A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver...

Although minerals are known to affect the environmental fate and transformation of heavy-metal ions, little is known about their interaction with the heavily exploited silver nanoparticles (AgNPs). Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric methods for imaging the distribution of AgNPs on various mineral surfaces and their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was tested on two macro- and microsized mineral models, muscovite [KAl₂(AlSi₃O₁₀)(OH)₂] and corundum (α-Al₂O₃), under key environmental conditions (ionic strength and pH). Both AgNPs^- and AgNPs⁺ were found to covalently attach to corundum (pH_pzc = 9.1) through the formation of Ag-O-Al- bonds and thereby to potentially experience reduced environmental mobility. Because label-free Raman imaging showed no molecular interactions between AgNPs^- and muscovite (pH_pzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce spatial distribution of AgNPs^- on such mineral surfaces. Raman maps comprising of n = 625-961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source software, and the results were confirmed via ICP-OES, AFM, and SEM-EDX. The proposed Raman-based imaging requires minimum to no sample preparation; is sensitive, noninvasive, cost-effective; and might be extended to other environmentally relevant systems.