A water-soluble dual-channel fluorescence-enhanced biosensor for heparin based on a cationic conjugated polyelectrolyte containing tetraphenylethene and 2, 1, 3-benzothiadiazole

A conjugated polymer (P0) containing tetraphenylethene (TPE) group and benzothiadiazole (BT) group has been designed and synthesized by Suzuki coupling reaction of 1, 2-di[4-(6-bromohexyloxy) phenyl]-1, 2-di(4-bromophenyl) ethene (M1), 1, 2-di[4-(6-bromohexyloxy) phenyl]-1, 2-di(4-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl) phenyl) ethylene (M2) and 4, 7-dibromo-2, 1, 3-benzothiadiazole (M3). The ratio of TPE unit to BT unit at 0. 803:0. 197 was determined by ¹H-NMR spectrum. A watersoluble cationic conjugated polyelectrolyte (P1) is obtained by quaternization P0 with trimethylamine, which is completely soluble in water and insoluble in common organic solvents such as THF and acetone. When P1 is aggregated by adding a poor solvent such as THF into P1 aqueous solution, the fluorescence intensity and color of P1 change drastically. The intensity increases up to seven times and the maximum emission peak is redshifted up to 30 nm. The color is changing from khaki (no aggregation) to orange (aggregation) under ultraviolet irradiation. Therefore, dual-channel fluorescence response as the unique properties of P1 was applied to detect heparin, a highly sulfated glycosaminoglycan with the highest negative charge density among the known biological molecules. The fluorescence intensity gradually increased upon addition of heparin into P1 aqueous solution with a linear response to concentration of heparin, and the correlation coefficient was up to 0. 9999. In addition, a linear red shift of the maximum emission peak was observed as the concentration of heparin increased with a correlation coefficient up to 0. 9998. These results indicate that P1 is a potential material for developing dual-channel fluorescence probe for heparin quantification. The UV spectra of P1/heparin complexing titration don' t show any change. This indicates that there is no inter-chains or intra-chains π-π stacking of P1. DLS result indicates some aggregates can be formed through electrostatic interaction between P1 and heparin, which can reduce the radiative decay when the intramolecular rotation of phenyl group on the side chains is restricted and the fluorescent emission can correspondingly be enhanced. Moreover, due to existence of BT, a typical fluorescence acceptor in fluorescence resonance energy transfer, the fluorescence of TPE segments can effectively be transferred to BT segments when the aggregates are formed by electrostatic interaction between P1 and heparin, which leads to red-shifts of the maximum emission wavelength. Based on these different responses to electrostatic interaction between P1 and heparin, dual-channel signals can be obtained as the above discussion. This study thus provides a new concept for designing dual-channel fluorescence sensing material.