Detecting abnormal levels of folic acid (FA), Fe3+, and Ca2+ in the human body is crucial due to their association with various diseases. Orange peel carbon quantum dots (OP-CQDs) with blue fluorescence (& lambda;ex = 370 nm, & lambda;em = 440 nm) were synthesized using a one-step hydrothermal method when waste biomass orange peel was utilized as the carbon source. An "on-off" fluorescence sensor, referred to as OP-CQDs-FA-Fe3+/Ca2+, was developed for quantitative analysis of the drug FA, as well as the metal ions Fe3+ and Ca2+. Additionally, the sensor was utilized for fluorescence imaging of HepG2 liver cancer cells, demonstrating its exceptional biocompatibility. The concentration of FA displayed a linear relationship in the range of 5.00-90.00 & mu;mol L-1 and 0.30-2.50 mmol L-1, with a limit of detection (LOD) of 0.15 & mu;mol L-1. The spiked recovery in folic acid tablets ranged from 95.70% to 104.50%. The addition of Fe3+ or Ca2+ leads to a reduction or restoration of the fluorescence intensity in the OPCQDs-FA system, resulting in an "on-off" sensor mode. The spiked recoveries in human serum samples were 100.15%-100.48% for Fe3+ and 98.05%-98.18% for Ca2+. This study highlights the considerable potential of OP-CQDs for ion detection, drug analysis, and cell imaging.
