A fluorescent turn-on sensor for mercury (II) ions in near neutral poly(metharylic acid) solution

The conformational change of poly (methacrylic acid) (PMAA) at various pH values is well studied; however, the application of PMAA in the field of analytical chemistry has been very limited. This investigation takes advantage of the conformational change of PMAA at various pH levels and the conformational change induced by metal ions. By adjusting the pH, thiophene-phenylanilide-acridinium molecules can serve as turn-on sensors for Hg²⁺ ions. In pH 7.4 buffer with PMAA molecules, the sensor is selectively turned on by Hg²⁺ ions to display strong charge shift state (CSH) emission at 560 nm. The intensity shows linear response to the concentration of Hg²⁺ ions between 0.020 mM and 0.151 mM with a detection limit in nanomolar range. The photophysical properties of sensor molecules in PMAA/mercury (II) mixture at near neutral pH are comparable to those in PMAA solution in acidic condition without mercury (II) ions. The effect of pH, temperature, polymer size, and polymer concentration on emission intensity were investigated. The sensor showed excellent percent recovery (98.4 % to 103 %) of spiked mercury (II) ions in real water samples. The sensing mechanism is likely through intrachain and interchain coordination of mercury (II) ions with the carboxyl groups on the side chain of PMAA to induce an extended coil conformation of PMAA. Calculations support the conclusion that the size and geometry of the binding sites formed inside PMAA are suitable to incorporate sensor molecules and enhance the charge shift state emission of sensor molecules.