Author: Yao, Yuan
Title: Synthesis and characterization of novel polyethylenimine-based photoluminescent nanoparticles for bioimaging
Advisors: Li, Pei (ABCT)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Water-soluble polymers
Nanostructured materials
Department: Department of Applied Biology and Chemical Technology
Pages: xviii, 172 pages : color illustrations
Language: English
Abstract: In the past two decades, numerous types of nanostructured photoluminescent nanomaterials have been developed and demonstrated their significant advantages over the conventional organic dyes. Among these emerging nanomaterials, carbon dots and nonconjugated photoluminescent polymer dots have received increasing attention in recent years due to their excellent optical properties such as high quantum yield and excitation-dependent emission, low cytotoxicity and ease of preparation by solution-based synthesis. Polyethyleneimine (PEI) is a water-soluble polymer containing various amino groups. It is regarded as one of the most promising polycations for gene delivery. In recent years, increasing research efforts have been made to enhance intrinsic photoluminescent properties of PEI-based nanomaterials for advanced biological applications such as cell imaging and image-guided gene delivery. This thesis describes our research on the development of novel PEI containing photoluminescent particles based on carbon dots and nonconjugated polymer systems. These particles possess excellent water dispersibility, outstanding optical performance, and feasibility for scale-up production. The photoluminescent mechanisms were studied through systematic investigation of various reaction parameters and careful characterization of their properties. The potential applications of the photoluminescent PEI-based nanoparticles for cell imaging and intracellular tracking of gene delivery have also been explored. The first chapter reviews emerging photoluminescent nanomaterials with an emphasis on the synthesis and properties of carbon dots (CDs) and photoluminescent nonconjugated polymer dots. The photoluminescent mechanisms behind the CDs and nonconjugated polymer dots are comprehensively discussed. The drawbacks, controversies, and challenges of CDs and nonconjugated polymer dots are also presented. Chapter two describes the design rationale of our synthetic approaches towards multiple CDs crosslinked PEI nanogels and photoluminescent nonconjugated nanoparticles derived from n-butyl acrylate modified PEI (PEI-nBA). The specific objectives of the thesis are also presented.
Chapter three reports ultra-bright nanogels that consist of multiple carbon dots homogeneously distributed within a polyethyleneimine network in the aqueous system. The nanogels are firstly prepared through the synthesis of methyl methacrylate swollen amphiphilic polyethyleneimine-graft-poly (methyl methacrylate) (PEI-g-PMMA) nanoparticles via a tert-butyl hydroperoxide-induced graft polymerization and in-situ self-assembly into nanoparticles The preformed nanoparticles then underwent a hydrothermal treatment at 150 °C for an appropriate duration. The carbon dots generated from methyl methacrylate (as carbon source) were able to crosslink the PEI through amidation reaction, giving a multiple-carbon dots-crosslinked PEI nanogel (mCDs@PEI). The nanogels have hydrodynamic diameters ranging from 88 to 106 nm with narrow size distribution. They possess highly positive surface charges (ξ-potential > +35 mV) and are very stable in water. This type of water-dispersible mCDs@PEI nanogels exhibit several distinctive fluorescence properties: ultra-bright nanogel with high quantum yield to up to 66%, contributing from the synergistic effect of N-doped carbon dots and immobilized PEI network; excitation-dependent emission, stable fluorescence intensity in a wide pH range, and excellent photobleaching resistance. These ultra-bright mCDs@PEI nanogels enabled us to study intracellular trafficking of DNA/nanocarrier complexes in HeLa cells with a confocal microscopy, leading to an insightful understanding of the endosomal escape step in gene transfection of nonviral systems. Chapter four exploits a simple, scalable strategy to synthesize water-dispersible nonconjugated photoluminescent polymer dots through a Michael addition reaction between PEI and hydrophobic n-butyl acrylate (nBA), followed by an in-situ self-assembly process to form nBA modified PEI nanoparticles. The nonconjugated photoluminescent PEI-nBA polymer dots have an average particle size around 25 nm with positive charges in aqueous (ξ-potential > +20 mV). This type of nonconjugated polymer dots exhibit not only ultra-bright photoluminescence with high quantum yield to up to 44% as compared to the weak intrinsic fluorescence of PEI (QY < 1%), they also possess multicolor photoluminescence behaviors when varying excitation wavelengths. Furthermore, photoluminescent mechanism of the PEI-nBA polymer dots has been systematically investigated. Results indicated that the enhanced photoluminescent properties of the PEI-nBA nanoparticles is a result of the synergistic effect that includes aggregation of the carbonyl and amine groups to form nanoparticles with the new surface state, and the restriction of the movement of PEI chains. Finally, the potential application of the PEI-nBA nanoparticles for multicolor cell imaging has been successfully achieved. The final chapter summarizes the main conclusions and major contributions of this work. It also includes suggestions and recommendations for future studies.
Rights: All rights reserved
Access: open access

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