A highly Sensitive Turn-on Fluorescent Sensor for Ba²⁺ Based on G-Quadruplexes

A G-quadruplex-based fluorescent biosensor for highly sensitive detection of barium ion (Ba²⁺) was constructed for the first time. In the absence of Ba²⁺, the G-quadruplex-specific fluorescence ligand N-methyl mesoporphyrin IX (NMM) remained weakly fluorescent when coexisted with a single-stranded G-quadruplex sequence AGRO100. Upon addition of Ba²⁺, AGRO100 was folded into G-quadruplex structures with the aid of Ba²⁺, which bound with NMM by stacking forces and significantly enhanced its fluorescence. The maximum fluorescence intensity of NMM was increased by ca. 22-fold in response to 1 μM Ba²⁺. This simple method exhibites a good linear relationship in the range of 0-600 nM with the detection limit of 4 nM. The detection method is turn-on, fast, economic, high in signal-to-noise ratio and free of participation of toxic organic solvents, demonstrating its great potential for on-site and real-time Ba²⁺ detection.