Quasi-Spherical and Upconversion g-CNNPs for Ultrasensitive Detection of Fipronil in Soil Samples and Bioimaging in Zebrafish

Herein, we report a large-scale solid-state synthesis method for water-soluble graphitic carbon nitride nanoparticles (g-CNNPs) using urea and trisodium citrate, adhering to the principle of atom economy. The as-synthesized g-CNNPs, approximately 3.0 nm in size with a quasi-spherical structure and having a high water-soluble property, exhibit strong blue fluorescence emission. The surface of g-CNNPs contains six surface active sites and serves as an effective fluorescence sensor for the detection of the insecticide fipronil (FPN) at pH 7.0. The size variation of g-CNNPs (2–4.5 nm) induces size-dependent surface effects, contributing to their upconversion properties. The quasi-spherical g-CNNPs (ΦF 21.08%) demonstrate high selectivity and sensitivity for FPN detection, with a limit of detection (LoD) of 2.79 nM (R2 = 0.99511) and a limit of quantification (LoQ) of 9.30 nM within a linear concentration range of 1 × 104 to 9 × 104 nM (ΦF 17.40%), and an association constant (Ka) of 1.356 × 102 M–1. The observed fluorescence quenching during the sensitivity study is likely due to intermolecular hydrogen bonding between g-CNNPs and FPN. Time-correlated single photon counting (TCSPC) analysis confirms the static quenching of g-CNNPs (lifetime 5.53–5.60 ns) upon FPN detection. To confirm the pH-dependent behavior of g-CNNPs and FPN interaction, the TCSPC technique was carried out in particular at pH 3.0 and 11. Density functional theory studies were conducted to confirm the interaction between g-CNNPs and FPN molecules. Additionally, the g-CNNPs demonstrated practical applicability by detecting FPN in soil samples with recovery rates ranging from 102.5 to 129.7%. Due to their low cytotoxicity and good cell permeability, g-CNNPs were successfully applied for selective cytotoxic effects and bioimaging in zebrafish.