Enhanced Raman Scattering by ZnO Superstructures: Synergistic Effect of Charge Transfer and Mie Resonances

A remarkable enhancement of Raman scattering is achieved by submicrometer‐sized spherical ZnO superstructures. The secondary superstructures of ZnO particles with a uniform diameter in the range of 220–490 nm was formed by aggregating ca. 13 nm primary single crystallites. By engineering the superstructure size to induce Mie resonances, leading to an electromagnetic contribution to the SERS enhancement. Meanwhile, a highly efficient charge‐transfer (CT) contribution derived from the primary structure of the ZnO nanocrystallites was able to enhance the SERS signals as well. The highest Raman enhancement factor of 10⁵ was achieved for a non‐resonant molecule by the synergistic effect of CT and Mie resonances. The Mie resonances scattered near‐field effect investigated in the present study provides not only an important guide for designing novel SERS‐active semiconductor substrates, but also a coherent framework for modelling the electromagnetic mechanism of SERS on semiconductors.     

半导体纳米材料作为表面增强拉曼散射基底的研究进展

在总结半导体纳米材料作为表面增强拉曼散射(SERS)基底的一些相关研究工作的基础上, 讨论了半导体纳米粒子SERS基底的增强效应与纳米材料的种类、尺寸的相关性; 对半峰宽、激发波长进行了分析, 并对半导体纳米材料作为SERS基底时, 化学增强、电磁增强、纳米半导体缺陷和激子波尔半径的影响等进行了阐述.     

Defect engineering in semiconductor-based SERS

Semiconductor-based surface enhanced Raman spectroscopy (SERS) platforms take advantage of the multifaceted tunability of semiconductor materials to realize specialized sensing demands in a wide range of applications. However, until quite recently, semiconductor-based SERS materials have generally exhibited low activity compared to conventional noble metal substrates, with enhancement factors (EF) typically reaching 10³, confining the study of semiconductor-based SERS to purely academic settings. In recent years, defect engineering has been proposed to effectively improve the SERS activity of semiconductor materials. Defective semiconductors can now achieve noble-metal-comparable SERS enhancement and exceedingly low, nano-molar detection concentrations towards certain molecules. The reason for such success is that defect engineering effectively harnesses the complex enhancement mechanisms behind the SERS phenomenon by purposefully tailoring many physicochemical parameters of semiconductors. In this perspective, we introduce the main defect engineering approaches used in SERS-activation, and discuss in depth the electromagnetic and chemical enhancement mechanisms (EM and CM, respectively) that are influenced by these defect engineering methods. We also introduce the applications that have been reported for defective semiconductor-based SERS platforms. With this perspective we aim to meet the imperative demand for a summary on the recent developments of SERS material design based on defect engineering of semiconductors, and highlight the attractive research and application prospects for semiconductor-based SERS.

Defect engineering strategies are used to boost the SERS activity of a wide variety of semiconductors including metal oxides, nitrides, carbon materials and transition metal dichalcogenides (TMDs), as discussed in this perspective.     

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.     

Plasmon-Induced Charge Transfer-Enhanced Raman Scattering on a Semiconductor: Toward Amplification-Free Quantification of SARS-CoV-2

Semiconductors demonstrate great potentials as chemical mechanism-based surface-enhanced Raman scattering (SERS) substrates in determination of biological species in complex living systems with high selectivity. However, low sensitivity is the bottleneck for their practical applications, compared with that of noble metal-based Raman enhancement ascribed to electromagnetic mechanism. Herein, a novel Cu₂O nanoarray with free carrier density of 1.78×10²¹ cm⁻³ comparable to that of noble metals was self-assembled, creating a record in enhancement factor (EF) of 3.19×10¹⁰ among semiconductor substrates. The significant EF was mainly attributed to plasmon-induced hot electron transfer (PIHET) in semiconductor which was never reported before. This Cu₂O nanoarray was subsequently developed as a highly sensitive and selective SERS chip for non-enzyme and amplification-free SARS-CoV-2 RNA quantification with a detection limit down to 60 copies/mL within 5 min. This unique Cu₂O nanoarray demonstrated the significant Raman enhancement through PIHET process, enabling rapid and sensitive point-of-care testing of emerging virus variants.     

Probing Innovative Microfabricated Substrates for their Reproducible SERS Activity

New types of microfabricated surface‐enhanced Raman spectroscopy (SERS) active substrates produced by electron beam lithography and ion beam etching are introduced. In order to achieve large enhancement factors by using the lightning rod effect, we prepare arrays consisting of sharp‐edged nanostructures instead of the commonly used dots. Two experimental methods are used for fabrication: a one‐stage process, leading to gold nanostar arrays and a two‐stage process, leading to gold nanodiamond arrays. Our preparation process guarantees high reproducibility. The substrates contain a number of arrays for practical applications, each 200×200 μm² in size. To test the SERS activity of these nanostar and nanodiamond arrays, a monolayer of the dye crystal violet is used. Enhancement factors are estimated to be at least 130 for the nanodiamond and 310 for the nanostar arrays.     

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.     

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages

Enhancement of the semiconductor–molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor‐based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a‐ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×10⁵. This remarkable SERS sensitivity can be attributed to high‐efficiency ICTP within a‐ZnO NC molecule system, which is caused by metastable electronic states of a‐ZnO NCs. First‐principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a‐ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn−S bonds peculiar to the mercapto molecule adsorbed a‐ZnO NCs, which has been verified through the X‐ray absorption near‐edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.     

Approaching the electromagnetic mechanism of surface-enhanced Raman scattering: from self-assembled arrays to individual gold nanoparticles

Surface-enhanced Raman scattering (SERS) has been intensively explored both in theory and applications and has been widely used in chemistry, physics and biology for decades. A variety of SERS substrates have been developed in order to investigate the mechanisms behind, which give rise to the enormous enhancement even enabling single molecule detection. The Raman enhancement, which involves an electromagnetic enhancement (EM) and a chemical enhancement (CM), reflects both the physical principle of light/metal interactions and the molecule/metal interactions. In this tutorial review, we focus on the EM enhancement of SERS active substrates made of colloidal gold nanoparticles (GNPs), varying from self-assembled arrays down to single particles, for the purpose of investigating the EM coupling effect and probing the distribution of the induced electric field of single GNPs.     

Highly reproducible surface-enhanced Raman scattering-active Au nanostructures prepared by simple electrodeposition: Origin of surface-enhanced Raman scattering activity and applications as electrochemical substrates

The fabrication of effective surface-enhanced Raman scattering (SERS) substrates has been the subject of intensive research because of their useful applications. In this paper, dendritic gold (Au) rod (DAR) structures prepared by simple one-step electrodeposition in a short time were examined as an effective SERS-active substrate. The SERS activity of the DAR surfaces was compared to that of other nanostructured Au surfaces with different morphologies, and its dependence on the structural variation of DAR structures was examined. These comparisonal investigations revealed that highly faceted sharp edge sites present on the DAR surfaces play a critical role in inducing a high SERS activity. The SERS enhancement factor was estimated to be greater than 10⁵, and the detection limit of rhodamine 6G at DAR surfaces was 10⁻⁸ M. The DAR surfaces exhibit excellent spot-to-spot and substrate-to-substrate SERS enhancement reproducibility, and their long-term stability is very good. It was also demonstrated that the DAR surfaces can be effectively utilized in electrochemical SERS systems, wherein a reversible SERS behavior was obtained during the cycling to cathodic potential regions. Considering the straightforward preparation of DAR substrates and the clean nature of SERS-active Au surfaces prepared in the absence of additives, we expect that DAR surfaces can be used as cost-effective SERS substrates in analytical and electrochemical applications.     

Fabrication and characterization of homogeneous surface-enhanced Raman scattering substrates by single pulse UV-laser treatment of gold and silver films

The fabrication of SERS-active substrates, which offer high enhancement factors as well as spatially homogeneous distribution of the enhancement, plays an important role in the expansion of surface-enhanced Raman scattering (SERS) spectroscopy to a powerful, quantitative, and noninvasive measurement technique for analytical applications. In this paper, a novel method for the fabrication of SERS-active substrates by laser treatment of 20, 40, and 60 nm thick gold and of 40 nm thick silver films supported on quartz glass is presented. Single 308 nm UV-laser pulses were applied to melt the thin gold and silver films. During the cooling process of the noble metal, particles were formed. The particle size and density were imaged by atomic force microscopy. By varying the fluence, the size of the particles can be controlled. The enhancement factors of the nanostructures were determined by recording self-assembled monolayers of benzenethiol. The intensity of the SERS signal from benzenethiol is correlated to the mean particle size and thus to the fluence. Enhancement factors up to 10(6) with a high reproducibility were reached. Finally we have analyzed the temperature dependence of the SERS effect by recording the intensity of benzenethiol vibrations from 300 to 120 K. The temperature dependence of the SERS effect is discussed with regard to the metal properties.     

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.     

SERS and FDTD simulation of gold nanoparticles grafted on germanium wafer via galvanic displacement

Uniform and dense Au nanoparticles grown on Ge (Au/Ge) were fabricated by a facile galvanic displacement method and employed as surface‐enhanced Raman scattering (SERS) substrates. The substrates exhibited excellent reproducibility in the detection of rhodamine 6G aqueous solution with a relative standard deviation of <20%. The substrate showed a high Raman enhancement factor of 3.44 × 10⁶. This superior SERS sensitivity was numerical confirmed by the three‐dimensional finite‐difference time‐domain method, which demonstrated a stronger electric field intensity (|E/E₀|²) distribution around the Au nanoparticles grown on Ge. This facile and low‐cost prepared Au/Ge substrate with high SERS sensitivity and reproducibility might have potential applications in monitoring in situ reaction in aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.     

Long‐Term Stable Silver Subsurface Ion‐Exchanged Glasses for SERS Applications

We report on the formation of silver subsurface ion‐exchanged metal oxide (silver SIMO) glasses and their surface‐enhanced Raman scattering (SERS) activity. The samples were prepared by a combined thermal and chemical three‐step methodology and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), environmental electron scanning microscopy (ESEM), and UV/Vis spectroscopy. This unique method provides SERS substrates with protection against contamination and strong, reliable and reproducible SERS enhancement. The Raman enhancement factors of the long‐term stable SIMO glasses were estimated to approximately 10⁷.     

Recent Advances in Metal Organic Frameworks Based Surface Enhanced Raman Scattering Substrates: Synthesis and Applications

Metal-organic frameworks (MOFs) are supramolecular nanomaterials, in which metal ions or clusters are connected by organic ligands to form crystalline lattices with highly ordered periodic porous network structure. MOFs have been widely applied in various fields, such as catalyst, sample preparation, and sensing. In recent years, MOFs based surface enhanced Raman scattering (SERS) substrates have attracted much attention since MOFs can largely improve the performance of metallic SERS substrates toward target enrichment and signal enhancement. MOFs have been exploited in SERS analysis to tackle some challenges that bare metal substrates cannot achieve. Combination of MOFs and SERS improved the sensitivity of traditional SERS analysis and extended the application scope of SERS. With the increasing exploration of MOFs based SERS substrates, there is a great demand to review the advances in these researches. Herein, this review concentrated on summarizing the preparation and applications of MOFs based SERS substrates. Representative researches were discussed to better understand the property of MOFs based SERS substrates. The advantages of MOFs based SERS substrates were highlighted, as well as their limitations. In addition, the challenges, opportunities, and future trends in MOFs based SERS analysis were tentatively discussed.     

Characterization of the surface enhanced raman scattering (SERS) of bacteria

The surface enhanced Raman scattering (SERS) of a number of species and strains of bacteria obtained on novel gold nanoparticle (approximately 80 nm) covered SiO(2) substrates excited at 785 nm is reported. Raman cross-section enhancements of >10(4) per bacterium are found for both Gram-positive and Gram-negative bacteria on these SERS active substrates. The SERS spectra of bacteria are spectrally less congested and exhibit greater species differentiation than their corresponding non-SERS (bulk) Raman spectra at this excitation wavelength. Fluorescence observed in the bulk Raman emission of Bacillus species is not apparent in the corresponding SERS spectra. Despite the field enhancement effects arising from the nanostructured metal surface, this fluorescence component appears "quenched" due to an energy transfer process which does not diminish the Raman emission. The surface enhancement effect allows the observation of Raman spectra of single bacterial cells excited at low incident powers and short data acquisition times. SERS spectra of B. anthracis Sterne illustrate this single cell level capability. Comparison with previous SERS studies reveals how the SERS vibrational signatures are strongly dependent on the morphology and nature of the SERS active substrates. The potential of SERS for detection and identification of bacterial pathogens with species and strain specificity on these gold particle covered glassy substrates is demonstrated by these results.     

Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications

This tutorial review discusses a new class of colloidal metal nanoparticles that is able to enhance the efficiencies of surface-enhanced Raman scattering (SERS) by as much as 10(14)-10(15) fold. This enormous enhancement allows spectroscopic detection and identification of single molecules located on the nanoparticle surface or at the junction of two particles under ambient conditions. Considerable progress has been made in understanding the enhancement mechanisms, including definitive evidence for the single-molecule origin of fluctuating SERS signals. For applications, SERS nanoparticle tags have been developed based on the use of embedded reporter molecules and a silica or polymer encapsulation layer. The SERS nanoparticle tags are capable of providing detailed spectroscopic information and are much brighter than semiconductor quantum dots in the near-infrared spectral window. These properties have raised new opportunities for multiplexed molecular diagnosis and in vivo Raman spectroscopy and imaging.     

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.     

Lithographically-defined 3D porous networks as active substrates for surface enhanced Raman scattering

Interferometric lithographically fabricated porous carbon acts as active substrates for Surface Enhanced Raman Scattering (SERS) applications with enhancement factors ranging from 7 to 9 orders of magnitude.     

Electrochemical synthesis and functionality evaluation of silver nanostructured layers

The functionality of silver nanostructures prepared by means of electrochemical deposition of silver into the pores of anodic alumina oxide (AAO) template was examined in correlation to electrodeposition conditions. The optical activity as well as the chemical separation ability of prepared nanostructured films was studied. The surface enhanced Raman spectroscopy (SERS) performance was evaluated by the signals of rhodamine 6G, 4‐aminothiophenol and 2,7‐dichlorfluorescein. Nanostructured silver substrates showed moderate surface enhancement for Raman scattering from adsorbed molecules with the magnitude of about 26.9. Moreover, a novel separation/pre‐concentration function of the silver nanowire structures was indicated. The identification and position detection of the model compounds were realised with SERS. The separation of single chemical components from the two‐component mixture over the examined silver nanostructured films was sufficiently approved. The results obtained demonstrated the potential of the prepared substrate as a SERS detection and separation probe for further implementation to any instrumentation. Copyright © 2014 John Wiley & Sons, Ltd.