pH-Induced Vesicle-to-Micelle Transition in Amphiphilic Diblock Copolymer: Investigation by Energy Transfer between <i>in Situ</i> Formed Polymer Embedded Gold Nanoparticles and Fluorescent Dye

The ability to regulate the formation of nanostructures through self-assembly of amphiphilic block copolymers is of immense significance in the field of biology and medicine. In this work, a new block copolymer synthesized by using reversible addition–fragmentation chain transfer (RAFT) polymerization technique from poly­(ethylene glycol) monomethyl ether acrylate (PEGMA) and Boc-l-tryptophan acryloyloxyethyl ester (Boc-l-trp-HEA) was found to spontaneously form pH-responsive water-soluble nanostructures after removal of the Boc group. While polymer vesicles or polymerosomes were formed at physiological pH, the micelles were formed at acidic pH (< 5.2), and this facilitated a pH-induced reversible vesicle-to-micelle transition. Formation of these nanostructures was confirmed by different characterization techniques, viz. transmission electron microscopy, dynamic light scattering, and steady-state fluorescence measurements. Further, these vesicles were successfully utilized to reduce HAuCl₄ and stabilize the resulting gold nanoparticles (AuNPs). These AuNPs, confined within the hydrophobic shell of the vesicles, could participate in energy transfer process with fluorescent dye molecules encapsulated in the core of the vesicles, thus forming a nanometal surface energy transfer (NSET) pair. Subsequently, following the efficiency of energy transfer between this pair, it was possible to monitor the process of transition from vesicles to micelles. Thus, in this work, we have successfully demonstrated that NSET can be used to follow the transition between nanostructures formed by amphiphilic block copolymers

Water-soluble polymeric chemosensor for detection of Cu²⁺ ions with high selectivity and sensitivity

<p>Development of water-soluble chemosensors that are selective and sensitive to Cu²⁺ ions is of tremendous importance owing to their potential applications in biological systems. In the present work, we report the synthesis of a new water-soluble polymer containing pendant rhodamine units that are capable of highly selective and sensitive detection of Cu²⁺ ions in aqueous medium. Poly(2-pyrrolidinemethyl acrylate) was prepared using RAFT polymerization technique. The pyrrolidine nitrogen group in the polymer was subjected to Aza-Michael type addition with ethyl acrylate that was followed by covalent linking of rhodamine units to the polymer. This polymer was completely water-soluble and found to be capable of sensing Cu²⁺ ions in aqueous medium. Cu²⁺-induced opening of the spirolactam ring of the rhodamine units resulted in rapid and easily noticeable colour change, thus enabling a highly selective detection of Cu²⁺ in μmol range. The ability of these polymeric systems to detect Cu²⁺ ions in complete aqueous media has more importance than use of organic solvents to solubilize the polymer as reported previously, and thus opened a new window for application of these systems in the detection of copper ions in biological systems.</p

Thermoregulated Formation and Disintegration of Cationic Block Copolymer Vesicles: Fluorescence Resonance Energy Transfer Study

Formation and disintegration of self-assembled nanostructures in response to external stimuli are important phenomena that have been widely explored for a variety of biomedical applications. In this contribution, we report the thermally triggered assembly of block copolymer molecules in aqueous solution to form vesicles (polymersomes) and their disassembly on reduction of temperature. A new thermoresponsive diblock copolymer of poly­(<i>N</i>-isopropylacrylamide) poly­((3-methacrylamidopropyl)­trimethylammonium chloride) (PNIPA-<i>b</i>-PMAPTAC) was synthesized by reversible addition–fragmentation chain transfer technique. The solution properties and self-assembling behavior of the block copolymer molecules were studied by turbidimetry, temperature-dependent proton nuclear magnetic resonance, fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. Fluorescence resonance energy transfer studies between coumarin-153 (C-153, donor) and rhodamine 6G (R6G, acceptor) have been performed by steady-state and picosecond-resolved fluorescence spectroscopy to probe the structural and dynamic heterogeneity of the vesicles. The occurrence of efficient energy transfer was evident from the shortening of donor lifetime in the presence of the acceptor. The capability of the vesicles to encapsulate both hydrophobic and hydrophilic molecules and release them in response to decrease in temperature makes them potentially useful as drug delivery vehicles