Development of optical pH sensors based on derivatives of hydroxyazobenzene, and the extended linear dynamic range using mixture of dyes

In order to fabricate efficient pH optical sensors, the characteristic features of the UV–vis spectra of six synthesized hydroxyazobenzene dyes were studied at different pH(s). Two optical pH sensors were developed using 4-[(p-nitrophenyl)azo]phenol and 2-methoxy-4-[(4-nitrophenyl)azo]phenol dyes immobilized on transparent triacetylcellulose (TAC) membrane, which showed reversible responses within the dynamic ranges of 8.5–10.5 and 6.5–8.5, respectively. A new optical pH sensor with a linear response over a range of 4.5 pH units (7–11.5) was also developed by immobilization of a mixture of the aforementioned dyes on a TAC membrane. All the fabricated optodes showed fast response time (less than 20 s), sharp and clear color change as well as excellent long-term stability (at least 3 months) under the applied conditions.