Responsive Polymeric Assemblies-Based Functional Materials For Biomedical And Detection Applications

Supramolecular assemblies fabricated from stimuli-responsive polymers have attracted considerable interests in the past decades due to their promising applications in diverse fields, such as catalysis, material preparation, and biomedicine, which render this interdisciplinary research subject as one of the promising scientific issues in the 21st century. The field of responsive polymers has nowadays been thoroughly and systematically explored, which evolved well beyond the demonstration of novel and interesting properties. Currently, the development 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, have emerged to be a more relevant subject. In this case, we reported on the facile fabrication of numerous well-defined specific functionalized polymers with varying chemical architectures in the combination of controlled/living radical polymerization, click chemistry and bioconjugation protocols, and investigated their self-assembly behavior in aqueous solution. Moreover, the following combination of stimuli-responsive polymers with inorganic nanoparticles and chemical sensors demonstrated that this kind of intelligent material can be utilized as promising functional nanocarriers for controlled drug delivery and ratiometric chemical sensing. Specifically, the dissertation includes the following four parts:1. We report a novel approach for the synthesis of amphiphilic and thermoresponsive tadpole-shaped linear-cyclic diblock copolymer, (c-PNIPAM)-b-PCL, consisting of hydrophobic linear poly(ε-caprolactone) (PCL) and thermoresponsive macrocyclic PNIPAM via the ring-opening polymerization (ROP) of CL monomer directly initiating from the cyclic PNIPAM precursor bearing a single hydroxyl functionality. We then investigated the self-assembly of (c-PNIPAM)-b-PCL in aqueous solution and thermal phase transition of c-PNIPAM corona within the micellar nanoparticles, and compared to those of the linear diblock copolymer, (l-PNIPAM)-b-PCL, with comparable molecular weight and composition. The temperature-dependent release profiles from drug-loaded micelles of (c-PNIPAM)-b-PCL and (l-PNIPAM)-b-PCL were also explored in detail.2. We report on the fabrication of thermoresponsive biohybrid double hydrophilic block copolymer (DHBC) via cofactor reconstitution approach. PNIPAM bearing a porphyrin moiety at the chain terminal, PPIXZn-PNIPAM, was synthesized via the combination of atom transfer radical polymerization (ATRP) and click chemistry. The subsequent cofactor reconstitution process between apomyoglobin and PPIXZn-PNIPAM afforded well-defined myoglobin-b-PNIPAM protein- polymer bioconjugates. Behaving as typical responsive DHBCs, the obtained myoglobin-b-PNIPAM biohybrid diblock copolymer exhibits thermo-induced aggregation behavior in aqueous solution due to the presence of thermoresponsive PNIPAM block. Moreover, we also reported the facilely fabrication of well-defined protein-polymer bioconjugates with different chain architecture, and investigated their binding steric crowding effects. First, a series of biotinylated homopolymers and diblock copolymers with varying architecture and molecular weight were synthesized via a combination of ATRP and click chemistry. The locations of biotin in polymer chains were precisely varied: at the chain end or in the middle of homopolymer, at the chain end or the junction point of diblock copolymer. Taking advantage of the special interaction between avidin and biotin, we facilely fabricated star polymers, star block copolymers, and heteroarm star polymers. However, as the hydrophilic biotinylated polymers dissolved molecularly in aqueous media and existed as extended random coil, the binding course between biotin and avidin would experience steric crowding effect in a certain extent. The effects of the DP of biotinylated polymer, and the location of biotin bound to the polymer chain on the conjugation efficiency were investigated in detail via standard avidin/HABA assays.3. We report on the fabrication of fluorescent pH-sensing organic/inorganic hybrid mesoporous silica nanoparticles (MSN) capable of tunable redox-responsive release of embedded guest molecules. Random copolymers composed of N-acryloxysuccinimide (NAS), oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA), and 1,8-naphthalimide-based fluorescent pH-sensing monomer (NaphMA) were anchored at the surface of MSN via surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization. The obtained hybrid MSN exhibits excellent water dispersibility and can act as sensitive fluorescent pH probe in the range of pH 4-8 due to the presence of NaphMA moieties. After loading with model drug molecules, rhodamine B (RhB), and crosslinking the polymer brushes with cystamine, the redox-responsive release of encapsulated guest molecules from organic/inorganic MSN can be facilely tuned by varying the concentrations of externally added dithiothreitol (DTT). In another case, we report on the fabrication of pH-disintegrable polyelectrolyte multilayer-coated MSN capable of triggered co-release of cisplatin and model drug molecules. The outer polyelectrolyte multilayer was assembled from permanently cationic polyelectrolyte, poly(allyl amine hydrochloride) (PAH), and negatively charged polyelectrolyte composed of N,N-dimethylacrylamide (DMA) and 3,4,5,6-tetrahydrophthalic anhydride-functionalized N-(3-aminopropyl)methacrylamide (TPAMA), which exhibits pH-induced charge conversion characteristics. Model drug molecule RhB was loaded into the interior mesopores of amine-functionalized MSN at first, this was followed by the layer-by-layer (LBL) deposition of P(DMA-co-TPAMA) and PAH at the outer surface of MSNs to effectively block the mesopore entrances. For cisplatin loading, it was mixed with the aqueous solution of P(DMA-co-TPAMA) and embedded into the polyelectrolyte multilayer in the LBL assembly process. The structural stability of TPAMA moieties within the negatively charged pH-triggerable charge conversion polymer, P(DMA-co-TPAMA) is highly pH-dependent, i.e, stable under neutral media and hydrolyzed into positively charged N-(3-aminopropyl)methacrylamide (APMA) moieties in weakly acidic media. Thus, the subtle alteration of solution pH from 7.4 to ⁵-6 can lead to the disintegration outer polyelectrolyte multilayers, accompanied with the co-release of cisplatin and RhB.4. We report on the fabrication of core cross-linked (CCL) micelles possessing thermoresponsive cores and their application as sensitive and selective ratiometric Hg²⁺ probes with thermo-tunable detection efficiency. Well-defined DHBC bearing naphthalimide-based Hg²⁺-reactive moieties (NUMA, 4), PEO-b- P(NIPAM-co-NAS-co-NUMA), was synthesized via RAFT polymerization, where PEO represents poly(ethylene oxide). The obtained DHBC can self-assemble into core-shell nanoparticles possessing thermoresponsive PNIPAM cores. After core cross-linking of the micellar nanoparticles formed at elevated temperatures, structurally stable CCL micelles with well-solvated PEO coronas and thermoresponsive cores embedded with Hg²⁺-reactive NUMA moieties were obtained. Upon Hg²⁺ addition, the aqueous dispersion of CCL micelles exhibit a colorimetric transition from yellowish to colorless and a fluorometric emission transition from green to bright blue. The Hg²⁺-sensing capability of PEO-b-P(NIPAM-co-NAS-co-NUMA) unimers and CCL micelles at temperatures below and above the critical phase transition temperature was then determined and compared in detail. The fluorescence imaging assay of Hg²⁺ ions in living cells was also investigated. In the last section, we reported on the synthesis of well-defined thermoresponsive polymers respectively labeled with fluorescence resonance energy transfer (FRET) pairs at chain middle and terminals, which can act as dual ratiometric fluorescent probes for pH and temperatures. Starting from difunctional initiator containing 7-nitro-2,1,3-benzoxadiazole (NBD) moiety, the ATRP process of OEGMA and di(ethylene glycol) monomethyl ether methacrylate (DEGMA), and the subsequent terminal group functionalization with RhB-ethylenediamine derivative afforded NBD-P(OEGMA-co-DEGMA)-RhB₂, which were labeled with FRET donor (NBD) and acceptor moieties (RhB) at the chain middle and terminals of the thermoresponsive polymer. The fluorescence emission of terminal RhB functionalities is highly pH-dependent, i.e, non-fluorescent in neutral or alkaline media (spirolactam form) and highly fluorescent in acidic media (ring-opened acyclic form), thus the off/on switching of FRET process can be facilely modulated by solution pH. Moreover, at acidic pH and highly dilute conditions, the thermo-induced chain collapse and extension of NBD-P(OEGMA-co- DEGMA)-RhB₂ can effectively modulate the spatial distance between FRET donor and acceptor moieties, leading to prominent changes in fluorescence intensity ratios. The incorporation of one FRET donor and two pH-switchable acceptors at the chain middle and terminals of thermoresponsive polymers allows for the effective off/on switching and the modulation of efficiency of FRET processes by dually playing with solution pH and temperatures.