Development of novel optical pH sensors based on coumarin 6 and nile blue A encapsulated in resin particles and...

Inspired by traditional pH papers, two types of ratiometric fluorescent pH sensors for neutral and alkaline pH ranges were developed in this study by using two fluorescent dyes, coumarin 6 (C6) and nile blue A (NB). These dyes were encapsulated into melamine-formaldehyde (MF) resin particles, which were then incorporated with polymer Nafion (Nf) or polyurethane hydrogel (PU) to prepare pH-sensing membranes. MF-C6 particles immobilized into polymer Nafion (i.e., MF-C6-Nf membrane) showed a dynamic ratiometric pH detection range of 4.5-7.5 through shift in fluorescence emission spectra at acidic and neutral pH solutions, as well as distinct color transition under normal visual sense from pink to yellow color. By contrast, MF-NB particles immobilized into polyurethane hydrogel (i.e., MF-NB-PU membrane) displayed a dynamic ratiometric fluorescence detection range of pH 9 - pH 12 via change in ratiometric fluorescence intensity at two emission band edges. The membrane also showed a clear change from blue to purple color under sunlight at high pH values. These pH-sensing membranes also exhibited high sensitivity, good reversibility, and stability. They were then applied to measure pH values in real urine samples and fermentation media.