Water-soluble pyrrolopyrrole cyanine (PPCy) near-infrared fluorescent pH indicators for strong acidity

Fluorescence based sensing has become an important technique in biomedicine. A number of these methods, such as ophthalmological angiography based on indocyanine green uorescence or intrasurgical tumor targeting based on administration of 5-aminolevulinic acid are now routinely used. In general, for all medical applications deep penetration depths of the techniques are important. Therefore, due to the reduced scattering and low absorption at long wavelengths, the application of near-infrared (NIR) uorophores is desirable. Monitoring pH is crucial for the understanding of physiological and pathological processes in organisms. Changes in pH values also play a pivotal role on cellular events, such as signal transduction, autophagy or apoptosis. While many pH uorescence indicators have been designed to work in the pH range of roughly 6 to 8 or in the weak acidic range from 4 to 6, hardly any pH-sensitive uorophores working in the strong acidic range below pH 4 are known. Such chromophores would, however, be of great interest for e.g. imaging the gastrointestinal tract. In this contribution, we report the development of novel water-soluble NIR uorophores which exhibit strong uorescence in the highly acidic range. The dyes are based on the recently developed pyrrolopyrrole cyanines (PPCys) which