Water-soluble Conjugated Polymer Fluorescence Sensor Materials Synthesis, Characterization And Fluorescence Quenching Studies

Water-soluble conjugated polymers (WSCPs), a new class of polyelectrolytes which consist of both polyions and electronically active conjugated backbones, attract much interest because of the potential application in the development of highly efficient chemo or biosensors. Because of lacking of receptor in the molecular structures of previous WSCPs, forming receptor/WSCP complex was inevitable to realize their high selectivity and sensitivity. Consequently, WSCPs functionalized with kinds of receptors were further developed to realize direct bioanalysis and illustrated good signal amplification. Thus, introducing special receptors into WSCP systems was anticipated to conveniently achieve detecting all kinds of analytes in water with good selectivity and high sensitivity. On the other hand, there's little research focused on the quenching behavior and quenching mechanism of WSCPs containing special receptors for detection in aqueous environments, especially the interchain aggregation which caused by static affinity in solution and then can cause fluorescence quenching by formation of nonemissive interchain excited states (excimers or interchain excitons). Understanding the quenching mechamsm of WSCPs in aqueous environments can help us to built WSCP-based sensing materials with high sensitivity. The initial research results showed that the efficiency of fluorescence quenching was influenced by many factors, such as effective conjugated length, size of conjugated polymer chain, charge on quencher, ionic strength, pH values, conjugated polymer concentration, concentration of surfactant or polyelectrolyte with counterions added into the WSCP system, and type of substrate introduced. As a result, the sensitivity could be controlled by chemical (design and synthesis) or physical methods (change of system environment). So it was very necessary to further investigate and explain the relationship between sensitivity and these factors. This dissertation is comprised of the following four parts:1. We have successfully designed and synthesized an anionic fluorine-based bipyridyl-containing water-soluble conjugated polymer (WSCP). This is the first time to introduce special receptors into WSCP's structure to realize convenient detection for transition metal ions in aqueous environments. The polymer showed obvious red-shifts of its absorption spectrum after adding transition metal ions, which was believed to be based on the chelation between the 2,2-bipyridyl units and the metal ions that made more planar and thus resulted in an increased effective conjugation length on the entire polymer chain. The increased red-shifts in aqueous solution compared with which in organic system may be explained by the decreased twist angle of the backbone resulting from the decreased mutual electrostatic repulsion of the anionic sulfonato pendant and the increased interchain aggregation after adding cationic metal ions and forming cationic metal complexes. The polymer's fluorescence in aqueous solution could be completely quenched upon addition of transition metal ions, even in a very low concentration of metal ions. The fluorescence quenching of the polymer when adding transition metal ions was most likely caused by energy or electron-transfer reactions between the polymer backbone and binding metal complexes, which was a nonradiative center and trapped the excitation energy passing through them. So we can conveniently detecting metal ions in water. The K_sv, of different transition metal ions in aqueous solution were much higher than previous reports in organic solutions and showed highest selectivity to Ni²⁺. The observed higher K_sv of the water-soluble polymer was believed to originate from the following factors: the good water solubility of the polymer, the strong static affinity between anionic polymer side chains and cationic metal ions, and the increased effective conjugation length of the polymer in aqueous solution. Considering the high sensitivity and good selectivity of this WSCP-based chemosensor in aqueous solution, our results opened opportunities for developing novel chemosensors by introducing selective fluorescent chromophore into the water-soluble conjugated backbone.2. Three sulfonato-containing fluorene-based anionic water-soluble conjugated polymers which are special designed to link fluorene with alternating moieties such as bipyridine (P1), pyridine (P2) and benzene (P3) have been firstly synthesized via the Pd-catalyst Sonogashira-coupling reaction, respectively. These polymers all had good solubility in water and showed different responses for transition metal ions with different valents in aqueous environments: the fluorescence of bipyridine-containing P1 can be completely quenched by addition of all transition metal ions selected and showed a good selectivity for Ni²⁺; the pyridine-containing P2 had little response for adding monovalent and bivalent metal ions and showed different fluorescence quenching with the addition of trivalent metal ions (with a special selectivity for Fe³⁺); P3 also had responses only for the trivalent metal ions within the ionic concentration we studied. The different responses of these polymers for transition metal ions are attributed to the different coordination ability of the units linked with fluorene in the main chain. Bipyridine units proved had much better coordinating ability than pyridine unit for transition metal ions in aqueous environments and the benzene units in P3 had no chelating ability for transition metal ions. The fluorescence quenching of pyridine and bipyridine-based polymers for low valent transition metal ions (monovalent and bivalent) were most likely caused by energy or electron-transfer reactions between the polymer backbone and binding metal complexes. With the compare of UV-vis absorption spectra, PL emission spectra, DLS and fluorescence lifetime of P1-P3 when adding transition metal ions we found that fluorescence quenching caused by interchain aggregation can take place when adding high valent transition metal ions and also caused partly fluorescence quenching at the same time. The origin of the fluorescence quenching of P1 and P2 for trivalent metal ions in aqueous solution arised from two factors: energy or electron transfer and interchain aggregation. These results revealed the origin of ionochromic effects of these polymers and suggested the potential application for these polymers as novel chemosensors with higher sensing sensitivity in aqueous environments.3. Three poly(p-phenyleneethynylene)(PPE) derivatives which are special designed to link benzene with alternating side chains such as tertiary amine group (P1), tri(ethylene glycol)methyl ether groups (P2) and alkoxy groups (P3) have been successfully synthesized via the Pd-catalyst Sonogashira-coupling reaction, respectively. With quaternization with bromoethane, we got their corresponding cationic polymers P1'-P3'. We also studied the fluorescence quenching of a water-soluble cationic poly(p-phenyleneethynylene) derivative (P1'-PPE-NEt₃⁺) at different concentrations by Fe(CN)₆^4-. A new UV-vis absorption peak appeared after adding Fe(CN)₆^4-. indicating that the polymer/quencher complex was formed. The static quenching constant K_sv^S of PPE-NEt₃⁺ for Fe(CN)₆^4- increased with the concentration decrease of PPE-NEt₃⁺ and was observed inverse proportional to PPE-NEt₃⁺ concentration (≥1μM) and deviation of the linearity at lower concentration (≤1μM) without the influence from aggregation. To account for this phenomenon, the concept of local quencher concentration was introduced into the Stern-Volmer equation and a new equation which successfully presented such a relationship between K_sv^S and [PPE-NEt₃⁺] was obtained. The value of association constant for Fe(CN)₆^4- binding to PPE-NEt₃⁺ can be obtained from the equation (K_b = 7.3×10⁶M^-1).4. We studied the optical properties and quenching behaviors of complexations of PPE-NEt₃Br with the oppositely charged polymer PAANa and PMAANa in aqueous solution. It was showed that addition of few PAANa induced the PPE-NEt3Br chain to vary from isolated state to aggregated state and then recovered to isolated state through increasing the amount of PAANa, which exhibited an obvious fluctuation of fluorescence intensity and K_sv values. While after adding PMAANa, PPE-NEt₃Br still exhibited the similar mutation of complex structure just as what appeared in PAANa/PPE-NEt₃Br complex except that more twisted conjugated main chain and no aggregation was formed. Such a significant structure difference showed that a little change of the structure of those anionic polymers will significantly influence the conformation and hence the optical and quenching properties of ionic conjugated polymers. Investigation of the percentage of inaccessible fluorescence is a good way to obtain the complex structure information that the mutation point for PAANa/PPE-NEt₃Br complex was at PAANa:PPE-NEt₃Br = 1.2, which is amount to the corresponding result from the variation of its fluorescence intensity, and that for PMAANa/PPE-NEt₃Br complex is at PMAANa:PPE-NEt₃Br = 1. Investigation of the sensitivity of those complexes showed that adding an appropriate amount of PAANa will be beneficial to enhance the K_sv value resulting from the PAANa-induced aggregation while no similar phenomenon was found in PMAANa/PPE-NEt₃Br system, which further demonstrated that the sensitivity of conjugated polymers could be controlled by the structure of the complexes formed between rod-like conjugated polyelectrolyte and oppositely charged polymers and thus by the structure of those oppositely charged polymers utilized in our system. Therefore, in practical application to develop good sensors with high sensitivity, it is important to choose a polyelectrolyte with suitable structure as biosensor platform and control its amount for obtaining complexes with good sensitivity.