Synthesis And Characterication Of Water-soluble Conjugated Polymer Brush For Sensor Application

Recently, conjugated polyelectrolytes (CPEs) have been widely used in chemical and biological sensing because of their intriguing optoelectronic and biocompatible properties. These materials haveÏ€-electron delocalized conjugated backbones, featuring molecule-wire like light-harvesting structures for rapid intrachain and interchain excitation migration via electron transfer (ET) or fluorescence resonance energy transfer (FRET). Thereby, CPEs can profoundly change their optical properties upon minor perturbation from external environment, ultimately leading to amplified fluorescence signals as compared to that for small molecule fluorophores. To promote the level of technologies for efficient, convenient and precise detection of biomolecules such as DNA and proteins, major efforts during past decade have been devoted to the development of CPEs with different linear structure such as poly(p-phenyleneethynylene) (PPE), poly(p-phenylenevinylene) (PPV), poly(thiophene) (PT) and poly(fluorene) (PF). However, linear conjugated polymers have several drawbacks. First, lower water-solubility induced by the hydrophobicity of congjugated backbones maybe limits their biomedical and sensing applications. Second, the aggregation of conjugated polymer in aquesous solution significantly induces weakned quantum yield, leading to lower brightness. Third, low specific surface area generated from the limited spatial configuration and poor dispersity in aqueous solution decreases the ability to associate with external molecules. Thus, by attaching highly-charged polymer to the conjugated backbone, it should be feasible to generate CPEs with high charge density that overcome the disadvantages of linear polyelectrolytes to have more desirable properties for various sensing applications. This paper is comprised of the following four parts.1. This paper designs and synthesizes three types of polyelectrolytes with high charge density including cantion, anion and zwitterion. Their structure and molecular weight are characterized by NMR and GPC, respectively. We study their optical spectra in aqueous solution, indicating that the difference in ionic segments may not influence electronic structure of conjugated polymers; we also study the influence of pH and ion strength on their optical properties, showing that the fluorescenc of zwitterion sustain in complex media; we measure their quantum yields in aqueous solution and the value obtained were 0.28, 0.67, 0.70 for cantion, anion and zwitterion, respectively. DLS and the time-resolved PL decay demonstrate their excellent dispersity in aqueous solution. Therefore, a new type of polymer brush with desired properties are successfully obtained, which favors the development of sensor with high perfromance in the field of biomedical and sensing applications2. A conjugated polyelectrolyte brush (PB3) composed of a polyfluorene backbone and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) side chains is synthesized via atom transfer radical polymerization (ATRP), and its properties are investigated and compared with its linear counterpart (PFB, poly[9,9′-bis(6-N,N,N-trimethylammoniumhexyl)fluorene] dibromide). Despite the same conjugated backbones for PB3 and P2, the polymer brush architecture of PB3 endows it with an extremely high charge density, consequently giving rise to better optical stability, higher water solubility (28 mg mL^-1) and higher quantum efficiency (52%) as compared to those of PFB; moreover, it induces stronger electrostatic attraction with oppositely-charged analytes, making PB3 contact the energy donor or quencher molecules much more efficiently than PFB does. As such, PB3 can afford not only higher FRET-amplified dye emission but also larger fluorescence quenching constant as compared to PFB. With its high water-solubility, the fluorescence of PB3 sustains in the presence of a large amount of ssDNA, showing its optical durability in complicated biological media. As a result, CPE brushes could constitute a new generation of water-soluble fluorescent macromolecules having desirable optical and biochemical properties for various sensing applications.3. We successfully advance a new concept of Hg2+ recognition that exploits anionic counterions in CPEs to perform highly selective and sensitive detection for Hg2+ without the incorporation of any external cofactors. This system operates by two necessary and successive processes: the formation of stable complex counterions with optical properties and further efficient interaction between CPEs and formed complex ions. We design and synthesize the new grafted polymer with excellent water-solubility (30 mg mL^-1), which is beneficial to improve sensitivity of array. I-, the counterion in CPEs, can form a stable complex with mercury ion as [HgI₄]^2-, leading to significant fluorescence quenching. We demonstrate that the fluorescence probe shows a remarkably high selectivity for Hg2+ over other metal ions. Owing to the introduction of no any external cofactors, our design affords high stability and extraordinarily low disturb for this sensor and at the same time the novel idea imparts high convenient detection for mercury ion to the fluorescence probe. According to the same mechanism, we realize the specific iodide detection. Potentially, the same concept might be able to be applied to create new sensors for monitoring other cationic or anionic ions.4. Cationic and zwitterionic conjugated polyelectrolytes comprising a polyfluorene backbones with a small fraction of 2,1,3-benzothiadiazole and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) side chains are synthesized via atom transfer radical polymerization (ATRP). Due to higher charge density generated from grafted side chains, two polymers show higher water-solubility and higher quantum yield. In comparison with cationic polyelectrolytes, zwitterionic polyelectrolytes are stable over a broad pH range from 1-13 and even in 1 M NaCl solution. The absence of FRET between zwitterionic polymers and dye-labeled ssDNA indicates their ultralow non-specific adsorption, while cationic polymer shows much stronger non-specific adsorption. The minimal cytotoxicity of zwitterionic polymers demonstrated by MTT assay exhibits their potential in long-term clinical application. Most importantly, zwitterionic polymer could be taken up by cells, whereas contionic polymer stain the surface of cell due to membrane disruption generated from positive charges. The results illustrate that zwitterionic conjugated polymer can serve as a novel type of highly efficient antifouling material with low cytotoxicity for labelling cell or potential biomedical applications.