Unconventional Fluorescent and Multi-responsive Polyethyleneimine with LCST and UCST Behavior: Synthesis, Characterization and Biological Applications

Non-aromatic fluorescent and multi-responsive materials, exhibiting inherent fluorescence emission and controlled phase change, have garnered significant attention in recent years. However, the underlying interaction between their fluorescent properties and phase transition remains unclear. In this study, we synthesized a series of catalyst-free aza-Michael addition-based polyethyleneimine (RFPEI) materials by reacting polyethyleneimine (PEI) with N-isopropyl acrylamide (NIPAM). The resulting RFPEI was comprehensively characterized, and demonstrated dual-phase transition behavior (LCST and UCST) in water, which could be finely tuned by adjusting its composition or external factors such as pH. Notably, upon UV irradiation (365 nm), RFPEI exhibited strong fluorescence emission. We further investigated the effects of NIPAM grafting percentage to PEI, polymer concentration, and pH on the LCST/UCST and fluorescent properties of RFPEI aqueous solutions. Moreover, we showcased the great potential of RFPEI as a versatile tool for physiological cell imaging, trace detection, and controlled release of doxorubicin. Our study presents a novel class of stimuli-responsive fluorescent materials with promising applications in the field of biomedicine.