Phosphate-dependent modulation of antibacterial strategy: a redox state-controlled toxicity of cerium oxide nanoparticles

Specific reactivity of cerium oxide nanoparticles with phosphate ions was used to design a novel antibacterial system. The redox sensitivity of cerium oxide nanoparticles (CeNPs) was used to irreversibly scavenge phosphate ions from the microbial growth media resulting in nutrient starvation in microbes. Cerium oxide nanoparticles surface was engineered with different ratios of (\(\hbox {Ce}\,{(+3)}/\hbox {Ce}\,{(+4)})\) cerium oxidation states and the effect of surface oxidation states was evaluated on the antibacterial activity. The nutrient depletion-based antibacterial activity is demonstrated selectively by CeNPs with higher \(\hbox {Ce}\,{(+3)}/\hbox {Ce}\,{(+4)}\) ratio on the surface. The surface chemistry of \(\hbox {Ce}\,{(+3)}\) is altered in the presence of phosphate, resulting in the irreversible formation of surface cerium phosphates leading to the loss of its intrinsic superoxide dismutase (SOD) activity. It is hypothesized that nutrient starvation by \(\hbox {Ce}\,{(+3)}\) leads to oxidative stress in microbes which is not neutralized by the altered surface chemistry of CeNPs with high \((\hbox {Ce}\,{(+3)}/\hbox {Ce}\,{(+4)})\) ratio. On the contrary, CeNPs with higher \((\hbox {Ce}\,{(+4)}/\hbox {Ce}\,{(+3)})\) ratio did not show any reactivity towards phosphate, thus depicted no antibacterial activity, confirming the hypothesis that surface chemistry, rather than size or morphology-dependent toxicity is the main reason for the observed antibacterial activity of CeNPs.