Calcium carbonate microparticles with embedded silver and magnetite nanoparticles as new SERS-active sorbent for solid phase extraction

Abstract

The authors describe a dual-use sorbent for solid phase extraction (SPE) in combination with surface-enhanced Raman spectroscopy (SERS). The sorbent consists of calcium carbonate microparticles with incorporated silver nanoparticles (AgNPs) and magnetic nanoparticles (Fe3O4). It can be simply prepared, is cost-effective, disposable, and possesses high sorption capabilities together with a Raman enhancement of 109 in the best case. Following magnetic separation of the microsphere from the liquid sample, the sorbent matrix (CaCO3) is dissolved in acid so that the AgNPs and the analyte are released. This provides an optimal interaction between them and warrants a strong SERS signal. A new SPE-SERS protocol was worked out and applied to the following model analytes: Rhodamine 6G (a fluorescent dye; R6G), Photosens (a photodynamic dye), and sulfadimethoxine (an antibiotic). The first two possess a high Raman cross-section, while the last one displays comparatively low intrinsic Raman intensity. It is found that coating the surface of the sorbent with polyethyleneimine (a cationic polyelectrolyte) leads to a 2-fold increase in the sorption of Photosens (from 40% to 90%) and to a 6-fold enhancement of the SERS signal. The limits of detection for R6G, Photosens, and sulfadimethoxine (with 78%, 94%, and 41% sorption values, respectively) are 29 pM, 12 nM, and 69 nM, respectively. Raman enhancements are in the range between 106 and 109 depending on the analyte.