Squaraine dyes as fluorescent turn-on sensors for the detection of porcine gastric mucin: A spectroscopic and kinetic study

Mucin-type glycoproteins are the principal components of mucus which cover all the mucosal surfaces of the human body. The mucus and mucins are essential mediators of the innate immune system, however in the last decades mucins have been identified even as an important class of cancer biomarkers. Luminogenic materials with fluorescence turn-on behavior are becoming promising materials because of their advantages of label free, relatively inexpensive and simple to use properties for biological detection and imaging. Squaraines are luminogens characterized by high fluorescence in organic media but poor emission in aqueous environments due to their tendency to self-aggregate. Herein we investigate the interaction between porcine gastric mucin (PGM) and several squaraines in aqueous media. While squaraine dyes showed low fluorescence intensity and quantum yield in water, as a result of the formation of aggregates, an enhancement of fluorescence up to 45-fold was achieved when PGM was added. PGM was detected in a linear range of 10–300 μg/mL with a limit of detection of 800 ng/mL. The assay was used to quantify mucin in diluted human serum samples and recoveries of 94.9–116.2% were achieved. To the best of our knowledge, this is the easiest and convenient method for mucin detection in the reported literature.