Zinc oxide nanotubes decorated with silver nanoparticles as an ultrasensitive substrate for surface-enhanced Raman scattering

Au-Coated ZnO Surface-Enhanced Raman Scattering (SERS) Substrates: Synthesis, Characterization, and Applications in Exosome Detection

Developing a biomolecular detection method that minimizes photodamage while preserving an environment suitable for biological constituents to maintain their physiological state is expected to drive new diagnostic and mechanistic breakthroughs. In addition, ultra-sensitive diagnostic platforms are needed for rapid and point-of-care technologies for various diseases. Considering this, surface-enhanced Raman scattering (SERS) is proposed as a non-destructive and sensitive approach to address the limitations of fluorescence, electrochemical, and other optical detection techniques. However, to advance the applications of SERS, novel approaches that can enhance the signal of substrate materials are needed to improve reproducibility and costs associated with manufacture and scale-up. Due to their physical properties and synthesis, semiconductor-based nanostructures have gained increasing recognition as SERS substrates; however, low signal enhancements have offset their widespread adoption. To address this limitation and assess the potential for use in biological applications, zinc oxide (ZnO) was coated with different concentrations (0.01-0.1 M) of gold (Au) precursor. When crystal violet (CV) was used as a model target with the synthesized substrates, the highest enhancement was obtained with ZnO coated with 0.05 M Au precursor. This substrate was subsequently applied to differentiate exosomes derived from three cell types to provide insight into their molecular diversity. We anticipate this work will serve as a platform for colloidal hybrid SERS substrates in future bio-sensing applications.

Ultrasensitive SERS Analysis of Liquid and Gaseous Putrescine and Cadaverine by a 3D-Rosettelike Nanostructure-Decorated Flexible Porous Substrate

Putrescine and cadaverine are toxic biogenic amines in spoiled food, which poses a serious threat to food security. In this work, we reported a highly sensitive three-dimensional (3D)-rosettelike surface-enhanced Raman spectroscopy (SERS) substrate functionalized with a p-mercaptobenzoic acid (p-MBA) monolayer to detect liquid and gaseous putrescine and cadaverine in pork samples. The SERS substrate was made by a combination of the merit of the 3D morphology of ZnO nanorod arrays on a flexible porous poly(vinylidene fluoride) (PVDF) membrane and the in situ chemical growth of Au nanoparticle seeds on Au film-coated ZnO nanorods, which produced a 3D-rosettelike BigAuNP/Au/ZnO/P heterostructure with abundant SERS-active hot spots that significantly enhanced the localized surface plasmonic resonance (LSPR) effect and charge-transfer (CT) effect of Raman enhancement. This SERS substrate showed high sensitivity, reproducibility, stability, and uniformity. With the p-MBA molecular monolayer as the sensing interface, our SERS substrate realized the highly sensitive and quantitative detection of liquid putrescine and cadaverine within 10 min, with a limit of detection (LOD) of 3.2 × 10^-16 and 1.6 × 10^-13 M, respectively. Additionally, the sensor showed efficient SERS responses to gaseous amine molecules at low concentrations (putrescine: 1.26 × 10^-9 M, cadaverine: 2.5 × 10^-9 M). Further, the sensor was successfully applied to determine the total content of putrescine and cadaverine. Moreover, the practicability of this SERS sensor was verified by the measurement of liquid and gaseous amines in pork samples, and it showed great potential applications for sensitive detection of food spoilage.

Monitoring the charge-transfer process in a Nd-doped semiconductor based on photoluminescence and SERS technology

Surface-enhanced Raman scattering (SERS) and photoluminescence (PL) are important photoexcitation spectroscopy techniques; however, understanding how to analyze and modulate the relationship between SERS and PL is rather important for enhancing SERS, having a great effect on practical applications. In this work, a charge-transfer (CT) mechanism is proposed to investigate the change and relationships between SERS and PL. Analyzing the change in PL and SERS before and after the adsorption of the probe molecules on Nd-doped ZnO indicates that the unique optical characteristics of Nd3+ ions increase the SERS signal. On the other hand, the observed SERS can be used to explain the cause of PL background reduction. This study demonstrates that modulating the interaction between the probe molecules and the substrate can not only enhance Raman scattering but also reduce the SERS background. Our work also provides a guideline for the investigation of CT as well as a new method for exploring fluorescence quenching.

Detect, remove and re-use: Sensing and degradation pesticides via 3D tilted ZMRs/Ag arrays

The objective of this study was to design a versatile and reusable pesticide detection surface-enhanced Raman scattering (SERS) substrate in combination with SERS enhancement and self-cleaning properties. In this paper, we present an inexpensive way to synthesize three-dimensional tilted ZnO micron rods with an Ag hierarchical structure (ZMRs/Ag arrays). Although expensive materials and complex methods were not used, the detection limit of thiram residue was 10^-11 M, with a quantitative relationship (R² = 0.9929) between the thiram concentration and the intensity of the SERS peaks. Additionally, the substrates exhibited fast and efficient photocatalytic activity for the degradation of adsorbed thiram, and the degradation rate in 30 min was close 100 % under visible-light irradiation. The enhancement and photocatalytic mechanism of this substrate were meticulously analyzed in detail. Furthermore, the residues of several mixed pesticides (e.g., thiram and methamidophos compounds) in various juices (such as grape, pear, orange, apple, and cherry juices) were quickly detected using ZMRs/Ag substrates. The main advantages of this substrate are recyclability, stability, selectivity, handiness, and cost-eff ;ectiveness. The substrate can prevent single-use problems associated with conventional SERS substrates and can be applied in pesticide residue and food security.

Plasmon-coupled Charge Transfer in FSZA Core-shell Microspheres with High SERS Activity and Pesticide Detection

A commercial SERS substrate does not only require strong enhancement, but also can be reused and recycled in actual application. Herein, Fe₃O₄/SiO₂/ZnO/Ag (FSZA) have been synthesised, which consisted of Fe₃O₄ core with strong magnetic field response and an intermediate SiO₂ layer as an electronic barrier to keep the stability of magnetite particles and outer ZnO and Ag as the effective layers for detecting pollutants. The SERS enhancement factor (EF) of the FSZA was ~8.2 × 10⁵. The enhancement mechanism of the FSZA core-shell microspheres were anatomized. The electromagnetic enhancement of surface deposited Ag, charge transfer, and molecular and exciton resonances act together to cause such high enhancement factors. For practical application, the FSZA core-shell microspheres were also used to detect thiram, moreover, which was collected and separated by an external magnetic field, and maintained the SERS activity without significant decline during multiple tests. So the good enhancement performance and magnetic recyclability make the FSZA core-shell microspheres a promising candidates for practical SERS detection applications.

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.

Improved Charge Transfer and Hot Spots by Doping and Modulating the Semiconductor Structure: A High Sensitivity and Renewability Surface-Enhanced Raman Spectroscopy Substrate

Here, we develop a new method to improve the surface-enhanced Raman spectroscopy (SERS) activity of ZnO using Mg doping combined with noble metals. Highly aligned silver nanoparticles (AgNPs) decorated on an array of Mg-doped ZnO (MZO@Ag) were fabricated. Using rhodamine 6G as the probe molecule, SERS indicated that the MZO@Ag substrate possesses perfect sensitivity, homogeneity, and chemical stability. The enhancement mechanism of this substrate was analyzed in detail, and finite-difference time-domain (FDTD) simulations were used to examine "hot spot" distribution which generated gaps between the balls, the rods, and the stems. FDTD simulation calculated ( E/ E₀)⁴ to be 2.5 × 10⁶. Furthermore, the prepared substrates could degrade the target molecules in situ irradiated by visible light irradiation over the course of 40 min and then efficiently recover detectability through a recycling process. Our substrates were easy to fabricate, self-cleaning, and reusable. They are expected to provide new opportunities for the use of SERS in biological sensors, biomedical diagnostics, and food safety.

Synthesis of wheatear-like ZnO nanoarrays decorated with Ag nanoparticles and its improved SERS performance through hydrogenation

Semiconductor/noble metal composite SERS substrates have been extensively studied due to their unique bifunctionality. In this work, wheatear-like ZnO nanoarrarys have been fabricated via a modified low-temperature solution method. The hierarchical nanostructures that are constructed by stacked nanoflakes and long whiskers of ZnO possess a substantial number of characteristic nano corners and edges, which are proved to be beneficial to deposit more Ag nanoparticles (NPs). Furthermore, hydrogenated wheatear-like ZnO/AgNP composite substrates are achieved via a safe and facile solid hydrogen source (NaBH4). The hydrogenated ZnO/AgNPs (H-ZnO/Ag) substrates exhibit greatly improved SERS activity in detecting R6G molecules with an enhancement factor (EF) up to ∼0.49 × 10(8), over two orders of magnitude higher than that of the substrates before hydrogenation. The outstanding SERS performance of wheatear-like H-ZnO/Ag substrates benefits from the emerging porous structure of ZnO and abundant surface defects introduced by hydrogenation. In addition, the as-prepared substrates also show high detection sensitivity, good repeatability and recyclability, indicating great potential for practical applications.

Ag-nanoparticle-decorated porous ZnO-nanosheets grafted on a carbon fiber cloth as effective SERS substrates

We report on the large-scale synthesis of Ag-nanoparticle (Ag-NP) decorated ZnO-mesoporous-nanosheets (NSs) grafted on a flexible carbon fiber cloth (CFC), as sensitive and reproducible surface enhanced Raman scattering (SERS) substrates with excellent flexibility. The composite SERS-substrates are achieved by a combination of atomic layer deposition of ZnO-seeds on each fiber of the CFC (denoted as ZnO-seeds@CFC), chemical bath deposition and subsequent pyrolysis for the creation of ZnO-mesoporous-NSs grafted on ZnO-seeds@CFC, and ion-sputtering of Ag-NPs on the ZnO-mesoporous-NSs. As abundant SERS "hot spots" are generated from the electromagnetic coupling of the densely distributed Ag-NPs, and the semiconducting ZnO-mesoporous-NSs also have chemical supporting enhancement and distinct molecule adsorbing abilities, the composite SERS-substrates demonstrate high SERS-sensitivity with good signal reproducibility. As a trial for potential applications, the composite SERS-substrates were used to identify pesticides and highly toxic polychlorinated biphenyls (PCBs), and low concentrations down to 10(-7) M for methyl parathion and 5 × 10(-6) M for PCB-77 were reached, respectively, showing promising potential for the SERS-based rapid detection of toxic organic pollutants in the environment.