Rapid and efficient removal of silver nanoparticles from plant surfaces using sodium hypochlorite and ammonium hydroxide solution

With the increasing likelihood of significant release of engineered nanomaterials (ENMs) into the environment, investigations focused on the behavior and impact of ENMs on plant species which have become more important and prevalent. One consistent challenge in these studies is how to separate the plant surface-attached and internalized ENMs. Unfortunately, there are no current effective methods to remove ENMs from plant surfaces. In this study, a protocol was developed to remove surface-attached silver nanoparticles (AgNPs) from spinach leaves. Spinach leaves were exposed to three kinds of AgNPs (2 μg per leaf) for 1 h and 7-day foliar exposure times, including 40 nm CIT- and PVP-AgNPs, and 100 nm CIT-AgNPs. Harvested AgNPs-contaminated spinach leaves were sequentially immersed in solutions of Clorox bleach (sodium hypochlorite, 200 mg/L, 5 min) and ammonium hydroxide (NH₃·H₂O, 2.8%, 1 min), with a final rinse in deionized (DI) water (1 min). The AgNPs removal efficiency was evaluated by inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy (SEM). Both ICP-MS results and SEM images demonstrated effective removal of 91–93% of the surface-attached Ag residuals from the contaminated spinach leaves. This value was significantly higher than removal with DI water (7%), Clorox bleach alone (36%), or NH₃·H₂O alone (36%). However, no significant changes were observed for the internalized AgNPs, indicating that this method is only surface active. This study provides essential information for the rapid and efficient removal of surface-attached AgNPs and furthers understanding of NP interactions with food crops. Importantly, all chemicals used in this developed washing protocol are generally considered as safe (GRAS), and could readily be used as a postharvest washing procedure to reduce Ag contaminants in leafy vegetables.