Plasmonic Nanoink-Coated Paper SERS Substrate for Pesticide Detection

Point-of-care sensors are a promising platform for detecting pesticide residues in food and agricultural products. Here, we demonstrate a paper-based Surface-Enhanced Raman Spectroscopy (SERS) substrate for the detection of thiram. We synthesized two plasmonic nanoinks─polyvinylpyrrolidone- and polyethyleneimine-capped silver nanoparticles (I-PVP and I-PEI-Ag). Various analytical techniques were employed to characterize the morphological and physicochemical properties of the nanoinks. I-PVP-Ag and I-PEI-Ag were evaluated for thiram sensing in aqueous solution using a handheld Raman spectrometer. The limit of detection (LoDs) was 8.09 × 10^–8 and 2.24 × 10^–10 M, with enhancement factors (EF) of 1.2 × 10⁵ and 1.37 × 10⁸, respectively. Among them, I-PEI-Ag exhibited a ∼3-fold higher EF due to selective binding of thiram through hydrogen bonding. We selected I-PEI-Ag ink for coating onto (2-dodecen-1-yl)-succinic anhydride (DDSA)-functionalized paper (SF-PEI/Ag) because DDSA forms ester linkages with the paper surface, thereby enhancing the ink’s binding affinity. The SF-PEI/Ag substrates were used to detect thiram in fruits, leafy vegetables, soil, and water, with sensitivities in the pM–nM range (10^–12–10^–9 M). Further, Density Functional Theory (DFT) calculations revealed a binding affinity of −62.75 kcal·mol^–1, confirming the chemisorption and hydrogen-bonding formation between I-PEI-Ag and the thiram molecules. Moreover, principal component analysis (PCA) plots confirmed the data sets' discrimination ability. Overall, this work demonstrates that combining experimental SERS data with theoretical methods (DFT and PCA) provides an effective approach for detecting pM levels of thiram residues in fruits, soil, and agricultural products.