In this communication, we report a strategy for the preparation of Pt nanoparticles encapsulated in Generation 4.5 (Polyamido amine) PAMAM dendrimer and subsequent chemical linking of the nanocomposite to the gold electrode through a self assembled cystamine monolayer. The modification resulted in the formation of a robust electrochemically active thin film with very high surface area, reflected by the enhanced hydrogen adsorption coverage. Interestingly, TEM images revealed selfassembly of Pt nanoparticles and the SAED (Selected Area Electron Diffraction) patterns showed the presence of Pt single crystals (111). The Pt-dendrimer nanocomposite film obtained using the novel modification procedure exhibited high electrocatalytic activity for the oxidation of organic fuels like methanol, ethanol and ethylene glycol. The film did not suffer from degradation even after repeated use in solution-phase voltammetry. It is however observed that the intermediate SAM layer and the bulky PAMAM dendrimer (generation 4.5) have slowed down the electron transfer kinetics which is reflected by a relatively high overpotential for methanol oxidation. Nevertheless this shortcoming is more than compensated by the existence of Pt(111) planes, which alleviate CO poisoning.
Formation of self assembled monolayers on gold surface by thiols and disulphides is a well known phenomenon and extensive research work has been carried out in this area with envisaged applications in the area of sensors, molecular electronics, lithography, device fabrication using bottom-up approach, etc. Recently, it has been established that thiophene molecules can self assemble on gold surface due to Au-S interactions. 3-(3-thienyl) acrylic acid, a bifunctional ligand is used in this work to form self-assembled monolayers on planar gold surfaces (two dimensional assemblies) and to prepare monolayer protected gold nano clusters (three-dimensional assemblies). The electron transfer blocking properties of the two-dimensional monolayers were evaluated by using standard redox probes like ferrocyanide anions and Ruthenium hexamine cations. The functionalisation of the two-dimensional and three-dimensional assemblies has been carried out with ferrocene carboxylic acid and the functionalised monolayers were characterized by Cyclic voltammetry. The formation of thienyl acrylic acid protected nanoclusters has been verified by TEM and surface plasmon resonance absorption. It has been observed that when thiophene based ligands are used as stabilizers for the formation of metal nanoparticles, they tend to aggregate as a result of pi-pi interactions between adjacent thiophene ligands. In this case it is found that aggregation is prevented. The substituent at the thiophene ring hinders pi-pi interactions. The quantised nature of electrochemical charging of these nanoparticles has been demonstrated by differential pulse voltammetry (DPV), which exhibit peak like features (coulomb's staircase). This work also explores the possibility of using 3-(3-thienyl) acrylic acid as building blocks or spacers on planar and colloidal gold surfaces for potential applications in the field of sensors and devices.
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