Understanding and exploring the decisive factors responsible for superlative catalytic efficiency is necessary to formulate active electrode materials for improved electrocatalysis and high-throughput sensing. This research demonstrates the ability of bud-shaped gold nanoflowers (AuNFs), intermediates in the bud-to-blossom gold nanoflower synthesis, to offer remarkable electrocatalytic efficiency in the oxidation of ascorbic acid (AA) at nanomolar concentrations. Multicomponent sensing in a single potential sweep is measured using differential pulse voltammetry while the kinetic parameters are estimated using electrochemical impedance spectroscopy. The outstanding catalytic activity of bud-structured AuNF [iAuNFp(Bud)/iGCp ≅ 100] compared with other bud-to-blossom intermediate nanostructures is explained by studying their structural transitions, charge distributions, crystalline patterns, and intrinsic irregularities/defects. Detailed microscopic analysis shows that density of crystal defects, such as edges, terraces, steps, ledges, kinks, and dislocation, plays a major role in producing the high catalytic efficiency. An associated ab initio simulation provides necessary support for the projected role of different crystal facets as selective catalytic sites. Density functional theory corroborates the appearance of inter- and intra-molecular hydrogen bonding within AA molecules to control the resultant fingerprint peak potentials at variable concentrations. Bud-structured AuNF facilitates AA detection at nanomolar levels in a multicomponent pathological sample.
The electrocatalytic performance of noble metal nanoparticles depends upon their size, shape, composition, and crystalline facets. Here we demonstrate the shape-dependent electrocatalytic activity of Au nanoparticles toward ascorbic acid oxidation in acidic medium, wherein the catalysis is strongly influenced by the shape of the nanoparticles. The synthesis of (popcorn, tetrapod, and bipod shaped) Au nanoparticles was carried out using a systematic variation of the surfactant concentrations based on the seed-mediated growth technique at room temperature. Due to the facile electrostatic interaction of the positively charged Au nanoparticles with glassy carbon electrode, the modification of the surface with variable-shaped Au nanoparticles is accomplished without involving any binding agents. Among variable-shaped face-centered cubic (fcc) crystalline AuNPs, bipod-shaped Au nanoparticles (GNBipd) exhibit a superior electrocatalytic performance over tetrapod-shaped (GNTepd) and popcorn-shaped (GNPop) nanoparticles as inferred from the differential pulse voltammetry and electrochemical impedance spectroscopy. The results have been explained by invoking the relative surface free energy (γ) with preferentially exposed crystal planes, relative surface area (A), zeta potential (ξ), and the curvature-induced charge density (σ q ) at the apex for individual variable-shaped gold nanoparticles.
We report here the permselectivity of overoxidized polyaniline obtained using anodic polarization of polyaniline on glassy carbon electrodes. The contrasting redox behavior of overoxidized polyaniline coated electrodes towards [Fe(CN)] and [Ru(NH)] has been analyzed using cyclic voltammetry, hydrodynamic voltammetry and electrochemical impedance spectroscopy. This permselectivity vis a vis anion exclusivity arises from the incorporation of counter anions rather than by the formation of new functional groups in the polymer upon overoxidation - as inferred from FT Raman and UV-Visible spectral data. The surface charges of the polymeric films are also deduced from the zeta potential analysis. The thickness-dependent anion exclusion behavior of overoxidized polyaniline is quantitatively interpreted using diffusion coefficient measurements with rotating disc electrodes. The mechanism pertaining to the non-trivial role of film thickness in influencing anion exclusion is confirmed by additional impedance spectroscopy carried out during the overoxidation of polyaniline.
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