▎ 摘　　要

The photoluminescence of graphene quantum dots (GQD) is widely explored for sensory applications. Among various metal ions to be detected in aqueous bodies, Ca2+ ions are often neglected which is evident from the lack of literature. However, owing to booming industrialization, the available potable water is mostly crossing the prescribed hardness limits. Presented report summarize the synthesis of GQD via modified hydrothermal cutting of graphene oxide, which was further employed for sensing Ca2+ ions in hard water samples. The as synthesized GQD exhibited its characteristic blue photoluminescence on excitation with longwave UV (lambda approximate to 365 nm) which was suppressed on the introduction of Ca2+ ions. The GQD as photoluminescent probe determined the concentration of Ca2+ ion in water with an accuracy of 96.74% +/- 3.26%. The LOD of the reported method was found to be 5 mu M and it was sensitive towards Ca2+ ions in the presence of interfering ions such as Al3+, Mg2+, and K+ which are commonly found in hard water samples. Therefore, the determination of hardness of water w.r.t Ca2+ using GQD based PL turn off sensing is a quick, viable and economical method.