Facing Challenges in Real-Life Application of Surface-Enhanced Raman Scattering: Design and Nanofabrication of Surface-Enhanced Raman Scattering Substrates for Rapid Field Test of Food Contaminants

Fabrication of highly sensitive and reproducible 3D surface-enhanced Raman spectroscopy substrates through in situ cleaning and layer-by-layer assembly of Au@Ag nanocube monolayer film

A highly sensitive and uniform three-dimensional (3D) surface-enhanced Raman spectroscopy (SERS) substrate has been fabricated by in situ ultraviolet ozone cleaning and layer-by-layer self-assembly. The SERS properties and the structural changes of the substrates were systematically studied by adjusting the cleaning time upon the in situ and post cleaning strategy. Under the optimal cleaning condition, the cleaning technology could give rise to clean and optimal surfaces for SERS analysis, thus obtaining efficient plasmonic films populated with a large number of homogeneous 'hot-spots'. Then with the optimal monolayer film, the SERS performance derived from plasmon coupling in multilayers of the Au@Ag nanocubes substrates was explored. It demonstrated that the plasmon coupling between layers (out-of-plane) contributed much to the SERS intensity, leading a more superior SERS enhancement from the 3D SERS substrates than that from the conventional two-dimensional SERS substrates. Also the in situ cleaning 3D SERS substrates displayed a nice uniformity and excellent time stability. With this method, the optimized substrates were further used to detect prohibited pigments in drink with an excellent linear relationship between characteristic peak intensity and analytes concentration over wide concentration ranges. Our experimental results clearly show that the in situ cleaning 3D SERS substrates provide an ideal candidate for SERS applications in food safety.

Recent Advances in 2D Inorganic Nanomaterials for SERS Sensing

Surface-enhanced Raman spectroscopy is a powerful and sensitive analytical tool that has found application in chemical and biomolecule analysis and environmental monitoring. Since its discovery in the early 1970s, a variety of materials ranging from noble metals to nanostructured materials have been employed as surface enhanced Raman scattering (SERS) substrates. In recent years, 2D inorganic materials have found wide use in the development of SERS-based chemical sensors owing to their unique thickness dependent physico-chemical properties with enhanced chemical-based charge-transfer processes. Here, recent advances in the application of various 2D inorganic nanomaterials, including graphene, boron nitride, semiconducting metal oxides, and transition metal chalcogenides, in chemical detection via SERS are presented. The background of the SERS concept, including its basic theory and sensing mechanism, along with the salient features of different nanomaterials used as substrates in SERS, extending from monometallic nanoparticles to nanometal oxides, is comprehensively discussed. The importance of 2D inorganic nanomaterials in SERS enhancement, along with their application toward chemical detection, is explained in detail with suitable examples and illustrations. In conclusion, some guidelines are presented for the development of this promising field in the future.

Metal-Organic Frameworks for Food Safety

Food safety is a prevalent concern around the world. As such, detection, removal, and control of risks and hazardous substances present from harvest to consumption will always be necessary. Metal-organic frameworks (MOFs), a class of functional materials, possess unique physical and chemical properties, demonstrating promise in food safety applications. In this review, the synthesis and porosity of MOFs are first introduced by some representative examples that pertain to the field of food safety. Following that, the application of MOFs and MOF-based materials in food safety monitoring, food processing, covering preservation, sanitation, and packaging is overviewed. Future perspectives, as well as potential opportunities and challenges faced by MOFs in this field will also be discussed. This review aims to promote the development and progress of MOF chemistry and application research in the field of food safety, potentially leading to novel solutions.

A porous Au-Ag hybrid nanoparticle array with broadband absorption and high-density hotspots for stable SERS analysis

Constructing high-density hotspots is of crucial importance in surface enhanced Raman scattering (SERS). In this paper, we present a large-area and broadband porous Au-Ag hybrid nanoparticle array which was fabricated by an ultra-thin alumina mask (UTAM) technique incorporated with annealing and galvanic replacement techniques. Experimental results and numerical simulations demonstrated that the porous Au-Ag hybrid nanoparticle array possessed enormous hotspots for high sensitivity, uniformity, and stability in SERS analysis. A large Raman enhancement factor of 2.2 × 107 was achieved with a relative standard deviation (RSD) of 7.7%, leading to excellent reliability for Raman detection. Furthermore, this novel substrate exhibited a long shelf time in an ambient environment and promising practical applications in many SERS-based quantitative analytical and biomedical sensing techniques.

Nanomaterial-based SERS sensing technology for biomedical application

Over the past few years, nanomaterial-based surface-enhanced Raman scattering (SERS) detection has emerged as a new exciting field in which theoretical and experimental studies of the structure and function of nanomaterials have become a focus.

Nanopillar-Assisted SERS Chromatography

Practical implementation of surfaced enhanced Raman spectroscopy (SERS) sensing is hindered by complexity of real-life samples, which often requires long and costly pretreatment and purification. Here, we present a novel nanopillar-assisted SERS chromatography (NPC-SERS) method for simultaneous quantitation of target molecules and analysis of complex, multicomponent fluids, e.g., human urine spiked with a model drug paracetamol (PAR). Gold-coated silicon nanopillar (AuNP) SERS substrates and a centrifugal microfluidic platform are tactfully combined, which allows (i) a precise and fully automated sample manipulation and (ii) spatial separation of different molecular species on the AuNP substrate. The NPC-SERS technique provides a novel approach for wetting the stationary phase (AuNP) using the "wicking effect", and thus minimizes dilution of analytes. Separation of PAR and the main human urine components (urea, uric acid, and creatinine) has been demonstrated. Quantitative detection of PAR with ultrawide linear dynamic range (0-500 ppm) is achieved by analyzing the spreading profiles of PAR on the AuNP surface. NPC-SERS transforms SERS into a sensing technique with general applicability, facilitating rapid and quantitative detection of analytes in complex biofluids, such as saliva, blood, and urine.

Simultaneous In Situ Extraction and Fabrication of Surface-Enhanced Raman Scattering Substrate for Reliable Detection of Thiram Residue

We report a novel strategy of simultaneous in situ extraction and fabrication of surface-enhanced Raman scattering substrate (IE-SERS) to perform selective and reliable on-site determination of thiram residue in soil, fruits, and vegetables. In this protocol, the thiram residue on complex surfaces can facilely diffuse into the solvent (dichloromethane (DCM)) and specifically bind to gold nanoparticles (AuNPs), affording the SERS substrate through the embedding of the thiram-trapped AuNPs into the cellulose p-toluenesulfonates (CTSAs) film through the evaporation of DCM. SERS signals of the specifically prepared CTSAs could be used as an internal standard to calibrate the absolute signal of thiram, which can avoid the fluctuation of SERS intensities caused by uneven and irregular morphology of SERS substrate. Thus, reliable quantitation of thiram through SERS detection and superior reproducibility in the SERS measurement (RSD = 4.21%) were achieved. As for directly sensing the thiram residue in soil, the established method shows strong anti-interference ability and a good linear response from 0.1 to 12 μg/g with a low limit of detection (LOD) of 50 ng/g, which is lower than that of all the previously reported methods. The recoveries range from 91.76 to 112.3% for thiram in paddy soils, indicating that the established IE-SERS method is reliable and applicable to the detection of thiram residue in real soil samples. In addition, the measurement of the residual thiram on strawberry and cucumber surface was also successfully accomplished by this strategy, indicating that the established method also has great potential in the in situ ultrasensitive detection of thiram on irregular fruits and vegetables.

Molecular Affinity Agents for Intrinsic Surface-Enhanced Raman Scattering (SERS) Sensors

Research at the interface of synthetic materials, biochemistry, and analytical techniques has enabled sensing platforms for applications across many research communities. Herein we review the materials used as affinity agents to create surface-enhanced Raman spectroscopy (SERS) sensors. Our scope includes those affinity agents (antibody, aptamer, small molecule, and polymer) that facilitate the intrinsic detection of targets relevant to biology, medicine, national security, environmental protection, and food safety. We begin with an overview of the analytical technique (SERS) and considerations for its application as a sensor. We subsequently describe four classes of affinity agents, giving a brief overview on affinity, production, attachment chemistry, and first uses with SERS. Additionally, we review the SERS features of the affinity agents, and the analytes detected by intrinsic SERS with that affinity agent class. We conclude with remarks on affinity agent selection for intrinsic SERS sensing platforms.