(537g) Light-Dependent Antibacterial Properties of Cu-Doped TiO2 Nanoparticles (NPs)
This study investigated the effects of different Cu-doped TiO2 nanoparticles' characteristics (composition, size and crystal structure) on the viability of Mycobacterium smegmatis (pathogenic bacteria) and Shewanella oneidensis MR-1 (metal reducing bacteria) under three light conditions (UV light, fluorescent light, and complete dark). We treated both bacteria with TiO2 NPs or Cu-doped TiO2 NPs (20 mg/L, ~35 nm, anatase crystalline structure) for 2 hr in a complete minimal medium without any carbon sources. The cell death rates triggered by NPs and light conditions were closely monitored via measurement of colony-forming unit (CFU). We observed the survival rate of M. smegmatis highly depends on light conditions. Compared to control experiments without NPs, TiO2 NPs did not significantly reduce M. smegmatis' viability under complete dark or fluorescent light. However, TiO2 NPs caused significant more lethality (~90%) to cell under UV light. Meanwhile, if TiO2 NPs were doped with copper, their antimicrobial activities were strongly enhanced. Under dark or fluorescent light, the lethality of NPs increased with the content of doped Cu in NPs, where M. smegmatis was completely unviable if the Cu doping amount is over 3.0%. Furthermore, M. smegmatis was able to survive with UV light, but they were completely damaged in the presence of both UV and TiO2 NPs (even with low amount of doped Cu, e.g., 0.25%). In addition, the antimicrobial activity of NPs was also size-dependent under three light conditions, while smaller size caused a decreased survival rate. Above observations indicate that nanotoxicity can be due to three factors: 1. Particle associated physical stresses (such as size); 2. Chemical stress from dissolved toxic metal ions; 3. Light activated oxidative stress. On the other hand, S. oneidensis MR-1 showed completely different survival responses to NPs stresses. First, neither TiO2 NPs nor Cu-doped TiO2 NPs can reduce Shewanella viability under dark or fluorescent light conditions. This is because Shewanella species has evolved strong capability to reduce toxic metal ions and oxidative stresses. Under UV light, S. oneidensis MR-1 was almost unviable. However, the presence of Cu-doped TiO2 NPs tended to dramatically increase their viability. 1% Cu-doped NPs showed highest improvement of cell survival by 196-fold. This phenomenon was possibly related with the absorbance of UV light by Cu-doped NPs. Particle size and particle compositions of NPs could tune the light absorbance and light scattering properties, and thus control their effectiveness for protection of Shewanella against UV stresses. The mechanism of stress reduction by NPs is still under investigation at molecular biology level.