Rational Design of Silver Gradient for Studying Size Effect of Silver Nanoparticles on Contact Killing

The cellular mechanism underlying bacteria responses to silver nanoparticles (AgNPs) has not been fully elucidated. Especially, it is difficult to distinguish the contact killing from release killing as Ag⁺ releases from AgNPs. In this paper, AgNPs gradient was designed for evaluating the size effect of AgNPs on contact killing. A size gradient of AgNPs (5-45 nm) was achieved on TiO₂ nanotubes (TNTs) by rational design of bipolar electrochemical reaction, including applied voltage, electrolyte concentration, and sample size. High-throughput investigation of cellular responses showed that the smallest AgNPs were the most efficient in suppressing bacteria whereas the largest AgNPs were more favorable for MC3T3-E1 cell adhesion and proliferation. As Ag⁺ concentration was the same for the entire gradient, the difference in cellular responses was dominated by the contact effect (rather than difference in released Ag⁺) which was tuned by AgNPs size. This method offers new prospect for efficient evaluation of the contact effect of nanoparticles, such as Ag, Au, and Cu.