Visualizing Electron Transfer through Silver Nanoparticle Formation and Photothermal Imaging: A Case Study of Nanoscale Zerovalent Iron

Electron release and transfer are pivotal to the efficiency of multiple biogeochemical and pollutant processes. Despite substantial efforts to develop electron-transfer characterization techniques, <i>in situ</i> visualization of electron transfer remains challenging. This study introduces an innovative strategy for mapping electron-transfer distance using nanoscale zerovalent iron (nZVI) as a case study. In this method, silver ions (Ag⁺) are employed as electron traps, forming silver nanoparticles. These nanoparticles can then be immobilized in the agarose-solidified gel used as a heterogeneous porous medium to simulate the underground aquifer and visualized through photothermal imaging. <i>In situ</i> detection has unveiled remarkably extensive electron transfer, reaching distances of up to centimeters from the nZVI source. Nationwide groundwater investigations have further confirmed this long electron-transfer distance, ranging from 1.7 ± 0.4 to 9.6 ± 0.1 mm (agarose gel may not fully replicate the complex and heterogeneous conditions of groundwater environments). Importantly, the efficiency of electron transfer from nZVI was primarily influenced by factors such as nZVI dosage and groundwater matrix conditions, particularly the abundance of electron-shuttling materials like natural organic matter. Surprisingly, the inherent properties of nZVI had a lesser impact on electron-transfer distance. Our work highlights the long-distance electron transfer from nZVI, revealing a previously overlooked but ubiquitous nZVI remediation process.