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In this work, a polyadenine-based fluorescent probe (pAFP) was designed for highly selective determination of Cu 2+  in various freshwater sources. The pAFP contained a fluorescent group carboxyfluorescein (FAM) and a guanine-cytosine-guanine-cytosine (GCGC) sequence which was linked to the ends of a polyadenine strand. The complexation between polyadenine strand and Cu 2+  induced the transition of polyadenine strand from a loose random coil to a compact structure, resulting in the proximity of the GCGC to the FAM and subsequent fluorescent quench of the FAM. Our work identified the polyadenine strand as a novel recognization element for Cu 2+ , which displayed high selectivity and appropriate affinity (10 4  ~ 10 5  M −1 ) to Cu 2+ , with a stoichiometric ratio of 1:1. Based on it, the pAFP provided a dynamic range from 80 nM to 20 µM for the determination of Cu 2+ and a detection limit of 62 nM, meeting the requirements of the World Health Organization (WHO) and Environmental Protection Agency (EPA). The analysis of Cu 2+ -spiked real samples from lentic (Yudai Lake) and a lotic freshwater system (Xiangjiang River) revealed the high accuracy of the method, evidencing the application of this proposed sensor towards wide-field freshwater detection. Compared to other reported fluorescent probes, our pAFP displays a much simpler composition and structure. Meanwhile, in the actual procedure, our method only requires a simple pretreatment of the water sample by a 0.22 μm syringe filter, and the whole test procedure can be completed within 10 min. Such a convenient, low-cost and time-saving method for the determination of Cu 2+ will greatly facilitates the on-site and real-time monitoring of Cu 2+  in different freshwater sources and subsequent water quality control.

Polyadenine-based fluorescent probe for high-selective determination of copper ion in freshwater