Quasi-Spherical and\nUpconversion g‑CNNPs for\nUltrasensitive Detection of Fipronil in Soil Samples and Bioimaging\nin Zebrafish

Herein, we report a large-scale solid-state synthesis\nmethod for\nwater-soluble graphitic carbon nitride nanoparticles (g-CNNPs) using\nurea and trisodium citrate, adhering to the principle of atom economy.\nThe as-synthesized g-CNNPs, approximately 3.0 nm in size with a quasi-spherical\nstructure and having a high water-soluble property, exhibit strong\nblue fluorescence emission. The surface of g-CNNPs contains six surface\nactive sites and serves as an effective fluorescence sensor for the\ndetection of the insecticide fipronil (FPN) at pH 7.0. The size variation\nof g-CNNPs (2–4.5 nm) induces size-dependent surface effects,\ncontributing to their upconversion properties. The quasi-spherical\ng-CNNPs (Φ<sub>F</sub> 21.08%) demonstrate high selectivity\nand sensitivity for FPN detection, with a limit of detection (LoD)\nof 2.79 nM (<i>R</i><sup>2</sup> = 0.99511) and a limit\nof quantification (LoQ) of 9.30 nM within a linear concentration range\nof 1 × 10<sup>4</sup> to 9 × 10<sup>4</sup> nM (Φ<sub>F</sub> 17.40%), and an association constant (<i>K</i><sub>a</sub>) of 1.356 × 10<sup>2</sup> M<sup>–1</sup>. The\nobserved fluorescence quenching during the sensitivity study is likely\ndue to intermolecular hydrogen bonding between g-CNNPs and FPN. Time-correlated\nsingle photon counting (TCSPC) analysis confirms the static quenching\nof g-CNNPs (lifetime 5.53–5.60 ns) upon FPN detection. To confirm\nthe pH-dependent behavior of g-CNNPs and FPN interaction, the TCSPC\ntechnique was carried out in particular at pH 3.0 and 11. Density\nfunctional theory studies were conducted to confirm the interaction\nbetween g-CNNPs and FPN molecules. Additionally, the g-CNNPs demonstrated\npractical applicability by detecting FPN in soil samples with recovery\nrates ranging from 102.5 to 129.7%. Due to their low cytotoxicity\nand good cell permeability, g-CNNPs were successfully applied for\nselective cytotoxic effects and bioimaging in zebrafish.