Crystal violet–G-quadruplex complexes as fluorescent sensors for homogeneous detection of potassium ion

A novel K⁺ detection method was reported using a label-free G-quadruplex-forming oligonucleotide and a triphenylmethane fluorescent dye crystal violet (CV). This method is based on the fluorescence difference of some CV/G-quadruplex complexes in the presence of K⁺ or Na⁺, and the fluorescence change with the variation of K⁺ concentration. According to the nature of the fluorescence change of CV as a function of ionic conditions, two K⁺ detection modes can be developed. One is a fluorescence-decreasing mode, in which T₃TT₃ (5′-GGGTTTGGGTGGGTTTGGG) is used, and the fluorescence of CV decreases with an increased concentration of K⁺. The other is a fluorescence-increasing mode, in which Hum21 (5′-GGGTTAGGGTTAGGGTTAGGG) is used, and the fluorescence of CV increases with an increased concentration of K⁺. Compared with some published K⁺ detection methods, this method has some important characteristics, such as lower cost of the test, higher concentrations of Na⁺ that can be tolerated, adjustable linear detection range and longer excitation and emission wavelengths. Preliminary results demonstrated that the method might be used in biological systems, for example in urine.