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We have demonstrated a simple yet versatile method for in situ generation of stable 2D assemblies of gold nanoparticles (AuNPs) inside polyethylenimine (PEI) silane monolayer templates that exhibit electro-catalytic activity towards methanol oxidation reaction (MOR). This methodology is currently extended to the in situ generation of other metal nanoparticles like Pt and Ag as well as for the synthesis of Au-Pt bimetallic and magnetic nanoparticles on silicon and indium tin oxide (ITO)-coated glass surfaces

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Nanoparticle (NP) assemblies have gained significant attention recently with the intention of comprehending the true potential applications of their unique physical, optical and electronic properties. The ultimate aim is to interface these assemblies to microscale and subsequently to macroscale by organizing them into higher-level structures, devices and systems with well-defined functionality. Although, significant progress have been made in the synthesis of NPs in solution with precise control over size and shape, the synthesis of functional, aggregation-free two dimensional (2D) NP assemblies with tunable size and inter particle spacing still remains a challenge, and an area of active research. In particular, the integration of metal NPs as 2D assemblies can find wide range of applications in a variety of sensors and fabrication of flexible nanodevices. Organized assemblies on solid surfaces provide an ideal platform for the synthesis of materials at the nanometer scale. Self-assembled monolayer (SAM) is one of the most convenient ways of generating functionalized interface through the immersion of appropriate surface in a dilute solution of the desired organic template. The present method is primarily based on self-assembly and takes advantage of the variation in solution pH to control the degree of ionization of the surface functional groups, which modulate the electrostatic interaction between the ions in solution and the immobilizing surface. Au and Au-Pt nanoparticle assemblies generated in this study are also catalytically active towards methanol oxidation reaction, which is relevant for direct methanol fuel cells (DMFCs).

It is to be noted that the reducing agent may have effect on the final size and monodispersity of the resulting surface bound nanoparticles. Sodium citrate used presently in our studies is considered to be a weak reducing agent compared to sodium borohydride, another well-known reducing agent for gold nanoparticle synthesis in solution. We are continuing our studies by using different common reducing agents generally used for synthesis of gold NPs in solution. Some of the representative reducing agents are ascorbic acid, methylaminophenol sulfate, sodium borohydride, glutathione, hydrazine hydrochloride, hydroxylamine hydrochloride, tetrakis(hydroxymethyl)phosphonium chloride (THPC). It is observed that the reducing agent has a profound influence on nanoparticle size and improving the monodispersity of these surface bound nanoparticles. It is observed that ascorbic acid produces relatively larger sized AuNPs     (~15 nm) and glutathione provides the smallest (~2 nm) NPs as assessed by Atomic Force Microscopy (AFM) image nanoparticle generated to silicon surface. We have also expanded our studies to determine the effect of incubation time in Au³⁺ solution on the final nanoparticle size and interparticle distance. In both cases, effect of variation of synthesis parameter on final nanoparticle morphology is assessed extensively through analysis of z-height (taken as diameter of individual nanoparticle), measuring interparticle distance and also the surface coverage of the generated nanoparticle from number of AFM images.

Bimetallic Au-Pt NPs are generated both on silicon and ITO coated glass surfaces by simultaneous in situ reduction of [AuCl₄]- and [PtCl6]^2- ions bound to the TSPEI functionalized surface. The multiple amine functionalities present at the polyethyleneimine (PEI) backbone of TSPEI entraps both [AuCl₄]^- and [PtCl₆]^2- ions from solution through electrostatic interaction at a lower pH. For Au-Pt bimetallic NP generation, a 1 x 10^-2 M HAuCl₄ and 1 x 10^-2 M H₂PtCl₆ solutions are made separately and mixed in different volumes to create the desired different Au:Pt mole ratio in the final solution. After incubating the surfaces for 8 hours followed by rinsing with water and drying under flow of argon, surfaces with adsorbed ions were placed in freshly prepared 1% (w/v) aqueous solution of sodium citrate for 8 hours to generate the Au-Pt NPs. Silicon and ITO surfaces are used for metallic nanoparticle assembly generation in order to evaluate structural, optical and electro-catalytic activity of the formed nanostructures.

Magnetic nanoparticle assemblies are also generated on silicon surface functionalized with N-(trimethoxysilylpropyl)ethylenediamine triacetic acid trisodium salt (TETA). Monolayer formation is achieved through silane coupling chemistry which lead to surfaces with carboxylic acid functional groups capable of entrapping Fe(II) and Fe(III) ions simultaneously from solution by electrostatic interaction. Functionalized surfaces are first immersed in mixed dilute solution of FeSO₄ (1.9 x 10^-5 M) and Fe₂(SO₄)₃ (2.1 x 10^-5 M)  so that Fe(II) and Fe(III) could adsorb simultaneously on the surface through attractive electrostatic interaction and subsequently air oxidation of surface bound species generated the magnetic nanoparticle in situ. In situ generation of magnetic nanoparticles can lead to the formation of mixture of different iron oxides namely hematite (a-Fe₂O₃), goethite (a-FeOOH) and magnetite (Fe₃O₄). XPS is used to evaluate the oxidation states of iron present in the structure and preliminary results suggest the formation of predominantly a-FeOOH.

The proposed research is continuing to provide opportunity for undergraduates to participate in research in the exciting multidisciplinary area of nanotechnology. Outcome of this project resulted in a number of poster and oral presentations by the PI and undergraduate students at the American Chemical Society (ACS) National Meetings, American Chemical Society Colloid and Surface Science (ACS CSS) Symposiums and the Annual Argonne Symposium for Undergraduates in Science, Engineering and Mathematics at the Argonne National Laboratory. In addition, three undergraduate students are co-authors on our two recent publications from this project in Colloids and Surfaces A: Physicochemical and Engineering Aspects (2009) and Nanoscale Research Letters (2009).