M. Lin, F. Alsammarraie, P. Liou
University of Missouri,
Keywords: gold nanorod, nanocellulose, SERS, pesticide, food
Summary:In recent years, there have been increasing concerns about pesticide residues in various foods. On the other hand, there is growing attention in utilizing novel nanomaterials as highly sensitive, low-cost, and reproducible substrates for surface-enhanced Raman spectroscopy (SERS) applications. The objective of this study was to develop novel nanosubstrates for SERS method that were used for rapid detection of pesticides in different types of food samples (fruit juice, milk, and apples). Two types of novel SERS substrates were developed: (i) gold nanorods assembled into standing arrays on a gold-coated silicon slide. The standing nanorod arrays were neatly arranged, and were able to generate strong electromagnetic field in SERS measurement. (ii) A nanocomposite based on cellulose nanofibers (CNFs) coated with silver nanoparticles (AgNPs) was developed in this study as a flexible and effective substrate. The substrates were utilized to detect pesticides (carbaryl and thiabendazole (TBZ)) in fruits juice (orange and grapefruit), milk, and apples. The results show that the concentrations of carbaryl spiked in fruit juice and milk were linearly correlated with the concentrations predicted by the partial least squares (PLS) models with R values of 0.91, 0.88, and 0.95 for orange juice, grapefruit juice, and milk, respectively. The SERS method was able to detect carbaryl that was extracted from food samples and the detection limits of carbaryl were 509, 617, and 391 ppb in orange juice, grapefruit juice, and milk, respectively. All detection limits are below the Maximum Residue Limits that were set by the U.S. EPA. The CNF-AgNP films were used in SERS analysis to detect TBZ residues in apples. The influence of pH on the SERS spectra of TBZ was investigated because TBZ is a neutral molecule that has a low affinity for AgNPs. The pH of TBZ solution was decreased to below the TBZ’s pKa, thus enabling the electrostatic attraction between TBZ and AgNPs. CNFs can prevent the uncontrolled aggregation of AgNPs in low pH environment and serve as an effective AgNP/nanocellulose platform for SERS analysis. The limit of detection of 5 ppm for TBZ was obtained, which is lower than the current tolerance ppm for apple and other citrus fruits (10 ppm). These results demonstrate that SERS coupled with the standing gold nanorod array substrates or CNF-AgNP nanocomposites is a rapid, reliable, sensitive, and reproducible method for the detection of pesticide residues in foods.