Author: Wong, Chun Him
Title: Novel water dispersible polymeric nanofluorophore : synthesis, characterization and potential application
Advisors: Li, Pei (ABCT)
Degree: Ph.D.
Year: 2015
Subject: Water -- Purification -- Technological innovations.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Biology and Chemical Technology
Pages: xxxiv, 258 pages : illustrations (some color)
Language: English
Abstract: This thesis aims to develop novel water dispersible photoluminescent polymer dots (PPD) in aqueous system without loading any known emitting moiety, and explore its potential application in chromium(VI) detection. Specifically, two synthetic strategies have been employed to prepare inherent PPDs that consist of hydrophobic grafted chains or molecules as cores and hydrophilic polyethylenimine (PEI) as shells. In the first approach, namely core-removal method, amphiphilic poly(methyl methacrylate)/polyethylenimine (PMMA/PEI) particles with well-defined core-shell nanostrucutre were first synthesized through a graft copolymerization of MMA from PEI, followed by removal of the PMMA homopolymers in a dichloromethane/H2O Obinary solvent system. After complete evaporation of dichloromethane (DCM) at 35°C, and filtration of insoluble homopolymer, water dispersible PEI-g-PMMA PPDs were isolated in an aqueous solution. In the second approach, the PPDs were generated through a hydrophobic modification of the PEI, followed by in-situ self-assembly of the modified PEI into nanogel in water. The chemical modification was based on a Michael Addition of aliphatic amines to α,β-unsaturated carbonyl compounds. The resulting PPDs contain a plenty of surface active amino groups, thus possessing water dispersibility in the absence of stabilizing agent. In addition, the PPDs can express stronger blue fluorescence emission than the native branched PEI in water due to its enhanced electron transfer efficiency in a confined space. Thus, these kinds of novel PPDs are an alternative and promising water dispersible nanofluorophores which may find diverse potential applications. Chapter one briefly introduces some general but important knowledge about fluorescence spectroscopy. For instance, the development and improvement of biotechnologies based on fluorescence spectroscopy. Some background information about fluorescence technology are described including the principle of fluorescence, typical structure of fluorescence spectra and classification of current fluorescent materials. This chapter also reviews synthetic approaches to prepare fluorescent materials in aqueous system. Drawbacks of these methods are also pointed out. A new concept of water dispersible nanofluorophore, namely photoluminescent polymer dots (PPDs), is proposed as a novel alternative photoluminescent material without encapsulating any traditional fluorescent dyes. The background of the PPDs including their synthesis, characterization and potential application in chromium(VI) detection are also discussed.
The second chapter presents the rationale of using highly branched PEI for the formation of photoluminescent polymer dots. Specific objectives of the research are discussed. Chapter three provides detailed research methodologies of the core-removal method in generation of PEI-g-PMMA PPDs. It includes synthesis and characterization using various analytical techniques such as dynamic light scattering for particle size and surface charge measurements; fourier transform infrared spectroscopy (FTIR); proton nuclear magnetic resonance (1H-NMR); field emission scanning electron microscopy (FESEM); transmission electron microscopy (TEM); UV-Vis spectroscopy and fluorospectroscopy. Results showed that the quantum yield of PEI-g-PMMA PPDs generated at 35°C was 0.093 which is 5.5 folds higher than that of native PEI (The relative quantum yield of the native PEI at pH 7 in water was 0.017). Chapter four describes the effect studies in order to enhance the quantum yield of PEI-g-PMMA PPDs through core-removal method. The effect studies include initiator concentration for the formation of PMMA/PEI core-shell particles, post-treating temperature, structure of hydrophobic monomer and solution pH. Results suggest that the highest degree of fluorescence of the PEI-g-PMMA PPDs can be obtained with the following conditions: Particles are synthesized with 0.125 mM initiator concentration; core-removal with DCM, followed by complete evaporation of DCM at 35°C at pH 7. Based on the results of these effect studies, fluorescence mechanism of PEI-g-PMMA PPDs is proposed. Chapter five provides detailed research methodologies on hydrophobic modification of PEI through a Michael Addition reaction to generate PPDs. Results showed that the quantum yield of n-butyl acrylate (nBA)-modified PPDs was as high as 0.115 which is 8 fold higher than that of native PEI (The relative quantum yield of native PEI at pH 10 in water was 0.015). Chapter six describes the studies of various reaction parameters on generation of water dispersible PPDs through different hydrophobic modification of PEI. These parameters include degree of hydrophobic modification, effect of atmosphere gas during the synthesis, solution pH and structure of hydrophobic monomers. Results suggest that nBA-modified PPDs with 48 % hydrophobic modification on PEI generated at pH 7 and under air exhibit the strongest degree of fluorescence. The quantum yield reached 0.194 under such conditions. Based on these effect studies, the fluorescence mechanism of hydrophobically modified PPDs is proposed. Chapter seven describes potential application using PEI-g-PMMA PPDs for chromium(VI) detection. Relationships of photoluminescence quenching efficiency of the PEI-g-PMMA PPDs with different heavy metal ions and chromium(VI) ion concentration were systematically studied. The detection limit of chromium ions with these PPDs was as low as 5 μM. Chapter eight summaries the key results obtained in this thesis and provide recommendation for further studies. Significance and implications of this work will also be discussed.
Rights: All rights reserved
Access: open access

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