A novel amphiphilic pH-responsive AIEgen for highly sensitive detection of protamine and heparin

Abstract

An amphiphilic aggregation-induced emission (AIE) tetraphenylbenzene derivative, namely 6-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl) hexanoic acid (TPHA), was designed. TPHA exhibits pH-responsive AIE properties in aqueous solutions. Its fluorescence intensity in dispersed systems of acidic THF/water increased with an increasing volume ratio of water due to the formation of aggregates, while only very weak fluorescence was observed in basic THF/water dispersions. The formation of aggregates in acidic dispersions could be attributed to the hydrophobic effect, intermolecular π-π interaction, and possible hydrogen bonding between the carboxyl groups. Deprotonation of the carboxyl moieties in basic dispersions resulted in the formation of molecularly dispersed systems. The fluorescence intensity of the dispersions in basic media increased dramatically (turn-on signals) when adding protamine, due to the formation of aggregates via electrostatic interactions between the deprotonated carboxyl groups and protein molecules. A good linear relationship between the fluorescence intensities and the protein concentrations in the range of 0–6 μg/mL was obtained, and the detection limit was calculated to be 4.78 ng/mL. In addition, heparin was found to quench the fluorescence (turn-off signals) of the TPHA-protamine system effectively. Therefore, this strategy can be used as fluorescence probes for both protamine and heparin detection.