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

Abstract

During the last decade, stimuli-responsible polymers based on poly(N-isopropylacrylamide) having conformational transition in the range of physiological temperature have been discussed as novel drug delivery nanosystems. A star-like copolymer with a dextran core and grafted poly(N-isopropylacrylamide) arms (D-g-PNIPAM) was synthesized, characterized, and used as a matrix for silver sol preparation. The comparative study of the behavior of individual D-g-PNIPAM and the nanohybrid system D-g-PNIPAM/silver nanoparticles has been done in the temperature range near the lower critical solution temperature (LCST). The methods of Dynamic Light Scattering, small angle X-ray scattering, and UV-VIS absorption spectroscopy have been used. The existence of single nanoparticles and aggregated nanoparticles located in a limited polymer macromolecular volume was established. The increase of the temperature leads to slight aggregation of the silver nanoparticles at the LCST transition. Single nanoparticles do not aggregate with the temperature increase. The thermally induced collapse of end-grafted poly(N-isopropylacrylamide) chains above the LCST do not affect significantly the size characteristics of silver nanoparticles incorporated into the polymer matrix.

Laser-Driven Aggregation in Dextran–Graft–PNIPAM/Silver Nanoparticles Hybrid Nanosystem: Plasmonic Effects

“…Correspondingly, the concentration of the single Ag NPs decreases, while the concentration of the aggregated ones increases. Let us note that the phenomenon of the temperature-induced aggregation of D-g-PNIPAM/AgNPs macromolecules was observed in our recent work [12]. We revealed in [12] that the direct heating at the temperatures higher than the LCST point does not change the size of D-g-PNIPAM/AgNPs aggregates.…”

“…The transformation of the extinction spectra of D-g-PNIPAM/AuNPs at a backward decrease in the laser intensity keeps its trend that proves the fact that the laser-induced formation of aggregates of D-g-PNIPAM/AuNPs macromolecules is irreversible. Thus, the present work together with our recent work [30] will show the irreversible character of laser-induced transformations (aggregation) in the Dg-PNIPAM/Ag(Au) NPs hybrids that differ from the temperature-induced transformations which usually are reversible [11][12][13]. Such difference has been rationalized to be a result of the action of attractive optical forces between the Ag NPs that promote the formation of aggregates and counteract their destruction.…”

“…The second one is a quite high relative concentration of single Ag NPs in the sample comparing to the sample. We have revealed in our recent work [12] that the temperature-induced LCST conformation transition in an aqueous solution of D-g-PNIPAM/AgNPs hybrid macromolecules leads to the aggregation of the macromolecules. Another important fact, revealed in our work [12], is that the direct heating at temperatures higher than the LCST point does not change the size of D-g-PNIPAM/AgNPs aggregates.…”

“…We have revealed in our recent work [12] that the temperature-induced LCST conformation transition in an aqueous solution of D-g-PNIPAM/AgNPs hybrid macromolecules leads to the aggregation of the macromolecules. Another important fact, revealed in our work [12], is that the direct heating at temperatures higher than the LCST point does not change the size of D-g-PNIPAM/AgNPs aggregates. Let us note that, as we show below, such behavior of the size of D-g-PNIPAM/AgNPs aggregates at temperatures higher that the LCST point differs essentially from the behavior at the laser illumination of this nanosystem.…”

“…Perhaps, the most famous among thermosensitive polymers is poly(N-isopropylacrylamide), (PNIPAM). This interest is due to the famous lower critical solution temperature (LCST) behavior, where heating an aqueous solution of PNIPAM above 32 ∘ C induces a phase transition [10][11][12][13]. Below this temperature, the PNIPAM macromolecules are hydrophilic, the polymer chains are swollen, and the polymer exists in the water soluble form.…”