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

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).

Biosynthesis of platinum nanoparticles by Escherichia coli MC4100: can such nanoparticles exhibit intrinsic surface enantioselectivity?

The biomanufacture of two types of platinum bionanoparticle (bioNP) using Escherichia coli MC4100(1% and 20% by mass metal loading) together with a method for both liberating the nanoparticles (NPs) from the bacterial layer and their subsequent critical cleaning is reported. The possibility of an enantiomeric excess of chiral kink sites forming on the surface of the Pt nanoparticles produced by the bacteria was investigated using the electrooxidation of D- and L-glucose as the chiral probe. Transmission electron microscopy revealed that the Pt bioNPs (after recovery and cleaning) were typically 2.3 ± 0.7 nm (1% loading) and 4.5 ± 0.7 nm (20% loading) in diameter. The D- and L-glucose electrooxidation measurements did not give rise to any chiral response using either of the Pt bioNPs types but did display differing CV profiles. This suggested that the overall surface morphology of each bioNP could be controlled by the degree of metal loading but that no enantiomeric excess of intrinsically chiral surface kink sites was present.

Biorefining of precious metals from wastes: an answer to manufacturing of cheap nanocatalysts for fuel cells and power generation via an integrated biorefinery?

Bio-manufacturing of nano-scale palladium was achieved via enzymatically-mediated deposition of Pd from solution using Desulfovibrio desulfuricans, Escherichia coli and Cupriavidus metallidurans. Dried 'Bio-Pd' materials were sintered, applied onto carbon papers and tested as anodes in a proton exchange membrane (PEM) fuel cell for power production. At a Pd(0) loading of 25% by mass the fuel cell power using Bio-Pd( D. desulfuricans ) (positive control) and Bio-Pd( E. coli ) (negative control) was ~140 and ~30 mW respectively. Bio-Pd( C. metallidurans ) was intermediate between these with a power output of ~60 mW. An engineered strain of E. coli (IC007) was previously reported to give a Bio-Pd that was >3-fold more active than Bio-Pd of the parent E. coli MC4100 (i.e. a power output of >110 mW). Using this strain, a mixed metallic catalyst was manufactured from an industrial processing waste. This 'Bio-precious metal' ('Bio-PM') gave ~68% of the power output as commercial Pd(0) and ~50% of that of Bio-Pd( D. desulfuricans ) when used as fuel cell anodic material. The results are discussed in relation to integrated bioprocessing for clean energy.

Involvement of hydrogenases in the formation of highly catalytic Pd(0) nanoparticles by bioreduction of Pd(II) using Escherichia coli mutant strains.

Escherichia coli produces at least three [NiFe] hydrogenases (Hyd-1, Hyd-2 and Hyd-3). Hyd-1 and Hyd-2 are membrane-bound respiratory isoenzymes with their catalytic subunits exposed to the periplasmic side of the membrane. Hyd-3 is part of the cytoplasmically oriented formate hydrogenlyase complex. In this work the involvement of each of these hydrogenases in Pd(II) reduction under acidic (pH 2.4) conditions was studied. While all three hydrogenases could contribute to Pd(II) reduction, the presence of either periplasmic hydrogenase (Hyd-1 or Hyd-2) was required to observe Pd(II) reduction rates comparable to the parent strain. An E. coli mutant strain genetically deprived of all hydrogenase activity showed negligible Pd(II) reduction. Electron microscopy suggested that the location of the resulting Pd(0) deposits was as expected from the subcellular localization of the particular hydrogenase involved in the reduction process. Membrane separation experiments established that Pd(II) reductase activity is membrane-bound and that hydrogenases are required to initiate Pd(II) reduction. The catalytic activity of the resulting Pd(0) nanoparticles in the reduction of Cr(VI) to Cr(III) varied according to the E. coli mutant strain used for the initial bioreduction of Pd(II). Optimum Cr(VI) reduction, comparable to that observed with a commercial Pd catalyst, was observed when the bio-Pd(0) catalytic particles were prepared from a strain containing an active Hyd-1. The results are discussed in the context of economic production of novel nanometallic catalysts.

Manufacture of stable palladium and gold nanoparticles on native and genetically engineered flagella scaffolds.

The use of bacterial flagella as templates for the immobilization of Pd and Au nanoparticles is described. Complete coverage of D. desulfuricans flagellar filaments by Pd(0) nanoparticles was obtained via the H(2)-mediated reduction of Pd(NH3)4]Cl2 but similar results were not obtained using HAuCl4. The introduction of additional cysteine-derived thiol residues in the E. coli FliC protein increased Au(III) sorption and reduction onto the surface of the flagellar filament and resulted in the production of stabilized Au(0) nanoparticles of approximately 20-50 nm diameter. We demonstrate the application of molecular engineering techniques to manufacture biologically passivated Au(0) nanoparticles of a size suitable for catalytic applications.

Biomass-supported palladium catalysts on Desulfovibrio desulfuricans and Rhodobacter sphaeroides.

A Rhodobacter sphaeroides-supported dried, ground palladium catalyst ("Rs-Pd(0)") was compared with a Desulfovibrio desulfuricans-supported catalyst ("Dd-Pd(0)") and with unsupported palladium metal particles made by reduction under H2 ("Chem-Pd(0)"). Cell surface-located clusters of Pd(0) nanoparticles were detected on both D. desulfuricans and R. sphaeroides but the size and location of deposits differed among comparably loaded preparations. These differences may underlie the observation of different activities of Dd-Pd(0) and Rs-Pd(0) when compared with respect to their ability to promote hydrogen release from hypophosphite and to catalyze chloride release from chlorinated aromatic compounds. Dd-Pd(0) was more effective in the reductive dehalogenation of polychlorinated biphenyls (PCBs), whereas Rs-Pd(0) was more effective in the initial dehalogenation of pentachlorophenol (PCP) although the rate of chloride release from PCP was comparable with both preparations after 2 h.

Dehalogenation of polychlorinated biphenyls and polybrominated diphenyl ethers using a hybrid bioinorganic catalyst.

The environmentally prevalent polybrominated diphenyl ether (PBDE) #47 and polychlorinated biphenyls (PCBs) #28 and #118 were challenged for 24 hours with a novel biomass-supported Pd catalyst (Bio-Pd(0)). Analysis of the products via GC-MS revealed the Bio-Pd(0) to cause the challenged compounds to undergo stepwise dehalogenation with preferential loss of the least sterically hindered halogen atom. A mass balance for PCB #28 showed that it is degraded to three dichlorobiphenyls (33.9%), two monochlorobiphenyls (12%), and biphenyl (30.7%). The remaining mass was starting material. In contrast, while PCB #118 underwent degradation to yield five tetra- and five trichlorinated biphenyls, no less chlorinated products or biphenyl were detected, and the total mass of degraded products was 0.3%. Although the Bio-Pd(0) material was developed for treatment of PCBs, a mass balance for PBDE #47 showed that the biocatalyst could prove a potentially useful method for treatment of PBDEs. Specifically, 10% of PBDE #47 was converted to identifiable lower brominated congeners, predominantly the tribrominated PBDE #17 and the dibrominated PBDE #4, 75% remained intact, while 15% of the starting mass was unaccounted for.