The influence of H2 partial pressure on biogenic palladium nanoparticle production assessed by single-cell ICP-mass spectrometry

The production of biogenic palladium nanoparticles (bio-Pd NPs) is widely studied due to their high catalytic activity, which depends on the size of nanoparticles (NPs). Smaller NPs (here defined as <100 nm) are more efficient due to their higher surface/volume ratio. In this work, inductively coupled plasma-mass spectrometry (ICP-MS), flow cytometry (FCM) and transmission electron microscopy (TEM) were combined to obtain insight into the formation of these bio-Pd NPs. The precipitation of bio-Pd NPs was evaluated on a cell-per-cell basis using single-cell ICP-MS (SC-ICP-MS) combined with TEM images to assess how homogenously the particles were distributed over the cells. The results provided by SC-ICP-MS were consistent with those provided by “bulk” ICP-MS analysis and FCM. It was observed that heterogeneity in the distribution of palladium over an entire cell population is strongly dependent on the Pd²⁺ concentration, biomass and partial H₂ pressure. The latter three parameters affected the particle size, ranging from 15.6 to 560 nm, and exerted a significant impact on the production of the bio-Pd NPs. The TEM combined with SC-ICP-MS revealed that the mass distribution for bacteria with high Pd content (144 fg Pd cell⁻¹) indicated the presence of a large number of very small NPs (D50 = 15.6 nm). These results were obtained at high cell density (1 × 10⁵ ± 3 × 10⁴ cells μl⁻¹) and H₂ partial pressure (180 ml H₂). In contrast, very large particles (D50 = 560 nm) were observed at low cell density (3 × 10⁴ ± 10 × 10² cells μl⁻¹) and H₂ partial pressure (10–100 ml H₂). The influence of the H₂ partial pressure on the nanoparticle size and the possibility of size-tuned nanoparticles are presented.