Assessing the risk and consequence of engineered nano-scale zinc oxide in phytological and bacterial systems

With the increased usage and production of engineered nanoparticles (ENPs), entry into the environment and hence contact with plant root systems is inevitable. Nano zinc oxide (nZnO) is widely used in commercial products, such as sunscreens, paints and coatings due to its high antimicrobial properties and wide electrical band-gap. Disposal down drains and into greywater leads to particle entry into the environment via waste water systems. Here, ENPs could potentially interact with plant root systems, which may lead to uptake, translocation and accumulation within plant tissues, and in the case of edible crops have consequences on human health. This study aimed to identify mechanisms of toxicity by employing whole-cell biosensors in conjunction with model bacteria and plant species. Furthermore, zeta potential (ZP), particle size, reactive oxygen species (ROS) release and solubility of the particles were determined and linked to both plant and bacterial toxicity. In Escherichia coli bacteria, it was demonstrated that growth inhibition from nano-scale ZnO treatment was similar to that from the bulk-scale ZnO and ionic zinc treatments, with the concentrations leading to 50 % inhibition (IC50) demonstrated to be 251, 282 and 298 mg/L for bulk, nano-scale and ZnSO4, respectively. It was demonstrated that the mode of nZnO toxicity in E. coli was bacteriostatic rather than bacteriotoxic. In barley plants, biomass was negatively impacted by up to 50 %, and significantly more zinc was able to enter root tissues as a result of hydroponic nZnO treatment, with 47 mg/L zinc detected in root tissues after 7 days treatment with 500 mg/L nZnO. Comparison of particle characteristics revealed that ROS, solubility, ZP, size and concentration were involved in toxicity, with ZP (charge) identified as a key parameter in both plant and bacterial toxicity