Vinyl-functionalized polyphenolic-carbon dot-based fluorometric turn-off-on biosensor for the dual detection of mercury and cysteine and their in vivo sensing in zebrafish larvae.

The fluorometric turn-off-on biosensor was developed for the ultra-sensitive detection of mercury (Hg) and cysteine (Cys) utilizing the highly fluorescent carbon dots (CDs). Herein, the sophisticated low-temperature reflux-mediated reaction was adopted using precursors namely citric acid (CA) and polyphenolic kaempferol (KMP) by using dimethylformamide (DMF) as a solvent. The resulting CDs (i.e., CKCDs) were in the highly negative charged groups (-OH) presented with a bright-orange fluorescence. These CKCDs were functionalized with 4-vinylaniline (4-VA) by employing EDC/NHS coupling reaction, which switched its photoluminescence (PL) towards the strong-blue colored emission and termed as V-CKCDs. The functionalized V-CKCDs can be capable enough to detect mercury via the strong electrostatic interactions between positively charged Hg cations and negatively charged anions (-OH groups). Hence, an adequate fluorescence quenching was observed in V-CKCDs with the lowest concentrations of Hg around 0.5 μM. Significantly, after adding the complex of V-CKCDs-Hg to the Cys, the fluorescence enhancement was observed. This might be attributed from the strong interactions between Hg in the fluorescence sensing system and thiol (-SH) moieties from the Cys. The developed V-CKCDs are highly sensitive for detecting Hg and Cys, which showed detection limits of 10.6 and 42. 48 nM, respectively. Also, the in vivo studies were investigated in zebrafish larvae using V-CKCDs for the detection of Hg and Cys. The V-CKCDs were investigated in the real water samples and human serum to detect Hg and Cys, respectively.