A “turn-on” fluorometric aptasensor for simultaneous detection of Hg(II) and Pb(II) in fruits and vegetables after digestion

A novel structure-switching “turn-on” fluorometric aptasensor was developed for simultaneous detection of Hg(II) and Pb(II) in grapes, broccoli, and cabbage. The sensing mechanism utilizes fluorescence resonance energy transfer (FRET) between a FAM-labeled aptamer (FAM-Apt) and a BHQ1-labeled complementary strand (CS-BHQ1). In the absence of target ions, FAM-Apt and CS- BHQ1 bind through complementary base pairing, bringing FAM and BHQ1 into close proximity, which induces FRET and quenches FAM’s fluorescence. Target ion binding induces a conformational change in the FAM-Apt, disrupting FRET and restoring fluorescence. Optimized parameters include reagent addition sequence, Apt and CS concentrations, incubation times, buffer conditions, and salt concentrations. Under optimized conditions, the aptasensor achieved limits of detection (LODs) of 2.29   μg/L for Hg(II) and 0.24   μg/L for Pb(II), much lower than the regulatory limits for fruits and vegetables, and the limits of quantification (LOQ) for food samples were 6.02   μg/kg and 2.07   μg/kg, respectively. Furthermore, the constructed aptasensor could rapidly complete detection within 22.5   min and demonstrated high selectivity against interfering ions. Spiked sample recoveries ranged from 98.07-104.12% for Hg(II) and 89.23-105.46% for Pb(II), with relative standard deviations of 0.95-5.64% and 1.21-14.53%, respectively. This aptasensor provides a sensitive, rapid, and cost-effective alternative to conventional detection methods.