Thin Film Optical Sensors Employing Polyelectrolyte Assembly

This work describes the development of thin film optical sensors for pH, metal ions (ferric and mercury), and 2,4-dinitrotoluene detection. To fabricate the pH sensor, a fluorescent molecule, 1-hydroxypyren-3,6,8-trisulfonate, was assembled with a polycation by an electrostatic layer-by-layer assembly technique. The fluorescent indicator molecule exhibits distinct and well-defined emission peaks for protonated and deprotonated forms. The relative peak positions and intensity of fluorescence of the protonated and deprotonated forms change in response to pH variations. For metal ion (ferric and mercury) and 2,4-dinitrotoluene sensing, the indicator molecules were covalently incorporated into poly(acrylic acid) and subsequently assembled with a polycation employing electrostatic layer-by-layer assembly. The sensor is based on the fluorescence quenching of indicator molecules by electron transfer from indicator to electron-deficient analytes such as ferric ions, mercury, and 2,4-dinitrotoluene. Fluorescence intensities decreased with increasing concentration of analytes. Quenching behavior follows Stern−Volmer bimolecular quenching kinetics. Linear increase in absorbance, film thickness, and emission intensity was observed as a function of number of bilayers deposited in all these films.