A wavelength-ratiometric pH sensitive probe based on the boronic acid moiety and suppressed sugar response

Abstract

We characterize a new water soluble fluorescent probe sensitive to changes in pH. The new probe shows spectral shifts and intensity changes in different pH media, in a wavelength ratiometric and colorimetric manner. Subsequently, changes in pH can readily be determined around the physiological level. The new probe's response is based on the ability of the boronic acid group to interact with strong bases like OH −, changing from the neutral form of the boronic acid group [R-B(OH) 2] to the anionic R-B −(OH) 3 form, which is an electron donating group. The presence of an electron deficient quaternary heterocyclic nitrogen center and a strong electron donating amino group in the 6-position, on the quinolinium backbone, provides for the spectral changes observed upon OH − complexation. In addition, the presence of the amino group in the 6-position of the quinolinium backbone, suppresses the response of the boronic acid containing probe towards mono saccharides such as glucose and fructose, which are present in many biological fluids, allowing for the predominant pH sensitivity. Finally we compare our findings to those of a control compound that does not contain the boronic acid group.