Alexander Wei

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Name: Wei, Alexander

Organization: Purdue University , USA

Department: Department of Chemistry

Title: (PhD)

TOPICS

Material Chemistry

Organic Chemistry

Calixarene

Chemicals
References

Protein Corona Analysis of Silver Nanoparticles Exposed to Fish Plasma

Co-reporter:Jiejun Gao, Lu Lin, Alexander Wei, and Maria S. Sepúlveda

Environmental Science & Technology Letters May 9, 2017 Volume 4(Issue 5) pp:174-174

Publication Date(Web):April 7, 2017

DOI:10.1021/acs.estlett.7b00074

Nanoparticles (NPs) in contact with biological fluids experience changes in surface chemistry that can impact their biodistribution and downstream physiological impact. One such change involves the formation of a protein corona (PC) on the surface of NPs. Here we present a foundational study of PC formation following the incubation of polyvinylpyrrolidone-coated AgNPs (PVP-AgNPs, 50 nm) in the plasma of smallmouth bass (Micropterus dolomieu). The level of PC formation increases with exposure time and is also affected by gender, with AgNPs incubated in male plasma having PCs slightly thinner than and ζ potentials less negative than those of AgNPs incubated in female plasma. Proteomic analysis also revealed gender-specific differences in PC composition: in particular, egg-specific proteins (vitellogenin and zona pellucida) were identified in only PCs derived from female plasma, raising the possibility of their roles in AgNP-related reproductive toxicity by promoting their accumulation in developing oocytes.

Micellization and Single-Particle Encapsulation with Dimethylammoniopropyl Sulfobetaines

Co-reporter:Jianxin Wang, Oscar Morales-Collazo, and Alexander Wei

ACS Omega April 2017? Volume 2(Issue 4) pp:1287-1287

Publication Date(Web):April 4, 2017

DOI:10.1021/acsomega.7b00288

Sulfobetaines (SBs) are a class of zwitterionic surfactants with a reputation for enhancing colloidal stability at high salt concentrations. Here, we present a systematic study on the self-assembly of SB amphiphiles (sultaines or hydroxysultaines) in aqueous solutions, as a function of chain length and composition, ionic strength, and in the presence of alkanethiol-coated Au nanoparticles (GNPs). The diameters of the micelles assembled from SB and amidosulfobetaine (ASB) generally increase monotonically with chain length, although ASB micelles are smaller relative to alkyl SB micelles with similarly sized tailgroups, and oleyl sulfobetaine (OSB) micelles are slightly larger. SB amphiphiles can stabilize alkanethiol-coated GNPs in physiologically relevant buffers at concentrations well below their CMC, with size increases corresponding to single-particle encapsulation. SB-encapsulated GNPs were prepared by three different methods with SB:GNP weight ratios of 10:1, followed by dispersion in water or 1 M NaCl. The low hydrodynamic size of the SB micelles and SB-coated NPs is within the range needed for efficient renal clearance.Topics: Amphiphiles; Ionic strength; Materials processing; Molecular structure-property relationship; Nanoparticles; Phase; Self-assembly; Surfactants;

Label-Free Detection and Discrimination of Bacterial Pathogens Based on Hemin Recognition

Co-reporter:Thora R. Maltais, Avijit K. Adak, Waleed Younis, Mohamed N. Seleem, and Alexander Wei

Bioconjugate Chemistry 2016 Volume 27(Issue 7) pp:1713

Publication Date(Web):June 23, 2016

DOI:10.1021/acs.bioconjchem.6b00236

Hemin linked to hexa(ethylene glycol)bishydrazide was patterned by inkjet printing into periodic microarrays, and evaluated for their ability to capture bacterial pathogens expressing various hemin receptors. Bacterial adhesion was imaged under darkfield conditions with Fourier analysis, supporting a label-free method of pathogen detection. Hemin microarrays were screened against a panel of 16 bacteria and found capable of capturing multiple species, some with limits of detection as low as 103 cfu/mL. Several Gram-positive strains including Staphylococcus aureus and Bacillus anthracis also exhibited rapid adhesion, enabling pattern recognition within minutes of exposure. This can be attributed to differences in hemin acquisition systems: aggressively adherent bacteria express cell-surface hemin receptors (CSHRs) that enable direct hemin binding and uptake, whereas other types of bacteria including most Gram-negative strains rely on the secretion and recapture of soluble proteins (hemophores) for hemin acquisition, with consequently longer times for ligand binding and detection.

Characterization of Asphaltene Deposits by Using Mass Spectrometry and Raman Spectroscopy

Co-reporter:James S. Riedeman, Naveen Reddy Kadasala, Alexander Wei, and Hilkka I. Kenttämaa

Energy & Fuels 2016 Volume 30(Issue 2) pp:805-809

Publication Date(Web):January 6, 2016

DOI:10.1021/acs.energyfuels.5b02002

Crude oil deposition in oil transfer pipelines and bore wells afflicts many oil reservoirs. Asphaltenes play a major role in this process because of their tendency to precipitate in pipelines upon changes in temperature and/or pressure. Asphaltenes are defined by their lack of solubility in n-alkane solvents, which means that they likely contain many compounds that do not actively contribute to the deposition of crude oil in pipelines. The preponderance of studies in the literature have focused on asphaltenes derived from crude oil, whereas far fewer investigations have focused on asphaltenes derived from oil deposits. In this study, structural parameters of oil-deposit asphaltenes were examined using Raman spectroscopy and tandem mass spectrometry and compared to results reported previously for petroleum asphaltenes. On the basis of D1 and G band intensities in the Raman spectrum of oil-deposit asphaltenes, the average aromatic sheet size of these molecules was 21.0 Å, slightly larger than earlier values reported for petroleum asphaltenes (15.2–18.8 Å). Mass spectrometric experiments of oil-deposit asphaltenes ionized via atmospheric pressure chemical ionization (APCI) using CS2 solvent were used to measure the molecular weight distribution (MWD), saturated carbon content, and the number of fused aromatic rings in the cores of the asphaltene molecules. The MWD was found to be 150–1050 Da with an average molecular weight (average MW) of 497 Da, which are significantly lower than those reported previously for petroleum asphaltenes (200–1500 Da and 570–700 Da, respectively). Aromatic core sizes were estimated to contain 8 fused rings on average for the most abundant species in oil-deposit asphaltenes, with 5–15 carbons in their alkyl side chains, as compared to averages of 3–7 aromatic rings and 17–41 alkyl carbons for petroleum asphaltenes.

Polymer–iron oxide composite nanoparticles for EPR-independent drug delivery

Co-reporter:Jinho Park, Naveen Reddy Kadasala, Sara A. Abouelmagd, Mark A. Castanares, David S. Collins, Alexander Wei, Yoon Yeo

Biomaterials 2016 101() pp: 285-295

Publication Date(Web):September 2016

DOI:10.1016/j.biomaterials.2016.06.007

Time-Resolved Proteomic Visualization of Dendrimer Cellular Entry and Trafficking

Co-reporter:Linna Wang; Li Yang; Li Pan; Naveen Reddy Kadasala; Liang Xue; Robert J. Schuster; Laurie L. Parker; Alexander Wei;W. Andy Tao

Journal of the American Chemical Society 2015 Volume 137(Issue 40) pp:12772-12775

Publication Date(Web):October 1, 2015

DOI:10.1021/jacs.5b07875

Our understanding of the complex cell entry pathways would greatly benefit from a comprehensive characterization of key proteins involved in this dynamic process. Here we devise a novel proteomic strategy named TITAN (Tracing Internalization and TrAfficking of Nanomaterials) to reveal real-time protein–dendrimer interactions using a systems biology approach. Dendrimers functionalized with photoreactive cross-linkers were internalized by HeLa cells and irradiated at set time intervals, then isolated and subjected to quantitative proteomics. In total, 809 interacting proteins cross-linked with dendrimers were determined by TITAN in a detailed temporal manner during dendrimer internalization, traceable to at least two major endocytic mechanisms, clathrin-mediated and caveolar/raft-mediated endocytosis. The direct involvement of the two pathways was further established by the inhibitory effect of dynasore on dendrimer uptake and changes in temporal profiles of key proteins.

Trace detection of tetrabromobisphenol A by SERS with DMAP-modified magnetic gold nanoclusters

Co-reporter:Naveen Reddy Kadasala and Alexander Wei

Nanoscale 2015 vol. 7(Issue 25) pp:10931-10935

Publication Date(Web):04 Jun 2015

DOI:10.1039/C4NR07658C

Magnetic gold nanoclusters (MGNCs) functionalized with 4-dimethylaminopyridine (DMAP) enables the trace detection of tetrabromobisphenol A (TBBPA), an environmental pollutant, using surface-enhanced Raman scattering (SERS) spectroscopy. The synthesis, cleansing, and functionalization of MGNCs are conducted in aqueous solutions; SERS samples are prepared by magnetic precipitation in the presence of trace analyte. The limit of detection (LOD) for TBBPA is greatly increased by the use of DMAP as a reporter molecule: DMAP-modified MGNCs can detect TBBPA at 10 pM in water, whereas the LOD for TBBPA by unfunctionalized Au is 1 nM. The reproducibility of picomolar TBBPA detection with DMAP-modified MGNCs is confirmed by two-dimensional correlation analysis. The high SERS sensitivity for TBBPA can be attributed to its capacity to modulate the Raman spectrum of adsorbed DMAP. This indirect mode of detection can also be applied toward the detection of other hydrophobic analytes, each identifiable by its characteristic SERS identity.

Cys34-PEGylated Human Serum Albumin for Drug Binding and Delivery

Co-reporter:Jonathan G. Mehtala, Chris Kulczar, Monika Lavan, Gregory Knipp, and Alexander Wei

Bioconjugate Chemistry 2015 Volume 26(Issue 5) pp:941

Publication Date(Web):April 28, 2015

DOI:10.1021/acs.bioconjchem.5b00143

Polyethylene glycol (PEG) derivatives were conjugated onto the Cys-34 residue of human serum albumin (HSA) to determine their effects on the solubilization, permeation, and cytotoxic activity of hydrophobic drugs such as paclitaxel (PTX). PEG(C34)HSA conjugates were prepared on a multigram scale by treating native HSA (n-HSA) with 5- or 20-kDa mPEG-maleimide, resulting in up to 77% conversion of the mono-PEGylated adduct. Nanoparticle tracking analysis of PEG(C34)HSA formulations in phosphate buffer revealed an increase in the number of nanosized aggregates relative to n-HSA, both in the absence and presence of PTX. Cell viability studies conducted with MCF-7 breast cancer cells indicated that PTX cytotoxicity was enhanced by PEG(C34)HSA when mixed at 10:1 mol ratios, up to a 2-fold increase in potency relative to n-HSA. The PEG(C34)HSA conjugates were also evaluated as PTX carriers across monolayers of HUVEC and hCMEC/D3 cells, and found to have permeation profiles nearly identical to those of n-HSA.

Calixarene-Mediated Synthesis of Cobalt Nanoparticles: An Accretion Model for Separate Control over Nucleation and Growth

Co-reporter:Zhenguo Chen, Jie Liu, Andrew J. Evans, Laura Alberch, and Alexander Wei

Chemistry of Materials 2014 Volume 26(Issue 2) pp:941

Publication Date(Web):December 19, 2013

DOI:10.1021/cm402484x

The nucleation and growth of crystalline cobalt nanoparticles (Co NPs) under solvothermal conditions can be separated into distinct stages by using (i) polynuclear clusters with multivalent capping ligands to initiate nucleation, and (ii) thermolabile organometallic complexes with low autonucleation potential to promote crystalline growth. Both nucleation and growth take place within an amorphous accretion, formed in the presence of polyvalent surfactants. At the prenucleation stage, a calixarene complex with multiple Co2–alkyne ligands (Co16–calixarene 1) undergoes thermal decomposition above 130 °C to form “capped cluster” intermediates that coalesce into well-defined Co nanoclusters, but are resistant to further aggregation. At the postnucleation stage, a monomer (pentyne–Co4(CO)10, or PTC) with a low thermal activation threshold but a high barrier to autonucleation is introduced, yielding ε-Co NPs with a linear relationship between particle volume and the Co mole ratio ([Cofinal]/[Coseed]). Co nanocrystals can be produced up to 40 nm with a 10–12% size dispersity within the accretion, but their growth rate depends on the activity of the supporting surfactant, with an octapropargyl calixarene derivative (OP-C11R) providing the most efficient transport of reactive Co species through the amorphous matrix. Postgrowth digestion with oleic acid releases the Co NPs from the residual accretion, which can then self-assemble by magnetic dipolar interactions into flux-closure rings when stabilized by calixarene-based surfactants. These studies demonstrate that organometallic complexes can be designed to establish rational control over the nucleation and growth of crystalline NPs within an intermediate accretion phase.Keywords: cobalt; growth; magnetic properties; nucleation; organometallic;

Glycosyl Dithiocarbamates: β-Selective Couplings without Auxiliary Groups

Co-reporter:Panuwat Padungros, Laura Alberch, and Alexander Wei

The Journal of Organic Chemistry 2014 Volume 79(Issue 6) pp:2611-2624

Publication Date(Web):February 18, 2014

DOI:10.1021/jo500032k

In this article, we evaluate glycosyl dithiocarbamates (DTCs) with unprotected C2 hydroxyls as donors in β-linked oligosaccharide synthesis. We report a mild, one-pot conversion of glycals into β-glycosyl DTCs via DMDO oxidation with subsequent ring opening by DTC salts, which can be generated in situ from secondary amines and CS2. Glycosyl DTCs are readily activated with Cu(I) or Cu(II) triflate at low temperatures and are amenable to reiterative synthesis strategies, as demonstrated by the efficient construction of a tri-β-1,6-linked tetrasaccharide. Glycosyl DTC couplings are highly β-selective despite the absence of a preexisting C2 auxiliary group. We provide evidence that the directing effect is mediated by the C2 hydroxyl itself via the putative formation of a cis-fused bicyclic intermediate.

Synthesis and Reactivity of 4′-Deoxypentenosyl Disaccharides

Co-reporter:Panuwat Padungros, Ren-Hua Fan, Matthew D. Casselman, Gang Cheng, Hari R. Khatri, and Alexander Wei

The Journal of Organic Chemistry 2014 Volume 79(Issue 11) pp:4878-4891

Publication Date(Web):May 5, 2014

DOI:10.1021/jo500449h

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4′-deoxypentenosyl (4′-DP) disaccharides, and we investigate their post-glycosylational C5′ additions using the DMDO oxidation/ring-opening sequence. The α-linked 4′-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4′-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4′-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4′-DP unit is established at a post-glycosylative stage.

Citrate-Stabilized Gold Nanorods

Co-reporter:Jonathan G. Mehtala, Dmitry Y. Zemlyanov, Joann P. Max, Naveen Kadasala, Shou Zhao, and Alexander Wei

Langmuir 2014 Volume 30(Issue 46) pp:13727-13730

Publication Date(Web):2017-2-22

DOI:10.1021/la5029542

Stable aqueous dispersions of citrate-stabilized gold nanorods (cit-GNRs) have been prepared in scalable fashion by surfactant exchange from cetyltrimethylammonium bromide (CTAB)-stabilized GNRs, using polystyrenesulfonate (PSS) as a detergent. The surfactant exchange process was monitored by infrared spectroscopy, surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopy (XPS). The latter established the quantitative displacement of CTAB (by PSS) and of PSS (by citrate). The Cit-GNRs are indefinitely stable at low ionic strength, and are conducive to further ligand exchange without loss of dispersion stability. The reliability of the surface exchange process supports the systematic analysis of ligand structure on the hydrodynamic size of GNRs, as described in a companion paper.

Nanometric Resolution in the Hydrodynamic Size Analysis of Ligand-Stabilized Gold Nanorods

Co-reporter:Jonathan G. Mehtala and Alexander Wei

Langmuir 2014 Volume 30(Issue 46) pp:13737-13743

Publication Date(Web):2017-2-22

DOI:10.1021/la502955h

The stability and hydrodynamic size of ligand-coated gold nanorods (GNRs; aspect ratio 3.6) have been characterized by nanoparticle tracking analysis (NTA)—a single-particle counting method that can measure size distributions with low nanometer resolution. Stable aqueous suspensions of citrate-stabilized GNRs (cit-GNRs) are amenable to surface functionalization without loss of dispersion control. Cit-GNRs can be treated with chemisorptive ligands (thiols and dithiocarbamates), nonionic surfactants (Tween 20), and proteins (human serum albumin), all of which produce stable suspensions at low surfactant concentrations. The precision of NTA (relative standard deviation 10–12%, standard error <2%) is sufficient to allow differences in the hydrodynamic size of coated GNRs to be interpreted in terms of surfactant structure and conformation.

Label-Free Detection of Staphylococcus aureus Captured on Immutable Ligand Arrays

Co-reporter:Avijit K. Adak, J. William Boley, David P. Lyvers, George T. Chiu, Philip S. Low, Ronald Reifenberger, and Alexander Wei

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 13) pp:6404

Publication Date(Web):June 17, 2013

DOI:10.1021/am4016236

The rapid capture and label-free detection of Staphylococcus aureus, an opportunistic bacterium that can infect upon contact, can be performed using periodic microarrays of ligand–protein conjugates created by noncontact (inkjet) printing, darkfield imaging conditions, and a FFT-based readout method. Ink solutes were prepared using bovine serum albumin (BSA) conjugated to a glycan with high affinity for bacterial adhesins and printed as dot-matrix arrays with periodicities of 80–120 μm using a thermal injection method. Upon exposing the glycan–BSA microarrays to live strains of S. aureus, patterns emerge that can be detected under optical darkfield conditions. These patterns can be decoded by fast Fourier transform (FFT) analysis to generate fault-tolerant readout signals that correspond to the capture of S. aureus, with a limit of detection between 102 and 103 cfu/mL. Inkjet printing provides independent control over array periodicity, enabling FFT signals to be assigned to specific frequencies in reciprocal k-space.Keywords: biosensors; Fourier transform; glycans; inkjet printing; microarrays; pathogens;

Differential response of macrophages to core–shell Fe3O4@Au nanoparticles and nanostars

Co-reporter:Wei Xia, Hyon-Min Song, Qingshan Wei and Alexander Wei

Nanoscale 2012 vol. 4(Issue 22) pp:7143-7148

Publication Date(Web):21 Sep 2012

DOI:10.1039/C2NR32070C

Murine RAW 264.7 cells were exposed to spheroidal core–shell Fe3O4@Au nanoparticles (SCS-NPs, ca. 34 nm) or nanostars (NSTs, ca. 100 nm) in the presence of bovine serum albumin, with variable effects observed after macrophagocytosis. Uptake of SCS-NPs caused macrophages to adopt a rounded, amoeboid form, accompanied by an increase in surface detachment. In contrast, the uptake of multibranched NSTs did not induce gross changes in macrophage shape or adhesion, but correlated instead with cell enlargement and signatures of macrophage activation such as TNF-α and ROS. MTT assays indicate a low cytotoxic response to either SCS-NPs or NSTs despite differences in macrophage behavior. These observations show that differences in NP size and shape are sufficient to produce diverse responses in macrophages following uptake.

Glycal Assembly by the in Situ Generation of Glycosyl Dithiocarbamates

Co-reporter:Panuwat Padungros, Laura Alberch, and Alexander Wei

Organic Letters 2012 Volume 14(Issue 13) pp:3380-3383

Publication Date(Web):June 11, 2012

DOI:10.1021/ol301349w

Glycal assembly offers an expedient entry into β-linked oligosaccharides, but epoxyglycal donors can be capricious in their reactivities. Treatment with Et2NH and CS2 enables their in situ conversion into glycosyl dithiocarbamates, which can be activated by copper triflate for coupling with complex or sterically congested acceptors. The coupling efficiency can be further enhanced by in situ benzoylation, as illustrated in an 11-step synthesis of a branched hexasaccharide from glucals in 28% isolated yield and just four chromatographic purifications.

Challenges and opportunities in the advancement of nanomedicines

Co-reporter:Alexander Wei, Jonathan G. Mehtala, Anil K. Patri

Journal of Controlled Release 2012 Volume 164(Issue 2) pp:236-246

Publication Date(Web):10 December 2012

DOI:10.1016/j.jconrel.2012.10.007

Nanomedicine-based approaches to cancer treatment face several challenges that differ from those encountered by conventional medicines during clinical development. A systematic exploration of these issues has led us to identify the following needs and opportunities for further development: (1) robust and general methods for the accurate characterization of nanoparticle size, shape, and composition; (2) scalable approaches for producing nanomedicines with optimized bioavailability and excretion profiles; (3) particle engineering for maintaining low levels of nonspecific cytotoxicity and sufficient stability during storage; (4) optimization of surface chemistries for maximum targeted delivery and minimum nonspecific adsorption; (5) practical methods for quantifying ligand density and distributions on multivalent nanocarriers; and (6) the design of multifunctional nanomedicines for novel combination therapies with supportable levels of bioaccumulation.

Sulfoform generation from an orthogonally protected disaccharide

Co-reporter:Runhui Liu, Oscar Morales-Collazo, Alexander Wei

Carbohydrate Research 2012 Volume 355() pp:19-27

Publication Date(Web):1 July 2012

DOI:10.1016/j.carres.2012.04.010

An orthogonally protected disaccharide (GlcN(α1→4)Glc) with a β-linked 2′-aminoethyl linker was used to generate a series of sulfated derivatives (sulfoforms), with a 6-O-sulfate on the glucose residue and one or more sulfate esters on the terminal glucosamine. Deprotection and sulfonation steps were performed in solution and in variable order, with isolated yields of 36–54% (85–90% per operation) after HPLC purification. The modular deprotection–sulfonation sequences can be performed with efficient recovery of the polysulfate products, and avoids complications associated with heterogeneous reactivity in solid-phase synthesis.Graphical abstractHighlights► Synthesis of an orthogonally protected disaccharide (GlcN(α1→4)Glc) with 2′-aminoethyl linker. ► Generation of sulfoforms with a 6-O-sulfate on glucose and multiple sulfate esters on glucosamine. ► Deprotection and sulfonation steps performed in variable order with 85–90% yield per operation. ► Microwave-assisted sulfonation provides significant improvement to overall yields.

Fabrication of Anisotropic Metal Nanostructures Using Innovations in Template-Assisted Lithography

Co-reporter:Zhao Tang and Alexander Wei

ACS Nano 2012 Volume 6(Issue 2) pp:998

Publication Date(Web):February 10, 2012

DOI:10.1021/nn300375r

Advances in the burgeoning field of plasmonics are increasingly dependent on the ability to fabricate metal nanostructures with precisely defined shapes and orientations, on a scale suitable for technological developments. Recent innovations in top-down lithography have created new windows of opportunity to produce anisotropic metal nanostructures en masse, with near-term applications in photonics, biosensing, and other nanotechnology-enabled pursuits. We focus specifically on C-shaped nanostructures (nanocrescents and split-ring resonators), which can be fabricated by using novel variants of shadow-mask lithography, substrate etching, or microcontact printing.

Photolithography of dithiocarbamate-anchored monolayers and polymers on gold

Co-reporter:Alexei P. Leonov and Alexander Wei

Journal of Materials Chemistry A 2011 vol. 21(Issue 12) pp:4371-4376

Publication Date(Web):14 Feb 2011

DOI:10.1039/C0JM04153J

Dithiocarbamate (DTC)-anchored monolayers and polymers were investigated as positive resists for UV photolithography on planar and roughened Au surfaces. DTCs were formed in situ by the condensation of CS2 with monovalent or polyvalent amines such as linear polyethyleneimine (PEI) under mildly basic aqueous conditions, just prior to surface passivation. The robust adsorption of the polyvalent PEI–DTC to Au surfaces supported high levels of resistance to photoablation, providing opportunities to generate thin films with gradient functionality. Treatment of photopatterned substrates with alkanethiols produced binary coatings, enabling a direct visual comparison of DTC- and thiol-passivated surfaces against chemically induced corrosion using confocal microscopy.

Self-assembly and flux closure studies of magnetic nanoparticle rings

Co-reporter:Alexander Wei, Takeshi Kasama and Rafal E. Dunin-Borkowski

Journal of Materials Chemistry A 2011 vol. 21(Issue 42) pp:16686-16693

Publication Date(Web):06 Sep 2011

DOI:10.1039/C1JM11916H

Thermoremanent magnetic nanoparticles (MNPs) can self-assemble into rings through dipolar interactions, when dispersed under appropriate conditions. Analysis of individual MNP rings and clusters by off-axis electron holography reveals bistable flux closure (FC) states at ambient temperatures, and their reversible switching by magnetic field gradients. We introduce a line-bond formalism to describe the coupling between MNPs.

Metal-Mesh Lithography

Co-reporter:Zhao Tang, Qingshan Wei, and Alexander Wei

ACS Applied Materials & Interfaces 2011 Volume 3(Issue 12) pp:4812

Publication Date(Web):November 21, 2011

DOI:10.1021/am201305x

Metal-mesh lithography (MML) is a practical hybrid of microcontact printing and capillary force lithography that can be applied over millimeter-sized areas with a high level of uniformity. MML can be achieved by blotting various inks onto substrates through thin copper grids, relying on preferential wetting and capillary interactions between template and substrate for pattern replication. The resulting mesh patterns, which are inverted relative to those produced by stenciling or serigraphy, can be reproduced with low micrometer resolution. MML can be combined with other surface chemistry and lift-off methods to create functional microarrays for diverse applications, such as periodic islands of gold nanorods and patterned corrals for fibroblast cell cultures.Keywords: chemisorption; lithography; microarrays; surface chemistry; wetting;

Stereoelectronic Factors in the Stereoselective Epoxidation of Glycals and 4-Deoxypentenosides

Co-reporter:Laura Alberch, Gang Cheng, Seung-Kee Seo, Xuehua Li, Fabien P. Boulineau, and Alexander Wei

The Journal of Organic Chemistry 2011 Volume 76(Issue 8) pp:2532-2547

Publication Date(Web):March 18, 2011

DOI:10.1021/jo102382r

Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a “majority rule”, in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.

Optical Imaging with Dynamic Contrast Agents

Co-reporter:Qingshan Wei ;Alexer Wei

Chemistry - A European Journal 2011 Volume 17( Issue 4) pp:1080-1091

Publication Date(Web):

DOI:10.1002/chem.201002521

Abstract

Biological imaging applications often employ molecular probes or nanoparticles for enhanced contrast. However, resolution and detection are still often limited by the intrinsic heterogeneity of the sample, which can produce high levels of background that obscure the signals of interest. Herein, we describe approaches to overcome this obstacle based on the concept of dynamic contrast: a strategy for elucidating signals by the suppression or removal of background noise. Dynamic contrast mechanisms can greatly reduce the loading requirement of contrast agents, and may be especially useful for single-probe imaging. Dynamic contrast modalities are also platform-independent, and can enhance the performance of sophisticated biomedical imaging systems or simple optical microscopes alike. Dynamic contrast is performed in two stages: 1) a signal modulation scheme to introduce time-dependent changes in amplitude or phase, and 2) a demodulation step for signal recovery. Optical signals can be coupled with magnetic nanoparticles, photoswitchable probes, or plasmon-resonant nanostructures for modulation by magnetomotive, photonic, or photothermal mechanisms, respectively. With respect to image demodulation, many of the strategies developed for signal processing in electronics and communication technologies can also be applied toward the editing of digital images. The image-processing step can be as simple as differential imaging, or may involve multiple reference points for deconvolution by using cross-correlation algorithms. Periodic signals are particularly amenable to image demodulation strategies based on Fourier transform; the contrast of the demodulated signal increases with acquisition time, and modulation frequencies in the kHz range are possible. Dynamic contrast is an emerging topic with considerable room for development, both with respect to molecular or nanoscale probes for signal modulation, and also to methods for more efficient image processing and editing.

Role of Frozen Spins in the Exchange Anisotropy of Core−Shell Fe@Fe3O4 Nanoparticles

Co-reporter:Quy Khac Ong ; Xiao-Min Lin

The Journal of Physical Chemistry C 2011 Volume 115(Issue 6) pp:2665-2672

Publication Date(Web):January 24, 2011

DOI:10.1021/jp110716g

Core−shell Fe@Fe3O4 nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core−shell interface. These spins are frozen into magnetic alignment with field cooling and are depinned in a temperature-dependent manner. The population of such frozen spins has a direct impact on both coercivity (HC) and the exchange-bias field (HE), which are modulated by external physical parameters such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect). Aging of the core−shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of HC and HE, as well as a large increase in the population of frozen spins. These changes are accompanied by a structural evolution from well-defined core−shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the Fe core is remarkably well preserved. The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. The exchange-bias field becomes negligible upon deliberate oxidation of Fe@Fe3O4 nanoparticles into yolk−shell particles, with a nearly complete physical separation of core and shell.

Formation of the ST12 phase in nanocrystalline Ge at ambient pressure

Co-reporter:Suk Jun Kim, Ong Khac Quy, Ling-Shao Chang, Eric A. Stach, Carol A. Handwerker and Alexander Wei

Journal of Materials Chemistry A 2010 vol. 20(Issue 2) pp:331-337

Publication Date(Web):03 Nov 2009

DOI:10.1039/B915841C

Amorphous nanophase germanium can crystallize into a high-energy tetragonal phase (Ge-III/ST12) upon thermal annealing, particularly when prepared by the naphthalide-mediated reduction of GeCl4 and subsequent treatment with t-BuMgCl. The formation of ST12-Ge is typically associated with high pressure, yet the nucleation and coalescence of this phase can be observed by in situ transmission electron microscopy at low pressure and at temperatures well below 200 °C, and can persist at temperatures of up to 500 °C. The low coalescence temperature of nanophase ST12-Ge suggests its compatibility with applications in direct-print electronics.

Bishydrazide Glycoconjugates for Lectin Recognition and Capture of Bacterial Pathogens

Co-reporter:Avijit Kumar Adak, Alexei P. Leonov, Ning Ding, Jyothi Thundimadathil, Sumith Kularatne, Philip S. Low, and Alexander Wei

Bioconjugate Chemistry 2010 Volume 21(Issue 11) pp:2065

Publication Date(Web):October 6, 2010

DOI:10.1021/bc100288c

Bishydrazides are versatile linkers for attaching glycans to substrates for lectin binding and pathogen detection schemes. The α,ω-bishydrazides of carboxymethylated hexa(ethylene glycol) (4) can be conjugated at one end to unprotected oligosaccharides, then attached onto carrier proteins, tethered onto activated carboxyl-terminated surfaces, or functionalized with a photoactive cross-linking agent for lithographic patterning. Glycoconjugates of bishydrazide 4 can also be converted into dithiocarbamates (DTCs) by treatment with CS2 under mild conditions, for attachment onto gold substrates. The immobilized glycans serve as recognition elements for cell-surface lectins and enable the detection and capture of bacterial pathogens such as Pseudomonas aeruginosa by their adsorption onto micropatterned substrates. A detection limit of 103 cfu/mL is demonstrated, using a recently introduced method based on optical pattern recognition.

Plasmon-Resonant Nanoparticles and Nanostars with Magnetic Cores: Synthesis and Magnetomotive Imaging

Co-reporter:Hyon-Min Song, Qingshan Wei, Quy K. Ong, and Alexander Wei

ACS Nano 2010 Volume 4(Issue 9) pp:5163

Publication Date(Web):August 6, 2010

DOI:10.1021/nn101202h

Plasmon-resonant gold nanostars (NSTs) with magnetic cores were synthesized by a multistep sequence from superparamagnetic Fe3O4 nanoparticles (NPs) and evaluated as optical contrast agents under magnetomotive (MM) imaging conditions. Core−shell Fe3O4@Au NPs were prepared in nonpolar organic solvents with nanometer control over shell thickness and with good epitaxy to the Fe3O4 surface. Anisotropic growth was performed in micellar solutions of cetyltrimethylammonium bromide (CTAB) under mildly reducing conditions, resulting in NSTs with physical features similar to those produced from colloidal gold seeds. NSTs could be produced below 100 nm from tip to tip, but seed size had a significant impact on growth habit, with larger seed particles producing submicrometer-sized “morning stars”. Both NSTs and aggregated core−shell NPs are responsive to in-plane magnetic field gradients and can provide enhanced near-infrared (NIR) contrast under MM conditions, but do so by different mechanisms. NSTs can modulate polarized NIR scattering with minimal translational motion, giving the appearance of a periodic but stationary “blinking”, whereas core−shell NP aggregates require lateral displacement for signal modulation. The polarization-sensitive MM imaging modality offers the dual advantage of enhanced signal quality and reduced background signal and can be applied toward the detection of magnetomotive NSTs in heterogeneous biological samples, as illustrated by their detection inside of granular cells such as macrophages.Keywords: imaging; magnetic nanoparticles; nanomedicine; nanostars; plasmonics; synthesis

Gyromagnetic Imaging: Dynamic Optical Contrast Using Gold Nanostars with Magnetic Cores

Co-reporter:Qingshan Wei ; Hyon-Min Song ; Alexei P. Leonov ; Jacob A. Hale ; Dongmyung Oh ; Quy K. Ong ; Kenneth Ritchie

Journal of the American Chemical Society 2009 Volume 131(Issue 28) pp:9728-9734

Publication Date(Web):May 12, 2009

DOI:10.1021/ja901562j

Plasmon-resonant nanoparticles with optical scattering in the near-infrared (NIR) are valuable contrast agents for biophotonic imaging and may be detected at the single-particle limit against a dark background, but their contrast is often limited in environments with high noise. Here we consider gyromagnetic imaging as a dynamic mode of optical contrast, using gold nanostars with superparamagnetic cores. The nanostars exhibit polarization-sensitive NIR scattering and can produce a frequency-modulated signal in response to a rotating magnetic field gradient. This periodic “twinkling” can be converted into Fourier-domain images with a dramatic reduction in background. We demonstrate gyromagnetic imaging of nanostars inside of tumor cells, using broadband excitation: while their time-domain signals are obscured by incoherent scattering, their Fourier-domain signals can be clearly resolved in less than a second. The gyromagnetically active nanostars do not cause a loss in viability, and can even have a mild stimulatory effect on cell growth.

Imaging gold nanorods in excised human breast carcinoma by spectroscopic optical coherence tomography

Co-reporter:Amy L. Oldenburg, Matthew N. Hansen, Tyler S. Ralston, Alexander Wei and Stephen A. Boppart

Journal of Materials Chemistry A 2009 vol. 19(Issue 35) pp:6407-6411

Publication Date(Web):20 Apr 2009

DOI:10.1039/B823389F

Plasmon-resonant gold nanorods (GNRs) can serve as imaging agents for spectroscopic optical coherence tomography (SOCT). The aspect ratio of the GNRs is adjusted for maximum absorption in the far red to create a partial spectral overlap with the short-wavelength edge of the near-infrared SOCT imaging band. The spectroscopic absorption profile of the GNRs is incorporated into a depth-resolved algorithm for mapping the relative GNR density within OCT images. This technique enables us to image GNR distributions in excised human breast carcinomas, demonstrating their potential as OCT contrast agents in heterogeneous, highly scattering tissues.

Prenucleation and coalescence of cobalt nanoclusters mediated by multivalent calixarene complexes

Co-reporter:Jie Liu and Alexander Wei

Chemical Communications 2009 (Issue 28) pp:4254-4256

Publication Date(Web):09 Jun 2009

DOI:10.1039/B903954F

The formation of cobalt nanoclusters from polynuclear Co–alkyne species can be directed by multivalent calixarene ligands; thermochemical studies of Con–calixarenes reveal the influence of multivalency in prenucleation and postnuclear growth processes.

Dithiocarbamate-Coated SERS Substrates: Sensitivity Gain by Partial Surface Passivation†

Co-reporter:Yan Zhao, James N. Newton, Jie Liu and Alexander Wei

Langmuir 2009 Volume 25(Issue 24) pp:13833-13839

Publication Date(Web):August 17, 2009

DOI:10.1021/la902087e

The surface-enhanced Raman scattering (SERS) activity of nanoporous gold (NPG) can be boosted by controlled surface passivation. The SERS activities of unfunctionalized NPG were first optimized by etching substrates with NaI/I2 (triiodide) and using 2-mercaptopyridine (2-MP) as the probing analyte. Gains in analyte sensitivity were then achieved by passivating the superficial regions of the NPG substrates with dimethyldithiocarbamate (Me2DTC) while leaving the more recessed “hot spots” available for SERS detection. Partial surface passivation with DTCs increased the substrate sensitivity to chemisorptive analytes such as 2-MP by an order of magnitude, whereas surface saturation lowered the sensitivity by an order of magnitude. The partially passivated NPG films can also be functionalized with supramolecular receptors for chemoselective SERS. Installation of a DTC-anchored terpyridine enabled the detection of divalent metal ions at trace levels, as determined by the complexation-induced shift of a characteristic Raman peak of the metal ion receptor.

Reversal of Flux Closure States in Cobalt Nanoparticle Rings With Coaxial Magnetic Pulses

Co-reporter:Takeshi Kasama;Rafal E. Dunin-Borkowski;Michael R. Scheinfein;Steven L. Tripp;Jie Liu;Alexer Wei

Advanced Materials 2008 Volume 20( Issue 22) pp:4248-4252

Publication Date(Web):

DOI:10.1002/adma.200702941

Probing osmotic effects on invertase with L-(−)-sucrose

Co-reporter:Seung-kee Seo and Alexander Wei

Organic & Biomolecular Chemistry 2008 vol. 6(Issue 18) pp:3362-3365

Publication Date(Web):25 Jul 2008

DOI:10.1039/B810158B

L-(−)-Sucrose was efficiently synthesized using intramolecular aglycon delivery and used to elucidate osmotic effects on the activity of invertase, which catalyzes the hydrolysis of D-(+)-sucrose. The osmotic effect imposed by L-sucrose was responsible for more than 30% of the activity loss ascribed otherwise to “substrate inhibition.”

Assembly of Dithiocarbamate-Anchored Monolayers on Gold Surfaces in Aqueous Solutions

Co-reporter:Heng Zhu, Donna M. Coleman, Christopher J. Dehen, Iris M. Geisler, Dmitry Zemlyanov, Jean Chmielewski, Garth J. Simpson and Alexander Wei

Langmuir 2008 Volume 24(Issue 16) pp:8660-8666

Publication Date(Web):July 11, 2008

DOI:10.1021/la801254b

Dithiocarbamates (DTCs) can be formed by the in situ condensation of polar alkylamines with CS2, and assembled into dithiocarbamate-anchored monolayers (DAMs) on Au substrates in aqueous solutions. Primary and secondary amines can both be used to prepare DTCs, but have significant differences in their reactivities and product stabilities. Ultraviolet absorption spectroscopy provides a convenient method for monitoring in situ DTC formation as well as the formation of potential byproducts. The kinetics of DAM assembly on Au substrates as measured by second harmonic generation (SHG) indicated first-order rate processes and saturation coverages similar to those of alkanethiols on Au. However, the rate of adsorption did not change with DTC concentration in a manner expected of Langmuir kinetics, and is attributed to the competitive adsorption of alkylammonium counterions to the freshly oxidized Au substrate. These analyses establish a practical range of conditions for preparing DAMs from polar amines using in situ DTC formation.

Detoxification of Gold Nanorods by Treatment with Polystyrenesulfonate

Co-reporter:Alexei P. Leonov, Jiwen Zheng, Jeffrey D. Clogston, Stephan T. Stern, Anil K. Patri and Alexander Wei

ACS Nano 2008 Volume 2(Issue 12) pp:2481

Publication Date(Web):December 1, 2008

DOI:10.1021/nn800466c

We address an outstanding issue associated with the biocompatibility of gold nanorods (GNRs), a promising agent for biomedical imaging and theragnostics. GNRs are typically prepared in the presence of cetyltrimethylammonium bromide (CTAB), a cationic surfactant whose rigorous removal is necessary due to its cytotoxicity and membrane-compromising properties. CTAB-stabilized GNRs can be partially purified by treatment with polystyrenesulfonate (PSS), an anionic polyelectrolyte often used as a surrogate peptizing agent, followed by chloroform extraction and ultrafiltration with minimal loss of dispersion stability. However, in vitro cytotoxicity assays of PSS-coated GNRs revealed IC50 values in the low to submicromolar range, with subsequent studies indicating the source of toxicity to be associated with a persistent PSS−CTAB complex. Further exchange of CTAB-laden PSS with fresh polyelectrolyte greatly improves biocompatibility, to the extent that 85 μg/mL of “CTAB-free” GNRs (the highest level evaluated) has comparable toxicity to a standard phosphate buffer solution. Ironically, PSS is not effective by itself at stabilizing GNRs in CTAB-depleted suspensions: while useful as a detergent for GNR detoxification, it should be replaced by more robust coatings for long-term stability under physiological conditions.Keywords: dispersion stability; nanobiotechnology; nanomedicine; nanorods; toxicity

Gold Nanorod Arrays as Plasmonic Cavity Resonators

Co-reporter:David P. Lyvers, Jeong-Mi Moon, Alexander V. Kildishev, Vladimir M. Shalaev and Alexander Wei

ACS Nano 2008 Volume 2(Issue 12) pp:2569

Publication Date(Web):December 9, 2008

DOI:10.1021/nn8006477

Hexagonal 2D arrays of Au nanorods support discrete plasmon resonance modes at visible and near-infrared wavelengths when coupled with light at normal incidence (kz). Reflectance spectra of nanorod arrays mounted on a thin Au baseplate reveal multiple resonant attenuations whose spectral positions vary with nanorod height and the dielectric medium. Simulations using 3D finite-element method calculations reveal harmonic sets of longitudinal standing waves in cavities between nanorods, reminiscent of acoustic waves generated by musical instruments. The nodes and antinodes of these quarter-wave plasmon modes are bounded, respectively, at the base and tips of the array. The number of harmonic resonances and their frequencies can be adjusted as a function of nanorod height, diameter-spacing ratio, and the refractive index of the host medium. Dispersion relations based on these standing-wave modes show strong retardation effects, attributed to the coupling of nanorods via transverse modes. Removal of the metal baseplate is predicted to result in resonant transmission through the Au nanorod arrays, at frequencies defined by half-wave modes within the open-ended cavities.Keywords: arrays; dispersion relation; nanophotonics; nanorods; plasmonics

Two-photon luminescence imaging of Bacillus spores using peptide-functionalized gold nanorods

Co-reporter:Wei He;Walter A. Henne;Qingshan Wei;Yan Zhao;Derek D. Doorneweerd

Nano Research 2008 Volume 1( Issue 6) pp:450-456

Publication Date(Web):2008 December

DOI:10.1007/s12274-008-8047-y

Bacillus subtilis spores (a simulant of Bacillus anthracis) have been imaged by two-photon luminescence (TPL) microscopy, using gold nanorods (GNRs) functionalized with a cysteine-terminated homing peptide. Control experiments using a peptide with a scrambled amino acid sequence confirmed that the GNR targeting was highly selective for the spore surfaces. The high sensitivity of TPL combined with the high affinity of the peptide labels enables spores to be detected with high fidelity using GNRs at femtomolar concentrations. It was also determined that GNRs are capable of significant TPL output even when irradiated at near infrared (NIR) wavelengths far from their longitudinal plasmon resonance (LPR), permitting considerable flexibility in the choice of GNR aspect ratio or excitation wavelength for TPL imaging.

Gold Nanorods Mediate Tumor Cell Death by Compromising Membrane Integrity

Co-reporter:L. Tong;Y. Zhao;M. N. Hansen;T. B. Huff;J.-X. Cheng;A. Wei

Advanced Materials 2007 Volume 19(Issue 20) pp:3136-3141

Publication Date(Web):17 OCT 2007

DOI:10.1002/adma.200701974

Folate-conjugated gold nanorods targeted to tumor cell surfaces produced severe membrane damage upon near-infrared irradiation. Photoinduced injury to the plasma membrane resulted in a rapid increase in intracellular calcium (shown in green) with subsequent disruption of the actin network, featured prominently by the formation of membrane blebs.

Calixarene-encapsulated nanoparticles: self-assembly into functional nanomaterials

Co-reporter:Alexander Wei

Chemical Communications 2006 (Issue 15) pp:1581-1591

Publication Date(Web):25 Jan 2006

DOI:10.1039/B515806K

Calixarenes are excellent surfactants for enhancing the dispersion and self-assembly of metal nanoparticles into well-defined structures, particularly those with unit length scales in the 10–100 nm size range. Particles within these ensembles are strongly coupled, giving rise to unique collective optical or magnetic properties. The self-assembled nanostructures described in this feature article include 2D arrays of colloidal Au nanoparticles with size-dependent plasmonic responses, and sub-100 nm Co nanoparticle rings with chiral magnetic states. These nanoparticle assemblies may be further developed for applications in chemical sensing based on surface-enhanced Raman scattering (SERS) and as binary elements for nonvolatile memory, respectively.

In vitro and in vivo two-photon luminescence imaging of single gold nanorods

Co-reporter:Haifeng Wang;Terry B. Huff;Daniel A. Zweifel;Wei He;Philip S. Low;Ji-Xin Cheng;

Proceedings of the National Academy of Sciences 2005 102(44) pp:15752-15756

Publication Date(Web):October 20, 2005

DOI:10.1073/pnas.0504892102

Gold nanorods excited at 830 nm on a far-field laser-scanning microscope produced strong two-photon luminescence (TPL) intensities, with a cos⁴ dependence on the incident polarization. The TPL excitation spectrum can be superimposed onto the longitudinal plasmon band, indicating a plasmon-enhanced two-photon absorption cross section. The TPL signal from a single nanorod is 58 times that of the two-photon fluorescence signal from a single rhodamine molecule. The application of gold nanorods as TPL imaging agents is demonstrated by in vivo imaging of single nanorods flowing in mouse ear blood vessels.

Nanoprobe implantation into mammalian cells by cationic transfection

Co-reporter:Yan Zhao, Bryce Sadtler, Min Lin, Gregory H. Hockerman and Alexander Wei

Chemical Communications 2004 (Issue 7) pp:784-785

Publication Date(Web):04 Mar 2004

DOI:10.1039/B317061F

Submicron-sized Au particles and Au/SiO2 nanocomposites (superparticles) as large as 670 nm have been introduced into tsA201 cells with minimal cell trauma by cationic transfection systems. Successful implantations can be characterized by the expression of co-transfected DNA.

Flux Closure in Self-Assembled Cobalt Nanoparticle Rings

Co-reporter:Steven L. Tripp Dr.;Rafal E. Dunin-Borkowski Dr.;Alexer Wei

Angewandte Chemie International Edition 2003 Volume 42(Issue 45) pp:

Publication Date(Web):3 NOV 2003

DOI:10.1002/anie.200352825

Closing the circle: Chiral magnetic domains are observed when ferromagnetic Co nanoparticles self-assemble into bracelet-like rings (see picture). The magnetic dipoles of the particles collectively contribute toward flux closure, a bistable state with potential utility for nonvolatile data storage.

Spherical ensembles of gold nanoparticles on silica: electrostatic and size effects

Co-reporter:Bryce Sadtler and Alexander Wei

Chemical Communications 2002 (Issue 15) pp:1604-1605

Publication Date(Web):25 Jun 2002

DOI:10.1039/B204760H

Core–shell ensembles of citrate-stabilized gold nanoparticles (20–80 nm) on submicron silica cores (330–550 nm) have been prepared by electrostatic self-assembly with shell packing densities as high as ϕ = 0.55.

Prenucleation and coalescence of cobalt nanoclusters mediated by multivalent calixarene complexes

Co-reporter:Jie Liu and Alexander Wei

Chemical Communications 2009(Issue 28) pp:NaN4256-4256

Publication Date(Web):2009/06/09