pH-sensitive optode membrane covalently incorporated with an ICT dye for low pH values

• pH optode membranes were prepared by the covalent immobilization of an ICT dye onto a HEMA-based polymer.
• The membranes exhibited fluorescence emission and ratiometric absorbance changes in an extreme acidic condition.
• A specially designed cross-liker (PSA) remarkably enhanced the pH-sensitivity of the membrane.
• The optimal membrane thickness for the high sensitivity was determined.

Highly sensitive pH optode membranes for operation at low pH values were fabricated by the covalent immobilization of an ICT dye (DPA) on a silanized glass surface. The photo-initiated copolymerization of DPA with 2-hydroxyethyl methacrylate (HEMA) and diacrylamidated poly(dimethylsiloxane) (PSA) or triethylene glycol dimethacrylate as a cross-linker gave pH-sensitive optode membranes (M1 and M2) capable of working in highly acidic condition. The fluorescence intensities of the membranes increased with increasing pH in the pH range of −1.0 to 2.0. Both membranes were useful as a ratiometric pH optode in the same acidic pH range. Optode membrane M1 prepared by using PSA was more sensitive to pH change than was M2. The prepared membranes were characterized in terms of pKa values, optimal thickness, effect of ionic strength, response time, and leaching of dye.