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 P EG . This sensing strategy using NC-Dots CHIT displayed a wide linear working range from 1.25 µ g/mL to 43.54 µ g/mL with a detection limit of 1.41 µ g/mL. The fluorescence of C-Dots P EG 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 ( λ exc: 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.