that the immobilized cCP1 (via the N-terminus) tends to bend and gradually lie down onto the SAM surface, due to the large structural fluctuation of the C-terminus induced by unfavorable interactions between the hydrophobic C-terminal residues and water. Differently, the tethered CP1c (via the C-terminus) more or less stands up on the surface, only tilting slightly even after 60 ns. The simulation results can be well correlated to the recent experimental results obtained from sum frequency generation (SFG) vibrational spectroscopic study. The current simulation data provide more atomic level details on how the hydrophobicity difference in the C-terminus and N-terminus of the amphiphilic peptide can lead to different structures of the same peptide tethered to the surface via different termini. This knowledge can be used to rationally design chemically immobilized peptides to achieve desired structure and functionality.
Co-reporter:Zunliang Wang, Xiaofeng Han, Nongyue He, Zhan Chen, and Charles L. Brooks III
The Journal of Physical Chemistry B 2014 Volume 118(Issue 42) pp:12176-12185
Publication Date(Web):September 29, 2014
DOI:10.1021/jp508550d
Our recent sum frequency generation (SFG) vibrational spectroscopic experiment ( J. Phys. Chem. B 2014, 118, 2904−2912) showed that immobilized antimicrobial peptide cecropin P1 (cCP1) on a self-assembled monolayer (SAM) surface via N-terminus exhibited significantly different conformational and/or orientational behaviors when exposed to pure water vs a 50% (v/v) 2,2,2-trifluoroethanol (TFE)/water mixture. Meanwhile, our recent molecular dynamics (MD) simulations ( J. Phys. Chem. B 2014, 118, 5670−5680) further revealed that the immobilized cCP1 via N-terminus in pure water largely adopts an overall bent structure lying down on the SAM surface, consistent with the SFG observation. Here, MD simulations were performed on the immobilized cCP1 on a SAM surface via N-terminus while in contact with a 50% (v/v) TFE/water mixture to further investigate the effects of environment (water vs TFE/water mixture) on the interfacial structure and orientation of immobilized peptide. The simulation results demonstrated that the immobilized cCP1 on the SAM surface via the N-terminus with two different starting states with different orientations and conformations, when exposed to a 50% (v/v) TFE/water mixture, was eventually able to maintain a linear α-helical structure, standing upright on the SAM surface. Taken with the corresponding SFG observation, our simulation results indicate that the conformational behavior of the immobilized peptide is mediated by the local hydrophobic environments resulting from the TFE aggregation around the peptide. Such knowledge can be used to regulate the surface conformation and functionality of immobilized peptides via changing surrounding chemical environments (e.g., TFE cosolvent), which is important for the microbial detection and killing based on surface-immobilized antimicrobial peptides.
Co-reporter:John N. Myers, Chi Zhang, Kang-Wook Lee, Jaimal Williamson, and Zhan Chen
Langmuir 2014 Volume 30(Issue 1) pp:165-171
Publication Date(Web):2017-2-22
DOI:10.1021/la4037869
Interfacial properties such as adhesion are determined by interfacial molecular structures. Adhesive interfaces in microelectronic packages that include organic polymers such as epoxy are susceptible to delamination during accelerated stress testing. Infrared–visible sum frequency generation vibrational spectroscopy (SFG) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were used to study molecular structures at buried epoxy interfaces during hygrothermal aging to relate molecular structural changes at buried interfaces to decreases in macroscopic adhesion strength. SFG peaks associated with strongly hydrogen bonded water were detected at hydrophilic epoxy interfaces. Ordered interfacial water was also correlated to large decreases in interfacial adhesion strength that occurred as a result of hygrothermal aging, which suggests that water diffused to the interface and replaced original hydrogen bond networks. No water peaks were observed at hydrophobic epoxy interfaces, which was correlated with a much smaller decrease in adhesion strength from the same aging process. ATR-FTIR water signals observed in the epoxy bulk were mainly contributed by relatively weakly hydrogen bonded water molecules, which suggests that the bulk and interfacial water structure was different. Changes in interfacial methyl structures were observed regardless of the interfacial hydrophobicity which could be due to water acting as a plasticizer that restructured both the bulk and interfacial molecular structure. This research demonstrates that SFG studies of molecular structural changes at buried epoxy interfaces during hygrothermal aging can contribute to the understanding of moisture-induced failure mechanisms in electronic packages that contain organic adhesives.
Co-reporter:Bei Ding, Alisa Glukhova, Katarzyna Sobczyk-Kojiro, Henry I. Mosberg, John J. G. Tesmer, and Zhan Chen
Langmuir 2014 Volume 30(Issue 3) pp:823-831
Publication Date(Web):2017-2-22
DOI:10.1021/la404055a
G protein-coupled receptor kinase 5 (GRK5) is thought to associate with membranes in part via N- and C-terminal segments that are typically disordered in available high-resolution crystal structures. Herein we investigate the interactions of these regions with model cell membrane using combined sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy. It was found that both regions associate with POPC lipid bilayers but adopt different structures when doing so: GRK5 residues 2–31 (GRK52–31) was in random coil whereas GRK5546–565 was partially helical. When the subphase for the GRK52–31 peptide was changed to 40% TFE/60% 10 mM phosphate pH 7.4 buffer, a large change in the SFG amide I signal indicated that GRK52–31 became partially helical. By inspecting the membrane behavior of two different segments of GRK52–31, namely, GRK52–24 and GRK525–31, we found that residues 25–31 are responsible for membrane binding, whereas the helical character is imparted by residues 2–24. With SFG, we deduced that the orientation angle of the helical segment of GRK52–31 is 46 ± 1° relative to the surface normal in 40% TFE/60% 10 mM phosphate pH = 7.4 buffer but increases to 78 ± 11° with higher ionic strength. We also investigated the effect of PIP2 in the model membrane and concluded that the POPC:PIP2 (9:1) lipid bilayer did not change the behavior of either peptide compared to a pure POPC lipid bilayer. With ATR-FTIR, we also found that Ca2+·calmodulin is able to extract both peptides from the POPC lipid bilayer, consistent with the role of this protein in disrupting GRK5 interactions with the plasma membrane in cells.
Co-reporter:Xiaoxian Zhang and Zhan Chen
Langmuir 2014 Volume 30(Issue 17) pp:4933-4944
Publication Date(Web):2017-2-22
DOI:10.1021/la500476u
Phthalates, the most widely used plasticizers in poly(vinyl chloride) (PVC), have been extensively studied. In this paper, a highly sensitive, easy, and effective method was developed to examine short-term phthalate leaching from PVC/phthalate films at the molecular level using sum frequency generation vibrational spectroscopy (SFG). Combining SFG and Fourier transform infrared spectroscopy (FTIR), surface and bulk molecular structures of PVC/phthalate films were also comprehensively evaluated during the phthalate leaching process under various environments. The leaching processes of two phthalates, diethyl phthalate (DEP) and dibutyl phthalate (DBP), from the PVC/phthalate films with various weight ratios were studied. Oxygen plasma was applied to treat the PVC/phthalate film surfaces to verify its efficacy on preventing/reducing phthalate leaching from PVC. Our results show that DBP is more stable than DEP in PVC/phthalate films. Even so, DBP molecules were still found to very slowly leach to the environment from PVC at 30 °C, at a rate much slower than DEP. Also, the bulk DBP content substantially influences the DBP leaching. Higher DBP bulk concentration yields less stable DBP molecules in the PVC matrix, allowing molecules to leach from the polymer film more easily. Additionally, DBP leaching is very sensitive to temperature changes; higher temperature can strongly enhance the leaching process. For most cases, the oxygen plasma treatment can effectively prevent phthalate leaching from PVC films (e.g., for samples with low bulk concentrations of DBP—5 and 30 wt %). It is also capable of reducing phthalate leaching from high DBP bulk concentration PVC samples (e.g., 70 wt % DBP in PVC/DBP mixture). This research develops a highly sensitive method to detect chemicals at the molecular level as well as provides surface and bulk molecular structural changes. The method developed here is general and can be applied to detect small amounts of chemical/biological environmental contaminants.
Co-reporter:Lei Shen, McKenna Schroeder, Tadeusz L. Ogorzalek, Pei Yang, Fu-Gen Wu, E. Neil G. Marsh, and Zhan Chen
Langmuir 2014 Volume 30(Issue 20) pp:5930-5938
Publication Date(Web):May 6, 2014
DOI:10.1021/la5016862
We demonstrate the control of enzyme orientation for enzymes chemically immobilized on surfaces. Nitro-reductase (NfsB) has the ability to reduce a broad range of nitro-containing compounds and has potential applications in a broad range of areas including the detection and decomposition of explosives. The enzyme was tethered through unique surface cysteine residues to a self-assembled monolayer (SAM) terminated with maleimide groups. One cysteine was introduced close to the active site (V424C), and the other, at a remote site (H360C). The surface-tethered NfsB variants were interrogated by a combination of surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) to determine how the mode of attachment altered the enzyme’s orientation. The activities of the two immobilized NfsB variants were measured and can be well correlated to the deduced orientations. The relationships among enzyme engineering, surface immobilization, enzyme orientation, and enzyme activity were revealed.
Co-reporter:Fu-Gen Wu, Pei Yang, Chi Zhang, Bolin Li, Xiaofeng Han, Minghu Song, and Zhan Chen
Langmuir 2014 Volume 30(Issue 28) pp:8491-8499
Publication Date(Web):2017-2-22
DOI:10.1021/la501718n
Sum frequency generation (SFG) vibrational spectroscopy was applied to study molecular interactions between amantadine and substrate supported lipid bilayers serving as model cell membranes. Both isotopically asymmetric and symmetric lipid bilayers were used in the research. SFG results elucidated how the water-soluble drug, amantadine, influenced the packing state of each leaflet of a lipid bilayer and how the drugs affected the lipid flip-flop process. It is difficult to achieve such detailed molecular-level information using other analytical techniques. Especially, from the flip-flop rate change of isotopically asymmetric lipid bilayer induced by amantadine, important information on the drug–membrane interaction mechanism can be derived. The results show that amantadine can be associated with zwitterionic PC bilayers but has a negligible influence on the flip-flop behavior of PC molecules unless at high concentrations. Different effects of amantadine on the lipid bilayer were observed for the negatively charged DPPG bilayer; low concentration amantadine (e.g., 0.20 mM) in the subphase could immediately disturb the outer lipid leaflet and then the lipid associated amantadine molecules gradually reorganize to cause the outer leaflet to return to the original orderly packed state. Higher concentration amantadine (e.g., 5.0 mM) immediately disordered the packing state of the outer lipid leaflet. For both the high and low concentration cases, amantadine molecules only bind to the outer PG leaflet and cannot translocate to the inner layer. The presence of amantadine within the negatively charged lipid layers has certain implications for using liposomes as drug delivery carriers for amantadine. Besides, by using PC or PG bilayers with both leaflets deuterated, we were able to examine how amantadine is distributed and/or oriented within the lipid bilayer. The present work demonstrates that SFG results can provide an in-depth understanding of the molecular mechanisms of interactions between water-soluble drugs and model cell membranes.
Co-reporter:Xiaolin Lu, John N. Myers, and Zhan Chen
Langmuir 2014 Volume 30(Issue 31) pp:9418-9422
Publication Date(Web):July 14, 2014
DOI:10.1021/la502037h
Understanding molecular structures of buried polymer/metal interfaces is important for the design and development of polymer adhesives used in advanced microelectronic devices and polymer anticorrosion coatings for metals. The buried interfacial molecular structure between polystyrene (PS) and silver (Ag) was investigated using infrared-visible sum frequency generation (SFG) vibrational spectroscopy via a “sandwiched” sample geometry. SFG resonant signals from the phenyl C–H stretching vibrational modes were detected from the PS/Ag interface, suggesting that the PS phenyl groups at this buried polymer/metal interface are ordered. Spectral analysis indicated that the phenyl groups at the buried PS/Ag interface tilt toward the interface, pointing away from the Ag side.
Co-reporter:Chi Zhang, John N. Myers, and Zhan Chen
Langmuir 2014 Volume 30(Issue 42) pp:12541-12550
Publication Date(Web):2017-2-22
DOI:10.1021/la502239u
Epoxies are widely used as main components in packaging underfills for microelectronics. Their strong adhesion to different substrate materials is an important factor for the functioning of electronic devices. Amines are commonly used cross-linking agents for epoxides. However, the molecular mechanisms of epoxide–amine mixture adhesion to substrate materials remain unclear. In this research we investigated the adhesion mechanism of epoxide–amine mixtures at poly(ethylene terephthalate) (PET) interfaces using attenuated total-internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy and sum frequency generation (SFG) vibrational spectroscopy. Results show that both epoxide and amine could diffuse into the PET film. They could also dissolve or modify the PET film at the interphase region. In the process of epoxy curing on PET, epoxide molecules could cross-link with the modified PET film, providing strong adhesion. This hypothesis was further confirmed by adding reactive and nonreactive silanes to the epoxies and measuring the adhesion strengths of such mixtures to PET. The reactive silanes could cross-link with the system, showing good adhesion, while the nonreactive silane prevented sufficient cross-linking, showing poor adhesion. This research developed an in-depth insight for molecular behaviors at the epoxy/PET interface which helped clarify the related adhesion mechanism.
Co-reporter:Jintao Yang, Mingzhen Zhang, Hong Chen, Yung Chang, Zhan Chen, and Jie Zheng
Biomacromolecules 2014 Volume 15(Issue 8) pp:
Publication Date(Web):June 26, 2014
DOI:10.1021/bm500598a
Numerous biocompatible antifouling polymers have been developed for a wide variety of fundamental and practical applications in drug delivery, biosensors, marine coatings, and many other areas. Several antifouling mechanisms have been proposed, but the exact relationship among molecular structure, surface hydration property, and antifouling performance of antifouling polymers still remains elusive. Here this work strives to provide a better understanding of the structure–property relationship of poly(N-hydroxyalkyl acrylamide)-based materials. We have designed, synthesized, and characterized a series of polyHAAA brushes of various carbon spacer lengths (CSLs), that is, poly(N-hydroxymethyl acrylamide) (polyHMAA), poly(N-(2-hydroxyethyl)acrylamide) (polyHEAA), poly(N-(3-hydroxypropyl)acrylamide) (polyHPAA), and poly(N-(5-hydroxypentyl)acrylamide) (polyHPenAA), to study the structural dependence of CSLs on their antifouling performance. HMAA, HEAA, HPAA, and HPenAA monomers contained one, two, three, and five methylene groups between hydroxyl and amide groups, while the other groups in polymer backbones were the same as each other. The relation of such small structural differences of polymer brushes to their surface hydration and antifouling performance was studied by combined experimental and computational methods including surface plasmon resonance sensors, sum frequency generation (SFG) vibrational spectroscopy, cell adhesion assay, and molecular simulations. Antifouling results showed that all polyHAAA-based brushes were highly surface resistant to protein adsorption from single protein solutions, undiluted blood serum and plasma, as well as cell adhesion up to 7 days. In particular, polyHMAA and polyHEAA with the shorter CSLs exhibited higher surface hydration and better antifouling ability than polyHPMA and polyHPenAA. SFG and molecular simulations further revealed that the variation of CSLs changed the ratio of hydrophobicity/hydrophilicity of polymers, resulting in different hydration characteristics. Among them, polyHMAA and polyHEAA with the shorter CSLs showed the highest potency for surface hydration and antifouling abilities, while polyHPenAA showed the lowest potency. The combination of both hydroxyl and amide groups in the same polymer chain provides a promising structural motif for the design of new effective antifouling materials.
Co-reporter:Chi Zhang ; Fu-Gen Wu ; Peipei Hu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 23) pp:12195-12205
Publication Date(Web):May 14, 2014
DOI:10.1021/jp502383u
Polyethylenimine (PEI) has been widely used as a transfection agent for gene delivery, but it is cytotoxic and can lead to cell apoptosis. Although several apoptotic mechanisms have been proposed, a molecular level understanding of PEI/cell membrane interaction can help develop further insight into such cytotoxicity. We combined sum frequency generation (SFG) vibrational spectroscopy and attenuated total-internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study the effect of PEI on lipid transbilayer movement in supported bilayers (serving as model cell membranes) as a function of lipid composition, PEI concentration, and temperature. For both dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylcholine (DSPC) bilayers, PEI molecules showed no significant effect on lipid translocation at room temperature (21 °C). In contrast, significant lipid translocation was observed near the physiological temperature (39 °C), indicating the ability of PEI to induce lipid translocation in both negatively charged and zwitterionic lipid bilayers, without the assistance of membrane proteins. Furthermore, results showed that PEI had strong interactions with negatively charged DPPG and weak interactions with zwitterionic DSPC. Concentration-dependent studies indicated that the lipid translocation rate had a linear dependence on the PEI concentration in the subphase. The effects of branched and linear PEIs were compared in the study, showing that branched PEI had a greater effect on the lipid translocation rate due to the higher charge density, which might be a possible indication of higher toxicity. ATR-FTIR spectroscopy verified that the results observed in SFG were mainly caused by lipid translocation, not bilayer damage or removal from the substrate. The combined SFG and ATR-FTIR study provides a powerful method to examine molecular interactions between lipid bilayers and polyelectrolytes at a molecular level. The results can help to develop further understanding on PEI’s cytotoxicity in biological systems.
Co-reporter:Pei Yang ; Andrew Boughton ; Kristoff T. Homan ; John J. G. Tesmer
Journal of the American Chemical Society 2013 Volume 135(Issue 13) pp:5044-5051
Publication Date(Web):March 5, 2013
DOI:10.1021/ja3116026
The manner in which the heterotrimeric G protein complexes Gβ1γ2 and Gαiβ1γ2 interact with membranes is likely related to their biological function. We combined complementary measurements from sum frequency generation (SFG) vibrational and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to determine the possible membrane orientations of Gβ1γ2 and the Gαiβ1γ2 heterotrimer more precisely than could be achieved using SFG alone. The most likely orientations of Gβ1γ2 and the Gαiβ1γ2 heterotrimer were both determined to fall within a similar narrow range of twist and tilt angles, suggesting that Gβ1γ2 may bind to Gαi without a significant change in orientation. This “basal” orientation seems to depend primarily on the geranylgeranylated C-terminus of Gγ2 along with basic residues at the N-terminus of Gαi, and suggests that activated G protein-coupled receptors (GPCRs) must reorient G protein heterotrimers at lipid bilayers to catalyze nucleotide exchange. The innovative methodologies developed in this paper can be widely applied to study the membrane orientation of other proteins in situ.
Co-reporter:Yuwei Liu ; Tadeusz L. Ogorzalek ; Pei Yang ; McKenna M. Schroeder ; E. Neil G. Marsh
Journal of the American Chemical Society 2013 Volume 135(Issue 34) pp:12660-12669
Publication Date(Web):July 24, 2013
DOI:10.1021/ja403672s
The immobilization of enzymes on solid supports is widely used in many applications, including biosensors, antifouling coatings, food packaging materials, and biofuel cells. Enzymes tend to lose their activity when in contact with a support surface, a phenomenon that has been attributed to unfavorable orientation and (partial) unfolding. In this work, specific immobilization of 6-phospho-β-galactosidase (β-Gal) on a self-assembled monolayer (SAM) containing maleimide end groups and oligo(ethylene glycol) spacer segments was achieved through a unique cysteinyl residue. A systematic means to characterize the interfacial orientation of immobilized enzymes has been developed using a combination of sum frequency generation vibrational spectroscopy and attenuated total reflectance FTIR-spectroscopy. The possible orientations of the immobilized β-Gal were determined and found to be well-correlated with the tested activity of β-Gal. This study will impact the development of an increasingly wide range of devices that use surface-immobilized enzymes as integral components with improved functions, better sensitivity, enhanced stability, and longer shelf life.
Co-reporter:Chuan Leng, Katherine A. Gibney, Yuwei Liu, Gregory N. Tew, and Zhan Chen
ACS Macro Letters 2013 Volume 2(Issue 11) pp:1011
Publication Date(Web):October 30, 2013
DOI:10.1021/mz400503z
Antibiofouling materials have a wide range of applications in biomedical devices and marine coatings. Due to the amphiphilic nature of proteins and organisms, amphiphilic materials have been designed to resist their unspecific adsorption. Surface restructuring behavior of amphiphilic materials in water is believed to play a key role in the antibiofouling mechanisms. In this work, the surface structures of several amphiphilic polybetaine coatings in water have been probed in situ using sum frequency generation (SFG) vibrational spectroscopy. These are novel polybetaines constructed from functionalized polynorbornenes. The polybetaines with oligo(ethylene glycol) (OEG), octyl (C8), or fluorinated (F13) side chains exhibit different surface restructuring behaviors upon contacting water due to their different surface hydrophobicity. The OEG and C8 chains were present and ordered at the water interface, while the F13 chain withdrew from water. The hydrophilic betaine group extended into the water and formed hydrogen bonds with water molecules. The surface restructuring of these materials detected using SFG can be well correlated to their antibiofouling performance, providing an understanding of their antibiofouling mechanisms.
Co-reporter:Chi Zhang, John N. Myers and Zhan Chen
Soft Matter 2013 vol. 9(Issue 19) pp:4738-4761
Publication Date(Web):11 Mar 2013
DOI:10.1039/C3SM27710K
Sum frequency generation (SFG) vibrational spectroscopy has been developed into an important technique to study surfaces and interfaces. It can probe buried interfaces in situ and provide molecular level structural information such as the presence of various chemical moieties, quantitative molecular functional group orientation, and time dependent kinetics or dynamics at such interfaces. This paper focuses on these three most important advantages of SFG and reviews some of the recent progress in SFG studies on interfaces related to polymer materials and biomolecules. The results discussed here demonstrate that SFG can provide important molecular structural information of buried interfaces in situ and in real time, which is difficult to obtain by other surface sensitive analytical techniques.
Co-reporter:Yuwei Liu, Chuan Leng, Bret Chisholm, Shane Stafslien, Partha Majumdar, and Zhan Chen
Langmuir 2013 Volume 29(Issue 9) pp:2897-2905
Publication Date(Web):February 8, 2013
DOI:10.1021/la304571u
Poly(dimethylsiloxane) (PDMS) materials have been extensively shown to function as excellent fouling-release (FR) coatings in the marine environment. The incorporation of biocide moieties, such as quaternary ammonium salts (QAS), can impart additional antibiofouling properties to PDMS-based FR coating systems. In this study, the molecular surface structures of two different types of QAS-incorporated PDMS systems were investigated in different chemical environments using sum frequency generation vibrational spectroscopy (SFG). Specifically, a series of PDMS coatings containing either a QAS with a single ammonium salt group per molecule or a quaternary ammonium-functionalized polyhedral oligomeric silsesquioxane (Q-POSS) were measured with SFG in air, water, and artificial seawater (ASW) to investigate the relationships between the interfacial surface structures of these materials and their antifouling properties. Although previous studies have shown that the above-mentioned materials are promising contact-active antifouling coatings, slight variations of the QAS structure can lead to substantial differences in the antifouling performance. Indeed, the SFG results presented here indicated that the surface structures of these materials depend on several factors, such as the extent of quaternization, the molecular weight of the PDMS component, and the functional groups of the QAS used for incorporation into the PDMS matrix. It was concluded that in aqueous environments a lower extent of Q-POSS quaternization and the use of ethoxy (instead of methoxy) functional groups for QAS incorporation facilitated the extension of the alkyl chains away from the nitrogen atom of the QAS on the surface. The SFG results correlated well with the antifouling activity studies that indicated that the coatings exhibiting a lower concentration of longer alkyl chains protruding out of the surface can neutralize microorganisms more effectively, ultimately leading to better antifouling performance. Furthermore, the results of this study provide additional evidence that incorporated QAS exert their antimicrobial activity through a two-step interaction. The first step is the adsorption of the bacteria on the surface as a result of the electrostatic attraction between the negatively charged microorganisms and the positively charged QAS nitrogen atoms on the surface. The second step is the disruption of the cell membranes by the penetration of the QAS long, extended alkyl chains.
Co-reporter:Xiaoxian Zhang, Chi Zhang, Jeanne M. Hankett, and Zhan Chen
Langmuir 2013 Volume 29(Issue 12) pp:4008-4018
Publication Date(Web):February 27, 2013
DOI:10.1021/la4000796
In this research, a variety of analytical techniques including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the surface and bulk structures of phthalate plasticized poly(vinyl chloride) (PVC) at the molecular level. Two types of phthalate molecules with different chain lengths, diethyl phthalate (DEP) and dibutyl phthalate (DBP), mixed with PVC in various weight ratios were examined to verify their different surface and bulk behaviors. The effects of oxygen and argon plasma treatment on PVC/DBP and PVC/DEP hybrid films were investigated on both the surface and bulk of films using SFG and CARS to evaluate the different plasticizer migration processes. Without plasma treatment, SFG results indicated that more plasticizers segregate to the surface at higher plasticizer bulk concentrations. SFG studies also demonstrated the presence of phthalates on the surface even at very low bulk concentration (5 wt %). Additionally, the results gathered from SFG, CARS, and XPS experiments suggested that the PVC/DEP system was unstable, and DEP molecules could leach out from the PVC under low vacuum after several minutes. In contrast, the PVC/DBP system was more stable; the migration process of DBP out of PVC could be effectively suppressed after oxygen plasma treatment. XPS results indicated the increase of C═O/C–O groups and decrease of C–Cl functionalities on the polymer surface after oxygen plasma treatment. The XPS results also suggested that exposure to argon plasma induced chemical bond breaking and formation of cross-linking or unsaturated groups with chain scission on the surface. Finally, our results indicate the potential risk of using DEP molecules in PVC since DEP can easily leach out from the polymeric bulk.
Co-reporter:Chi Zhang and Zhan Chen
Langmuir 2013 Volume 29(Issue 2) pp:610-619
Publication Date(Web):December 15, 2012
DOI:10.1021/la3041727
Because of the wide applications of silicone adhesives, it is important to study adhesion mechanisms of silicone elastomers to polymers. Adhesion properties are believed to be directly related to the molecular structures at the adhesive/substrate interfaces. To improve adhesion, adhesion promoters such as silanes are commonly used to modify the interfacial structures. It is difficult to study buried interfacial molecular structures between two materials in situ using conventional analytical techniques. In this study, sum frequency generation (SFG) vibrational spectroscopy was used to investigate molecular structures at buried silicone/poly(ethylene terephthalate) (PET) interfaces. Environmental-friendly epoxysilanes including (3-glycidoxypropyl)triethoxysilane (γ-GPES), (3-glycidoxypropyl)methyldiethoxysilane (γ-GPDES), and (3-glycidoxypropyl)dimethylethoxysilane (γ-GPDMES) and their mixtures with methylvinylsiloxanol (MVS) were used as adhesion promoters to modify silicone adhesion properties to PET. Various PET/silane, PET/uncured silicone, and PET/cured silicone interfaces were examined. The interfacial structures deduced from SFG spectra were correlated to adhesion testing results. It was found that silane headgroup order at the polymer interfaces is an important factor for improving adhesion. The decrease of silane headgroup order due to chemical reaction and disordering of such groups at the polymer interfaces can be associated with improved adhesion. The molecular level understanding on polymer/adhesive interfacial structures helps to design and develop adhesion promoters and polymer adhesives with improved performance.
Co-reporter:Chuan Leng, Yuwei Liu, Courtney Jenkins, Heather Meredith, Jonathan J. Wilker, and Zhan Chen
Langmuir 2013 Volume 29(Issue 22) pp:6659-6664
Publication Date(Web):May 10, 2013
DOI:10.1021/la4008729
Marine mussels deposit adhesive proteins containing 3,4-dihydroxyphenylalanine (DOPA) to attach themselves to different surfaces. Isolating such proteins from biological sources for adhesion purposes tends to be challenging. Recently, a simplified synthetic adhesive polymer, poly[(3,4-dihydroxystyrene)-co-styrene] (PDHSS), was developed to mimic DOPA-containing proteins. The pendant catechol group in this polymer provides cross-linking and adhesion much like mussel proteins do. In this work, sum frequency generation (SFG) vibrational spectroscopy was applied to reveal the structures of this DOPA-inspired polymer at air, water, and polymer interfaces. SFG spectroscopy results showed that when underwater, the catechol rings and the quinone rings were ordered, ready to adhere to surfaces. At the hydrophobic polystyrene interface, benzene π–π stacking is likely the adhesive force, whereas at the hydrophilic poly(allylamine) interface, primary amines may form hydrogen bonds with catechol or react with quinones for adhesion.
Co-reporter:Xiaofeng Han, Joshua R. Uzarski, Charlene M. Mello, and Zhan Chen
Langmuir 2013 Volume 29(Issue 37) pp:11705-11712
Publication Date(Web):August 6, 2013
DOI:10.1021/la401818k
Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) were used to investigate the orientation of N-terminus cysteine-modified cecropin P1 (cCP1) at the polystyrene maleimide (PS-MA)/peptide phosphate buffer solution interface. The cCP1 cysteine group reacts with the maleimide group on the PS-MA surface to chemically immobilize cCP1. Previously, we found that the C-terminus cysteine-modified cecropin P1 (CP1c) molecules exhibit a multiple-orientation distribution at the PS-MA/peptide phosphate buffer solution interface, due to simultaneous physical adsorption and chemical immobilization of CP1c on the PS-MA surface. Differently, in this research, it was found that the interfacial orientation of cCP1 molecules varied from a horizontal orientation to the “tilting” orientation to the “standing up” orientation and then to the “multiple-orientation” distribution as the peptide concentration increased from 0.19 to 3.74 μM. This research shows the different interaction mechanisms between CP1c and PS-MA and between cCP1 and PS-MA.
Co-reporter:Bei Ding, Jennifer E. Laaser, Yuwei Liu, Pengrui Wang, Martin T. Zanni, and Zhan Chen
The Journal of Physical Chemistry B 2013 Volume 117(Issue 47) pp:14625-14634
Publication Date(Web):October 25, 2013
DOI:10.1021/jp408064b
Sum-frequency generation (SFG) vibrational spectroscopy is often used to probe the backbone structures and orientations of polypeptides at surfaces. Using the ovispirin-1 polypeptide at the solid/liquid interface of polystyrene, we demonstrate for the first time that SFG can probe the polarization response of a single-isotope-labeled residue. To interpret the spectral intensities, we simulated the spectra using an excitonic Hamiltonian approach. We show that the polarization dependence of either the label or the unlabeled amide I band alone does not provide sufficient structural constraints to obtain both the tilt and the twist of the ovispirin helix at a solid/liquid interface, but that both can be determined from the polarization dependence of the complete spectrum. For ovispirin, the detailed analysis of the polarized SFG experimental data shows that the helix axis is tilted at roughly 138° from the surface normal, and the transition dipole of the isotope-labeled C═O group is tilted at 23° from the surface normal, with the hydrophobic region facing the polystyrene surface. We further demonstrate that the Hamiltonian approach is able to address the coupling effect and the structural disorder. For comparison, we also collected the FTIR spectrum of ovispirin under similar conditions, which reveals the enhanced sensitivity of SFG for structural studies of single monolayer peptide surfaces. Our study provides insight into how structural and environmental effects appear in SFG spectra of the amide I band and establishes that SFG of isotope-labeled peptides will be a powerful technique for elucidating secondary structures with residue-by-residue resolution.
Co-reporter:Jeanne M. Hankett, William R. Collin, and Zhan Chen
The Journal of Physical Chemistry B 2013 Volume 117(Issue 50) pp:16336-16344
Publication Date(Web):November 27, 2013
DOI:10.1021/jp409254y
Plasticized poly(vinyl chloride) (PVC) materials for industrial, medical, and household use are often intentionally exposed to UV light, though its impact on the molecular integrity and toxicity of the surface and bulk of PVC materials is still not well understood. This paper investigates the surface and bulk molecular changes of plasticized PVC films with 25, 10, or 0 wt % bis-2-ethylhexyl phthalate (DEHP) plasticizer after exposure to short wave (254 nm) or long wave (365 nm) UV light. Surface analytical techniques including sum frequency generation vibrational spectroscopy (SFG) revealed short wave UV exposure induced major molecular changes on the plasticized PVC surfaces, resulting in increased surface hydrophilicity and decreased CH3 content with increasing exposure time. Additionally, it was deduced from multiple techniques that the surface and the bulk of the plastic exposed to short wave UV contained phthalic monoesters and phthalic acid formed from multistep radical reactions. In contrast, when exposed to long wave UV, molecular content and ordering on the surfaces of the plastic remained relatively unchanged and the introduction of DEHP in plastic helped protect PVC chains from degradation. Results from this study demonstrate short wave UV exposure will result in plastic surfaces containing phthalates and phthalate-related products accessible to contact by living organisms.
Co-reporter:Pei Yang, Fu-Gen Wu, and Zhan Chen
The Journal of Physical Chemistry C 2013 Volume 117(Issue 33) pp:17039-17049
Publication Date(Web):July 20, 2013
DOI:10.1021/jp4047215
Alamethicin has been extensively studied as an antimicrobial peptide (AMP) and is widely used as a simple model for ion channel proteins. It has been shown that the antimicrobial activity of AMPs is related to their cell membrane orientation, which may be influenced by the phase of the lipid molecules in the cell membrane. The “healthy” cell membranes contain fluid phase lipids, while gel phase lipids can be found in injured or aged cells or in some phase-separated membrane regions. Thus, investigations on how the phase of the lipids influences the membrane orientation of AMPs are important to understand more details regarding the AMP’s action on cell membranes. In this study, we determined the orientational changes of alamethicin molecules associated with planar substrate supported single lipid bilayers (serving as model cell membranes) with different phases (fluid or gel) as a function of peptide concentration using sum frequency generation (SFG) vibrational spectroscopy. The phase changes of the lipid bilayers were realized by varying the sample temperature. Our SFG results indicated that alamethicin lies down on the surface of fluid and gel phase 1,2-dimyristoyl(d54)-sn-glycero-3-phosphocholine (d-DMPC) lipid bilayers when the lipid bilayers are in contact with a peptide solution with a low concentration of 0.84 μM. However, at a medium peptide concentration of 10.80 μM, alamethicin inserts into the fluid phase lipid bilayer. Its orientation switches from a transmembrane to an in-plane (or lying down) orientation when the phase of the lipid bilayer changes from a fluid state to a gel state. At a high peptide concentration of 21.60 μM, alamethicin adopts a transmembrane orientation while associated with both fluid and gel phase lipid bilayers. We also studied the structural changes of the fluid and gel phase lipid bilayers upon their interactions with alamethicin molecules at different peptide concentrations.
Co-reporter:Jeanne M. Hankett;Yuwei Liu;Xiaoxian Zhang;Chi Zhang
Journal of Polymer Science Part B: Polymer Physics 2013 Volume 51( Issue 5) pp:311-328
Publication Date(Web):
DOI:10.1002/polb.23221
Abstract
Industrial plastics, biomedical polymers and numerous other polymeric systems are contacted with water for everyday functions and after disposal. Probing the interfacial molecular interactions between widely used polymers and water yields valuable information that can be extrapolated to macroscopic polymer/water interfacial behaviors so scientists can better understand polymer bio-compatibility, hygroscopic tendencies and improve upon beneficial polymer behavior in water. There is an ongoing concerted effort to elucidate the molecular level behaviors of polymers in water by using sum frequency generation vibrational spectroscopy (SFG). SFG stands out for its utility in probing buried interfaces in situ and in real time without disrupting interfacial chemistry. Included in this review are SFG water interfacial studies performed on poly(methacrylate) and (acrylate)s, poly(dimethyl siloxane)s, poly(ethylene glycol)s, poly(electrolyte)s and other polymer types. The driving forces behind common water/polymer interfacial molecular features will be discussed as well as unique molecular reorientation phenomena and resulting macroscopic behaviors from microscopic polymer rearrangement. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013
Co-reporter:Pei Yang, Fu-Gen Wu, and Zhan Chen
The Journal of Physical Chemistry C 2013 Volume 117(Issue 7) pp:3358-3365
Publication Date(Web):January 24, 2013
DOI:10.1021/jp3099522
Alamethicin has been extensively studied as an antimicrobial peptide and is widely used as a simple model for ion channel proteins. It has been shown that the antimicrobial activity of peptides is related to their membrane orientation. In this study, we determined the relationship between the solution concentration of alamethicin and its membrane orientation in lipid bilayers using sum frequency generation (SFG) vibrational spectroscopy. Our SFG results indicated that the alamethicin molecules more or less lay down on the surface of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers at a low peptide concentration of 0.84 μM; the α-helix segment tilts at about 88°, and 310-helix segment tilts at about 58° versus the surface normal. However, when the peptide concentration was increased to 15.6 μM, we observed that alamethicin molecules further inserted into the lipid bilayers: the α-helical component changes its orientation to make a 37° tilt from the lipid bilayer normal, and the 310-helical component tilts at about 50° versus the surface normal. This is in agreement with the barrel-stave mode for the alamethicin–cell membrane interaction as reported previously. Additionally, we have also studied membrane orientation of alamethicin at other peptide concentrations with SFG. Our results showed that the membrane orientation of the alamethicin α-helical component changed substantially with the increase of the alamethicin concentration, while the membrane orientation of the 310-helical component remained more or less the same.
Co-reporter:Chi Zhang and Zhan Chen
The Journal of Physical Chemistry C 2013 Volume 117(Issue 8) pp:3903-3914
Publication Date(Web):February 1, 2013
DOI:10.1021/jp307472j
Silicone materials such as poly(dimethylsiloxane) (PDMS) are widely used in a variety of important applications such as polymer adhesives, packaging materials for microelectronics, polymer MEMS, microfluidics, biomedical implants, and marine antifouling coatings. In such applications, molecular structures of PDMS at buried interfaces will determine interfacial properties. Therefore, it is important to elucidate PDMS molecular structures at relevant buried interfaces. In this study, the interfacial structures of PDMS silicone elastomer in contact with silica and different polymer materials have been studied using sum frequency generation (SFG) vibrational spectroscopy. It was found that the PDMS methyl groups are ordered at the buried poly(ethylene terephthalate) (PET)/PDMS and fused silica/PDMS interfaces. However, these methyl groups tend to adopt different orientations at different interfaces. Using the SFG spectral fitting results, the possible ranges of tilt angles and twist angles of PDMS methyl groups at the buried PET/PDMS and silica/PDMS interfaces were determined. At the PET/PDMS interface, the methyl groups tend to have large tilt angles (>70°) with small twist angles (<20°). At the silica/PDMS interface, methyl groups tend to adopt a broad distribution of tilt angles along with large twist angles. The absolute orientations of the PDMS methyl groups at the buried interfaces were determined from the interference pattern of the PDMS SFG signal with the nonresonant signal from a TiO2 thin film. PDMS methyl groups tend to orient toward the PDMS bulk rather than the contacting substrates at both the PET/PDMS and silica/PDMS interfaces. However, at the polystyrene/PDMS and poly(methyl methacrylate)/PDMS interfaces, PDMS methyl groups orient toward the hydrophobic polymer substrate surfaces. The different orientations of PDMS methyl groups at the investigated buried interfaces were correlated to interfacial polar interactions determined by substrate surface hydrophobicities.
Co-reporter:XiaoXian Zhang;XiaoFeng Han;FuGen Wu;Joshua Jasensky
Science Bulletin 2013 Volume 58( Issue 21) pp:2537-2556
Publication Date(Web):2013 July
DOI:10.1007/s11434-013-5700-y
Research in biology and medicine is a rapidly expanding field incorporating some of the most fundamental questions concerning structure, function, and purpose. The forefront of new research demands access to advanced techniques and instrumentation capable of probing these unanswered questions. Over the past several decades, nano-scale materials and devices ranging from quasi-one dimensional quantum dots to two dimensional graphene sheets have been engineered and have found applications in nano-bio imaging and spectroscopy. In this review, the incorporation of nanomaterials into three influential spectroscopic and microscopic techniques including fluorescence microscopy, surface plasmon resonance, and sum frequency generation will be introduced. Fluorescence imaging has visualized nanomaterials as compliments or replacements to comparable organic fluorphores, act as a quencher for FRET-based sensing, and serve as a nanoscaffold for molecular beacons. Their versatility in coating materials makes nanomaterials an excellent targeting molecule for any cellular macromolecule or structure. In addition to the targeting capabilities of nanomaterials in fluorescence imaging, surface plasmon resonance has incorporated nanomaterials for applications in signal enhancement, selectivity of target molecules, and the development of more refined and accurate detection. Functionalized nanoparticles enhance the capabilities of sum frequency generation vibrational spectroscopy by providing unique surface chemistry which alters target molecule interactions and orientations. In summary, the incorporation of nanomaterials has greatly enhanced the field of biology and medicine and has allowed for the continual advancement of not only research but instrument development.
Co-reporter:Shuji Ye ; Hongchun Li ; Feng Wei ; Joshua Jasensky ; Andrew P. Boughton ; Pei Yang
Journal of the American Chemical Society 2012 Volume 134(Issue 14) pp:6237-6243
Publication Date(Web):March 15, 2012
DOI:10.1021/ja2110784
Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/310-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 310-helix at the C-terminus tilt at about 72° (θ1) and 50° (θ2) versus the surface normal, respectively. When pH increases to 11.9, θ1 and θ2 decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ1 for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes.
Co-reporter:Chi Zhang, Jeanne Hankett, and Zhan Chen
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 7) pp:3730
Publication Date(Web):June 18, 2012
DOI:10.1021/am300854g
It is important to understand the buried interfacial structures containing epoxy underfills as such structures determine the interfacial adhesion properties. Weak adhesion or delamination at such interfaces leads to failure of microelectronic devices. Sum frequency generation (SFG) vibrational spectroscopy was used to examine buried interfaces at polymer/model epoxy and polymer/commercial epoxy resins (used as underfills in flip chip devices) at the molecular level. We investigated a model epoxy: bisphenol A digylcidyl ether (BADGE) at the interfaces of poly (ethylene terephthalate) (PET) before and after curing. Furthermore, small amounts of different silanes including (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-Aminopropyl)trimethoxysilane (ATMS), Octadecyltrimethoxysilane (OTMS(18C)), and Octyltrimethoxysilane (OTMS(8C)) were mixed with BADGE. Silane influences on the polymer/epoxy interfacial structures were studied. SFG was also used to study molecular interfacial structures between polymers and two commercial epoxy resins. The interfacial structures probed by SFG were correlated to the adhesion strengths measured for corresponding interfaces. The results indicated that a small amount of silane molecules added to epoxy could substantially change the polymer/epoxy interfacial structure, greatly affecting the adhesion strength at the interface. It was found that ordered methyl groups at the interface lead to weak adhesion, and disordered interfaces lead to strong adhesion.Keywords: adhesion; epoxy; interface; polymer; silane; sum frequency generation (SFG);
Co-reporter:Jeanne M. Hankett, Chi Zhang, and Zhan Chen
Langmuir 2012 Volume 28(Issue 10) pp:4654-4662
Publication Date(Web):February 7, 2012
DOI:10.1021/la2045527
Polyvinyl chloride (PVC) is a widely used polymer to which various phthalates are extensively applied as plasticizers. PVC materials are often treated with plasma to vary the hydrophobicity or for cleaning purposes, but little is known of the nature of the surface molecular structures after treatment. This research characterizes molecular surface structures of PVC and bis-2-ethylhexyl phthalate (DEHP)-plasticized PVC films in air before annealing, after annealing, and after exposure to air-generated glow discharge plasma using sum frequency generation (SFG) vibrational spectroscopy. In addition, we compare the vibrational molecular signatures on the surfaces of PVC with DEHP (at a variety of percent loadings) to those of the bulk detected using coherent anti-Stokes Raman scattering (CARS). X-ray photoelectron spectroscopy (XPS) and contact angle measurements have been used to analyze PVC surfaces to supplement SFG data. Our results indicate that DEHP was found on the surfaces of PVC films even at low weight percentages (5 wt %) and that DEHP segregates on surfaces after annealing. The treatment of these films with glow discharge plasma resulted in surface-sensitive reactions involving the removal of chlorine atoms, the addition of oxygen atoms, and C–H bond rearrangement. CARS data demonstrate that the bulk of our films remained undisturbed during the plasma treatment. For the first time, we probed the molecular structure of the surface and the bulk of a PVC material using combined SFG and CARS studies on the same sample in exactly the same environment. In addition, the methodology used in this research can be applied to characterize various plasticizers in a wide variety of polymer systems to understand their surface and bulk structures before and after systematic applications of heat, plasma, or other treatments.
Co-reporter:Yuwei Liu, Joshua Jasensky, and Zhan Chen
Langmuir 2012 Volume 28(Issue 4) pp:2113-2121
Publication Date(Web):December 15, 2011
DOI:10.1021/la203823t
Interfacial peptides and proteins are critical in many biological processes and thus are of interest to various research fields. To study these processes, surface sensitive techniques are required to completely describe different interfacial interactions intrinsic to many complicated processes. Sum frequency generation (SFG) spectroscopy has been developed into a powerful tool to investigate these interactions and mechanisms of a variety of interfacial peptides and proteins. It has been shown that SFG has intrinsic surface sensitivity and the ability to acquire conformation, orientation, and ordering information about these systems. This paper reviews recent studies on peptide/protein–substrate interactions, peptide/protein–membrane interactions, and protein complexes at interfaces and demonstrates the ability of SFG on unveiling the molecular pictures of complicated interfacial biological processes.
Co-reporter:Chi Zhang, Nick E. Shephard, Susan M. Rhodes, and Zhan Chen
Langmuir 2012 Volume 28(Issue 14) pp:6052-6059
Publication Date(Web):March 16, 2012
DOI:10.1021/la300004x
Sum frequency generation (SFG) vibrational spectroscopy was used to study the effect of silane headgroups on the molecular interactions that occur between poly(ethylene terephthalate) (PET) and various epoxy silanes at the PET/silane and PET/silicone interfaces. Three different silanes were investigated: (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-glycidoxypropyl) methyl-dimethoxysilane (γ-GPMS), and (3-glycidoxypropyl) dimethyl-methoxysilane (γ-GPDMS). These silanes share the same backbone and epoxy end group but have different headgroups. SFG was used to examine the interfaces between PET and each of these silanes, as well as silanes mixed with methylvinylsiloxanol (MVS). We also examined the interfaces between PET and uncured or cured silicone with silanes or silane–MVS mixtures. Silanes with different headgroups were found to exhibit a variety of methoxy group interfacial segregation and ordering behaviors at various interfaces. The effect of MVS was also dependent upon silane headgroup choice, and the interfacial molecular structures of silane methoxy headgroups were found to differ at PET/silane and PET/silicone interfaces. Epoxy silanes have been widely used as adhesion promoters for polymer adhesives; therefore, the molecular structures probed using SFG were correlated to adhesion testing results to understand the molecular mechanisms of silicone-polymer adhesion. Our results demonstrated that silane methoxy headgroups play important roles in adhesion at the PET/silicone interfaces. The presence of MVS can change interfacial methoxy segregation and ordering, leading to different adhesion strengths.
Co-reporter:Bei Ding and Zhan Chen
The Journal of Physical Chemistry B 2012 Volume 116(Issue 8) pp:2545-2552
Publication Date(Web):January 31, 2012
DOI:10.1021/jp209604m
We investigated the molecular interactions of a cell penetrating peptide (CPP) Pep-1 with model cell membranes using sum frequency generation (SFG) vibrational spectroscopy, supplemented by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR). Hydrogenated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG and dDPPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (POPG) were used in the experiments to represent gel-phase and fluid-phase lipid bilayers, respectively. Our SFG results indicated that Pep-1 molecules adopted a β-sheet conformation when adsorbed to the surface of gel-phase DPPG lipid bilayers. When interacting with fluid-phase POPG lipid bilayers, Pep-1 adopted a mix of α-helical and β-sheet structures over a broad range of peptide concentrations. The orientation distribution of the α-helical Pep-1 segment associated with the fluid-phase bilayers was found to depend on the peptide concentration. SFG orientation analysis showed that Pep-1 molecules adopted an orientation nearly perpendicular to the plane of the bilayer for peptide concentrations of 0.28 and 1.4 μM. When the Pep-1 concentration was increased to 7.0 μM, combined SFG and ATR-FTIR measurements showed that Pep-1 molecules were associated with the bilayer with a broad orientation distribution. Our results demonstrated that lipid bilayer phase and peptide concentration affect the conformation and orientation of Pep-1 molecules associated with model cell membranes, which is crucial to the translocation process of CPPs. A combination of SFG and ATR-FTIR studies can be used to determine the conformation and orientation of CPPs interacting with model cell membranes in situ.
Co-reporter:Anne V. Vázquez, Brad Holden, Cornelius Kristalyn, Mike Fuller, Brett Wilkerson, and Zhan Chen
ACS Applied Materials & Interfaces 2011 Volume 3(Issue 5) pp:1640
Publication Date(Web):April 19, 2011
DOI:10.1021/am2001899
Flip chip technology has greatly improved the performance of semiconductor devices, but relies heavily on the performance of epoxy underfill adhesives. Because epoxy underfills are cured in situ in flip chip semiconductor devices, understanding their surface and interfacial structures is critical for understanding their adhesion to various substrates. Here, sum frequency generation (SFG) vibrational spectroscopy was used to study surface and buried interfacial structures of two model epoxy resins used as underfills in flip chip devices, bisphenol A digylcidyl ether (BADGE) and 1,4-butanediol diglycidyl ether (BDDGE). The surface structures of these epoxies were compared before and after cure, and the orientations of their surface functional groups were deduced to understand how surface structural changes during cure may affect adhesion properties. Further, the effect of moisture exposure, a known cause of adhesion failure, on surface structures was studied. It was found that the BADGE surface significantly restructured upon moisture exposure while the BDDGE surface did not, showing that BADGE adhesives may be more prone to moisture-induced delamination. Lastly, although surface structure can give some insight into adhesion, buried interfacial structures more directly correspond to adhesion properties of polymers. SFG was used to study buried interfaces between deuterated polystyrene (d-PS) and the epoxies before and after moisture exposure. It was shown that moisture exposure acted to disorder the buried interfaces, most likely due to swelling. These results correlated with lap shear adhesion testing showing a decrease in adhesion strength after moisture exposure. The presented work showed that surface and interfacial structures can be correlated to adhesive strength and may be helpful in understanding and designing optimized epoxy underfill adhesives.Keywords: adhesion; epoxy underfills; moisture exposure; molecular structures; SFG spectroscopy; surfaces and interfaces
Co-reporter:Christopher W. Avery, Edmund F. Palermo, Amanda McLaughlin, Kenichi Kuroda, and Zhan Chen
Analytical Chemistry 2011 Volume 83(Issue 4) pp:1342
Publication Date(Web):January 13, 2011
DOI:10.1021/ac1025804
Sum frequency generation (SFG) vibrational spectroscopy was used to analyze interactions between solid-supported lipid bilayers acting as models for cellular membranes and several membrane-active random copolymers with different lipophilic side chains, named 0R (no group), 33Me (methyl group), 11Bz (benzyl group), and 33Bu (butyl group), according to both the identity and percentage of the side chains within the polymer. Biological tests of the minimum inhibitory concentration (MIC) and hemolytic concentration were performed. The inherent surface sensitivity of SFG allowed for independent monitoring of isotopically labeled lipid bilayer leaflets as a function of concentration to study polymer−bilayer interaction mechanisms. Concentrations at which each bilayer leaflet was disrupted were quantitatively determined for each copolymer. Spectroscopic evidence of interaction with the bilayer below the critical concentrations was observed for the 11Bz polymer. The lipophilic butyl side chain of the 33Bu polymer was found to be oriented parallel to the surface normal. This research shows that SFG is a useful analytical technique which provides unique details regarding the interaction mechanisms of these membrane-active copolymers and lipid bilayers.
Co-reporter:Cornelius B. Kristalyn, Shannon Watt, Sarah A. Spanninga, Rachel A. Barnard, Khoi Nguyen, Zhan Chen
Journal of Colloid and Interface Science 2011 Volume 353(Issue 1) pp:322-330
Publication Date(Web):1 January 2011
DOI:10.1016/j.jcis.2010.09.057
Semifluorinated self-assembled (FAS SA) films fabricated from trifunctional precursors are frequently used in myriad applications, yet an understanding of the effects of fabrication conditions, including deposition time, on adsorption mechanisms and molecular architectures is still being developed. In this work we prepared SA films based on the F(CF2)8(CH2)2SiCl3 (FAS-17) precursor and characterized these films using a suite of surface analytical techniques. Contact angle, sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ellipsometry results are consistent with the formation of disordered sub-monolayer structures at short deposition times, well-ordered monolayers at intermediate deposition times, and inhomogeneous multilayers at long deposition times. Correlation of SFG and XPS results demonstrates a change in FAS-17 chain orientation as the deposition time increases from 2 s to 5 min. Group theory-based calculations, SFG studies, and Fourier-transform infrared (FTIR) results also afford additional evidence in support of the assignment of the SFG signals at ∼1345 and ∼1370 cm−1 to the asymmetric stretching mode of the semifluorinated silane chain’s terminal CF3 group rather than to its axial CF2 stretches. To our knowledge, this is the first report of SFG studies on semifluoroalkyl silane self-assembled films in the C–F stretching frequency region.Graphical abstractSemifluorinated self-assembled films fabricated from trifunctional precursors have disordered sub-monolayer structure at short deposition times, well-ordered monolayer structure at intermediate deposition times, and inhomogeneous multilayer structure at long deposition times.Research highlights► Ordered semifluorinated silane monolayers formed at 5-min deposition time. ► Inhomogeneous semifluorinated silane multilayers formed at 20-min deposition time. ► Signals at 1345 and 1370 cm−1 from the asymmetric stretching mode of the CF3 group. ► Orientation changed for semifluorinated films with different deposition times.
Co-reporter:Andrew P. Boughton;Pei Yang;Bei Ding;Valerie M. Tesmer;John J. G. Tesmer
PNAS 2011 Volume 108 (Issue 37 ) pp:
Publication Date(Web):2011-09-13
DOI:10.1073/pnas.1108236108
Few experimental techniques can assess the orientation of peripheral membrane proteins in their native environment. Sum Frequency
Generation (SFG) vibrational spectroscopy was applied to study the formation of the complex between G protein-coupled receptor
(GPCR) kinase 2 (GRK2) and heterotrimeric G protein β1γ2 subunits (Gβγ) at a lipid bilayer, without any exogenous labels. The most likely membrane orientation of the GRK2-Gβγ complex
differs from that predicted from the known protein crystal structure, and positions the predicted receptor docking site of
GRK2 such that it would more optimally interact with GPCRs. Gβγ also appears to change its orientation after binding to GRK2.
The developed methodology is widely applicable for the study of other membrane proteins in situ.
Co-reporter:Xiaofeng Han, Lauren Soblosky, Morris Slutsky, Charlene M. Mello, and Zhan Chen
Langmuir 2011 Volume 27(Issue 11) pp:7042-7051
Publication Date(Web):May 9, 2011
DOI:10.1021/la200388y
Sum frequency generation (SFG) vibrational spectroscopy has been applied to the investigation of peptide immobilization on a polymer surface as a function of time and peptide conformation. Surface immobilization of biological molecules is important in many applications such as biosensors, antimicrobial materials, biobased fuel cells, nanofabrication, and multifunctional materials. Using C-terminus-cysteine-modified cecropin P1 (CP1c) as a model, we investigated the time-dependent immobilization behavior in situ in real time. In addition, potassium phosphate buffer (PB) and mixtures of PB and trifluoroethanol were utilized to examine the effect of peptide secondary structure on CP1c immobilization to polystyrene maleimide (PS-MA). The orientation of immobilized CP1c on PS-MA was determined using polarized SFG spectra. It was found that the peptide solution concentration, solvent composition, and assembly state (monomer vs dimer) prior to immobilization all influence the orientation of CP1c on a PS-MA surface. The detailed relationship between the interfacial peptide orientation and these immobilization conditions is discussed.
Co-reporter:Pei Yang, Ayyalusamy Ramamoorthy, and Zhan Chen
Langmuir 2011 Volume 27(Issue 12) pp:7760-7767
Publication Date(Web):May 19, 2011
DOI:10.1021/la201048t
Antimicrobial peptides (AMPs) selectively disrupt bacterial cell membranes to kill bacteria whereas they either do not or weakly interact with mammalian cells. The orientations of AMPs in lipid bilayers mimicking bacterial and mammalian cell membranes are related to their antimicrobial activity and selectivity. To understand the role of AMP–lipid interactions in the functional properties of AMPs better, we determined the membrane orientation of an AMP (MSI-78 or pexiganan) in various model membranes using sum frequency generation (SFG) vibrational spectroscopy. A solid-supported single 1,2-dipalmitoyl-an-glycero-3-[phospho-rac-(1-glycerol)] (DPPG) bilayer or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) bilayer was used as a model bacterial cell membrane. A supported 1,2-dipalmitoyl-an-glycero-3-phosphocholine (DPPC) bilayer or a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was used as a model mammalian cell membrane. Our SFG results indicate that the helical MSI-78 molecules are associated with the bilayer surface with ∼70° deviation from the bilayer normal in the negatively charged gel-phase DPPG bilayer at 400 nM peptide concentration. However, when the concentration was increased to 600 nM, MSI-78 molecules changed their orientation to make a 25° tilt from the lipid bilayer normal whereas multiple orientations were observed for an even higher peptide concentration in agreement with toroidal-type pore formation as reported in a previous solid-state NMR study. In contrary, no interaction between MSI-78 and a zwitterionic DPPC bilayer was observed even at a much higher peptide concentration (∼12 000 nM). These results demonstrate that SFG can provide insights into the antibacterial activity and selectivity of MSI-78. Interestingly, the peptide exhibits a concentration-dependent membrane orientation in the lamellar-phase POPG bilayer and was also found to induce toroidal-type pore formation. The deduced lipid flip-flop from SFG signals observed from lipids also supports MSI-78-induced toroidal-type pore formation.
Co-reporter:Arthur A. McClelland, Vilmalí López-Mejías, Adam J. Matzger, and Zhan Chen
Langmuir 2011 Volume 27(Issue 6) pp:2162-2165
Publication Date(Web):February 15, 2011
DOI:10.1021/la105067x
Sum frequency generation vibrational spectroscopy (SFG-VS) has been applied to investigate the selective crystallization of two forms of acetaminophen (ACM) on polymer surfaces. To our knowledge, this is the first account of SFG-VS being applied to study a polymer−crystal interface. SFG elucidates the molecular-level interactions governing phase selection at this buried interface, providing insight into the process of polymer-induced heteronucleation (PIHn) in solution as well as from the vapor phase. ACM heteronucleates from supersaturated aqueous solution in the metastable orthorhombic crystal form on poly(methyl methacrylate) (PMMA) surfaces, whereas the thermodynamically stable monoclinic crystal form is observed to form on poly(n-butyl methacrylate) (PBMA) surfaces. When the ACM crystals were grown by sublimation, only the monoclinic form was observed on both PMMA and PBMA. SFG-VS results indicate that hydrogen bonds are formed between PMMA C═O groups and the orthorhombic ACM crystals at the PMMA−ACM interface. At PBMA−monoclinic ACM interfaces, no hydrogen bond formation was observed. This research demonstrates that SFG-VS can be used to probe molecular interactions at polymer−crystal interfaces. Understanding the interfacial molecular interactions will ultimately provide a rational basis for improving methods for polymorph discovery and selection based on heteronucleation on polymer surfaces.
Co-reporter:Andrew P. Boughton, Khoi Nguyen, Ioan Andricioaei, and Zhan Chen
Langmuir 2011 Volume 27(Issue 23) pp:14343-14351
Publication Date(Web):November 4, 2011
DOI:10.1021/la203192c
Recent advances in the collection and interpretation of surface-sensitive vibrational spectroscopic measurements have made it possible to study the orientation of peptides and proteins in situ in a biologically relevant environment. However, interpretation of sum frequency generation (SFG) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) vibrational spectroscopy is hindered by the fact that orientation cannot be inferred without some prior knowledge of the protein structure. In this work, molecular dynamics simulations were used to study the interfacial orientation and structural deformation of the short β-sheet peptide tachyplesin I at the polystyrene/water interface. By combining these results with ATR-FTIR and SFG measurements, reasonable agreement was found with the simulation results, suggesting that tachyplesin I lies parallel to the surface, although the simulation results imply a broader distribution of peptide twist angles than could be characterized using available experimental measurements. The interfacial structure was found to be deformable even when disulfide bonds were preserved, and these local deviations from a purely extended β-sheet conformation may be of importance to future developments in the interpretation of SFG and ATR-FTIR spectra.
Co-reporter:Ting Wang ; Dawei Li ; Xiaolin Lu ; Alexander Khmaladze ; Xiaofeng Han ; Shuji Ye ; Pei Yang ; Gi Xue ; Nongyue He
The Journal of Physical Chemistry C 2011 Volume 115(Issue 15) pp:7613-7620
Publication Date(Web):March 24, 2011
DOI:10.1021/jp200546h
Planar solid supported single lipid bilayers on mica, glass, or other inorganic surfaces have been widely used as models for cell membranes. To more closely mimic the cell membrane environment, soft hydrophilic polymer cushions were introduced between the hard inorganic substrate and the lipid bilayer to completely avoid the possible substrate−lipid interactions. In this Article, sum frequency generation (SFG) vibrational spectroscopy was used to examine and compare single lipid bilayers assembled on the CaF2 prism surface and on poly(L-lactic acid) (PLLA) cushion. By using asymmetric lipid bilayers composed of a hydrogenated 1,2-dipalmitoyl-sn-glycerol-3-phosphoglycerol (DPPG) leaflet and a deuterated 1,2-dipalmitoyl-(d62)-sn-glycerol-3-phosphoglycerol (d-DPPG) leaflet, it was shown that the DPPG lipid bilayers deposited on the CaF2 and PLLA surfaces have similar structures. SFG has also been applied to investigate molecular interactions between an antimicrobial peptide Cecropin P1 (CP1) and the lipid bilayers on the above two different surfaces. Similar results were again obtained. This research demonstrated that the hydrophilic PLLA cushion can serve as an excellent substrate to support single lipid bilayers. We believe that it can be an important cell membrane model for future studies on transmembrane proteins, for which the possible inorganic substrate−bilayer interactions may affect the protein structure or function.
Co-reporter:Xiaolin Lu ; Matthew L. Clarke ; Dawei Li ; Xinping Wang ; Gi Xue
The Journal of Physical Chemistry C 2011 Volume 115(Issue 28) pp:13759-13767
Publication Date(Web):June 23, 2011
DOI:10.1021/jp202416z
On the basis of the development in the literature, a thin film model was used to interpret the thickness-dependent sum frequency generation (SFG) spectra collected from the air/silica/poly(n-butyl methacrylate) (PBMA)/water system. By taking into account the Fresnel coefficients of the PBMA layers at the silica/PBMA and PBMA/water interfaces, the SFG spectra of the silica/PBMA and PBMA/water interfaces were obtained. The side chain methyl vibrational modes at the two interfaces were found to have opposite phases. This suggests that the side chain methyl groups at the two interfaces adopt different absolute orientations. It is believed that methyl groups at both interfaces point toward the PBMA bulk considering the unfavorable interactions between the hydrophobic methyl groups and hydrophilic silica surface and water. Orientation analysis indicates the side chain methyl groups at both interfaces tilt more toward the interfaces rather than be perpendicular to them. Applying the thin film model to the air/PBMA/silica system, we found that the surface signals from PBMA in air dominate the SFG output on account of the relatively higher Fresnel coefficients at this surface over those of the buried PBMA/silica interface.
Co-reporter:Zhan Chen
Progress in Polymer Science 2010 Volume 35(Issue 11) pp:1376-1402
Publication Date(Web):November 2010
DOI:10.1016/j.progpolymsci.2010.07.003
This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.
Co-reporter:Khoi Tan Nguyen ; Ronald Soong ; Sang-Choul lm ; Lucy Waskell ; Ayyalusamy Ramamoorthy
Journal of the American Chemical Society 2010 Volume 132(Issue 43) pp:15112-15115
Publication Date(Web):October 8, 2010
DOI:10.1021/ja106508f
In addition to providing a semipermeable barrier that protects a cell from harmful stimuli, lipid membranes occupy a central role in hosting a variety of biological processes, including cellular communications and membrane protein functions. Most importantly, protein−membrane interactions are implicated in a variety of diseases and therefore many analytical techniques were developed to study the basis of these interactions and their influence on the molecular architecture of the cell membrane. In this study, sum frequency generation (SFG) vibrational spectroscopy is used to investigate the spontaneous membrane insertion process of cytochrome b5 and its mutants. Experimental results show a significant difference in the membrane insertion and orientation properties of these proteins, which can be correlated with their functional differences. In particular, our results correlate the nonfunctional property of a mutant cytochrome b5 with its inability to insert into the lipid bilayer. The approach reported in this study could be used as a potential rapid screening tool in measuring the topology of membrane proteins as well as interactions of biomolecules with lipid bilayers in situ.
Co-reporter:Anne V. Vázquez, Andrew P. Boughton, Nick E. Shephard, Susan M. Rhodes and Zhan Chen
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 1) pp:96
Publication Date(Web):December 18, 2009
DOI:10.1021/am900612r
Silane adhesion promoters are commonly used to enhance the adhesion of elastomeric materials to polymers in many industrial applications. However, it is difficult to study the molecular-level mechanisms underlying adhesion promotion because adhesion occurs at the boundary between two layers, a buried interface that is difficult to probe with most techniques. Here, a surface/interface-sensitive optical technique, sum frequency generation vibrational spectroscopy, was used to probe the buried interfaces between the silicone elastomer and (3-glycidoxypropyl)trimethoxysilane (γ-GPS) as well as a known silane adhesion-promoting mixture of γ-GPS and methylvinylsiloxane (MVS). The γ-GPS methoxy groups were found to order at the silicone interface both in the neat silane and in the mixture with MVS. The interfacial structures between the silicone elastomer and two other silanes not used as adhesion promoters, n-octadecyltrimethoxysilane (OTMS) and (tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane (TDFTMS), and their mixtures with MVS, were also compared to those of γ-GPS and the γ-GPS/MVS mixture. It was found that these silanes behaved differently than the known adhesion-promoting mixture. Further, molecular dynamics simulations confirmed that all silanes showed broad, random orientation distributions at the silicone interface. Because only the known adhesion-promoting mixture of γ-GPS and MVS exhibited methoxy order at the silicone interface, as well as at the poly(ethylene terephthalate) interface, as shown in a previous publication, it is inferred that this ordering may be a necessary condition for adhesion promotion.Keywords: molecular dynamics simulations; polymer adhesion; polymer interfaces; silane adhesion promoters; silicone elastomer; sum frequency generation vibrational spectroscopy
Co-reporter:Sarah A. Spanninga, David C. Martin and Zhan Chen
The Journal of Physical Chemistry C 2010 Volume 114(Issue 35) pp:14992-14997
Publication Date(Web):August 17, 2010
DOI:10.1021/jp104591d
Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomaterial coatings because of its outstanding electrical properties and chemical stability. In our previous research, X-ray photoelectron spectroscopy (XPS) was used to investigate the incorporation of counterions into PEDOT. In this research, XPS was applied to systematically investigate the chemical composition of electrochemically polymerized PEDOT films to determine counterion affinity. The counterions probed here included polyanions such as poly(sodium 4-styrenesulfonate) (PSSNa) and poly(acrylic acid) (PAA), and small anions including lithium perchlorate, lithium bromide, sodium nitrate, sodium thiosulfate, sodium chloride, sodium phosphate monohydrate, sodium phosphate dibasic hepthydrate, sodium acetate trihydrate, sodium p-toluenesulfonate (TosNa), calcium carbonate, and sodium carbonate. In addition to these counterions, an ion mixture, phosphate-buffered solution (PBS), was also examined. In counterions with ion mixtures containing PSSNa during PEDOT electrochemical polymerization, PSSNa was found to be the dominant counterion incorporated into PEDOT due to its polymeric nature over the monomeric-like version TosNa, though bromide anions were found to act as PEDOT counterions even in the presence of PSSNa (or PAA). The overall qualitative PEDOT counterion affinity for anions with one negative charge observed in the XPS spectra was the following: ClO4− and Br− over smaller contributions of Cl−, Tos−, and COO− (acetate), with no phosphate or NO3− contributions. As for the anions with two negative charges, S2O32− dominated over both carbonate and phosphate anions. The trends found in counterion affinity did follow the general trend for anionic hydration suggested by the Hofmeister series. Systematic investigations on counterion incorporation into PEDOT can greatly improve the basic understanding of the chemical composition of various PEDOT films when different counterions or ion mixtures are used in the electrochemical polymerization process, aiding in the design of optimized PEDOT films with tailored properties.
Co-reporter:Sarah A. Spanninga, David C. Martin and Zhan Chen
The Journal of Physical Chemistry C 2010 Volume 114(Issue 35) pp:14998-15004
Publication Date(Web):August 19, 2010
DOI:10.1021/jp104592n
Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomedical device coatings because of its outstanding electrical properties and chemical stability. In our previous research, X-ray photoelectron spectroscopy (XPS) was used to investigate the incorporation of various counterions into electrochemically polymerized PEDOT. In this research, XPS was used to further investigate the chemical composition of electrochemically polymerized PEDOT films to determine whether anionic hydration determined counterion affinity in a more systematic way. These counterions probed in this research included sodium citrate tribasic dIhydrate (Na3C6H5O7 2H2O), potassium citrate tribasic monohydrate (K3C6H5O7 H2O), sodium carbonate (Na2CO3), calcium carbonate (CaCO3), sodium thiosulfate (Na2S2O3), sodium acetate trihydrate (NaC2H3O2 3H2O), sodium phosphate dibasic hepthydrate (Na2HPO4 7H2O), sodium phosphate monohydrate (NaH2PO4 H2O), sodium chloride (NaCl), sodium bromide (NaBr), lithium bromide (LiBr), sodium perchlorate (NaClO4), lithium perchlorate (LiClO4), and sodium nitrate (NaNO3). Various mixtures containing the above anions were also studied in detail. The thiosulfate was found to act as the dominate counterion for electrochemically polymerized PEDOT within every mixture it was present in regardless of the anionic charge, cation, or anionic hydration. From our systematic study, we deduced the general qualitative trend of PEDOT counterion affinity (from strongest to weakest) as: S2O32− > COO− (citrate), Br−, ClO4− > Cl−, COO− (acetate), CO32− > NO3− > H2PO4−/HPO42−. These results did not follow the Hofmeister Series rigidly, but some general trends were observed such as the dominance of Br− and ClO4− in all mixtures not containing thiosulfate, which matches the conclusions reported in our previous publication. The dominance of the S2O32− (hydrated) and citrate COO− (strongly hydrated) as well as the lack of NO3− (weakly hydrated) contributions were the major points of deviation from the Hofmeister series, suggesting that anion hydration alone is not the only factor determining PEDOT counterion affinity.
Co-reporter:Cornelius B. Kristalyn, Xiaolin Lu, Craig J. Weinman, Christopher K. Ober, Edward J. Kramer and Zhan Chen
Langmuir 2010 Volume 26(Issue 13) pp:11337-11343
Publication Date(Web):May 13, 2010
DOI:10.1021/la100701b
Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate surface structures of an amphiphilic surface-active block copolymer (SABC) film deposited on a CaF2 substrate, in air and in water in situ. Developed as a surface-active component of an antifouling coating for marine applications, this amphiphilic triblock copolymer contains both hydrophobic fluorinated alkyl groups as well as hydrophilic ethoxy groups. It was found that surface structures of the copolymer film in air and in water cannot be probed directly using the SFG experimental geometry we adopted because SFG signals can be contributed from the polymer/air (or polymer/water) interface as well as the buried polymer/CaF2 substrate interface. Using polymer films with varied thicknesses, structural information about the polymer surfaces in air and in water can be deduced from the detected SFG signals. With SFG, surface restructuring of this polymer has been observed in water, especially the methyl and methylene groups change orientations upon contact with water. However, the hydrophobic fluoroalkyl group was present on the surface in both air and water, and we believe that it was held near the surface in water by its neighboring ethoxy groups.
Co-reporter:Shuji Ye, Khoi Tan Nguyen, Andrew P. Boughton, Charlene M. Mello and Zhan Chen
Langmuir 2010 Volume 26(Issue 9) pp:6471-6477
Publication Date(Web):December 4, 2009
DOI:10.1021/la903932w
A surface sensitive second order nonlinear optical technique, sum frequency generation vibrational spectroscopy, was applied to study peptide orientation on polymer surfaces, supplemented by a linear vibrational spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy. Using the antimicrobial peptide Cecropin P1 as a model system, we have quantitatively demonstrated that chemically immobilized peptides on polymers adopt a more ordered orientation than less tightly bound physically adsorbed peptides. These differences were also observed in different chemical environments, for example, air versus water. Although numerous studies have reported a direct correlation between the choice of immobilization method and the performance of an attached biological molecule, the lack of direct biomolecular structure and orientation data has made it difficult to elucidate the relationship between structure, orientation, and function at a surface. In this work, we directly studied the effect of chemical immobilization method on biomolecular orientation/ordering, an important step for future studies of biomolecular activity. The methods for orientation analysis described within are also of relevance to understanding biosensors, biocompatibility, marine-antifouling, membrane protein functions, and antimicrobial peptide activities.
Co-reporter:Andrew P. Boughton, Ioan Andricioaei, and Zhan Chen
Langmuir 2010 Volume 26(Issue 20) pp:16031-16036
Publication Date(Web):September 21, 2010
DOI:10.1021/la1024394
We combined molecular dynamics based free energy calculations with sum frequency generation (SFG) spectroscopy to study the orientational distribution of solvated peptides near hydrophobic surfaces. Using a simplified atomistic model of the polystyrene (PS) surface, molecular dynamics simulations have been applied to compute the orientational probability of an α-helical peptide, magainin 2, with respect to the PS/water interface. Free energy calculations revealed that the preferred (horizontal) peptide orientation was driven by the favorable interactions between the hydrophobic PS surface and the hydrophobic residues on the helix, and additional simulations examined the importance of small aggregate formation. Concentration-dependent measurements obtained via SFG vibrational spectroscopy suggest that, at very low peptide concentrations, magainin molecules tend to lie down at the PS/solution interface, which correlates well with the simulation results. When the concentration is increased, peptides exhibit behavior not captured by MD simulations using single helical peptides. A combination of simulations and experiments was shown to yield more reliable results with molecular-level insights into interaction between peptides and polymer surfaces.
Co-reporter:Shuji Ye, Partha Majumdar, Bret Chisholm, Shane Stafslien, and Zhan Chen
Langmuir 2010 Volume 26(Issue 21) pp:16455-16462
Publication Date(Web):March 26, 2010
DOI:10.1021/la1001539
Poly(dimethylsiloxane) (PDMS) materials containing chemically bound (‘‘tethered’’) quaternary ammonium salt (QAS) moieties are being developed as new contact-active antimicrobial coatings. Such coatings are designed to inhibit the growth of microorganisms on surfaces for a variety of applications which include ship hulls and biomedical devices. The antimicrobial activity of these coatings is a function of the molecular surface structure generated during film formation. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study polymer surface structures at the molecular level in different chemical environments. SFG was successfully used to characterize the surface structures of PDMS coatings containing tethered QAS moieties that possess systematic variations in QAS chemical composition in air, in water, and in a nutrient growth medium. The results indicated that the surface structure was largely dependent on the length of the alkyl chain attached to the nitrogen atom of the QAS moiety as well as the length of alkyl chain spanning between the nitrogen atom and silicon atom of the QAS moiety. The SFG results correlated well with the antimicrobial activity, providing a molecular interpretation of the activity. This research showed that SFG can be effectively used to aid in the development of new antimicrobial coating technologies by correlating the chemical structure of a coating surface to its antimicrobial activity.
Co-reporter:Shuji Ye, Khoi Tan Nguyen and Zhan Chen
The Journal of Physical Chemistry B 2010 Volume 114(Issue 9) pp:3334-3340
Publication Date(Web):February 17, 2010
DOI:10.1021/jp911174d
Structures of membrane-associated peptides and molecular interactions between peptides and cell membrane bilayers govern biological functions of these peptides. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study such structures and interactions at the molecular level. In this research, SFG has been applied, supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to characterize the interactions between alamethicin (a model for larger channel proteins) and different lipid bilayers in the absence of membrane potential. The orientation of alamethicin in lipid bilayers has been determined using SFG amide I spectra detected with different polarization combinations. It was found that alamethicin adopts a mixed α-helical and 310-helical structure in fluid-phase lipid bilayers. The helix (mainly α-helix) at the N-terminus tilts at about 63° versus the surface normal in a fluid-phase 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine-1,1,2,2-d4-N,N,N-trimethyl-d9 (d-DMPC)/1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer. The 310-helix at the C-terminus (beyond the Pro14 residue) tilts at about 43° versus the surface normal. This is the first time to apply SFG to study a 310-helix experimentally. When interacting with a gel-phase lipid bilayer, alamethicin lies down on the gel-phase bilayer surface or aggregates or both, which does not have significant insertion into the lipid bilayer.
Co-reporter:Khoi Tan Nguyen, John Thomas King and Zhan Chen
The Journal of Physical Chemistry B 2010 Volume 114(Issue 25) pp:8291-8300
Publication Date(Web):May 26, 2010
DOI:10.1021/jp102343h
Structural information such as orientations of interfacial proteins and peptides is important for understanding properties and functions of such biological molecules, which play crucial roles in biological applications and processes such as antimicrobial selectivity, membrane protein activity, biocompatibility, and biosensing performance. The α-helical and β-sheet structures are the most widely encountered secondary structures in peptides and proteins. In this paper, for the first time, a method to quantify the orientation of the interfacial β-sheet structure using a combined attenuated total reflectance Fourier transformation infrared spectroscopic (ATR-FTIR) and sum frequency generation (SFG) vibrational spectroscopic study was developed. As an illustration of the methodology, the orientation of tachyplesin I, a 17 amino acid peptide with an antiparallel β-sheet, adsorbed to polymer surfaces as well as associated with a lipid bilayer was determined using the regular and chiral SFG spectra, together with polarized ATR-FTIR amide I signals. Both the tilt angle (θ) and the twist angle (ψ) of the β-sheet at interfaces are determined. The developed method in this paper can be used to obtain in situ structural information of β-sheet components in complex molecules. The combination of this method and the existing methodology that is currently used to investigate α-helical structures will greatly broaden the application of optical spectroscopy in physical chemistry, biochemistry, biophysics, and structural biology.
Co-reporter:Christopher W. Avery, Abhigyan Som, Yongjiang Xu, Gregory N. Tew and Zhan Chen
Analytical Chemistry 2009 Volume 81(Issue 20) pp:8365
Publication Date(Web):September 15, 2009
DOI:10.1021/ac901271f
Sum frequency generation (SFG) vibrational spectroscopy was used to study interactions between solid-supported lipid bilayers mimicking microbial and erythrocyte cellular membranes and synthetic antimicrobial arylamide oligomers named 2, 3, and 4, designed with the facial amphiphilicity common to naturally occurring antimicrobial peptides. The three compounds have the same backbone structure but varied side chains. The inherent interfacial sensitivity of SFG allowed for simultaneous monitoring of lipid ordering in the individual bilayer leaflets and orientation of 2, 3, and 4 upon interaction with the bilayer. Critical concentrations at which the inner leaflet is disrupted were determined for each oligomer. Spectral evidence of the oligomers’ interaction with the bilayer below the critical concentrations was also found. Oligomers 2 and 3 tilted toward the bilayer surface normal, in agreement with previous experimental and simulation results. These oligomers selectively interact with microbial membrane models over erythrocyte membrane models, correlating well to previously published SFG studies on antimicrobial oligomer 1. It was shown that the oligomers interact with the lipid bilayers differently, indicating their different activity and selectivity. This research further shows that SFG is a particularly useful technique for the investigation of interaction mechanisms between cell membranes and membrane-active molecules. Additionally, SFG provides details of the specific interactions between these novel antimicrobials and lipid bilayers.
Co-reporter:Anne V. Vázquez, Nick E. Shephard, Cheryl L. Steinecker, Dongchan Ahn, Sarah Spanninga, Zhan Chen
Journal of Colloid and Interface Science 2009 Volume 331(Issue 2) pp:408-416
Publication Date(Web):15 March 2009
DOI:10.1016/j.jcis.2008.11.065
The use of silane adhesion promoters to improve adhesion of elastomeric materials to polymers has become increasingly common in many industrial applications. However, little is understood about the molecular-level mechanisms of how adhesion promoters enhance adhesion. Here, sum frequency generation (SFG) vibrational spectroscopy was used to probe the buried interface between poly(ethylene terephthalate) (PET) and (3-glycidoxypropyl)trimethoxysilane (γ-GPS), and the interface between PET and a mixture of γ-GPS and a methylvinylsiloxanol (MVS), a known adhesion-promoting mixture. Furthermore, the interfaces between PET and uncured silicone with incorporated silane or silane mixture and the interfaces between PET and cured silicone with incorporated silane or silane mixture were studied. The γ-GPS methoxy groups were found to order at the polymer interface and the presence of MVS increased the interfacial segregation and/or order of γ-GPS. For comparison, two other silanes, N-octadecyltrimethoxysilane (OTMS) and (tridecafluoro-1,1,2,2-tetrahydroctyl)trimethoxysilane (TDFTMS), as well as their mixtures with MVS were also studied at the various interfaces, and were found to exhibit different interfacial behaviors than γ-GPS and the known silane adhesion-promoting mixture of γ-GPS and MVS. Further, X-ray photoelectron spectroscopy (XPS) was used to investigate the exposed PET surfaces resulting from peeling the PET/cured silicone elastomer with TDFTMS and with the TDFTMS/MVS mixture interfaces, and it was shown that the fluorinated silane does segregate to the polymer interface. When correlated to adhesion testing results, it is inferred that segregation and ordering of the silane methoxy groups at the polymer/silane and polymer/silicone elastomer interfaces is crucial for adhesion promotion in this system.SFG spectra have been successfully collected from the buried PET/cured PDMS (with silane or silane mixture) interface. Only γ-GPS molecules are ordered at this buried interface. No SFG signal was detected from two other types of silane molecules.
Co-reporter:Qing Shi, Shuji Ye, Sarah A. Spanninga, Yanlei Su, Zhongyi Jiang and Zhan Chen
Soft Matter 2009 vol. 5(Issue 18) pp:3487-3494
Publication Date(Web):13 Jul 2009
DOI:10.1039/B823045E
Surface properties of polymer materials are important for many applications such as biomedical materials, marine antifouling coatings, polymer membranes for biological and chemical molecule separations, and polymer adhesives. Surface properties are highly dependent on molecular surface structures. Presently, surface polymerization is one of the most effective methods to tailor and optimize molecular surface structures of many polymers. Herein, the surface structure of polydimethylsiloxane (PDMS) is modified by tethering polyelectrolytes (PEs) through surface-initiated ultraviolet (UV) polymerization. Using dimethylacrylamide, acrylic acid, and [2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium as monomers, cationic, anionic, and zwitterionic PEs are grafted onto PDMS surfaces using the “graft-from” method. Successful grafting of PEs onto PDMS surfaces has been verified by several analytical techniques, including Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurement. In particular, sum-frequency generation (SFG) vibrational spectroscopy has been employed to probe the molecular structure of modified PDMS surfaces in both the dry and hydrated states. It was found that in air, the surface dominating –Si–CH3groups of unmodified PDMS segregate to the PDMS surface along with the grafted PEs. These methyl groups have similar orientations to those on the unmodified PDMS surface: they more or less stand up on the surface, along the surface normal. Upon exposure to water, only SFG signals from the surface-tethered PE chains on the modified PDMS surface are observed, showing a substantial surface restructuring behavior. More details regarding such surface restructuring behavior can be deduced through SFG data analysis. With numerous PDMS applications in aqueous environments, it is of particular importance to modify its surface structures and characterize such surface structures in aqueous environments in situ.
Co-reporter:Xiaolin Lu, Dawei Li, Cornelius B. Kristalyn, Jianglong Han, Nick Shephard, Susan Rhodes, Gi Xue and Zhan Chen
Macromolecules 2009 Volume 42(Issue 22) pp:9052-9057
Publication Date(Web):October 14, 2009
DOI:10.1021/ma901757w
We developed a methodology to directly probe molecular ordering at the buried polymer/metal interface. Using sum frequency generation (SFG) vibrational spectroscopy, we observed ordering of ester methyl groups at the buried poly(methyl acrylate) (PMA)/silver (Ag) interface. In order to directly probe the PMA/Ag interface, we collected SFG signal from a thin PMA film sandwiched between a fused silica substrate and a silver surface. It was found that the observed SFG signal intensity does not depend on the PMA film thickness. According to the calculated Fresnel coefficients of the fused silica/PMA and PMA/Ag interfaces with PMA films of different thicknesses, it was shown that SFG signals are solely contributed by the PMA/Ag interface. Further studies indicated that the ester methyl groups at the PMA/Ag interface tilt away from the Ag surface.
Co-reporter:Khoi Tan Nguyen, Stéphanie V. Le Clair, Shuji Ye and Zhan Chen
The Journal of Physical Chemistry B 2009 Volume 113(Issue 36) pp:12169-12180
Publication Date(Web):August 3, 2009
DOI:10.1021/jp904153z
In this paper, we systematically presented the orientation determination of protein helical secondary structures using vibrational spectroscopic methods, particularly, nonlinear sum frequency generation (SFG) vibrational spectroscopy, along with linear vibrational spectroscopic techniques such as infrared spectroscopy and Raman scattering. SFG amide I signals can be collected using different polarization combinations of the input laser beams and output signal beam to measure the second-order nonlinear optical susceptibility components of the helical amide I modes, which are related to their molecular hyperpolarizability elements through the orientation distribution of these helices. The molecular hyperpolarizability elements of amide I modes of a helix can be calculated based on the infrared transition dipole moment and Raman polarizability tensor of the helix; these quantities are determined by using the bond additivity model to sum over the individual infrared transition dipole moments and Raman polarizability tensors, respectively, of the peptide units (or the amino acid residues). The computed overall infrared transition dipole moment and Raman polarizability tensor of a helix can be validated by experimental data using polarized infrared and polarized Raman spectroscopy on samples with well-aligned helical structures. From the deduced SFG hyperpolarizability elements and measured SFG second-order nonlinear susceptibility components, orientation information regarding helical structures can be determined. Even though such orientation information can also be measured using polarized infrared or polarized Raman amide I signals, SFG has a much lower detection limit, which can be used to study the orientation of a helix when its surface coverage is much lower than a monolayer. In addition, the combination of different vibrational spectroscopic techniques, for example, SFG and attenuated total reflectance Fourier transform infrared spectroscopy, provides more measured parameters for orientation determination, aiding in the deduction of more complicated orientation distributions. In this paper, we discussed two types of helices, the α-helix and 3−10 helix. However, the orientation determination method presented here is general and thus can be applied to study other helices as well. The calculations of SFG amide I hyperpolarizability components for α-helical and 3−10 helical structures with different chain lengths have also been performed. It was found that when the helices reached a certain length, the number of peptide units in the helix should not alter the data analysis substantially. It was shown in the calculation, however, that when the helix chain is short, the SFG hyperpolarizability component ratios can vary substantially when the chain length is changed. Because 3−10 helical structures can be quite short in proteins, the orientation determination for a short 3−10 helix needs to take into account the number of peptide units in the helix.
Co-reporter:Arthur A. McClelland, Seokhoon Ahn, Adam J. Matzger and Zhan Chen
Langmuir 2009 Volume 25(Issue 22) pp:12847-12850
Publication Date(Web):October 26, 2009
DOI:10.1021/la902479v
Sum frequency generation vibrational spectroscopy (SFG) has been applied to study two-dimensional (2D) crystals formed by an isophthalic acid diester on the surface of highly oriented pyrolytic graphite, providing complementary measurements to scanning tunneling microscopy (STM) and computational modeling. SFG results indicate that both aromatic and C═O groups in the 2D crystal tilt from the surface. This study demonstrates that a combination of SFG and STM techniques can be used to gain a more complete picture of 2D crystal structure, and it is necessary to consider solvent−2D crystal interactions and dynamics in the computer models to achieve an accurate representation of interfacial structure.
Co-reporter:Xiaolin Lu, Jianglong Han, Nick Shephard, Susan Rhodes, Alex D. Martin, Dawei Li, Gi Xue and Zhan Chen
The Journal of Physical Chemistry B 2009 Volume 113(Issue 39) pp:12944-12951
Publication Date(Web):September 9, 2009
DOI:10.1021/jp9058092
Epoxy and phenolic resins are extensively used for modern microelectronics, for example, as packaging materials. Humidity may greatly alter or degrade their function and application, leading to failure of the device. A nonlinear optical laser technique, sum frequency generation (SFG) vibrational spectroscopy, was used to investigate the molecular surface structures of the epoxy and phenolic resins after exposure to humid air. It was found that the adsorbed water molecules at the phenolic resin surface can induce substantial surface restructuring. The surface phenyl groups were reoriented closer to a perpendicular position to the surface after exposure to humid air from a more parallel position in air. Epoxide group surface restructuring was not observed.
Co-reporter:Sarah A. Spanninga, David C. Martin and Zhan Chen
The Journal of Physical Chemistry C 2009 Volume 113(Issue 14) pp:5585-5592
Publication Date(Web):2017-2-22
DOI:10.1021/jp811282f
Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomaterial coatings because of its outstanding electrical properties and chemical stability. In this research, X-ray photoelectron spectroscopy (XPS) was used to investigate incorporation of different counterions during electrochemical polymerization into PEDOT. The counterions probed included both a polyanion, poly(sodium 4-styrenesulfonate) (PSSNa), and small anions including lithium perchlorate, sodium chloride, and sodium phosphate monobasic monohydrate. In addition to these counterions, an ion mixture, phosphate-buffered saline solution (PBS), was also examined. For such mixtures, the chlorine anion from sodium chloride was found to act as the counterion during PEDOT electrochemical polymerization in PBS solution. This is important because PEDOT is being considered for biomedical applications, which may be prepared in the presence of PBS or other mixtures of ions. Various mixtures of PSSNa, lithium perchlorate, sodium p-toluenesulfonate (TosNa) and PBS counterions were investigated. We detected that the polyanion, PSS−, preferentially incorporated into PEDOT in comparison to ClO4− and Cl− anions when ion mixtures were used. Results were supplemented by those obtained via other analytical techniques including electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron microscopy.
Co-reporter:Khoi Tan Nguyen, Stéphanie V. Le Clair, Shuji Ye and Zhan Chen
The Journal of Physical Chemistry B 2009 Volume 113(Issue 36) pp:12358-12363
Publication Date(Web):August 3, 2009
DOI:10.1021/jp904154w
In this paper, we investigated the molecular interactions of magainin 2 with model cell membranes using sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Symmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-[Phospho-rac-(1-glycerol)] (POPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, which model the bacterial and mammalian cell membranes, respectively, were used in the studies. It was observed by SFG that magainin 2 orients relatively parallel to the POPG lipid bilayer surface at low solution concentrations, around 200 nM. When increasing the magainin 2 concentration to 800 nM, both SFG and ATR-FTIR results indicate that magainin 2 molecules insert into the POPG bilayer and adopt a transmembrane orientation with an angle of about 20° from the POPG bilayer normal. For the POPC bilayer, even at a much higher peptide concentration of 2.0 μM, no ATR-FTIR signal was detected. For this concentration on POPC, SFG studies indicated that magainin 2 molecules adopt an orientation nearly parallel to the bilayer surface, with an orientation angle of about 75° from the surface normal. This shows that SFG has a much better detection limit than ATR-FTIR and can therefore be applied to study interfacial molecules with a much lower surface coverage. This magainin 2 orientation study and further investigation of the lipid bilayer SFG signals support the proposed toroidal pore model for the antimicrobial activity of magainin 2.
Co-reporter:Mark A. Even, Jie Wang and Zhan Chen
Langmuir 2008 Volume 24(Issue 11) pp:5795-5801
Publication Date(Web):May 7, 2008
DOI:10.1021/la800138x
Mytilus edulis foot protein Mefp-3 serves as a primer in the formation of adhesive plaques that attach the mussel to solid surfaces in its immediate environment. The adsorption behavior of this protein on various materials of different hydrophobicity was studied using sum frequency generation (SFG) vibrational spectroscopy. By collecting SFG signals from side chains of these amino acids and from secondary structures of the protein, we have determined that this protein adopts different conformations at different interfaces, depending on hydrophobicity of the contact medium and specific chemical group interactions. We have also demonstrated that SFG has the potential to track the interfacial conformations of a single amino acid in a protein.
Co-reporter:Xiaolin Lu, Nick Shephard, Jianglong Han, Gi Xue and Zhan Chen
Macromolecules 2008 Volume 41(Issue 22) pp:8770-8777
Publication Date(Web):October 27, 2008
DOI:10.1021/ma801680f
Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate the molecular structure of the buried poly(methyl methacrylate) (PMMA)/silver (Ag) interface. To elucidate such a structure, PMMA films with different thicknesses deposited on Ag substrates have been studied in the experiments. SFG signals collected from such PMMA films are interference results of the signals generated from the PMMA/air and PMMA/Ag interfaces. Such signals also include some nonresonant contributions from the samples. When the PMMA film thickness is changed, such an interference effect also varies. SFG signals from the buried PMMA/Ag interface have been successfully deconvoluted from such detected interference results, from which molecular structure of the PMMA/Ag interface can be inferred. The signals from the ester methyl groups dominate the SFG signals deduced for the PMMA/Ag interface, indicating the dominating presence of the ester methyl groups at this interface. The ester methyl groups point away from the Ag surface with a large tilt angle. They lie down more toward the interface compared to those on the PMMA surface in air. Methylene and α-methyl groups are also detected at the PMMA/Ag interface. This research demonstrates that SFG is a viable technique to elucidate molecular structures of buried polymer/metal interfaces.
Co-reporter:Qing Shi, Shuji Ye, Cornelius Kristalyn, Yanlei Su, Zhongyi Jiang and Zhan Chen
Langmuir 2008 Volume 24(Issue 15) pp:7939-7946
Publication Date(Web):July 11, 2008
DOI:10.1021/la800570a
We blended Pluronic F127 into polyethersulfone (PES) to improve surface properties of PES, which has been extensively used in biomaterial and other applications. The molecular surface structures of PES/Pluronic F127 blends have been investigated by sum-frequency generation (SFG) vibrational spectroscopy. The molecular orientation of surface functional groups of PES changed significantly when blended with a small amount of Pluornic F127. Pluronic F127 on the blend surface also exhibited different features upon contacting with water. The entanglement of PES chains with Pluronic F127 molecules rendered the blends with long-term surface stability in water in contrast to the situation where a layer of Pluronic F127 adsorbed on the PES surface. Atomic force microscopy (AFM) and quartz crystal microbalance (QCM) measurements were included to determine the relative amount of protein that adsorbed to the blend surfaces. The results showed a decreased protein adsorption amount with increasing Pluronic F127 bulk concentration. The correlations between polymer surface properties and detailed molecular structures obtained by SFG would provide insight into the designing and developing of biomedical polymers and functional membranes with improved fouling-resistant properties.
Co-reporter:Shuji Ye, Arthur McClelland, Partha Majumdar, Shane J. Stafslien, Justin Daniels, Bret Chisholm and Zhan Chen
Langmuir 2008 Volume 24(Issue 17) pp:9686-9694
Publication Date(Web):July 31, 2008
DOI:10.1021/la800769z
Polymer surface properties are controlled by the molecular surface structures. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study polymer surface structures at the molecular level in different chemical environments. In this research, SFG has been used to study the surface segregation of biocide moieties derived from triclosan (TCS) and tetradecyldimethyl (3-trimethoxysilylpropyl) ammonium chloride (C-14 QAS) that have been covalently bound to a poly(dimethylsiloxane) (PDMS) matrix. PDMS materials are being developed as coatings to control biofouling. This SFG study indicated that TCS-moieties segregate to the surface when the bulk concentration of TCS-moieties exceeds 8.75% by weight. Surface segregation of C-14 QAS moieties was detected after 5% by weight incorporation into a PDMS matrix. SFG results were found to correlate well with antifouling activity, providing a molecular interpretation of such results. This research showed that SFG can aid in the development of coatings for controlling biofouling by elucidating the chemical structure of the coating surface.
Co-reporter:Cheryl L. Loch, Dongchan Ahn, Anne V. Vázquez, Zhan Chen
Journal of Colloid and Interface Science 2007 Volume 308(Issue 1) pp:170-175
Publication Date(Web):1 April 2007
DOI:10.1016/j.jcis.2006.12.029
The surface-sensitive technique of sum frequency generation (SFG) vibrational spectroscopy has been applied to study the buried interfaces between different polymers including deuterated polystyrene (d-PS) and deuterated poly(methyl methacrylate) (d-PMMA) and a two-component silane adhesion-promoting mixture (SAPM) composed of (3-glycidoxypropyl)trimethoxysilane (γ-GPS) and a methylvinylsiloxanol (MVS). Because of the dissolution of d-PS, no SFG CH stretching signals could be collected from the d-PS/γ-GPS interface, and SFG signals collected from the d-PS/SAPM interface gradually disappeared over time. SFG results also showed that γ-GPS can diffuse through the d-PMMA film. The diffusion of γ-GPS through the d-PMMA film was confirmed by SFG studies on the interface between γ-GPS and a d-PMMA/PS two-polymer layer system. Initially the SFG signal from the PS layer was detected. However, after γ-GPS diffused through the d-PMMA film, the PS film was dissolved by the silane, and thus the SFG signal from PS was lost. Similar experiments have been carried out at the interface between the SAPM and the d-PMMA/PS two-polymer layer system and it was found that the diffusion time of the γ-GPS in the SAPM through the d-PMMA film was significantly longer. These results were much different to those from previous SFG studies on the analogous PET interfaces and appear consistent with differences in solubility parameters calculated for these systems.
Co-reporter:Zhan Chen
Polymer International 2007 Volume 56(Issue 5) pp:
Publication Date(Web):29 DEC 2006
DOI:10.1002/pi.2201
This paper reviews recent progress in the studies on polymer surfaces/interfaces using sum frequency generation (SFG) vibrational spectroscopy. SFG theory, technique, and some experimental details have been presented. The review is focused on the SFG studies on buried interfaces involving polymer materials, such as polymer–water interfaces and polymer–polymer interfaces. Molecular interactions between polymer surfaces and adhesion promoters as well as biological molecules such as proteins and peptides have also been elucidated using SFG. This review demonstrates that SFG is a powerful technique to characterize molecular level structural information of complicated polymer surfaces and interfaces in situ. Copyright © 2006 Society of Chemical Industry
Co-reporter:Xiaoyun Chen;Jie Wang;Zoltan Paszti;Fulin Wang
Analytical and Bioanalytical Chemistry 2007 Volume 388( Issue 1) pp:65-72
Publication Date(Web):2007 May
DOI:10.1007/s00216-006-0999-8
Electrostatic interactions between negatively charged polymer surfaces and factor XII (FXII), a blood coagulation factor, were investigated by sum frequency generation (SFG) vibrational spectroscopy, supplemented by several analytical techniques including attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), quartz crystal microbalance (QCM), ζ-potential measurement, and chromogenic assay. A series of sulfonated polystyrenes (sPS) with different sulfonation levels were synthesized as model surfaces with different surface charge densities. SFG spectra collected from FXII adsorbed onto PS and sPS surfaces with different surface charge densities showed remarkable differences in spectral features and especially in spectral intensity. Chromogenic assay experiments showed that highly charged sPS surfaces induced FXII autoactivation. ATR-FTIR and QCM results indicated that adsorption amounts on the PS and sPS surfaces were similar even though the surface charge densities were different. No significant conformational change was observed from FXII adsorbed onto surfaces studied. Using theoretical calculations, the possible contribution from the third-order nonlinear optical effect induced by the surface electric field was evaluated, and it was found to be unable to yield the SFG signal enhancement observed. Therefore it was concluded that the adsorbed FXII orientation and ordering were the main reasons for the remarkable SFG amide I signal increase on sPS surfaces. These investigations indicate that negatively charged surfaces facilitate or induce FXII autoactivation on the molecular level by imposing specific orientation and ordering on the adsorbed protein molecules.
Co-reporter:Chunyan Chen, Matthew L. Clarke, Jie Wang and Zhan Chen
Physical Chemistry Chemical Physics 2005 vol. 7(Issue 11) pp:2357-2363
Publication Date(Web):06 May 2005
DOI:10.1039/B501910A
Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate and compare the chemical structures of poly(ethyl methacrylate)
(PEMA) and poly(ethyl acrylate)
(PEA) in air, in water, and in a non-polar solvent, FC-75. SFG spectra from both polymer surfaces in air are dominated by vibrational modes from the ester ethyl side groups. The average orientation of these ester ethyl groups on the two polymer surfaces is slightly different. In water, the two polymers show markedly different restructuring behavior. The ester ethyl side chains on the PEMA surface in water reorient to tilt more toward the surface, yet remain ordered. Such a restructuring of the PEMA surface in water is reversible. However, no SFG signal was detected from the PEA/water interface, showing that the surface of PEA becomes disordered upon contacting water, and this process is irreversible. SFG results collected from the CO range indicate that hydrogen bonding is observed for both polymer/water interfaces, but the order of CO at the PEA/water interface is much lower than that at the PEMA/water interface. Supplemental experiments support our hypothesis that the PEA surface becomes rough and loses order gradually as it interacts with water. We have demonstrated, for the first time, that the loss of surface structural order is due to the interaction between soft PEA chains with water molecules followed by reorganization of the polymer backbone. This causes the polymer surface to become rough and disordered. However, the surface structures of PEMA and PEA in FC-75 are similar and are also similar to those in air. This indicates that not only Tg, but also the contacting medium plays an important role in determining the surface restructuring behavior of polymer materials.
Co-reporter:Jie Wang, Matthew L. Clarke, Xiaoyun Chen, Mark A. Even, William C. Johnson, Zhan Chen
Surface Science 2005 Volume 587(1–2) pp:1-11
Publication Date(Web):1 August 2005
DOI:10.1016/j.susc.2005.04.034
Sum frequency generation (SFG) vibrational spectroscopy has been applied to examine the molecular structures of proteins adsorbed at various interfaces, especially polymer/protein solution interfaces in situ. A thin film model was adopted to interpret interfacial protein SFG spectra. A polarization mapping method was developed that can help to analyze complicated protein SFG spectra more reliably, from which more structural information of various interfacial proteins can be deduced. SFG signals in different frequency ranges, including C–H, N–H, O–H stretching and amide I ranges, can be collected from proteins at interfaces. Different proteins, including bovine serum albumin, ubiquitin, fibrinogen, factor XII, and some selectively or randomly deuterated proteins, have been examined in SFG studies. Our research shows that proteins can have different conformations at various interfaces, and also that time-dependent structural changes of proteins after adsorption can be varied. SFG can be developed into a powerful and unique technique to elucidate interfacial protein structures at the molecular level in situ after continued success in the application of SFG to examine interfacial protein structures.
Co-reporter:Jie Wang;Xiaoyun Chen;Matthew L. Clarke;
Proceedings of the National Academy of Sciences 2005 102(14) pp:4978-4983
Publication Date(Web):March 25, 2005
DOI:10.1073/pnas.0501206102
In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational
spectra from interfacial proteins and peptides at the solid/liquid interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen molecules. By adopting
an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between
the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen
was mainly contributed by the β-sheet structure. For a β-sheet peptide tachyplesin I, which may be quite ordered at the solid/liquid
interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed
by electric dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to
our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG
vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein
and peptide structures.
Co-reporter:Jie Wang, Sarah M. Buck and Zhan Chen
Analyst 2003 vol. 128(Issue 6) pp:773-778
Publication Date(Web):28 Apr 2003
DOI:10.1039/B212551J
The air–BSA solution interface has been investigated by various techniques for years. From these studies we know that BSA molecules segregate at the BSA solution–air interface, and the surface coverage increases with the increase of the bulk solution concentration. However, questions still remain as to whether the protein changes conformation, orientation, or a combination of the two upon adsorption. In this paper, by using sum frequency generation (SFG) vibrational spectroscopy we found that the conformation of interfacial BSA molecules changes dramatically at the solution–air interface, compared to that of the native BSA in solution. The hydrophobic methyl groups of BSA molecules at this interface tend to align along the surface normal. The degree of such conformational changes of surface BSA molecules depend on the surface coverage, indicating that the protein–protein interaction plays a very important role in determining the conformation of interfacial protein molecules. At very low surface concentration, the adsorbed BSA molecules unfold substantially. Our results can provide a molecular interpretation of results obtained from other studies such as protein layer thickness and surface tension measurements of protein solution.
Co-reporter:Shuji Ye, Khoi Tan Nguyen, Stéphanie V. Le Clair, Zhan Chen
Journal of Structural Biology (October 2009) Volume 168(Issue 1) pp:61-77
Publication Date(Web):1 October 2009
DOI:10.1016/j.jsb.2009.03.006
Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study the molecular structures of surfaces and interfaces in different chemical environments. This review summarizes recent SFG studies on hybrid bilayer membranes and substrate-supported lipid monolayers and bilayers, the interaction between peptides/proteins and lipid monolayers/bilayers, and bilayer perturbation induced by peptides/proteins. To demonstrate the ability of SFG to determine the orientations of various secondary structures, studies on the interactions between different peptides/proteins (melittin, G proteins, alamethicin, and tachyplesin I) and lipid bilayers are discussed. Molecular level details revealed by SFG in these studies show that SFG can provide a unique understanding on the interactions between a lipid monolayer/bilayer and peptides/proteins in real time, in situ and without any exogenous labeling.
Co-reporter:Xiaoyun Chen, Jie Wang, Cornelius B. Kristalyn, Zhan Chen
Biophysical Journal (1 August 2007) Volume 93(Issue 3) pp:
Publication Date(Web):1 August 2007
DOI:10.1529/biophysj.106.099739
Interactions between membrane bilayers and peptides/proteins are ubiquitous throughout a cell. To determine the structure of membrane bilayers and the associated peptides/proteins, model systems such as supported lipid bilayers are often used. It has been difficult to directly investigate the interactions between a single membrane bilayer and peptides/proteins without exogenous labeling. In this work we demonstrate that sum frequency generation vibrational spectroscopy can be employed to study the interactions between peptides/proteins and a single lipid bilayer in real time, in situ, and without exogenous labeling. Using melittin and a dipalmitoyl phosphatidylglycerol bilayer as a model system, we monitored the C-H and C-D stretching signals from isotopically symmetric or asymmetric dipalmitoyl phosphatidylglycerol bilayers during their interaction with melittin. It has been found that the extent and kinetics of bilayer perturbation induced by melittin are very sensitive to melittin concentration. Such concentration dependence is correlated to melittin's mode of action. Melittin is found to function via the early and late stage of the carpet model at low and high concentrations, respectively, whereas the toroidal model is probable at intermediate concentrations. This research illustrates the potential of sum frequency generation as a biophysical technique to monitor individual leaflet structure of lipid bilayers in real time during their interactions with biomolecules.
Co-reporter:Xiaolin Lu ; Gi Xue ; Xinping Wang ; Jianglong Han ; Xiaofeng Han ; Jeanne Hankett ; Dawei Li
Macromolecules () pp:
Publication Date(Web):July 24, 2012
DOI:10.1021/ma301088g
Previously, we developed several methods to use sum frequency generation (SFG) vibrational spectroscopy to probe buried polymer/metal interfaces in situ by depositing polymer films with different thicknesses on metal surfaces or sandwiching a polymer thin film between a metal surface and a fused silica window. In this study, we developed a new and easier method to directly probe the polymer/metal interface by collecting ppp SFG spectra using a poly(ethyl methacrylate) (PEMA)/silver (Ag) interface as an example. We confirmed that for a thin polymer film on metal, the dominant SFG signals were contributed from the polymer surface in air and/or the polymer metal interface, while the contribution from the polymer bulk could be ignored. Previously, we showed that the ssp spectra were contributed by both the polymer/air and polymer/metal interfaces. Here we demonstrated that the SFG ppp spectra were dominated by signals from the buried polymer/metal interface from which the structural information on the buried interface can be deduced. This method to probe the buried polymer/metal interface via SFG is relatively simple compared to our previous sample preparation techniques and/or data analysis methods.
Co-reporter:Nathan W. Ulrich, John Andre, Jaimal Williamson, Kang-Wook Lee and Zhan Chen
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 19) pp:NaN12155-12155
Publication Date(Web):2017/04/19
DOI:10.1039/C7CP00567A
Adhesion is important in many industrial applications including those in the microelectronics industry. Flip-chip assemblies commonly utilize epoxy underfills to promote reliability and the buried interfacial structure of underfills is crucial to device lifetime. Poor adhesion at this interface can cause premature device failure. One method to increase adhesion strength is to plasma treat the substrate attached to underfills, however, the mechanism of this increase in adhesion strength has not been thoroughly investigated at the molecular level in situ, because it is difficult to probe a buried interface where the adhesion occurs. In this work, sum frequency generation (SFG) vibrational spectroscopy was utilized to investigate the buried polymer/epoxy resin interface at the molecular level. Plasma treatment was performed on the polymer surfaces and the effects were examined. The buried interfaces between the polymer surface before and after plasma treatment and epoxy were then investigated to understand if the effects of the treatment can be observed using SFG. It was found that the molecular structure of the buried interface of the pristine polymer surface in contact with epoxy is drastically different from the buried interface of the plasma treated surface with epoxy. The buried interface containing the plasma treated polymer surface was found to be considerably more disordered and had much higher adhesion strength. This research elucidates the plasma treatment effects on structures and properties of buried polymer/epoxy interfaces, providing in-depth understanding on the mechanism of adhesion strength increase facilitated by plasma treatment.
Co-reporter:Xiaoguang Wang, Pei Yang, Frederic Mondiot, Yaoxin Li, Daniel S. Miller, Zhan Chen and Nicholas L. Abbott
Chemical Communications 2015 - vol. 51(Issue 94) pp:NaN16847-16847
Publication Date(Web):2015/09/29
DOI:10.1039/C5CC06996C
We report that assemblies formed by eight oligopeptides at phospholipid-decorated interfaces of thermotropic liquid crystals (LCs) trigger changes in ordering of the LCs that are dependent on the secondary structures of the oligopeptides (as characterized in situ using infrared-visible sum-frequency spectroscopy).
Co-reporter:Jeanne M. Hankett, Xiaolin Lu, Yuwei Liu, Emily Seeley and Zhan Chen
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 37) pp:NaN20106-20106
Publication Date(Web):2014/08/08
DOI:10.1039/C4CP03206C
Upon water contact, phthalate-plasticized poly(vinyl chloride) (PVC) surfaces are highly unstable because the plasticizer molecules are not covalently bound to the polymer network. As a result, it is difficult to predict how the surface polymer chains and plasticizers may interact with water without directly probing the plastic/water interface in situ. We successfully studied the molecular surface restructuring of 10 wt% and 25 wt% bis 2-ethylhexyl phthalate (DEHP)-plasticized and pure PVC films (deposited on solid substrates) in situ due to water contact using sum frequency generation (SFG) vibrational spectroscopy. SFG spectral signals from both the top and the bottom of the plastic film were obtained simultaneously, so a thin-film model spectral analysis was applied to separately identify the molecular changes of plastics at the surface and the plastic/substrate interface in water. It was found that in water both the structures of the plastic surface and the buried plastic/substrate interface changed. After removing the samples from the water and exposing them to air again, the surface structures did not completely recover. Further SFG experiments confirmed that small amounts of DEHP were transferred into the water. The leached DEHP molecules could reorder and permanently transfer to new surfaces through water contact. Our studies indicate that small amounts of phthalates can transfer from surface to surface through water contact in an overall scope of minutes. This study yields vital new information on the molecular surface structures of DEHP plasticized PVC in water, and the transfer behaviors and environmental fate of plasticizers in polymers.
Co-reporter:Xiaoxian Zhang, John N. Myers, Qinghuang Lin, Jeffery D. Bielefeld and Zhan Chen
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 39) pp:NaN26139-26139
Publication Date(Web):2015/09/08
DOI:10.1039/C5CP03649F
Fully understanding the effect and the molecular mechanisms of plasma damage and silylation repair on low dielectric constant (low-k) materials is essential to the design of low-k dielectrics with defined properties and the integration of low-k dielectrics into advanced interconnects of modern electronics. Here, analytical techniques including sum frequency generation vibrational spectroscopy (SFG), Fourier transform infrared spectroscopy (FTIR), contact angle goniometry (CA) and X-ray photoelectron spectroscopy (XPS) have been employed to provide a comprehensive characterization of the surface and bulk structure changes of poly(methyl)silsesquioxane (PMSQ) low-k thin films before and after O2 plasma treatment and silylation repair. O2 plasma treatment altered drastically both the molecular structures and water structures at the surfaces of the PMSQ film while no bulk structural change was detected. For example, ∼34% Si–CH3 groups were removed from the PMSQ surface, and the Si–CH3 groups at the film surface tilted toward the surface after the O2 plasma treatment. The oxidation by the O2 plasma made the PMSQ film surface more hydrophilic and thus enhanced the water adsorption at the film surface. Both strongly and weakly hydrogen bonded water were detected at the plasma-damaged film surface during exposure to water with the former being the dominate component. It is postulated that this enhancement of both chemisorbed and physisorbed water after the O2 plasma treatment leads to the degradation of low-k properties and reliability. The degradation of the PMSQ low-k film can be recovered by repairing the plasma-damaged surface using a silylation reaction. The silylation method, however, cannot fully recover the plasma induced damage at the PMSQ film surface as evidenced by the existence of hydrophilic groups, including C–O/CO and residual Si–OH groups. This work provides a molecular level picture on the surface structural changes of low-k materials after plasma treatment and the subsequent silylation repair.
Co-reporter:Peipei Hu, Xiaoxian Zhang, Chi Zhang and Zhan Chen
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 15) pp:NaN9884-9884
Publication Date(Web):2015/03/05
DOI:10.1039/C5CP00477B
The interactions between nanoparticles (NPs) and cells are of huge interest because NPs have been extensively researched for biomedical applications. For the cellular entry of NPs, it remains unclear how the cell membrane molecules respond to the exposure of NPs due to a lack of appropriate surface/interface-sensitive techniques to study NP–cell membrane interactions in situ in real time. In this study, sum frequency generation (SFG) vibrational spectroscopy was employed to examine the interactions between lipid bilayers (serving as model mammalian cell membranes) and Au NPs of four different sizes with the same mass, or the same NP number, or the same NP surface area. It was found that lipid flip-flop was induced by Au NPs of all four sizes. Interestingly, the lipid flip-flop rate was found to increase as the Au NP size increased with respect to the same particle number or the same NP surface area. However, the induced lipid flip-flop rate was the same for Au NPs with different sizes with the same mass, which was interpreted by the same “effective surface contact area” between Au NPs and the model cell membrane. We believe that this study provided the first direct observation of the lipid flip-flop induced by the interactions between Au NPs and the model mammalian cell membrane.
Co-reporter:Minyu Xiao, Joshua Jasensky, Xiaoxian Zhang, Yaoxin Li, Cayla Pichan, Xiaolin Lu and Zhan Chen
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 32) pp:NaN22099-22099
Publication Date(Web):2016/07/13
DOI:10.1039/C6CP04155H
The molecular structures of organic semiconducting thin films mediate the performance of various devices composed of such materials. To fully understand how the structures of organic semiconductors alter on substrates due to different polymer side chains and different interfacial interactions, thin films of two kinds of polythiophene derivatives with different side-chains, poly(3-hexylthiophene) (P3HT) and poly(3-potassium-6-hexanoate thiophene) (P3KHT), were deposited and compared on various surfaces. A combination of analytical tools was applied in this research: contact angle goniometry and X-ray photoelectron spectroscopy (XPS) were used to characterize substrate dielectric surfaces with varied hydrophobicity for polymer film deposition; X-ray diffraction and UV-vis spectroscopy were used to examine the polythiophene film bulk structure; sum frequency generation (SFG) vibrational spectroscopy was utilized to probe the molecular structures of polymer film surfaces in air and buried solid/solid interfaces. Both side-chain hydrophobicity and substrate hydrophobicity were found to mediate the crystallinity of the polythiophene film, as well as the orientation of the thiophene ring within the polymer backbone at the buried polymer/substrate interface and the polymer thin film surface in air. For the same type of polythiophene film deposited on different substrates, a more hydrophobic substrate surface induced thiophene ring alignment with the surface normal at both the buried interface and on the surface in air. For different films (P3HT vs. P3KHT) deposited on the same dielectric substrate, a more hydrophobic polythiophene side chain caused the thiophene ring to align more towards the surface at the buried polymer/substrate interface and on the surface in air. We believe that the polythiophene surface, bulk, and buried interfacial molecular structures all influence the hole mobility within the polythiophene film. Successful characterization of an organic conducting thin film surface, buried interfacial, and bulk structures is a first crucial step in understanding the structure–function relationship of such films in order to optimize device performance. An in-depth understanding on how the side-chain influences the interfacial and surface polymer orientation will guide the future molecular structure design of organic semiconductors.
Co-reporter:Xiaoxian Zhang, Yaoxin Li, Jeanne M. Hankett and Zhan Chen
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 6) pp:NaN4482-4482
Publication Date(Web):2015/01/02
DOI:10.1039/C4CP05287K
Tributyl acetyl citrate (TBAC), a widely-used “green” plasticizer, has been extensively applied in products for daily use. In this paper, a variety of analytical tools including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), contact angle goniometry (CA), and Fourier transform infrared spectroscopy (FTIR) were applied together to investigate the molecular structures of TBAC plasticized poly(vinyl chloride) (PVC) and the migration behavior of TBAC from PVC–TBAC mixtures into water. We comprehensively examine the effects of oxygen and argon plasma treatments on the surface structures of PVC–TBAC thin films containing various bulk percentages of plasticizers and the leaching behavior of TBAC into water. It was found that TBAC is a relatively stable PVC plasticizer compared to traditional non-covalent plasticizers but is also surface active. Oxygen plasma treatment increased the hydrophilicity of TBAC–PVC surfaces, but did not enhance TBAC leaching. However, argon plasma treatment greatly enhanced the leaching of TBAC molecules from PVC plastics to water. Based on our observations, we believe that oxygen plasma treatment could be applied to TBAC plasticized PVC products to enhance surface hydrophilicity for improving the biocompatibility and antibacterial properties of PVC products. The structural information obtained in this study will ultimately facilitate a molecular level understanding of plasticized polymers, aiding in the design of PVC materials with improved properties.
Co-reporter:Yaoxin Li, Xiaoxian Zhang, John Myers, Nicholas L. Abbott and Zhan Chen
Chemical Communications 2015 - vol. 51(Issue 55) pp:NaN11018-11018
Publication Date(Web):2015/06/04
DOI:10.1039/C5CC03274A
Sugar coatings can stabilize the “native” structure and control the orientation of surface-immobilized peptides in air, providing a potential approach to retain biological functions of surface-immobilized biomolecules in air. This method is general and applicable to complex enzymes.
Co-reporter:Chuan Leng ; Xiaofeng Han ; Qing Shao ; Yongheng Zhu ; Yuting Li ; Shaoyi Jiang
The Journal of Physical Chemistry C () pp:
Publication Date(Web):
DOI:10.1021/jp504293r
Surface hydration has been proposed as the key nonfouling mechanism of zwitterionic materials. Because these materials have various chemical structures and will be used in complex environments, in situ probing of their surfaces in different aqueous environments is crucial to understanding their surface hydration properties. In this work, the surfaces of three zwitterionic polymer brushes in aqueous solutions with salts or varied pH were probed using sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra indicate the ordering of the polymer brushes in water and the strong hydrogen bonding of the interfacial water molecules. The ordering of water at the surfaces of carboxybetaine polymers changed with pH, but at the sulfobetaine polymer surface, this ordering was not affected by pH. The interfacial ordering of water also decreased when salt ions were associated with the polymers. Ions from different salts had different interfacial binding affinities depending on the ion type and the polymer structure, as deduced from the interfacial water signals. These SFG results reveal important surface hydration properties of zwitterionic polymers that will guide their applications in complex environments.
.jpg)
![Bisbenz[5,6]indeno[1,2,3-cd:1',2',3'-lm]perylene, 5,10,15,20-tetraphenyl-](/data/chemimg/116100/175606-05-0.png)
![Bisbenz[5,6]indeno[1,2,3-cd:1',2',3'-lm]perylene, 5,10,15,20-tetraphenyl-](/data/chemimg/116100/175606-05-0_b.png)
![3,5,9-Trioxa-4-phosphapentacosan-1-aminium,4-hydroxy-N,N,N-trimethyl-10-oxo-7-[(1-oxohexadecyl)oxy]-, inner salt, 4-oxide](http://img.cochemist.com/ccimg/2700/2644-64-6.png)
![3,5,9-Trioxa-4-phosphapentacosan-1-aminium,4-hydroxy-N,N,N-trimethyl-10-oxo-7-[(1-oxohexadecyl)oxy]-, inner salt, 4-oxide](http://img.cochemist.com/ccimg/2700/2644-64-6_b.png)
![1,2-Propanediol, 3-[3-(ethoxydimethylsilyl)propoxy]-](http://img.cochemist.com/ccimg/184900/184870-14-2.png)
![1,2-Propanediol, 3-[3-(ethoxydimethylsilyl)propoxy]-](http://img.cochemist.com/ccimg/184900/184870-14-2_b.png)
![1-Dodecanaminium,N,N-dimethyl-N-[2-[(2-methyl-1-oxo-2-propenyl)oxy]ethyl]-, bromide](http://img.cochemist.com/ccimg/96600/96526-35-1.png)
![1-Dodecanaminium,N,N-dimethyl-N-[2-[(2-methyl-1-oxo-2-propenyl)oxy]ethyl]-, bromide](http://img.cochemist.com/ccimg/96600/96526-35-1_b.png)
![(6R,9AR)-OCTAHYDRO-2H-PYRIDO[1,2-A]PYRAZIN-6-YLMETHANOL](http://img.cochemist.com/ccimg/13700/13699-48-4.png)
![(6R,9AR)-OCTAHYDRO-2H-PYRIDO[1,2-A]PYRAZIN-6-YLMETHANOL](http://img.cochemist.com/ccimg/13700/13699-48-4_b.png)
![Hexadecanoic acid,1,1'-[1-[[[(2-aminoethoxy)hydroxyphosphinyl]oxy]methyl]-1,2-ethanediyl] ester](http://img.cochemist.com/ccimg/3100/3026-45-7.png)
![Hexadecanoic acid,1,1'-[1-[[[(2-aminoethoxy)hydroxyphosphinyl]oxy]methyl]-1,2-ethanediyl] ester](http://img.cochemist.com/ccimg/3100/3026-45-7_b.png)
![Poly[oxy(ethenylmethylsilylene)]](http://img.cochemist.com/ccimg/28400/28323-46-8.png)
![Poly[oxy(ethenylmethylsilylene)]](http://img.cochemist.com/ccimg/28400/28323-46-8_b.png)
![Hexadecanoic acid,1,1'-[1-[[[(2,3-dihydroxypropoxy)hydroxyphosphinyl]oxy]methyl]-1,2-ethanediyl]ester](http://img.cochemist.com/ccimg/4600/4537-77-3.png)
![Hexadecanoic acid,1,1'-[1-[[[(2,3-dihydroxypropoxy)hydroxyphosphinyl]oxy]methyl]-1,2-ethanediyl]ester](http://img.cochemist.com/ccimg/4600/4537-77-3_b.png)
![3,5,8-Trioxa-4-phosphahexacos-17-en-1-aminium,4-hydroxy-N,N,N-trimethyl-9-oxo-7-[[(1-oxohexadecyl)oxy]methyl]-, inner salt,4-oxide, (7R,17Z)-](http://img.cochemist.com/ccimg/26900/26853-31-6.png)
![3,5,8-Trioxa-4-phosphahexacos-17-en-1-aminium,4-hydroxy-N,N,N-trimethyl-9-oxo-7-[[(1-oxohexadecyl)oxy]methyl]-, inner salt,4-oxide, (7R,17Z)-](http://img.cochemist.com/ccimg/26900/26853-31-6_b.png)
