Synthesis And Catalytic Property Of Highly Fluorescent, Water-soluble Carbon Quantum Dots For Free Radical Polymerizations

Carbon quantum dots (usually called carbon nanodots) is a new class of carbon nanomaterials with size below 10 nm, consisting of crystalline or non-crystalline carbon core and an amorphous shell comprised of abundant oxygen-, nitrogen- or sulfur-containing organic groups. In 2004, carbon quantum dots were firstly found by Walter A. Scrivens et al and later were named as "carbon nanodots" by Ya-Ping Sun et al in 2006. Carbon nanodots have become a new star in the family of carbon nanomaterials in the past decade since they have potential applications in various areas such as bioimaging, detection of metal ions, biochemistry testing and photocatalysis. Carbon nanodots not only have unique physicochemical properties, such as high quantum yield, excellent up- or down-conversion photoluminescence (PL) and photo-induced electron transfer property, but also possess many desired advantages such as lower toxicity, superior biocompatibility/biosafty, lower cost and easier preparation process compared with classical semiconductor quantum dots (e.g., CdS/CdSe QDs).In this thesis, green tea was used as a new carbon source to prepare monodispersed, water-soluble carbon nanodots (named as T-CNDs) in an average size of 3.83 nm by a "one-step" pyrolysis method. T-CNDs contain abundant phenolic hydroxyl groups and carboxylic acid moieties on the surfaces and show excellent optical properties as follows:(1) A high quantum yield of 64% in water at 330 nm excitation when choosing quinoline sulfate as a standard; (2) An excitation-dependent PL behavior with pH-switched luminescence, while showing the strongest emission in basic solution with pH=10; (3) Up-conversion PL when the excitation wavelength is longer than 600 nm; (4) A high selectivity of Fe3+ ions, fluorescence quenching happened when 0.5μM Fe3+ was added into the aqueous solution containing T-CNDs, which is similar to our previous report on carbon nanodots prepared from konjac flour; (5) Photo-induced electron transfer property, no matter an electron donor (N,N-diethylaniline, DEA) or an electron acceptor (2,4-dinitrotoluene) was added into the T-CNDs solution, PL from T-CNDs decreased remarkably. Accordingly, the decay time of T-CNDs was shortened from 4.5 ns to 2.0 ns and 2.2 ns, respectively. This result indicates that photoexcited T-CNDs are both excellent electron donors and electron acceptors.Although carbon nanodots have been extensively applied in photocatalysis, it was recently reported that carbon nanodot is not only an excellent photocatalyst but also has obvious chemical catalytic effect on organic condensation reaction because of abundant surface hydroxyl groups. In this thesis, the catalytic effect of T-CNDs on free radical polymerization is firstly investigated. Regarding good water solubility of T-CNDs, we select a commercial water-soluble monomer, sodium 4-styrenesulfonate (NaSS), as a model. Compared with polymerization without T-CNDs, both monomer conversion rate and polymer molecular weight have remarkably increased in presence of T-CNDs, in the meanwhile, polymer molecular weight distribution, that is polydispersity index (PDI=Mw/Mn), also becomes narrower. The possible catalytic mechanism of T-CNDs is proposed as follows:As T-CNDs are both excellent electron donors and electron acceptors, they can combine with the oxidative initiator, potassium persulfate (KPS), and then form redox initiators (T-CND/KPS) at the initial stage of polymerization. The formed redox initiators will generate SO4-· radicals more quickly than KPS alone. On the other hand, T-CNDs can activate the double-bond of NaSS because of conjugation between T-CNDs and NaSS, which makes NaSS transform into monomer radicals more easily. The above-mentioned two reasons lead to a faster radical generation as well as a higher monomer conversion rate. According to the classical theory of polymer chemistry, polymer molecular weight decrease inversely with the radical concentration in the reaction system. One molecule of KPS in aqueous solution dissociates to give two radicals, however, a pair of T-CND/KPS redox initiator can give only one radical of SO4-·. In addition, T-CNDs can trap free radicals of OH· induced by SO4-· in the aqueous solution, which means total concentration of radical species in T-CND/KPS-initiated polymerization is lower than that in KPS-initiated polymerization. Therefore, the molecular weight of PSSNa polymerized with T-CNDs is much higher than the control polymerized without T-CNDs while the PDI becomes narrower. Furthermore, we have applied T-CNDs-catalyzed free radical polymerizations to other vinyl monomers such as acrylamide (AM) and methyl methacrylate (MMA). It is found that water-soluble AM shows a faster gelling rate with T-CNDs than the control without T-CNDs in polymerization system. Nevertheless, no obvious catalytic effect can be observed for PMMA synthesized with T-CNDs. The reason lies in the fact that polymerization of MMA was performed in toluene using oil-soluble initiator azobisisobutyronitrile (AIBN), but hydrophilic T-CNDs has a poor solubility in toluene, which prevent them from combining with AIBN and MMA. This result has demonstrated their versatility of T-CNDs in catalysing polymerizations of water-soluble vinyl monomers.In addition, we also have found the catalytic capability of T-CNDs in ring-opening polymerization of ε-caprolactone (CL) initiated by benzyl alcohol, where T-CNDs can act as a co-catalyst to assist with organic acid catalysts such as salicylic acid, tartaric acid, and citric acid. Compared with the ring-opening polymerization without T-CNDs, the addition of T-CNDs in the polymerization not only leads to a higher conversion rate of monomer, but also a higher molecular weight of PCL with a narrower PDI can be obtained. Although the internal mechanism of T-CND as a co-catalyst is not very clear yet, it is probably related to abundant hydroxyl groups on their surfaces.