Abstract
Bimodal sized elemental copper (Cu) nanoparticles (NPs) were synthesized from
inexpensive oxidized copper salts by an extracellular metal-reduction process using
anaerobic Thermoanaerobacter sp. X513 bacteria in aqueous solution. The bacteria nucleate
NPs outside of the cell, and they control the Cu2+ reduction rate to form uniform crystallites
with an average diameter of 1.75±0.46 μm after 3-day incubation. To control the size and
enhance air stability of Cu NPs, the reaction mixtures were supplemented with
nitrilotriacetic acid as a chelator, and the surfactant capping agents oleic acid, oleylamine,
ascorbic acid, or L-cysteine. Time-dependent UV-visible absorption measurements and XPS
studies indicated well-suspended, bimodal colloidal Cu NPs (70–150 and 5–10 nm) with
extended air-stability up to 300 min and stable Cu NP films surfaces with 14% oxidation
after 20 days. FTIR spectroscopy suggested that these capping agents were effectively
adsorbed on the NP surface providing oxidation resistance in aqueous and dry conditions.
Compared to previously reported Cu NP syntheses, this biological process substantially
reduced the requirement for hazardous organic solvents and chemical reducing agents, while
reducing the levels of Cu oxide impurities in the product. This process was highly
reproducible and scalable from 0.01 to 1-L batches.
