| Carbon dots (C-Dots) are recognized as excellent, zero-dimensional, bio-compatible nanoparticles with diameters less than 10 nm. Since its discovery in 2004 by unintentional single-wall nanotube purification, C-Dots have been extensively studied and applied in many aspects in the past couple decades. C-Dots are mainly made from carbon source and nitrogen/sulfur doping, which made them bio-compatible and non-toxic nanomaterials, in comparison to other nanoparticles, such as semi-conductor quantum dots. In addition to non-toxicity, C-Dots possess excellent photostability and high fluorescence (QY up to 94 %), which is comparable to quantum dots for bio-imaging applications. In contrast to quantum dots, the synthesis of C-Dots is usually simple and inexpensive. This lowers the cost of commercializing C-Dots as biomarkers, metal sensors, optical sensing, light harvesting, fluorescent ink, anti-counterfeiting, and potential drug delivery system for example. To further gain fundamental understanding and for better utilization of C-Dots' optical properties, the molecular origin of C-Dots' photoluminescence properties is indispensable. Studies revealed that C-Dots can be formed at high temperature and high pressure and have similar photophysical behavior in spite of having different starting materials. It would be beneficial to look further into both the physical and optical properties of C-Dots. In terms of physical properties of C-Dots, their sizes and shapes are well-studied. However, C-Dots synthesized from citric acid and ethylenediamine at particular experimental conditions, have a unique physical gel-like property. Herein, the physical (viscosity) properties are examined in depth to understand its gel-like characteristics. Besides the special physical properties, the issue of C-Dots favoring fluorescence remains debatable. To obtain a further understanding of the C-Dot photoluminescence mechanism, two series of synthetic building blocks are proposed and tested. A series of carbon-source building blocks were selected to study the influence of varying numbers of carbon atoms or carboxyl/hydroxyl groups on the fluorescence mechanism of C-Dots. The conclusion provides insights for future research when certain photophysical properties are desired. C-Dots are often synthesized using a carbon source as well as a doping agent, e.g. nitrogen. In addition to carbon-source building blocks, another series of nitrogen-source building blocks were chosen to understand further the synthetic mechanism. Fluorescent amino acid, namely tryptophan, and two different nitrogen dopants, ethylenediamine and urea, were chosen as nitrogen-source building blocks to investigate the C-Dot photoluminescence mechanism in detail. |
