Flexible Cellulose Nanofibril-Based Surface-Enhanced Raman Scattering Substrate via Silver-Ammonia-Mediated In SituPhotoreduction for Pesticide Residue Detection and Full-Spectrum Thiram Quantification.

Uniform and reproducible Ag nanostructures are essential for flexible cellulose-based surface-enhanced Raman scattering (SERS) substrates. Herein, a flexible SERS substrate was fabricated by in situ photoreduction of a silver-ammonia precursor on TEMPO-oxidized cellulose nanofibril (TOCNF) films without external reducing agents or surfactants during Ag photoreduction. Compared with an AgNO3 precursor control, the silver-ammonia system reached its optimal SERS response within a shorter irradiation time (25 vs 70 min) and produced more homogeneous AgNP assemblies. The optimized substrate enabled detection of thiram and thiabendazole with limits of detection of 1.39 × 10-8 and 1.16 × 10-7 M, respectively while characteristic signals remained distinguishable on tomato surfaces at 0.48 and 1.61 ng/cm2. To improve thiram quantification over a broad concentration range, full-spectrum PLS, linear SVR, and RFR models were evaluated. RFR provided the best predictive performance (Rp2 = 0.9859, RMSEP = 0.1839, and RPD = 8.50) and yielded recoveries of 108.28% and 103.37% for tomato samples spiked at 96 and 9.6 ng/cm2, respectively. These results demonstrate precursor-dependent photochemical growth of SERS-active Ag nanostructures on flexible TOCNF films and the utility of full-spectrum nonlinear regression for thiram quantification.