Squaraine Dyes as Fluorescent Turn-on Probes for Mucins: A Step Toward Selectivity^†

Mucins are a family of long polymeric glycoproteins which can be overexpressed in several types of cancers, and over recent years, great attention was addressed to identify mucins as an important biomarker of adverse prognosis. Fluorometric detection mediated by fluorescent probes could represent a winning strategy in the early diagnosis of different pathologies. Among promising biological fluorescent probes, squaraines are gaining particular attention, thanks to their sharp and intense absorption and emission in the NIR region. In this contribution, three squaraine dyes bearing different substituents and with different lipophilicity have been investigated for their ability to detect mucin. The turn-on response upon the addition of mucin has been investigated by means of absorbance and fluorescence spectroscopy. After a preliminary screening, the squaraine (S6) bearing bromine as a substituent and C4 aliphatic chains showed the highest fluorescence turn-on and highest affinity for mucin than albumin. To further highlight the selectivity of S6 for mucin, the fluorescence response has been evaluated in the presence of serum and site-specific proteins different than albumin. Absorption spectroscopy was used to characterize the binding mechanism of squaraine to mucin.