Thermal responsive fluorescent block copolymer for intracellular temperature sensing

In this study, a novel fluorescent and temperature responsive block copolymer has been designed and synthesized by a reversible addition–fragmentation chain transfer (RAFT) polymerization method in terms of the strategy that N-isopropylacrylamide (NIPAm), maleic anhydride (MAn) and 7-amino-4-methylcoumarin (AMC) act as the temperature responsive unit, the hydrophilic unit and the fluorescent unit, respectively. The successfully synthesized block copolymer was characterized by gel permeation chromatography (GPC) and nuclear magnetic resonance (1H NMR) spectroscopy. Meanwhile, the self-aggregation behaviour in aqueous solution and the thermo-responsive property of the block copolymer were demonstrated by particle size measurement, transmission electron microscopy (TEM) observations and lower critical solution temperature (LCST) determination, respectively. Then the variation of fluorescence intensity with temperature was confirmed. With increasing temperature, shrinking of PNIPAm chains caused the block copolymer to become more hydrophobic above the LCST, assembling larger aggregates with lower interfacial curvature. Thus a part of the fluorescent groups would be embedded inside the enlarged block copolymer micelles, resulting in lower fluorescence intensity. Furthermore, the superior hydrophilicity and biocompatibility of the block copolymer as a thermometer have been demonstrated by application in intracellular temperature sensing of MDCK cells ranging from 24 °C to 38 °C.