In Situ Arsenic Speciation using Surface-enhanced Raman Spectroscopy and the Coffee Ring Effect

The coffee ring effect (CRE) phenomenon is originated from the nonuniform solvent evaporation of a sessile droplet deposited onto the flat substrate. Once the evaporation starts, the droplet suspended particles move outwards by the radial flow and concentrate in the edge region of the evaporating droplet, resulting in the formation of the so-called CRE stains. In this work we have expanded the applications of the CRE from separation of particles and macromolecules to small molecules, in particular, coupled to surface-enhanced Raman spectroscopy (SERS). Herein, we have developed a theoretical framework to describe the CRE-driven separation process of small molecules, using SERS analysis of dimethylarsinic acid (DMA^V), dimethylmonothioarsinic acid (DMMTA^V), and dimethyldithioarsinic acid (DMDTA^V) on gold nanofilm (AuNF) as an example. By combining the CRE theory for the radial flow and the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory for mass transfer between solution and AuNF surface, we adapted the conventional chromatographic theory to derive a modified van Deemter equation for the CRE-driven separation. By using this model, we predicted the travel distances of arsenicals based on the different affinity of analytes to AuNF and evaluated the possibility of separation of unknown analytes by CRE-based SERS, demonstrating the successful adaptation of classic chromatographic theory to CRE-driven nanochromatography.

Furthermore, we have extrapolated the application of the developed method for the speciation of the peptide-like arsenic-based anticancer drug Darinaparsin (DAR) and its major breakdown product dimethylarsino-cysteine (DMAC). Despite the overlap of DAR and DMAC’s Raman spectra in the final CRE deposit, we were able to identify each compound due to their unique SERS fingerprint. Overall, the developed method was able not only to separate and identify the S-conjugated arsenicals, but at the same time to preserve the DAR and DMAC’s As^III oxidations state and the fragile As – S moiety, thus providing an alternative speciation method for unstable thioarsenicals. The key advantage of the application of this method for peptides separation is the separation coupled with the mild detection providing the rich structural information of each metabolite.