A neoteric dual-channel spectral sensor for wide-range pH detection based on variables in UV–vis peak and intensity

Abstract

A water-soluble 2′,4′,6′-trihydroxy phenylazo luminol dye (THPL) with a D–π–A conjugated structure was developed for the first time for the dual-channel spectral sensing of a wide-range of pH values in aqueous solutions, using phloroglucinol as the electron-donating group, heterocycle amide luminol as the electron-accepting one and –NN– as the π-conjugated bridge. Its UV–vis absorption spectrum, especially its spectral response to pH, was investigated in detail. The results confirmed that the proposed THPL exhibited superior UV–vis spectral properties, a strong molar absorption coefficient (ε_max = 3.35 × 10⁴ L mol⁻¹ cm⁻¹) and a small half-band width of ca. 126 nm. In particular, either the absorption intensity (Abs) or peak (λ_max) changed linearly with pH values from 3.0 to 10.0 under the optimized conditions. Notably, THPL expressed an exclusive dual-channel spectral response to pH, i.e., Abs (a.u.) = 0.1316 + 0.0278 pH (R² = 0.9921), λ_max = 441.5 − 6.64 pH (R² = 0.9968, pH = 3.0–6.0) and λ_max = 359.6 + 7.38 pH (R² = 0.9879, pH = 6.0–10.0). Under the optimized conditions, THPL was applied to detect pH values in some aqueous and fruit juice samples with satisfactory results. The reversible recognition mechanism was deduced by UV–vis titration, ¹H NMR titration and theoretical calculation.