New fluorescent pH sensors based on covalently linkable PET rhodamines

Abstract

A new class of rhodamines for the application as indicator dyes in fluorescent pH sensors is presented. Their pH-sensitivity derives from photoinduced electron transfer between non-protonated amino groups and the excited chromophore which results in effective fluorescence quenching at increasing pH. The new indicator class carries a pentafluorophenyl group at the 9-position of the xanthene core where other rhodamines bear 2-carboxyphenyl substituents instead. The pentafluorophenyl group is used for covalent coupling to sensor matrices by “click” reaction with mercapto groups. Photophysical properties are similar to “classical” rhodamines carrying 2′-carboxy groups. pH sensors have been prepared with two different matrix materials, silica gel and poly(2-hydroxyethylmethacrylate). Both sensors show high luminescence brightness (absolute fluorescence quantum yield Φ _F ≈0.6) and high pH-sensitivity at pH 5–7 which makes them suitable for monitoring biotechnological samples. To underline practical applicability, a dually lifetime referenced sensor containing Cr(III)-doped Al ₂O ₃ as reference material is presented.

Highlights

► New rhodamine pH-indicators employing photoinduced electron transfer are prepared. ► Indicators are covalently immobilised to the sensor support. ► Obtained pH optrodes feature high brightness, excellent sensitivity and good operation stability. ► Dynamic range (pH 5–7) is particularly useful for biotechnological applications.