Inverse Hysteresis of Phase Transition in Thermosensitive PNIPAM copolymer / Au nanoparticles Hybrid Nanosystem Revealed by Surface Plasmon Resonance Spectroscopy

The extinction spectra of Au nanoparticles in dilute aqueous solution of dextrane-grafted-PNIPAM/AuNPs copolymer hybrid macromolecules have been measured at slow successive heating and cooling of the sample in the temperature range of 23 – 46 °C. Additionally, the dynamic light scattering measurements have been performed in the same temperature range. Both spectroscopic methods show the fact of existence of the thermally induced LCST structural phase transition in PNIPAM macromolecule which is accompanied by its shrinking. It has been shown that the macromolecule shrinking at heating leads to unexpected decrease of surface plasmon resonance (SPR) peak area, its blue shift and narrowing. Such effects have been rationalized as the result of the sharp jump-like decrease of the refractive index of PNIPAM polymer occurring at the phase transition.

Moreover, an anomalous inverse hysteresis behavior of the observed LCST phase transition in D-g-PNIPAM macromolecules with Au nanoparticles has been observed by surface plasmon extinction and dynamic light scattering spectroscopy. Namely, the phase transition at heating occurs at temperature lower than one occurs at cooling. Meanwhile, the phase transition in PNIPAM macromolecules without Au NPs is common, i.e. the direct one. The inverse character of observed hysteresis has been assumed to be caused by Au NPs which play the key role in the change of the “sign” of the hysteresis. The possible cause of such effect can be the surface charge of Au NPs which leads to the repulsion of the Au NPs and, correspondingly, to the volume expansion of the PNIPAM macromolecules. Finally, another important obtained result is the reversible character of the observed LCST phase transition that is confirmed both by the reversible temperature dependences of the SPR peak spectral characteristics and hydrodynamic size of PNIPAM macromolecules.