A luminescent europium complex for wide-range pH sensors and sensor microtiterplates

Abstract

In this work, we show a new concept of a luminescent wide-range pH indicator based on a europium complex and its implementation in sensors and sensor microtiterplates. For this purpose, we complexed europium with the ligands gallic acid (Gall) and pyridine dicarboxylic acid (PDA) that carry groups with wide ranging pK_a values and the antenna ligand theonyl trifluoroacetone (TTA). This yields an indicator that overcomes the typical drawbacks of many optical pH indicator dyes which mostly cover three pH units, only. The spectral characterization of the new europium complex Eu³⁺–TTA–Gall–PDA in a molar ratio of 1 : 3 : 1 : 1 in aqueous buffer delivers a luminescence emission that is dependent on pH over up to 8 orders of magnitude from pH 2–10 in aqueous solution. It was therefore embedded into sensor membranes made of cellulose acetate (CA) which respond reversibly from pH 2–7 over 5 orders of magnitude within several minutes. Moreover, the first high-throughput pH sensor microtiterplate based on this indicator showed a dynamic range from pH 2–8 over 6 orders of magnitude and is suitable for up to 24 h of continuous use.