In this work, water soluble carbon quantum dots (wsCQDs) were synthesized from lemon peel waste using a facile and cost effective hydrothermal process. As synthesized wsCQDs were 1-3 nmin size with spherical morphology and oxygen rich surface functionalities. These wsCQDs manifest excellent photoluminescence (PL) properties and exhibited quantum yield (QY) similar to 14% with high aqueous stability. wsCQDs were further used to design an economic, green and highly sensitive fluorescent probe for the detection of Cr6+ ions with a detection limit of similar to 73 nM. This wsCQDs based fluorescent probe could provide a simple, rapid, convenient technique for the sensitive and selective detection of Cr6+ in water purification processes. Further, wsCQDs were immobilized over electrospun TiO2 nanofibers and the photocatalytic activity for such a TiO2-wsCQDs composite was demonstrated using methylene blue (MB) dye as a model pollutant. Photocatalytic activity for the TiO2-wsCQDs composite was found to be similar to 2.5 times more than that of TiO2 nanofibers. The synthesis method for wsCQDs could be easily scaled up for gram scale synthesis of carbon quantum dots.