A novel fluorescence ratiometric pH sensor based on covalently immobilized piperazinyl-1,8-napthalimide and benzothioxanthene

Two compounds with a terminal double bond, N-allyl-4-piperazinyl-1,8-naphthalimide (APN) and N-(2-methacryloxyethyl)benzo[k,l]thioxanthene-3,4-dicarboximide (MBTD), a benzothioxanthene derivative, have been synthesized as fluorophores for hydrogen ions sensing. To avoid the leakage of the fluorophores, the APN (a pH-sensitive carrier) and MBTD (a pH-insensitive carrier) which served as a referencing fluorophore were co-polymerized with acrylamide, hydroxyethyl methacrylate and triethylene glycol dimethacrylate on the glass surface treated with a silanizing agent. The ratiometric method can offset the influence caused by the external environment and instrument conditions and can improve the measuring precision. The fluorescence enhancement of APN with an increase in hydrogen ions concentration is based on the hindering of photo-induced electron transfer from aliphatic amine group to the napthalimide after its protonation. The proposed pH sensor shows sufficient selectivity, repeatability and short response time. The sensing membrane shows a good stability with a lifetime of at least 1 month. The sensor can be used for the determination of pH between 5.80 and 8.80. Most commonly co-existing inorganic ions and some organic species do not interfere with the determination of pH. The sensor has been applied to the analysis of water and soil samples.