Characterization of intracellular palladium nanoparticles synthesized by Desulfovibrio desulfuricans and Bacillus benzeovorans

Early studies have focused on the synthesis of palladium nanoparticles within the periplasmic layer or on the outer membrane ofDesulfovibrio desulfuricansand on the S-layer protein ofBacillus 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 ofD. desulfuricansand an aerobic strain ofB. benzeovoransusing 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. WithD. desulfuricansNCIMB 8307 (but notD.desulfuricansNCIMB 8326) and withB. benzeovoransNCIMB 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.