Laboratory for Advanced Molecular Processing

Carbon Quantum Dots

History of CQDs

Carbon quantum dots (CQDs) represent a promising class of recently discovered nanocarbons composed of discrete, quasi-spherical nanoparticles with sizes below 10 nm. Commonly showing size and excitation wavelength dependent photoluminescence (PL) behavior, CQDs show high potential in the application of biological labeling and photovoltaics as replacements for the toxic metal-based quantum dots. Ironically, CQDs were discovered first by Walter Scrivens et al. in 2004 derived from arc-discharge soot during the purification of single-walled carbon nanotubes(SWCNTs). Timeline showing recent activity regarding CQDs is illustrated below.
History of CQDs
Typically, CQDs include many peripheral carbonyl moieties at their surface, so having them with good water solubility and the possibility for desirable functionalization with various organic, inorganic, polymeric, or biological species. Unlike nanodiamonds, which possess a sp3 hybridized core, CQDs have a core of sp2 carbon, a representative feature of nanocrystalline graphite.
History of CQDs History of CQDs

Nature of CQDs

Selected-area electron-diffration (SAED) experiments on CQDs with a size of about 3nm fabricated by a one-step laser ablation/passivation method uncovered a ring pattern, implies a diamond-like structure. The lattice spacings observed varied from 0.2-0.23 nm. Raman studies show that both the G band related to in-plane vibration of sp2 carbon is at 1590 /cm, and the D band related to the presence of sp3 defects is at 1320 /cm. Lattice spacings of 0.208 nm for CQDs made from oxidizing candle soot is revealed. By using C NMR, they were able to find the presence of sp2 carbon with signals in the 90-180 ppm range, while the absence of signals in the 8-80 ppm range led to the assertion of a lack of detectable sp3 carbon. Raman studies of CQDs also showed both the D and G bands. It can generally be concluded that CQDs consist of an amorphous to nanocrystalline core with predominantly sp2 carbon.

Optical properties

Optical properties CQDs typically show strong optical absorption in the UV region, with a tail extending out into the visible range. CQDs produced from a one-step laser-passivation method have an excitation edge at 280nm, and ones produced from electrochemical oxidation show an absorption band at 270nm. One of the most fascinating features or CQDs is their photoluminescence (PL). One unifying feature of the PL of CQDs is that emission wavelength and intensity depend on excitation wavelength. The requirement for surface passivation is linked to the fabrication method of CQDs. The resulting PL emission spectra ranged from the visible into the near-infrared (NIR), are generally spectrally broad, and dependent upon the excitation wavelength.
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According to fabrication method and surface chemistry involved, the quantum yield of CQDs varies. CQDs of about 5 nm in size, produced by laser ablation had quantum yields between 4 and 10 %. In contrast, 7nm CQDs produced using one-step thermal decomposition methods gave a quantum yield of only 3 %. Smaller 4-6 nm CQDs made by thermal decomposition on NaY supports even lower quantum yield. Quantum yield is dependent on surface passivation. CQDs from laser ablation passivated with PPEI-EI had lower quantum yields than those passivated with PEG1500N. When coated with ZnO or ZnS and PEG1500N, the quantum yield rises to 45 and 50 %, respectively.

Application - Bioimaging

Varieties of the bioimaging agents, for example quantum dots, gold nanoparticles, magnetic iron oxide nanoparticles, and carbon materials, have been developed and applied in many researches. Gold nanoparticles have been studied as a nano carrier and also as a potential contrast agent for X-ray imaging. They are nontoxic and have a high X-ray absorption coefficient than typical iodine-based contrast agents. Hainfeld et al. showed the biodistribution of small gold nanoparticles detected by X-ray, and Eck et al. have shown that intravenously injected anti-CD4-targeted gold nanoparticles can be detected in X-ray computed tomography. Also, superparamagnetic iron oxide nanoparticles have been utilized for cellular labeling and cancer imaging [5,6]. Semiconductor nanocrystal quantum dots (QDs) have been widely used in biological application. Because of their superb optical properties, for example size dependent emission profiles, resistance to photobleaching, high quantum yield, and etc. QDs have attracted much attention in in vitro and in vivo optical imaging studies. A major concern for the QDs, however, is toxicity from cadmium or other heavy metal constituents. Therefore, there has been a strong need to find greener imaging agents.
Similary, CQDs can also be applied in the field of bioimaging. CQDs have the advantages in their chemical inertness, lack of blinking, potentially low cytotoxicity and excellent biocompatibility. They typically display size and excitation wavelength dependent photoluminescence behavior and have excellent water solubility.