Photo-formed Metal Nanoparticle Arrays in Monolithic Silica-Biopolymer Aerogels
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Photo-formed Metal Nanoparticle Arrays in Monolithic Silica-Biopolymer Aerogels Xipeng Liu, Yu Zhu, Chunhua Yao and William M. Risen, Jr. Department of Chemistry, Brown University Providence, RI 02912, U.S.A. ABSTRACT Transparent monolithic aerogels based on silica, the bioderived polymer chitosan, and coordinated ions have been employed to serve as a three-dimensional scaffold decorated with Au, Pt and Pd ions. The coordination states of metal ions and the initial gel in one or several of the ions are established in the thickness of the monolith that is produced by supercritical CO2 extraction to form the aerogels. Also in this work, it has been found that the Au(III) aerogels can be imaged photolytically in the two planar dimensions. The spatially controlled photolysis produces nanoparticles, (Au)n in the range of from 5 to 85 nm, for example, that constitute two dimensionally imaged arrays whose volumetric concentration also varies in the third dimension. These images microarrays of nanoparticles provide a basis for localization and detection of thiols, disulfides, amino acids and protein molecules. The formation of these arrays, including the dependence of their properties on light intensity, frequency and exposure, and distribution of target molecules and ions in the initial aerogel is presented.
INTRODUCTION Metal nanoparticles, formed as colloids by reactions in solutions and suspensions, are widely known. They have been produced by chemical reductions and photolytically induced-reductions applied to solutions of salts of Au(III), Pt(II), Pd(II) and other metal ions. Chemical reductions are the most common, with such reactions as the citric acid reduction of Au(III) salts being among the most frequently used methods to obtain gold nanoparticles, but photolytic methods using γ-ray or ultraviolet irradiation have been reported as well. For example, ultra fine metal particles have been prepared by Itakura et al. [1] by UV irradiation of salts dissolved in ethanol. The reaction is accelerated by using benzoin as a photoinitiator; indeed, the observed gold nanoparticle size changes from 7nm to 17 nm with increasing benzoin concentration. Nanoparticles have been formed in solid matrices of soft materials as well. For example, nanoparticles of Pt, Rh, Ru and Ag, have been obtained by chemical reduction in ionomer matrices [2-3]. Studies of gold nanoparticles have been quite fruitful for understanding molecular adsorption onto their surfaces and for utilizing these adsorption reactions for detection and other applications. In some cases, the adsorption reactions are used to control or at least found to control the sizes of the particles formed during the reduction process, because the adsorbed molecules can compete with the growth process for the addition of atoms to the surface of nucleated Au(0) clusters. For example, a gold sol was obtained by Pal [4] by UV irradiation of HAuCl4 in aqueous Triton X-100, which acts as both a reductant and a stabilizer. Without it, gold (III) failed to form gold particles a
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