Carbon quantum dots (CQDs) have emerged out as promising fluorescent probes for hazardous heavy metals detection in recent past. In this study, water soluble CQDs were synthesized by facile microwave pyrolysis of citric acid & cysteamine, and functionalized with ditheritheritol to impart thiol functionalities at surface for selective detection of toxic arsenite in water. Microscopic analysis reveals that the synthesized CQDs are of uniform size (diameter similar to 5 nm) and confirmed to have surface SH groups by FT-IR. The functionalized probe is then demonstrated for arsenite detection in water by "Turn-On" read out mechanism, which reduces the possibility of false positive signals associated with "turnoff' probes reported earlier. The blue luminescent functionalized CQDs exhibit increase in fluorescence intensity on arsenite addition in 5-100 ppb wide detection range. The probe can be used for sensitive detection of arsenite in environmental water to a theoretical detection limit (3s) of 0.086 ppb (R-2 = 0.9547) with good reproducibility at 2.6% relative standard deviation. The presented reliable, sensitive, rapid fCQDs probe demonstrated to exhibit high selectivity towards arsenite and exemplified for real water samples as well. The analytical performance of the presented probe is comparable to existing organic & semiconductor based optical probes. (C) 2017 Elsevier B.V. All rights reserved.