Graphene quantum dots and their applications in bioimaging and catalysis

• Main Author: Chen, Jie
• Other Authors: Chen Peng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2019
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136527
• Institution: Nanyang Technological University

Graphene quantum dot (GQD) is an emerging zero-dimensional material and the latest addition to the family of carbon materials. Owing to its unique and intriguing properties, GQD promises a wide spectrum of novel applications. For example, its tunable photoluminescence (PL), high photostability, excellent biocompatibility and molecular size make GQD a desirable candidate for bioimaging. In the first result chapter of this thesis, we have successfully synthesized single-crystalline GQDs by using a bottom-up approach. The as-prepared GQDs have a high quantum yield (up to ~31 %), a small average size (~5.4 nm) and sufficient exposed carboxyl groups. Subsequently, for the first time in the world, they are conjugated with monosaccharide moieties, galactose and mannose, respectively. Further, the GQD-sugar conjugates are employed to reveal the distribution and dynamic trafficking of carbohydrate receptors on live cells. GQD has an excellent dispersity in aqueous solutions due to its abundant oxygen-containing groups and molecular sizes. Taking advantage of that, we for the first time demonstrate the use of GQDs and sulfonated GQDs (SGQDs) as a quasi-homogeneous catalyst for chemo-catalytic biomass conversion. Specifically, we demonstrate that GQDs can selectively and effectively degrade carbohydrates (e.g., fructose, glucose, and cellulose) into 5-hydroxymethylfurfural (5-HMF). For example, dehydration of fructose (up to 20 wt% concentration) to 5-HMF is catalyzed by SGQDs with a product yield of 51.7 % and a conversion rate of 91.8 %. The high catalytic performance is attributed to unique physicochemical and structural properties of GQDs, i.e., they uniquely combine the merits of homogenous catalysts and heterogeneous catalysts. On one hand, highly dispersive SGQDs resemble homogenous catalysts in the reaction. On the other hand, they can be recycled for reuse like heterogeneous catalysts. The DFT calculations reveal that intra-molecular hydrogen bonded hydroxyl groups and the adjacent sulfonic groups on SGQDs play a synergistical role in their high catalytic efficiency. This study suggests the potential of SGQD as an efficient and green carbon catalyst for one-pot biomass transformation and more generally the unique potential of functionalized GQDs as novel quasi-homogeneous catalysts for various reactions. Furthermore, we have designed two different functionalization strategies to systematically tailor the bandgap structures of GQDs whereby making them snugly suitable for particular applications. Specifically, we conjugate GQDs with poly aromatic molecules to enlarge π-conjugated sp2-carbon systems thus narrow the GQD bandgap by lowering the π* orbital. We also introduce the intermediate n orbital between π and π* orbitals by coupling GQDs with electron-donating functional groups. The functionalized GQDs with a narrow bandgap form a unique intramolecular Z-scheme structure, which consists of p-type oxygenated groups, n-type conjugates, and sp2 ohmic contact (mediator) in between. Then, the GQDs are employed as the efficient photocatalysts for water splitting and carbon dioxide reduction under illumination of visible light. GQDs as photocatalysts have several advantages. They are made from coal, an earth-abundant and cheap material. Besides, they are very small, and their large surface area allows more exposed active sites to intimately contact with the substrates.