Chemical transformations of nanoscale zinc oxide in simulated sweat and its impact on the antibacterial efficacy

Abstract

Nanoscale zinc oxide (n-ZnO) is widely used in personal care products and textiles, thus, it would likely be released into human sweat. To better evaluate the potential human health risks of n-ZnO, it is essential to understand its chemical transformations in physiological solutions, such as human sweat, and the resulting changes in the n-ZnO bioavailability. Here, two types of n-ZnO, ZnO nanoparticles (ZnO-NPs) and nanorod-based ZnO nanospheres (ZnO-NSs) were synthesized and incubated in 3 types of simulated sweat with different pH values and phosphate concentrations. The content of Zn₃(PO₄)₂ in the transformed n-ZnO was quantified by selective dissolution of Zn₃(PO₄)₂ in 0.35 M ammonia solution where 100% and 5.5% of Zn₃(PO₄)₂ and ZnO were dissolved, respectively. The kinetics analysis indicated that by 24-48 h the content of Zn₃(PO₄)₂ reached the maximum, being 15-21% at pH 8.0 and 45-70% at pH 5.5 or 4.3. Interestingly, no correlation was observed between the rate constants of Zn₃(PO₄)₂ formation and the specific surface areas of n-ZnO, implying that chemical transformations from n-ZnO to Zn₃(PO₄)₂ in the simulated sweat might not be simply attributed to dissolution and precipitation. Using a variety of characterization techniques, we demonstrated the formation of a ZnO‒Zn₃(PO₄)₂ core-shell structure with the shell consisting of amorphous Zn₃(PO₄)₂ at pH 8.0 and additionally of crystalline Zn₃(PO₄)₂ and Zn₃(PO₄)₂•4H₂O at pH 5.5 or 4.3. The phosphate-induced transformation of n-ZnO in the simulated sweat at pH 5.5 and 4.3 greatly reduced the antibacterial efficacy of n-ZnO through moderating the nanoparticle dissolution, indicating limited bioavailability of the NPs upon transformation. The results improve the understanding of the fate and hazards of n-ZnO.

Keywords: Bioavailability; Nanoscale zinc oxide; Phosphate-induced transformation; Selective dissolution; Simulated sweat.