Rylene bisimide-based nanoparticles with cross-linked core and thermoresponsive shell using poly(vinyl amine)-based block copolymers

Threonine-Based Stimuli-Responsive Nanoparticles with Aggregation-Induced Emission-Type Fixed Cores for Detection of Amines in Aqueous Solutions

Stimuli-responsive polymeric nanoparticles (NPs) exhibit reversible changes in the dispersion or aggregation state in response to external stimuli. In this context, we designed and synthesized core-shell NPs with threonine-containing weak polyelectrolyte shells and fluorescent cross-linked cores, which are applicable for the detection of pH changes and amine compounds in aqueous solution. Stable and uniform NP(dTh) and NP(Fl), consisting of fluorescent symmetric diphenyl dithiophene (dTh) and diphenyl fluorene (Fl) cross-linked cores, were prepared by site-selective Suzuki coupling reactions in self-assembled block copolymer. NP(Fl) with the Fl unit in the core showed a high fluorescence intensity in different solvents, which is regarded as an aggregation-induced emission-type NP showing strong emission in aggregated states in the cross-linked core. Unimodal NPs were observed in water at different pH values, and the diameter of NP(Fl) changed from 122 (pH = 2) to 220 nm (pH = 11). Furthermore, pH-dependent changes of the fluorescence peak positions and intensities were detected, which may be due to the core aggregation derived from the deprotonation of the threonine-based shell fragment. Specific interactions between the threonine-based shell of NP(Fl) and amine compounds (triethylamine and p-phenylenediamine) resulted in fluorescence quenching, suggesting the feasibility of fluorescent amine detection.

Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers

Two series of poly(vinyl amine) (PVAm)-based block copolymers with zwitterionic and thermoresponsive segments were synthesized by the reversible addition-fragmentation chain transfer polymerization. A mixture of the two copolymers, poly(N-acryloyl-l-lysine) (PALysOH) and poly(N-isopropylacrylamide) (PNIPAM), which have the same cationic PVAm chain but different shell-forming segments, were used to prepare mixed polyplex micelles with DNA. Both PVAm-b-PALysOH and PVAm-b-PNIPAM showed low cytotoxicity, with characteristic assembled structures and stimuli-responsive properties. The cationic PVAm segment in both block copolymers showed site-specific interactions with DNA, which were evaluated by dynamic light scattering, zeta potential, circular dichroism, agarose gel electrophoresis, atomic force microscopy, and transmission electron microscopy measurements. The PVAm-b-PNIPAM/DNA polyplexes showed the characteristic temperature-induced formation of assembled structures in which the polyplex size, surface charge, chiroptical property of DNA, and polymer-DNA binding were governed by the nitrogen/phosphate (N/P) ratio. The DNA binding strength and colloidal stability of the PVAm-b-PALysOH/DNA polyplexes could be tuned by introducing an appropriate amount of zwitterionic PALysOH functionality, while maintaining the polyplex size, surface charge, and chiroptical property, regardless of the N/P ratio. The mixed polyplex micelles showed temperature-induced stability originating from the hydrophobic (dehydrated) PNIPAM chains upon heating, and remarkable stability under salty conditions owing to the presence of the zwitterionic PALysOH chain on the polyplex surface.

Synthesis, Assembled Structures, and DNA Complexation of Thermoresponsive Lysine-Based Zwitterionic and Cationic Block Copolymers

A series of anionic, zwitterionic, and cationic lysine-based block copolymers with a thermoresponsive segment were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization of N-acryloyl- N-carbobenzoxy-l-lysine [A-Lys(Cbz)-OH], which contains a carboxylic acid and a protected amine-functionality in the monomer unit. Carboxylic acid-containing homopolymers, poly(A-Lys(Cbz)-OH), with predetermined molecular weights with relatively low polydispersities were initially synthesized by RAFT polymerization of A-Lys(Cbz)-OH. The chain extension of the dithiocarbamate-terminated poly(A-Lys(Cbz)-OH) to N-isopropylacrylamide (NIPAM) via the RAFT process and subsequent deprotection afforded the zwitterionic block copolymer composed of thermoresponsive poly(NIPAM) and poly(A-Lys-OH), which exhibited switchability among the zwitterionic, anionic, and cationic states by pH change. The assembled structures and thermoresponsive and chiroptical properties of these block copolymers were evaluated by dynamic light scattering, circular dichroism, and turbidity measurements. Finally, the cationic block copolymer, poly(A-Lys-OMe)- b-poly(NIPAM), was obtained by the methylation of the carboxylic acid group in the zwitterionic poly(A-Lys-OH) segment. Selective interactions of DNA with the cationic poly(A-Lys-OMe) segment in the lysine-based block copolymer were further evaluated by agarose gel electrophoresis and atomic force microscopy measurements, which revealed characteristic assembled structures and temperature-responsive properties of the polyplexes.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.