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.     

Electrodeposition of dendritic gold/silver nanaoparticles on disposable screen‐printed carbon electrode and its application of 4‐mercaptopyridine in in situ electrochemical surface‐enhanced Raman scattering

Column electrodes pretreated through oxidation–reduction cycles were traditionally used in electrochemical surface‐enhanced Raman scattering (SERS). In this study, a disposable screen‐printed carbon electrode was introduced into in situ electrochemical SERS through the electrodeposition of dendritic gold/silver nanoparticles (Au/AgNPs) onto the surface of the carbon working electrode to induce the SERS enhancement effect on the electrode. Scanning electron microscopy images showed that dendritic Au/AgNPs nanostructures could be fabricated under appropriate electrodeposition conditions and could present a minimum SERS factor of 4.25 × 10⁵. Furthermore, the absorbed behavior of 4‐mercaptopyridine was investigated under different potentials. The adsorption configuration was inferred to transform from ‘vertical’ to ‘lying‐flat’. The proposed new electrode combined with a portable Raman spectrometer could be useful in the identifying products or intermediates during electrochemical synthesis or electrochemical catalysis in in situ electrochemical SERS. Copyright © 2016 John Wiley & Sons, Ltd.     

Silver Nanoparticle Thin Films with Nanocavities for Surface‐Enhanced Raman Scattering

The formation of nanometer‐sized gaps between silver nanoparticles is critically important for optimal enhancement in surface‐enhanced Raman scattering (SERS). A simple approach is developed to generate nanometer‐sized cavities in a silver nanoparticle thin film for use as a SERS substrate with extremely high enhancement. In this method, a submicroliter volume of concentrated silver colloidal suspension stabilized with cetyltrimethylammonium bromide (CTAB) is spotted on hydrophobic glass surfaces prepared by the exposure of the glass to dichloromethysilane vapors. The use of a hydrophobic surface helps the formation of a more uniform silver nanoparticle thin film, and CTAB acts as a molecular spacer to keep the silver nanoparticles at a distance. A series of CTAB concentrations is investigated to optimize the interparticle distance and aggregation status. The silver nanoparticle thin films prepared on regular and hydrophobic surfaces are compared. Rhodamine 6G is used as a probe to characterize the thin films as SERS substrates. SERS enhancement without the contribution of the resonance of the thin film prepared on the hydrophobic surface is calculated as 2×10⁷ for rhodamine 6G, which is about one order of magnitude greater than that of the silver nanoparticle aggregates prepared with CTAB on regular glass surfaces and two orders of magnitude greater than that of the silver nanoparticle aggregates prepared without CTAB on regular glass surfaces. A hydrophobic surface and the presence of CTAB have an increased effect on the charge‐transfer component of the SERS enhancement mechanism. The limit of detection for rhodamine 6G is estimated as 1.0×10^?8 M . Scanning electron microscopy and atomic force microscopy are used for the characterization of the prepared substrate.     

Local and Remote Charge‐Transfer‐Enhanced Raman Scattering on One‐Dimensional Transition‐Metal Oxides

The one‐dimensional (1D) transition‐metal oxide MoO₃ belt is synthesized and characterized with X‐ray diffraction, scanning electron microscopy, and Raman spectroscopy. Charge‐transfer‐(CT) enhanced Raman scattering of 4‐mercaptobenzoic acid (4‐MBA) on a 1D MoO₃ belt was investigated experimentally and theoretically. The chemical enhancement of surface‐enhanced Raman scattering (SERS) of 4‐MBA on the MoO₃ belt by CT is in the order of 10³. The SERS of 4‐MBA was investigated theoretically by using a quantum chemical method. The remote SERS of 4‐MBA along the 1D MoO₃ belt (the light excitation to one side of the MoO₃ belt, and the SERS spectrum is collected on the other side of the MoO₃ belt) is also shown experimentally, which provides potential applications of SERS. The incident polarization dependence of remote SERS spectra has also been investigated experimentally.     

Visible‐light‐driven self‐cleaning SERS substrate of silver nanoparticles and graphene oxide decorated nitrogen‐doped titania nanotube array

Graphene oxide (GO) and silver nanoparticles (Ag NPs) sequentially decorated nitrogen‐doped titania nanotube array (N‐TiO₂ NTA) had been designed as visible‐light‐driven self‐cleaning surface‐enhanced Raman scattering (SERS) substrate for a recyclable SERS detection application. N‐TiO₂ NTA was fabricated by anodic oxidation and then doping nitrogen treatment in ammonia atmosphere, acting as a visible‐light‐driven photocatalyst and supporting substrate. Ag/GO/N‐TiO₂ NTA was prepared by decorating GO monolayer through an impregnation process and then depositing Ag NPs through a polyol process on the surface of N‐TiO₂ NTA, acting as the collection of organic molecule and Raman enhancement. The SERS activity of Ag/GO/N‐TiO₂ NTA was evaluated using methyl blue as an organic probe molecule, revealing the analytical enhancement factor of 4.54 × 10⁴. Ag/GO/N‐TiO₂ NTA was applied as active SERS substrate to determine a low‐affinity organic pollutant of bisphenol A, revealing the detection limit of as low as 5 × 10^?7 m . Ag/GO/N‐TiO₂ NTA could also achieve self‐cleaning function for a recycling utilization through visible‐light‐driven photocatalytic degradation of the adsorbed organic molecules. Ag/GO/N‐TiO₂ NTA has been successfully reused for five times without an obvious decay in accuracy and sensitivity for organic molecule detection. The unique properties of this SERS substrate enable it to have a promising application for the sensitive and recyclable SERS detection of low‐affinity organic molecules. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Immobilization of silver nanoparticles into POEGMA polymer brushes as SERS‐active substrates

Surface‐enhanced Raman scattering (SERS) has attracted a great deal of interest during the past four decades and emerged as an ultrasensitive optical technique for chemical and biomedical analysis. It is widely accepted that the facile fabrication of SERS substrates with high activity and good reproducibility is of crucial importance for their applications. Herein, we report on a fast and robust method for the synthesis and immobilization of silver nanoparticles (AgNPs) into poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes under mild conditions without using any reducing agents. POEGMA brushes of different chain lengths were synthesized directly on silicon wafers by surface‐initiated atom transfer radical polymerization with various reaction time. X‐ray photoelectron spectroscopy and field emission scanning electron microscope measurements indicated that the AgNPs were firmly and homogeneously embedded into POEGMA brushes. The resulting POEGMA–AgNP hybrid films were employed as SERS substrates for the detection of 4‐aminothiophenol, giving rise to an enhancement factor of up to 1.9 × 10⁶. The influence of the POEGMA's chain length on SERS performance was also investigated. Copyright © 2016 John Wiley & Sons, Ltd.     

3D aluminum/silver hierarchical nanostructure with large areas of dense hot spots for surface‐enhanced raman scattering

Plasmonic nanomaterials possessing large‐volume, high‐density hot spots with high field enhancement are highly desirable for ultrasensitive surface‐enhanced Raman scattering (SERS) sensing. However, many as‐prepared plasmonic nanomaterials are limited in available dense hot spots and in sample size, which greatly hinder their wide applications in SERS devices. Here, we develop a two‐step physical deposition protocol and successfully fabricate 3D hierarchical nanostructures with highly dense hot spots across a large scale (6 × 6 cm²). The nanopatterned aluminum film was first prepared by thermal evaporation process, which can provide 3D quasi‐periodic cloud‐like nanostructure arrays suitable for noble metal deposition; then a large number of silver nanoparticles with controllable shape and size were decorated onto the alumina layer surfaces by laser molecular beam epitaxy, which can realize large‐area accessible dense hot spots. The optimized 3D‐structured SERS substrate exhibits high‐quality detection performance with excellent reproducibility (13.1 and 17.1%), whose LOD of rhodamine 6G molecules was 10^?9 M. Furthermore, the as‐prepared 3D aluminum/silver SERS substrate was applied in detection of melamine with the concentration down to 10^?7 M and direct detection of melamine in infant formula solution with the concentration as low 10 mg/L. Such method to realize large‐area hierarchical nanostructures can greatly simplify the fabrication procedure for 3D SERS platforms, and should be of technological significance in mass production of SERS‐based sensors.     

Structure of 4‐biphenylthiolate on Au nanoparticle surfaces studied by UV‐Vis absorption spectroscopy,transmission electron microscopy and surface‐enhanced Raman scattering

Structure of 4‐biphenylthiolate on Au nanoparticle surfaces has been studied by UV‐Vis absorption spectroscopy, transmission electron microscopy and surface‐enhanced Raman scattering (SERS). 4‐Biphenylthiolate is found to have a standing geometry on Au from the presence of the benzene ring CH stretching band identified at ～3060 cm^?1. The ν_8a band at 1597 cm^?1 in the ordinary Raman spectrum was found to split clearly into two features at 1599 and 1585 cm^?1. This result suggests that orientation of the phenyl rings in 4‐biphenylthiolate may be quite different and should not lie in the same plane on Au nanoparticle surfaces. On the basis of the electromagnetic enhancement factor, the dihedral angle could be estimated with a reported value of the tilt angle. Copyright © 2004 John Wiley & Sons, Ltd.     

Glucose‐induced and fructose‐induced deboronation reaction of 4‐mercaptophenylboronic acid assembled on silver investigated by surface‐enhanced Raman scattering

We examined the deboronation reaction of 4‐mercaptophenylboronic acid (4MPBA) via fructose and glucose on silver surfaces by means of surface‐enhanced Raman scattering (SERS) at the excitation wavelengths of 488, 514, and 633 nm. The SERS spectra on silver nanoparticles clearly exhibited specific spectral signatures of thiophenol (TP) peaks, indicating a deboronation reaction of 4MPBA on the surfaces, whereas no strong TP peaks were observed on gold nanoparticles. The vibrational bands at 417, 999, 1021, and 1574 cm^?1 in the Ag SERS spectra could correspond to the in‐plane aromatic ring modes in TP. X‐ray photoelectron spectroscopy also supported the surface reaction on Ag by referring the B1s peaks at ~193 eV. The ratiometric Raman measurements of the band at 1574 cm^?1, with respect to that at 1587 cm^?1, revealed fructose and glucose quantification in the concentration range of 1–10 mm . We did not identify such changes for mannose, sucrose, and sialic acid. The SERS peaks of 4MPBA on roughened Ag plates also exhibited TP bands to show the time‐dependent spectral change. Our findings indicate that the deboronation of 4MPBA and conjugation with fructose and glucose may be facilitated efficiently on silver surfaces for their quantification. Copyright © 2016 John Wiley & Sons, Ltd.     

A fast and low‐cost spray method for prototyping and depositing surface‐enhanced Raman scattering arrays on microfluidic paper based device

In this study, a fast, low‐cost, and facile spray method was proposed. This method deposits highly sensitive surface‐enhanced Raman scattering (SERS) silver nanoparticles (AgNPs) on the paper‐microfluidic scheme. The procedures for substrate preparation were studied including different strategies to synthesize AgNPs and the optimization of spray cycles. In addition, the morphologies of the different kinds of paper substrates were characterized by SEM and investigated by their SERS signals. The established method was found to be favorable for obtaining good sensitivity and reproducible results. The RSDs of Raman intensity of randomly analyzing 20 spots on the same paper or different filter papers depositing AgNPs are both below 15%. The SERS enhancement factor is approximately 2 × 10⁷. The whole fabrication is very rapid, robust, and does not require specific instruments. Furthermore, the total cost for 1000 pieces of chip is less than $20. These advantages demonstrated the potential for growing SERS applications in the area of environmental monitoring, food safety, and bioanalysis in the future.     

Silver Coated Platinum Core–Shell Nanostructures on Etched Si Nanowires: Atomic Layer Deposition (ALD) Processing and Application in SERS

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface‐enhanced Raman spectroscopy (SERS)‐based sensing. As host material for the plasmonically active nanostructures we use dense single‐crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core‐shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core‐shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning‐ and transmission electron microscopy. Optimized core–shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.     

Excitation‐wavelength‐dependent charge transfer resonance of bipyridines on silver nanoparticles: surface‐enhanced Raman scattering study

Charge transfer (CT) resonance mechanisms of 2,2′‐bipyridine (2,2′‐BiPy), 2,4′‐bipyridine (2,4′‐BiPy), and 4,4′‐bipyridine (4,4′‐BiPy) on silver nanoparticle surfaces have been comparatively investigated by means of surface‐enhanced Raman scattering (SERS) at the excitation wavelengths of 457, 514, 633, and 785 nm. A combination of the electromagnetic (EM) and charge transfer (CT) contributions should affect the SERS intensities for the bipyridine compounds adsorbed on silver nanoparticle surfaces. The CT resonance is assumed to occur in dissimilar ways for the bipyridine compounds, as evidenced from their different excitation‐wavelength‐dependent SERS enhancement factors. Ab initio density functional theory (DFT) calculations at the level of B3LYP/LANL2DZ have been carried out for the bipyridine‐Ag complexes. Copyright © 2007 John Wiley & Sons, Ltd.     

New Tools for Investigating Electromagnetic Hot Spots in Single‐Molecule Surface‐Enhanced Raman Scattering

Surface‐enhanced Raman scattering (SERS) is quickly growing as an analytical technique, because it offers both molecular specificity and excellent sensitivity. For select substrates, SERS can even be observed from single molecules, which is the ultimate limit of detection. This review describes recent developments in the field of single‐molecule SERS (SM‐SERS) with a focus on new tools for characterizing SM‐SERS‐active substrates and how they interact with single molecules on their surface. In particular, techniques that combine optical spectroscopy and microscopy with electron microscopy are described, including correlated optical and transmission electron microscopy, correlated super‐resolution imaging and scanning electron microscopy, and correlated optical microscopy and electron energy loss spectroscopy.     

Cetylpyridinium Chloride Activated Trinitrotoluene Explosive Lights Up Robust and Ultrahigh Surface‐Enhanced Resonance Raman Scattering in a Silver Sol

Surface‐enhanced resonance Raman scattering (SERRS) is not realized for most molecules of interest. Here, we developed a new SERRS platform for the fast and sensitive detection of 2,4,6‐trinitrotoluene (TNT), a molecule with low Raman cross section. A cationic surfactant, cetylpyridinium chloride (CPC) was modified on the surface of silver sols (CP‐capped Ag). CPC not only acts as the surface‐seeking species to trap sulfite‐sulfonated TNT, but also undergoes complexation with it, resulting in the presence of two charge‐transfer bands at 467 and 530 nm, respectively. This chromophore absorbs the visible light that matches with the incident laser and plasmon resonance of Ag sols by the use of a 532.06 nm laser, and offered large resonance Raman enhancement. This SERRS platform evidenced a fast and accurate detection of TNT with a detection limit of 5×10^?11 M under a low laser power (200 μW) and a short integration time (3 s). The CP‐capped Ag also provides remarkable sensitivity and reliable repeatability. This study provides a facile and reliable method for TNT detection and a viable idea for the SERS detection of various non‐resonant molecules.     

Rolling‐Circle Amplification Detection of Thrombin Using Surface‐Enhanced Raman Spectroscopy with Core‐Shell Nanoparticle Probe

An ultrasensitive surface‐enhanced Raman spectroscopy (SERS) sensor based on rolling‐circle amplification (RCA)‐increased “hot‐spot” was developed for the detection of thrombin. The sensor contains a SERS gold nanoparticle@Raman label@SiO₂ core‐shell nanoparticle probe in which the Raman reporter molecules are sandwiched between a gold nanoparticle core and a thin silica shell by a layer‐by‐layer method. Thrombin aptamer sequences were immobilized onto the magnetic beads (MBs) through hybridization with their complementary strand. In the presence of thrombin, the aptamer sequence was released; this allowed the remaining single‐stranded DNA (ssDNA) to act as primer and initiate in situ RCA reaction to produce long ssDNAs. Then, a large number of SERS probes were attached on the long ssDNA templates, causing thousands of SERS probes to be involved in each biomolecular recognition event. This SERS method achieved the detection of thrombin in the range from 1.0×10^?12 to 1.0×10^?8 M and a detection limit of 4.2×10^?13 M , and showed good performance in real serum samples.     

Silver–platinum core–shell nanoparticles for surface-enhanced Raman spectroscopy

Core–shell Ag@Pt nanoparticles have been synthesised by the means of seed-growth reaction including reduction of PtCl₄²⁻ with silver and replacing Ag atoms with Pt. Surface-enhanced Raman scattering (SERS) spectra of pyridine (which gives slightly different spectra when interacting with various metals) adsorbed on synthesised Ag@Pt clusters were measured. SERS measurements have revealed that deposition of the platinum layer causes near elimination of the spectral interferences from pyridine directly interacting with the silver core. The average SERS enhancement factor for pyridine adsorbed on the Ag@Pt clusters was estimated as equal to about 10³–10⁴, significantly higher than the SERS enhancement factor achievable on the pure platinum nanostructures. Using the silver core (instead of the previously used gold cores) allows for measurement of strong SERS spectra on the Pt covered nanostructures for the wider range of the excitation radiation. This procedure of platinum deposition was tested with various silver nanoparticles – produced with borohydride, citrate and citrate/borohydride methods – which substantially differ in size distribution. The application of formed Ag@Pt structures for obtaining intense Raman spectra for molecules adsorbed on only slightly modified platinum surfaces is discussed.     

Biocompatible Triplex Ag@SiO2@mTiO2 Core–Shell Nanoparticles for Simultaneous Fluorescence‐SERS Bimodal Imaging and Drug Delivery

Herein, we report the synthesis of biocompatible triplex Ag@SiO₂@mTiO₂ core–shell nanoparticles (NPs) for simultaneous fluorescence‐surface‐enhanced Raman scattering (F‐SERS) bimodal imaging and drug delivery. Stable Raman signals were created by typical SERS tags that were composed of Ag NPs for optical enhancement, a reporter molecule of 4‐mercaptopyridine (4‐Mpy) for a spectroscopic signature, and a silica shell for protection. A further coating of mesoporous titania (mTiO₂) on the SERS tags offered high loading capacity for a fluorescence dye (flavin mononucleotide) and an anti‐cancer drug (doxorubicin (DOX)), thereby endowing the material with fluorescence‐imaging and therapeutic functions. The as‐prepared F‐SERS dots exhibited strong fluorescence when excited by light at 460 nm whilst a stable, characteristic 4‐Mpy SERS signal was detected when the excitation wavelength was changed to longer wavelength (632.8 nm), both in solution and after incorporation inside living cells. Their excellent biocompatibility was demonstrated by low cytotoxicity against MCF‐7 cells, even at a high concentration of 100 μg mL^?1. In vitro cell cytotoxicity confirmed that DOX‐loaded F‐SERS dots had a comparable or even greater therapeutic effect compared with the free drug, owing to the increased cell‐uptake, which was attributed to the possible endocytosis mechanism of the NPs. To the best of our knowledge, this is the first proof‐of‐concept investigation on a multifunctional nanomedicine that possessed a combined capacity for fast and multiplexed F‐SERS labeling as well as drug‐loading for cancer therapy.     

Porosity‐Controlled Eggshell Membrane as 3D SERS‐Active Substrate

We report on the fabrication of a surface‐enhanced Raman scattering (SERS) platform, comprised of a three‐dimensional (3D) porous eggshell membrane (ESM) scaffold decorated with Ag nanoparticles (NPs). Both native and treated ESM were used, where the treated ESM pore size and fiber crossing density was controlled by timed exposure to hydrogen peroxide (H₂O₂). Ag NPs were synthesized in situ by reduction of silver nitrate with ascorbic acid. Our results demonstrate that H₂O₂‐treated Ag‐ESM provides a more densely packed 3D network of active material, which leads to consistently higher SERS enhancement than untreated Ag‐ESM substrates.     

Towards Reliable and Quantitative Surface‐Enhanced Raman Scattering (SERS): From Key Parameters to Good Analytical Practice

Experimental results obtained in different laboratories world‐wide by researchers using surface‐enhanced Raman scattering (SERS) can differ significantly. We, an international team of scientists with long‐standing expertise in SERS, address this issue from our perspective by presenting considerations on reliable and quantitative SERS. The central idea of this joint effort is to highlight key parameters and pitfalls that are often encountered in the literature. To that end, we provide here a series of recommendations on: a) the characterization of solid and colloidal SERS substrates by correlative electron and optical microscopy and spectroscopy, b) on the determination of the SERS enhancement factor (EF), including suitable Raman reporter/probe molecules, and finally on c) good analytical practice. We hope that both newcomers and specialists will benefit from these recommendations to increase the inter‐laboratory comparability of experimental SERS results and further establish SERS as an analytical tool.