A Facile Aptasensor for Instantaneous Determination of Cadmium Ions Based on Fluorescence Amplification Effect of MOPS on FAM-Labeled Aptamer

Analytical performance and efficiency are two pivotal issues for developing an on-site and real-time aptasensor for cadmium (Cd(2+)) determination. However, suffering from redundant preparations, fabrications, and incubation, most of them fail to well satisfy the requirements. In this work, we found that fluorescence intensity of 6-carboxyfluorescein(FAM)-labeled aptamer (FAM-aptamer) could be remarkably amplified by 3-(N-morpholino)propane sulfonic acid (MOPS), then fell proportionally as Cd(2+) concentration introduced. Importantly, the fluorescence variation occurred immediately after addition of Cd(2+), and would keep stable for at least 60 min. Based on the discovery, a facile and ultra-efficient aptasensor for Cd(2+) determination was successfully developed. The sensing mechanism was confirmed by fluorescence pattern, circular dichroism (CD) and intermolecular interaction related to pK(a). Under the optimal conditions, Cd(2+) could be determined rapidly from 5 to 4000 ng mL(−1). The detection limit (1.92 ng mL(−1)) was also lower than the concentration limit for drinking water set by WHO and EPA (3 and 5 ng mL(−1), respectively). More than a widely used buffer, MOPS was firstly revealed to have fluorescence amplification effect on FAM-aptamer upon a given context. Despite being sensitive to pH, this simple, high-performance and ultra-efficient aptasensor would be practical for on-site and real-time monitoring of Cd(2+).