A Fluorescence Indicator at Extreme Low pH Region Based on Dissolution of Zn Tetra-2,3-Pyridoporphyradine (TPP) Nanocrystal Suspension

Conventional organic pH indicators are widely used for chemical synthesis and analysis based on change of pi conjugation length depending on pH values. However, there are a few organic pH indicators for strong acid conditions, because organic compounds are usually decomposed at extreme low pH. Tetra-2,3-pyridoporphyradine (TPP) has high chemical stability and crystallinity, and its nano-crystal is widely used as stable pigments. ZnTPP exhibits red fluorescence, but ZnTPP crystals don't exhibit fluorescence due to concentration quenching. In this paper, we show a UV-assisted synthesis of ZnTPP nano-crystals and application for fluorescence pH indicator at extreme low pH condition by quaternization of ZnTPP nano-crystals. ZnTPP crystals were successfully synthesized and then dispersed into 1 to 10 M HCl aq. UV-Vis and fluorescence spectroscopy revealed that absorption and fluorescence change were observed depending on HCl concentrations at extreme low pH conditions due to quarternization of ZnTPP and dissolution of ZnTPP nano-crystals. These results indicate that ZnTPP nano-crystals can be used for fluorescence pH indicators at extreme low pH.