Abstract
We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).
Highlights
Until recently, only chemical and physical synthesis methods were available to produce metallic nanoparticles (NPs) but increasing pressure to develop ‘clean’ nanomaterial synthesis methods has led to a growing interest in biotransformations as a route to controlled growth of nanoscale structures
For energy dispersive X-ray (EDX) microanalysis, metallized cells were dispersed in water and deposited on a carbon thin film coating copper transmission electron microscopy (TEM) grids (Agar Scientific, grid thickness: 20– 30 nm)
Size-controlled and cost-efficient method to synthesize Pd – Au core/shell nanostructures using E. coli which, in contrast to Desulfovibrio desulfuricans [21], can grow to high density at scale, and does not produce H2S, a catalyst poison
Summary
Only chemical and physical synthesis methods were available to produce metallic nanoparticles (NPs) but increasing pressure to develop ‘clean’ nanomaterial synthesis methods has led to a growing interest in biotransformations as a route to controlled growth of nanoscale structures. Bioreductive routes use the ability of some bacterial cells to reduce metal precursors enzymatically to the zero-valent state via an electron donor [5,6], usually leading to the formation of metallic NPs at the cell surface that exhibit a catalytic behaviour similar or superior to metallic NPs prepared using chemical methods in a wide range of reactions [7,8,9,10,11]. Bimetallic NPs that exhibit a core/shell structure [12] were recently shown to possess increased catalytic activity in several reactions [13,14,15]. This work presents a simple, facile, two-step biochemical and chemical hybrid route to produce ordered Au –Pd core/shell nanostructures with strong catalytic activity in the oxidation of benzyl alcohol
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