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 (lambda 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.