Films of Reduced Graphene Oxide with Metal Oxide Nanoparticles Formed at a Liquid/Liquid Interface as Reusable Surface Enhanced Raman Scattering Substrates for Dyes

Flower-like Ag-decked non-stoichiometric Bi2O3-x/rGO hybrid nanocomposite SERS substrates for an effective detection of Rhodamine 6G dye molecules

In early years, SERS-active substrates were generally noble metals. However, their practical applications were limited due to their poor biocompatibility, low uniformity and high cost. Recently, the utilization of semiconductor SERS-active substrates has greatly expanded the applications of SERS in many fields. However, metal-free SERS-active substrates have a low enhancement factor (EF), which can be overcome by adjusting their oxygen deficiency or through the effective preparation of non-stoichiometric semiconducting oxide materials. This is the key strategy and may work as an efficient and simple way to achieve high sensitivity and obtain an enhancement factor (G-factor) comparable to that of noble metals. Here, we report the preparation of flower-like rGO-Bi2O3/Bi2O2.75 and rGO-Ag-Bi2O3/Bi2O2.75 hybrid thin film nanocomposites using a liquid/liquid interface method (LLI) for the first time. In addition to the synergic effect of different enhancement mechanisms, the 3-D flower-like morphology of the substrate shows more favourable properties to improve the G-factor due to the existence of more hotspots. The rGO-Ag-Bi2O3/Bi2O2.75 hybrid thin-film nanocomposites show an EF of 1.8 × 109 with a detection ability of up to 1 nM towards Rhodamine 6G (R6G), which is highly toxic to humans and the aquatic environment.

Improved charge-transfer resonance in graphene oxide/ZrO2 substrates for plasmonic-free SERS determination of methyl parathion

This work reports a sensitive SERS substrate based on graphene oxide (GO) and quantum-sized ZrO₂ nanoparticles (GO/ZrO₂) for label-free determination of the organophosphate pesticide methyl parathion (MP). The enhanced light-matter interactions and the consequent SERS effect in these substrates resulted from the effective charge transfer (CT) mechanism attributed to synergistic contributions of three main factors: i) the strong molecular adherence of the MP molecules and the ZrO₂ surface which allows the first layer-effect, ii) the relatively abundant surface defects in low dimensional ZrO₂ semiconductor NPs, which act as intermediate electronic states that reduce the large bandgap barrier, and iii) the hindered charge recombination derived from the transference of the photoinduced holes to the GO layer. This mechanism allowed an enhancement factor of 8.78 × 10⁴ for GO/ZrO₂-based substrates, which is more than 5-fold higher than the enhancement observed for platforms without GO. A detection limit of 0.12 μM was achieved with an outstanding repeatability (variation ≤4.5%) and a linear range up to 10 μM, which is sensitive enough to determine the maximal MP concentration permissible in drinking water according to international regulations. Furthermore, recovery rates between 97.4 and 102.1% were determined in irrigation water runoffs, strawberry and black tea extracts, demonstrating the reliability of the hybrid GO/ZrO₂ substrate for the organophosphate pesticides quantification in samples related to agri-food sectors and environmental monitoring.

Self-assembled Cr2O3@nanogel/Au nanozymes to simulate peroxidase activity as a H2O2 sensor

The low temperature solvothermal method synthesized Cr₂O₃ NPs has not only peroxidase activity, but also oxidase activity. Then, the oxidase activity of Cr₂O₃ NPs is effectively shielded by nanogel immobilization using three monomers acrylamide, NIPAAM (N-isopropylacrylamide) and MBA (N,N'-methylene bisacrylamide) in HEPES (4-(2-hydroxyerhyl)piperazine-1-erhanesulfonic acid) buffer. Ultimately, the enzymatic activity of Cr₂O₃@nanogel/Au is significantly enhanced after doping Au NPs by SERS (Surface Enhanced Raman Spectroscopy) evaluation. A SERS strategy was proposed for the detection of H₂O₂ by Cr₂O₃@nanogel/Au. The linear range was 10^-8 mol·L^-1-10^-1 mol·L^-1.

Low-cost electrochemical detection of l-tyrosine using an rGO-Cu modified pencil graphite electrode and its surface orientation on a Ag electrode using an ex situ spectroelectrochemical method

A low cost reduced graphene oxide–copper hybrid nano thin-film modified Pencil Graphite Electrode has been employed to detect the l-tyrosine enantiomer. The free-standing rGO–Cu hybrid nano-thin film was prepared by a simple one-step liquid–liquid interface method. Electrochemical Cyclic Voltammetry, Differential Pulse Voltammetry, pH-dependent and scan rate dependent studies on bare PGE, Cu, rGO, and rGO–Cu for l-tyrosine have been explained in detail. The rGO–Cu modified PGE based biosensor exhibits good detection of l-tyrosine. The linear range detection limit was estimated to be 1 × 10⁻⁷ M. The calculated sensitivity is 0.4 μA ppm⁻¹ mm². This electroactive biosensor is easily fabricated and controlled and is cost-effective. The surface orientation of l-tyrosine on the Ag electrode at a particular potential and its comparison with vibrational DFT calculations have been studied for the first time.

A low cost reduced graphene oxide–copper hybrid nano thin-film modified pencil graphite electrode has been employed to detect the l-tyrosine enantiomer.

Single sea urchin-MoO3 nanostructure for surface enhanced Raman spectroscopy of dyes

Enhancing the surface-enhanced Raman scattering (SERS) activity of semiconductor metal oxide nanostructures by controlling the morphology and oxygen vacancies towards trace detection of organics is of significant interest. In this study, MoO₃ with a novel sea urchin morphology is synthesized employing chemical bath deposition and consists of hundreds of ∼15 μm long spikes originating from the core forming 20-40 micron globular structures. The spikes taper to form 20 nm sharp tips. SERS of rhodamine 6G (R6G) over MoO₃ sea urchins has been investigated and compared to that of 1D h-MoO₃ nanorod arrays. The SERS activity is morphology dependent and the sea urchin-like morphology exhibits higher SERS activity with an enhancement factor (EF) of the order 10⁵ and a detection limit of 100 nM, while for h-MoO₃ nanorods, the corresponding values are 10³ and 1 μM, respectively. X-ray photoelectron spectroscopy reveals a high concentration of Mo⁺⁵ states in sea urchins indicating lattice oxygen vacancies. The observed EF is quite high for a metal oxide substrate and is attributed to the enhanced charge transfer between analyte molecules and the substrate promoted by the oxygen vacancies along with surface defects and hydroxyl groups on MoO₃ sea urchins providing more active sites for the adsorption of probe molecules. The role of oxygen vacancies is confirmed by the lower EF value exhibited by the stoichiometric 1D h-MoO₃. Raman mapping of a single sea urchin is achieved with good R6G intensity and indicates that the tips of spiky features are involved in SERS enhancement. The reusability of substrates is shown for repeated cycles of R6G adsorption by UV irradiation exploiting the photocatalytic activity of MoO₃ nanostructures.

Biological application of water-based electrochemically synthesized CuO leaf-like arrays: SERS response modulated by the positional isomerism and interface type

Cupric oxide leaf-like nanostructures (CuONSs) (average dimensions: 80-180 nm in width and 400-750 nm in length) were synthesized via anodic electrochemical dissolution of copper in an ethanol solution containing LiCl electrolyte and water. Ultraviolet-visible (UV-Vis), Fourier-transform infrared (FT-IR), and Raman spectroscopies as well as scanning electron microscope (SEM), high-resolution transmission electron microscopy with energy dispersive X-ray (HD-TEM-EDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) were used to explore the metal surface plasmon, size, rheology, and structure of CuONSs. Then, pyridine α-aminophosphinic acid isomers (α-, β-, and γ-NHPy) were synthesized and assembled on the CuONS/air and CuONS/aqueous solution interfaces at the pH level of solution = 7. Differences in adsorption and thus in the spectral response resulting from positional isomerism were examined by surface-enhanced Raman scattering (SERS) with an excitation wavelength of 785 nm. The manner of interaction of the investigated isomers with CuONSs in an aqueous solution was discussed in detail and compared with that at the CuONS/air interface. For γ-NHPy, at the CuONS/water interface, the time-dependent changes in the spectral profile were observed and analyzed. For β-NHPy at the CuONS/air interface, tip-enhanced Raman scattering (TERS) measurements were performed. These measurements allowed observing single molecule behavior and avoiding interference from the molecule's surrounding environment.

Improved surface-enhanced Raman and catalytic activities of reduced graphene oxide-osmium hybrid nano thin films

Reduced graphene oxide-osmium (rGO-Os) hybrid nano dendtrites have been prepared by simple liquid/liquid interface method for the first time. The method involves the introduction of phase-transfered metal organic precursor in toluene phase and GO dispersion in the aqueous phase along with hydrazine hydrate as the reducing agent. Dendritic networks of Os nanoparticles and their aggregates decorating rGO layers are obtained. The substrate shows improved catalytic and surface-enhanced activities comparable with previous reports. The catalytic activity was tested for the reduction of p-nitroaniline into p-phenyldiamine with an excess amount of NaBH₄. The catalytic activity factors of these hybrid films are 2.3 s^-1 g^-1 (Os film) and 4.4 s^-1 g^-1 (rGO-Os hybrid film), which are comparable with other noble metal nanoparticles such as Au, Ag, but lower than Pd-based catalysts. Surface-enhanced Raman spectroscopy (SERS) measurements have been done on rhodamine 6G (R6G) and methylene blue dyes. The enhancement factor for the R6G adsorbed on rGO-Os thin film is 1.0 × 10⁵ and for Os thin film is 7 × 10³. There is a 14-fold enhancement observed for Os hybrids with rGO. The enhanced catalytic and SERS activities of rGO-Os hybrid thin film prepared by simple liquid/liquid interface method open up new challenges in electrocatalytic application and SERS-based detection of biomolecules.