A simple fluorometric technique has been adopted for cysteine (Cys) sensing in alkaline medium down to the nM level. The huge fluorescent signal of the solution is a consequence of fluorescent carbon dots (CDs) produced in situ from modified hydrothermal (MHT) reaction between Cys and dopamine (DA). It has been observed that the inherent fluorescence of DA is drastically quenched in alkaline solution. Cys can selectively rescue the fluorescence of DA. Thus, Cys determination in a straightforward way, but only to a micro molar (10(-7) M i.e. 0.1 μM) level is possible through such fluorescence enhancement. Sensitive Cys determination remains associated with the in situ generated CDs, but the external addition of pre-formed CDs to Cys solution fails miserably towards Cys detection. However, CDs prepared from the Cys-DA system in alkaline solution admirably increase the limit of detection (LOD) of Cys at least two orders higher (10(-9) M) than that observed without hydrothermal technique i.e., without CDs. This method finds applications for Cys determination in biological samples and pharmaceutical preparations.