A novel biosensor for sensitively detecting potassium ion (K+) has been developed based on fluorescent copper nanoparticles (Cu NPs). In our design, we employ a label-free single-strand DNA (ssDNA) that contains two parts. One is 3'-terminus structure-switching aptamers (SSAs) that can fold into G-quadruplex after binding with its target K+. The other is 5'-terminus poly thymine (polyT) which works as a template to construct fluorescent Cu NPs. After incubating with K+, the part SSAs go through target-induced conformational changes. Benifiting from the exceptional digestion ability of exonuclease I (Exo I), the G-quadruplexes display effective resistance to nuclease digestion, so that 5'-terminus polyT remains and the in situ formation of Cu NPs provides a turn-on fluorescent signal that is used to evaluate the concentration of K+. The recovery of the fluorescence intensity is linearly correlated with the K+ concentration in the range of 0.05 to 1 mM with a detection limit of 0.05 mM. Compared with some methods, this assay is cost-effective and facile with high specificity. Meanwhile, this excellent strategy shows a great potentiality in other sensing approaches that can study the interaction between similar SSAs and different specific targets.
Keywords: Exo I; Fluorescent copper nanoparticles; G-quadruplex; potassium ion; terminal protection.