Polyoxovanadate fabricated gold nanoparticles: Application in SERS

Small Au nanoparticles to be modified with decavanadate for sensitive and stable SERS detection

Surface enhanced Raman spectroscopy (SERS) has been a powerful vibrational optical spectroscopic technique for trace determination. And the application and promotion of SERS technology are closely related to plasmonic substrates. To combine the distinct electronic properties of polyoxovanadates with the plasmonic properties of Au NPs, the small cit-Au NPs (reduced by sodium citrate) was modified with decavanadate (V10) via ligand displacement. The structure of V10 was controlled by pH adjustment. X-ray photoelectron spectroscopy (XPS) and SERS were introduced for studying interactions of V10 with the surface of Au NPs. Compared with cit-Au NPs, V10-Au NPs owned higher absolute value of zeta potential and presented greater stability in CH3OH/H2O (V/V = 1:10) solvent environment. Even after 60 min of soaking, the V10-Au NPs still exhibited a typical coffee ring effect during the evaporation process with SERS enhancement. The stability of V10-Au NPs (in a solution with pH 4.8) was tested over a period of 5 days to indicate their ability to maintain SERS activity. Moreover, the V10-Au NPs also showed great sensitivity for cationic dye molecules and antibiotics, and the detection levels could be as low as μg⋅L-1. This study lays the foundation for the screening low-concentration cations in mixed solutions and monitoring the morphological changes of polyoxometalates during their application via SERS technology.

Self-assembly of hard anions around cationic gold nanorods: potential structures for SERS

The placement of polyoxometalates next to the surface of noble metallic nanoparticles has been found to enhance the surface-enhanced Raman scattering (SERS) effect. The enhancement is believed to stem from either charge (electrostatic attraction) or chemical effects. Anisotropic gold nanorods are recognized as useful nanostructures for SERS, mainly due to the high electric field enhancement at their ends. The presented work examines the use of a polyoxometalate encapsulated gold nanorod for SERS, to assess whether the two enhancement pathways would be synergetic. For this, a gold nanorod-polyoxometalate composite was synthesized by coating cetyltrimethylammonium bromide-stabilized gold nanorods with a silicotungstic Keggin anion through electrostatic attraction. The structure was characterized, confirming that the nanorods have been fully encapsulated by the polyoxometalate. The SERS performance of the composite was assessed in solution using crystal violet as a SERS indicator, finding an analytical enhancement factor of 1.8 × 104 in colloidal solution. The enhancement mechanism was examined first by comparison to gold nanorods stabilized by a cetyltriethylammonium bromide bilayer, cationic thiol bound polyoxometalate, and polyelectrolyte coating. Next, composites made using polyoxometalates of different atomic composition and charge were examined. It was concluded that the polyoxometalate charge had a noticeable effect on the enhancement while the atomic composition did not. Furthermore, high enhancement is observed mainly in cases where the nanorod monolayer allows the sequestration of the dye molecule into the nanoparticle's ligand layer. The proposed mechanism therefore involves the negative charge of the polyoxometalate attracting the positively charged dye, and facilitating the sequestration of the dye within the ligand bilayer, closer to the nanorod's surface.

Recent Advances in Hybrid Materials of Metal Nanoparticles and Polyoxometalates

Polyoxometalates (POMs), anionic metal-oxygen nanoclusters that possess various composition-dependent properties, are widely used to modify the existing properties of metal nanoparticles and to endow them with new ones. Herein, we present an overview of recent advances in hybrid materials that consist of metal nanoparticles and POMs. Following a brief introduction on the inception of this area and its development, representative properties and applications of these materials in various fields such as electrochemistry, photochemistry, and catalysis are introduced. We discuss how the combination of two classic inorganic materials facilitates cooperative and synergistic behavior, and we also give personal perspectives on the future development of this field.

Hybrid Gold Nanoparticle-Polyoxovanadate Matrices: A Novel Surface Enhanced Raman/Surface Enhanced Infrared Spectroscopy Substrate

Bare gold nanoparticles were embedded into an iron-polyoxovanadate matrix and used to enhance both the infrared and Raman signatures of a model analyte. A detailed characterization of the matrix-embedded nanoparticles revealed that they retained a plasmon resonance at 564 nm. The enhancement of vibrational signatures of the model analyte crystal violet using bare and embedded gold nanoparticles was compared for both surface enhanced infrared (SEIRA) spectroscopy and surface enhanced Raman spectroscopy (SERS) yielding enhancement factors of 2.2 for SEIRA and 77 for SERS. In contrast, the bare gold nanoparticles revealed significantly lower enhancements (1.6 for SEIRA; 20 for SERS). Hence, it was shown that embedding nanoparticles within an inorganic polyoxometalate-based matrix is an innovative strategy to amplify their signal enhancement properties in vibrational spectroscopies.

In Situ Synthesis of SERS-Active Au@POM Nanostructures in a Microfluidic Device for Real-Time Detection of Water Pollutants

We present a simple, versatile, and low-cost approach for the preparation of surface-enhanced Raman spectroscopy (SERS)-active regions within a microfluidic channel 50 cm in length. The approach involves the UV-light-driven formation of polyoxometalate-decorated gold nanostructures, Au@POM (POM: H₃PW₁₂O₄₀ (PW) and H₃PMo₁₂O₄₀ (PMo)), that self-assemble in situ on the surface of the polydimethylsiloxane (PDMS) microchannels without any extra functionalization procedure. The fabricated LoCs were characterized by scanning electron microscopy (SEM), UV-vis, Raman, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. The SERS activity of the resulting Au@POM-coated lab-on-a-chip (LoC) devices was evaluated in both static and flow conditions using rhodamine R6G. The SERS response of Au@PW-based LoCs was found to be superior to Au@PMo counterparts and outstanding when compared to reported data on metal@POM nanocomposites. We demonstrate the potentialities of both Au@POM-coated LoCs as analytical platforms for real-time detection of the organophosphorous pesticide paraoxon-methyl at 10^-6 M concentration level.

Carbon Quantum Dots Prepared with Chitosan for Synthesis of CQDs/AuNPs for Iodine Ions Detection

Water-soluble and reductive carbon quantum dots (CQDs) were fabricated by the hydrothermal carbonization of chitosan. Acting as a reducing agent and stabilizer, the as-prepared CQDs were further used to synthesize gold nanoparticles (AuNPs). This synthetic process was carried out in aqueous solution, which was absolutely "green". Furthermore, the CQDs/AuNPs composite was used to detect iodine ions by the colorimetric method. A color change from pink to colorless was observed with the constant addition of I- ions, accompanied by a decrease in the absorbance of the CQDs/AuNPs composite. According to the absorbance change, a favorable linear relationship was obtained between ΔA and I- concentration in the range of 20⁻140 μM and 140⁻400 μM. The detection limit of iodide ions, depending on the 3δ/slope, was estimated to be 2.3 μM, indicating high sensitivity to the determination of iodide. More importantly, it also showed good selectivity toward I- over other anion ions, and was used for the analysis of salt samples. Moreover, TEM results indicated that I- ions induced the aggregation of CQDs/AuNPs, resulting in changes in color and absorbance.