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This article describes the synthesis of branched flower-like gold (Au) nanocrystals and their electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Gold nanoflowers (GNFs) were obtained by a one-pot synthesis using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) as a reducing/stabilizing agent. The GNFs have been characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electrochemical measurements. The UV-visible spectra show two bands corresponding to the transverse and longitudinal surface plasmon (SP) absorption at 532 and 720 nm, respectively, for the colloidal GNFs. The GNFs were self-assembled on a sol-gel-derived silicate network, which was preassembled on a polycrystalline Au electrode and used for electrocatalytic applications. The GNFs retain their morphology on the silicate network; the UV-visible diffuse reflectance spectra (DRS) of GNFs on the silicate network show longitudinal and transverse bands as in the case of colloidal GNFs. The GNFs show excellent electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Oxidation of methanol in alkaline solution was observed at approximately 0.245 V, which is much less positive than that on an unmodified polycrystalline gold electrode. Reduction of oxygen to H2O2 and the further reduction of H2O2 to water in neutral pH were observed at less negative potentials on the GNFs electrode. The electrocatalytic activity of GNFs is significantly higher than that of the spherically shaped citrate-stabilized Au nanoparticles (SGNs).

A nonenzymatic electrochemical method is described for the detection of glucose by using gold (Au) nanoparticles self-assembled on a three-dimensional (3D) silicate network obtained by using sol-gel processes. The nanosized Au particles have been self-assembled on the thiol tail groups of the silicate network and enlarged by hydroxylamine. The Au nanoparticles efficiently catalyze the oxidation of glucose at less-positive potential (0.16 V) in phosphate buffer solution (pH 9.2) in the absence of any enzymes or redox mediators. The Au nanoparticle-modified transducer (MPTS-nAuE) was successfully used for the amperometric sensing of glucose and it showed excellent sensitivity with a detection limit of 50 nM. The common interfering agent ascorbate (AA) does not interfere with the detection of glucose. The MPTS-nAuE transducer showed individual voltammetric responses for glucose and AA. This transducer responded linearly to glucose in the range of 0-8 mM and the sensitivity of the transducer was found to be 0.179 nA cm(-2) nM(-1). Excellent reproducibility, and long-term storage and operational stability was observed for this transducer.

Development of efficient electrocatalyst based on non-precious metal that favors the four-electron pathway for the reduction of oxygen in alkaline fuel cell is a challenging task. Herein, we demonstrate a new facile route for the synthesis of hybrid functional electrocatalyst based on nitrogen-doped reduced graphene oxide (N-rGO) and Mn3O4 with pronounced electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution. The synthesis involves one-step in situ reduction of both graphene oxide (GO) and Mn(VII), growth of Mn3O4 nanocrystals and nitrogen doping onto the carbon framework using a single reducing agent, hydrazine. The X-ray photoelectron (XPS), Raman and FTIR spectral, and X-ray diffraction measurements confirm the reduction of GO and growth of nanosized Mn3O4. The XPS profile reveals that N-rGO has pyridinic (40%), pyrrolic (53%), and pyridine N oxide (7%) types of nitrogen. The Mn3O4 nanoparticles are single crystalline and randomly distributed over the wrinkled N-rGO sheets. The hybrid material has excellent ORR activity and it favors the 4-electron pathway for the reduction of oxygen. The electrocatalytic performance of the hybrid catalyst is superior to the N-rGO, free Mn3O4 and their physical mixture. The hybrid material shows an onset potential of -0.075 V, which is 60-225 mV less negative than that of the other catalyst tested. It has excellent methanol tolerance and high durability. The catalytic current density achieved with the hybrid material at 0.1 mg cm(-2) is almost equivalent to that of the commercial Pt/C (10%). The synergistic effect of N-rGO and Mn3O4 enhances the overall performance of the hybrid catalyst. The nitrogen in N-rGO is considered to be at the interface to bridge the rGO framework and Mn3O4 nanoparticles and facilitates the electron transfer.

Development of a highly sensitive nanostructured electrochemical biosensor based on the integrated assembly of dehydrogenase enzymes and gold (Au) nanoparticle is described. The Au nanoparticles (AuNPs) have been self-assembled on a thiol-terminated, sol-gel-derived, 3-D, silicate network and enlarged by hydroxylamine seeding. The AuNPs on the silicate network efficiently catalyze the oxidation of NADH with a decrease in overpotential of approximately 915 mV in the absence of any redox mediator. The surface oxides of AuNP function as an excellent mediator, and a special inverted "V" shape voltammogram at less positive potential was observed for the oxidation of NADH. The AuNP self-assembled sol-gel network behaves like a nanoelectrode ensemble. The nanostructured electrode shows high sensitivity (0.056 +/- 0.001 nA/nM) toward NADH with an amperometric detection limit of 5 nM. The electrode displays excellent operational and storage stability. A novel methodology for the fabrication of a NADH-dependent dehydrogenase biosensor based on the integration of dehydrogenase enzyme and AuNPs with the silicate network is developed. The enzymatically generated NADH is, in turn, electrocatalytically detected by the AuNPs on the silicate network. The integrated assembly has been successfully used for the amperometric biosensing of lactate and ethanol at a potential of -5 mV. The biosensor is very stable and highly sensitive, and it has a fast response time. The excellent performance validates the integrated assembly as an attractive sensing element for the development of new dehydrogenase biosensors.

We describe the development of a highly sensitive amperometric biosensor based on the hybrid material derived from nanoscale Pt particles (nPt) and graphene for the sensing of H 2 O 2 and cholesterol. The biosensing platform was developed using the hybrid material and enzymes cholesterol oxidase and cholesterol esterase. Chemically synthesized graphene has been decorated with nanosized Pt particles. The electron microscopic measurements show that the Pt nanoparticles on graphene have an average size of 12 nm and are randomly distributed throughout the surface. The Pt nanoparticle based hybrid material modified electrode efficiently catalyzes the electrochemical oxidation of H 2 O 2 at the potential of 0.4 V, which is >100 mV less positive with respect to the bulk Pt electrode. The sensing platform is highly sensitive and shows linear response toward H 2 O 2 up to 12 mM with a detection limit of 0.5 nM [S/N (signal-to-noise ratio) ) 3] in the absence of any redox mediator or enzyme. The combination of electronically highly conductive graphene and catalytically active Pt nanoparticle favors the facilitated electron transfer for the oxidation of H 2 O 2 . The cholesterol biosensor was developed by immobilizing cholesterol oxidase and cholesterol esterase on the surface of graphene-nanoparticle hybrid material. The bienzyme integrated nanostructured platform is very sensitive, selective toward cholesterol, and it has a fast response time. The sensitivity and limit of detection of the electrode toward cholesterol ester are 2.07 ( 0.1 µA/µM/cm 2 and 0.2 µM, respectively. The apparent Michaelis-Menten constant (K m app ) was calculated to be 5 mM. The sensor does not suffer from the interference due to other common electroactive species and is highly stable. The analytical performance of the hybrid material was further evaluated using screen-printed electrodes with 50 µL of electrolyte.

A rapid and facile route for the synthesis of reduced graphene oxide sheets (rGOs) at room temperature by the chemical reduction of graphene oxide using Zn/acid in aqueous solution is demonstrated.

Recent progress in the development of a new class of inexpensive metal-free and non-noble metal-based electrocatalysts for the cathodic reduction of oxygen is discussed.