Pyrolysis and Solvothermal Synthesis for Carbon Dots: Role of Purification and Molecular Fluorophores

Abstract

Over the last decade, the interest in carbon dots, graphene dots, or similar carbon-based nanoparticles has increased considerably. This interest is based on potentially high fluorescent quantum yields, controllable excitation-dependent emission, low toxicity, and convenient reaction conditions. Carbon dots are often seen as a promising alternative to classical semiconductor quantum dots that are typically made from toxic semiconductor materials. Surprisingly, aspects like the atomic structure, composition, mechanism of formation, and precise understanding of the photophysical properties of carbon dots are still mostly unknown. The large number of different precursor systems and the variety in synthesis routes make a direct comparison of different systems difficult. To advance this, we went for a systematic approach and compared the results of four synthesis routes using two different precursor systems. We used different spectroscopy and microscopy methods including fluorescence correlation spectroscopy to characterize the different reaction products. We found that for syntheses solely based on citric acid as the precursor, we obtain particles where the emission wavelength is strongly dependent on the excitation wavelength despite relatively low quantum yields. In comparison, when urea is added as a nitrogen doping reactant, we observe vastly increased quantum yields. By making use of a combination of dialysis and column chromatography, we were able to isolate various luminescent species with high quantum yields and verify the existence of different molecular fluorophores. A detailed and consistent characterization of the reaction products during the course of purification revealed strong interactions between molecular fluorophores and larger reaction products.

The role of molecular fluorophores in the photoluminescence of carbon dots derived from citric acid: current state-of-the-art and future perspectives

“…85 Then, Rogach's group proved that citrazinic acid is the main fluorophore responsible for the blue fluorescence of CDs prepared from CA and urea (recent reports also indicate the presence of oligomeric ureas, 5-oxopyrrolidine-3-carboxylic acid and other blue PL emitting species in these materials). [86][87][88][89] After careful evaluation of the optical properties of CDs obtained at different reaction times, it was concluded that at the initial stages of CD synthesis molecular fluorophores dominate their absorption and fluorescence characteristics, while for CDs prepared at longer reaction times more and more pronounced contribution of carbonized species becomes apparent (Fig. 2).…”

Luminescent Carbon Nanodots Doped with Gadolinium (III): Purification Criteria, Chemical and Biological Characterization of a New Dual Fluorescence/MR Imaging Agent

“…However, it is now clearly emerging that this is not sufficient for many CDs preparations and a combination of alternative purification techniques (i.e., gel electrophoresis and chromatography) should be employed more routinely. [ 7–10 ]…”

“…However, it is now clearly emerging that this is not sufficient for many CDs preparations and a combination of alternative purification techniques (i.e., gel electrophoresis and chromatography) should be employed more routinely. [7][8][9][10] Here, we aim at employing the technology of CDs to develop new multifunctional materials as contrast agents (CAs) in magnetic resonance imaging (MRI). As well known, most of CAs currently used in MR medical imaging, are gadolinium (III) (Gd) based complexes (Gadolinium-based Contrast Agents, GDCAs), whilst alternative systems, such as iron oxide and manganese compounds are still to be confirmed for a wide use in clinics.…”

Water Soluble PMPC-Derived Bright Fluorescent Nitrogen/Phosphorous-Doped Carbon Dots for Fluorescent Ink (Anti-Counterfeiting) and Cellular Multicolor Imaging

“…CDs are prepared using either top-down or bottom-up methods including laser ablation [ 20 ], ultrasonication [ 21 , 22 ], arc discharge [ 23 ], simple heating [ 24 ], and solvothermal [ 25 , 26 ]. Chemical precursors such as acid derivatives, thiourea derivatives, and carbohydrates were employed to prepare CDs [ 27 , 28 , 29 ].…”