Spiropyran-Based Fluorescent Anion Probe and Its Application for Urinary Pyrophosphate Detection

In recent decades, numerous spiropyran derivatives have been designed and utilized for optical sensing of metal ions. However, there is still less research on spiropyran-based anion sensors. In this work, a new spiropyran compound (L) appended with a pendant bis(2-pyridylmethyl)amine was synthesized and used in fluorescent sensing of pyrophosphate ion (PP(i)) in aqueous solution. The molecular recognition and signal transduction are based on the cooperative ligation interactions and the ligation-induced structural conversion of the spiropyran, which leads to a significant change in the photophysical property of the spiropyran. In an ethanol/water solution (30:70, v/v) at pH 7.4, ligation of L with Zn(2+) causes an intense fluorescence emission at 620 nm at the expense of the original fluorescence at 560 nm. Once PP(i) was introduced, interaction between PP(i) and the L-Zn(2+) complex leads to full quenching of the 620 nm band emission which was concomitant with recovery of the 560 nm band emission, and the fluorescence intensity ratio, F(560)/F(620), is proportional to the PP(i) concentration. Under the optimum condition, the L-Zn(2+) complex responds to PP(i) over a dynamic range of 1.0 x 10(-6) to 5.0 x 10(-4) M, with a detection limit of 4.0 x 10(-7) M. The fluorescence response is highly selective for PP(i) over other biologically related substrates, especially the structurally similar anions, such as phosphate and adenosine triphosphate. The mechanism of interaction among L, Zn(2+), and PP(i) was primarily studied by (1)H NMR, (31)P NMR, and HRMS. To demonstrate the analytical application of this approach, the PP(i) concentration in human urine was determined. It was on the order of 3.18 x 10(-5) M, and the mean value for urinary PP(i) excretion by three healthy subjects was 62.4 micromol/24 h.