Recyclable SERS substrates based on Au-coated ZnO nanorods

Ag/ZnO microcavities with high sensitivity and self-cleaning properties for fast repetitive SERS detection

A SERS substrate with high sensitivity and reusability was proposed. The chip consists of multiple ZnO microcavities loaded with silver particles. Based on structural characteristics, this coupling between cavity modes and localized surface plasmon modes can highly localize the electric field, where experimental results revealed a detection limit of 10-11 M for R6G. In addition, during carrier control in semiconductors with localized electromagnetic fields, our substrate also exhibits high self-cleaning efficiency and in situ detection stability. Even in a dry environment, it exhibits excellent light-mediated cleaning ability across multiple reuse test cycles. The convenient, rinse-free substrate, with its cost-effective and sustainable features, shows great promise for the study on detection and degradation of active materials.

Solid-phase microextraction - a future technique in pharmacology and coating trends

Traditional sample preparation techniques based on liquid-liquid extraction (LLE) or solid-phase extraction (SPE) often suffer from a major error due to the matrix effects caused by significant co-extraction of matrix components. The implementation of a modern extraction technique such as solid-phase microextraction (SPME) was aimed at reducing analysis time and the use of organic solvents, as well as eliminating pre-analytical and analytical errors. Solid-phase microextraction (SPME) is an innovative technique for extracting low molecular weight compounds (less than 1500 Da) from highly complex matrices, including biological matrices. It has a wide range of applications in various types of analysis including pharmaceutical, clinical, metabolomics and proteomics. SPME has a number of advantages over other extraction techniques. Among the most important are low environmental impact, the ability to sample and preconcentrate analytes in one step, simple automation, and the ability to extract multiple analytes simultaneously. It is expected to become, in the future, another method for cell cycle research. Numerous available literature sources prove that solid-phase microextraction can be a future technique in many scientific fields, including pharmaceutical sciences. This paper provides a literature review of trends in SPME coatings and pharmacological applications.

ZnO Nanocages Decorated with Au@AgAu Yolk-Shell Nanomaterials for SERS-Based Detection of Hyperuricemia

Surface-enhanced Raman scattering (SERS) is widely recognized as a highly sensitive technology for chemical detection and biological sensing. In SERS-based biomedical applications, developing highly efficient sensing platforms based on SERS plays a pivotal role in monitoring disease biomarker levels and facilitating the early detection of cancer biomarkers. Hyperuricemia, characterized by abnormally high concentrations of uric acid (UA) in the blood, was associated with a range of diseases, such as gouty arthritis, heart disease, and acute kidney injury. Recent reports have demonstrated the correlation between UA concentrations in blood and tears. In this work, we report the fabrication of SERS substrates utilizing ZnO nanocages and yolk-shell-structured plasmonic nanomaterials for the noninvasive detection of UA in tears. This innovative SERS substrate enables noninvasive and sensitive detection of UA to prevent hyperuricemia-related diseases.

Flexible Surface-Enhanced Raman Scattering Tape Based on Ag Nanostructured Substrate for On-Site Analyte Detection

Surface-enhanced Raman scattering (SERS) has emerged as a powerful surface analytical technique that amplifies Raman scattering signals of molecules adsorbed onto metal nanostructured surfaces. The droplet reaction method has recently been employed to fabricate large-scale microring patterns of silver (Ag) nanostructures on rigid substrates, which enables sensitive detection within the ring area. However, these rigid substrates present limitations for direct on-site detection of analyte residues on irregular sample surfaces. There is a need to develop soft and flexible SERS substrates that can intimately conform to arbitrary surfaces. In this study, we presented a SERS substrate using flexible and adhesive tape as the supporting material. This SERS tape was fabricated by repeatedly transferring presynthesized Ag nanostructures from a rigid substrate to the tape. For a model compound adenine, our SERS tape exhibited a good linear response from 5 × 10-4 M to 5 × 10-5 M with a low limit of detection (LOD) of 5 × 10-7 M and displayed a SERS enhancement factor (EF) of 3.2 × 105. The relative standard deviation (RSD) of SERS intensity achieved was as low as 1.93%, indicating its outstanding uniformity. The as-prepared SERS tape was used for in situ detection of pesticide residue on an apple surface and dye residue on human hair. Leveraging the large surface area of Ag nanostructure patterns from the droplet reaction, the developed SERS tape demonstrates excellent performance in terms of sensitivity and uniformity. The successful detection of analyte residues on arbitrary surfaces of apple and human hair highlights the potential of this flexible SERS tape for real-world applications across various industries for enhanced diagnostic accuracy.

Surface functionalized 3D printed metal structures as next generation recyclable SERS substrates

Combining the design flexibility and rapid prototyping capabilities of additive manufacturing with photocatalytic and plasmonic functionalities is promising for the development of next-generation SERS applications such as point of care diagnostics and in situ monitoring of chemical reactions in fuels and chemical processing. Laser powder bed fusion (LPBF) is a well-matured additive manufacturing technique which generates metallic structures through localised melting and joining of metal powders using a laser. LPBF reduces material wastage during manufacturing, is applicable to a wide range of metals and alloys, and allows printing of complex internal structures. This feature article elaborates the use of soot templating, chemical vapour deposition and electroless plating techniques for grafting plasmonic and semiconductor nanoparticles on the surface of LPBF manufactured metallic substrates. The capability to fabricate different types of intricate metallic lattices using additive manufacturing is demonstrated and technical challenges in their adequate functionalization are elaborated. The developed methodology allows tailoring of the substrate structure, composition, morphology, plasmonic and photocatalytic activities and thus unveils a new class of recyclable SERS substrates.

Ultralow-background SERS substrates for reliable identification of organic pollutants and degradation intermediates

Chemical methods for preparing SERS substrates have the advantages of low cost and high productivity, but the strong background signals from the substrate greatly limit their applications in characterization and identification of organic compounds. Herein, we developed a one-step synthesis method to prepare silver nanoparticle substrates with ultralow SERS background using anionic ligands as stabilizing agents and applied the SERS substrate for the reliable and reproducible identification of typical organic pollutants and corresponding degradation intermediates. The synthesis method shows excellent universality to different reducing agents cooperating with different anionic ligands (Cl-, Br-, I-, SCN-). As model applications, the machine learning algorithm can realize the precise prediction of six organophosphorus pesticides and eight sulfonamide antibiotics with 100% accuracy based on SERS training data. More importantly, the ultralow-background SERS substrate enables one to detect and identify the time-dependent degradation intermediates of organophosphorus pesticides by combining them with density functional theory (DFT) calculations. All the results indicate that the ultralow-background SERS substrate will greatly push the development of SERS characterization applications.

Umbrella-frame silicon nanorod arrays decorated with Au nanoparticles as recyclable SERS substrates

Surface-enhanced Raman scattering (SERS) is a powerful technique for detection and identification of trace amounts of molecules with high specificity. A variety of two- and three-dimensional (3D) SERS substrates have been developed. Among these SERS substrates, to further develop new morphology of 3D SERS-active substrate with robust SERS functionality is still desired and necessary. In this paper, what we believe to be a novel and effective SERS-active substrate based on large-scale 3D Si hierarchical nanoarrays in conjunction with homogeneous Au nanoparticles (AuNPs) was proposed. Its building block shaped like the umbrella-frame structure was fabricated by a simple and cost-effective top-down nanofabrication method. Such umbrella-frame structure achieved excellent SERS performance with high sensitivity and spatial uniformity. For R6G molecules, the detection limit can be as low as 10^-14 M, with an enhancement factor of up to 10⁷. The relative standard deviation can reach about 11% above 30 positions across an area of 100×100 μm². This is mainly attributed to much more active-sites provided by the umbrella-frame structure for adsorption of target molecules and AuNPs, and sufficient 3D hotspots generated by the coupling between the SiNRs guided mode and AuNPs localized surface plasmon resonance (LSPR), as well as that between AuNPs LSPR. Especially by introducing the umbrella-ribs SiNRs and AuNPs, the light field can be greatly confined to the structure surface, creating strongly enhanced and even zero-gap fields in 3D space. Moreover, the proposed SERS-active substrate can be erased and reused multiple times by plasma cleaning and exhibits typically excellent recyclability and stability for robust SERS activity. The experimental results demonstrate the proposed substrate may serve as a promising SERS platform for trace detection of chemical and biological molecules.

Effect of Ti and Au buffer layers on controlling the density and wettability of well-aligned ZnO nanorod arrays grown on different substrates

ZnO nanorod arrays (NRAs) have potential applications as building blocks for nanoscale electronic, optoelectronic, and sensing applications. The density of ZnO NRAs is controlled by a simple low-cost hydrothermal growth process. It is shown that Ti and Au thin buffer layers can be used to control ZnO NRA density up to an order of magnitude on a wide variety of substrates including bare glass AZO, ZnO seeded AZO, FTO and ITO substrates, respectively. We investigate surface morphological, structural and optical properties of ZnO NRAs by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, and photoluminescence spectroscopy measurements, respectively. To highlight the importance of NRA density, wettability measurements show large dependence on density and static water contact angles range from as low as ∼23° to as large as ∼142°. These results indicate that the capability to control the density of ZnO NRAs, and thus their wettability, can have additional implications such as in their use in biosensors, field emission, dye-sensitized solar-cells (DSSCs), and photocatalytic activity in addition to potential light trapping effects over wide spectral ranges.

A Review on Integrated ZnO-Based SERS Biosensors and Their Potential in Detecting Biomarkers of Neurodegenerative Diseases

Surface-enhanced Raman spectroscopy (SERS) applications in clinical diagnosis and spectral pathology are increasing due to the potential of the technique to bio-barcode incipient and differential diseases via real-time monitoring of biomarkers in fluids and in real-time via biomolecular fingerprinting. Additionally, the rapid advancements in micro/nanotechnology have a visible influence in all aspects of science and life. The miniaturization and enhanced properties of materials at the micro/nanoscale transcended the confines of the laboratory and are revolutionizing domains such as electronics, optics, medicine, and environmental science. The societal and technological impact of SERS biosensing by using semiconductor-based nanostructured smart substrates will be huge once minor technical pitfalls are solved. Herein, challenges in clinical routine testing are addressed in order to understand the context of how SERS can perform in real, in vivo sampling and bioassays for early neurodegenerative disease (ND) diagnosis. The main interest in translating SERS into clinical practice is reinforced by the practical advantages: portability of the designed setups, versatility in using nanomaterials of various matter and costs, readiness, and reliability. As we will present in this review, in the frame of technology readiness levels (TRL), the current maturity reached by semiconductor-based SERS biosensors, in particular that of zinc oxide (ZnO)-based hybrid SERS substrates, is situated at the development level TRL 6 (out of 9 levels). Three-dimensional, multilayered SERS substrates that provide additional plasmonic hot spots in the z-axis are of key importance in designing highly performant SERS biosensors for the detection of ND biomarkers.

ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs). Benefiting from the synergistic effect of electromagnetic enhancement and charge transfer effect, the Ag NFs@ZnO NRs substrate exhibits a low detection limit (10^-13 M) for crystal violet molecules. This SERS substrate has good uniformity with a relative standard deviation of 7.463 %. Besides, owning to the photocatalytic property of ZnO NRs, the hybrid substrate can degrade probe molecules after SERS detection and realize recyclability. As a demonstration, we employed our SERS substrate for the trace detection of pesticide residues on apple surface and in river water. This study provides a new idea for improving the SERS performance of hybrid substrates.

Recent State and Challenges in Spectroelectrochemistry with Its Applications in Microfluidics

This review paper presents the recent developments in spectroelectrochemical (SEC) technologies. The coupling of spectroscopy and electrochemistry enables SEC to do a detailed and comprehensive study of the electron transfer kinetics and vibrational spectroscopic fingerprint of analytes during electrochemical reactions. Though SEC is a promising technique, the usage of SEC techniques is still limited. Therefore, enough publicity for SEC is required, considering the promising potential in the analysis fields. Unlike previously published review papers primarily focused on the relatively frequently used SEC techniques (ultraviolet-visible SEC and surface-enhanced Raman spectroscopy SEC), the two not-frequently used but promising techniques (nuclear magnetic resonance SEC and dark-field microscopy SEC) have also been studied in detail. This review paper not only focuses on the applications of each SEC method but also details their primary working mechanism. In short, this paper summarizes each SEC technique's working principles, current applications, challenges encountered, and future development directions. In addition, each SEC technique's applicative research directions are detailed and compared in this review work. Furthermore, integrating SEC techniques into microfluidics is becoming a trend in minimized analysis devices. Therefore, the usage of SEC techniques in microfluidics is discussed.

Green Synthesis of Three-Dimensional Au Nanorods@TiO2 Nanocomposites as Self-Cleaning SERS Substrate for Sensitive, Recyclable, and In Situ Sensing Environmental Pollutants

In this work, a simple, low-cost, green, and mild method for the preparation of three-dimensional nanocomposite materials of gold nanorods (Au NRs)@TiO₂ is reported. The surface of Au NRs was coated with TiO₂ in situ reduction at room temperature without a complicated operation. The synthetic Au NRs@TiO₂ nanocomposites were used as surface-enhanced Raman spectroscopy (SERS) active substrates for the reusable and sensitive detection of environmental pollutants. The results showed that the pollutants on Au NRs@TiO₂ nanocomposites have higher SERS activity and reproducibility than those on the Au NR substrate without the presence of TiO₂. Moreover, the SERS substrate can be readily recycled by UV-assisted self-cleaning to remove residual analyte molecules. Malachite green (MG) and crystal violet (CV) were used as examples to demonstrate the feasibility of the proposed sensor for the sensitive detection of environmental pollutants. The results showed that the limit of detections (LODs) were 0.75 μg/L and 0.50 μg/L for MG and CV, respectively, with the recoveries ranging from 86.67% to 91.20% and 83.70% to 89.00%. Meanwhile, the SERS substrate can be easily regenerated by UV light irradiation. Our investigation revealed that within three cycles, the Au NRs@TiO₂ substrates still maintained the high SERS enhancement effect that they showed when first used for SERS detection. These results indicated that the method can be used to detect MG and CV in really complex samples. Due to the high sensitivity, reusability, and portability and the rapid detection property of the proposed sensor, it can have potential applications in the on-site detection of environmental pollutants in a complex sample matrix.

Experimental and theoretical evaluation of crystal facet exposure on the charge transfer and SERS activity of ZnO films

Semiconductors exhibit great potential as a surface enhanced Raman scattering (SERS) substrate due to their low cost, good stability and biocompatibility. However, the extensive application of semiconductors has been restricted by their intrinsically low SERS sensitivity. It is urgently required to design uniform metal oxide substrates with enhanced charge transfer and SERS activity. Herein, three facet-defined ({101̄0}, {0001} and {101̄1}) ZnO films were synthesized via an electrodeposition procedure with the assistance of KCl or ethylenediamine. According to the results, the ZnO films with {0001} and {101̄1} exposed facets exhibit appreciable SERS enhancement factors (EFs) of 1.6 × 10⁴ and 2.8 × 10⁴ for 4-nitrobenzenethiol (4-NBT), as well as a relatively low limit of detection (LOD) down to 1 × 10^-6 M and 5 × 10^-7 M, respectively. Simultaneously, the electrodeposited ZnO films deliver good repeatability and SERS stability, with relative standard deviation (RSD) less than 6% and 85.2% of their original activity retained after 40 days. Theoretical calculations verified that the {0001} and {101̄1} facets can transfer more electrons from ZnO to the molecules on account of their low facet-related electronic work functions, thus generating the noticeable improvement of SERS activity. The current study provides theoretical and technical support for the crystal facet engineering and property improvement of semiconductors.

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

The Wavelength-Dependent SERS Template Based on a Nanopillar Array

Surface-enhanced Raman spectroscopy (SERS) can be regarded as a powerful tool for probing chemical molecules by effectively enhancing Raman signals. However, the enhancement factors depend on the SERS template, the probed molecular structures, and the excitation laser wavelength. Herein, we proposed a simple and easily fabricated nanostructured template for SERS and analyzed the wavelength-dependent factors. Three types of golden nanopillar arrays on silicon wafers were designed and manufactured. The SERS signals of the Rhodamine 6G (R6G) molecules were extracted. Three laser sources, a blue 17 mW 458 nm diode laser, a green 20 mW 532 nm laser, and a red 6 mW 633 nm laser, were employed as the excitation laser sources. The 458 nm laser was located far from the resonate spectrum of R6G. The optical intensity distributions for the different SERS templates excited by three laser beams were also simulated. The enhancement factors (EFs) of R6G on the three nanostructured templates were measured and compared. The photoluminescence spectrum of the nanostructured templates and SERS signals of R6G were also measured. In addition, the experimental results concerned optical simulations. The analysis tool that was used was a convolution profile of multiple Lorentzian line shapes with a Gaussian profile. It is helpful to understand the SERS signals when the excitation laser wavelength is located out of the resonance region of molecules. It can also provide a new design approach to fabricate an SERS Template with a nanopillar array for different excitation wavelengths.

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.

MOF-Derived hierarchical porous 3D ZnO/Ag nanostructure as a reproducible SERS substrate for ultrasensitive detection of multiple environmental pollutants

The three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate for trace molecule detection has recently attracted considerable interest; however, these substrates generally either show poor sensitivity or require a complex preparation process. In this work, we have fabricated a 3D ZnO/Ag substrate using porous zeolite imidazole frameworks (ZIF-8) derived ZnO nanoparticles (NPs) followed by evaporation-induced self-assembly of Ag NPs over it, which can detect multiple environmental pollutants by a facile and cost-effective method. This 3D porous substrate showed an ultra-sensitivity for detecting various types of molecules, e.g., rhodamine 6G (R6G), crystal violet (CV), tetracycline, and thiram, simultaneously suggesting its generality. Notably, the lowest detectable concentration (LDC) attained for R6G is 10^-13 M, and the enhancement factor (EF) reaches up to 1.8 × 10⁸. The most important reason for ultra-sensitivity is that ZnO derived from ZIF-8 has a hierarchical porous structure and large surface area to provide more "hot spots" and absorb more probe molecules. Consequently, the ZnO/Ag nanostructures show excellent photocatalytic performance. The detected probe molecules could be completely degraded in situ within a short UV exposure time (<30 min), thereby enabling outstanding reusability of this substrate. Finite-different time-domain (FDTD) simulations were used to understand the underlying mechanism of the substrate by calculating electric fields and hot spot distributions. The simulations suggested that the widespread hot spots structures on the substrate are the main reason for its SERS ultra-sensitivity.

Facile synthesis of nanocellulose-based Cu2O/Ag heterostructure as a surface-enhanced Raman scattering substrate for trace dye detection

Semiconductor metal-oxide/metal heterostructures with synergetic properties have potential applications in photocatalysis and optical sensors. Here, Cu₂O sub-micro cubes were synthesized under environmentally benign conditions using 2, 2, 6, 6-tetramethylpyperdine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils as a reducing and stabilizing agent. Then the surface of the Cu₂O cubes was decorated with silver nanoparticles (AgNPs) by a substitution reaction. The Cu₂O/Ag heterostructures within the cellulose nanofibrils (CNFs) network were employed as a promising surface-enhanced Raman scattering (SERS) assay for efficient sensing of methylene blue (MB), reaching a maximum enhancement factor (EF) of 4.0 × 10⁴. Their SERS intensities depended on the coverage density of AgNPs and the wavelength of the excitation laser. The excellent SERS performance may result from the charge transfer between Ag and Cu₂O molecules and the strong electromagnetic field at the interface. The CNF-Cu₂O/Ag substrates were capable of detecting MB dye down to 10^-8 M level with a relative standard deviation of 10-15%, demonstrating great sensitivity and reproducibility.

A stable naked-eye colorimetric sensor for monitoring release of extracellular gamma-aminobutyric acid (GABA) neurotransmitter from SH-SY5Y cells

A novel optical γ-aminobutyric acid (GABA)-based sensor was developed on interacting thiol compounds and o-phthalaldehyde (OPA) to form thiacetal compounds. Then, the thiacetal interacts with the GABA molecule to form an isoindole compound. The effects of four thiol compounds on the stability of the resulting isoindole compound were assessed. The 2-mercaptoethanol, "one of the most used derivatizing agents," is unexpectedly the least stable; while, 16-mercaptohexadecanoic acid resulted in the most durable isoindole compound. The developed sensor showed the capability for detecting GABA within a wide concentration range spanning from 500 nmol L^-1 to 100 µmol L^-1. The detection limit was about 330 nmol L^-1, which indicated the high sensitivity of the developed sensor compared with those previously reported. The findings illustrated the ability to detect GABA at the physiological pH (pH = 7.4) without adjusting the pH value, opening the door for real applications. Furthermore, the sensor could detect various GABA concentrations in human serum with good recovery percentages (98% to 101.4%). In addition, this assay was applied to monitor GABA release from the SH-SY5Y cell line to convert glutamate into GABA. This result indicates the capability of the proposed assay for visually monitoring the release of GABA neurotransmitters.

Zinc oxide nanostructures for fluorescence and Raman signal enhancement: a review

Since the initial discovery of surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF), these techniques have shown huge potential for applications in biomedicine, biotechnology, and optical sensors. Both methods rely on the high electromagnetic fields created at locations on the surface of plasmonic metal nanoparticles, depending on the geometry of the nanoparticles, their surface features, and the specific location of analyte molecules. Lately, ZnO-based nanostructures have been exploited especially as SERS substrates showing high enhancement factors and increased charge transfer effect. Additionally, applications focused on enhancing the fluorescence of analyte molecules as well as on tuning the photoluminescence properties of ZnO nanostructures through combination with metal nanoparticles. This review covers the major recent results of ZnO-based nanostructures used for fluorescence and Raman signal enhancement. The broad range of ZnO and ZnO-metal nanostructures synthesis methods are discussed, highlighting low-cost methods and the recyclability of ZnO-based nanosubstrates. Also, the SERS signal enhancement by ZnO-based nanostructures and the influences of lattice defects on the SERS signal are described. The photoluminescence enhancement of ZnO in the presence of noble metal nanoparticles and the molecular fluorescence enhancement in the presence of ZnO alone and in combination with metal nanoparticles are also reviewed.

A recyclable graphene/Ag/TiO2 SERS substrate with high stability and reproducibility for detection of dye molecules

Magnetron sputtering combined with the wet chemical transfer of graphene successfully prepared a multilayer composite material and an efficient photocatalytic renewable SERS substrate. It has excellent photocatalytic activity against dye molecules.

Effect of the Duty Cycle of Flower-like Silver Nanostructures Fabricated with a Lyotropic Liquid Crystal on the SERS Spectrum

Surface-enhanced Raman scattering (SERS) has been widely reported to improve the sensitivity of Raman spectra. Ordinarily, the laser is focused on the sample to measure the Raman spectrum. The size of the focused light spot is comparable with that of micro-nano structures, and the number of micro-nano structures contained in the light spot area (defined as duty cycle) will severely affect the spectrum intensity. In this study, flower-like silver nanostructures were fabricated with a soft lyotropic liquid crystal template in order to investigate the effect of duty cycle. They were observed under a scanning electron microscope, and their spectrum enhancement factor was computed with the obtained Raman spectrum. Then, their duty cycles were measured using a SERS substrate at different locations. A formula was derived to represent the relation between the duty cycle of the nanoflowers and the Raman spectral intensity. This work could promote the actual applications of SERS in high-sensitivity spectrum testing.

Photothermal microfluidic-assisted self-cleaning effect for a highly reusable SERS sensor

The synergistic integration of optofluidic and surface enhanced Raman scattering (SERS) sensing is a new analytical technique that provides a number of unique characteristics for enhancing the sensing performance and simplifying the design of microsystems. Here, we propose a reusable optofluidic SERS sensor by integrating Au nanoisland substrate (AuNIS)-coated fiber into a microfluidic chip. Through both systematic experimental and theoretical analysis, the sensor enables efficient self-cleaning based on its optical-to-heat-hydrodynamic energy conversion property. Besides, the sensor exhibits the instrument detection limit down to 10^-13mol/L and enhancement factor of 10⁶ for Rhodamine 6G. Our optofluidic SERS sensor with such a photothermal microfluidic-assisted self-cleaning method has the advantages of portability, simple operation, and high cleaning efficiency, which will provide a new, to the best of our knowledge, concept and approach for cost-effective and reusable sensors.

Nanostructure-Based Surface-Enhanced Raman Spectroscopy Techniques for Pesticide and Veterinary Drug Residues Screening

Pesticide and veterinary drug residues in food and environment pose a threat to human health, and a rapid, super-sensitive, accurate and cost-effective analysis technique is therefore highly required to overcome the disadvantages of conventional techniques based on mass spectrometry. Recently, the surface-enhanced Raman spectroscopy (SERS) technique emerges as a potential promising analytical tool for rapid, sensitive and selective detections of environmental pollutants, mostly owing to its possible simplified sample pretreatment, gigantic detectable signal amplification and quick target analyte identification via finger-printing SERS spectra. So theoretically the SERS detection technology has inherent advantages over other competitors especially in complex environmental matrices. The progress in nanostructure SERS substrates and portable Raman appliances will promote this novel detection technology to play an important role in future rapid on-site assay. This paper reviews the advances in nanostructure-based SERS substrates, sensors and relevant portable integrated systems for environmental analysis, highlights the potential applications in the detections of synthetic chemicals such as pesticide and veterinary drug residues, and also discusses the challenges of SERS detection technique for actual environmental monitoring in the future.

Preventing Memory Effects in Surface-Enhanced Raman Scattering Substrates by Polymer Coating and Laser-Activated Deprotection

The development of continuous monitoring systems requires in situ sensors that are capable of screening multiple chemical species and providing real-time information. Such in situ measurements, in which the sample is analyzed at the point of interest, are hindered by underlying problems derived from the recording of successive measurements within complex environments. In this context, surface-enhanced Raman scattering (SERS) spectroscopy appears as a noninvasive technology with the ability of identifying low concentrations of chemical species as well as resolving dynamic processes under different conditions. To this aim, the technique requires the use of a plasmonic substrate, typically made of nanostructured metals such as gold or silver, to enhance the Raman signal of adsorbed molecules (the analyte). However, a common source of uncertainty in real-time SERS measurements originates from the irreversible adsorption of (analyte) molecules onto the plasmonic substrate, which may interfere in subsequent measurements. This so-called "SERS memory effect" leads to measurements that do not accurately reflect varying conditions of the sample over time. We introduce herein the design of plasmonic substrates involving a nonpermeable poly(lactic-co-glycolic acid) (PLGA) thin layer on top of the plasmonic nanostructure, toward controlling the adsorption of molecules at different times. The polymeric layer can be locally degraded by irradiation with the same laser used for SERS measurements (albeit at a higher fluence), thereby creating a micrometer-sized window on the plasmonic substrate available to molecules present in solution at a selected measurement time. Using SERS substrates coated with such thermolabile polymer layers, we demonstrate the possibility of performing over 10,000 consecutive measurements per substrate as well as accurate continuous monitoring of analytes in microfluidic channels and biological systems.

Photocatalytic activity and photoelectrochemical properties of Ag/ZnO core/shell nanorods under low-intensity white light irradiation

Zinc oxide (ZnO) nanorod thin films were prepared by CBD onto glass and FTO/glass substrates. Silver (Ag) nanoparticles were synthesized on the surface of the prepared ZnO nanorod thin films using electrochemical methods. The scanning electron microscopy images of the Ag/ZnO/glass core/shell nanostructure confirmed that the average particles size is 20 nm while it was 41 nm for Ag NPs that synthesized onto ZnO/FTO NRs. The photocatalytic activity of the prepared Ag/ZnO core/shell nanostructure was studied by analyzing the degradation of methylene blue (MB) dye under visible light. Various pH values (6 and 10) and exposure time (30-240) min were controlled to investigate the photocatalytic activity of as-prepared Ag/ZnO core/shell nanostructure and that annealed at 200 °C and 300 °C for 1 h. It was observed that when the pH was 6, the degradation rate increased with the annealing temperature and irradiation time reaching 51% at the annealing temperature of 300 °C and exposure time of 240 min. In other hands, when the pH was 10, and the sample was annealed at 200 °C, it showed a good degradation rate of 100% at the irradiation time of 90 min. By contrast, the sample annealed at 300 °C required 180 min to degrade the MB dye completely. The photoelectrochemical cell measurement based on photocurrent density revealed a slight response to light. Cycle voltammetry (CV) measurement was conducted, and the CV curves of the Ag/ZnO core/shell electrodes indicated nonfaradaic and pseudocapacitance behavior. The electrodes showed nearly rectangular CV curves, which indicated the dominance of the nonfaradaic capacitance behavior. The specific capacitance of the electrodes remained at approximately 99%. Mott-Schottky analysis revealed that the semiconductor was an n-type with dependence on flat band potential V _FB deviation in the negative direction.

Silver nanoparticle on zinc oxide array for label-free detection of opioids through surface-enhanced raman spectroscopy

Opioid abuse is a significant public health problem. Over two million Americans have some form of addiction to opioids; however, despite governmental programs established to treat overdoses and restrict opioid distribution, there are still few screening tools that are quantitative, portable and easy to use for high-throughput mapping and monitoring this ongoing crisis. In this paper, we demonstrated a plasmonic zinc oxide (ZnO) arrays-on-silicon sensor for the label-free detection of opioids through surface-enhanced Raman spectroscopy (SERS), and evaluated the chips' opioid sensing performance. Specifically, we tested our device with oxycodone, a potent and commonly abused opioid, dissolved in methanol and blood serum as a proof-of-concept study. Ag particles were in situ patterned onto the ZnO array to form the completed sensing platform. The resulting Ag@ZnO arrays were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDS), and element mapping. In addition, the enhanced electric field induced by the localized surface plasmonic resonance at the Ag particle decorated ZnO is simulated using COMSOL. Opioid-containing samples at varying concentrations, from 900 μg mL-1 to 90 ng mL-1 were tested using SERS to characterize the chip's accuracy and sensitivity. We demonstrated that the sensor can reliably detect opioid concentrations as low as 90 ng mL-1 with great accuracy and sensitivity even spiked into blood serum. The chips could provide a cost-effective, high-throughput method for detecting opiate oxycodone, thereby providing a powerful tool to monitor and control the emerging public health threats.

Toward the Limitation of Dealloying: Full Spectrum Responsive Ultralow Density Nanoporous Gold for Plasmonic Photocatalytic SERS

Plasmon-mediated chemical reaction has a great potential to create self-cleaning surface-enhanced Raman scattering (SERS) substrates. However, few works have been reported to promote this goal. Here, we report ultralow density nanoporous gold (ULDNPG) that possesses an impressive full spectrum responsive characteristic with a reflectivity lower than 5% in the waveband of 300-900 nm. ULDNPG was fabricated by a sandwich dealloying strategy from ultradilute Au-Ag solid solutions with the Au content as low as 1-5 at.%. The prepared ULDNPG presents excellent SERS properties, including high sensitivity, high uniformity, and reproducibility. The full spectrum responsive characteristic of ULDNPG leads to an obvious plasmonic photocatalytic activity. The short lifetime of the SP-excited hot carriers causes a restricted self-cleaning SERS property and a strong photothermal effect for ULDNPG structures.

Substrates for Surface-Enhanced Raman Scattering Formed on Nanostructured Non-Metallic Materials: Preparation and Characterization

The efficiency of the generation of Raman spectra by molecules adsorbed on some substrates (or placed at a very close distance to some substrates) may be many orders of magnitude larger than the efficiency of the generation of Raman spectra by molecules that are not adsorbed. This effect is called surface-enhanced Raman scattering (SERS). In the first SERS experiments, nanostructured plasmonic metals have been used as SERS-active materials. Later, other types of SERS-active materials have also been developed. In this review article, various SERS substrates formed on nanostructured non-metallic materials, including non-metallic nanostructured thin films or non-metallic nanoparticles covered by plasmonic metals and SERS-active nanomaterials that do not contain plasmonic metals, are described. Significant advances for many important applications of SERS spectroscopy of substrates based on nanostructured non-metallic materials allow us to predict a large increase in the significance of such nanomaterials in the near future. Some future perspectives on the application of SERS substrates utilizing nanostructured non-metallic materials are also presented.

Zinc Oxide Nanorod Surface-Enhanced Raman Scattering Substrates without and with Gold Nanoparticles Fabricated through Pulsed-Laser-Induced Photolysis

We fabricated surface-enhanced Raman scattering (SERS) substrates using gold nanoparticle (AuNP)-decorated zinc oxide (ZnO) nanorods (NRs). Prior to decoration with AuNPs, ZnO NRs on the glass substrate fabricated using the sol–gel method could enhance the SERS signal for detecting 10−5 M rhodamine 6G (R6G). Microscopic analysis revealed that the thermal-annealing process for fabricating the seed layers of ZnO facilitated the growth of ZnO NRs with the highly preferred c-axis (002) orientation. A decrease in the diameter of ZnO NRs occurred because of the use of annealed seek layers further increased the surface-to-volume ratio of ZnO NRs, resulting in an increase in the SERS signal for R6G of 10−5 M. To combine the localized surface plasmon resonance (LSPR) mode with the charge transfer (CT) mode, ZnO NRs were decorated with AuNPs through pulsed-laser-induced photolysis (PLIP). However, the preferred vertical (002) orientation of ZnO NRs was prone to the aggregation of AuNPs, which hindered the SERS signal. The experimental results revealed that ZnO NRs with the crystalline structure of horizontal (100) and (101) orientations facilitated the growth of homogeneous, independent and isolated AuNPs which serves as “hot spots” for SERS signal of detecting R6G at a low concentration of 10−9 M. Comparing to previous fabrication of SERS substrate, our method has advantage to fabricate AuNP-decorated ZnO NR in a short time. Moreover, the optimization of the SERS behaviors for different fabrication conditions of AuNPs using the PLIP method was investigated in detail.

In Situ Recyclable Surface-Enhanced Raman Scattering-Based Detection of Multicomponent Pesticide Residues on Fruits and Vegetables by the Flower-like MoS2@Ag Hybrid Substrate

Pesticides, extensively used in agriculture production, have received enormous attention because of their potential threats to the environment and human health. Hence, in this study, a kind of highly sensitive and stable hybrid surface-enhanced Raman scattering (SERS)-active substrates constructed with flower-like two-dimensional molybdenum sulfide and Ag (MoS₂@Ag) has been developed, and then the above substrate was sequentially utilized in the recyclable detection of pesticide residues on several kinds of fruits and vegetables. In the first place, the excellent photocatalytic performance of the MoS₂@Ag hybrid substrate was demonstrated, which was attributed to the inhibition of electron-hole combination after the formation of Schottky barrier between the Ag NPs and MoS₂ matrix. Thereafter, two calibration curves with ultra-low limits of detection (LOD) as 6.4 × 10^-7 and 9.8 × 10^-7 mg/mL were established for the standard solutions of thiram (tetramethylthiuram disulfide, TMTD) and methyl parathion (MP), and then the recyclable assay of their single and mixed residues on eggplant, Chinese cabbage, grape, and strawberry was successfully realized. It is interesting to note that the detection recoveries from 95.5 to 63.1% for TMTD and 92.3 to 62.6% for MP are greatly dependent on the size and surface roughness of these foods. In a word, the MoS₂@Ag composite matrix shows attractive SERS and photocatalysis performance, and it is expected to have the potential application on food safety monitoring.

Flexible and Reusable Ag Coated TiO2 Nanotube Arrays for Highly Sensitive SERS Detection of Formaldehyde

Quantitative analysis of formaldehyde (HCHO, FA), especially at low levels, in various environmental media is of great importance for assessing related environmental and human health risks. A highly efficient and convenient FA detection method based on surface-enhanced Raman spectroscopy (SERS) technology has been developed. This SERS-based method employs a reusable and soft silver-coated TiO₂ nanotube array (TNA) material, such as an SERS substrate, which can be used as both a sensing platform and a degradation platform. The Ag-coated TNA exhibits superior detection sensitivity with high reproducibility and stability compared with other SERS substrates. The detection of FA is achieved using the well-known redox reaction of FA with 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT) at room temperature. The limit of detection (LOD) for FA is 1.21 × 10⁻⁷ M. In addition, the stable catalytic performance of the array allows the degradation and cleaning of the AHMT-FA products adsorbed on the array surface under ultraviolet irradiation, making this material recyclable. This SERS platform displays a real-time monitoring platform that combines the detection and degradation of FA.

The effect of ZnO/ZnSe core/shell nanorod arrays photoelectrodes on PbS quantum dot sensitized solar cell performance

ZnO nanorod (NR) based inorganic quantum dot sensitized solar cells have gained tremendous attention for use in next generation solar cells. ZnO/ZnSe-core/shell NR arrays (NRAs) with various densities were grown on an Au@ZnO seed layer (Au = 0.0, 4.0, 8.0 and 16.0 nm) on glass supported fluorine-doped tin oxide (FTO) substrates using low cost hydrothermal and ion-exchange approaches. PbS quantum dots (QDs) were loaded into the ZnO/ZnSe core/shell NRAs via a successive ionic layer adsorption and reaction (SILAR) method. The morphology, structural and optical properties of the core/shell NRAs were investigated using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-vis spectroscopy measurements. It was observed that the density of the ZnO/ZnSe NRAs decreases with increasing Au buffer layer thickness. The absorption decreases along with a decrease in the ZnO/ZnSe NRA density. The ZnO NRs/PbS QD photoelectrode performs poorly; however, after introducing a ZnSe shell on the core-ZnO, the solar cells parameters changed according to the ZnO/ZnSe NRA density. Values of η = ∼0.88%, J _SC = 14.60 mA cm^-2, and V _OC = 190 mV, and η = ∼0.25%, J _SC = 6.77 mA cm^-2, and V _OC = 115 mV were obtained for the highest and lowest NRA densities, respectively. Although the photovoltaic performance of these photoelectrodes is still inferior, further improvement of the device would be possible by suppressing surface defects, and through quality optimization of the ZnO/ZnSe NRAs, PbS QDs, counter electrode and electrolyte.

Nanographene oxide-TiO2 photonic films as plasmon-free substrates for surface-enhanced Raman scattering

The development of nanostructured semiconductors with tailored morphology and electronic properties for surface-enhanced Raman scattering (SERS) has been attracting significant attention as a promising alternative to conventional coinage metal SERS substrates. In this work, functionalized TiO₂ photonic crystals by graphene oxide nanocolloids (nanoGO) are demonstrated as highly sensitive, recyclable, plasmon-free SERS substrates that combine slow-photon amplification effects with the high adsorption capacity and surface reactivity of GO nanosheets. Comparative evaluation of photonic band gap engineered nanoGO-TiO₂ inverse opal films was performed on methylene blue SERS detection under different laser excitations in combination with rigorous theoretical simulations of the photonic band structure. A very low detection limit of 6 × 10^-7 M and an enhancement factor of 5 × 10⁴ along with excellent self-cleaning performance and reusability could be achieved by the interplay of slow-photon effects assisted by interfacial charge transfer between the analyte and the nanoGO-TiO₂ semiconducting substrate. Slow-photon management in combination with judicious engineering of chemical enhancement in photonic nanostructures is accordingly proposed as an advanced approach for the design of efficient dielectric SERS substrates.

Two Hybrid Au-ZnO Heterostructures with Different Hierarchical Structures: Towards Highly Efficient Photocatalysts

A new paradigm for photocatalysts based on two different hierarchically structured honeycomb and porous cylindrical Au-ZnO heterostructures was successfully developed via a straightforward and cost-effective hydrothermal method under different preparation conditions, which can be promising for industrial applications. The photocatalytic performance of all as-prepared samples under the illumination of sunlight was evaluated by the photocatalytic degradation of rhodamine B (RhB) and malachite green (MG) aqueous solutions. The results show that the photocatalytic degradation efficiency of RhB and MG was 55.3% and 40.7% for ZnO, 95.3% and 93.4% for the porous cylindrical Au-ZnO heterostructure, and 98.6% and 99.5% for the honeycomb Au-ZnO heterostructure, respectively. Compared with those from the ZnO, the results herein demonstrate an excellent reduction in the photoluminescence and improvement in the photocatalysis for the Au-ZnO hybrids with different morphologies. These results were attributed not only to the greatly improved sunlight utilization efficiency due to the surface plasmon resonance (SPR) absorption of Au nanoparticles in the visible region coupled with the UV light utilization by the ZnO nanostructures and multi-reflections of the incident light in the pore structures of their interior cavities but also to the high charge separation efficiency and low Schottky barrier generated by the combination of Au nanoparticles and ZnO micromaterials. Moreover, the honeycomb Au-ZnO heterostructure had a high Au content, surface area and surface oxygen vacancy (O_V), which enabled photocatalytic properties that were higher than those of the porous cylindrical Au-ZnO heterostructures. In addition, two different formation mechanisms for the morphology and possible photocatalytic mechanisms are proposed in this paper.

Electrical Tuning of the SERS Enhancement by Precise Defect Density Control

Surface-enhanced Raman scattering (SERS) has been widely established as a powerful analytical technique in molecular fingerprint recognition. Although conventional noble metal-based SERS substrates show admirable enhancement of the Raman signals, challenges on reproducibility, biocompatibility, and costs limit their implementations as the preferred analysis platforms. Recently, researches on SERS substrates have found that some innovatively prepared metal oxides/chalcogenides could produce noble metal comparable SERS enhancement, which profoundly expanded the material selection. Nevertheless, to tune the SERS enhancement of these materials, careful experimental designs and sophisticated processes were needed. Here, an electrically tunable SERS substrate based on tungsten oxides (WO_3-x) is demonstrated. An electric field is used to introduce the defects in the oxide on an individual substrate, readily invoking the SERS detection capability, and further tuning the enhancement factor is achieved through electrical programming of the oxide leakage level. Additionally, by virtue of in situ tuning the defect density and enhancement factor, the substrate can adapt to different molecular concentrations, potentially improving the detection range. These results not only help build a better understanding of the chemical mechanism but also open an avenue for engaging non-noble metal materials as multifunctional SERS substrates.

Si1-xGex nanoantennas with a tailored Raman response and light-to-heat conversion for advanced sensing applications

Active light-emitting all-dielectric nanoantennas recently have demonstrated great potential as highly efficient nanoscale light sources owing to their strong luminescent and Raman responses. However, their large-scale fabrication faces a number of problems related to productivity limits of existing lithography techniques. Thus, high-throughput fabrication strategies allowing in a facile way to tailor of the nanoantenna emission and thermal properties in the process of their fabrication are highly desirable for various applications. Here, we propose a cost-effective approach to large-scale fabrication of Si1-xGex alloyed Mie nanoresonators possessing an enhanced inherent Raman response which can be simply tailored via tuning the Ge concentration. Moreover, by tailoring the relative Ge composition one can gradually change a complex refractive index of the produced Si1-xGex alloy, which affects the ratio between radiative and nonradiative losses in Si1-xGex nanoantennas, which is crucial for optimization of their optical heating efficiency. Composition-tunable Si1-xGex nanoantennas with an optimized size, light-to-heat conversion and Raman response are implemented for non-invasive sensing of 4-aminothiophenol molecules with a temperature feedback modality and high subwavelength spatial resolution. The results are important for advanced multichannel optical sensing, providing information on analyte's composition, analyte-nanoantenna temperature response and spatial position.

Role of TiO2 Nanoparticles and UV Irradiation in the Enhancement of SERS Spectra To Improve Levamisole and Cocaine Detection on Au Substrates

The original goal of this study was the employment of surface-enhanced Raman spectroscopy (SERS) for the analysis of real cocaine samples (containing adulterants) on composite Au-TiO₂ nanomaterials to achieve low detection limits suitable for the analysis of illicit drugs and controlled substances and to exploit the photodegradation activity of TiO₂ to recycle the SERS substrate for repeated analyses. The photodegradation (self-cleaning) effects of the Au-TiO₂ composite nanomaterials by ultraviolet (UV) radiation are known. These effects were investigated on large-area SERS substrates immersed in the TiO₂ nanoparticle aqueous suspension. The cocaine samples were measured on electrochemically gold-plated platinum targets. Surprisingly, the intensity of SERS spectra of the pure cocaine did not change after immersion in a suspension of TiO₂ under UV irradiation. However, for some real cocaine samples, the overall intensity of the SERS spectra was even higher after the treatment by TiO₂ and UV radiation as compared to the usual Au substrate. This unexpected signal amplification (valuable for illicit drug detection) was found to be caused mainly by the contained levamisole, which is used as a medical drug and is one of the frequent adulterants of cocaine. Both the sole effect of TiO₂ on the levamisole spectrum intensity and the role of UV irradiation were inspected separately. Finally, an investigation of both the TiO₂ and UV radiation treatments was performed, demonstrating (i) the necessity of both factors for selective SERS signal enhancement of the adulterant and (ii) the revision of general anticipation of the role of TiO₂ in SERS systems.