Naked eye and smartphone applicable detection of toxic mercury ions using fluorescent carbon nanodots

Abstract

Chitosan passivated carbon nanodots (C-Dots$_{CHIT})$ were synthesized from expired molasses via a simple and green thermal synthesis procedure. As-synthesized C-Dots were nitrogen-doped (NC-Dots$_{CHIT})$ by posttreatment with liquid ammonia and used as nanoprobes for fluorometric detection of mercury ions (Hg(II)$_{aq.})$. Fluorescence response of NC-Dots$_{CHIT}$ in the presence of mercury was evaluated and compared with that of the polyethylene glycol passivated C-Dots$_{PEG}$. This sensing strategy using NC-Dots$_{CHIT}$ displayed a wide linear working range from 1.25 $\mu $g/mL to 43.54 $\mu $g/mL with a detection limit of 1.41 $\mu $g/mL. The fluorescence of C-Dots$_{PEG}$ did not show any significant change upon mercury addition. Selectivity of as-synthesized NC-Dots$_{CHIT}$ to Hg(II)$_{aq.}$ was assessed by comparing the level of fluorescence quenching in the presence of four other divalent cations (cadmium(II), zinc(II), nickel(II), and copper(II)). Finally, synthesized nanoprobes were embedded into the cross-linked alginate hydrogels and test strips were formed on the FTO-coated glass. Images captured under a UV light source (λexc: 365 nm) were successfully processed by a smartphone application. Color codes generated by the app showed a close resemblance to the data gathered from fluorescence spectroscopy. The proposed detection system was applied satisfactorily to both a certified calibration standard and real water samples. The methodology developed within this study could be a potential candidate for detection of mercury concentration in water samples with high recovery rates reaching up to 98%. This smartphone applicable detection platform that uses carbon nanodots as cheap yet sensitive nanoprobes could lead to more advanced lab-on-site systems for water or food sample analysis that can be performed by anyone, anywhere, anytime.