Efficient immobilization of silver nanoparticles on metal substrates through various thiol molecules to utilize a gap mode in surface enhanced Raman scattering

To utilize a gap mode in surface enhanced Raman scattering, we elucidated the interaction between adsorbed species and Ag nanoparticles (AgNPs). Various thiol molecules such as normal alkanethiols, thiols with a phenyl, cyclohexane or naphthalene ring on Ag films immobilized AgNPs through van der Waals force, and electrostatic interaction. Immobilized AgNPs provided enormous Raman enhancement by a factor of 10⁷–10¹⁰ for thiol molecules at a nanogap, in consistent with that anticipated by finite difference time domain calculations. Only alkanethiols with a tert-methyl group and those with a carboxylic group did not immobilize any AgNPs probably owing to steric hindrance. A gap mode is relevant for a variety of metals even with large damping like Pt and Fe, indicating a crucial role of electric multipoles in AgNPs generated by a localized surface plasmon and induced mirror images in metal substrates for markedly enhanced electric field at a nanogap.     

Adsorbed state of p-mercaptobenzoic acid on silver nanoparticles

To exploit a gap mode plasmon in flocculates of metal nanoparticles most efficiently, the interaction between adsorbed chemical species and metal nanostructures were adjusted. We successfully formed flocculates of Ag nanoparticles (AgNPs) using electrostatic interaction between dissociated p-mercaptobenzoic acid (PMBA), protonated p-aminothiophenol (PATP) and their counter ions (Mⁿ⁺, X^m−), as well as van der Waals force between neutral PMBA, PATP and AgNPs. Detailed adsorbed state of PMBA and PATP in addition to trapped counter ions was characterized using enormous SERS enhancement in a flocculation method.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.     

Contribution of charge-transfer mechanisms to surface-enhanced Raman scattering with near-IR excitation.

Surface-enhanced Raman scattering under near-IR excitation is investigated for p-aminothiophenol (PATP) molecules that are either adsorbed on an electrochemically roughened silver electrode or embedded in an Au/PATP/Ag molecular junction assembled on an indium-doped tin oxide electrode. The contribution from chemical enhancement can be amplified relative to the contribution from electromagnetic enhancement, because the energy of the near-IR excitation is far from the surface plasmon resonance of the nanosized metal particles. The energy required for the charge-transfer process for the Au/PATP/Ag molecular junction is much lower than that of the PATP molecules adsorbed on the electrochemically roughened silver electrode. Coadsorption of chloride ions on the metal nanoparticles may result in an alteration of the local Fermi level of the metal nanoparticles, thus leading to better energy matching between the energy level of the interconnecting PATP molecules and the Fermi level of the metal nanoparticles.     

Surface-enhanced resonance Raman scattering and background light emission coupled with plasmon of single Ag nanoaggregates

We investigated the optical properties of isolated single aggregates of Ag nanoparticles (Ag nanoaggregates) on which rhodamine 6G molecules were adsorbed to reveal experimentally a correlation among plasmon resonance Rayleigh scattering, surface-enhanced resonance Raman scattering (SERRS), and its background light emission. From the lack of excitation-laser energy dependence of background emission maxima we concluded that the background emission is luminescence, not Raman scattering. The polarization dependence of both SERRS and background emission was the same as that of the lowest-energy plasmon resonance maxima, which is associated with a longitudinal plasmon. From the common polarization dependence, we identified that the lowest-energy plasmon is coupled with both SERRS and background emission. In addition, we revealed that the lowest-energy plasmon with a higher quality factor (Q factor) yields larger SERRS and background emission intensity. Also, we identified that the Q factor dependence of the SERRS intensity was similar to that of the background emission intensity. This similarity directly supported us to demonstrate an enhancement of both SERRS and background emission by coupling with a common plasmon radiative mode.     

Photon-driven charge transfer and Herzberg-Teller vibronic coupling mechanism in surface-enhanced Raman scattering of p-aminothiophenol adsorbed on coinage metal surfaces: a density functional theory study

The chemical enhancement effects in surface-enhanced Raman scattering of p-aminothiophenol (PATP, it is also called p-mercaptoaniline or p-aminobenzenthiol) adsorbed on coinage metal surfaces with single thiol end or trapped into metal-molecule-metal junctions with both thiol and amino groups have been studied by density functional theory (DFT). We focus on the influence of photon-driven charge transfer (PDCT) and chemical bonding interaction (ground-state charge transfer) on the intensity enhancement and frequency shift in the surface Raman spectra of PATP. For comparison, the electronic structures and transitions of free PATP are studied first. The simulated pre-resonance UV Raman spectra illustrate that b(2) modes can be selectively enhanced via vibronic coupling. The fundamentals of all the b(2) modes in the frequency range of 1000 to 1650 cm(-1) are assigned in detail. For PATP adsorbed on coinage metals, the time-dependent-DFT calculations indicate that the low-lying CT excited state arises from the π bonding orbital of molecule to the antibonding s orbital of metallic clusters. Our results further show that the PDCT resonance-like Raman scattering mechanism enhances the totally symmetric vibrational modes and the NH(2) wagging vibration. Finally, the effect of chemical bonding interaction is also investigated. The amino group binding to metals gives a characteristic band of the NH(2) wagging mode with the large blueshift frequency and an intense Raman signal.     

Self-assembly of lambda-DNA networks/Ag nanoparticles: hybrid architecture and active-SERS substrate

In this article, highly rough and stable surface enhanced Raman scattering (SERS)-active substrates had been fabricated by a facile layer-by-layer technique. Unique lambda-DNA networks and CTAB capped silver nanoparticles (AgNP) were alternatively self-assembled on the charged mica surface until a desirable number of bilayers were reached. The as-prepared hybrid architectures were characterized by UV-vis spectroscopy, tapping mode atomic force microscopy (AFM) and confocal Raman microscopy, respectively. Linear increases of the maximum absorbance of DNA band with the number of bilayers present a common LBL assembly feature. The red-shift of surface plasmon of silver nanoparticles within the hybrid films was mainly due to the aggregation effect. With the increase of number of bilayers, the surface coverage of nanoparticles on the substrate became larger, as well as the rising of total amount of nanoparticles and the surface roughness of hybrid films. These rough metallic hybrid architectures could be utilized as SERS-active substrates. A significant enhanced Raman scattering effect of the adsorbed analytes, e.g., methylene blue (MB), on these hybrid films was easily exploited by the confocal Raman microscopy. The enhancement factor depended on the surface coverage of nanoparticles and number of bilayers of lambda-DNA/AgNP.     

Seeded-growth approach to fabrication of silver nanoparticle films on silicon for electrochemical ATR surface-enhanced IR absorption spectroscopy

Ag nanoparticle films (simplified as nanofilms hereafter) on Si for electrochemical ATR surface enhanced IR absorption spectroscopy (ATR-SEIRAS) have been successfully fabricated by using chemical deposition, which incorporates initial embedding of Ag seeds on the reflecting plane of an ATR Si prism and subsequent chemical plating of conductive and SEIRA-active Ag nanofilms. Two alternative methods for embedding initial Ag seeds have been developed: one is based on self-assembly of Ag colloids on an aminosilanized Si surface, whereas the other the reduction of Ag+ in a HF-containing solution. A modified silver-mirror reaction was employed for further growth of Ag seeds into Ag nanofilm electrodes with a theoretically average thickness of 40-50 nm. Both Ag seeds and as-deposited Ag nanofilms display island structure morphologies facilitating SEIRA, as revealed by AFM imaging. The cyclic voltammetric feature of the as-prepared Ag nanofilm electrodes is close to that of a polycrystalline bulk Ag electrode. With thiocyanate as a surface probe, enhancement factors of ca. 50-80 were estimated for the as-deposited Ag nanofilms as compared to a mechanically polished Ag electrode in the conventional IRAS after reasonable calibration of surface roughness factor, incident angles, surface coverage, and polarization states. As a preliminary example for extended application, the pyridine adsorption configuration at an as-deposited Ag electrode was re-examined by ATR-SEIRAS. The results revealed that pyridine molecules are bound via N end to the Ag electrode with its ring plane perpendicular or slightly tilted to the local surface without rotating its C2 axis about the surface normal, consistent with the conclusion drawn by SERS in the literature.     

The adsorption of methamphetamine on Ag nanoparticles dispersed in agarose gel – Detection of methamphetamine in fingerprints by SERS

Surface-enhanced Raman scattering (SERS) has been used to investigate the adsorption of methamphetamine hydrochloride (MA) on AgNPs surfaces characterized by the dispersion of AgNPs on agarose gel (AgNPs/Agar). The AgNPs/Agar was characterized by transmission electron microscopy (TEM) as being formed by AgNPs with a mean diameter of 13.5 nm. The AgNPs/Agar films presented a surface plasmon resonance absorption band centered at 421 nm. SERS spectra, excited at 632.8 nm, of MA adsorbed onto AgNPs/Agar films were recorded for MA concentrations down to 1.0 × 10⁻⁵ mol L^-1. The results have also shown that MA adsorbs on the Ag surface forming ionic pairs with adsorbed chloride following a Frumkin adsorption isotherm with a ΔG_ads of −24 kJ mol^-1 and a g parameter characteristic of attractive lateral interaction. The AgNPs/Agar SERS substrate was further evaluated for MA detection on latent fingerprints (LFP). The AgNPs/Agar films prove to be a suitable substrate for recording fingerprints contaminated with MA making possible the detection of ca. 190 μg of MA, before and after LFP development. The SERS signal of MA adsorbed onto AgNPs/Agar films remained stable for at least 180 days.     

pH Dependent plasmon-driven surface-catalysis reactions of p,p′-dimercaptoazobenzene produced from para-aminothiophenol and 4-nitrobenzenethiol

In this paper,we studied the pH dependent plasmon-driven surface-catalysis(PDSC) reactions of p,p’-dimercaptoazobenzene(DMAB) produced from para-aminothiophenol(PATP) and 4-nitrobenzenethiol(4NBT) both theoretically and experimentally.The surface enhanced Raman spectrum(SERS) of DMAB produced from PATP and 4NBT on Ag films in solutions with various pH values has been measured.The simulation and experimental results indicated that the pH dependence of PATP appeared in acidic environment and came from the amino group NH2.Furthermore,the ratio of intensity of Raman peak caused by PATP and DMAB indicated that this acidic sensor had higher pH sensitivity when it was excited by photons of higher energy.     

Novel Silver/Poly(vinyl alcohol) Nanocomposites for Surface‐Enhanced Raman Scattering‐Active Substrates

Summary: Surface‐enhanced Raman scattering (SERS)‐active substrates with high enhancement were prepared by an in situ reduction method. Novel silver/poly(vinyl alcohol) (PVA) nanocomposite films were obtained, in which the silver nitrate, poly(γ‐glutamic acid) (PGA), and PVA acted as precursor, stabilizer, and polyol reducant, respectively. The UV‐visible spectra of the as‐fabricated films showed that the surface plasmon resonance (SPR) absorption band was narrow and of a stronger intensity, which indicates that the Ag nanoparticle size distribution on the substrate was highly uniform. This finding was further confirmed by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and field‐emission scanning electron microscope (FE‐SEM) measurements. It was found that a PGA‐stabilized PVA nanocomposite film revealed the presence of well‐dispersed spherical silver nanoparticles with an average diameter of 90 nm. The new substrate presents high SERS enhancement and the enhanced factor is estimated to be 10⁶ for the detection of benzoic acid.

The Raman scattering enhancement factor for the Raman spectra of benzoic acid on the various nanocomposite films.     

Effect of Ag and Au nanoparticles on the SERS of 4-aminobenzenethiol assembled on powdered copper

Raman scattering measurements were conducted for 4-aminobenzenethiol (4-ABT) assembled on powdered copper substrates. Initially, very weak Raman peaks were detected, but upon attaching Ag nanoparticles probably via NH2 groups onto 4-ABT/Cu, distinct Raman spectra were observed. Considering the fact that no Raman peak was identified when Ag nanoparticles were adsorbed on 4-aminophenyl-derivatized silane monolayers assembled on silica powders, the Raman spectra observed for Ag@4-ABT/Cu should be surface-enhanced Raman scattering (SERS) spectra, occurring by an electromagnetic coupling of the localized surface plasmon of Ag nanoparticles with the surface plasmon polariton of Cu powders. The extra enhancement factor attainable by the attachment of a single Ag nanoparticle is estimated to be as large as 1.4 x 10(5) in the case when 632.8-nm radiation is used as the excitation source. When Au nanoparticles were attached onto 4-ABT/Cu, at least an order of magnitude weaker Raman spectra were obtained at all excitation wavelengths, however, indicating that the Au-to-Cu coupling should be far less effective than the Ag-to-Cu coupling for the induction of SERS.     

Surface-enhanced raman scattering on dendrimer/metallic nanoparticle layer-by-layer film substrates

In this paper, the fabrication, characterization, and application of unique layer-by-layer (LBL) films of dendrimers and metallic nanoparticles is reported. Silver nanoparticles (d = approximately 20 nm) are produced in solution by sodium citrate reduction and incorporated into thin films with generation 1 and 5 DAB-Am dendrimers (polypropylenimine dendrimers with amino surface groups) by the LBL technique. The resulting nanocomposite films are characterized by UV-visible surface plasmon absorption and atomic force microscopy (AFM) measurements, and employed as substrates for surface-enhanced Raman scattering (SERS) of 2-naphthalenethiol. Through variation of the molecular size (dendrimer generation) and concentration of the cross-linker used, as well as the number of layers produced, the optical properties of several different possible architectures are studied. In the films, Ag nanoparticles are shown to be effectively immobilized and stabilized with increased control over their spacing and aggregation. Moreover, the films are shown to be excellent substrates for SERS measurements, demonstrating significant enhancement capability. As expected, large electromagnetic enhancement of Raman scattering signals is found to be strongly dependent on interparticle coupling between neighboring metallic nanoparticles. Finally, the possibility of detecting SERS signals from architectures with intervening layers between the metal nanoparticles and analyte molecules is explored. It is shown that although there are decreases in intensity with increasing number of intervening layers (as is expected from the distance dependence of SERS), electromagnetic enhancement is still able to function at these distances, thus offering the possibility of developing sensors with external layers that are chemically selective for specific analytes.     

Tailoring Size and Coverage Density of Silver Nanoparticles on Monodispersed Polymer Spheres as Highly Sensitive SERS Substrates

Silver nanoparticles (AgNPs) were deposited onto the monodispersed carboxylic polystyrene (CPS) spheres by an improved in situ reduction method. The size and coverage density of the AgNPs on the surface of CPS spheres could be easily tailored by tuning the concentrations of carboxylic functional groups and silver precursor. The morphologies and structures of the resulting CPS/Ag hybrid particles were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), UV‐Vis‐NIR spectrometer and X‐ray photoelectron spectroscopy (XPS), etc. The surface enhanced Raman scattering (SERS) performances of the resulting uniform CPS/Ag hybrid particles were investigated using 4‐aminobenzenethiol (4‐ABT) as the probe molecule. The optimized CPS/Ag hybrid particles show high enhancement factor (EF) of 2.71×10⁷, low limit of detection (LOD) of 10^?10 m and good reproducibility with relative standard deviation (RSD) of 9.64 %. The good SERS improvement properties demonstrate these hybrid particles could be employed as simple and effective substrates in the SERS spectroscopy.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag/Au nanoparticle and macroscopically smooth Au substrate

Raman scattering measurements were conducted for a 4-aminobenzenethiol (4-ABT) monolayer assembled on a macroscopically smooth Au substrate. Although no peak was detected at the beginning, Raman peaks were distinctly observed by attaching Ag or Au nanoparticles onto the 4-ABT monolayer (Ag(Au)@4-ABT/Au(flat)). Considering the fact that no Raman signal is observed when Ag (Au) nanoparticles are adsorbed on a (4-aminophenyl)silane monolayer assembled on a silicon wafer, the Raman spectrum observed for Ag(Au)@4-ABT/Au(flat) must be a surface-enhanced Raman scattering (SERS) spectrum, derived from the electromagnetic coupling of the localized surface plasmon of Ag (Au) nanoparticles with the surface plasmon polariton of the underneath Au metal. The electromagnetic coupling responsible for SERS appeared to be governed more by the bulk Au substrate than the sparsely distributed Ag or Au nanoparticles. The chemical enhancement appeared on the other hand to be derived more from the formation of Au-S bonds than any charge-transfer interaction between the protonated amine group and the Au or Ag nanoparticles. The enhancement factors derived from the attachment of a single Ag or Au nanoparticle onto 4-ABT on Au were estimated to be as large as 8.3 x 10(5) and 5.0 x 10(5), respectively, (for the ring 3 band (b(2)) near 1390 cm(-1)) in which a factor of approximately 10(2) was presumed to be due to the chemical effect, with the remaining contributed by the electromagnetic effect.     

Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering

Sandwich surface-enhanced Raman scattering (SERS) substrates (3S) utilizing coupling between continuous metal films and plasmonic particles were fabricated using silver mirrors, electrochemically roughened films, and various sizes of silver nanoparticles. The effect of excitation wavelength and nanoparticle size on SERS spectra of poly(vinylpyridine), selected as a model compound, was studied to determine the optimum conditions for the strongest SERS signal. The Raman enhancement resulted from the plasmon coupling of silver nanoparticles to the underlying continuous film as well as the lateral plasmon coupling between the silver nanoparticles. The formation of the charge transfer complex was also observed. The 3S configuration was used to obtain SERS spectra of dipicolinic acid (DPA), a chemical signature for Bacillus anthracis.     

Spectral variations in background light emission of surface-enhanced resonance hyper Raman scattering coupled with plasma resonance of individual silver nanoaggregates

We demonstrate the origin of spectral variations in background light emission of surface enhanced resonance hyper Raman scattering (SERHRS) from single Ag nanoaggregates. Ag nanoaggregate-by-nanoaggregate variations in background light emission spectra are related to plasma (plasmon) resonance spectra. Temporal variations in background light emission spectra with temporal blueshifts in plasma resonance spectra are also observed under continuous laser excitation. Both types of the variations in background light emission are reproduced by multiplying background light emission spectra measured from a Ag microaggregate by Lorentz function spectra derived from plasma resonance spectra. The reproduction reveals that second electromagnetic (EM) enhancement by plasma resonance is the origin of the variations. Additionally, spectral variations in background light emission of SERHRS are similar to that of surface enhanced resonance Raman scattering (SERRS). The similarity indicates that both types of background light emission commonly obtain second EM enhancement from identical plasma resonance.     

Mechanistic study of photomediated triangular silver nanoprism growth

This article presents a mechanistic study of the photomediated growth of silver nanoprisms. The data show that the photochemical process is driven by silver redox cycles involving reduction of silver cations by citrate on the silver particle surface and oxidative dissolution of small silver particles by O2. Bis(p-sulfonatophenyl)phenylphosphine increases the solubility of the Ag(+) by complexing it and acts as a buffer to keep the concentration of Ag(+) at 20 microM. The silver particles serve as photocatalysts and, under plasmon excitation, facilitate Ag(+) reduction by citrate. Higher Ag(+) concentrations favor a competitive thermal process, which results in increased prism thickness.     

Enhanced Raman scattering and photocatalytic activity of Ag/ZnO heterojunction nanocrystals

In this work, we study the enhancement of Raman signals and photocatalytic activity of Ag/ZnO heterojunctions with an Ag content of 1 at.%, which were synthesized by photochemical deposition of Ag nanoparticles onto pre-synthesized ZnO nanorods. A strong interaction between Ag and ZnO nanocrystals were evidenced by XPS and UV-vis spectroscopy. The binding energy of Ag nanoparticles shifts toward lower energy compared to that of pure Ag nanoparticles, revealing that electrons transfer from Ag to the ZnO nanocrystals. The red shift of the plasmon absorption peak of Ag nanoparticles in Ag/ZnO heterojunctions further confirms the strong interaction between the two components. This strong interaction, arising from the coupling between Ag and ZnO nanocrystals, is responsible for the enhancement of Raman signals and photocatalytic activity of the Ag/ZnO heterojunctions.     

复合SERS基底的构建及性质

通过自组装方法以对巯基苯胺(PATP)为偶联分子, 在石英基片上构筑了多种形貌的银钠米粒子单层结构和三明治结构. 研究了组装膜在不同激发线下表面增强拉曼散射(SERS)的增强差异. 研究结果表明, 单层基底和三明治基底中偶联分子的SERS信号因银纳米粒子间的电磁场耦合而显著增强, 且在三明治结构中增强更加明显. 对复合SERS基底增强因子进行计算可知, 复合SERS基底的表面等离子体共振(SPR)峰与激发线的匹配程度越好, 其增强因子越大. 在三明治结构中更易发生PATP分子转变为对巯基偶氮苯(DMAB)分子的激光诱导催化偶联反应. 另外, 该激光诱导催化偶联反应与激发波长密切相关.