Preparation of cellulose based hydrogel 3D SERS flexible substrate for the detection of urea in sweat

Au NRs/HA/CMC-PVA hydrogel membranes were successfully prepared by mixing sodium hyaluronate (HA) and carboxymethyl cellulose (CMC) solutions for immersion crosslinking on the basis of freeze-thaw crosslinking of polyvinyl alcohol (PVA) with gold nanorods (Au NRs). By adjusting the density of Au NRs and the concentrations of CMC and HA, more 'hot spots' could be generated to enhance the SERS signals, thus improving the sensitivity and homogeneity of the hydrogels (low detection limit of 10^-9 mol/L Rhodamine 6G (R6G), RSD = 3.04 %). Meanwhile, the strong hydrogen bonds formed in the hydrogel network were utilized to increase the cross-linking degree of the hydrogels, which led to the improvement of the mechanical properties and flexural flexibility of the hydrogels (elongation at break of 308.07 % and tensile strength of 26.09 MPa). The hydrophilic groups of the Au NRs/HA/CMC-PVA hydrogel film were able to adsorb sweat and diffuse urea molecules from sweat into the hydrogel. It showed high sensitivity for the detection of urea, exhibiting a linear relationship at concentrations of 1000-1 mM, with a LOD = 7.83 × 10^-5 M.