DOI: 10.14457/tu.the.2019.522
title: Utilization of carbon dots for selective chemical sensing applications

description: Carbon dots are carbon-based nanomaterials with strong, tunable photoluminescence and low toxicity. The promising optoelectronic properties of carbon dots enable them to replace traditional inorganic quantum dots in various applications, including light-emitting diodes, photocatalysis, sensing, solar energy harvesting, bioimaging, and drug-delivery systems. The carbon dots exhibit excellent biocompatibility, low environmental toxicity, superior photochemical stability, ease of synthesis, and surface engineering. In this work, we demonstrated simple, economical synthetic routes to produce fluorescent, water-soluble carbon dots from a variety of chemical-based and natural materials using different methods. All as-prepared carbon dots were characterized using routinely used characterization techniques, such as Fourier transform infrared spectroscopy, UV-visible spectroscopy, fluorescencespectroscopy, and X-ray photoelectron spectroscopy. It was found that all carbon dots showed blue fluorescence emission under UV irradiation but different in quantum yields depending on the precursors and synthesis conditions. First, the carbon dots were synthesized from maleic anhydride and tetraethylenepentamine via one-step pyrolysis method with sulfuric acid. These precursors were considered as small molecules. Due to the high content of nitrogen from tetraethylenepentamine, the nitrogen-doped carbon dots were produced with a high quantum yield of 21%. These nitrogen-doped carbon dots were applied as selective Cu2+ sensor by utilizing the fluorescence quenching effects. The selective detection was studied in aqueous solution to yield a detection limit of 0.16 μM, and a surface coverage of 1.6 ions·nm-2. Paper-based sensor was also developed which was capable to detect Cu2+ at the concentration of 100 μM. Another chemical precursor used for the carbon dot synthesis in this dissertation was nylon 6, a nitrogen-containing step polymer. The nylon 6-derived carbon dots were produced using two-step carbonization, pyrolysis and hydrothermal approaches. These carbon dots had a quantum yield of 3%. The carbon dots in this work were grafted on SiO2 substrate by thermal annealing method. The carbon dot-grafted substrates were used for the adsorption of Cu2+. It was found that the surface coverage was 2 ions·nm-2. Carbon dots-grafted layer was furtherfunctionalized with long-chain hydrocarbon, showing that the surface-grafted carbon dots can be tailored to enhance hydrophobicity. Natural precursors were also desirable to be used for carbon dot synthesis. In this work, rice husk and jack fruit were used to synthesize the carbon dots with a quantum yield of 3% and 5%, respectively. Due to their excellent absorption stability, the rice husk- and jackfruit-derived carbon dots were demonstrated as sensing materials in optical electronic nose system for the realtime detection of volatile organic compounds. The carbon dots from rice husk were capable of detecting alcohol vapors at room temperature, and of distinguishing between methanol, ethanol, and several volatile organic compounds. The carbon dots produced from jackfruit were used to acetone vapor and can distinguish acetone from hexane, ethanol, methanol, and water and between different concentrations of acetone in ethanol and aqueous solutions. To confirm the interactions between the carbon dots and solvent vapors, density functional theory was used to calculate the interaction energy, time-dependent density functional theory was also used to compute the absorption spectra of carbon dot model. It revealed that an increase incarbon dot’s extinction coefficients in implicit solvents was in a good agreement with the optical electronic nose results. The carbon dots synthesized from different carbon sources showed relatively similar structures with the presence of hydrophilic functional groups including oxygen- and nitrogen-containing groups, allowing carbon dots to have high water solubility. Size of carbon dots was found to be 5-10 nm, measured using dynamic light scattering and transmission electron microscopy. Based on these results, the carbon dots synthesized from chemical precursors containing nitrogen atoms have a great potential for metal ion sensing while the carbon dots synthesized from naturalsources showed the excellent absorption properties, and were able to prepare sensing layers for the solvent vapor detection using electronic nose system. The use of readily available, low-cost precursors for the synthesis of functional carbon dots will be useful for large-scale applications.