Fabrication Of Stimuli-Responsive Supramolecular Assemblies And Their Chemosensing Applications

The past two decades have evidenced a tremendous growth in the field of responsive polymers, which can exhibit reversible or irreversible changes in physical properties and/or chemical structures to an external stimulus such as pH, temperature, ionic strength, light irradiation, mechanical forces, electric and magnetic fields, specific analytes, external additives (ions, bioactive molecules, etc.), or a combination of them. Responsive polymers can exist in the form of solutions, gels, self-assembled nanoparticles,(multilayer) films, and bulk solids. The field of responsive polymers has nowadays evolved well beyond the demonstration of novel and interesting properties. Currently, the exploitation of useful and advanced functions, e.g., drug or gene carriers with triggered release properties, catalysis, detection and imaging, environmentally adaptive coatings, and self-healing materials, has emerged to be a more relevant subject. In this dissertation, we focus on the synthesis and self-assembly of responsive polymers, as well as the sensing and imaging applications. Specifically, the dissertation can be further categorized into the following four parts.1. A robust approach to the size-selective and template-free synthesis of asymmetrically functionalized ultrasmall (<4nm) gold nanoparticles (AuNPs) stably anchored with a single amphiphilic triblock copolymer chain per nanoparticle was reported. Directed nanoparticle self-assembly in aqueous solution can be facilely accomplished to afford organic/inorganic hybrid micelles, vesicles, rods, and large compound micelles by taking advantage of the rich microphase separation behavior of as-synthesized gold nanoparticle hybrid amphiphilic triblock copolymers, PEO-AuNP-PS, which act as the polymer-metal-polymer analogue of conventional amphiphilic triblocks. Factors affecting the size-selective fabrication and self-assembling characteristics, and the time-dependent morphological evolution of nanoparticle assemblies were thoroughly explored.2.(1)We report on a novel type of responsive double hydrophilic block copolymer (DHBC)-based multifunctional chemosensors to Hg2+ions, pH, and temperatures and investigate the effects of thermo-induced micellization on the detection sensitivity. Well-defined DHBCs bearing rhodamine B-based Hg2+-reactive moieties (RhBHA) in the thermo-responsive block, poly(ethylene oxide)-b-poly(N-isopropylacrylamide-co-RhBHA)(PEO-b-P(NIPAM-co-RhBHA)), were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Nonfluorescent RhBHA moieties are subjected to selective ring-opening reaction upon addition of Hg2+ions or lowering solution pH, producing highly fluorescent acyclic species. Thus, at room temperature PEO-b-P(NIPAM-co-RhBHA) DHBCs can serve as water-soluble multifunctional and efficient fluorescent chemosensors to Hg2+ions and pH. Upon heating above the lower critical solution temperature (-36℃) of the PNIPAM block, they self-assemble into micelles possessing P(NIPAM-co-RhBHA) cores and well-solvated PEO coronas, which were fully characterized by dynamic and static laser light scattering. It was found that the detection sensitivity to Hg2+ions and pH could be dramatically improved at elevated temperatures due to fluorescence enhancement of RhBHA residues in the acyclic form, which were embedded within hydrophobic cores of thermo-induced micellar aggregates.(2) An amphiphilic thermo-responsive diblock copolymer micelle-based multifunctional ratiometric fluorescent chemosensors for metal ions (Hg2+and Cu2+), pH, and temperatures was fabricated. A fluorescence resonance energy transfer (FRET) pair consisting of4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE) donor and rhodamine B-based potential acceptor (RhBHA) in the spirolactam form with pH and Hg2+(Cu2+)-reactive characteristics were respectively copolymerized into the hydrophobic PS and thermoresponsive PNIPAM block of P(St-co-NBDAE)-b-P(NIPAM-co-RhBHA) amphiphilic diblock copolymers, where PS and PNIPAM represent polystyrene and poly(N-isopropylacrylamide). In aqueous solution, the FRET pair-labeled diblock copolymer self-assembles into nanosized micelles with NBDAE moieties located in the micellar cores and RhBHA in the thermoresponsive coronas. Because of that Hg2+ions and acidic pH can induce the transformation of RhBHA from the non fluorescent spirolactam form to highly fluorescent acyclic form, and the FRET process between NBDAE and RhBHA moieties, located respectively within micellar cores and coronas, can be efficiently switched on. Thus, these nanosized micelles can serve as excellent ratiometric fluorescent probes for Hg2+ions and pH, accompanied by fluorometric transition from green to orange and colorimetric change from almost colorless to pink. At a micellar concentration of0.05g/L and25℃, the detection limit of Hg2+ions can be down to-14.8ppb. On the other hand, Cu2+ions can quantitatively induce the ring-opening of RhBHA moieties and afford non fluorescent residues, which can efficiently quench the NBDAE emission. On the basis of the relative changes in NBDAE emission intensities, the Cu2+detection limit can be down to-4.3ppb. Most importantly, the spatial distance of the FRET pair can be facilely tuned via thermo-induced collapse of PNIPAM micellar coronas, which dramatically increase the FRET efficiency and enhance the pH detection sensitivity.(3) A new type of polymeric fluorescent Hg2+probe covering a broad effective concentration range from nanomolar to micromolar levels and exhibiting considerably enhanced detection selectivity was prepared. Two amphiphilic diblock copolymers colabeled with Hg2+-reactive caged dye (RhBHA) and Hg2+-catalyzed caged fluorophore (HCMA) in the hydrophilic segments, PS-b-P(DMA-co-HCMA) and PS-b-P(DMA-co-RhBHA), were synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization, where PS, DMA, HCMA, and RhBHA are polystyrene, N,N-dimethylacrylamide, hydrazone-caged coumarin, and rhodamine B (RhB) derivatives, respectively. The two amphiphilic diblock copolymers can spontaneously self-assemble into mixed micelles in aqueous solution possessing hydrophobic PS cores and HCMA and RhBHA moieties colabeled hydrophilic PDMA coronas. Fluorescence emissions of caged RhBHA and HCMA moieties can effectively turn on in the presence of low and high Hg2+concentrations via Hg2+-induced ring-opening reaction and Hg2+-catalyzed hydrolysis mechanisms, respectively. The drastically different, but self-complementary reaction kinetics and optimum working concentration ranges of RhBHA and HCMA moieties endow the sensing system with high selectivity and broad sensing concentration range (from nanomolar to micromolar). In addition, the Hg2+-sensing platform can be further employed for the fl uorescent ratiometric detection of Cu2+ion via its selective quenching of the emission of acyclic RhBHA moieties. This work presents a new example of ensembling two partially selective chemical reaction-based fluorometric sensing motifs to achieve enhanced metal ion sensing selectivity and broadened working concentration ranges, which can be further generalized for the construction of other highly selective and broad dynamic range sensing systems.3.(1) We report on a novel colorimetric and fluorometric chemosensor for fluoride ions based on4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE)-labeled polymers. Upon gradual addition of fluoride ions (F-), the green fluorescence emission of NBDAE moieties can be dramatically quenched, accompanied with the distinct colorimetric transition from green to yellow. NBDAE moieties are capable of selectively recognizing F-ions via hydrogen-bonding (H-bonds) interactions at low F" concentration and subjected to further deprotonation process at high F" concentration. NBDAE-labeled polymers in organic solvents possess high selectivity and fluorescence "turn-off" characteristics toward the sensing of F" ions with the detection limit down to～0.8μM.(2) A novel type of ratiometric fluorescent polymeric probes for fluoride ions (F) based on self-assembled micellar nanoparticles of P(MMA-co-NBDAE)-b-PF-b-P(MMA-co-NBDAE) coil-rod-coil triblock copolymer, where MMA, NBDAE, and PF are methyl methacrylate,4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole, and polyfluorene, respectively. Blue-emitting conjugated PF block and green-emitting NBDAE moieties with F-turn-off characteristics within the PMMA block serve as fluorescence resonance energy transfer (FRET) donors and switchable acceptors, respectively. For coil-rod-coil triblock copolymer in a good solvent such as THF, the blue emission of PF block dominates due to unimolecularly dissolved state associated with ineffective FRET process. The addition of F-ions only leads to～2.92-fold decrease of fluorescence intensity ratio,I515/I417, of characteristic NBDAE and PF emission bands. In acetone, the triblock copolymer spontaneously self-assembles into micelles possessing PF cores and NBDAE-labeled PMMA coronas. In the absence of F-ions, effective FRET processes between micellar cores and coronas occurs, resulting in prominently enhanced NBDAE emission. Upon addition of F-ions, the quenching of NBDAE emission bands leads to～8.75-fold decrease in the emission intensity ratio, I515/I417, which is also accompanied by naked eye-discernible fluorometric transition from cyan to blue emissions and colorimetric transition from green to yellowish. At a micellar concentration of0.1g/L in acetone at25℃, the detection limit of F-ions can be down to-4.78μM (-0.09ppm). This work presents a new example of polymeric micelles-based optical F-probes and manifests that, upon proper structural design and optimization of spatial distribution of FRET donors and acceptors, self-assembled micelles of coil-rod-coil triblock copolymers serve as better ratiometric fluorescent F-ion sensors possessing visual detection capability, as compared to that of molecularly dissolved chains.