Robust nanofibrillated cellulose composite SERS substrate for capillary preconcentration and trace level detection of organic molecules

Preconcentration of organic analytes from an aqueous solution onto a substrate surface can significantly improve trace level analyte detection by Raman spectroscopy. Nanofibrillated cellulose (NFC)-based three dimensional (3D) substrates have great potential for this application since they can readily absorb water when exposed to an aqueous analyte solution while adsorbing organic molecules from the solution. However, the transport of organic analytes inside the substrate along with water, loss of mechanical robustness, and disintegration of the 3D structure in water limit the use of porous NFC substrates in aqueous environments. To overcome these deficiencies, a chemically crosslinked network of methacrylated carboxymethyl cellulose was incorporated into the NFC matrices, which improves the stability and robustness of the substrates in water. Application of a polydimethyl siloxane-based hydrophobic coating on four of the five analyte exposed surfaces further improves preconcentration efficiency by forcing the analyte solutions to pass through one hydrophilic surface only. Samples with a range of porosities were investigated to optimize sampling time, solution uptake volume, and substrate robustness in water. Using this substrate, parts-per-million detection sensitivity for organic probe molecules in aqueous solution was possible. Incorporation of silver nanoparticles within the substrates further enhanced substrate sensitivity to parts-per-trillion level detection of probe molecules, due to the Raman signal enhancement by surface enhanced Raman scattering (SERS) effect. A model is presented here which describes the linearity, saturation, and depletion of the SERS signal.