A review of developments and applications of thin‐film microextraction coupled to surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) greatly expands the applications of Raman spectroscopy and is a promising technique for food safety, environmental analysis, and public safety. Thin‐film microextraction (TFME) provides an efficient sample preparation method for SERS to improve its selectivity and detection efficiency. This review comprehensively describes the development and applications of SERS and TFME, including the history, mechanisim, and active substrate of SERS and the theory, device, forms, and practical applications of TFME. The applications of TFME‐SERS in food safety and environment monitoring are discussed, which could improve their advantages. TFME extracts and enriches the target analytes to eliminate the interfering substance, providing a facial way for SERS to analyze the target analytes in complex matrices. The development of TFME‐SERS technology not only expands the application range of TFME, but greatly improves the anti‐interference ability and detection sensitivity of SERS. Thus, the established methods are fast, convenient, and highly sensitive. This technology is potential to be applied in the on‐site and real‐time detection.     

Ultrasensitive SERS Detection of Lysozyme by a Target‐Triggering Multiple Cycle Amplification Strategy Based on a Gold Substrate

An ultrasensitive surface enhanced Raman scattering (SERS) method has been designed to selectively and sensitively detect lysozyme. The gold chip as the detection substrate, the aptamer‐based target‐triggering cascade multiple cycle amplification, and gold nanoparticles (AuNPs) bio‐barcode Raman probe enhancement on the gold substrate are employed to enhance the SERS signals. The cascade amplification process consists of the nicking enzyme signaling amplification (NESA), the strand displacement amplification (SDA), and the circular‐hairpin‐assisted exponential amplification reaction (HA‐EXPAR). With the involvement of an aptamer‐based probe, two amplification reaction templates, and a Raman probe, the whole circle amplification process is triggered by the target recognition of lysozyme. The products of the upstream cycle (NESA) could act as the “DNA trigger” of the downstream cycle (SDA and circular HA‐EXPAR) to generate further signal amplification, resulting in the immobility of abundant AuNPs Raman probes on the gold substrate. “Hot spots” are produced between the Raman probe and the gold film, leading to significant SERS enhancement. This detection method exhibits excellent specificity and sensitivity towards lysozyme with a detection limit of 1.0×10^?15 M . Moreover, the practical determination of lysozyme in human serum demonstrates the feasibility of this SERS approach in the analysis of a variety of biological specimens.     

Microarray‐Based Detection of Dye‐Labeled DNA by SERRS Using Particles Formed by Enzymatic Silver Deposition

The growing interest in DNA diagnostics is addressed today by microarrays with fluoresence detection. In our approach, we utilize spatially defined arrays of short oligonucleotides on a modified glass surface. Surface enhanced resonance Raman scattering (SERRS) is used to obtain molecularly specific spectra of the Raman‐active dye‐labeled DNA. Nanoparticles produced by enzymatic silver deposition are used as SERS‐active substrate. They grow directly on the modified oligonucleotides and only in the spatially defined areas on the chip. Furthermore, they potentially offer several advantages for SERS detection. The nanoparticles are characterized and their ability for use as SERS‐ and SERRS‐active substrate is estimated. Three different Raman‐active dyes are investigated for their potential for involvement in sequence specific DNA analysis.     

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.     

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.     

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.     

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.     

Rapid Detection of Sildenafil Drugs in Liquid Nutraceuticals Based on Surface‐Enhanced Raman Spectroscopy Technology

In this study, surface‐enhanced Raman spectroscopy (SERS ) technology was used to rapidly detect illegally added sildenafil drugs. A detailed attribution analysis by density functional theory (DFT ) was used to guide specific experiments. The Raman signals were obtained from a silver colloid (Ag col) substrate, and they increased in the presence of the mineral salt, potassium iodide (KI ). These methods detected sildenafil in aqueous solutions as low as 1 µg/mL with high signal uniformity (RSD =3.77%). Prior to this study, traditional Raman techniques detected substances in solid samples only. Here, Raman technology detected low contents of sildenaful drugs in liquid nutraceuticals. Therefore, SERS technology has great potential for on‐site and real‐time detection of illegal drugs in water and in liquid nutraceuticals.     

Noble‐Metal‐Free Materials for Surface‐Enhanced Raman Spectroscopy Detection

Surface‐enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single‐molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble‐metal‐free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble‐metal‐free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene‐based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble‐metal‐free materials for enhancing detection sensitivity can further fuel the development of SERS‐related applications.     

Multilayer Substrate‐Mediated Tuning Resonance of Plasmon and SERS EF of Nanostructured Silver

A thin‐film of dielectric on a reflecting surface constituting a multilayer substrate modulates light intensity due to the interference effect. A nanostructure consisting of randomly oriented silver particles of different shapes, sizes, and interparticle spacings supports multiple plasmon resonances and is observed to have a broad extinction spectrum that spans the entire visible region. Combining the two systems by fabricating the nanostructure on the thin‐dielectric film of the multilayer substrate yields a new composite structure which is observed to modulate both the extinction spectrum and the SERS EF (surface enhanced Raman scattering enhancement factor) of the nanostructure as the thickness of the thin‐film dielectric is varied. The frequency and intensity of the visible extinction spectrum vary dramatically with the dielectric thickness and in the intermediate thickness range the spectrum has no visible band. The SERS EF determined for the composite structure as a function of the thin‐film dielectric thickness varies by several orders of magnitude. Strong correlation between the magnitude of the SERS EF and the extinction intensity is observed over the entire dielectric thickness range indicating that the extinction spectrum corresponds to the excitation of the plasmon resonances of the nanostructure. A significant finding which has potential applications is that the composite structure has synergic effect to boost SERS EF of the nanostructure by an order of magnitude or more compared to the same nanostructure on an unlayered substrate.     

Gold Nanoparticles as Dual Functional Sensor to Detect E.coliDH5α as a Model for Gram‐negative Bacteria

4‐aminothiophenol‐modified gold nanoparticles (PATP‐AuNPs) were used as colorimetric and Surface Enhanced Raman Scattering (SERS) probes for the sensitive detection of Escherichia coliDH5α, as a model for Gram‐negative bacteria. The nano‐probes were easy to prepare through Au‐S bonding. Under optimized conditions, the PATP‐AuNPs surface with positive charge can bind with negatively charged E.coliDH5α via electrostatic adhesion, resulting in a quick color change from red to blue, and also a dramatic SERS signal enhancement from thousands of AuNPs aggregated on the surface of bacteria, which was utilized for both colorimetric and SERS detection of E.coliDH5α. For colorimetric analysis, it is the first time that the classical partial least square (PLS) regression was utilized to deal with the relationship between adsorption and E.coliDH5α concentrations. Excellent linear relationship was observed from 1.1 x 10⁷ to 1.3 x 10⁸ cfu mL^‐1 with the average relative error (ARE) of 5.430, which was more accurate than the traditional extinction ration method. When coupled with confocal Raman microscope, this PATP‐AuNPs probes could be used to detection SERS signals produced from even one single bacterium. This bioassay is rapid, less expensive and convenient for bacteria detection and analysis. Therefore, PATP‐AuNPs system as a novel, versatile, on‐site and real‐time Gram‐negative bacteria sensor, would have a wide range of practical applications.     

Chiral Carbonaceous Nanotubes Modified with Titania Nanocrystals: Plasmon‐Free and Recyclable SERS Sensitivity

Chiral carbonaceous nanotubes (CNT) were successfully used in plasmon‐free surface‐enhanced Raman scattering (SERS) for the first time. Further modification of TiO₂ nanocrystals on the chiral CNTs successfully realized the recycling of SERS substrate as chiral CNT/TiO₂ hybrids. The high SERS sensitivity of methylene blue (MB) over the chiral CNT/TiO₂ hybrids is ascribed to the laser‐driven birefringence induced by the helical structure, which provides much more opportunities for the occurrence of Raman scattering. The TiO₂ nanocrystals highly dispersed on the surface and inside the hollow cavity of chiral CNTs can completely degrade the MB under the solar light irradiation, leading to the self‐cleaning of SERS substrate. The present research opens a new way for the application of chiral inorganic materials in plasmon‐free SERS detection.     

Localized Plasmon‐Stimulated Nanochemistry of Graphene Oxide on a SERS Substrate

In recent years, there has been remarkable progress in the reduction and functionalization of graphene oxide (GO) using nanoparticles and high‐energy optical photons. Most of these reactions are carried out in solutions, whereas the local modification of GO on solid substrates still remains a challenge. In this work, we demonstrate the local reduction of GO and its further destruction, leading to the synthesis of polyaromatic hydrocarbons (PAHs) stimulated by localized surface plasmons (LSPs). The reduction of GO and the synthesis of PAHs have been carried out on a substrate designed for surface‐enhanced Raman spectroscopy (SERS). We found that LSPs initiate the destruction of water molecules entrapped in the nanogaps between silver nanoparticles after the deposition of GO from the aqueous suspension. It was demonstrated that OH radicals, as a result of water decomposition, initiate the reduction of GO, leading to the synthesis of PAHs. The reactions have been observed in real time by using SERS. The measurement of current–voltage (I–V) characteristics through conductive atomic force microscopy (AFM), recorded in an LSP‐stimulated area, have shown the increased electrical conductivity (more than ten times) compared with the conductivity of GO. The synthesis of new compounds in the LSP‐stimulated area has been confirmed by the appearance of new peaks in the Raman spectra and nonlinear I–V characteristics typical for PAHs. We show that the used method allows the local modification of electrical properties of GO and controlled nanopattering of organic compounds on the surface.     

用表面增强拉曼散射研究电解法制备的纳米银膜

用一种廉价的电解方法制备了纳米银膜,并详细研究了在这种银膜上的表面增强拉曼散射效果．结晶紫为本实验的检测性分子．通过实验发现,这种银膜用便携式拉曼光谱仪测试并计算出的表面增强拉曼散射的增强因子为603,并对结晶紫的最小检出限为0．1nmol／L．     

Highly Reproducible Surface‐Enhanced Raman Spectra on Semiconductor SnO2 Octahedral Nanoparticles

Highly reproducible surface‐enhanced Raman scattering (SERS) spectra are obtained on the surface of SnO₂ octahedral nanoparticles. The spot‐to‐spot SERS signals show a relative standard deviation (RSD) consistently below 20 % in the intensity of the main Raman peaks of 4‐mercaptobenzoic acid (4‐MBA) and 4‐nitrobenzenethiol (4‐NBT), indicating good spatial uniformity and reproducibility. The SERS signals are believed to mainly originate from a charge‐transfer (CT) mechanism. Time‐dependent density functional theory (TD‐DFT) is used to simulate the SERS spectrum and interpret the chemical enhancement mechanism in the experiment. The research extends the application of SERS and also establishes a new uniform SERS substrate.     

Fabrication and Formation Mechanism of Ag Nanoplate‐Decorated Nanofiber Mats and Their Application in SERS

We report a new simple method to fabricate a highly active SERS substrate consisting of poly‐m‐phenylenediamine/polyacrylonitrile (PmPD/PAN) decorated with Ag nanoplates. The formation mechanism of Ag nanoplates is investigated. The synthetic process of the Ag nanoplate‐decorated PmPD/PAN (Ag nanoplates@PmPD/PAN) nanofiber mats consists of the assembly of Ag nanoparticles on the surface of PmPD/PAN nanofibers as crystal nuclei followed by in situ growth of Ag nanoparticles exclusively into nanoplates. Both the reducibility of the polymer and the concentration of AgNO₃ are found to play important roles in the formation and the density of Ag nanoplates. The optimized Ag nanoplates@PmPD/PAN nanofiber mats exhibit excellent activity and reproducibility in surface‐enhanced Raman scattering (SERS) detection of 4‐mercaptobenzoic acid (4‐MBA) with a detection limit of 10^?10 m , making the Ag nanoplates@PmPD/PAN nanofiber mats a promising substrate for SERS detection of chemical molecules. In addition, this work also provides a design and fabrication process for a 3D SERS substrate made of a reducible polymer with noble metals.     

Novel Bottom‐Up SERS Substrates for Quantitative and Parallelized Analytics

Surface‐enhanced Raman spectroscopy (SERS) is an emerging technology in the field of analytics. Due to the high sensitivity in connection with specific Raman molecular fingerprint information SERS can be used in a variety of analytical, bioanalytical, and biosensing applications. However, for the SERS effect substrates with metal nanostructures are needed. The broad application of this technology is greatly hampered by the lack of reliable and reproducible substrates. Usually the activity of a given substrate has to be determined by time‐consuming experiments such as calibration or ultramicroscopic studies. To use SERS as a standard analytical tool, cheap and reproducible substrates are required, preferably with a characterization technique that does not interfere with the subsequent measurements. Herein we introduce an innovative approach to produce low‐cost and large‐scale reproducible substrates for SERS applications, which allows easy and economical production of micropatterned SERS active surfaces on a large scale. This approach is based on an enzyme‐induced growth of silver nanostructures. The special structural feature of the enzymatically deposited silver nanoparticles prevents the breakdown of SERS activity even at high particle densities (particle density >60 %) that lead to a conductive layer. In contrast to other approaches, this substrate exhibits a relationship between electrical conductivity and the resulting SERS activity of a given spot. This enables the prediction of the SERS activity of the nanostructure ensemble and therewith the controllable and reproducible production of SERS substrates of enzymatic silver nanoparticles on a large scale, utilizing a simple measurement of the electrical conductivity. Furthermore, through a correlation between the conductivity and the SERS activity of the substrates it is possible to quantify SERS measurements with these substrates.     

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.     

ZnGa2O4 Nanorod Arrays Decorated with Ag Nanoparticles as Surface‐Enhanced Raman‐Scattering Substrates for Melamine Detection

The availability of sensitive, reproducible, and stable substrates is critically important for surface‐enhanced Raman spectroscopy (SERS)‐based applications, but it presently remains a challenge. In this work, well‐aligned zinc gallate (ZnGa₂O₄) nanorod arrays grown on a Si substrate by chemical vapor deposition were used as templates to fabricate SERS substrates by deposition of Ag nanoparticles onto the ZnGa₂O₄ nanorod surfaces. The coverage of the Ag nanoparticles on the ZnGa₂O₄ nanorod surfaces was easily controlled by varying the amount of AgNO₃. SERS measurements showed that the number density of Ag nanoparticles on the ZnGa₂O₄ nanorod surfaces had a great effect on SERS activity. The SERS signals collected by point‐to‐point and SERS mapping images showed that as‐prepared SERS substrates exhibited good spatial uniformity and reproducibility. Detection of melamine molecules at low concentrations (1.0×10^?7 M ) was used as an example to show the possible application of such a substrate. In addition, the effect of benzoic acid on the detection of melamine was also investigated. It was found that the SERS signal intensity of melamine decreased greatly as the concentration of benzoic acid was increased.     

Untargeted Tumor Metabolomics with Liquid Chromatography–Surface‐Enhanced Raman Spectroscopy

Metabolomics is a powerful systems biology approach that monitors changes in biomolecule concentrations to diagnose and monitor health and disease. However, leading metabolomics technologies, such as NMR and mass spectrometry (MS), access only a small portion of the metabolome. Now an approach is presented that uses the high sensitivity and chemical specificity of surface‐enhanced Raman scattering (SERS) for online detection of metabolites from tumor lysates following liquid chromatography (LC). The results demonstrate that this LC‐SERS approach has metabolite detection capabilities comparable to the state‐of‐art LC‐MS but suggest a selectivity for the detection of a different subset of metabolites. Analysis of replicate LC‐SERS experiments exhibit reproducible metabolite patterns that can be converted into barcodes, which can differentiate different tumor models. Our work demonstrates the potential of LC‐SERS technology for metabolomics‐based diagnosis and treatment of cancer.