The pH response of near infrared 5,5’-bisulfonic heptamethine indocyanines in water, CTAB solution and metal oxide-based sol under extremely acidic and basic conditions

Abstract

In order to detect extreme pH in biosystems, waste water and/or sol/gel formation, six N-p-carboxybenzyl (or ethyl)-5,5’-bisulfonic heptamethine cyanines (SCy-Cls, and their derivatives SCy-Os and SCy-Ns) with spectral absorption and emission in near infrared range were developed, in which the structures of SCy-Os had not been reported before. They all exhibited pH response in water, methanol and/or ethanol, but in different extreme pH conditions. SCy-Os (pKa in 3.02–3.09) reversibly turned to their tautomers (SCy-OHs) at pH 2.11–4.28 accompanying a great change in their maxima absorption and emission wavelengths (from 504–639 nm to 709–762 nm) and the corresponding intensity and in the color of their solutions (from red to green) which made the detection processes recognized by naked eyes. In contrast, SCy-Cls and SCy-Ns took their corresponding tautomerizations at strong basic (pH 9.80–13.90, pKa in 10.41–11.93) conditions rather than acidic condition because of the difference in the meso-atoms (Cl, N, O) and conjugated systems of the cyanine molecules. The interactions between molecules of the cyanines and CTAB in aqueous solutions made not only the absorption and emission wavelengths and intensity of the cyanines vary, but also the pH response turn to more extremely acidic conditions, meanwhile made the CMC of CTAB remarkably decrease. SiO2 sol and TiO2 sol also affected the pH response due to the interaction of cyanines and the sol particles. So they might have special sensitive pH response in extremely complex acidic and basic environment, and be applied as pH probes in the fields and/or in special materials manufactures.