Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

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 H(2) 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).

Figure 1.

Electron microscopy of metallized cells of E. coli MC4100. (a) TEM of cells of E. coli MC4100 following the sequential reduction of Pd(II) and Au(III) (5%/5% Pd–Au on biomass w/w); untreated cells are shown in inset (b). Scale bars are 500 nm. (c) EDX mapping of two Pd–Au particles showing superimposed Au and Pd distributions: yellow, X-ray signal intensity from the characteristic L_α transitions of Au; blue, the characteristic L_α transitions of Pd. The particle on the right-hand side has segregation between Pd and Au with a clearly observed Pd-rich region. The particle on the left-hand side shows homogeneous mixing between Pd and Au. Regions of Pd are apparent at the surface of the nanoparticles (arrowed) and some areas between the NPs also indicated the presence of Pd (circled). Individual distributions of Au and Pd, together with complementary high-angle annular dark field microscopy, which provides atomic number contrast, were described previously [21].

Figure 2.

Surface analysis of bioPd–Au using cyclic voltammetry (CV). (a) Voltammetric profile of bioPd–Au in 0.1 M H₂SO₄ for the first cycle (solid line), the tenth cycle (dashed line), the last cycle (dotted line) and the glassy carbon support (dash-dotted line). Scan rate 100 mV s⁻¹. (b) Voltammetric profile of bioPd–Au in 0.1 M H₂SO₄ from scan 10 onwards. The arrows show the increase/decrease of oxide peaks. Scan rate 100 mV s⁻¹.

Figure 3.

EXAFS analysis of bioPd–Au and reference compounds. K³-weighted EXAFS spectra (left panel) and corresponding FT (right panel) of bioPd–Au sample and reference compound at the (a) Pd K-edge and (b) Au L_III-edge. Blue lines, data; red lines, fit.