Synthesis And Sensing Applications Of Stimuli-Responsive Polymeric Materials

Supramolecular assemblies fabricated from stimuli-responsive polymers offer great promises in the fields of materials science, information science, and life science, which render this interdisciplinary research subject as one of the key scientific issues in the21st century. The self-assembly behavior of responsive polymers has been thoroughly investigated in the past two decades. Currently, the exploitation of their useful and advanced functions have emerged to be a more relevant subject. In this dissertation, a series of well-defined stimuli-responsive polymers with varying chain topologies were synthesized via a combination of controlled radical polymerization and click chemistry. Their self-assembly behaviors in aqueous solution were thoughtfully investigated. On the basis of stimuli-responsive polymer materials, several kinds of colorimetric or fluorometric probes were fabricated. Moreover, a new general strategy to construct fluorogenic sensors was proposed in this dissertation. Specifically, this dissertation can be further categorized into the following five parts.1. Two well-defined nonlinear-shaped responsive polymers, ABC miktoarm star terpolymer and asymmetric centipede-shaped polymer brush, were synthesized via a combination of ATRP and click chemistry. All the intermediate and final products were characterized by Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatograph (GPC), and’H NMR. The ABC miktoarm, PEG(-b-PtBMA)-b-PNIPAM, star terpolymer consisting of hydrophilic poly(ethylene glycol), hydrophobic poly(tert-butyl methacrylate), and thermoresponsive poly(N-isopropylacrylamide) arms. In aqueous solution, it supramolecularly self-assembled into micelles consisting of PtBMA cores and hybrid PEG/PNIPAM coronas, which were characterized by dynamic and static laser light scattering (LLS), and transmission electron microscopy (TEM). Upon heating to above the phase transition temperature of PNIPAM in the corona, the initially formed micelles at lower temperatures further underwent structural rearrangement and fused into much larger aggregates, which were stabilized solely by PEG coronas. While, the asymmetric centipede-shaped polymer brush as the rod segment, PEO-b-[PGMA-g-(PDEA)(PMEO2MA)] possesses a pH-responsive PDEA graft and thermoresponsive PMEO2MA graft at each repeating unit. Atomic force microscopy (AFM) analysis revealed that PEO-b-[PGMA-g-(PDEA)(PMEO2MA)] Coil-Rod diblock unimer chains adopt a worm-like conformation in aqueous solution at pH4and room temperature. Possessing pH-responsive PDEA and thermo-responsive PMEO2MA grafts arranged in an asymmetric centipede manner within the rod segment, PEO-b-[PGMA-g-(PDEA)(PMEO2MA)] self-assembles into two types spherical aggregates in aqueous solution, depending on solution pH and temperatures. The multi-responsive switching between worm-like unimers and two types of micellar aggregates were characterized by temperature-dependent optical transmittance, dynamic laser light scattering (DLS), and AFM.2. Well-defined thermoresponsive water-soluble diblock copolymer and homopolymers precisely functionalized with controlled numbers of C60moieties at predetermined positions were synthesized via the combination of ATRP and click chemistry. Azide-containing polymer precursors including monoazide-terminated and α,α-diazide-terminated PNIPAM,N3-PNIPAM and (N3)2-PNIPAM, as well as PEG-b-PNIPAM with one azide moiety at the diblock junction, PEG(-N3)-b-PNIPAM, were synthesized via ATRP using specific azide-functionalized small molecule and polymeric initiators. On the other hand, the reaction of4-prop-2-ynyloxy-benzaldehyde with pristine C60in the presence of glycine afforded alkynyl-modified C60, alkynyl-C60. Subsequently, the click reaction of N3-PNIPAM,(N3)2-PNIPAM, and PEG(-N3)-b-PNIPAM led to the facile preparation of thermoresponsive diblock copolymer and homopolymers embedded with controlled numbers of C60at designed positions, including C60-PNIPAM,(C60)2-PNIPAM, and PEG(-C60)--PNIPAM. All the intermediate and final products were characterized by1H NMR, Fourier transform infrared spectroscopy (FT-IR), UV-Vis spectroscopy, thermogravimetric analysis (TGA), and gel permeation chromatograph (GPC) equipped with UV/RI dual detectors. C60-containing hybrid nanoparticles were then fabricated via supramolecular self-assembly of C60-PNIPAM,(C60)2-PNIPAM, and PEG(-C60)-b-PNIPAM in aqueous solution, which were characterized by dynamic and static laser light scattering (LLS) and transmission electron microscopy (TEM). These novel fullerenated polymers retain the thermoresponsiveness of PNIPAM-based precursors and self-assembled hybrid nanoparticles exhibit thermo-induced collapse/aggregation behavior due to the lower critical solution temperature (LCST) phase transition of PNIPAM chains.3. Thermoresponsive PNIPAM nanogel-based dual fluorescent sensors for temperature and Hg2+ions, and the effects of thermo-induced nanogel collapse on the detection sensitivity of Hg2+ions were fabricated in this dissertation. Near-monodisperse thermoresponsive nanogels were prepared via emulsion polymerization of NIPAM and a novel1,8-naphthalimide-based polarity-sensitive and Hg2+-reactive fluorescent monomer (NPTUA). At room temperature, PNIPAM nanogels labeled with a single type of naphthalimide-based dye (NPTUA) can act as ratiometric Hg2+probes at the nanomolar level. Upon heating above the phase transition temperature, the fluorescence intensity of NPTUA-labeled nanogels in the absence of Hg2+exhibit～3.4-fold increase due to that NPTUA moieties are now located in a more hydrophobic microenvironment. Moreover, it was observed that the detection sensitivity to Hg2+can be further improved above the nanogel phase transition temperature. At a nanogel concentration of0.05g/L and in the same Hg2+concentration range (0-3.0equiv.),～10fold and-57fold increase in fluorescence emission intensity ratio changes can be achieved at25and40℃, respectively. A convenient glucose colorimetric sensor based on polydiacetylene (PDA) liposomes with rapid, selective, and sensitive detection capabilities was also synthesized in the dissertation.4. Thermoresponsive luminescent diblock copolymer micelles-based multi-chromophore luminescent system exhibiting reversible three-state switching of fluorescence emission (green, yellow, orange, and red) via the external stimuli-modulation (e.g., pH, temperature, light irradiation) of two independent FRET processes was fabricated. The FRET system consists of one donor (NBDAE) and two acceptors (RhBAM and SPMA), which were precisely spatial organized in the polymeric micelles, and on/off fluorescence switching of the latter two can be respectively controlled by pH and light irradiation. The diblock copolymer micelles can serve as sensitive ratiometric fluorescent dual probes to pH and temperatures. Moreover, the detection sensitivities can be facilely adjusted via thermo-induced collapse of responsive micellar coronas due to closer proximity between FRET donors and acceptors. This novel type of multicolor luminescent polymeric assemblies augurs well for its practical applications in cell imaging, biosensing, and clinical diagnosis. We also constructed an adamantine and enzyme dual-responsive FRET system based on the host-guest interaction between adamantine and beta-cyclodextrin.5. We designed a novel type of water-soluble charge-generation polymers (termed as CGPs) possessing pendent carbamate-masked amine functionalities, which can undergo stimuli-triggered transition from the initially neutral state to the charged one in the presence of a specific analyte of interest. The charge-generation process was then coupled with the induced aggregation of Au nanoparticles (AuNPs) to design colorimetric probes of hydrogen peroxide (H2O2) and glucose by taking advantage of the unique distance-dependent surface plasmon absorption properties of AuNPs. Compared to previously reported AuNP-based colorimetric assay protocols, the current approach is novel due to the integration of H2O2-triggerable water-soluble CGP and it possesses combined advantages such as high sensitivity and selectivity, ease of fabrication, and cost-effective. On the other hand, since aggregation-induced emission (AIE) is a fluorogenic process of emerging importance in which non-fluorescent dyes in the molecularly dissolved state in solution can be rendered highly emissive via aggregation or absorbing onto bulky substrates, we then coupled the charge-generation process with the induced aggregation of AIE fluorogen (TPE-COOH4) to design reliable, cost-effective, quantitative, and sensitive aqueous-based fluorogenic probes. It is worthy of noting that a variety of analyte-specific carbamate-based amine-caging motifs could be incorporated into the CGP design. We envisage that these proof-of-concept examples can be further generalized to the design of more sophisticated sensing systems.