Fast <i>On-Site</i> Speciation and High\nSpatial Resolution Imaging of Labile Arsenic in Freshwater and Sediment\nUsing the DGT-SERS Sensor

Diffusive gradients in thin films (DGT) technique is\nrenowned for <i>in situ</i> passive sampling but not for\nrapid <i>on-site</i> analysis, whereas surface-enhanced\nRaman spectroscopy (SERS) excels\nin ultrasensitive <i>on-site</i> detection but is limited\nby substrate contamination from complex matrices. Here, a hierarchical\nnanostructure of silver (Ag) mirror-supported large Ag nanoparticles\n(∼120 nm) was grown <i>in situ</i> in polyacrylamide\nhydrogel with a restricted pore size (PAM/Ag mirror/AgNPs) to serve\nas both the DGT binding phase and the SERS substrate. The substrate\nexhibited a maximum electric field enhancement factor of 9.9 ×\n10<sup>8</sup> and a signal relative standard error of 4.8%. Using\nthe DGT-SERS sensor, As(III) and As(V) in freshwater were simultaneously\ndetected at limits of 0.9 and 0.8 μg L<sup>–1</sup>,\nrespectively, applicable across a wide range of environmental conditions.\nThe DGT-SERS effectively mitigated the interfacial reduction of As(V)\ncaused by humic acid by excluding it from plasmonic hotspots through\nsize exclusion of the diffusive layer. The Raman analysis of a DGT\nsample in the field requires only 2 s using a portable spectrometer\nwithout DGT device disassembly. More importantly, the DGT-SERS captured\nthe first two-dimensional image of As(III) and As(V) in one DGT at\nthe micron scale resolution, revealing their spatially supplementary\ndistribution patterns at the sediment-water interface. This study\npaves the way for next-generation speciation imaging DGT and the application\nof SERS in complex environments.