Green and cost-effective synthesis of carbon dots from date kernel and their application as a novel switchable fluorescence probe for sensitive assay of Zoledronic acid drug in human serum and cellular imaging.

A new label-free, sensitive and selective off and on signaling fluorescence platform for assay of trace levels of Zoledronic acid (ZA) drug in human biological samples based on nitrogen doped carbon dots (N-CDs) - ferric ions (Fe) was designed. The fluorescence probe, N-CDs, was synthesized for the first time through a facile, eco-friendly and one-step hydrothermal treatment using date kernel as the precursor without any need to use chemical reagents. These CDs exhibited excellent water solubility, ionic and photo stability in various circumstances and a highly relative quantum yield of 12.5%. In the presence of Fe, the fluorescence intensity (FL) for N-CDs was strongly quenched due to the interaction between ferric ions and the functional groups at the N-CDs (switch off). Afterwards, by the addition of ZA, the fluorescence sensor status turned to "ON" (switch on) due to the dominance of ZA in the competition between functional groups on the surface of N-CDs and phosphate groups in ZA in the interaction with Fe results in removing Fe from the surface of N-CDs. Under the optimized conditions, the proposed fluorescence probe (N-CDs-Fe) exhibited good sensing performance for ZA assay with a linearity from 0.1 μM to 10.0 μM, a detection limit of 0.04 μM and the precision of 2.70%. The developed N-CDs-Fe sensor was successfully used for the assay of ZA contents with good recoveries and selectivity in human serum samples. Meanwhile, the in vitro cytotoxic activity and cellular uptake of N-CDs were investigated on human osteosarcoma (MG-63) cell line.