Self-assembly of metal nanoparticles has attracted considerable attention because of its unique applications in technologies such as plasmonics, surface-enhanced optics, sensors, and catalysts. However, fabrication of ordered nanoparticle structures remains a significant challenge. Thus, developing an efficient approach for the assembly of large-scale Au nanoparticles films for theoretical studies and for various applications is highly desired. In this paper, a facial approach for fabricating a monolayer film of Au nanoparticles was developed successfully. Using the surfactant polyvinylpyrrolidone (PVP), a large-scale monolayer film of well-ordered, uniform-sized Au nanoparticles was fabricated at the air/water interface. The film exhibited a two-dimensional (2D) hexagonal close-packed (HCP) structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform “hot spots” for surface-enhanced Raman scattering (SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The low spot-to-spot and substrate-to-substrate variations of intensity (<10%), the large surface enhancement factor (∼106), and the high stability (∼45 days) make the substrate suitable for SERS measurements. Transfer of the monolayer film onto a glassy carbon electrode produced an Au electrode with clean, well-defined nanostructure suitable for electrochemical SERS measurements. The adsorption process of ionic liquids on the electrode with the monolayer film is similar to that on bulk metal electrodes. The present strategy provides an effective way for self-assembly of Au nanoparticles into well-defined nanostructures that may form optimal reproducible SERS substrates for quantitative analysis. It also provides an electrode with clean, well-defined nanostructure for electrochemical investigations.

•Silica encapsulated Au with mercaptobenzoic acid core-shell nanoparticles (Au-MBA@SiO2) was developed for the SERS-based immunoassay.•The silica shell was employed to protect SERS labels, and to block the aggregation of Au nanoparticles and the nonspecific bingding.•MBA molecules played dual roles as SERS labels and to facilitate the formation of silica shell•The detection sensitivity was improved by about 1– 2 orders of magnitude by comparing to the traditional approach based on naked Au-MBA nanoparticles.•This kind of labels embedded core-shell nanoparticles could be developed as the versatile nanotags for the bioanalysis and bioimaging.Traditional “sandwich” structure immunoassay is mainly based on the self-assembly of “antibody on solid substrate-antigen-antibody with nanotags” architectures, and the sensitivity of this strategy is critically depended on the surface enhanced Raman scattering (SERS) activities and stability of nanotags. Therefore, the rational design and fabrication on the SERS nanotags attracts the common interests to the bio-related detecting and imaging. Herein, silica encapsulated Au with mercaptobenzoic acid (MBA) core-shell nanoparticles (Au-MBA@SiO2) are fabricated instead of the traditional naked Au or Ag nanoparticles for the SERS-based immunoassay on human and mouse IgG antigens. The MBA molecules facilitate the formation of continuous pinhole-free silica shell and are also used as SERS labels. The silica shell is employed to protect MBA labels and to isolate Au core from the ambient solution for blocking the aggregation. This shell also played the similar role to BSA in inhibiting the nonspecific bindings, which allowed the procedures for constructing “sandwich” structures to be simplified. All of these merits of the Au-MBA@SiO2 brought the high performance in the related immunoassay. Benefiting from the introduction of silica shell to encapsulate MBA labels, the detection sensitivity was improved by about 1– 2 orders of magnitude by comparing with the traditional approach based on naked Au-MBA nanoparticles. This kind of label-embedded core-shell nanoparticles could be developed as the versatile nanotags for the bioanalysis and bioimaging.

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays. Relationships among SERS effect, SPR catalysis, and gap distance were experimentally and theoretically studied.

•SHINERS technology is extended to Au(111) surface in nonaqueous solutions.•SPR-induced “hot electrons” catalyze hydrogenation reaction on a roughened Au surface.•Shell-isolated nanoparticles isolate “hot electrons” from Au nanoparticles.•Aprotic acetonitrile has no hydrogen source for catalytic hydrogenation.Surface-enhanced Raman spectroscopy (SERS) studies of electrode/solution interfaces are important for understanding electrochemical processes. However, revealing the nature of reactions at well-defined single crystal electrode surfaces, which are SERS-inactive, remains challenging. In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used for the first time to study electrochemical adsorption and hydrogenation reactions at single crystal surfaces in nonaqueous solvents. A roughened Au surface was also studied for comparison. The experimental results show that the hydrogenation of adsorbed p-ethynylaniline (PEAN) on roughened Au electrode surfaces occurred at very negative potentials in methanol because of the catalytic effect of surface plasmon resonance (SPR). However, because “hot electrons” were blocked by the silica shell of Au@SiO2 nanoparticles and aprotic acetonitrile was an ineffective hydrogen source, surface reactions at Au(111) were inhibited in the systems studied. Density functional theory (DFT) calculations revealed that the PEAN triple bond opened, allowing adsorption in a flat configuration on the Au(111) surface via two carbon atoms. This work provides an advanced understanding of electrochemical interfacial processes at single crystal surfaces in nonaqueous systems.Download high-res image (358KB)Download full-size image

The elimination of β-agonist has attracted considerable interest due to its harmfulness to human health when it existed in pork. Here, a strategy based on immuno-magnetic nanoparticles has been successfully developed for the selective and successive magnetic separation of two kinds of β-agonists, clenbuterol (CL) and salbutamol (SAL). The calibration curve of competitive immunoassay was determined for the estimation of the final concentration of targets after the separation, in which the limit of detection (LOD) and half maximal inhibitory concentration (IC50) were about 17 fg mL−1 and 193 pg mL−1, respectively. The specific interaction between the target and the complementary antibody attached to Fe3O4@Au nanoparticles resulted in the aggregation of Fe3O4@Au nanoparticles carried with targets. The magnetic collection of the aggregation of Fe3O4@Au nanoparticles decreased the concentration of targets significantly. The results revealed that the final concentration of remaining targets was lower than the LOD. This strategy was employed to separate CL and SAL molecules in mixed solutions simultaneously or successively with high efficiency. The results demonstrate that it provides a selective and effective approach for the removal of harmful residues in practical samples.

The assembly of simple nanostructures has already attracted significant interest in the field of plasmonic devices and other relevant areas. From the viewpoint of theoretical simulation and practical application, the precise control of nanoparticles still remains a significant challenge. Herein, a strategy was successfully developed to fabricate in situ a polymer-encapsulated Au nanoparticle dimer based on the C–C coupling reaction of p-aminophenylacetylene (p-APAC). The balance between the polymerization processes and the coupling reaction resulted in Au nanoparticle assemblies with different configurations, such as monomer, dimer, and multimer, depending on the concentration of p-APAC. The gap distance of about 1.8 nm was well consistent with the length of the coupling products of p-APAC, i.e. the gap distance was about double the length of a single p-APAC molecule. The observation of a longitudinal peak in the UV-vis spectrum demonstrated that the aspect ratio of the Au nanoparticles was about 2.5, indicating the formation of Au dimers with reasonable yield. Moreover, the thickness of the polymer shell was well-controlled via changing the concentration of p-APAC. The gap of the dimer resulted in a very large coupling effect of the localized surface plasmon resonance (LSPR), and the surface enhanced Raman spectroscopy (SERS) signal of the molecules was accordingly enhanced in the gap areas, which served as the hot spots. Based on the characteristic spectral feature of the coupling products, the single Au nanoparticle dimer was positioned via SERS mapping. The large LSPR coupling effect in the gap area allowed the conversion of p-nitrothiophenol (PNTP) to dimercaptoazobenzene (DMAB) with high efficiency. Thus, it was confirmed that the SPR-catalyzed coupling reaction preferentially occurred on a hot spot area. The proposed approach is expected to be developed into a promising tool for precisely controlling the gap distance of a nanoparticle assembly, and it may serve as a simple model for theoretical consideration in understanding the SERS mechanism(s).

Plasmonic hot spots, capable of confining strong electromagnetic fields near metallic surfaces, are particularly essential to a variety of enhanced spectroscopic techniques. Understanding the electric field distributions in the hot spot plays a crucial role in controlling the fabrication of plasmonic nanostructures for a variety of plasmon-based applications. The investigation of plasmonic hot spots in metallic nanosystems has not been fully evaluated. Here, we develop a facile approach by experimental means for investigating the distribution of plasmonic hot spots in surface-enhanced Raman spectroscopy (SERS) based on the dual-probe strategy by coupling a p-mercaptobenzoic acid-embedded Au@SiO2 ((Au-pMBA)@SiO2) nanoparticle monolayer film with thiophenol-modified gold single crystal plates (TP-GSCPs). We demonstrated, for the first time, the heterogeneous distribution of SERS effect in the gap between Au@SiO2 monolayer film and GSCPs. As increasing the gap distance by changing the thickness of silica spacer, the SERS effect of the probe on the gold nanoparticles decayed with a slower rate than that of the other probe attached onto the GSCPs. It mainly originated from the difference of localized dielectric environments and curvatures. By deliberately controlling the silica shell thickness, the switchable plasmonic coupling effect can be achieved between “particle–particle” gap mode and “particle-surface” gap mode. The results reveal that the transfer effect is more evident for (Au-pMBA)@SiO2 films with thinner silica shell thickness and for 785 nm illumination as excitation wavelength than 633 nm. Moreover, the introduction of NaOH solution to (Au-pMBA)@SiO2 films leads to the transfer of the hot spots to the areas between neighboring nanoparticles again due to the removal and dissolution of silica shells. The understanding gained from our experimental observation provides keen insight into the plasmonic hot spots in coupled nanostructures, offering guidance for rational design of plasmonic substrates for ultrasensitive SERS detections.

The performance of a surface enhanced Raman spectroscopy (SERS) based magnetic immunoassay is critically dependent upon the properties of the magnetic nanoparticles, in which the plasmon enhanced optics and magnetism are integrated together. Tuning SERS activity and magnetism together still remains a significant challenge. Herein, a facile approach for the fabrication of Ni@Au magnetic nanoparticles was developed as the immune substrate for a competitive magnetic immunoassay. Immune Ni@Au nanoparticles and 4-mercaptobenzoic acid (MBA)-labelled immune Au nanoparticles (immune Au-MBA) were employed for detection of aflatoxin B1 (AFB1) through a SERS based competitive magnetic immunoassay. In the immune system, competitive binding with immune Au nanoparticles appeared between the free AFB1 and coating antigen modified Ni@Au nanoparticles; the concentration of AFB1 was determined by comparing the extent of the decrease in the SERS intensity of MBA labels. Based on the relationship between SERS intensity and AFB1 concentrations, the inhibitory concentration 50 (IC50) was determined to be about 27.1 fg mL−1 (around 0.03 ppt) with a reasonable correlation coefficient of 0.997 and the limit of detection was about 0.05 fg mL−1. The observation of unobvious cross-reactions suggested the high specificity of this strategy. By comparing to the traditional determination techniques, the present approach based on Ni@Au nanoparticles exhibited the highest sensitivity. In the spiking experiments, the recoveries ranged from 87.4% to 111.7% after the addition of standard AFB1 at different concentrations in fresh maize samples. The results were also verified by the commonly accepted LC-MS technique. It was revealed that the competitive magnetic immunoassay exhibited the distinct advantage of high sensitivity. The proposed approach is expected to be developed into a promising tool for quasi-quantitative detection of the trace residues of AFB1 in food.

Much attention has been paid to surface enhanced Raman spectroscopy (SERS) in recent years due to its capability to detect trace amounts of chemical and biological analytes with single molecule sensitivity and its rapid response. Unfortunately, sensitive SERS detection is often limited to molecules that can strongly adsorb onto the surface of plasmonic substrates. The detection of molecules without specific adsorption groups still remains a great challenge, especially for vapor molecules in a flowing system. Herein, we present a novel PDMS/C3H7S-assisted SERS amplification process for on-line sensitive detection of aromatic vapors. Specifically, the SERS amplification process relies on a PDMS-coated 1-propanethiol-modified Au@Ag nanoparticle monolayer film (PDMS/C3H7S–Au@Ag MLF) as the SERS substrate, in which the PDMS and 1-propanethiol layers are capable of capturing vapor molecules from flowing air and placing them in close proximity in plasmonic hot spots. By taking advantage of the high adsorption capability of PDMS and strong plasmonic properties of the nanoparticle film, our PDMS/C3H7S-assisted SERS amplification can result in about two orders of magnitude enhancement of the SERS signal in comparison with that of the naked Au@Ag MLF. We also demonstrate that our developed plasmonic nanoparticles film could be applicable for on-line SERS detection of a series of aromatic vapors in the flowing system. These results provide exciting opportunities for in situ monitoring of volatile organic pollutants in the atmospheric environment.

•Fe3O4@Au nanoparticles monolayer film was fabricated at hexane/water interface.•The interparticle spacing was dynamically tuned by external magnetic field.•The response of SERS effect to the magnetic field was completely reversible.•The magnetism responsive film was used as substrate with tunable optical properties.A large surface-enhanced Raman scattering (SERS) effect is critically dependent on the gap distance of adjacent nanostructures, i.e., “hot spots”. However, the fabrication of dynamically controllable hot spots still remains a remarkable challenge. In the present study, we employed an external magnetic field to dynamically control the interparticle spacing of a two-dimensional monolayer film of Fe3O4@Au nanoparticles at a hexane/water interface. SERS measurements were performed to monitor the expansion and shrinkage of the nanoparticles gaps, which produced an obvious effect on SERS activities. The balance between the electrostatic repulsive force, surface tension, and magnetic attractive force allowed observation of the magnetic-field-responsive SERS effect. Upon introduction of an external magnetic field, a very weak SERS signal appeared initially, indicating weak enhancement due to a monolayer film with large interparticle spacing. The SERS intensity reached maximum after 5 s and thereafter remained almost unchanged. The results indicated that the observed variations in SERS intensities were fully reversible after removal of the external magnetic field. The reduction of interparticle spacing in response to a magnetic field resulted in about one order of magnitude of SERS enhancement. The combined use of the monolayer film and external magnetic field could be developed as a strategy to construct hot spots both for practical application of SERS and theoretical simulation of enhancement mechanisms.

•The vibrational Stark effect of νCN was determined based on molecule ruler strategy.•EC-SERS was developed to distinguish the orientation and measure potential dependent Stark effect.•The interfacial electric field completely decayed before reaching the nitrile group location of longer probe.•The potential drop occurred over a range of 5–6 Å away from electrode surface in RTILs.Room temperature ionic liquids (RTILs) played an important role in the electrochemistry because of the advantageous properties. Due to the different electrochemical behavior from the dilute solution, it was still remained a significant challenge to obtain the insight into the interfacial structure. Although the spectroelectrochemistry offered the rich spectroscopic information for obtaining the interfacial structure at molecular level, the lack of the spatial information of those techniques at nano/subnanometer scale led the difficulties in the determination of the spatial structure of double layer. Here, the surface enhanced Raman spectroscopy (SERS) Stark tuning rates of CN stretching frequency were employed to determine the electric double layer structure on Au electrode, and probes with different lengths, involving cyanide ion (CN−), thiocyanate (SCN−), 3-aminopropanenitrile (3-APN) and 6-aminohexanenitrile (6-AHN), were used as rulers to measure the spatial structure of double layer. In this approach, SERS combined with electrochemical control was performed to distinguish the orientation of surface species and detect the vibrational frequency of the nitrile stretching mode. For shorter probes (such as CN− and SCN−), the nitrile group were located in the electric double layer region, resulting in measureable Stark tuning rates (dνCN/dE), while for longer probe molecules (such as 3-APN and 6-AHN), the interfacial potential completely decayed before reaching the nitrile group location, resulting in the negligible Stark tuning rates. The vibrational Stark tuning rates suggested that the potential drop occurred over a range of 5–6 Å from the metal surface into the bulk phase of ionic liquids. The results demonstrated that the overall potential mostly dropped across the first layer and only a small fraction across the second layer. The preliminary results allowed us to obtain an insight into the physical picture of the interfacial structure of RTILs/metal system.

Surface enhanced Raman spectroscopy (SERS) has been considered as a promising tool for detecting targets with single molecule sensitivity. However, the SERS detection on targets without a specific adsorption group has still remained a significant challenge. In this paper, we reported a facile strategy to fabricate a PDMS film-coated Au nanoparticle monolayer film (Au MLF) composite substrate for improving SERS detection of aromatic molecules in water and in the atmosphere. Toluene, benzene and nitrobenzene were used as the targets to evaluate the performance of the composite substrate. The results indicated that the PDMS film played the vital role to capture and preconcentrate these targets for improving the capability in SERS detection of these targets. The performance was critically dependent on the hydrophobicity, functional groups and the permeability of the targets. This composite substrate was more favorable for the detection of toluene and nitrobenzene than benzene. The limit of detection (LOD) for toluene and nitrobenzene was decreased by about two orders of magnitude on the PDMS-Au MLF compared to that on the naked Au MLF, and by one order of magnitude for benzene. It was estimated to be 0.5 ppm, 0.6 ppm and 78 ppm for toluene, nitrobenzene and benzene, respectively. The results demonstrated that this approach could be developed as a promising tool to detect numerous targets which were non-specifically adsorbed onto metallic nanostructures. It opened a window towards the general application of SERS for in situ monitoring of pollutants in water and in the atmosphere.

Rapid separation and detection of analytes have been the focus of a growing body of investigation for potential applications including food safety and environment science. However, the development of a robust analytical technique for simultaneous rapid separation and on-line detection remains a formidable challenge. Herein, we report a rational design based on the combination of high performance liquid chromatography (HPLC) and surface-enhanced Raman spectroscopy (SERS) for the rapid separation and on-line detection of multi-analytes. In particular, a plasmonic nanoparticle-modified capillary (NPMC) is fabricated through a self-assembly process and connected to a HPLC effluent-end port. After separation by HPLC, the analytes are adsorbed onto plasmonic nanoparticles in the capillary and then detected by SERS. The resulting HPLC-SERS coupled detection system can simultaneously achieve rapid separation and provide on-line molecular structural information of multi-analytes. In addition, we also demonstrate the on-line detection of a pesticide molecule (thiram) in an orange using this combined system. Importantly, the detection limit can be down to 10−7 mol L−1. These findings indicate that our coupled HPLC-SERS system offers a promising analytical technique in modern analytical science and technology.

Human epididymis protein 4 (HE4), as a serological marker, has been proposed to be the most promising tumor marker in ovarian cancer diagnosis. An approach based on surface enhanced Raman spectroscopy (SERS) and a magnetic immunoassay technique was developed successfully for rapid detection and separation of HE4 with high sensitivity and selectivity. The detection was involved in the construction of a unique sandwich structure using a bottom-up method, which consisted of HE4 antibody and SERS reporter coated Au nanoparticles (A) and target HE4 antigen and HE4 antibody-modified magnetic core–shell Fe3O4@Au nanoparticles (B). The sandwich structure was effectively enriched by using a magnet for SERS detection. This approach exhibited an extremely high specificity in the detection of HE4 due to the strong specific interaction between the antibody and the corresponding antigen. The results revealed that the limit of detection (LOD) of the present approach was as low as 100 fg mL−1 and demonstrated a linear relationship between SERS intensities and lgc in a concentration range of 1 pg mL−1 to 10 ng mL−1. Accompanied by the magnetic enrichment procedure after the assembling of the sandwich structure, almost all of the HE4 protein was removed. The immuno Fe3O4@Au nanoparticles were regenerated by releasing the HE4 from the sandwich structure into the acidified methanol solution, and it could be used for magnetic enrichment and SERS detection for at least five times. Moreover, two kinds of immuno nanoparticles (A and B) could be developed as reagent kits in the clinical diagnosis of ovarian cancer.

A femtogram level and specific surface enhanced Raman spectroscopy (SERS) based competitive immunoassay was developed to detect Hg(II) in aqueous solution for the first time. This novel approach provides an alternative, ultrasensitive and specific analytical method for the detection of Hg(II).

Surface plasmon plays an important role in surface catalysis reactions, and thus the tuning of plasmon on metal nanostructures and the extension of plasmon induced surface catalysis reactions have become important issues. Au nanoparticle monolayer film was fabricated by the assembling of Au nanoparticles at the liquid–air interface with numerous “hot spots” for strong surface plasmon coupling. A facile approach was developed to achieve the decarboxylation reaction driven by appropriate surface plasmon on the Au nanoparticle monolayer film surface, and surface enhanced Raman spectroscopy (SERS) has been developed as a sensitive tool for the in situ monitoring of the plasmon induced surface reaction. The effects of the power and wavelength of the laser and solution pH on the decarboxylation reaction were investigated. With laser illumination, para-mercaptobenzoic acid (PMBA) was transformed to thiophenol (TP), and the decarboxylation was enhanced on increasing the laser power and illumination time. The results revealed that the carboxylate groups of the adsorbed PMBA molecules were removed to produce TP, which were still adsorbed onto Au surfaces. The solution pH values exhibited a significant influence on the decarboxylation reaction. In air and neutral solution, decarboxylation proceeded at a slow rate to transform PMBA to TP, while it was absent in acidic solution. The deprotonated carboxylate group accelerated the decarboxylation for producing TP with a fast rate in alkaline solution. As a comparison, a similar plasmon driven decarboxylation reaction was observed on a Ag nanoparticle monolayer film surface. These results suggested that the transformation from PMBA to TP molecules on an Au nanoparticle film surface under laser illumination was associated with a surface-catalyzed reaction driven by local surface plasmon.

•In situ SERS was developed to probe surface coordination chemistry.•The competitive coadsorption was observed between 22BPY and benzil.•The interfacial configurations of 22BPY, BA and benzoin were proposed.The surface configurations and properties are recognized of importance to well understand the surface processes of the heterogeneous reaction. Consequently, the surface sensitive technique is highly desired for obtaining the distinct interfacial pictures. The in situ electrochemical surface-enhanced Raman spectroscopy (SERS) was developed to investigate the adsorption behavior of 2,2′-bipyridine (22BPY) in the presence and absence of benzoin (benzoin = 2-hydroxy-2-phenyl acetophenone) and benzoic acid (BA) on Cu electrode surface in nonaqueous solution. Based on the changes in the spectral feature, the surface coordination processes between the Cu surface and 22BPY with BA, benzoin were deduced. In a wide potential range (−1.2 V to 0.1 V), 22BPY adsorbed vertically on the surface through the two nitrogen atoms of cis-configuration molecules. The introduction of benzoin and BA caused the dramatic change in the surface coordination processes of 22BPY to the Cu electrode. With the positive movement of potential, the benzoin was oxidized to benzil (benzil = 1,2-diphenyl-ethane-1,2-dione) accomplishing with the electrochemical oxidation of Cu electrode. The coadsorption of 22BPY, benzil and BA was observed around −0.2 V, while the anion of ClO4− was induced to coadsorb on the electrode surface. The results revealed that the cis-configuration of 22BPY and the deprotonated BA were contributed to the coadsorption behavior, and benzil participated into the coadsorption layer in a neutral formation. The induced coadsorption was transformed to the competitive adsorption between BA at about 0 V. The surface spectral feature was compared to that of the complex [(22BPY)2Cu(BA)]ClO4⋅(benzil) synthesized by direct electrochemical oxidation of Cu electrode in a similar condition. It suggested the surface adsorption and coordination processes were well agreement with the structure of the surface complex. The interfacial configurations of 22BPY, BA and benzoin in different potential regions were proposed.

•In situ SERS was developed to probe electric double layer and pzc of RTILs.•The minimum of camel-shaped capacitance curves was assigned to pzc.•SERS was well correlated with the capacitance–potential curves.•The pzc was depended on the length of alkyl chain of imidazolium based RTILs.The electrochemical differential capacitance and in situ surface enhanced Raman spectroscopy (SERS) have been combined to investigate the structure of electric double layer (EDL) at a series of imidazolium based ionic liquids/Ag electrode interface. By associating with the change in spectral feature of the imidazolium ring, the potential at local minimum between the two maxima in the camel-shaped differential capacitance curves was assigned to the potential of zero charge (pzc). The pzc shifted positively with increasing the length of the alkyl chain substituted to the imidazolium ring, and the approximate linear relation was observed between the pzc values and the carbon numbers of the alkyl chain. The values of the minimum capacitance at the pzc decreased with increasing the alkyl chain length, which was mainly contributed by the smaller values of dielectric constant (ε) for the RTILs with long alkyl chain. By employing in-situ SERS, the potential dependent spectral feature could be served as the criteria for resolving the adsorption behavior of the cations. With the movement of the potential from relative positive to negative potential, the cation adsorption configuration changed from vertical to nearly flat on. The reorientation could be considered as the evidence for estimating the pzc values. The spectroscopic investigation revealed that the onset potentials for the reorientation were shifted to positive direction as increasing the length of the alkyl chain. The tendency in changing the potential at the minimum capacitance or the reorientation was contributed to the π-electron interaction of imidazolium ring with the Ag electrode, in which the adsorption of cation acted like the behavior of a specific adsorbed anion. The weaker the interaction (long alkyl chain) was, the more the pzc shifted to positive potential direction. The above facts indicated that the classical differential capacitance curves and in situ SERS technique were well correlated for obtaining a deeper insight into the interfacial structure of RTILs at Ag electrodes.

Chiral amplification and discrimination are great challenges in both scientific and technological research fields such as chemical synthesis, chiral catalysis, and biomedicine. By mimicking protein superstructures in nature, chiral conducting polyaniline (PANI) molecules induced by chiral dopants were self-assembled to ultra-ordered superhelical microfibers. The induced homochirality is observed to be amplified into different hierarchies, from chiral molecules to helical nanostructures, and to superhelical microstructures. Furthermore, both experimental and theoretical results indicated that the gas sensor made from a single PANI helical microfiber showed enantioselective discrimination to chiral aminohexane, giving it great potential for applications in online chiral discrimination.

•A facile approach was developed to fabricate stacking faults enriched Ag nanowires.•The mechanism of the formation stacking faults was proposed.•The high catalytic and SERS activities were originated from the enriched stacking faults.A facile approach based on seed-mediated method for synthesis of stacking faults enriched Ag nanowires (SFEANWs) was successfully developed. SFEANWs were formed and attached onto the seed (α-Fe2O3/Au) surfaces through the reduction of AgNO3 by ascorbic acid (AA) in the presence of sodium polyacrylate (PAANa). Their length can be tuned with different concentrations of AgNO3 or PAANa. According to the effects of seeds and PAANa, the plausible growth mechanism of SFEANWs was discussed. The catalytic activity of SFEANWs comparing with fivefold twinned Ag nanowires (FFTANWs) was evaluated through reducing p-nitrophenol by NaBH4. The activation energy of the classical reaction catalyzed by SFEANWs was calculated through the Arrhenius equation. In addition, these SFEANWs exhibited excellent surface enhanced Raman scattering (SERS) activities due to the hot spots located in the cross of the twist wires. The detection limit of by SERS for 1,4-benzenedithiol (1,4-BDT) was estimated about 10−7 mol/L.Graphical abstract

•A facile approach has been developed to fabricate multifunctional Fe3O4@AuAg nanoparticles.•The nanocompositions integrated the stronger magnetism and tunable SERS effect.•The combination of magnetism, SERS and microfluidic chip improved the enrichment efficiency and detection sensitivity.A facile approach has been developed to fabricate multifunctional Fe3O4@AuAg alloy core–shell nanoparticles, owning the magnetism of the core and the surface enhanced Raman spectroscopy (SERS) activities of the alloy shell. By changing the amount of HAuCl4 and AgNO3, Fe3O4@AuAg alloy nanoparticles with different component ratios of Au and Ag were successfully prepared. The surface plasmon resonance of the composition was linearly tuned in a wide range by varying the molar fraction of Ag and Au, suggesting the formation of AuAg alloy shell. SERS and magnetic enrichment effects were investigated by using thiophenol (TP) as the probe molecule. The SERS intensity was strongly dependent on the molar ratios of Au and Ag and the excitation line. Enrichment for the molecules with low concentration and on line SERS monitoring experiments were performed through combining the magnetism of the core and the SERS effect of the alloy shell. The results revealed that the magnetic enrichment efficiency was dramatically increased due to the strong magnetism of Fe3O4 core. In addition, the Fe3O4@AuAg nanoparticles were also used in the microfluidic chip to continuously detect different flowing solution in the channel. The detection time and amount of analyte were successfully decreased.Graphical abstract

The corrosion inhibition behavior of benzotriazole (BTAH) on Ag electrodes and the influence of triphenylphosphane (pph3) were investigated by electrochemical method, in situ surface-enhanced Raman spectroscopy (SERS) and direct electrochemical synthesis of surface complexes in nonaqueous solution. The results indicated that the BTA− ion was coordinated to the Ag surface to form a highly cross-linked surface polymer complex of [Ag(BTA)]n, which suppressed the dissolution and oxidation of Ag effectively. The introduction of a neutral ligand of pph3 blocked the surface coordination processes of BTAH with the Ag electrode. It resulted in a decrease of inhibition efficiency to Ag surface. The ligand of pph3 played a negative role on the corrosion inhibition of BTAH to the Ag electrode. The SERS results were well consistent with the cyclic voltammetry and polarization curves measurements. For modeling, two different surface complexes were prepared in acetonitrile with and without pph3 by direct electrochemical synthesis. A polymer-like complex of [Ag(BTA)]n attached to the Ag surface was obtained in the absence of pph3, which suppressed the dissolution and oxidation of Ag effectively. A new binuclear compound, Ag2(BTA)2(pph3)4, was produced in acetonitrile with pph3 and the final coordination process occurred in solution leading to difficulties in forming a compact surface film, thus decreasing the corrosion inhibition efficiency of BTAH. The role of pph3 and the mechanism were proposed.Graphical abstractThe inhibition behavior of benzotriazole (BTAH) on Ag electrodes was investigated by electrochemical method, in situ surface-enhanced Raman spectroscopy (SERS) and direct electrochemical synthesis of surface complexes in nonaqueous solution. The role of pph3 and the surface mechanism were proposed.Highlights► In situ SERS and synthesis have been developed to evaluate the inhibition corrosion efficiency. ► The pph3 molecules played a negative role on the inhibition corrosion of BTAH. ► Composition of surface complex was obtained for elucidating inhibition mechanisms.

A facile approach was developed to prepare novel multifunctional Fe2O3/Au/Ag nanostructures integrated with isolated functions involving magnetic and optical properties. The Fe2O3/Au/Ag hybrid nanoparticles with different thicknesses of Ag shell were prepared by adjusting the amount of the AgNO3. Surface structures were varied from the rough with pinhole to smooth and pinhole free surfaces with increasing amounts of AgNO3. The surface plasmon resonance was tuned in a very wide region from that of Au to Ag. Surface enhanced Raman scattering (SERS) effects were also investigated, employing thiophenol (TP) and aminothiophenol (PATP) as probe molecules. It was revealed that the SERS intensity was strongly depended on the molar ratio of Ag and Au. With an increase in the Ag molar fractions, SERS signals were enhanced to the maximum due to the surface plasmon resonance of the pinhole structure. The magnetic enrichment for on line SERS monitoring the molecules with low concentration was performed based on the magnetic core and the SERS activity of the bimetallic shells. This enrichment procedure improved efficiently the limits of the SERS detection. It was shown that the multicomponent nanoparticles have potential applications in the fields of optical devices and magnetic separation.Graphical abstractHighlights► A facile approach was developed to fabricate multifunctional Fe2O3/Au/Ag nanostructures. ► The composite nanostructures exhibited optimal magnetism and tunable SERS activities. ► The combination of magnetic enrichment and SERS improved the limit of SERS detection.

A novel gene delivery system based on graphene oxide chemically-functionalized with branched polyethylenimine (PEI-GO) is reported. The PEI-GO conjugate was formed by the covalent linking of PEI and GO via an amide bond by widely used EDC chemistry. Thus-prepared PEI-GO exhibits an excellent ability to condense DNA at a low mass ratio with a positive potential of 49 mV. A WST assay reveals that PEI-GO is significantly less cytotoxic than PEI 25 kDa. Finally, the transfection efficiency of PEI-GO was evaluated. It is demonstrated that the luciferase expression of PEI-GO is comparable or even higher than that of the PEI 25 kDa at optimal mass ratio. Moreover, intracellular tracking of Cy3-labelled pGL-3 indicates that PEI-GO could effectively deliver plasmid DNA into cells and be localized in the nucleus. These findings suggest that PEI-GO is a promising candidate for efficient gene delivery.

A novel and highly sensitive immunoassay method based on surface enhanced Raman spectroscopy (SERS) and magnetic particles has been developed. This method exhibits great potential application in bio-separation and immunoassay.

The adsorption of three cyanopyridine isomers on Pt electrodes was investigated using electrochemical surface-enhanced Raman spectroscopy (SERS) within a wide potential region. It was revealed that the adsorption configurations depend on the applied potential and on the positions of CN group. The 4-cyanopyridine (4-CP) and 3-cyanopyridine (3-CP) molecules adsorb on Pt surfaces via the N atoms of the pyridine ring, while the 2-cyanopyridine (2-CP) molecules attach to the surface with the N atoms of both the pyridine ring and the CN group. In the case of 4-CP, the tilted adsorption mode is dominant under the potential of −0.8–0 V. It is converted to the vertical mode when the potential is moved to 0–0.8 V. For 3-CP, the vertical adsorption mode is found to be preferential in the full potential window. The CN stretching vibrational band of 2-CP is absent in the observed SER spectra. This is mainly caused by the characteristic π-electron delocalization as both N atoms are anchored to the Pt surface to form a heterogeneous ring. The hydrogen atoms are co-adsorbed with the cyanopyridine isomers on the Pt surface in extremely negative potential region.Highlights► Isomer dependent adsorption configuration was probed. ► Cyanopyridine coadsorbed with hydrogen at negative potential. ► Different Stark effect was measured for the isomers.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to the Fe electrode/electrolyte interface in neutral and weakly acidic solutions for the first time. The SEIRA-active Fe film electrode was obtained through a potentiostatic electrodeposition of a virtually pinhole-free 40 nm-thick Fe overfilm onto a 60 nm-thick Au underfilm chemically predeposited on the reflecting plane of an ATR Si prism. The infrared absorption for CO adlayer at the Fe film electrode measured with ATR-SEIRAS was enhanced by a factor of larger than 34, as compared to that at a Fe bulk electrode with external infrared absorption spectroscopy in the literature. More importantly, the unipolar band shape enabled the reliable determination of the Stark tuning rates of CO adlayer at Fe electrode. In situ ATR-SEIRAS was also applied to study the electrosorption of a typical corrosion inhibitor benzotriazole (BTAH) on Fe electrode as a function of potential, providing additional spectral information at positive potentials in support of the formation of a polymer-like surface complex FeIIm(BTA)n as the corrosion-resistant layer.

A facile approach has been developed for the synthesis of monodisperse spindle-shaped α-Fe2O3 and multibranched spindle FeO(OH) as well as their core–shell structures. The formation of unique multibranched spindle FeO(OH) particles depended on the smooth liberation of hydroxide ions by urea which inhibited the nucleation and growth rate of crystals. The amount of different metal ions and oleic acid exhibited significant effect on the shape of the particles, in which the formation of multibranched ends was induced. Their core–shell structures were fabricated by attaching the small Au nanoseeds onto the core surfaces followed with the catalyzed reduction to form continuous Au shell. The FeO(OH) and FeO(OH)@Au core–shell crystals were characterized using transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). The surface-enhanced Raman scattering (SERS) activities from different core–shell substrates were investigated. The results revealed that the intensity of SERS signal was dependent on the shell thickness. The multibranched core–shell particles exhibited optimum SERS activity and could serve as candidate substrate for both theoretical and practical applications in SERS.

The complexes of benzotriazole (BTAH) with Ag, Cu, Fe, Zn and Ni were prepared respectively in the non-aqueous solution by the direct electrochemical synthesis and characterized by microanalysis and normal Raman spectroscopy. The influence of the neutral ligand of triphenylphosphine (pph3) on the coordination process was deduced by the normal Raman spectroscopy and electrochemistry. The metals were classified into two categories. For the first type, such as Cu and Ag, CuBTA and AgBTA were obtained in the solution without pph3. The introduction of pph3 led to its participation in the coordination processes of BTAH with Cu or Ag and appeared in the final complex. Fe, Zn and Ni belonged to the second type, there was no influence on the coordination of BTAH with Zn, Ni and Fe, i.e., the final complexes were Fe(BTA)2, Zn(BTA)2 and Ni(BTA)2, respectively, in the solution with/without pph3. The electrochemical results revealed that the BTAH can inhibit the corrosion of all the above metals, and the introduction of pph3 resulted in the decrease of inhibition efficiency to Cu surface, while no influence was observed on the Ni surface. The different role of pph3 was explained in terms of hard/soft acids and bases rule and coordination mechanism was proposed.

Magnetic Fe2O3/Au core/shell nanoparticles can be particularly used in biological separation, but the development of an appropriate technique including a production process for higher efficient separation and the subsequent immunoassay for lower level still represent a great challenge. In this article, Fe2O3/Au core/shell nanoparticles with different Au ratios were prepared by reducing HAuCl4 on the surface of γ-Fe2O3 nanoparticles. Scanning electron microscopy (SEM) images and surface-enhanced Raman spectroscopy (SERS) spectra clearly show that the surfaces of Fe2O3 nanoparticles were covered by Au. SERS signals of pyridine (Py) have been obtained on the Fe2O3/Au nanoparticles, and it has been found that the SERS intensity enhanced with the increase of iterative additions of HAuCl4. The antigens in test solution have been effectively separated by the magnetic Fe2O3/Au core/shell nanoparticles, and subsequent rapid detection was examined by immunoassay analysis based on SERS. The result demonstrates that the magnetic bioseparation program used by this magnetic Fe2O3/Au core/shell nanoparticles could separate almost all of the antigens in test solution. The ease of operation and good separation efficiency of this effective method has shown a potential application for magnetic Fe2O3/Au core/shell nanoparticles in bioseparation.

The adsorption and reorientation behavior of benzonitrile on an electrochemical roughened Pt electrode were studied by surface-enhanced Raman spectroscopy (SERS) over a wide potential range. The results revealed that benzonitrile was adsorbed on the surface in a vertical orientation through the nitrogen atom of CN group in the relative positive potential region (>−0.6 V), while the benzonitrile molecule changed its orientation with the negative movement of potential (<−0.8 V). In the hydrogen evolution region, the intensity of the Raman bands from the benzene ring and CN group decreased dramatically, and the frequency of CN red-shifted about 160 cm−1. It indicated that the benzonitrile was adsorbed on the Pt electrode in a flat orientation by interacting with the electrode surface via CN and/or benzene ring. A schematic model about the adsorption configurations of benzonitrile in different potential regions was proposed.

The two tautomers of 7H-[1,3]dioxolo[4′,5′,4,5]benzo[1,2-d]thiazole-6-thione (DBTT) have been investigated by FT-IR and FT-Raman combined with density functional theory (DFT) calculations at the B3LYP level and 6-311+G** basis sets. On the basis of optimized structures, the harmonic force fields, vibrational frequencies and Raman intensities were calculated and scaled. The assignment of the fundamental vibrations for this molecule in its thione form was performed according to the potential energy distribution (PED) analysis. We have also discussed the adsorption behavior of DBTT on gold by means of SERS and DFT calculations at the same level. It revealed that the DBTT exhibited stable conformation of benzothiazole-2-thione (BTT) form both in solid and on gold surfaces; in addition, DBTT molecule is chemisorbed to the gold through both N and the exocyclic S atoms in its thione form and its molecular plane is perpendicular to the surface as BTT. The results show that the substituted groups in the phenyl ring have changed the characters such as the charge density in heterocyclic atoms and so on, but have little influence on the tautomeric preference of the BTT molecule and adsorption orientation on gold.

The activities of chemical systems can be evaluated successfully by combining vibrational spectroscopic analysis and quantum chemical calculation based on density functional theory (DFT). Two tautomers of 5-fluorobenzo[d]thiazole-2(3H)-thione (FBTT), 7H-[1,3]dioxolo[4′,5′,4,5]benzo[1,2-d]thiazole-6-thione (DBTT) and 5-chloro-6-methylbenzo[d]thiazole-2(3H)-thione (CMBTT) were investigated by FT-Raman spectroscopy and DFT calculations at B3LYP/6-311 + G** level. The molecular properties and activity relationships were determined by the HOMO energies, Mulliken charges and the binding energies with metal. It is concluded that three derivatives exhibited stable conformation of the thione form both in the isolated powder monomers and in their complexes with gold. The binding capability with gold was in the order of DBTT > BTT ≈ CMBTT > FBTT. The derivatives with the electron-donor substitutes in benzene ring were favorable to metal for the p–π conjugate effect.

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays. Relationships among SERS effect, SPR catalysis, and gap distance were experimentally and theoretically studied.

A femtogram level and specific surface enhanced Raman spectroscopy (SERS) based competitive immunoassay was developed to detect Hg(II) in aqueous solution for the first time. This novel approach provides an alternative, ultrasensitive and specific analytical method for the detection of Hg(II).

A novel gene delivery system based on graphene oxide chemically-functionalized with branched polyethylenimine (PEI-GO) is reported. The PEI-GO conjugate was formed by the covalent linking of PEI and GO via an amide bond by widely used EDC chemistry. Thus-prepared PEI-GO exhibits an excellent ability to condense DNA at a low mass ratio with a positive potential of 49 mV. A WST assay reveals that PEI-GO is significantly less cytotoxic than PEI 25 kDa. Finally, the transfection efficiency of PEI-GO was evaluated. It is demonstrated that the luciferase expression of PEI-GO is comparable or even higher than that of the PEI 25 kDa at optimal mass ratio. Moreover, intracellular tracking of Cy3-labelled pGL-3 indicates that PEI-GO could effectively deliver plasmid DNA into cells and be localized in the nucleus. These findings suggest that PEI-GO is a promising candidate for efficient gene delivery.

A novel and highly sensitive immunoassay method based on surface enhanced Raman spectroscopy (SERS) and magnetic particles has been developed. This method exhibits great potential application in bio-separation and immunoassay.

Human epididymis protein 4 (HE4), as a serological marker, has been proposed to be the most promising tumor marker in ovarian cancer diagnosis. An approach based on surface enhanced Raman spectroscopy (SERS) and a magnetic immunoassay technique was developed successfully for rapid detection and separation of HE4 with high sensitivity and selectivity. The detection was involved in the construction of a unique sandwich structure using a bottom-up method, which consisted of HE4 antibody and SERS reporter coated Au nanoparticles (A) and target HE4 antigen and HE4 antibody-modified magnetic core–shell Fe3O4@Au nanoparticles (B). The sandwich structure was effectively enriched by using a magnet for SERS detection. This approach exhibited an extremely high specificity in the detection of HE4 due to the strong specific interaction between the antibody and the corresponding antigen. The results revealed that the limit of detection (LOD) of the present approach was as low as 100 fg mL−1 and demonstrated a linear relationship between SERS intensities and lgc in a concentration range of 1 pg mL−1 to 10 ng mL−1. Accompanied by the magnetic enrichment procedure after the assembling of the sandwich structure, almost all of the HE4 protein was removed. The immuno Fe3O4@Au nanoparticles were regenerated by releasing the HE4 from the sandwich structure into the acidified methanol solution, and it could be used for magnetic enrichment and SERS detection for at least five times. Moreover, two kinds of immuno nanoparticles (A and B) could be developed as reagent kits in the clinical diagnosis of ovarian cancer.