Optimization of Nanoparticle-Based SERS Substrates through Large-Scale Realistic Simulations

Bimetallic Single-Atom Catalysts for Water Splitting

Green hydrogen from water splitting has emerged as a critical energy vector with the potential to spearhead the global transition to a fossil fuel-independent society. The field of catalysis has been revolutionized by single-atom catalysts (SACs), which exhibit unique and intricate interactions between atomically dispersed metal atoms and their supports. Recently, bimetallic SACs (bimSACs) have garnered significant attention for leveraging the synergistic functions of two metal ions coordinated on appropriately designed supports. BimSACs offer an avenue for rich metal-metal and metal-support cooperativity, potentially addressing current limitations of SACs in effectively furnishing transformations which involve synchronous proton-electron exchanges, substrate activation with reversible redox cycles, simultaneous multi-electron transfer, regulation of spin states, tuning of electronic properties, and cyclic transition states with low activation energies. This review aims to encapsulate the growing advancements in bimSACs, with an emphasis on their pivotal role in hydrogen generation via water splitting. We subsequently delve into advanced experimental methodologies for the elaborate characterization of SACs, elucidate their electronic properties, and discuss their local coordination environment. Overall, we present comprehensive discussion on the deployment of bimSACs in both hydrogen evolution reaction and oxygen evolution reaction, the two half-reactions of the water electrolysis process.

Reviewing advances in nanophotonic biosensors

Biosensing, a promising branch of exploiting nanophotonic devices, enables meticulous detection of subwavelength light, which helps to analyze and manipulate light-matter interaction. The improved sensitivity of recent high-quality nanophotonic biosensors has enabled enhanced bioanalytical precision in detection. Considering the potential of nanophotonics in biosensing, this article summarizes recent advances in fabricating nanophotonic and optical biosensors, focusing on their sensing function and capacity. We typically classify these types of biosensors into five categories: phase-driven, resonant dielectric nanostructures, plasmonic nanostructures, surface-enhanced spectroscopies, and evanescent-field, and review the importance of enhancing sensor performance and efficacy by addressing some major concerns in nanophotonic biosensing, such as overcoming the difficulties in controlling biological specimens and lowering their costs for ease of access. We also address the possibility of updating these technologies for immediate implementation and their impact on enhancing safety and health.

DNA origami-templated gold nanorod dimer nanoantennas: enabling addressable optical hotspots for single cancer biomarker SERS detection

The convergence of DNA origami and surface-enhanced Raman spectroscopy (SERS) has opened a new avenue in bioanalytical sciences, particularly in the detection of single-molecule proteins. This breakthrough has enabled the development of advanced sensor technologies for diagnostics. DNA origami offers a highly controllable framework for the precise positioning of nanostructures, resulting in superior SERS signal amplification. In our investigation, we have successfully designed and synthesized DNA origami-based gold nanorod monomer and dimer assemblies. Moreover, we have evaluated the potential of dimer assemblies for label-free detection of a single biomolecule, namely epidermal growth factor receptor (EGFR), a crucial biomarker in cancer research. Our findings have revealed that the significant Raman amplification generated by DNA origami-assembled gold nanorod dimer nanoantennas facilitates the label-free identification of Raman peaks of single proteins, which is a prime aim in biomedical diagnostics. The present work represents a significant advancement in leveraging plasmonic nanoantennas to realize single protein SERS for the detection of various cancer biomarkers with single-molecule sensitivity.

Quantitative 3D structural analysis of small colloidal assemblies under native conditions by liquid-cell fast electron tomography

Electron tomography has become a commonly used tool to investigate the three-dimensional (3D) structure of nanomaterials, including colloidal nanoparticle assemblies. However, electron microscopy is typically done under high-vacuum conditions, requiring sample preparation for assemblies obtained by wet colloid chemistry methods. This involves solvent evaporation and deposition on a solid support, which consistently alters the nanoparticle organization. Here, we suggest using electron tomography to study nanoparticle assemblies in their original colloidal liquid environment. To address the challenges related to electron tomography in liquid, we devise a method that combines fast data acquisition in a commercial liquid-cell with a dedicated alignment and reconstruction workflow. We present the advantages of this methodology in accurately characterizing two different systems. 3D reconstructions of assemblies comprising polystyrene-capped Au nanoparticles encapsulated in polymeric shells reveal less compact and more distorted configurations for experiments performed in a liquid medium compared to their dried counterparts. A similar expansion can be observed in quantitative analysis of the surface-to-surface distances of self-assembled Au nanorods in water rather than in a vacuum, in agreement with bulk measurements. This study, therefore, emphasizes the importance of developing high-resolution characterization tools that preserve the native environment of colloidal nanostructures.

From Genotype to Phenotype: Raman Spectroscopy and Machine Learning for Label-Free Single-Cell Analysis

Raman spectroscopy has made significant progress in biosensing and clinical research. Here, we describe how surface-enhanced Raman spectroscopy (SERS) assisted with machine learning (ML) can expand its capabilities to enable interpretable insights into the transcriptome, proteome, and metabolome at the single-cell level. We first review how advances in nanophotonics-including plasmonics, metamaterials, and metasurfaces-enhance Raman scattering for rapid, strong label-free spectroscopy. We then discuss ML approaches for precise and interpretable spectral analysis, including neural networks, perturbation and gradient algorithms, and transfer learning. We provide illustrative examples of single-cell Raman phenotyping using nanophotonics and ML, including bacterial antibiotic susceptibility predictions, stem cell expression profiles, cancer diagnostics, and immunotherapy efficacy and toxicity predictions. Lastly, we discuss exciting prospects for the future of single-cell Raman spectroscopy, including Raman instrumentation, self-driving laboratories, Raman data banks, and machine learning for uncovering biological insights.

The Role of the Polyethylene Glycol in the Organization of Gold Nanorods at the Air-Water and Air-Solid Interfaces

The organization of metallic nanoparticles into assembled films is a complex process. The type of nanoparticle stabilizing ligand and the method for creating an organized layer can profoundly affect the optical properties of the resulting nanoparticle assembly. Investigations of the ligand structure and nanoparticle interactions can provide a greater understanding of the design of the assembly process and the quality of the resulting materials. One of the functionalization methods in the preparation of specific gold nanorods is the utilization of thiol-terminated poly(ethylene glycol). This generates gold nanorods capable of forming stable monolayers at the air-water interface upon dispersion in a suitable organic solvent. Herein, we show that depending on the molecular weight of the poly(ethylene glycol), the structures obtained at the air-water and air-solid interfaces differ in the arrangement. The studied structures were characterized by using spectroscopic and microscopic techniques, and the structural type was correlated with the polymer type. Insoluble and stable Langmuir monolayers composed of higher-molecular-weight gold nanorods with poly(ethylene glycol) were formed only in the presence of an additional stabilizer that prevented the formation of gold nanorods in aqueous solutions. At the air-solid interface, conformational changes in poly(ethylene glycol) induced the aggregation of gold nanorods, which became closely packed under the influence of surface pressure. The presented results suggested that the arrangement of two-dimensional layers of gold nanorods could be tailored using poly(ethylene glycol) of various molecular weights.

In Situ Surface-Enhanced Raman Spectroscopy on Organic Mixed Ionic-Electronic Conductors: Tracking Dynamic Doping in Light-Emitting Electrochemical Cells

In the domain of organic mixed ionic-electronic conductors (OMIECs), simultaneous transport and coupling of ionic and electronic charges are crucial for the function of electrochemical devices in organic electronics. Understanding conduction mechanisms and chemical reactions in operational devices is pivotal for performance enhancement and is necessary for the informed and systematic development of more promising materials. Surface-enhanced Raman spectroscopy (SERS) is a potent tool for monitoring electrochemical evolution and dynamic doping in operational devices, offering enhanced sensitivity to subtle spectral changes. We demonstrate the utility of SERS for in situ tracking of doping in OMIECs in an organic light-emitting electrochemical cell (LEC) containing a conjugated polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]; MEH-PPV), a molecular anion (lithium triflate), and an electrolyte network (poly(ethylene oxide); PEO). SERS enhancement is achieved via an interleaved layer of gold particles formed by spontaneous breakup of a deposited thin gold film. The results successfully highlight the ability of SERS to unveil time-resolved MEH-PPV doping and polaron formation, elucidating the effects of triflate ion transfer in the operating device and validating the electrochemical doping model in LECs.

Growing Gold Nanostars on 3D Hydrogel Surfaces

Nanocomposites comprising hydrogels and plasmonic nanoparticles are attractive materials for tissue engineering, bioimaging, and biosensing. These materials are usually fabricated by adding colloidal nanoparticles to the uncured polymer mixture and thus require time-consuming presynthesis, purification, and ligand-exchange steps. Herein, we introduce approaches for rapid synthesis of gold nanostars (AuNSt) in situ on hydrogel substrates, including those with complex three-dimensional (3D) features. These methods enable selective AuNSt growth at the surface of the substrate, and the growth conditions can be tuned to tailor the nanoparticle size and density (coverage). We additionally demonstrate proof-of-concept applications of these nanocomposites for SERS sensing and imaging. High surface coverage with AuNSt enabled 1-2 orders of magnitude higher SERS signals compared to plasmonic hydrogels loaded with premade colloids. Importantly, AuNSt can be prepared without the addition of any potentially cytotoxic surfactants, thereby ensuring a high biocompatibility. Overall, in situ growth becomes a versatile and straightforward approach for the fabrication of plasmonic biomaterials.

Functionalized Single Crystal Perovskite Materials for SERS and Their Potential Detection Applications

Recent advances in detection and diagnostic tools have improved understanding and identification of plant physiological and biochemical processes. Effective and safe Surface Enhanced Raman Spectroscopy (SERS) can find objects quickly and accurately. Raman enhancement amplifies the signal by 1014-1015 to accurately quantify plant metabolites at the molecular level. This paper shows how to use functionalized perovskite substrates for SERS. These perovskite substrates have lots of surface area, intense Raman scattering, and high sensitivity and specificity. These properties eliminate sample matrix component interference. This study identified research gaps on perovskite substrates' effectiveness, precision, and efficiency in biological metabolite detection compared to conventional substrates. This article details the synthesis and use of functionalized perovskites for plant metabolites measurement. It analyzes their pros and cons in this context. The manuscript analyzes perovskite-based SERS substrates, including single-crystalline perovskites with enhanced optoelectronic properties. This manuscript aims to identify this study gap by comprehensively reviewing the literature and using it to investigate plant metabolite detection in future studies.

Crystallinity as a factor of SERS stability of silver nanoparticles formed by Ar+ irradiation

The plasmonic sensors based on silver nanoparticles are limited in application due to their relatively fast degradation in the ambient atmosphere. The technology of ion-beam modification for the creation of monocrystalline silver nanoparticles (NPs) with stable plasmonic properties will expand the application of silver nanostructures. In the present study, highly-stable monocrystalline NPs were formed on the basis of a thin silver film by low-energy ion irradiation. Combined with lithography, this technique allows the creation of nanoparticle ensembles in variant forms. The characterization of the nanoparticles formed by ion-beam modification showed long-term outstanding for Ag nanoparticles stability of their plasmonic properties due to their monocrystalline structure. According to optical spectroscopy data, the reliable plasmonic properties in the ambient atmosphere are preserved for up to 39 days. The mapping of crystal violet dye via surface-enhanced Raman spectroscopy (SERS) revealed a strong amplification factor sustaining at least thrice as long as the one of similarly sized polycrystalline silver NPs formed by annealing. The plasmonic properties sustain more than a month of storage in the ambient atmosphere. Thus, ion-beam modification of silver film makes it possible to fabricate NPs with stable plasmonic properties and form clusters of NPs for sensor technology and SERS applications.

Recent developments and applications of surface enhanced Raman scattering spectroscopy in safety detection of fruits and vegetables

This article reviewed the latest research progress of Surface-enhanced Raman Spectroscopy (SERS) in the security detection of fruits and vegetables in recent years, especially in three aspects: pesticide residues, microbial toxin contamination and harmful microorganism infection. The binding mechanism and application potential of SERS detection materials (including universal type and special type) and carrier materials (namely rigid and flexible materials) were discussed. Finally, the application prospect of SERS in fruit and vegetable safety detection was explored, and the problems to be solved and development trends were put forward. The poor stability and reproducibility of SERS substrates make it difficult for practical applications. It is necessary to continuously optimize SERS substrates and develop small and portable Raman spectroscopy analyzers. In the future, SERS technology is expected to play an important role in human health, food safety and economy.

Accurate Quantification and Imaging of Cellular Uptake Using Single-Particle Surface-Enhanced Raman Scattering

Understanding the uptake, distribution, and stability of gold nanoparticles (NPs) in cells is of fundamental importance in nanoparticle sensors and therapeutic development. Single nanoparticle imaging with surface-enhanced Raman spectroscopy (SERS) measurements in cells is complicated by aggregation-dependent SERS signals, particle inhomogeneity, and limited single-particle brightness. In this work, we assess the single-particle SERS signals of various gold nanoparticle shapes and the role of silica encapsulation on SERS signals to develop a quantitative probe for single-particle level Raman imaging in living cells. We observe that silica-encapsulated gap-enhanced Raman tags (GERTs) provide an optimized probe that can be quantifiable per voxel in SERS maps of cells. This approach is validated by single-particle inductively coupled mass spectrometry (spICP-MS) measurements of NPs in cell lysate post-imaging. spICP-MS also provides a means of measuring the tag stability. This analytical approach can be used not only to quantitatively assess nanoparticle uptake on the cellular level (as in previous digital SERS methods) but also to reliably image the subcellular distribution and to assess the stability of NPs in cells.

Wearable Plasmonic Sensors Engineered via Active-Site Maximization of TiVC MXene for Universal Physiological Monitoring at the Molecular Level

Two-dimensional transition metal carbon/nitrides (MXenes) are promising candidates to revolutionize next-generation wearable sensors as high-performance surface-enhanced Raman scattering (SERS) substrates. However, low sensitivity of pure MXene nanosheets and weak binding force or uncontrolled in situ growth of plasmonic nanoparticles on hybrid MXene composites limit their progress toward universal and reliable sensors. Herein, we designed and manufactured a highly sensitive, structurally stable wearable SERS sensor by in situ fabrication of plasmonic nanostructures on the flexible TiVC membranes via the maximization of chemically reducing sites using alkaline treatment. DFT calculations and experimental characterization demonstrated that the hydroxyl functional groups on the surface of MXenes can facilitate the reduction of metal precursors and the nucleation of gold nanoparticles (AuNPs) and can be covalently attached to AuNPs. Thus, the fabricated flexible TiVC-OH-Au sensor satisfied the rigorous mechanical requirements for wearable sensors. In addition, combining the electromagnetic (EM) enhancement from dense AuNPs formed by the activation of nucleation sites and charge transfer (CT) between target molecule and substrate induced by the abundant DOS near the Fermi level of TiVC, the fabricated sensor exhibits ultrasensitivity, long-term stability, good signal repeatability, and excellent mechanical durability. Moreover, the proof-of-concept application of the wearable SERS sensor in sweat sensing was demonstrated to monitor the content of nicotine, methotrexate, nikethamide, and 6-acetylmorphine in sweat at the molecular level, which was an important step toward the universality and practicality of the wearable sensing technology.

Advancing Brain Research through Surface-Enhanced Raman Spectroscopy (SERS): Current Applications and Future Prospects

Surface-enhanced Raman spectroscopy (SERS) has recently emerged as a potent analytical technique with significant potential in the field of brain research. This review explores the applications and innovations of SERS in understanding the pathophysiological basis and diagnosis of brain disorders. SERS holds significant advantages over conventional Raman spectroscopy, particularly in terms of sensitivity and stability. The integration of label-free SERS presents promising opportunities for the rapid, reliable, and non-invasive diagnosis of brain-associated diseases, particularly when combined with advanced computational methods such as machine learning. SERS has potential to deepen our understanding of brain diseases, enhancing diagnosis, monitoring, and therapeutic interventions. Such advancements could significantly enhance the accuracy of clinical diagnosis and further our understanding of brain-related processes and diseases. This review assesses the utility of SERS in diagnosing and understanding the pathophysiological basis of brain disorders such as Alzheimer's and Parkinson's diseases, stroke, and brain cancer. Recent technological advances in SERS instrumentation and techniques are discussed, including innovations in nanoparticle design, substrate materials, and imaging technologies. We also explore prospects and emerging trends, offering insights into new technologies, while also addressing various challenges and limitations associated with SERS in brain research.

Role of Surface Curvature in Gold Nanostar Properties and Applications

Gold nanostars (AuNSs) are nanoparticles with intricate three-dimensional structures and shape-dependent optoelectronic properties. For example, AuNSs uniquely display three distinct surface curvatures, i.e. neutral, positive, and negative, which provide different environments to adsorbed ligands. Hence, these curvatures are used to introduce different surface chemistries in nanoparticles. This review summarizes and discusses the role of surface curvature in AuNS properties and its impact on biomedical and chemical applications, including surface-enhanced Raman spectroscopy, contrast agent performance, and catalysis. We examine the main synthetic approaches to generate AuNSs, control their morphology, and discuss their benefits and drawbacks. We also describe the optical characteristics of AuNSs and discuss how these depend on nanoparticle morphology. Finally, we analyze how AuNS surface curvature endows them with properties distinctly different from those of other nanoparticles, such as strong electromagnetic fields at the tips and increased hydrophilic environments at the indentations, together making AuNSs uniquely useful for biosensing, imaging, and local chemical manipulation.

Surface Functionalization and Escherichia coli Detection Using Surface-Enhanced Raman Spectroscopy Driven by Functional Organic Polymer/Gold Nanofilm-Based Microfluidic Chip

In this work, a microfluidic prototype based on polymeric materials was developed to monitor surface processes using surface-enhanced Raman spectroscopy (SERS), keeping the reagents free of environmental contamination. The prototype was fabricated on poly(methyl methacrylic acid) (PMMA). A micrometric membrane of a functional organic polymer (FOP) based on p-terphenyl and bromopyruvic acid monomers was formed on the PMMA surface to promote the formation of metal nanoclusters. Au nanosized film was deposited on the FOP membrane to give rise to the SERS effect. A microchannel was formed on another piece of PMMA using micromachining. A representative 3D model of the prototype layer arrangement was built and simulated in COMSOL Multiphysics® to approximate the electric field distribution and calculate the power enhancement factor as the Au film changes over time. The fabrication process was characterized using UV-visible and Raman spectroscopies and XPS. The prototype was tested using a Raman microscope and liquid solutions of cysteamine and Escherichia coli (E. coli). The simulation results demonstrated that the morphological characteristics of the Au layer give rise to the SERS effect, and the power enhancement factor reaches values as high as 8.8 × 105 on the FOP surface. The characterization results showed the formation of the FOP and the Au film on PMMA and the surface functionalization with amine groups. The Raman spectra of the prototype showed temporal evolution as different compounds were deposited on the upper wall of the microchannel. Characteristic peaks associated with these compounds were detected with continuous monitoring over time. This prototype offers many benefits for applications like monitoring biological processes. Some advantages include timely surface evaluation while avoiding environmental harm, decreased use of reagents and samples, minimal interference with the process by measuring, and detecting microorganisms in just 1 h, as demonstrated with the E. coli sample.

Nanostructure Tuning of Gold Nanoparticles Films via Click Sintering

Colloidal metal nanoparticles dispersions are commonly used to create functional printed electronic devices and they typically require time-, energy- and equipment-consuming post-treatments to improve their electrical and mechanical properties. Traditional methods, e.g. thermal, UV/IR, and microwave treatments, limit the substrate options and may require expensive equipment, not available in all the laboratories. Moreover, these processes also cause the collapse of the film (nano)pores and interstices, limiting or impeding its nanostructuration. Finding a simple approach to obtain complex nanostructured materials with minimal post-treatments remains a challenge. In this study, a new sintering method for gold nanoparticle inks that called as "click sintering" has been reported. The method uses a catalytic reaction to enhance and tune the nanostructuration of the film while sintering the metallic nanoparticles, without requiring any cumbersome post-treatment. This results in a conductive and electroactive nanoporous thin film, whose properties can be tuned by the conditions of the reaction, i.e., concentration of the reagent and time. Therefore, this study presents a novel and innovative one-step approach to simultaneously sinter gold nanoparticles films and create functional nanostructures, directly and easily, introducing a new concept of real-time treatment with possible applications in the fields of flexible electronics, biosensing, energy, and catalysis.

Plasmonic Nanostructure Biosensors: A Review

Plasmonic nanostructure biosensors based on metal are a powerful tool in the biosensing field. Surface plasmon resonance (SPR) can be classified into localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (PSPP), based on the transmission mode. Initially, the physical principles of LSPR and PSPP are elaborated. In what follows, the recent development of the biosensors related to SPR principle is summarized. For clarity, they are categorized into three groups according to the sensing principle: (i) inherent resonance-based biosensors, which are sensitive to the refractive index changes of the surroundings; (ii) plasmon nanoruler biosensors in which the distances of the nanostructure can be changed by biomolecules at the nanoscale; and (iii) surface-enhanced Raman scattering biosensors in which the nanostructure serves as an amplifier for Raman scattering signals. Moreover, the advanced application of single-molecule detection is discussed in terms of metal nanoparticle and nanopore structures. The review concludes by providing perspectives on the future development of plasmonic nanostructure biosensors.

Surface-enhanced Raman imaging through sprayed probes for the application in chemical visualization of methamphetamine within fingerprints

For fingerprint-involved forensic investigations, cyanoacrylates and inorganic nanophosphors are mostly used for fingerprint visualization. However, methods to simultaneously report fingerprint images and the corresponding specific chemical information have yet to be realized. In this work, chemical visualization of the analytes in fingerprints is achieved through surface-enhanced Raman spectroscopy (SERS) measurements with the aid of spray-dispersed gold nanorods (AuNRs). The optimal coverage of AuNRs was studied by theoretical simulations and experimental operations. A rapid sampling of fingerprints with the chemical of interest was developed by tuning the spray parameters. In particular, the SERS imaging of methamphetamine in fingerprint latent was attempted by addressing the SERS spectral features of methamphetamine. This chemical visualization method reflects both the graphical and chemical characteristics of fingerprints in a single batch measurement, in which methamphetamine can be detected and mapped at the concentration of 10^-5 M. The data processing approach was also modified by employing relevant logical judgments. The improved SERS images with sharpened patterns of fingerprints were obtained by involving the scored multi-peak judgments.

SERS performance of GaN/Ag substrates fabricated by Ag coating of GaN platforms

The results of comparative studies on the fabrication and characterization of GaN/Ag substrates using pulsed laser deposition (PLD) and magnetron sputtering (MS) and their evaluation as potential substrates for surface-enhanced Raman spectroscopy (SERS) are reported. Ag layers of comparable thicknesses were deposited using PLD and MS on nanostructured GaN platforms. All fabricated SERS substrates were examined regarding their optical properties using UV-vis spectroscopy and regarding their morphology using scanning electron microscopy. SERS properties of the fabricated GaN/Ag substrates were evaluated by measuring SERS spectra of 4-mercaptobenzoic acid molecules adsorbed on them. For all PLD-made GaN/Ag substrates, the estimated enhancement factors were higher than for MS-made substrates with a comparable thickness of the Ag layer. In the best case, the PLD-made GaN/Ag substrate exhibited an approximately 4.4 times higher enhancement factor than the best MS-made substrate.

Ultra-trace SERS detection of cocaine and heroin using bimetallic gold-silver nanostars (BGNS-Ag)

A rapid, in-field, and reliable method for the detection of illegal drugs of abuse in biological fluids without any sample pretreatment would potentially be helpful for law enforcement, drug control officials, and public healthcare. In this study, we presented a cost-effective and highly reproducible solution-based surface-enhanced Raman scattering (SERS) platform utilizing a portable Raman instrument for fast sensitive SERS detection of illegal drugs, such as cocaine, and heroin in human urine without any sample preprocessing. The SERS platform was constructed for the first time by combining the superior SERS enhancement properties of bimetallic silver-coated gold nanostars (BGNS-Ag) and the advantages of suitable alkaline metal salts such as NaI for SERS signal amplification. The effects of the silver thickness of BGNS-Ag and alkaline salts on the SERS performance were investigated in detail; we observed that the maximum SERS enhancement was obtained for BGNS-Ag with the maximum silver thickness (54 ± 5 nm) in presence of NaI salt. Our SERS platform shows ultra-high sensitivity of cocaine and heroin with a limit of detection (LOD) as low as 10 pg/mL for cocaine and 100 pg/mL for heroin, which was 100 times lower than that of the traditional silver nanoparticle-based illegal drug detection. As a demonstration, the platform was further applied to detect cocaine and heroin spiked in human urine without any sample preprocessing achieving a LOD of 100 pg/mL for cocaine and 1 ng/mL for heroin. Overall, our SERS detection platform shows potential for rapid, onsite, ultra-low-cost portable applications for trace detection of illegal drugs and biomarkers.

Application of SERS-based nanobiosensors to metabolite biomarkers of CKD

A clinical diagnosis of chronic kidney disease (CKD) is commonly achieved by estimating the serum levels of urea and creatinine (CR). Given the limitations of the conventional diagnostic assays, it is imperative to seek alternative, economical strategies for the detection of CKD-specific biomarkers with high specificity and selectivity. In this respect, surface-enhanced Raman spectroscopy (SERS) can be regarded as an ideal choice. SERS signals can be greatly amplified by noble metal nanoparticles (e.g., gold nanoparticles (GNPs)) of numerous sizes, shapes, and configurations to help achieve ultra-sensitive single molecule-level detection at 10^-15 M (up to 10 orders of magnitude more sensitive than fluorescence-based detection). The irregular geometry of GNPs with spike-like tips, dimers, and aggregates with small nanogaps (i.e., due to plasmon coupling such as Raman hot spots) play a pivotal role in enhancing the specificity and sensitivity of SERS. This review critically outlines the performance of SERS-based biosensors in the ultrasensitive detection of CKD biomarkers in various body fluids in terms of basic quality assurance parameters (e.g., limit of detection, figure of merit, enhancement factor, and stability of the biosensor). Moreover, the challenges and perspectives are described with respect to the expansion of such sensing techniques in practical clinical settings.

A hybrid plasmonic nanoprobe using polyvinylpyrrolidone-capped bimetallic silver-gold nanostars for highly sensitive and reproducible solution-based SERS sensing

Practical solution-based assays using surface-enhanced Raman spectroscopy (SERS) with portable instrumentation are currently of particular interest for rapid, efficient, and low-cost detection of analytes. However, current assays still have limited applicability due to their poor sensitivity and reproducibility. Herein, we demonstrate highly stable polyvinylpyrrolidone (PVP)-capped bimetallic silver-coated gold nanostars (BGNS-Ag-PVP) as a solution-based SERS nanoprobe that is capable of producing a strong, uniform, and reproducible SERS signal using a portable Raman instrument. The developed hybrid BGNS-Ag-PVP nanostructure shows tunable optical properties with improved SERS sensitivity and reproducibility as compared to gold nanostars. We have synthesized bimetallic nanoprobes BGNS-Ag-PVP having three different silvers, referred to as BGNS-Ag-PVP-1, BGNS-Ag-PVP-2, and BGNS-Ag-PVP-3. The SERS performance of BGNS-Ag-PVP was studied using methylene blue (Meb) as a probe molecule, and we achieved a detection limit of up to 10 nM indicating the high sensitivity of the solution-based SERS platform. The application of such bimetallic nanoparticles is demonstrated via the sensitive detection of the antithyroid drug methimazole (Mz) used as a model analyte system. We have achieved a detection limit of 1 nM for Mz spiked with human urine indicating three orders of magnitude lower than previously reported solution-based SERS detection methods. Furthermore, the SERS performance was reproducible over 3 months indicating excellent stability and repeatability. The result illustrates the potential of this solution-based SERS detection platform as a promising sensing tool for analytes such as illicit drugs, and biomarkers that have affinity to bind on nanoprobes.

Interface Design of 3D Flower-like Ag@ZnSe Composites: SERS and Photocatalytic Performance

In this work, we cosputtered Ag and ZnSe on a polystyrene template to form a three-dimensional (3D) Ag@ZnSe (x) structure. The 3D surface morphology and material composition that provided abundant "hot spots" were controlled by adjusting the sputtering power of the ZnSe, which was confirmed by finite-difference time-domain (FDTD) simulation. The introduction of ZnSe into the noble metal Ag also introduced a charge-transfer (CT) effect into the system, and the CT path was proven with the two-dimensional correlation spectroscopy (2D-COS)-surface-enhanced Raman scattering (SERS) technique. In addition, the substrate exhibited excellent catalytic activity due to the CT effect. The catalyzed degradation of malachite green (MG) was due to the CT effect in the system, and the catalytic process was successfully monitored by in situ SERS. Most importantly, the catalytic degradation by Ag@ZnSe (x) with different parameters was proportional to the degree of CT (ρ_CT). The SERS and catalytic mechanisms were analyzed in depth with the 2D-COS-SERS technique, which was also useful in verifying the CT process. The catalytic sites for MG were successfully monitored with the 2D-COS-SERS technique. This study provides a reference for studies of the synergistic effects of the electromagnetic mechanism and CT, as well as a new perspective on photocatalysis with dye molecules and monitoring of the catalytic processes.

Designing structures that maximize spatially averaged surface-enhanced Raman spectra

We present a general framework for inverse design of nanopatterned surfaces that maximize spatially averaged surface-enhanced Raman (SERS) spectra from molecules distributed randomly throughout a material or fluid, building upon a recently proposed trace formulation for optimizing incoherent emission. This leads to radically different designs than optimizing SERS emission at a single known location, as we illustrate using several 2D design problems addressing effects of hot-spot density, angular selectivity, and nonlinear damage. We obtain optimized structures that perform about 4 × better than coating with optimized spheres or bowtie structures and about 20 × better when the nonlinear damage effects are included.

Discontinuous Galerkin integral equation method for light scattering from complex nanoparticle assemblies

This paper presents a discontinuous Galerkin (DG) integral equation (IE) method for the electromagnetic analysis of arbitrarily-shaped plasmonic assemblies. The use of nonconformal meshes provides improved flexibility for CAD prototyping and tessellation of the input geometry. The formulation can readily address nonconformal multi-material junctions (where three or more material regions meet), allowing to set very different mesh sizes depending on the material properties of the different subsystems. It also enables the use of h-refinement techniques to improve accuracy without burdening the computational cost. The continuity of the equivalent electric and magnetic surface currents across the junction contours is enforced by a combination of boundary conditions and local, weakly imposed, interior penalties within the junction regions. A comprehensive study is made to compare the performance of different IE-DG alternatives applied to plasmonics. The numerical experiments conducted validate the accuracy and versatility of this formulation for the resolution of complex nanoparticle assemblies.

Microfluidic SERS devices: brightening the future of bioanalysis

A new avenue has opened up for applications of surface-enhanced Raman spectroscopy (SERS) in the biomedical field, mainly due to the striking advantages offered by SERS tags. SERS tags provide indirect identification of analytes with rich and highly specific spectral fingerprint information, high sensitivity, and outstanding multiplexing potential, making them very useful in in vitro and in vivo assays. The recent and innovative advances in nanomaterial science, novel Raman reporters, and emerging bioconjugation protocols have helped develop ultra-bright SERS tags as powerful tools for multiplex SERS-based detection and diagnosis applications. Nevertheless, to translate SERS platforms to real-world problems, some challenges, especially for clinical applications, must be addressed. This review presents the current understanding of the factors influencing the quality of SERS tags and the strategies commonly employed to improve not only spectral quality but the specificity and reproducibility of the interaction of the analyte with the target ligand. It further explores some of the most common approaches which have emerged for coupling SERS with microfluidic technologies, for biomedical applications. The importance of understanding microfluidic production and characterisation to yield excellent device quality while ensuring high throughput production are emphasised and explored, after which, the challenges and approaches developed to fulfil the potential that SERS-based microfluidics have to offer are described.

Toward Quantitative Surface-Enhanced Raman Scattering with Plasmonic Nanoparticles: Multiscale View on Heterogeneities in Particle Morphology, Surface Modification, Interface, and Analytical Protocols

Surface-enhanced Raman scattering (SERS) provides significantly enhanced Raman scattering signals from molecules adsorbed on plasmonic nanostructures, as well as the molecules' vibrational fingerprints. Plasmonic nanoparticle systems are particularly powerful for SERS substrates as they provide a wide range of structural features and plasmonic couplings to boost the enhancement, often up to >10⁸-10¹⁰. Nevertheless, nanoparticle-based SERS is not widely utilized as a means for reliable quantitative measurement of molecules largely due to limited controllability, uniformity, and scalability of plasmonic nanoparticles, poor molecular modification chemistry, and a lack of widely used analytical protocols for SERS. Furthermore, multiscale issues with plasmonic nanoparticle systems that range from atomic and molecular scales to assembled nanostructure scale are difficult to simultaneously control, analyze, and address. In this perspective, we introduce and discuss the design principles and key issues in preparing SERS nanoparticle substrates and the recent studies on the uniform and controllable synthesis and newly emerging machine learning-based analysis of plasmonic nanoparticle systems for quantitative SERS. Specifically, the multiscale point of view with plasmonic nanoparticle systems toward quantitative SERS is provided throughout this perspective. Furthermore, issues with correctly estimating and comparing SERS enhancement factors are discussed, and newly emerging statistical and artificial intelligence approaches for analyzing complex SERS systems are introduced and scrutinized to address challenges that cannot be fully resolved through synthetic improvements.

Gold Nanostar Characterization by Nanoparticle Tracking Analysis

We demonstrate the application of nanoparticle tracking analysis (NTA) for the quantitative characterization of gold nanostars (GNSs). GNSs were synthesized by the seed-mediated growth method using triblock copolymer (TBP) gold nanoparticles (GNPs). These GNPs (≈ 10 nm) were synthesized from Au³⁺ (≈ 1 mM) in aqueous F127 (w/v 5%) containing the co-reductant ascorbic acid (≈ 2 mM). The GNS tip-to-core aspect ratio (AR) decreased when higher concentrations of GNPs were added to the growth solution. The AR dependency of GNSs on Au³⁺/Au(seed) concentration ratio implies that growth is partly under kinetic control. NTA measured GNS sizes, concentrations, and relative scattering intensities. Molar absorption coefficients ∼ 10⁹-10¹⁰ M^-1 cm^-1 (ε_400 nm) for each batch of GNSs were determined using the combination of extinction spectra and NTA concentrations for heterogeneous samples. NTA in combination with UV-vis was used to derive the linear relationships: (1) hydrodynamic size versus localized surface plasmon peak maxima; (2) ε_400 nm versus localized surface plasmon peak maxima; (3) ε_400 nm versus hydrodynamic size. NTA for quantitative characterization of anisotropic nanoparticles could lead to future applications, including heterogeneous colloidal catalysis.

Morphological and Optical Transitions during Micelle-Seeded Chiral Growth on Gold Nanorods

Chiral plasmonics is a rapidly developing field where breakthroughs and unsolved problems coexist. We have recently reported binary surfactant-assisted seeded growth of chiral gold nanorods (Au NRs) with high chiroptical activity. Such a seeded-growth process involves the use of a chiral cosurfactant that induces micellar helicity, in turn driving the transition from achiral to chiral Au NRs, from both the morphological and the optical points of view. We report herein a detailed study on both transitions, which reveals intermediate states that were hidden so far. The correlation between structure and optical response is carefully analyzed, including the (linear and CD) spectral evolution over time, electron tomography, the impact of NR dimensions on their optical response, the variation of the absorption-to-scattering ratio during the evolution from achiral to chiral Au NRs, and the near-field enhancement related to chiral plasmon modes. Our findings provide further understanding of the growth process of chiral Au NRs and the associated optical changes, which will facilitate further study and applications of chiral nanomaterials.

Emerging SERS biosensors for the analysis of cells and extracellular vesicles

Cells and their derived extracellular vesicles (EVs) or exosomes contain unique molecular signatures that could be used as biomarkers for the detection of severe diseases such as cancer, as well as monitoring the treatment response. Revealing these molecular signatures requires developing non-invasive ultrasensitive tools to enable single molecule/cell-level detection using a small volume of sample with low signal-to-noise ratio background and multiplex capability. Surface-enhanced Raman scattering (SERS) can address the current limitations in studying cells and EVs through two main mechanisms: plasmon-enhanced electric field (the so-called electromagnetic mechanism (EM)), and chemical mechanism (CM). In this review, we first highlight these two SERS mechanisms and then discuss the nanomaterials that have been used to develop SERS biosensors based on each of the aforementioned mechanisms as well as the combination of these two mechanisms in order to take advantage of the synergic effect between electromagnetic enhancement and chemical enhancement. Then, we review the recent advances in designing label-aided and label-free SERS biosensors in both colloidal and planar systems to investigate the surface biomarkers on cancer cells and their derived EVs. Finally, we discuss perspectives of emerging SERS biosensors in future biomedical applications. We believe this review article will thus appeal to researchers in the field of nanobiotechnology including material sciences, biosensors, and biomedical fields.

Multiplex Sensing Based on Plasmonic Optics of Noble Metallic Nanostructures

Since the colorimetric method has the characteristics of being simple and low cost, the fluorescence spectrum has the characteristics of a strong signal, and Surface-enhanced Raman scattering (SERS) detection has the characteristics of high sensitivity and strong specificity, people usually use these three methods for detection, but the detection of a single sample takes more time. If multiple samples can be tested at the same time, the detection efficiency and sensitivity can be improved, and the selectivity and reliability will be greatly improved. Multiplex sensing also provides a new direction for researchers. To fully understand the research of multiplex sensing based on the plasmonic optics of noble metal nanostructures, this review summarizes all the results previously reported in this field. It also discusses the principles of various detection methods and the biochemical application of multiple detections and finally summarizes the challenges and prospects.

Recent developments in the use of gold and silver nanoparticles in biomedicine

Gold and silver nanoparticles (NPs) are widely used in the biomedical research both in the therapeutic and the sensing/diagnostics fronts. Both metals share some common optical properties with surface plasmon resonance being the most widely exploited property in therapeutics and diagnostics. Au NPs exhibit excellent light‐to‐heat conversion efficiencies and hence have found applications primarily in precision oncology, while Ag NPs have excellent antibacterial properties which can be harnessed in biomaterials' design. Both metals constitute excellent biosensing platforms owing to their plasmonic properties and are now routinely used in various optical platforms. The utilization of Au and Ag NPs in the COVID‐19 pandemic was rapidly expanded mostly in biosensing and point‐of‐care platforms and to some extent in therapeutics. In this review article, the main physicochemical properties of Au and Ag NPs are discussed with selective examples from the recent literature.

This article is categorized under:

Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease

Diagnostic Tools > In Vitro Nanoparticle‐Based Sensing

Nanotechnology Approaches to Biology > Nanoscale Systems in Biology

Simultaneous determination of benzimidazole fungicides in food using signal optimized label-free HAu/Ag NS-SERS sensor

Extensively employed pesticide in agriculture causes residue in food products that would threaten public health safety. The surface-enhanced Raman scattering (SERS) signal reliant on double sensing of carbendazim and thiabendazole in a single step is achieved without the aid of any bio-recognition element. A label-free anisotropic bimetallic hollow Au/Ag nanostars (HAu/Ag NS) SERS substrate was synthesized with numerous hot spots for Raman molecule through a galvanic displacement-free deposition. The individual and mixed analyte calibration results were compared based on the identified peak at 1224 (carbendazim) and 778 (thiabendazole) cm^-1 and exhibited insignificant differences. The sensor could detect carbendazim and thiabendazole up to 4.28 × 10^-4 and 6.04 × 10^-4 µg·g^-1 or µg·mL^-1 in both individual and mixture of their extract. The recovery for accuracy and precision analysis was 91.54-98.26 % in rice and water. Finally, validation results were achieved satisfactorily (p > 0.05) with HPLC.

Highly Sensitive Flexible SERS-Based Sensing Platform for Detection of COVID-19

COVID-19 continues to spread and has been declared a global emergency. Individuals with current or past infection should be identified as soon as possible to prevent the spread of disease. Surface-enhanced Raman spectroscopy (SERS) is an analytical technique that has the potential to be used to detect viruses at the site of therapy. In this context, SERS is an exciting technique because it provides a fingerprint for any material. It has been used with many COVID-19 virus subtypes, including Deltacron and Omicron, a novel coronavirus. Moreover, flexible SERS substrates, due to their unique advantages of sensitivity and flexibility, have recently attracted growing research interest in real-world applications such as medicine. Reviewing the latest flexible SERS-substrate developments is crucial for the further development of quality detection platforms. This article discusses the ultra-responsive detection methods used by flexible SERS substrate. Multiplex assays that combine ultra-responsive detection methods with their unique biomarkers and/or biomarkers for secondary diseases triggered by the development of infection are critical, according to this study. In addition, we discuss how flexible SERS-substrate-based ultrasensitive detection methods could transform disease diagnosis, control, and surveillance in the future. This study is believed to help researchers design and manufacture flexible SERS substrates with higher performance and lower cost, and ultimately better understand practical applications.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

Optimization of laser deposited silver nanoparticle substrates for surface-enhanced raman spectroscopy

Creating sensitive and reproducible substrates for surface-enhanced Raman spectroscopy (SERS) has been a challenge in recent years. While SERS offers significant benefits over traditional Raman spectroscopy, certain hindrances have limited their commercial use, especially in settings where low limits of detection are necessary. We studied a variety of laser-deposited silver microstructured SERS substrates with different morphology as a means to optimize analyte detection. We found that using a 405 nm laser to deposit lines of silver nanoparticles (AgNPS) from a 2 mM silver nitrate and sodium citrate solution offered not only the best enhancement, but also the most consistent and reproducible substrates. We also found that the probability of deposition by laser was wavelength dependent and that longer wavelengths were less likely to deposit than shorter wavelengths. This work offers a better understanding of the laser deposition process as well as how substrate shape and structure effect SERS signals.

Fast synthesis of gold nanostar SERS substrates based on ion-track etched membrane by one-step redox reaction

Surface-enhanced Raman scattering (SERS), due to its high detecting sensitivity and rapid data acquisition ability, has been considered as a powerful technique for label-free ultrasensitive detection of chemical and biochemical analytes. As an important part, the uniform SERS substrate is the prerequisite for this technology being used in all the related areas. Therefore, seeking the fast, convenient and low-cost way to obtain the SERS substrate with high performance and reproducibility never stops in recent decades. In this work, the PC membrane with uniform nanopores obtained by ion irradiation and chemical etching (i.e., ion-track etched PC membrane) was first used to prepare the gold nanostar SERS substrate. The monolayer gold nanostars can be obtained through a one-step redox reaction on the surface of the PC membrane, which not only can act as the base of SERS substrate but also can work as the reaction adjuster. By optimizing the growth conditions, the SERS substrate with uniform monolayer gold nanostars can be fabricated without any complicated procedures and costly equipment fast (in 20 mins). Meanwhile, the prepared flexible gold nanostar SERS substrate exhibits excellent Raman performance, which can effectively detect the analyte R6G with the concentration as low as 1 × 10^-10 M and the SERS enhancement factors can be around 3.70 × 10⁵. The new facile SERS substrate preparation method is cost-effective, convenient, fast and easily scale up, which can satisfy the requests of the real applications in many fields.

Machine Learning-Assisted Sampling of Surfance-Enhanced Raman Scattering (SERS) Substrates Improve Data Collection Efficiency

Surface-enhanced Raman scattering (SERS) is a powerful technique for sensitive label-free analysis of chemical and biological samples. While much recent work has established sophisticated automation routines using machine learning and related artificial intelligence methods, these efforts have largely focused on downstream processing (e.g., classification tasks) of previously collected data. While fully automated analysis pipelines are desirable, current progress is limited by cumbersome and manually intensive sample preparation and data collection steps. Specifically, a typical lab-scale SERS experiment requires the user to evaluate the quality and reliability of the measurement (i.e., the spectra) as the data are being collected. This need for expert user-intuition is a major bottleneck that limits applicability of SERS-based diagnostics for point-of-care clinical applications, where trained spectroscopists are likely unavailable. While application-agnostic numerical approaches (e.g., signal-to-noise thresholding) are useful, there is an urgent need to develop algorithms that leverage expert user intuition and domain knowledge to simplify and accelerate data collection steps. To address this challenge, in this work, we introduce a machine learning-assisted method at the acquisition stage. We tested six common algorithms to measure best performance in the context of spectral quality judgment. For adoption into future automation platforms, we developed an open-source python package tailored for rapid expert user annotation to train machine learning algorithms. We expect that this new approach to use machine learning to assist in data acquisition can serve as a useful building block for point-of-care SERS diagnostic platforms.

Centrifugation-induced assembly of dense hotspots based SERS substrate for enhanced Raman scattering and quenched fluorescence

Hanging (aggregation stuck to the centrifugal tube) in the centrifugation process is always regarded as an unwanted condition. In this work, we develop a centrifugation-induced assembly of dense hotspots surface-enhanced Raman scattering (SERS) substrates from the hanging phenomenon. We discovered interesting sintering-resistant behavior (maintain the sharp nanotip features) of star-like Au nanoparticles after centrifugation-induced assembly, which is in stark contrast with the sintering phenomenon of sphere-like nanoparticles. We also found that one side of centrifugal-induced Au assemblies is two-dimensional (2D, root mean square (rms) roughness down to ∼10 nm), while the other is three-dimensional (3D, rms roughness more than 100 nm). The close-packed feature of the Au assemblies makes them candidates as dense hotspots based SERS substrates. Through systematic investigation of SERS performance of centrifugation-induced assemblies with different morphology (star-like and sphere-like, 2D and 3D), it was found that the 3D side of star-like Au nanoparticles assembly exhibits the highest SERS enhancement together with quenched fluorescence. The star-like SERS substrate also displays high detection uniformity (with 10^-7M Rhodamine 6G) and a low detection limit (down to 10^-12M Rhodamine 6G).

Universal Theory of Light Scattering of Randomly Oriented Particles: A Fluctuational-Electrodynamics Approach for Light Transport Modeling in Disordered Nanostructures

Disordered nanostructures are commonly encountered in many nanophotonic systems, from colloid dispersions for sensing to heterostructured photocatalysts. Randomness, however, imposes severe challenges for nanophotonics modeling, often constrained by the irregular geometry of the scatterers involved or the stochastic nature of the problem itself. In this Article, we resolve this conundrum by presenting a universal theory of averaged light scattering of randomly oriented objects. Specifically, we derive expansion-basis-independent formulas of the orientation-and-polarization-averaged absorption cross section, scattering cross section, and asymmetry parameter, for single or a collection of objects of arbitrary shape. These three parameters can be directly integrated into traditional unpolarized radiative energy transfer modeling, enabling a practical tool to predict multiple scattering and light transport in disordered nanostructured materials. Notably, the formulas of average light scattering can be derived under the principles of fluctuational electrodynamics, allowing the analogous mathematical treatment to the methods used in thermal radiation, nonequilibrium electromagnetic forces, and other associated phenomena. The proposed modeling framework is validated against optical measurements of polymer composite films with metal-oxide microcrystals. Our work may contribute to a better understanding of light-matter interactions in disordered systems, such as plasmonics for sensing and photothermal therapy, photocatalysts for water splitting and CO₂ dissociation, photonic glasses for artificial structural colors, and diffuse reflectors for radiative cooling, to name just a few.

Metal-organic framework engineered corn-like SERS active Ag@Carbon with controllable spacing distance for tracking trace amount of organic compounds

Probing water-soluble organic compounds via Surface-enhanced Raman scattering (SERS) technique could be helpful to prevent harmful impacts of polluted water. A key limitation of restraining SERS technique in probing these pollutants is the difficulty to control the spacing distance of plasmonic nanoparticles within 10 nm so that SERS effect can be efficiently induced. Herein, a strategy of mass-producing Ag-based SERS active material with tunable spacing distance is reported. In brevity, metal-organic framework (MOF) engineered corn-like Ag@Carbon is synthesized by simply thermal treating Ag-MOF. The thermal treatment in-situ turns Ag⁺ into Ag nanoparticles (NPs), resulting in Ag NPs well-dispersed on the surface of the carbonized MOF and forming ordered SERS hotspots. Due to the spatial distance of Ag⁺ directly depends on the molecular diameter of MOF organic ligands, spacing distance of Ag NP is fixed at around 7 nm. Theoretical analysis and experimental study confirm that the uniformly distributed Ag NPs lead to desirable SERS activity. Further study evidences the presented corn-like Ag@Carbon could be a good candidate for tacking organic compounds with satisfactory sensitivity, specificity and low detection limit (10^-8 M). Conclusively, these impressive results indicate a bright future of adopting the proposed strategy to design future SERS active materials.

Surfactant-free Synthesis of Spiky Hollow Ag-Au Nanostars with Chemically Exposed Surfaces for Enhanced Catalysis and Single-Particle SERS

Spiky/hollow metal nanoparticles have applications across a broad range of fields. However, the current bottom-up methods for producing spiky/hollow metal nanoparticles rely heavily on the use of strongly adsorbing surfactant molecules, which is undesirable because these passivate the product particles' surfaces. Here we report a high-yield surfactant-free synthesis of spiky hollow Au-Ag nanostars (SHAANs). Each SHAAN is composed of >50 spikes attached to a hollow ca. 150 nm diameter cubic core, which makes SHAANs highly plasmonically and catalytically active. Moreover, the surfaces of SHAANs are chemically exposed, which gives them significantly enhanced functionality compared with their surfactant-capped counterparts, as demonstrated in surface-enhanced Raman spectroscopy (SERS) and catalysis. The chemical accessibility of the pristine SHAANs also allows the use of hydroxyethyl cellulose as a weakly bound stabilizing agent. This produces colloidal SHAANs that remain stable for >1 month while retaining the functionalities of the pristine particles and allows even single-particle SERS to be realized.

Recent Advances in Metallic Nanoparticle Assemblies for Surface-Enhanced Spectroscopy

Robust and versatile strategies for the development of functional nanostructured materials often focus on assemblies of metallic nanoparticles. Research interest in such assemblies arises due to their potential applications in the fields of photonics and sensing. Metallic nanoparticles have received considerable recent attention due to their connection to the widely studied phenomenon of localized surface plasmon resonance. For instance, plasmonic hot spots can be observed within their assemblies. A useful form of spectroscopy is based on surface-enhanced Raman scattering (SERS). This phenomenon is a commonly used in sensing techniques, and it works using the principle that scattered inelastic light can be greatly enhanced at a surface. However, further research is required to enable improvements to the SERS techniques. For example, one question that remains open is how to design uniform, highly reproducible, and efficiently enhancing substrates of metallic nanoparticles with high structural precision. In this review, a general overview on nanoparticle functionalization and the impact on nanoparticle assembly is provided, alongside an examination of their applications in surface-enhanced Raman spectroscopy.

Hollow Gold Nanosphere Templated Synthesis of PEGylated Hollow Gold Nanostars and Use for SERS Detection of Amyloid Beta in Solution

Hollow gold nanospheres (HGNs) have been used as the template for seed-mediated growth of multibranched hollow gold nanostars (HNS). The HGNs were synthesized via anerobic reduction of cobalt chloride to cobalt nanoparticles and then formation of a gold shell via galvanic replacement followed by the oxidation of the cobalt core. We obtained control of the inner core size of the HGNs by increasing the size of the sacrificial cobalt core and by varying the ratio of B(OH)₃/BH₄ using boric acid rather than 48 h aged borohydride. We synthesized the HNS by reducing Au³⁺ ions in the presence of Ag⁺ ions using ascorbic acid, creating a spiky morphology that varied with the Au³⁺/Ag⁺ ratio. A broadly tunable localized surface plasmon resonance was achieved through control of both the inner core and the spike length. Amyloid beta (Aβ) was conjugated to the HNS by using a heterobifunctional PEG linker and identified by the vibrational modes associated with the conjugated ring phenylalanine side chain. A bicinchoninic acid assay was used to determine the concentration of Aβ conjugated to HNS as 20 nM, which is below the level of Aβ that negatively affects long-term potentiation. Both the core size and spike length were shown to affect the optical properties of the resulting nanostructures. This HGN templated method introduced a new parameter for enhancing the plasmonic properties of gold nanostars, namely, the addition of a hollow core. Hollow gold nanostars are highly desirable for a wide range of applications, including high sensitivity disease detection and monitoring.

Universal Fabrication of Highly Efficient Plasmonic Thin-Films for Label-Free SERS Detection

The development of novel, highly efficient, reliable, and robust surface enhanced Raman scattering (SERS) substrates containing a large number of hot spots with programmed size, geometry, and density is extremely interesting since it allows the sensing of numerous (bio-)chemical species. Herein, an extremely reliable, easy to fabricate, and label-free SERS sensing platform based on metal nanoparticles (NPs) thin-film is developed by the layer-by-layer growth mediated by polyelectrolytes. A systematic study of the effect of NP composition and size, as well as the number of deposition steps on the substrate's performance, is accomplished by monitoring the SERS enhancement of 1-naphtalenethiol (532 nm excitation). Distinct evidence of the key role played by the interlayer (poly(diallyldimethylammonium chloride) (PDDA) or PDDA-functionalized graphene oxide (GO@PDDA)) on the overall SERS efficiency of the plasmonic platforms is provided, revealing in the latter the formation of more uniform hot spots by regulating the interparticle distances to 5 ± 1 nm. The SERS platform efficiency is demonstrated via its high analytical enhancement factor (≈10⁶ ) and the detection of a prototypical substance(tamoxifen), both in Milli-Q water and in a real matrix, viz. tap water, opening perspectives towards the use of plasmonic platforms for future high-performance sensing applications.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Applications of Raman Spectroscopy in Bacterial Infections: Principles, Advantages, and Shortcomings

Infectious diseases caused by bacterial pathogens are important public issues. In addition, due to the overuse of antibiotics, many multidrug-resistant bacterial pathogens have been widely encountered in clinical settings. Thus, the fast identification of bacteria pathogens and profiling of antibiotic resistance could greatly facilitate the precise treatment strategy of infectious diseases. So far, many conventional and molecular methods, both manual or automatized, have been developed for in vitro diagnostics, which have been proven to be accurate, reliable, and time efficient. Although Raman spectroscopy (RS) is an established technique in various fields such as geochemistry and material science, it is still considered as an emerging tool in research and diagnosis of infectious diseases. Based on current studies, it is too early to claim that RS may provide practical guidelines for microbiologists and clinicians because there is still a gap between basic research and clinical implementation. However, due to the promising prospects of label-free detection and noninvasive identification of bacterial infections and antibiotic resistance in several single steps, it is necessary to have an overview of the technique in terms of its strong points and shortcomings. Thus, in this review, we went through recent studies of RS in the field of infectious diseases, highlighting the application potentials of the technique and also current challenges that prevent its real-world applications.