The adsorption of 3,4,9,10-perylene tetracarboxylic acid (PTCA) on Ag and Au self-assembled films has been investigated through surface-enhanced Raman scattering (SERS). The spectrum of PTCA reveals a strong excitation wavelength dependence. On the basis of Herzberg–Teller selection rules, it is deduced that appearance of a new b1 mode of PTCA is selectively enhanced by charge-transfer resonance. Moreover, density functional theory (DFT) calculations were also performed to identify the expected vibrational modes and electronic transitions. The information obtained from the adsorption of PTCA on Ag/Au by SERS is helpful to understand the Herzberg–Teller selection rule. Meanwhile, it provides evidence to explain Raman vibronic modes observed when charge-transfer resonances appear in metal–dye systems.

Among all coating methods, atomic layer deposition (ALD), which can provide a precise thickness control at the angstrom or the monolayer level, appears to be one of the most promising techniques. To investigate the interfacial charge-transfer mechanism from semiconductor–molecule–metal systems, the order of different layers is very essential because the charge-transfer process can be affected by the interfacial contact order of different materials. Also, for TiO2/MBA/Ag charge-transfer (CT) investigation, homogeneous assembling of TiO2 with precisely controllable thickness is of great importance because the energy level of semiconductor is sensitive to its size at the nanoscale. Here, unlike previous 3D composite CT models, our semiconductor–molecule–metal interfacial CT models are fabricated with the ALD and e-beam evaporation techniques, which ensures the accuracy of the CT investigation. The surface-enhanced Raman scattering (SERS) technique is adopted in the investigation of the interfacial charge-transfer process through the changes of CT-sensitive bands. In TiO2/MBA/Ag, the SERS signal of MBA molecules and the Raman spectra of TiO2 phonon vibrational mode exhibit evident CT-driven changes. To confirm these phenomena, 4 nm thickness of wide-bandgap HfO2 and Al2O3 are inserted as isolated layers. Also, the possible CT mechanisms and the charge-transfer degree in different systems are discussed. This work not only suggests a role for ALD in fabricating CT models but also promotes the application of SERS in more intensive fields.

A coupled semiconductor nanoparticle system composed of TiO2 NPs, 4-mercaptobenzoic acid (4-MBA) molecules and PbS NPs has been fabricated as a model to study the involved charge transfer (CT) effect using a Surface-Enhanced Raman Scattering (SERS) technique. In this TiO2/MBA/PbS system, the MBA molecules function as both a linker between TiO2 and PbS NPs and a SERS probe to reveal the CT process. For the SERS enhancement behaviors of the CT-sensitive vibrational mode in 4-MBA and the Raman spectra of the phonon vibrational mode in TiO2 NPs, there are some evident changes after adding PbS to the TiO2/MBA system. These changes are due to the interaction between TiO2 NPs and PbS NPs. We found that these differences, closely related to the natures of TiO2 and PbS NPs, are a reflection of the enhanced TiO2-to-molecule CT process in SERS. The SERS spectra of the TiO2/MBA/CdS system were also investigated and the result is in good accordance with our theory. This work will not only help in constructing nanoscale models to study interfacial CT processes but also be of considerable value for practical application of SERS technology.

Recently, surface-enhanced Raman scattering (SERS) research based on semiconductor TiO2 has received increasing attention. However, the practical application of TiO2 SERS-active substrates has been hampered due to their lower surface performance and detection sensitivity. Here we report a new strategy to enhance the SERS activity of TiO2 nanoparticles (NPs) by means of formation of a mesoporous framework via reductive calcination of polymer coated nanocrystals. The 4-mercaptobenzoic acid (4-MBA) probe molecules on the mesoporous TiO2 substrate exhibit larger SERS enhancement compared with that on the ordinary TiO2 NP substrate; this considerable SERS enhancement mainly stems from the contributions of rich surface active sites of mesoporous TiO2, which can provide more effective adsorption sites for the molecules and promote the charge transfer between the substrate and the adsorbed molecule, and thus remarkably enhance SERS signals. On the mesoporous TiO2, a detection limit of 4-MBA as low as 1 × 10−8 mol L−1 can be achieved, which is the highest sensitivity among those reported for semiconducting substrates and even comparable with those of noble metal substrates. And, it is proved that the mesoporous TiO2 substrates are characterized by high stability and self-cleaning properties, and can be repeatedly used in SERS detection without losing their activity.

In this study, Au deposited TiO2 (Au–TiO2) nanocomposites were prepared by photocatalytic reduction of Au species (HAuCl4) on TiO2 nanoparticles (NPs) and served as efficient SERS-active substrates. The observed considerable SERS enhancement of 4-MBA molecules on Au–TiO2 NPs compared with that on pure TiO2 NPs can be attributed to the synergistic contribution of semiconductors and metals by the charge-transfer mechanism from the substrate to the molecule. In the preparation of Au–TiO2 nanocomposites, the photo-reduction time and concentration of HAuCl4 have an important influence on the SERS enhancement effect of the composite substrate. Moreover, for the recycle of substrates, the photocatalytic experiment of Au–TiO2 nanocomposites was carried out. The results show that these nanocomposite substrates are characterized by a self-cleaning property and can be repeatedly used in SERS experiments without losing their activity and with high stability.

A number of recent studies have focused on improving the performance of dye-sensitized solar cells (DSSCs). Cells with a ZnO-TiO2/N3/Ag structure have attracted particular attention because of their excellent power conversion efficiencies. Using a dendritic crystal ZnO-TiO2 composite semiconductor and Ag in conjunction leads to different charge-transfer (CT) processes, and this is the main theoretical basis for the improvement of DSSC performances. Thus, in the present study, TiO2/N3, ZnO/N3, ZnO-TiO2/N3, TiO2/N3/Ag, ZnO/N3/Ag, and ZnO-TiO2/N3/Ag assemblies have been fabricated and their CT processes have been monitored by using surface-enhanced Raman scattering (SERS) spectra, with particular focus on the differences caused by the synergistic effect of the ZnO-TiO2 component. The dye loading capacity of the dendritic crystal ZnO-TiO2 is much larger than that of TiO2. There are extra enhancements in the SERS intensity and degree of CT (ρCT) in ZnO-TiO2/N3 compared to ZnO + TiO2/N3 (based on a simulation curve for the physically mixed TiO2 and ZnO semiconductors) with 476.5 nm excitation due to the synergistic effect of the ZnO-TiO2 component. And these enhancements in ZnO-TiO2/N3/Ag compared to ZnO + TiO2/N3/Ag appear with 476.5 and 532 nm excitation, which are particularly large with 532 nm excitation. Accordingly, the participation of Ag in this synergistic effect can reduce its energy threshold, which will make it easier to appear. Finally, to rationalize these extra enhancements, the models describing the CT mechanism have been proposed. Thus, the use of the dendritic crystal ZnO-TiO2 composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3 compared to that with TiO2. Meanwhile, the synergistic effect of ZnO-TiO2 and Ag can promote the CT process, demonstrating that ZnO-TiO2/N3/Ag is an excellent structure for DSSCs.

In this study, highly-dispersed TiO2 nanoparticles (NPs) with abundant active sites were synthesized by a simple sol-hydrothermal method with the assistance of surfactant polyethylene glycol (PEG) which served as an effective SERS-active substrate for the first time. The observed considerable SERS enhancement of 4-mercaptobenzoic acid (4-MBA) probe molecules on TiO2 NPs is attributed to the contribution of the charge transfer mechanism from the substrate to the probe molecule. It is suggested that PEG can act as a protective agent in the reductive calcination of the surfactant-coated nanocrystallite and consequently brings about abundant surface oxygen vacancies for TiO2 NPs, which provide more effective sites for the adsorption of probe molecules and promote the charge transfer effect and its consequent SERS enhancement. Moreover, the prepared TiO2 NPs can serve as an effective substrate for highly sensitive detection of p-aminobenzoic acid (PABA). The detection limit of PABA is as low as 1 × 10−8 M, which is the highest sensitivity among the reported determination methods. And, it proves that the prepared TiO2 substrate with abundant active sites is characterized by high stability, which offers a unique long service lifetime for SERS detection without losing its activity.

•The detailed structural changes of 4-MPBA in different pH environments have been studied.•The contribution of the charge transfer process to the system has been explored.•The method provides a theoretical basis for 4-MPBA-based research.4-Mercaptophenylboronic Acid (4-MPBA) plays pivotal role in various fields. The orientation and existing form of the 4-MPBA strongly depend on the pH value of the media. The general aim of this work is to obtain information about the structure changes of 4-MPBA absorbed on Ag nanoparticles in different pH environment. Surface-enhanced Raman spectroscopy (SERS) technique is a simple and rapid method to study adsorption phenomena at molecule level. The investigation is done by means of SERS. In order to interpret the experimental information, a series of SERS spectra is carried out. The relative intensities of the totally symmetric (a1 mode) and non-totally symmetric (b2 mode) bands in the SERS spectra of 4-MPBA change depend on the environmental pH values, which is a manifestation of charge transfer (CT) processes. The degree of charge transfer increases with the pH value of the media changing from acidity to alkalinity. The structure changes of MPBA had been carried out in different pH environment. We envision that this approach will be of great significance in related fields of 4-MPBA-involved detection.Download high-res image (55KB)Download full-size image

•A SERS method coupled with a two-step pretreatment of sample was first proposed for residue detection of penicillin G (PENG) in real milk.•The two-step pretreatment of sample is essential for the detection of PENG residue.•Detection limit of PENG residue is 0.85 μg/kg, far lower than standard of the EU.•There is a good linear relation in the concentration range of 1.0×10−8~1.0×10−3 M.•The proposed method is an excellent alternative approach for PENG residue detection.The antibiotic residue in animal source foods (milk, meat, etc.) is threatening people's health due to its abusing in livestock breeding more and more seriously. In this study, a simple and sensitive SERS method coupled with a two-step pretreatment process of sample was proposed for the residue detection of penicillin G (PENG) in real milk sample. It can be found that the two-step pretreatment process of sample is an essential procedure for the successful detection of PENG residue in milk, which can effectively avoid interference from other components in the sample and achieve the trace-level detection of PENG residue by SERS. Under the optimal test conditions, the limit of detection of PENG residue is 2.54×10−9 mol/L (equal to 0.85 μg/kg), which is lower than the standard of the European Union (4 μg/kg). And, there is a good linear relationship (R2=0.9902) in the concentration range of 1.0×10−8~1.0×10−3 mol/L. By this method, the recovery of PENG residue ranges from 76% to 97% with relative standard deviation between 4.8% and 2.1%. The proposed SERS method can be effectively applied for determination of PENG residue in milk.

In our previous study, we proposed a label-free enantioselective discrimination methodology for chiral alcohols based on surface-enhanced Raman scattering (SERS) spectroscopy. This method does not use either chiral reagents or circularly polarized light (chiral light). In the present study, a series of SERS experiments were carried out with an achiral SERS probe molecule, p-aminobenzenethiol (PATP), as the chiral selector molecule to develop this method further and to explore the possible mechanism of enantioselective discrimination. The relative intensities of the nontotally symmetric (b2 mode) bands in the SERS spectra of PATP change depending on the presence of different enantiomeric environments, which is a manifestation of charge transfer (CT) processes. From laser excitation wavelength-dependent and concentration-dependent SERS experiments, we have further verified that this CT-induced chiral discrimination magnifies the differences between two chiral alcohol enantiomers interacting with PATP by SERS technology. We propose that the directions of CT from Ag nanoparticles to PATP molecules are different in different enantiomeric conditions, which results in differences in the CT transitions and significantly different SERS spectra. Therefore, this work allows discrimination between two enantiomers and breaks the traditional notion that chiral discrimination requires other chiral entities as chiral selectors or the involvement of chiral light in the system. We envision that this approach will be of great significance in the field of chiral separation and chiral catalysis.

•TiO2 and Zn doped TiO2 nanoparticles were synthesized and served as SERS-active substrate for the detection of levofloxacin (LVFX) drug molecules.•Both experimental and theoretical methods are used to study the enhanced mechanism and interaction between substrate and drug molecule.•The detection limit (S/N = 3) of LVFX can be reduced to 1.29 × 10−7 mol/L by this method.•There is a good linear relationship in the range of 1 × 10−3–1 × 10−7 mol/L concentration for SERS of LVFX on TiO2.In this work, TiO2 and Zn doped TiO2 nanoparticles (Zn-TiO2 NPs) were synthesized and served as SERS-active substrate for the detection of levofloxacin (LVFX) drug molecules. Both experimental and theoretical methods are used to study the enhanced mechanism and interaction between substrate and drug molecule. The results indicate that the LVFX is close to the substrate surface through the carboxyl group. The levofloxacin molecules adsorbed on Zn-TiO2 NPs exhibit the largest SERS enhancement, when the Zn content is 3%, the adsorption time is 7 h and pH value is 6.78. The detection limit can be reduced to 1.29 × 10−7 mol/L by this method. And, a quantitative detection method of LVFX can be established. There is a good linear relationship in the range of 1 × 10−3–1 × 10−7 mol/L concentration.

The interfaces of metal–dye molecule–semiconductor sandwich structure are very important in the investigation of dye-sensitized solar cells (DSSCs) where metals are used to enhance absorption. In this work, we first designed and synthesized Ag/N719 and Ag/N719/TiO2 sandwich systems to investigate the chemical binding type at the interfaces of Ag/N719/TiO2. The results of the Raman spectra under the laser excitations of 532, 633, and 785 nm clarified that the SCN groups adsorbed on the Ag surface via the S terminal and the TiO2 layer possibly bound to Ag/N719 via the ester linkage (—O—C═O) of the COOH group in N719. Then, we optimized the Ag substrate as an SERS detection platform and selected the Ag sol film as the substrate. Last, the relationship between the “degree of CT (ρCT)” in the SERS spectra and the charge transfer (CT) process was investigated by tuning the contribution from the chemical effect. We found that, owing to the introduction of TiO2, the intensity and ρCT first increased (n = 0–2) and then decreased (n = 2–3) with the increase of the number of TiO2 layers under 532 and 633 nm laser excitations. However, the intensity decreased (n = 0–3) with the increase of the number of TiO2 layers under the laser excitation of 785 nm, and there was no obvious change about ρCT. Meanwhile ρCT became higher with the increase of the laser excitation energy at the interfaces with the same TiO2 layer number. In order to explain these variations about ρCT, we utilize ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectra to calculate energy levels for better understanding the charge transfer (CT) process, and the calculation result is in accordance with the variation tendency of ρCT.

Semiconductor materials have been successfully used as surface-enhanced Raman scattering (SERS)-active substrates, providing SERS technology with a high flexibility for application in a diverse range of fields. Here, we employ a dye-sensitized semiconductor system combined with semiconductor-enhanced Raman spectroscopy to detect metal ions, using an approach based on the “turn-off” SERS strategy that takes advantage of the intrinsic capacity of the semiconductor to catalyze the degradation of a Raman probe. Alizarin red S (ARS)-sensitized colloidal TiO2 nanoparticles (NPs) were selected as an example to show how semiconductor-enhanced Raman spectroscopy enables the determination of Cr(VI) in water. Firstly, we explored the SERS mechanism of ARS–TiO2 complexes and found that the strong electronic coupling between ARS and colloidal TiO2 NPs gives rise to the formation of a ligand-to-metal charge-transfer (LMCT) transition, providing a new electronic transition pathway for the Raman process. Secondly, colloidal TiO2 nanoparticles were used as active sites to induce the self-degradation of the Raman probe adsorbed on their surfaces in the presence of Cr(VI). Our data demonstrate the potential of ARS–TiO2 complexes as a SERS-active sensing platform for Cr(VI) in an aqueous solution. Remarkably, the method proposed in this contribution is relatively simple, without requiring complex pretreatment and complicated instruments, but provides high sensitivity and excellent selectivity in a high-throughput fashion. Finally, the ARS–TiO2 complexes are successfully applied to the detection of Cr(VI) in environmental samples. Thus, the present work provides a facile method for the detection of Cr(VI) in aqueous solutions and a viable application for semiconductor-enhanced Raman spectroscopy based on the chemical enhancement they contribute.

In this paper, TiO2 nanoparticles (NPs) with different crystallinity served as SERS-active substrates for SERS detection of ciprofloxacin (CIP) drug molecules for the first time. CIP is close to the surface of the TiO2 substrate through the carboxyl group. The mutual SERS enhancement behaviors between CIP molecules and TiO2 NPs were discovered, which are attributed to the contribution of the TiO2-to-molecule charge-transfer mechanism. The crystallinity of TiO2 NPs, the pH value of adsorption solution and the adsorption time have significant influences on the interaction and the SERS behavior between CIP and TiO2. When the calcination temperature of TiO2 NPs is 450 °C, the pH value of adsorption solution is 6 and the adsorption time is 9 h, the CIP molecules on TiO2 NPs exhibit the largest SERS enhancement.

In this paper, TiO2 nanoparticles (NPs) in the anatase phase were synthesized by a very simple sol-hydrothermal method with the assistance of H2SO4. And importantly, the crystallinity and the surface defects (surface state) of anatase TiO2 NPs can be regulated and controlled by this means for improving their SERS performances. The 4-MBA molecule adsorbed on 0.14 mL-H2SO4–TiO2 NPs exhibits the highest SERS enhancement, as compared with the native enhancement on unmodified TiO2 NPs. The introduction of H2SO4 not only enriches the surface defects of anatase TiO2 and the adsorption site, but also improves the separation efficiency of photo-generated carriers, which all can promote the TiO2-to-molecule charge transfer and are mainly responsible for the improved SERS performances.

•Most of the fundamentals vibrations agree well with the predicted frequencies.•HOMO–LUMO gap are predicted.•The UV–Vis spectra of title molecule are investigated experimentally and theoretically.In this work, the structural parameters of the title molecule (MTT) have been obtained at the B3LYP/6-311++G** level of theory. The two phenyl rings and the two heterocyclic rings are found in four different planes. The tetrazolium ring was connected with other three rings and their distances are also different in decrease order of 1.461 Å (from tetrazolium ring to carbon connected phenyl) > 1.445 Å (from tetrazolium ring to nitrogen connected phenyl) > 1.425 Å (from tetrazolium ring to thiazolyl ring). Fourier transform infrared (FTIR) and Raman spectra of the compound were obtained experimentally. All FTIR and Raman bands of the compound obtained experimentally were assigned based on the modeling results obtained at the B3LYP/6-311++G** level. The calculated vibrational frequencies were in good agreement with the experimental values. In addition, the UV–Vis spectra were obtained experimentally and theoretically. Considering the effect of the PCM modelling error, the calculated absorbance peaks obtained at the B3LYP/6-311++G** level were also in good agreement with the experimental values. The HOMO–LUMO gap was predicted to be 1.83 eV at the B3LYP/6-311++G** level.Graphical abstract3-[4,5-Dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide (MTT) was designed as a model molecule for theoretical and experimental studies of the molecule structure. Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Most of the fundamentals vibrations agree well with the predicted frequencies.

•Most of the fundamentals vibrations agree well with the predicted frequencies.•Hydrogen bond donors and acceptors are predicted.•The vibrational spectra of ninhydrin SERS are investigated experimentally and theoretically.In this paper, ninhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Density functional theory (DFT) calculations have been performed to predict the IR and Raman spectra for the molecule. In addition, Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Based on the modeling results obtained at the B3LYP/6-311++G** level, all FTIR and Raman bands of the compound obtained experimentally were assigned. Our calculated vibrational frequencies are in good agreement with the experimental values. The molecular electrostatic potential surface calculation was performed and the result suggested that the ninhydrin had two potential hydrogen bond donors and four potential hydrogen bond acceptors. HOMO–LUMO gap was also obtained theoretically at B3LYP/6-311++G** level.Graphical abstractNinhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Most of the fundamentals vibrations agree well with the predicted frequencies.

With the explosive development of analysis and detection techniques based on surface-enhanced Raman scattering (SERS), the further understanding and exploitation of the chemical mechanism becomes particularly important. We investigated the charge transfer (CT) effect on SERS in a semiconductor–molecule–metal system constructed with Ag NPs, 4-mercaptobenzoic acid (MBA) molecule, and atomic level TiO2. To ensure more ordering, the system is constructed by a layer-by-layer self-assembly method. After introducing TiO2, we found that the relative band intensity of some peaks displayed a distinct difference, which is attributed to the Herzberg–Teller contribution that occurs via CT. We also proposed a possible mechanism responsible for the selective enhancement observed in the SERS spectra of the Ag NPs/MBA/TiO2 system. This work will not only provide much deeper insight into the CT mechanism in SERS but also help in the development of a method to construct metal–semiconductor-based SERS substrates.

•The layer-by-layer self-assembly technique and particle lithography were employed as the fabricating strategy.•The micropatterned polyelectrolyte films were obtained for the buildup of surface micropatterns with multicomponents.•The Au/Ag nanoparticles were deposited on the micropatterned polyelectrolyte films.•The SERS was investigated on the obtained nanostructures.The site-specific deposition of polyelectrolytes (PEs) or PE/noble metallic nanoparticle (NP) composites at nanoscale is highly desirable owing to their diverse applications in nanodevices. Herein, we demonstrated a simple yet effective method to fabricate different types of surface micropatterns with PE/noble metallic NP composites via particle lithography. The positively charged PE of poly(diallyldimethylammonium chloride) and negatively charged NPs of gold/silver (Au/Ag) were used as a model system. The layer-by-layer self-assembly technique was employed as the fabricating strategy. The final nanostructures, consisting of Au NP films with inlaid circular vacant island microarrays or Au NP films with inlaid circular Ag NP island microarrays, were fabricated, respectively. Scanning electron microscopy and atomic force microscopy were used to investigate the structural morphology, microstructure, and the NP growth. Furthermore, the surface-enhanced Raman scattering performance has been obtained based on the as-prepared Au/Ag nanopatterns. Our method displayed an excellent opportunity for the simple and effective production of nanopatterns on the surfaces demonstrating significant advantages in designing novel nanodevices.

We investigated the influence of hydrogen bonds (H-bonds) on the intermolecular interactions of a system comprising p-aminothiophenol (PATP) and benzoic acid (BA) using surface-enhanced Raman scattering (SERS) for the first time. In this system, H-bonds form through intermolecular interactions between the –NH2 and –COOH groups and promote the charge-transfer (CT) transition from the Ag substrate to the adsorbed PATP molecules. Accordingly, the intensities of the non-totally symmetric vibrations (the b2-type bands) of PATP are influenced through the Herzberg–Teller contribution. This is clearly a BA concentration-dependent phenomenon. This behaviour can be attributed to an increase in the degree of conjugation of the system, which facilitates the CT process in the system with H-bonds. Furthermore, temperature-dependent SERS experiments and their two-dimensional (2D) correlation analysis confirmed that the formation of H-bonds facilitated the CT transition between the adsorbed molecules and substrate. The degree of CT was reduced by H-bond breakage that occurred with increasing temperature. An additional SERS experiment involving substituted BA molecules yielded similar conclusions.

•Most of the fundamentals vibrations agree well with the predicted frequencies.•Hydrogen bond donors and acceptors are predicted.•The vibrational spectra of 4-MPH are investigated experimentally and theoretically.In this paper, 4-mercaptophenol (4-MPH) was designed as a model molecule for theoretical and experimental studies of the molecule structure. Density functional theory (DFT) calculations have been performed to predict the IR and Raman spectra for the molecule. In addition, Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. All FTIR and Raman bands of the compound obtained experimentally were assigned based on the modeling results obtained at the B3LYP/6-311 + G** level. Our calculated vibrational frequencies are in good agreement with the experimental vales. The molecular electrostatic potential surface calculation was performed and the result suggested that the 4-MPH has two hydrogen bond donors and three hydrogen bond acceptors. HOMO–LUMO gap was also obtained theoretically at B3LYP/6-311 + G** level.4-Mercaptophenol (4-MPH) was designed as a model molecule for theoretical and experimental studies of the molecule structure. Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Most of the fundamentals vibrations agree well with the predicted frequencies. Hydrogen bond donors and acceptors are predicted.

•A new, fast, simple and sensitive method for melamine in milk detection was developed.•Hollow gold chip was fabricated and used as a melamine detection platform for the first time.•No pretreatment or separation steps is a feature of this melamine detection method.A hollow gold (HG) chip with high surface-enhanced Raman scattering (SERS) capability was fabricated and used to monitor the adulteration of milk with melamine. This chip was fabricated with self-assembled hollow gold nanospheres (HGNs) on glass wafers through electrostatic interaction. There are two important advantages for the use of this HG chip as a detection platform. First, HGNs show a strong SERS enhancement from individual particles due to their capability to localize the electromagnetic fields around the pinholes in hollow shells. Second, the HG chip improves the limit of detection through the enrichment effect. The characteristic SERS peak of melamine was used to distinguish it from other kinds of proteins or amino acids, and its intensity was used to monitor the percentage of melamine in milk. With its simple detection procedure (no pretreatment or separation steps), decreased processing time and low detection limit, this HG chip shows a strong potential for broad applications in melamine detection from real samples.

We report the significant effect of intermolecular hydrogen bonding (H-bonding) on surface-enhanced Raman scattering (SERS) spectra in which the vibrational frequencies and intensities of some characteristic peaks of p-mercaptobenzoic acid (MBA) change with varying concentrations of aniline. These changes can be attributed to modifications in the electronic structure of the MBA molecule and the conjugation of the system under the influence of H-bonding. Of remarkable note is that the nontotally symmetric (b2) mode of MBA is dramatically enhanced, which can be considered as a manifestation of the charge-transfer (CT) transition process in the system. By comparing SERS spectra obtained under normal and basic conditions, the effect caused by H-bonding can be further understood. These results manifest that the CT resonance between MBA and Ag NPs through Herzberg–Teller contributions can be promoted by H-bonding. The current work may, therefore, be instructive for studying the influence of H-bonding on the electronic structure of molecules in a system via SERS technique.

•Silver nanotriangles array with long ranged periodicity was fabricated on the surface of glass wafer.•High enhancement of the fluorescent intensity of the target antibodies was obtained by the silver nanotriangles array.•High sensitivity and good reproducibility for antibodies detection was obtained by the 2D silver nanotriangles array.Long range ordered silver nanotriangles array was fabricated for surface-enhanced fluorescent immunoassay in this paper. By polystyrene (PS) microspheres based LB template method, the silver nanotriangle array with about 100 nm in height was constructed on the surface of glass slide. On the surface of Ag nanotriangles array, the immune reaction of antigens and labeled antibodies was carried out. Based on the interaction of fluorophores from antibodies with the plasmon resonance from Ag nanotriangles and the enrichment effect of this patterned array, 3.11 times enhancement of the fluorescent intensity of the target antibodies was obtained. According to the fitting curve of fluorescent intensities and logarithmic concentrations of labeled antibodies from 100 pg/mL to 10 μg/mL, it concludes that the limit of detection by this Ag nanotriangles array for immune complex is 100 pg/mL. Due to the advantages of high sensitivity, good reproducibility, and convenient fabrication, the 2D silver nanotriangles array could be an exciting platform for bioassays in proteomics, drug discovery and diagnostics.Graphical abstract

Based on bovine serum albumin (BSA)-modified Au NPs, a simple and cost-effective approach was proposed to fabricate an anti-aggregated Au NP sensing platform for the detection of metal ions. It exhibits excellent stability even under highly ionic conditions due to its electrostatic stabilization, as well as the steric stabilization.

SERRS (surface-enhanced resonance Raman scattering) has been used to develop and optimize a novel and quantitative MTT assay for living cell viability. This highly sensitive method derives from two factors for formazan signal enhancing: the addition of Au nanoparticles and the resonance effect by 632.8 nm of excitation. The results show that the background elements, such as excessive MTT residues, serum, and the drug, did not interfere with the detection of formazan. Moreover, the detection limit of formazan is as low as 1 ng/mL. With the use of this method to quantify metabolically viable cells, dose–response curves of treated and untreated cells with the drug were constructed on the human lung cancer cell A549. The results also show that the Raman signal generated is dependent on the degree of activation of the cells. In comparison to the traditional method, the main advantages of this method are its rapidity (30 min), high-selectivity, high-precision, and cost-effectiveness (0.1 mg/mL MTT) without time-consuming steps and any modifying or labeling procedure. This work reports on an improved research tool that may help researchers apply this method for in situ cell assays.

A simple and effective surface-enhanced Raman scattering (SERS)-based protocol for the detection of protein–small molecule interactions has been developed. We employed silver-coated magnetic particles (AgMNPs), which can provide high SERS activity as a protein carrier to capture a small molecule. Combining magnetic separation and the SERS method for protein detection, highly reproducible SERS spectra of a protein–small molecule complex can be obtained with high sensitivity. This time-saving method employs an external magnetic field to induce the AgMNPs to aggregate to increase the amount of atto610-biotin/avidin complex in a unit area with the SERS enhancement. Because of the contribution of the AgMNP aggregation to the SERS, this protocol has great potential for practical high-throughput detection of the protein–small molecule complex and the antigen–antibody immunocomplex.Graphical abstractHighlights► Development of AgMNPs as protein carrier. ► External magnetic field induced the aggregation of AgMNPs can provide high SERRS enhancement. ► This is a promising potential of applications for quantitatively immunoassays based on SERRS.

A novel surface-enhanced Raman scattering (SERS)-based detection method for proteins attached to magnetic glass beads have been developed in this study. Silica coated magnetic nanoparticles (Fe3O4) are prepared and aldehyde-functionalized for capturing proteins in solutions by Schiff reaction, which is more biocompatible than previous magnetic nanoparticle and SERS-based detection methods. Target protein and molecule with a specially recognized feature are captured and separated by the magnetic nanoparticles, followed by SERS-based detection. Moreover, these biomagnetic glass beads and detection protocol are found to be useful in protein immunoassay and protein-ligand recognition even in a mixture system (two protein ligands). The proposed magnetic nanoparticles are biocompatible, manipulatable by an external magnetic force and highly sensitive after silver nanoparticles staining, which makes the protocol quite promising for high throughput protein assays.

TiO2 and Mn-doped TiO2 (1%, 3% and 5%) nanoparticles (NPs) were synthesized by a sol-hydrothermal method for surface-enhanced Raman spectroscopy study. When using the 4-mercaptobenzoic acid (4-MBA) as the probing molecule, optimum SERS signals were observed on the Mn–TiO2 (3%) samples. On the 3% Mn doped TiO2 substrate, 4-MBA molecules exhibit a higher SERS intensity by a factor of six as compared with the native enhancement of 4-MBA adsorbed on pure TiO2 NPs. The possible mechanism for the phenomena is also discussed. This study opens up a new dimension for the family of SERS substrates.Graphical abstractWe reported significant SERS signals were observed when the probing molecules of 4-mercaptobenzoic acid were adsorbed on the surface of Mn-doped TiO2 nanoparticles. Additionally, the crystalline defects of the Mn-TiO2 nanoparticles caused by Mn dopant play an important role in the SERS activity. The observation is attributed to the classical charge-transfer mechanism.Highlights► Mn doped TiO2 nanoparticles as SERS substrates have been investigated. ► Mn–TiO2 substrates exhibit stronger SERS activity than pure TiO2 nanoparticles. ► The surface defects of the Mn–TiO2 nanoparticles caused by Mn dopant play an important role in the SERS activity. ► This work brings in a new member for SERS substrates family.

By the aim of constructing surfaces for multi-component and multifunctional bioassay, a microsphere lithography technique was employed to control the surface morphology. Two kinds of protein molecules (antibodies) were used as building blocks. As a result, dual-component biocompatible surfaces with alternate immunoglobulin micropatterns were fabricated. The employed antibodies included human Immunoglobulin G (IgG) and rabbit IgG, which composed nanometer scale surface arrays on the surfaces. The antibodies were identified specially by immunoreactions with labeled antigens of fluorescein isothiocyanate (FITC)-antihuman IgG and tetramethylrhodamine-5-(and 6)-isothiocyanate (TRITC)-antirabbit IgG. The immune responses were confirmed by confocal fluorescence (FL) microscopy. A study on the sensitivity and quantification was done by using surface-enhanced resonance Raman scattering (SERRS) spectroscopy. The obtained SERRS spectra showed satisfactory resolution in the multi-component detection objects. No interference was observed from inner- or interactions of detecting molecules. The detection limits for both of the antigens reached to as low as 1 ng/mL, which was comparable to FL method. Meanwhile, a good linear relationship between SERRS peak intensity and the logarithm of antigens’ concentrations (from 1 ng/mL to 1 mg/mL) were observed. The results demonstrated that SERRS is a very promising detection technique for multi-component immunoassay, and has great potential applications in biotechnology and biochemistry.Graphical abstractHighlights► The samples with shape protein patterns are similar to natural biomolecules. ► Samples have alternative regions which contained separate antibodies. ► Both SERRS spectra of two kinds biomolecules can be obtained simultaneously. ► Relationship of SERRS intensity and proteins are found.

Traditional metal SERS-active substrate (Ag or Au) and novel semiconductor SERS-active substrate (TiO2) are combined into the composite system for their synergetic contribution to SERS. A series of assemblies with 4-mercaptobenzoic acid (4-MBA) molecule, TiO2, and/or Ag (Au) nanoparticles (NPs) have been fabricated by a self-assembly method. In the sandwich-structure assemblies (TiO2/MBA/Ag(Au) and Ag(Au)/MBA/TiO2), the SERS signals of 4-MBA molecule exhibit obvious difference in not only the intensity but also Raman frequency as compared with that SERS enhancement in the TiO2/MBA, which is attributed to the introduce of metals and its interaction/synergistic action with TiO2 NPs. SERS enhancement behaviors of 4-MBA in the sandwich-structure assemblies strongly depend on the natures of metals and 4-MBA molecule, which can result in a influence on the TiO2-to-molecule charge transfer and consequent additional EM field effect. This work would like to be interesting and of considerable value for both theory development and practice application of SERS technology.

Detailed understanding of the underlying mechanisms of surface-enhanced Raman scattering (SERS) remains challenging for different experimental conditions. In this study, a novel laser-driven photoinduced interfacial charge transfer (CT) was observed based on UV–visible–infrared excitation wavelengths (325, 488, 514, 633, and 785 nm) through surface modification of ZnO nanorods by 4-aminothiophenol (PATP). SERS spectra combined with well-characterized surface morphology and optical spectroscopy indicate that the chemical enhancement occurs at visible-infrared excitation but at ultraviolet excitation (325 nm) multiphonon resonant Raman Scattering (MRRS) results in additional strong enhancements of particular Raman transitions through Cu–ZnO–PATP model. The relationships between the excitation photon energies (3.82, 2.54, 2.41, 1.96, and 1.58 eV), and its Raman shift were discussed. We found the strong dependence of the Raman shifts with the exchanges of excitation photon energies. These results highlight the role of excitation energy in determining the interface enhanced Raman scattering for semiconductor-molecule models. This implies that copper sheet under the ZnO improve the interfacial CT in ZnO-molecule and act as an effective donor for inhibiting reversible CT, and there was a strong interaction, which might be regarded as CT resonance process, between PATP molecules and the ZnO surface. This work not only shows a possibility for further understanding the origin of the SERS mechanism from semiconductor substrates but also for exhibits a situ characterization technique for probing the photoinduced interfacial charge-transfer processes.

A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a series of metal-charge-transfer (CT) complex systems fabricated by a self-assembly method. The developed Ag/4-mercaptophenols (MPH)/n-TiO2 system presented layer number-dependent SERS spectra. By using the electron density values of the Ag13/MPH and Ag13/MPH/TiO2 system calculated using the density functional theory (DFT) and by using these values in combination with the results of our previous investigations on the mechanism of the Ag/MPH/TiO2 system, the absorption threshold of the CT complexes was clearly defined. The degree of CT was selected to study the layer number-dependent SERS spectra. Based on the layer number-dependent SERS data, it has been inferred that the degree of CT represents a resonance phenomenon. In addition, the CT resonance occurs at higher energy in the Ag/MPH/n-TiO2 system than in the monolayer TiO2 system owing to the blue-shift of CT states with the continuous introduction of TiO2. Thus, we provide a good example of the use of a CT complex system to investigate the chemical mechanism of SERS.

The charge-transfer resonance of Raman measurements in nanosized semiconductor–molecule–metal interfaces as a function of the excitation energy with four models (Cu–ZnO–PATP–Ag, Cu–Ag–PATP–ZnO, Cu–ZnO–Ag–PATP, and Cu–Ag–ZnO–PATP assemblies) to describe this dependence provides a powerful tool to study the chemical mechanism of surface enhanced Raman scattering (SERS). We measured the SERS spectra of self-assembled p-aminothiophenol (PATP) molecule junctions at 488, 514, 633, and 785 nm excitation wavelengths. We followed changes at the molecule junctions during the conditioning and eventually effect of charge-transfer (CT) through molecule–ZnO interfaces. Our results demonstrate that the interaction between the semiconductor bands and molecular energy levels can lead to novel charge behavior. The typical ZnO-PATP interfacial electron–hole recombination causes an increase in the CT resonance enhancement of Raman scattering, which is mainly responsible for the drastic change in molecular polarizability. We also proposed a complementary interpretation of the mechanism responsible for the highly variable enhancement observed in SERS.

The existence of pH-dependent surface-enhanced Raman scattering (SERS) of p-aminobenzenethiol (PATP) on Ag nanoparticles has been confirmed by numerous studies, but its mechanism still remains to be clarified. Discussion of the mechanism is at a standstill because of the lack of a systematic investigation of the process behind the pH-induced variation of the PATP behavior. Two-dimensional correlation spectroscopy is one of the most powerful and versatile spectral analysis methods for investigating perturbation-induced variations in dynamic data. Herein, we have analyzed the pH-dependent behavior of PATP using a static buffer solution with pH ranging from 3.0 to 2.0. The order of the variations in the different vibrational intensities was carefully investigated based on 2D correlation SERS spectroscopy. These results have demonstrated that the very first step of the pH-response process involves protonation of the amine group. The pH-response mechanism revealed is an important new component to our understanding of the origin of the b2-type bands of PATP.Keywords: charge-transfer contribution; p-aminobenzenethiol; surface-enhanced Raman scatting; two-dimensional correlation spectroscopy;

With the explosive development of analysis and detecting techniques based on SERS, the further understanding and exploit of the chemical mechanism becomes particularly important. We investigated the contribution of semiconductor to surface-enhanced Raman scattering (SERS) in metal and semiconductor heterostructure consisting of Ag/4-mercaptophenol (MPH)/TiO2. For this, we used the distinctive property, where the band edge position of an oxide semiconductor such as TiO2 is sensitive to the pH value, to control the charge-transfer (CT) contribution. It was found that increasing the pH of the buffer solution negatively shifts the conduction band edge of TiO2, thereby increasing the conductive band electron density at an equilibrium state. Thus, the relative band intensities of Ag/MPH/TiO2 increase in the SERS spectrum, which is attributed to the Herzberg–Teller contribution that occurs via CT. Moreover, because of the slower transport of cations from the pH buffer solution to the surface of TiO2, which results from the space-charge limitation, there is a decay time that is associated with the pH-response process.

We have fabricated a charge-transfer system using the self-assembly method. We find the nontotally symmetric b2 modes are selectively enhanced in SERS spectroscopy and represent a laser wavelength-dependent property.

We introduce a new approach that combines the self-assembly and seeding methods for silver plating of the proposed Fe2O3@Au-seed substrate. By adjusting the silver plating time between 0 and 360 s, the optical resonance of the substrates clearly varied from visible to near-infrared. To investigate the potential of these substrates for use in surface-enhanced Raman scattering (SERS) applications, SERS spectra of thiophenol (TP), 4-aminothiophenol (PATP) and rhodamine 6G (R6G) were evaluated at three excitation wavelengths (532, 633 and 785 nm). Analysis of the SERS spectra clearly demonstrated that the SERS effect depends strongly on the nature of the substrate surface and the nature and electronic resonance of the probe molecules. The calculated enhancement factor (EF) of TP adsorbed onto an Fe2O3@Ag substrate was ∼107 following three laser excitations. More importantly, these SERS substrates can successfully be utilized for the detection of small molecules at very low concentrations (∼100 pg mL−1).

A new kind of Ag/Cu2S composite micro/nanostructures has been prepared via a convenient galvanic reduction method. SEM images of these micro/nanostructures showed that Ag nanoparticles with the size of around 50–100 nm were well deposited on the surface of Cu2S micro/nanostructures. The SEM images also indicated that the Ag nanoparticles were preferentially grown on the big polygonal Cu2S microstructures, which could be explained by a localization of the electrons on the surface of the polygonal Cu2S microstructures after the electron transfer step. Owing to the introduction of Ag nanoparticles on the surface of Cu2S micro/nanostructures, the resulting Ag/Cu2S composite micro–nanostructures could be used as a versatile substrate for surface enhanced Raman scattering.Graphical abstractHighlights► Ag/Cu2S composite micro/nanostructures were prepared by a galvanic reduction method. ► The SERS spectra of PATP and 4-MPy molecules were studied. ► The mechanism of the SERS effect was proposed.

In this study, a nanoscale protein chip is prepared by using an etched polystyrene (PS) template. This protein chip can be directly used for immunoassay, with the help of Surface Enhanced Raman Scattering (SERS) spectra. Some glass slides submerged in aldehyde is initially prepared, modified with antibodies, human immunoglobulin G (IgG). Then PS arrays are self-assembled on these slides with the Langmuir–Blodgett method. The PS template pattern is transferred to the human IgG substrate using an etching process—slides are exposed to O2 plasma for 90 s. The PS nanoparticles are then washed away using phosphate buffered saline solution. Next, the slides are dipped into bovine serum albumin solution to ensure that the anti IgG would bond only to the human IgG. At this moment, a patterned protein chip is obtained. When used for protein detection, the protein chip could be immersed into labeled specificity antigen solution. Here we chose fluorescein isothiocyanate anti-human IgG. After washing, only bonded antigens remain. Fluorescence microscopy and SERS is used to characterize the samples. The SERS spectra intensity shows liner correlation with the concentration of anti-human IgG. All the experiments are conducted in a phosphate buffered saline solution at 37 °C for 2 h.Graphical abstractHighlights► This protein chip can be directly used for immunoassay. ► The samples are round shape which is more similar to natural biomolecules. ► We use glass slide as the substrate, and constructed protein patterns on a plat. ► We use SERS spectra here; fingerprints of biomolecule can be obtained. ► A liner correlation of SERS intensity and concentration is also found.

A model metal–semiconductor–molecule–metal assembly has been designed for probing the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS). We measured the SERS of ZnO–PATP–Ag, Au–ZnO–PATP–Ag, and Cu–ZnO–PATP–Ag assemblies at excitation wavelengths of 514.5, 785, and 1064 nm. Our results demonstrate that the metal–semiconductor contact can alter the charge distribution through p-aminothiophenol (PATP) molecules. This is attributed to the chemical SERS enhancement mechanism with additional electrical transport properties within these assemblies. These inhibit the CT from the metal to the molecule, resulting in the different degrees to which CT contributes to the overall SERS enhancement of PATP.

Coomassie brilliant dyes have high affinity to proteins and high Raman activity, and on the basis of which, we have employed brilliant blue R-250 (BBR) and brilliant blue G-250 (BBG) as surface-enhanced Raman scattering (SERS) labels to probe protein−ligand recognitions. This method differs from previously proposed methods in that target proteins are labeled rapidly before biological recognitions without procedures of separation and purification, rather than attaching Raman labels to metal nanoparticles, which significantly simplifies the Raman dye labeling procedure. In typical assays, ligand-functionalized metal nanoparticles assemble by target protein-specific bindings and this assembly sequentially turns on electromagnetic enhancement of Raman scattering of the proposed labels. The method with its advantages of rapidness, high sensitivity, and spectral multiplexing has great potential in probing protein−protein and protein−small molecule recognitions not only in solution systems but also on flexible solid substrates.

One-dimensional (1D) ZnO/4-mercaptopyridine (4-Mpy)/Ag assemblies have been prepared by immersing ZnO rod arrays modified by 4-MPy into Ag nanoparticles colloid. Scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the formation process of ZnO/4-Mpy/Ag assemblies. It shows that 4-Mpy molecules are adsorbing on the surface of the ZnO rod arrays, then 4-Mpy molecules are partially dissociated from ZnO rod arrays by desorption and connect with Ag nanoparticles via the S atom. The analysis of XPS confirms the result obtained from Raman spectra of ZnO/4-Mpy/Ag assemblies. This research provides a direct spectroscopic evidence for the formation of mixed 1D ZnO/4-Mpy/Ag assemblies.This research provides a direct spectroscopic evidence for the formation of mixed 1D ZnO/4-Mpy/Ag assemblies using Raman and X-ray photoelectron spectroscopy techniques.

Nanoscale Ag semishell arrays with controlled size and tunable interparticle distance were prepared by combining nanosphere lithography with reactive ion etching. First, a large-area ordered monolayer of polystyrene (PS) nanospheres was deposited on glass substrates using the Langmuir−Blodgett (LB) technique. The PS spheres with different diameters were employed in LB procedures. Second, the monolayers of PS spheres were etched to control the diameter and tune the interparticle distance. Finally, a Ag layer was evaporated on the etched PS templates. Ag films with periodical nanostructures were obtained and can be used as surface-enhanced Raman scattering (SERS) substrates. These substrates exhibited homogeneity and good enhancement ability. SERS enhancement factor (EF) was represented on the order of 104−105. The correlation between nanoscale morphology and SERS activity of the substrates was investigated. When the size of Ag-semishell was fixed, the EF value decreased with the increase of interparitcle distance. Both local surface plasmon mode and delocalized surface plasmon mode contributed to the total enhancement. The controlled size, tunable interparticle distance, and large-area ordered arrays of these substrates suggest their promising applications as functional components in spectroscopy, immunoassay, biosensors, and biochips.

Pefloxacin mesylate, a broad-spectrum antibacterial fluoroquinolone, has been widely used in clinical practice. Therefore, it is very important to detect the concentration of Pefloxacin mesylate. In this research, the near-infrared spectroscopy (NIRS) has been applied to quantitatively analyze on 108 injection samples, which was divided into a calibration set containing 89 samples and a prediction set containing 19 samples randomly. In order to get a satisfying result, partial least square (PLS) regression and principal components regression (PCR) have been utilized to establish quantitative models. Also, the process of establishing the models, parameters of the models, and prediction results were discussed in detail. In the PLS regression, the values of the coefficient of determination (R2) and root mean square error of cross-validation (RMSECV) of PLS regression are 0.9263 and 0.00119, respectively. For comparison, though applying PCR method to get the values of R2 and RMSECV we obtained are 0.9685 and 0.00108, respectively. And the values of the standard error of prediction set (SEP) of PLS and PCR models are 0.001480 and 0.001140. The result of the prediction set suggests that these two quantitative analysis models have excellent generalization ability and prediction precision. However, for this PFLX injection samples, the PCR quantitative analysis model achieved more accurate results than the PLS model. The experimental results showed that NIRS together with PCR method provide rapid and accurate quantitative analysis of PFLX injection samples. Moreover, this study supplied technical support for the further analysis of other injection samples in pharmaceuticals.

For surface-enhanced Raman scattering (SERS)-based protein identification, immunoassay, and drug screening, metal sandwich substrates bridged by proteins have been created in the present study. The sandwich architectures are fabricated based on a layer-by-layer (LbL) technique. The first gold monolayer is prepared by the self-assembling of gold nanoparticles on a poly(diallyldimethylammonium chloride) (PDDA)-coated glass slide. The second gold or silver layer is produced by the interactions between proteins in the middle layer of the sandwich architecture and the metal nanoparticles. Highly reproducible surface-enhanced resonance Raman scattering (SERRS) and SERS spectra can be obtained by the present gold-protein-gold (Au/Au) and gold-protein-silver (Au/Ag) sandwiches, and we find that the latter yields about 7 times stronger SERRS than the former. Because of contributions from the two metal layers to the SERS, this sandwich strategy holds great potential in highly sensitive and reproducible protein detections.

We detected concentration-dependent surface-enhanced Raman scattering (SERS) spectra of several label-free proteins (lysozyme, ribonuclease B, avidin, catalase, and hemoglobin) for the first time in aqueous solutions. Acidified sulfate was used as an aggregation agent to induce high electromagnetic enhancement in SERS. Strong SERS spectra of simple and conjugated protein samples could easily be accessed after the pretreatment with the aggregation agent. The detection limits of the proposed method for lysozyme and catalase were as low as 5 μg/mL and 50 ng/mL, respectively. This detection protocol for label-free proteins has combined simplicity, sensitivity, and reproducibility and allows routine qualitative and relatively quantitative detections. Thus, it has great potential in practical high-throughput protein detections.

Using an electroless replacement deposition method, large-area superhydrophobic metal substrate could be obtained. The superhydrophobic surfaces were prepared via a replacement reaction between copper substrate and HAuCl4 solution. The roughness of the copper substrate increased much after the replacement reaction. X-ray powder diffraction (XRD) pattern and energy dispersive X-ray (EDX) spectroscopy have proved that gold, CuCl and Cu2O formed on the surface of copper substrate after the replacement reaction. The surface showed remarkable superhydrophobic properties with a contact angle higher than 150 degrees without any modification with a self-assembled monolayer (SAM) of long chain thiol or perfluoro molecules.Large-area superhydrophobic metal substrate with a contact angle higher than 150 degrees without any modification with long chain thiol or perfluoro molecules has been obtained.

Surface-enhanced Raman scattering (SERS) of 4,4′-bipyridine (BPy) on silver foil substrate was measured using the 488, 514.5, and 1064 nm excitation lines. Density functional theory (DFT) methods were used to calculate the structure and vibrational spectra of Ag–BPy, Ag3–BPy and Ag4–BPy complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of BPy were assigned on the basis of the calculation of potential energy distribution. The calculated spectra of Ag–BPy and Ag4–BPy complexes were much closer to the experimental results of BPy adsorbed on silver surface than that of Ag3–BPy complexes. The vibrational frequencies that are sensitive to the planar or non-planar structure of BPy and to the dihedral angle of two pyridyl rings were discussed. The DFT results showed that the angles between two pyridyl rings for Ag–BPy and Ag4–BPy were skewed by about 38.44° and 37.1°, respectively. The energy gaps of the HOMO and LUMO from DFT were 415–912 nm for BPy–Ag complexes. The relative intensities of SERS bands changed with different excitation laser lines. Thus, a chemical enhancement mechanism should play an important role in the SERS of BPy on silver substrate.

Proteins are essential components of organisms and they participate in every process within cells. The key characteristic of proteins that allows their diverse functions is their ability to bind other molecules specifically and tightly. With the development of proteomics, exploring high-efficiency detection methods for large-scale proteins is increasingly important. In recent years, rapid development of surface-enhanced Raman scattering (SERS)-based biosensors leads to the SERS realm of applications from chemical analysis to nanostructure characterization and biomedical applications. For proteins, early studies focused on investigating SERS spectra of individual proteins, and the successful design of nanoparticle probes has promoted great progress of SERS-based immunoassays. In this review we outline the development of SERS-based methods for proteins with particular focus on our proposed protein-mediated SERS-active substrates and their applications in label-free and Raman dye-labeled protein detection.

In this paper, a series of silver-deposited TiO2 (Ag−TiO2) nanoparticles (NPs) with a varying content of Ag were prepared by a photoreduction method and were attempted to serve as SERS-active substrates for the first time. SERS signals of 4-MBA molecules adsorbed on Ag−TiO2 NPs were further enhanced considerably relative to those enhancements on pure TiO2 NPs. The surface-deposited Ag on TiO2 can inject additional electrons into molecules adsorbed on the TiO2 surface through the conduction band of TiO2 NPs because of plasmon resonance absorption of Ag under incident visible laser, besides the intrinsic TiO2-to-molecule charge-transfer (CT) contribution. The two contributions mentioned are responsible for the whole SERS intensity of the molecules adsorbed on Ag−TiO2 NPs. This work is valuable in developing nanosized TiO2 used as a promising, nontoxic and biologically compatible SERS-active substrate as well as in studying the CT mechanism between Ag and TiO2 for potential photoelectrochemical applications.

Surface-enhanced Raman scattering (SERS) of 4,4′-azopyridine (AZPY) on silver foil substrate was measured under 1064 nm excitation lines. Density-functional theory (DFT) methods were used to calculate the structure and vibrational spectra of models such as Ag–AZPY, Ag4–AZPY and Ag6–AZPY complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of AZPY were identified on the ground of analog computation of potential energy distribution. The calculated spectra of Ag4–AZPY and Ag6–AZPY models were much approximated to the experimental results than that of Ag–AZPY model. The DFT results showed that the angles between two pyridyl rings keep 0° from AZPY to Ag–AZPY, Ag4–AZPY and Ag6–AZPY model. The energy gaps between the HOMO and LUMO changed from 363 to 1140 nm for AZPY-Ag complexes according to the DFT results. An conclusion was conceived that chemical enhancement mechanism may play an important role in the SERS of AZPY on silver substrate.

In this paper, a simple method to fabricate a three-dimensional (3D) nanostructure decorated with Ag nanoparticles for surface-enhanced Raman scattering (SERS) is demonstrated. Highly ordered porous anodic aluminum oxide (AAO) templates were employed to construct these compound nanostructures. First, the AAO templates were fabricated using a two-step anodization approach. Second, an alternating current (AC) electrochemical deposition was used to fill AAO templates with Ag nanoparticles. Taking 4-mercaptopyridine (4-MPy) as the probing molecule, high-quality SERS spectra were observed. The UV−vis mirror reflection spectra were measured to investigate the surface plasma resonance (SPR) absorbance. An interesting phenomenon of SPR-affected thin film interference was observed. SERS mapping was performed to characterize the homogeneity of as-prepared substrates. Good homogeneity and stability make these substrates good candidates for SERS spectroscopy.

The assembly of metal/semiconductor/molecule was fabricated with gold nanoparticles, ZnO, and 4-aminothiophenol (PATP) molecules through the layer-by-layer self-assembly method. Raman spectra of PATP adsorbed on Au/ZnO were considerably enhanced relative to those observed on Au particles alone. By examining the relative enhancement of the nontotally symmetric (b2) modes, this effect is shown to arise from a charge transfer (CT) contribution from the metal to molecule.

Silver oxide film was electrodeposited onto the indium tin oxide (ITO) glass. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine the structure. The argentic oxide was identified to be AgO. Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) were obtained on the AgO film after the 514.5-nm laser activation. Ag nanoparticles produced from the photo-active decomposition of AgO was the substrate responsible for the enhancement of Raman signal of 4-mercaptobenzoic acid. The evolution of 4-MBA SERS spectra with time was observed due to development of Ag nanoparticles against the activation time. SERS spectra of 4-MBA, p-aminothiophenol (PATP) and benzoic acid (BA) were analyzed to determine the adsorption and orientation behaviors of above molecules. Both the electromagnetic and chemical enhancement mechanisms were proposed to be responsible for the observed enhancement of Raman signals. The enhancement factor for 4-mercaptopyridine (4-MPY) was estimated on the order of magnitude of 103.

The structure and vibrational frequencies of the chiral anti-ferroelectric liquid crystal molecule (AFLC) (1-methylheptyl)4-(4′-decyloxy-4-biphenyl) oxymethylene benzoate (MHDBOB) have been investigated by using the density functional theory (DFT) with the B3LYP/6-31G(d,p) level. The observed vibrational spectra have been resolved and assigned in detail by comparison to the computed values. The results indicate that the computed and observed spectra are in good agreement with each other. The molecular stable structure obtained by using DFT theory, shows that the two hydrocarbon chains are all-trans zigzag conformer and nearly perpendicular to each other. The orientation of the mesogen part and the hydrocarbon chains for MHDBOB at 109 °C was investigated by employing the polarization-angle-dependent infrared spectra in the electric field induced and the two-dimensional (2D) correlation spectroscopy. To combine the experimental with theoretical results, it can be concluded that the azimuth of the achiral and chiral chain is opposite to each other, the orientation of the achiral chain is in the same direction as the mesogen core and that of the chiral chain is perpendicular to the mesogen part. Their axes are parallel and perpendicular to the average molecular long axis, respectively. Two CH2 chains (achiral and chiral) are both a probable all-trans zigzag conformer. These results are quite new and different from the previous results, which show that the hydrocarbon chains of AFLC and/or ferroelectric liquid crystals (FLC) are in disorder.

It is important to monitor quality of tobacco during the production of cigarette. Therefore, in order to scientifically control the tobacco raw material and guarantee the cigarette quality, fast and accurate determination routine chemical of constituents of tobacco, including the total sugar, reducing sugar, Nicotine, the total nitrogen and so on, is needed. In this study, 50 samples of tobacco from different cultivation areas were surveyed by near-infrared (NIR) spectroscopy, and the spectral differences provided enough quantitative analysis information for the tobacco. Partial least squares regression (PLSR), artificial neural network (ANN), and support vector machine (SVM), were applied. The quantitative analysis models of 50 tobacco samples were studied comparatively in this experiment using PLSR, ANN, radial basis function (RBF) SVM regression, and the parameters of the models were also discussed. The spectrum variables of 50 samples had been compressed through the wavelet transformation technology before the models were established. The best experimental results were obtained using the (RBF) SVM regression with γ = 1.5, 1.3, 0.9, and 0.1, separately corresponds to total sugar, reducing sugar, Nicotine, and total nitrogen, respectively. Finally, compared with the back propagation (BP-ANN) and PLSR approach, SVM algorithm showed its excellent generalization for quantitative analysis results, while the number of samples for establishing the model is smaller. The overall results show that NIR spectroscopy combined with SVM can be efficiently utilized for rapid and accurate analysis of routine chemical compositions in tobacco. Simultaneously, the research can serve as the technical support and the foundation of quantitative analysis of other NIR applications.

A general method for the generation of two-dimensional (2D) ordered silver nanoparticles (av 45 nm) ring array has been demonstrated via controllable self-assembly. The selective self-assembly is conducted on the edges of a gold coated polyelectrolyte film. This film is fabricated using the monolayer polystyrene (PS) spheres (av 600 nm) on a substrate as template, followed by depositing a positively charged polyelectrolyte and gold colloids (av 17 nm) via the layer-by-layer (LbL) self-assembly technique, and finally by eliminating the PS monolayer. This gold coated polyelectrolyte film has a regular pattern of sharp edges, and those edges are composed of abundant polyelectrolyte. This heterogeneous surface is easily prepared and universal for site-selective absorption of nanoparticles (silver nanoparticles in this paper, av 45 nm). This surface-guided self-assembly is powerful for fabricating micro/nanostructures on the edges of prepatterns. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the products.

In this paper, triangular and hexagonal silver nanoplates were prepared on the surface of quartz substrate using photoreduction of silver ions in the presence of silver seeds. The obtained silver nanoplates were characterized by atomic force microscopy and UV–vis spectroscopy. It was found that the silver seeds played an important role in the formation of triangular and hexagonal silver nanoplates. By varying the irradiation time, nanoplates with different sizes and shapes could be obtained. The growth mechanism for triangular and hexagonal nanoplates prepared on quartz substrate was discussed.

Surface-enhanced Raman scattering from molecules adsorbed on TiO2 nanoparticles has been observed. This is attributed to the dominant contribution of the TiO2-to-molecule charge-transfer mechanism. The charge-transfer process is largely dependent on the intrinsic nature of the adsorbed molecules and the surface properties of the semiconductor. Both the stronger electron attracting ability of groups para- to the mercapto group bonded with TiO2 surface and the plentiful surface states of TiO2 nanoparticles are favorable to TiO2-to-molecule charge-transfer and SERS for molecules adsorbed on TiO2.

In this article, we report a simple wet-chemical method to prepare silver microflowers and large spherical particles. The formation of the two different microstructures of silver is based on the reduction of AgNO3 by para-phenylenediamine in aqueous medium at room temperature. The controlling of the silver microstructures can be achieved only by adjusting the concentration of the reactants. It is found that the two different silver microstructures display opposite wetting properties. Large spherical silver particles exhibit superhydrophilic properties with a contact angle (CA) of close to 0°, microflower-like silver particles exhibit highly hydrophobic properties with CA about 132°. X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV–vis spectra are used to characterize the chemical structure of the obtained products.Large spherical and microflower-like silver particles were obtained by simple wet-chemical method. It was found large spherical silver particles exhibit superhydrophilic properties with a contact angle (CA) of close to 0°, microflower-like silver particles exhibit highly hydrophobic properties with CA about 132°.

Reported here is the study on the structure of Langmuir–Blodgett (LB) films of double-armed dibenzo-18-crown-6 contain biphenyl (DACE) which are newly synthesized and mixed with stearic acid (SA). In addition, the miscibility of the two compounds was also tested by the measurement of the surface pressure–area (π–Aπ–A) isotherms of DACE and DACE/SA with various proportions. It is noted that there is no phase segregation in the mixed monolayer film of DACE/SA. Upon calculation of the excess surface area, it is concluded that the SA molecule can enter into the crown ether ring, while an 18C6 ring can host a maximum of one SA molecule. The difference of spectroscopic properties of DACE in LB films and bulk solution has been investigated by ultraviolet–visible (UV–vis) and Fourier-transfer infrared (FTIR) measurements. Molecules of DACE exist in the mixed LB films as monomers in contrast to those in the concentrated solution as aggregates. The hydrocarbon chains in DACE and DACE/SA LB films are tilted to the normal of the substrate surface, but perpendicular to the dipole moment of CO. Both CO bonds in the phenyl ethers and carboxylic ester of DACE, and the long axes of phenyl rings are aligned nearly perpendicular to the substrate surface. Infrared spectra of mixed LB films of DACE/SA present further evidence that the SA molecules enter into the crown ether rings of DACE.The structure of Langmuir–Blodgett films of DACE mixed with SA was studied. The amount of SA molecules located within crown ether rings increases with the molar ratio of SA.

The molecular geometry and vibrational frequencies of trans-1,2-bis(4-pyridyl)-ethylene (t-BPE) in the ground state were calculated using density functional theory (DFT) methods with 6–31++G(2d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes nonlocal three-parameter exchange and correlation functional and Lee–Yang–Parr correlation functional (B3LYP). Fourier transform Infrared (FTIR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of t-BPE on the silver foil substrate were recorded. All FTIR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(2d,p) method. The vibrational frequencies obtained by DFT(B3LYP) are in good agreement with observed results. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of t-BPE on the silver foil substrate surface.

We obtained the high-quality Raman spectra of 4-mercaptopyridine (4-Mpy) adsorbed on CdS microclusters. The Raman signals were enhanced relative to the same molecules in solution. We compared the Raman spectra of 4-Mpy molecules adsorbed on CdS microclusters and Ag substrate. The difference of 4-Mpy molecules adsorbed on semiconductor and metal substrate was revealed. The results demonstrated that adsorbed species on semiconductor CdS can be detected by SERS spectroscopy.

The molecular geometry and vibrational frequencies of 4,4′-bipyridine (BPE) in the ground state were calculated using density functional theory (DFT) methods (B3LYP) with 6-31++G(d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes non-local three parameter exchange and correlation functional and Lee–Yang–Parr correlation functional. Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of BPE on the silver foil substrate have been recorded. All FT-IR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(d,p) method. The vibrational frequencies obtained by DFT(3LYP) are in good agreement with observed results. The NIR-SERS of BPE excited by 1064 nm laser line is little difference with that excited by visible laser line. This phenomenon is result to the increase of the contribution of CHEM enhancement effect. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of BPE on the silver foil substrate surface. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPE, and to the dihedral angle were concluded.

The molecular geometry and vibrational frequencies of 4,4′-azopyridine (AZPY) were calculated using the density functional theory (DFT) methods. Fourier transform infrared (FTIR), fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of AZPY on the silver foil substrate have been recorded. On the basis of the computed results, FTIR, FT-Raman and NIR-SERS bands were assigned. It is shown that the calculated and observed frequencies are in good agreement with each other for FTIR and FT-Raman spectra. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of AZPY on the silver foil substrate surface.

A substrate for surface-enhanced Raman scattering (SERS) has been developed. Based on the surface-catalyzed reduction of Ag+ by citrate on the silver nanoparticles surface under light irradiation, small silver seeds on a quartz slide can be enlarged. The optical properties and characteristics of the silver films have been investigated by ultraviolet-visible spectroscopy, scan electron microscope and atomic force microscopy (AFM). The results indicate that the particle size and shape are different at different reduction time. At the first 3 h, some triangular and hexagonal nanoparticles formed; with the reduction proceeding, the shape of the silver particles became irregular and the size became larger. The silver films obtained are very suitable as SERS active substrate. The relationship between SERS intensity and the reduction time has been investigated for 1,4-bis[2-(4-pyridyl)ethenyl]-benzene molecule adsorbed on the silver film. The SERS intensity reached a maximum at 8 h reduction. The AFM measurements indicate that roughness features with an average size of 100 nm are present on the surface, which yielded the strongest SERS signal. Pyridine was used as a probe molecule to investigate the enhancement factor (EF) of the silver films. According to the formalism of Tian and co-workers, the EF of the silver films is estimated to be 3.4 × 105. The silver film that can remain active for more than 50 days would seem to be suitable for various analytical applications.

Langmuir films of a side-chain ferroelectric liquid crystalline polysiloxane of 4-(10-undecen-1-oyloxy)-4′-(s)-2-methybutoxybiphonyl (P(III)) and its monomer 4-(10-undecen-1-oyloxy)-4′-(s)-2-methybutoxybiphonyl (M(III)) were transferred successfully at surface pressures of 11 and 8 mN m− 1 onto solid substrates, respectively. The π–A isotherms of P(III) and M(III) have shown that they have different properties on the air/water interface. The monolayer film on the air/water interface of M(III) shows a clear orientation transition while that of P(III) does not. Infrared (IR) transmission and reflection–absorption spectra of Langmuir–Blodgett (LB) films of P(III) and M(III) indicate that the long axis of the chromophoric part of P(III) is almost perpendicular to the substrate surface while that of M(III) is tilted with respect to the substrate normal. The alkyl parts of both P(III) and M(III) are tilted considerably from the substrate normal. It also indicated from the IR spectra of the LB films that the liquid crystalline molecules of P(III) and M(III) are highly ordered and oriented in the monolayer and multilayer LB films. Temperature-dependent IR experiments show that the alkyl chains have anisotropic property in both P(III) and M(III). This may be due to the influence from the chiral groups of the molecules rather than from the substrates, and the side chains of P(III) play an important role in enhancing the anisotropic property.

We describe a simple method to synthesize triangular silver nanoparticles by photoreducing the silver ions by citrate. A noteworthy difference of the present method as compared with the previous photo-induced methods is that good shape control over the nanoparticles can be realized in the absence of soft templates or polymer directing agents. The formation process of the silver nanoparticles was investigated by UV–vis spectroscopy and transmission electron microscopy (TEM). It was found that the concentration of reactant plays important role in the morphology control of produced silver nanoparticles. As one of the applications of these nanoparticles, they were used as surface-enhanced Raman scattering substrates and 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BVPP) was used as a Raman probe to evaluate the enhancement ability of the triangular silver nanoparticles.

We investigated the fabrication of self-assembled monolayers of 5,10,15,20-tetra-(p-chlorophenyl)-porphyrin metal hydroxyl compounds (MOH; M=Gd, Tb, Er, Lu) on amino-terminated silanized quartz surfaces using ultraviolet–visible (UV–vis) and X-ray photoelectron spectroscopy (XPS). The orientation of MOH molecules in the films, the kinetics of the adsorption of MOH from a chloroform solution on an amino-terminated quartz substrate, and the stability of the metalloporphyrin films under natural light, UV light, and acidic and basic conditions were studied by UV–vis spectroscopy. The results indicate that the central metal is crucial in the formation of self-assembled porphyrin films and that the stability of the MOH SAMs also depends on the central metal. Under natural and UV light irradiation conditions, the stability of the MOH SAMs depends on the strength of the MN bonds between the central metal and pyrrole nitrogens or between the central metal and the axial ligand. In the acidic conditions, the UV–vis spectra of the MOH SAMs show decreases in the absorbance and blue shifts. These spectral changes lead us to suggest that the four MN bonds between the central metal and the pyrrole nitrogens of the porphyrins are cleaved and the resulted porphyrins are protonated under acidic conditions. Thus, it is likely that the protonated porphyrins and MOH form π−π complexes with a parallel stacking of the macrocycles through the electrostatic attractive interaction in the SAMs, resulting in the blue shifts of the Soret bands of the SAMs.

A novel surface-enhanced Raman scattering (SERS)-based detection method for proteins attached to magnetic glass beads have been developed in this study. Silica coated magnetic nanoparticles (Fe3O4) are prepared and aldehyde-functionalized for capturing proteins in solutions by Schiff reaction, which is more biocompatible than previous magnetic nanoparticle and SERS-based detection methods. Target protein and molecule with a specially recognized feature are captured and separated by the magnetic nanoparticles, followed by SERS-based detection. Moreover, these biomagnetic glass beads and detection protocol are found to be useful in protein immunoassay and protein-ligand recognition even in a mixture system (two protein ligands). The proposed magnetic nanoparticles are biocompatible, manipulatable by an external magnetic force and highly sensitive after silver nanoparticles staining, which makes the protocol quite promising for high throughput protein assays.

We introduce a new approach that combines the self-assembly and seeding methods for silver plating of the proposed Fe2O3@Au-seed substrate. By adjusting the silver plating time between 0 and 360 s, the optical resonance of the substrates clearly varied from visible to near-infrared. To investigate the potential of these substrates for use in surface-enhanced Raman scattering (SERS) applications, SERS spectra of thiophenol (TP), 4-aminothiophenol (PATP) and rhodamine 6G (R6G) were evaluated at three excitation wavelengths (532, 633 and 785 nm). Analysis of the SERS spectra clearly demonstrated that the SERS effect depends strongly on the nature of the substrate surface and the nature and electronic resonance of the probe molecules. The calculated enhancement factor (EF) of TP adsorbed onto an Fe2O3@Ag substrate was ∼107 following three laser excitations. More importantly, these SERS substrates can successfully be utilized for the detection of small molecules at very low concentrations (∼100 pg mL−1).

Based on bovine serum albumin (BSA)-modified Au NPs, a simple and cost-effective approach was proposed to fabricate an anti-aggregated Au NP sensing platform for the detection of metal ions. It exhibits excellent stability even under highly ionic conditions due to its electrostatic stabilization, as well as the steric stabilization.

We have fabricated a charge-transfer system using the self-assembly method. We find the nontotally symmetric b2 modes are selectively enhanced in SERS spectroscopy and represent a laser wavelength-dependent property.

Semiconductor materials have been successfully used as surface-enhanced Raman scattering (SERS)-active substrates, providing SERS technology with a high flexibility for application in a diverse range of fields. Here, we employ a dye-sensitized semiconductor system combined with semiconductor-enhanced Raman spectroscopy to detect metal ions, using an approach based on the “turn-off” SERS strategy that takes advantage of the intrinsic capacity of the semiconductor to catalyze the degradation of a Raman probe. Alizarin red S (ARS)-sensitized colloidal TiO2 nanoparticles (NPs) were selected as an example to show how semiconductor-enhanced Raman spectroscopy enables the determination of Cr(VI) in water. Firstly, we explored the SERS mechanism of ARS–TiO2 complexes and found that the strong electronic coupling between ARS and colloidal TiO2 NPs gives rise to the formation of a ligand-to-metal charge-transfer (LMCT) transition, providing a new electronic transition pathway for the Raman process. Secondly, colloidal TiO2 nanoparticles were used as active sites to induce the self-degradation of the Raman probe adsorbed on their surfaces in the presence of Cr(VI). Our data demonstrate the potential of ARS–TiO2 complexes as a SERS-active sensing platform for Cr(VI) in an aqueous solution. Remarkably, the method proposed in this contribution is relatively simple, without requiring complex pretreatment and complicated instruments, but provides high sensitivity and excellent selectivity in a high-throughput fashion. Finally, the ARS–TiO2 complexes are successfully applied to the detection of Cr(VI) in environmental samples. Thus, the present work provides a facile method for the detection of Cr(VI) in aqueous solutions and a viable application for semiconductor-enhanced Raman spectroscopy based on the chemical enhancement they contribute.

We investigated the influence of hydrogen bonds (H-bonds) on the intermolecular interactions of a system comprising p-aminothiophenol (PATP) and benzoic acid (BA) using surface-enhanced Raman scattering (SERS) for the first time. In this system, H-bonds form through intermolecular interactions between the –NH2 and –COOH groups and promote the charge-transfer (CT) transition from the Ag substrate to the adsorbed PATP molecules. Accordingly, the intensities of the non-totally symmetric vibrations (the b2-type bands) of PATP are influenced through the Herzberg–Teller contribution. This is clearly a BA concentration-dependent phenomenon. This behaviour can be attributed to an increase in the degree of conjugation of the system, which facilitates the CT process in the system with H-bonds. Furthermore, temperature-dependent SERS experiments and their two-dimensional (2D) correlation analysis confirmed that the formation of H-bonds facilitated the CT transition between the adsorbed molecules and substrate. The degree of CT was reduced by H-bond breakage that occurred with increasing temperature. An additional SERS experiment involving substituted BA molecules yielded similar conclusions.

In this paper, TiO2 nanoparticles (NPs) with different crystallinity served as SERS-active substrates for SERS detection of ciprofloxacin (CIP) drug molecules for the first time. CIP is close to the surface of the TiO2 substrate through the carboxyl group. The mutual SERS enhancement behaviors between CIP molecules and TiO2 NPs were discovered, which are attributed to the contribution of the TiO2-to-molecule charge-transfer mechanism. The crystallinity of TiO2 NPs, the pH value of adsorption solution and the adsorption time have significant influences on the interaction and the SERS behavior between CIP and TiO2. When the calcination temperature of TiO2 NPs is 450 °C, the pH value of adsorption solution is 6 and the adsorption time is 9 h, the CIP molecules on TiO2 NPs exhibit the largest SERS enhancement.

A nanoscale metal–molecule–semiconductor assembly (Ag/4-mercaptophenol/TiO2) has been fabricated over Au nanoparticle (NP) films as a model to study the interfacial charge transfer (CT) effects involved in Ag/MPH/TiO2. Due to the interaction between Au NPs and Ag NPs, some distinct differences occur in the SERS spectra. We also measured the SERS of Ag/MPH (4-mercaptophenol), Ag/MPH/TiO2, and Au/Ag/MPH/TiO2 assemblies at excitation wavelengths of 477, 514, 532, 633, and 785 nm. We found that the changes in the CT process, caused by the introduction of TiO2 and Au, can be reflected in SERS. Then in combination with other detection methods, we proposed a possible CT process involved in the Ag/MPH, Ag/MPH/TiO2, and Au/Ag/MPH/TiO2 assemblies. A Pt/Ag/MPH/TiO2 assembly was also constructed to verify our proposed CT mechanism. This work not only provides more details about CT between metal–molecule–semiconductor interfaces but also aids in constructing nanoscale models to study interfacial problems with the SERS technique.

Recently, surface-enhanced Raman scattering (SERS) research based on semiconductor TiO2 has received increasing attention. However, the practical application of TiO2 SERS-active substrates has been hampered due to their lower surface performance and detection sensitivity. Here we report a new strategy to enhance the SERS activity of TiO2 nanoparticles (NPs) by means of formation of a mesoporous framework via reductive calcination of polymer coated nanocrystals. The 4-mercaptobenzoic acid (4-MBA) probe molecules on the mesoporous TiO2 substrate exhibit larger SERS enhancement compared with that on the ordinary TiO2 NP substrate; this considerable SERS enhancement mainly stems from the contributions of rich surface active sites of mesoporous TiO2, which can provide more effective adsorption sites for the molecules and promote the charge transfer between the substrate and the adsorbed molecule, and thus remarkably enhance SERS signals. On the mesoporous TiO2, a detection limit of 4-MBA as low as 1 × 10−8 mol L−1 can be achieved, which is the highest sensitivity among those reported for semiconducting substrates and even comparable with those of noble metal substrates. And, it is proved that the mesoporous TiO2 substrates are characterized by high stability and self-cleaning properties, and can be repeatedly used in SERS detection without losing their activity.

In this study, Au deposited TiO2 (Au–TiO2) nanocomposites were prepared by photocatalytic reduction of Au species (HAuCl4) on TiO2 nanoparticles (NPs) and served as efficient SERS-active substrates. The observed considerable SERS enhancement of 4-MBA molecules on Au–TiO2 NPs compared with that on pure TiO2 NPs can be attributed to the synergistic contribution of semiconductors and metals by the charge-transfer mechanism from the substrate to the molecule. In the preparation of Au–TiO2 nanocomposites, the photo-reduction time and concentration of HAuCl4 have an important influence on the SERS enhancement effect of the composite substrate. Moreover, for the recycle of substrates, the photocatalytic experiment of Au–TiO2 nanocomposites was carried out. The results show that these nanocomposite substrates are characterized by a self-cleaning property and can be repeatedly used in SERS experiments without losing their activity and with high stability.