Abstract
Early studies have focused on the synthesis of palladium nanoparticles within the periplasmic layer or on the outer membrane of Desulfovibrio desulfuricans and on the S-layer protein of Bacillus sphaericus. However, it has remained unclear whether the synthesis of palladium nanoparticles also takes place in the bacterial cell cytoplasm. This study reports the use of high-resolution scanning transmission electron microscopy with a high-angle annular dark field detector and energy dispersive X-ray spectrometry attachment to investigate the intracellular synthesis of palladium nanoparticles (Pd NPs). We show the intracellular synthesis of Pd NPs within cells of two anaerobic strains of D. desulfuricans and an aerobic strain of B. benzeovorans using hydrogen and formate as electron donors. The Pd nanoparticles were small and largely monodispersed, between 0.2 and 8 nm, occasionally from 9 to 12 nm with occasional larger nanoparticles. With D. desulfuricans NCIMB 8307 (but not D. desulfuricans NCIMB 8326) and with B. benzeovorans NCIMB 12555, the NPs were larger when made at the expense of formate, co-localizing with phosphate in the latter, and were crystalline, but were amorphous when made with H2, with no phosphorus association. The intracellular Pd nanoparticles were mainly icosahedrons with surfaces comprising {111} facets and about 5 % distortion when compared with that of bulk palladium. The particles were more concentrated in the cell cytoplasm than the cell wall, outer membrane, or periplasm. We provide new evidence for synthesis of palladium nanoparticles within the cytoplasm of bacteria, which were confirmed to maintain cellular integrity during this synthesis.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-015-3067-5) contains supplementary material, which is available to authorized users.
Highlights
There is an increasing demand for precious metals in the areas of electronics and catalysis
Few Pd nanoparticles were visible at the surface of B. benzeovorans NCIMB 12555 (Fig. 1a, b), while D. desulfuricans NCIMB 8307 (Fig. 1e, f) supported surface-bound Pd (0) but none was apparent on D. desulfuricans NCIMB 8326 (Fig. 1i, j)
This study focuses on the synthesis and characterization of intracellular palladium nanoparticles in aerobic and anaerobic bacteria made via hydrogen and formate as electron donors, whereas all previous studies have been focused on surface Pd NPs
Summary
There is an increasing demand for precious metals in the areas of electronics and catalysis. The ability of palladium to catalyse a wide range of reactions in chemical synthesis makes it the most sought after precious metal in a range of industrial applications (Holt et al 1999). The high demand for palladium has led to a parallel increase in price (Cowley 2013) resulting in increasing focus on the clean and sustainable recovery of palladium from waste sources (Cui and Zhang 2008; Macaskie et al 2011; Lee and Pandey, 2012) with the potential for bioconversion of wastes into new catalytic materials (Mabbet et al 2006; Murray et al 2015). Recovery traditionally involves the dissolution of metals from spent catalysts using acids at high temperatures and application of strong chemical reductants, which are toxic (De corte et al 2013) and generate undesirable waste products. Bacterial cells act as reducing agents using enzymes, avoiding the use of toxic reagents, and controlling particle growth via ‘bio-patterning’ (Mikheenko et al 2008) while minimizing artefacts from surfactants and capping agents (Narayanan and Sakthivel 2010)
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