Anomalous Inverse Hysteresis of Phase Transition in Thermosensitive Dextran-graft-PNIPAM Copolymer/Au Nanoparticles Hybrid Nanosystem

Abstract

An anomalous inverse hysteresis behavior of the thermally induced LCST structural phase transition in dextran-graf t-PNIPAM copolymer macromolecules with Au nanoparticles has been observed by means of surface plasmon extinction and dynamic light scattering spectroscopy. Namely, the phase transition at heating occurs at temperature lower than one occurs at cooling. The inverse character of observed hysteresis has been rationalized to be caused by Au nanoparticles which play the key role in the change of the "sign" of the hysteresis. Also, Au NPs counteract the formation of PNIPAM aggregates in the solution. Moreover, it has been shown that the PNIPAM macromolecule shrinking at heating leads to an unexpected decrease of the surface plasmon 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. It has been found that the observed LCST phase transition is reversible.

Laser-driven structural transformations in dextran-graft-PNIPAM copolymer/Au nanoparticles hybrid nanosystem: the role of plasmon heating and attractive optical forces

“…36 and 37 the large Au@PNIPAM core-shell NPs with gold 100 nm core have been used. Also, the observed behavior of D-g-PNIPAM/AuNPs system at the laser heating is completely opposite to our recent results 46 on the LCST phase transition in the same system occurring at their usual heating-cooling cycle when no formation of aggregates has been found and structural transformations have been found to be reversible. The observed difference has been rationalized to be result of the action of laser induced attractive optical forces between the Au NPs that promote the formation of aggregates and counteract their destruction.…”

“…Thus, taking the change of Dn ¼ À0.0035 and the value of dn/dT from ref. 46, we estimated the highest temperature reached in our experiments at laser intensity of 425 W cm À2 to be about 70 C. Let us note that the laser induced rise of temperature reached in our experiments is quite moderate. This fact explains why there is no destruction of D-g-PNIPAM macromolecules under the laser irradiation of the sample that is mentioned above.…”

“…However, since the concentration of studied solution is lower than critical concentration needed for macromolecules aggregation, the usual heating does not lead to the formation of their aggregates and, respectively, to the aggregation of the Au NPs. 46 Meanwhile, the laser illumination leads not only to the heating of D-g-PNIPAM/AuNPs system but the laser electric eld induces the plasmonic dipoles in the Au NPs. The induced dipoles cause an appearance of the attractive forces acting between the NPs as it is discussed in the Introduction.…”

“…45 Below the critical concentration (Guinier regime) the PNIPAM exists in solution in the form of separate macromolecules. 45,46 PNIPAM-based polymers are a promising material in medical applications. The unique properties of PNIPAM-based polymers and copolymers indicate them as an innovative drug carrier for drug delivery systems, diagnostic substance carriers, and also as biosensors.…”

Temperature Driven Transformation in Dextran-Graft-PNIPAM/Embedded Silver Nanoparticle Hybrid System

“…Hybrid nanocomposites containing polymer and noble metal nanoparticles attract a great interest as material with unique properties [17][18][19][20]. It is known that Ag nanoparticles (AgNPs) possess a remarkable biomedical application [20].…”