Hierarchical Pattern Formation in the Diffusion-Controlled Reduction of HAuCl₄ in Poly(vinyl alcohol) Hydrogels

Abstract

Gold nanoparticles (AuNPs) were incorporated in poly(vinyl alcohol) (PVOH) hydrogel cylinders via diffusion-controlled reductions of tetrachloroauric acid dissolved in the gels using sodium borohydride or ascorbic acid. At certain reagent concentrations, the two reducing agents formed very different hierarchical structured patterns due to their different chemical nature. Sodium borohydride reduction, which likely follows the classical "supersaturation" mechanism, results in the formation of spherical and monodisperse AuNPs of ∼4 nm in diameter located in micrometer-scale stripes in the outer region of the gels. The mobility of the small colloids in the gels allows the formation of alternating particle-rich and particle-depleted stripes. The reaction of sodium borohydride with water and the -OH groups of the gel matrix diminishes its reducing ability over time and limits the AuNP formation to the outer region of the gels. AuNPs of >20 nm in diameter are formed throughout the gel matrices by ascorbic acid reduction, which is consistent with an "organizer" mechanism. Concentric bands of different colors from the outer to the inner regions of the gels;along the direction of ascorbic acid diffusion;are formed as the result of increased particle size and percentage of nonspherical shapes. The lack of stripes on the micrometer scale in the ascorbic acid system is likely due to the impeded mobility of the larger AuNPs. The structural features observed in this study are attributed primarily to the nature of the reaction matrix: reduction is controlled by the diffusion of reducing agents in the hydrogel matrix and the PVOH matrix polymer facilitates the dispersion and stabilization of the AuNPs formed.

Silver nanoparticles embedded polymer sorbent for preconcentration of uranium from bio-aggressive aqueous media

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