In situ and real time investigation of foliarly applied silver nanoparticles on and in spinach leaves by surface enhanced Raman spectroscopic mapping.

Understanding the behavior and biological fate of silver nanoparticles (AgNPs) applied on plant surfaces is significant for their risk assessment. Our study's objective is to investigate the interactions between AgNPs and plant biomolecules as well as to monitor and quantify the penetration of AgNPs in spinach by an in situ and real-time surface enhanced Raman spectroscopic (SERS) mapping technique. AgNPs (2 μg per leaf) of different surface coatings (citrate, CIT, and polyvinylpyrrolidone, PVP) and sizes (40 and 100 nm) were foliarly applied onto spinach leaves with different exposure times (1-48 h). Cysteine is the major biomolecule that interacts with AgNPs in spinach based on the in situ and in vitro SERS pattern recognition. The interaction between CIT-AgNPs and cysteine happened in as early as 1 h after AgNP foliar deposition, which is faster than the interaction between PVP-AgNPs and cysteine. Also, the SERS depth mapping shows that particle size rather than surface coating determines the penetration capability of AgNPs in spinach, in which 40 nm AgNPs show a deeper penetration than the 100 nm ones. Last but not least, based on the results of SERS mapping, we detected significantly higher amounts of 40 nm CIT-/PVP-AgNPs than 100 nm CIT-AgNPs internalized in the leaf tissues after 1 h exposure. The estimated percentage of internalized AgNPs (0.2-0.8%) was significantly smaller than that of the total residual Ag (9-12%), indicating the potential transformation of the AgNPs into other Ag species inside the plant tissues. This study facilitates a better understanding of the behavior and biological fate of AgNPs in plant tissues.