Fluorescence nonradiative energy transfer analysis of crosslinker heterogeneity in core–shell hydrogel nanoparticles Original Research Article

Hybrid thermoresponsive core–shell nanoparticles composed of a non-responsive poly(butyl methacrylate) (pBMA) core and a thermoresponsive poly(N-isopropylacrylamide) (pNIPAm) shell have been synthesized by two-stage free radical polymerization. A polymerizable fluorescence donor (phenanthrene) and a polymerizable acceptor (anthracene) have been introduced during polymerizations and thus covalently localized in the core and shell, respectively. In comparison with photon correlation spectroscopy (PCS), which probes the average particle hydrodynamic radius as a function of solution temperature, nonradiative energy transfer (NRET) probes the core/shell interfacial deswelling behavior, resulting in sharper phase transition curves at higher temperatures than the PCS-monitored response. Furthermore, PCS shows no dependence of phase transition behavior on the pNIPAm shell thickness, whereas the transition observed by NRET technique has been found to shift to higher temperatures as the pNIPAm shell thickness increases. NRET measurements also show a pronounced volume phase transition hysteresis between the heating and cooling cycles, which was found to increase with the thickness of the pNIPAm shell. This is in contrast to the lack of hysteresis observed when the phase transitions are probed by PCS. These results are ascribed to the internal/interfacial structure of pNIPAm shell, which is proposed to have a radial polymer density gradient associated with a decreasing crosslinking density progressing from the particle interior to the periphery.