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.

Paper/GO/e-Au flexible SERS sensors for in situ detection of tricyclazole in orange juice and on cucumber skin at the sub-ppb level: machine learning-assisted data analysis

Despite being an excellent surface enhanced Raman scattering (SERS) active material, gold nanoparticles were difficult to be loaded onto the surface of filter paper to fabricate flexible SERS substrates. In this study, electrochemically synthesized gold nanoparticles (e-AuNPs) were deposited on graphene oxide (GO) nanosheets in solution by ultrasonication, resulting in the formation of a GO/Au hybrid material. Thanks to the support of GO, the hybrid material could adhere onto the surface of filter paper, which was immersed into a GO/Au solution for 24 h and dried naturally at room temperature. The paper-based materials were then employed as substrates for a surface enhanced Raman scattering (SERS) sensing platform to detect tricyclazole (TCZ), a widely used pesticide, resulting in better sensitivity compared to the use of paper/Au SERS sensors. With the most optimal GO content of 4%, paper/GO/Au SERS sensors could achieve a limit of detection of 1.32 × 10-10 M in standard solutions. Furthermore, the filter paper-based SERS sensors also exhibited significant advantages in sample collection in real samples. On one hand, the sensors were dipped into orange juice, allowing TCZ molecules in this real sample to be adsorbed onto their SERS active surface. On the other hand, they were pasted onto cucumber skin to collect the analytes. As a result, the paper/GO/Au SERS sensors could sense TCZ in orange juice and on cucumber skin at concentrations as low as 10-9 M (∼2 ppb). In addition, a machine learning model was designed and developed, allowing the sensing system to discriminate TCZ from nine other organic compounds and predict the presence of TCZ on cucumber skin at concentrations down to 10-9 M.

High-performance SERS chips for sensitive identification and detection of antibiotic residues with self-assembled hollow Ag octahedra

High-performance SERS chips via self-assembled hollow Ag octahedra on PDMS were employed to achieve the sensitive identification and detection of antibiotic residues. The developed SERS chips were successfully applied in the detection of ciprofloxacin (CIP), amoxicillin (AMX) and cefazolin (CZL) in wastewater and tap water samples, as well as enrofloxacin (ENR) in milk, demonstrating the sensitive determination of antibiotics in the real environment. From this perspective, these SERS chips are expected to expand the on spot sensitive detection and identification field of antibiotic residues.

The Convenience of Polydopamine in Designing SERS Biosensors with a Sustainable Prospect for Medical Application

Polydopamine (PDA) is a multifunctional biomimetic material that is friendly to biological organisms and the environment, and surface-enhanced Raman scattering (SERS) sensors have the potential to be reused. Inspired by these two factors, this review summarizes examples of PDA-modified materials at the micron or nanoscale to provide suggestions for designing intelligent and sustainable SERS biosensors that can quickly and accurately monitor disease progression. Undoubtedly, PDA is a kind of double-sided adhesive, introducing various desired metals, Raman signal molecules, recognition components, and diverse sensing platforms to enhance the sensitivity, specificity, repeatability, and practicality of SERS sensors. Particularly, core-shell and chain-like structures could be constructed by PDA facilely, and then combined with microfluidic chips, microarrays, and lateral flow assays to provide excellent references. In addition, PDA membranes with special patterns, and hydrophobic and strong mechanical properties can be used as independent platforms to carry SERS substances. As an organic semiconductor material capable of facilitating charge transfer, PDA may possess the potential for chemical enhancement in SERS. In-depth research on the properties of PDA will be helpful for the development of multi-mode sensing and the integration of diagnosis and treatment.

Understanding Metal-Semiconductor Plasmonic Resonance Coupling through Surface-Enhanced Raman Scattering

Although there has been intense research on plasmon-induced charge transfer within metal/semiconductor heterostructures, previous studies have all focused on the surface plasmonic resonance (SPR) of only noble metals. Herein and for the first time, we observe and take into account the plasmonic coupling between SPR of both noble-metal and semiconductor nanostructures. A W₁₈O₄₉/Ag heterostructure composed of metallic Ag nanoparticles (Ag NPs) and semiconducting W₁₈O₄₉ nanowires (W₁₈O₄₉ NWs) is designed and fabricated, which exhibits a broad and strong SPR absorption in the visible wavelength range. This SPR band is attributed to the SPR coupling between the SPR of both Ag NPs and W₁₈O₄₉ NWs. Surface-enhanced Raman scattering (SERS) is then used to reveal the interactions between the metal SPR, semiconductor SPR, and the heterostructure's charge transfer (CT) process, demonstrating that such coupled SPR enhanced the heterostructure's internal CT and SERS signals. Finally, we proposed a new coupled-plasmon-induced charge transfer mechanism to interpret the improved CT efficiency between the SERS substrate and molecules. Our work provides insight for further studies on plasmonic effects and interfacial charge transfer in metal/semiconductor heterostructures.

Defect-Induced Self-Poling in a W18O49/PVDF Piezoelectric Energy Harvester

W₁₈O₄₉ nanostructures, previously used for electrocatalysis, energy storage, electrochromic, and gas sensing applications, are incorporated in poly(vinylidene fluoride) (PVDF) in this work for mechanical energy-harvesting applications. X-ray diffraction spectroscopy (XRD), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, differential scanning calorimetry (DSC), and the polarization-electric (P-E) field loop test prompts the addition of W₁₈O₄₉ nanorods in PVDF nucleates and stabilizes the piezoelectric polar γ-phase in the nanocomposite. Electrochemical experiments were employed for the first time to relate the event of the evolution of crystalline phases in PVDF to the transfer of electrons to the electrolyte from PVDF using the data from cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). High dielectric constant (ε') and low dielectric loss (ε″) values were obtained proportionately for different weight percentage additions of W₁₈O₄₉ nanorods in PVDF. DSC was employed to study the crystallization kinetics of γ-phase evolution. Piezoresponse force microscopy (PFM) was used to compare the piezoelectric responses from the PVDF nanocomposites. The W₁₈O₄₉/PVDF nanocomposite could generate a peak open circuit voltage of ∼6 V and a peak short circuit current of ∼700 nA. The W₁₈O₄₉/PVDF nanocomposite could light two commercial blue-light-emitting diodes (LEDs) with hand impulse imparting.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Hydrophilic modification of PVDF-based SERS imprinted membrane for the selective detection of L-tyrosine

The polyvinylidene difluoride (PVDF) membrane has received considerable attention as a flexible surface enhanced Raman scattering (SERS) substrate due to its excellent mechanical and physicochemical properties. However, the poor fouling resistance of PVDF membrane due to its intrinsic hydrophobic property limits its practical application. To address this, in this investigation, a SERS imprinted membrane is synthesized based on W₁₈O₄₉/Ag composites. Firstly, to promote hydrophilicity, N-vinyl-2-pyrrolidone (NVP) and triethoxyvinylsilane (VTES) are copolymerized by hydrolysis condensation and linked with engineered polyvinypyrrolidone (PVP) chains exposed on the surface of membrane. Furthermore, W₁₈O₄₉/Ag composites are dispersed on the membrane under the assistance of polydopamine (pDA) to promote the pollution resistance. Subsequently, in order to demonstrate the practical detection property, W₁₈O₄₉/Ag/PVDF membrane is selected as the SERS substrate to synthesize SERS imprinted membrane by precipitation polymerization for the selective detection of L-tyrosine. The characteristic results reveal that the SERS-imprinted membrane exhibits satisfactory hydrophilicity, and it can effectively degrade the pollutant molecules absorbed on its surface under ultraviolet light illumination. It is proved from the detection results that the LOD of WADP-MIMs for L-tyrosine reached 10^-9 mol L^-1 when the concentration of L-tyrosine changed between 10^-3-10^-9 mol L^-1. The correlation coefficient (R²) is 0.994 and the limit of detection is 10^-9 mol L^-1. Meanwhile, it can be applied for the selective detection of L-tyrosine in mixture samples. Overall, this study presents a novel approach for the hydrophilic modification and pollution resistance enhancement of PVDF-based SERS imprinted membrane, which can be effectively utilized for the selective detection of practical samples.

Fabrication of antireflective silver-capped tin oxide nano-obelisk arrays as high sensitive SERS substrate

Hybrid noble metal-semiconductor oxide nanostructures often provide unique and synergetic functionalities that are highly desirable in various practical applications. However, the fabrication of such systems with desired functionalities using cost-effective techniques is still a great challenge. In this work, we report a facile route for the preparation of novel Ag/SnO₂ nano-obelisk arrayed thin films on silicon substrates by spray pyrolysis and thermal evaporation techniques. The prepared samples exhibited broadband antireflectance in both UV and visible regions attributed to the refractive index gradient and scattering provided by the nano-obelisk arrays. The localised surface plasmon resonance of silver nanocaps further enhanced the light absorption contributing to the antireflective property of the hybrid system. Ag/SnO₂ nano-obelisk arrayed thin film exhibited excellent SERS performance with an enhancement factor of 1.13 × 10⁸ with a limit of detection value of 10^-12 M for the trace detection of R6G dye. In addition, Ag/SnO₂ nano-obelisk arrayed thin film based SERS substrate exhibited good homogeneity across the measured spots and outstanding stability which are essential for quantitative field analysis. The results indicate that the Ag/SnO₂ nano-obelisk arrayed thin films are efficient SERS substrates with the merits of having the ease of production, high sensitivity and stability for various practical sensing applications.

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.

Enhanced Raman scattering on lead iodide film

Surface-enhanced Raman scattering (SERS) on organic-inorganic hybrid perovskites have been proved a promising SERS-active substrate. As its inorganic skeleton, lead iodide (PbI₂) also represents a novel semiconductor substrate material that serves as either a complement or an alternative to conventional metal-based SERS. Here, for the first time, enhanced Raman scattering (ERS) of 1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on PbI₂ film is observed. To our surprise, we obtained the same ERS spectra on the PbI₂ as SERS spectra obtained on the perovskites, which demonstrates the enhancement of substrate comes from inorganic skeleton rather than organic-inorganic hybrid perovskites. Compared with the normal Raman spectrum of BPE molecules, we find the B_u mode of the BPE molecule was selectively enhanced. We demonstrate that it involves a charge transfer (CT) resonance mechanism and takes place between the HOMO of BPE and the conduction band edge of PbI₂. Moreover, the PbI₂ substrate performs good reproducibility, uniformity and good Raman enhancement (EF ≥ 10⁵). It is believed that this unusual discovery not only enables PbI₂ to be an enhanced substrate, which also provides a new advance toward a better understanding of SERS chemical mechanism.

A Flexible, Self-Floating Composite for Efficient Water Evaporation

A flexible, self-floating W18O49/carbon foam composite is fabricated by calcining melamine foam with W18O49 as an adsorbate in N2 atmosphere. This self-floating property is simply realized by a carbonization process other than the complicated surface modification process. The simple synthesis procedure helps to increase not only the solar absorption but also the retention of W18O49 in the porous net structure. This composite absorbs almost the whole solar spectrum and generates localized heat at the surface, which is beneficial for water evaporation. Its water evaporation rate is 6.6 times higher than that of pure water. It has a stable cyclic performance over ten cycles under the illumination of simulated sunlight (500 W Xe lamp). Its flexibility makes it easy to reuse and transfer, which is evaluated by the bending deformation test. The W18O49/carbon foam composite is a prospective material in solar energy conversion field, and the preparation procedure is feasible to scale-up.