Semiconductor-enhanced Raman scattering: active nanomaterials and applications

High Electromagnetic Field Enhancement of TiO2 Nanotube Electrodes

We present the fabrication of TiO₂ nanotube electrodes with high biocompatibility and extraordinary spectroscopic properties. Intense surface-enhanced resonance Raman signals of the heme unit of the redox enzyme Cytochrome b₅ were observed upon covalent immobilization of the protein matrix on the TiO₂ surface, revealing overall preserved structural integrity and redox behavior. The enhancement factor could be rationally controlled by varying the electrode annealing temperature, reaching a record maximum value of over 70 at 475 °C. For the first time, such high values are reported for non-directly surface-interacting probes, for which the involvement of charge-transfer processes in signal amplification can be excluded. The origin of the surface enhancement is exclusively attributed to enhanced localized electric fields resulting from the specific optical properties of the nanotubular geometry of the electrode.

Indirect glyphosate detection based on ninhydrin reaction and surface-enhanced Raman scattering spectroscopy

Glyphosate is one of the most commonly-used and non-selective herbicides in agriculture, which may directly pollute the environment and threaten human health. A simple and effective approach to assessment of its damage to the natural environment is thus quite necessary. However, traditional chromatography-based detection methods usually suffer from complex pretreatment procedures. Herein, we propose a simple and sensitive method for the determination of glyphosate by combining ninhydrin reaction and surface-enhanced Raman scattering (SERS) spectroscopy. The product (purple color dye, PD) of the ninhydrin reaction is found to SERS-active and directly correlate with the glyphosate concentration. The limit of detection of the proposed method for glyphosate is as low as 1.43×10^-8mol·L^-1 with a relatively wider linear concentration range (1.0×10^-7-1.0×10^-4mol·L^-1), which demonstrates its great potential in rapid, highly sensitive concentration determination of glyphosate in practical applications for safety assessment of food and environment.

Self-Assembled Ag-Cu₂O Nanocomposite Films at Air-Liquid Interfaces for Surface-Enhanced Raman Scattering and Electrochemical Detection of H₂O₂

We employ a facile and novel route to synthesize multifunctional Ag-Cu₂O nanocomposite films through the self-assembly of nanoparticles at an air-liquid interface. In the ethanol-water phase, AgNO₃ and Cu(NO₃)₂ were reduced to Ag-Cu₂O nanoparticles by NaBH₄ in the presence of cinnamic acid. The Ag-Cu₂O nanoparticles were immediately trapped at the air-liquid interface to form two-dimensional nanocomposite films after the reduction reaction was finished. The morphology of the nanocomposite films could be controlled by the systematic regulation of experimental parameters. It was found that the prepared nanocomposite films serving as the substrates exhibited strong surface-enhanced Raman scattering (SERS) activity. 4-aminothiophenol (4-ATP) molecules were used as the test probes to examine the SERS sensitivity of the nanocomposite films. Moreover, the nanocomposite films synthesized by our method showed enhanced electrocatalytic activity towards hydrogen peroxide (H₂O₂) and therefore could be utilized to fabricate a non-enzymatic electrochemical H₂O₂ sensor.

Polyacrylamide Gel-Contained Zinc Finger Peptide as the "Lock" and Zinc Ions as the "Key" for Construction of Ultrasensitive Prostate-Specific Antigen SERS Immunosensor

In this work, we adopted polyacrylamide gel-contained zinc finger peptide (PZF) as a "lock" of Raman signal and zinc ions (Zn²⁺) as a sensitive "key", which was converted from target-captured ZnO NPs, to achieve the measurement of prostate-specific antigen (PSA). Owing to the lock effect from PZF, the surface-enhanced Raman scattering (SERS) tag toluidine blue (TB) connected on Ag NP-coating silica wafer was sheltered leading to low Raman response. Meanwhile, target PSA can specifically connect with antibody 2-coupled ZnO nanocomplexes (ZnO@Au@Ab₂) and antibody 1-coupled magnetic (CoFe₂O₄@Au@Ab₁) nanocomposite through sandwich immunoassay. In the presence of HCl, the ZnO NPs would convert into Zn²⁺ to open the PZF because Zn²⁺ can specifically react with zinc finger peptide to destroy the PZF structure forming abundant pores. In this way, Zn²⁺ could act as the key of Raman signal to open the PZF structure obtaining a strong Raman signal of TB. The proposed SERS sensor can have a quantitative detection of PSA within the range of 1 pg mL^-1 to 10 ng mL^-1 with a detection limit of 0.65 pg mL^-1. The interaction between zinc finger peptide and Zn²⁺ was firstly applied in SERS sensor for the sensitive detection of PSA. These results demonstrated that the new designed SERS biosensor could be a promising tool in biomarker diagnosis.

SERS study of surface plasmon resonance induced carrier movement in Au@Cu2O core-shell nanoparticles

A plasmon induced carrier movement enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a charge-transfer (CT) enhancement mechanism. Here, we designed a strategy to study SERS in Au@Cu₂O nanoshell nanoparticles with different shell thicknesses. Among the plasmonically coupled nanostructures, Au spheres with Cu₂O shells have been of special interest due to their ultrastrong electromagnetic fields and controllable carrier transfer properties, which are useful for SERS. Au@Cu₂O nanoshell nanoparticles (NPs) with shell thicknesses of 48-56nm are synthesized that exhibit high SERS activity. This high activity originates from plasmonic-induced carrier transfer from Au@Cu₂O to 4-mercaptobenzoic acid (MBA). The CT transition from the valence band (VB) of Cu₂O to the second excited π-π* transition of MBA, and is of b₂ electronic symmetry, which was enhanced significantly. The Herzberg-Teller selection rules were employed to predict the observed enhanced b₂ symmetry modes. The system constructed in this study combines the long-range electromagnetic effect of Au NPs, localized surface plasmon resonance (LSPR) of the Au@Cu₂O nanoshell, and the CT contribution to assist in understanding the SERS mechanism based on LSPR-induced carrier movement in metal/semiconductor nanocomposites.

Toward Universal SERS Detection of Disease Signaling Bioanalytes Using 3D Self-Assembled Nonplasmonic near-Quantum-Scale Silicon Probe

Currently, the quantum-scale surface-enhanced Raman scattering (SERS) properties of Si materials have yet to be discovered for universal biosensing applications. In this study, a potential universal biosensing probe is generated by activating the SERS functionality of Si nanostructures through near quantum-scale (nQS) engineering. We introduce herein 3D nonplasmonic Si nanomesh structure with nQS defects for SERS biosensing applications. Through ionization of a single-crystal defect-free Si wafer, highly defect-rich Si subnano-orbs (sNOs) are fabricated and self-assemble as connective 3D Si nanomesh structures with enhanced SERS biosensing activity. By amending the laser ionization and ion-ion interactions, we observe the controlled synthesis of engineered nQS defects in the form of nQS-grain boundary disorder or surface nQS voids within the interconnected Si sNOs. To our knowledge, it is shown here for the first time that defect-rich Si nanomesh structures exhibit enhanced Raman activity, with the nQS morphological and crystallographic defects acting as the prime SERS contributors without a plasmonic contribution. The SERS biosensing sensitivity with the synthesized defect-rich Si nanomesh structures without an additional plasmonic material was evaluated using of a tripeptide biomarker l-glutathione (GSH); we observe an enhancement factor value of ∼10² for the GSH biomolecules with 10^-9 M sensitivity, a phenomena to our knowledge that has yet to be reported. Additionally, the SERS detection of multiple disease-signaling biomolecules (cysteine, tryptophan, and methionine) is achieved at very low analyte concentration (10^-9 M). These results indicate a potential new dimension to universal SERS biosensing applications with these unique nonplasmonic defect-rich 3D nQS-Si nanostructures.

Fabrication of Semiconductor ZnO Nanostructures for Versatile SERS Application

Since the initial discovery of surface-enhanced Raman scattering (SERS) in the 1970s, it has exhibited a huge potential application in many fields due to its outstanding advantages. Since the ultra-sensitive noble metallic nanostructures have increasingly exposed themselves as having some problems during application, semiconductors have been gradually exploited as one of the critical SERS substrate materials due to their distinctive advantages when compared with noble metals. ZnO is one of the most representative metallic oxide semiconductors with an abundant reserve, various and cost-effective fabrication techniques, as well as special physical and chemical properties. Thanks to the varied morphologies, size-dependent exciton, good chemical stability, a tunable band gap, carrier concentration, and stoichiometry, ZnO nanostructures have the potential to be exploited as SERS substrates. Moreover, other distinctive properties possessed by ZnO such as biocompatibility, photocatcalysis and self-cleaning, and gas- and chemo-sensitivity can be synergistically integrated and exerted with SERS activity to realize the multifunctional potential of ZnO substrates. In this review, we discuss the inevitable development trend of exploiting the potential semiconductor ZnO as a SERS substrate. After clarifying the root cause of the great disparity between the enhancement factor (EF) of noble metals and that of ZnO nanostructures, two specific methods are put forward to improve the SERS activity of ZnO, namely: elemental doping and combination of ZnO with noble metals. Then, we introduce a distinctive advantage of ZnO as SERS substrate and illustrate the necessity of reporting a meaningful average EF. We also summarize some fabrication methods for ZnO nanostructures with varied dimensions (0–3 dimensions). Finally, we present an overview of ZnO nanostructures for the versatile SERS application.

An enhanced degree of charge transfer in dye-sensitized solar cells with a ZnO-TiO2/N3/Ag structure as revealed by surface-enhanced Raman scattering

A number of recent studies have focused on improving the performance of dye-sensitized solar cells (DSSCs). Cells with a ZnO-TiO₂/N3/Ag structure have attracted particular attention because of their excellent power conversion efficiencies. Using a dendritic crystal ZnO-TiO₂ composite semiconductor and Ag in conjunction leads to different charge-transfer (CT) processes, and this is the main theoretical basis for the improvement of DSSC performances. Thus, in the present study, TiO₂/N3, ZnO/N3, ZnO-TiO₂/N3, TiO₂/N3/Ag, ZnO/N3/Ag, and ZnO-TiO₂/N3/Ag assemblies have been fabricated and their CT processes have been monitored by using surface-enhanced Raman scattering (SERS) spectra, with particular focus on the differences caused by the synergistic effect of the ZnO-TiO₂ component. The dye loading capacity of the dendritic crystal ZnO-TiO₂ is much larger than that of TiO₂. There are extra enhancements in the SERS intensity and degree of CT (ρ_CT) in ZnO-TiO₂/N3 compared to ZnO + TiO₂/N3 (based on a simulation curve for the physically mixed TiO₂ and ZnO semiconductors) with 476.5 nm excitation due to the synergistic effect of the ZnO-TiO₂ component. And these enhancements in ZnO-TiO₂/N3/Ag compared to ZnO + TiO₂/N3/Ag appear with 476.5 and 532 nm excitation, which are particularly large with 532 nm excitation. Accordingly, the participation of Ag in this synergistic effect can reduce its energy threshold, which will make it easier to appear. Finally, to rationalize these extra enhancements, the models describing the CT mechanism have been proposed. Thus, the use of the dendritic crystal ZnO-TiO₂ composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3 compared to that with TiO₂. Meanwhile, the synergistic effect of ZnO-TiO₂ and Ag can promote the CT process, demonstrating that ZnO-TiO₂/N3/Ag is an excellent structure for DSSCs.

Detection of Pesticide Residues in Food Using Surface-Enhanced Raman Spectroscopy: A Review

Pesticides directly pollute the environment and contaminate foods ultimately being absorbed by the human body. Their residues contain highly toxic substances that have been found to cause serious problems to human health even at very low concentrations. The gold standard method, gas/liquid chromatography combined with mass spectroscopy, has been widely used for the detection of pesticide residues. However, these methods have some drawbacks such as complicated pretreatment and cleanup steps. Recent technological advancements of surface-enhanced Raman spectroscopy (SERS) have promoted the creation of alternative detection techniques. SERS is a useful detection tool with ultrasensitivity and simpler protocols. Present SERS-based pesticide residue detection often uses standard solutions of target analytes in conjunction with theoretical Raman spectra calculated by density functional theory (DFT) and actual Raman spectra detected by SERS. SERS is quite a promising technique for the direct detection of pesticides at trace levels in liquid samples or on the surface of solid samples following simple extraction to increase the concentration of analytes. In this review, we highlight recent studies on SERS-based pesticide detection, including SERS for pesticide standard solution detection and for pesticides in/on food samples. Moreover, in-depth analysis of pesticide chemical structures, structural alteration during food processing, interaction with SERS substrates, and selection of SERS-active substrates is involved.

Controllable Charge Transfer in Ag-TiO₂ Composite Structure for SERS Application

The nanocaps array of TiO₂/Ag bilayer with different Ag thicknesses and co-sputtering TiO₂-Ag monolayer with different TiO₂ contents were fabricated on a two-dimensional colloidal array substrate for the investigation of Surface enhanced Raman scattering (SERS) properties. For the TiO₂/Ag bilayer, when the Ag thickness increased, SERS intensity decreased. Meanwhile, a significant enhancement was observed when the sublayer Ag was 10 nm compared to the pure Ag monolayer, which was ascribed to the metal-semiconductor synergistic effect that electromagnetic mechanism (EM) provided by roughness surface and charge-transfer (CT) enhancement mechanism from TiO₂-Ag composite components. In comparison to the TiO₂/Ag bilayer, the co-sputtered TiO₂-Ag monolayer decreased the aggregation of Ag particles and led to the formation of small Ag particles, which showed that TiO₂ could effectively inhibit the aggregation and growth of Ag nanoparticles.

Electrospun magnetic CoFe2O4/Ag hybrid nanotubes for sensitive SERS detection and monitoring of the catalytic degradation of organic pollutants

We report on the synthesis of magnetic CoFe₂O₄/Ag hybrid nanotubes as both SERS substrate and catalyst to monitor the catalytic degradation process of organic pollutants.