Detection of Isocarbophos Using Porous Silicon Bragg Mirror Combined with Upconversion-Nanoparticle-Based...

To address the difficulty of simultaneously achieving low cost, high sensitivity, and rapid response in the detection of isocarbophos residues in agricultural products and environmental water samples, a portable fluorescence sensing platform integrating a porous silicon Bragg mirror (PSBM) and upconversion nanoparticles (UCNPs) was constructed in this work. First, a PSBM was fabricated by electrochemical etching and then functionalized. Owing to the photonic bandgap effect of the PSBM, which enhances the local optical field, the enhancement effect of the PSBM on upconversion fluorescence was significantly superior to that of single-layer porous silicon. Subsequently, the isocarbophos aptamer was immobilized on the inner walls of the pores of the PSBM, and UCNPs were conjugated with the isocarbophos complementary strand to prepare fluorescent probes. Then, the probe solution was dropped onto the inner walls of the PSBM pores, and through specific binding between the isocarbophos aptamer and the complementary strand, the UCNPs and the complementary strand were indirectly immobilized on the inner walls of the PSBM pores. After that, isocarbophos standard solutions of different concentrations were added. Because isocarbophos molecules have stronger specific binding affinity for the isocarbophos aptamer, the UCNPs and complementary strand originally immobilized on the inner walls of the PSBM pores were displaced by isocarbophos molecules, resulting in a decrease in the number of UCNPs on the inner walls of the PSBM pores and thereby attenuating the upconversion fluorescence. Finally, the surface of the porous silicon sample was irradiated with a 980 nm laser to excite the UCNPs on the inner walls of the pores to generate upconversion fluorescence. Fluorescence images before and after the reaction were recorded by an image acquisition device, and the change in mean grayscale value was calculated using image processing software to determine the isocarbophos concentration. The limit of detection of this method was 0.27 μg/L. Under identical conditions, the signal change caused by 100 μg/L chlorpyrifos, a structurally similar organophosphate pesticide, was negligible, indicating that the sensor has good selectivity for ICP. These results demonstrate that the proposed platform is a promising low-cost, highly sensitive, and rapid tool for pesticide detection. The anti-interference experiment against structurally similar organophosphate pesticides confirmed the selectivity of the aptamer sensing mechanism, while systematic evaluation of matrix effects in real samples remains an important direction for future research.