A novel dual-mode colorimetric and ratiometric fluorescence method for detecting isocarbophos based on S, N-CDs inhibiting Fe3O4@Cu-BTC nanozyme activity.

Although many colorimetric methods have been developed for on-site pesticide detection, most exhibit low sensitivity and limited resistance to interference. To address these issues, this study developed a novel dual-mode colorimetric-ratiometric fluorescence method for isocarbophos (ICP) detection using sulfur and nitrogen co-doped carbon nanodots (S, N-CDs) to inhibit catalytic Fe3O4@Cu-BTC nanozyme activity. The synthesized Fe3O4@Cu-BTC nanozymes with higher peroxidase-like activity were inhibited by S, N-CDs. The inhibitory mechanism was deeply investigated and was ascribed to (1) the restriction of the reaction between Fe3O4@Cu-BTC and hydrogen peroxide (H2O2), which decreased the ·OH production, and (2) the ·OH scavenging ability of the S, N-CDs, forming the basis of the dual-mode method. The Fe3O4@Cu-BTC/Apt-complementary strand (CS)/S, N-CDs hybridization complexes demonstrated minimal to no peroxidase-like activity, preventing H2O2 from oxidizing o-phenylenediamine (OPD) to produce yellow fluorescent 2,3-diaminophenazine (DAP). Meanwhile, the detection system demonstrated a low fluorescence intensity ratio (I575/I465) at an excitation wavelength of 390 nm. However, the presence of ICP caused the CS/S, N-CDs to separate from the Fe3O4@Cu-BTC/Apt due to the high affinity between Apt and ICP. Then, the Fe3O4@Cu-BTC/Apt-ICP complexes recovered peroxidase-like activity, which promoted DAP production, accompanied by a pronounced fluorescence intensity ratio (I575/I465). In optimum conditions, the detection limits (LODs) were 2.79 ng/mL (colorimetric) and 0.26 ng/mL (ratiometric fluorescence), respectively. The feasibility of the proposed method was verified via three spiked samples, yielding recovery rates of between 86.08 % and 112.64 %, indicating the significant potential for the highly sensitive, accurate on-site detection of ICP in actual samples.