Cu-Mn codoped ZnS quantum dots-based ratiometric fluorescent sensor for folic acid

In the design of new fluorescent sensors, exploration of new materials with excellent properties and smart designs is of continuing interest. Herein, we designed a dual-emission ratiometric fluorescent sensor by doping ZnS quantum dots (QDs) with Cu²⁺ and Mn²⁺ ions. The codoped ZnS QDs with two separated dopant emissions were used as ratiometric fluorescent sensors for folic acid (FA). With addition of FA, the Cu dopant emission was quenched while the Mn dopant emission was enhanced. The fluorescence intensity ratios versus the FA concentrations could be fitted to a linear regression equation ranging from 0.01 to 5 μM (R² = 0.995) and the detection limit was calculated to be 6 nM. Thus, this method generated ratiometric fluorescence signals for quantitative detection of FA. To understand the specific fluorescence response of the sensors for FA, the sensing mechanism was proposed based on electron transfer between QDs and FA. Additionally, the sensors were successfully applied to the determination of FA in spiked samples and recoveries were ranged from 95.5 to 102.2%. Therefore, the sensors show high sensitivity and selectivity for FA and the proposed method has good potential for extension to the detection of other biological molecules after further development.