A pyrene-based pH fluorescence probe with continuous multiple responses under acidic conditions and its application for environmental water systems and cells

A dual-pKa values pH fluorescence probe based on pyrene, N-(2'-morpholinoethyl)-pyrene-l-carboxamide (MPCA), was fabricated through facile one-step reaction. Interestingly, MPCA showed a pKa(1) of 1.88 under extremely acidic conditions and a pKa(2) of 5.81 under weak acidic conditions in deionized water, which indicate that MPCA has a high sensitivity to change of pH value. When the pH value was decreased further from 4.05 to 0.52, the fluorescence response of MPCA was quenched by 93.8% due to quenching of fluorescence emission of the pyrene monomer and apparent conversion of monomer/excimer of pyrene group. The results show that pyrene excimer of MPCA can be produced in extremely acidic environment. When the pH value of the solution was decreased from 7.03 to 4.05, the fluorescence response of MPCA was enhanced due to the inhibition of photoinduced electron transfer (PET) from N atom on morpholine ring to pyrene fluorophore. In addition, MPCA has a short response time, good repeatability and anti-cation interference for pH monitoring. MPCA was then applied successfully to monitor the acidity of environmental water systems (tap water and Pearl River water) and the living GS cells of epinepheluscoioides. Finally, a fluorescence pH test paper based on the colorful changes of MPCA at different pHs was developed, which can conveniently monitor the pHs of aqueous solutions.

Automated summary

A pH fluorescence probe based on pyrene with dual-pKa values was developed, showing high sensitivity and ability to generate pyrene excimers under acidic conditions. The fluorescence response of MPCA is quenched in acidic environments and enhanced under neutral conditions.