Squaraine dye as a fluorescent probe for highly sensitive detection of pyrophosphate and alkaline phosphatase

We synthesized a squaraine dye (F-0) to develop a method for detecting pyrophosphate (PPi) and alkaline phosphatase (ALP) by modulating the fluorescence of F-0. The fluorescence intensity of the F-0 system was quenched upon the addition of Cu²⁺ ions; however, it was restored when PPi was introduced due to the formation of a complex between PPi and Cu²⁺. Since ALP can hydrolyze PPi, the fluorescence of the system was quenched again upon the addition of ALP. Based on these principles, we established a fluorescent probe that exhibits an "off-on-off" fluorescence response. The detection limits of this method for PPi and ALP were 103 nmol dm^-3 and 0.18 U dm^-3, respectively. Moreover, this method demonstrates good selectivity and specificity and can be applied to the detection of PPi in actual samples.