Evaluation of Different Bottom-up Routes for the Fabrication of Carbon Dots

Abstract

<jats:p>Carbon dots (CDs) are carbon-based nanoparticles with very attractive luminescence features. Furthermore, their synthesis by bottom-up strategies is quite flexible, as tuning the reaction precursors and synthesis procedures can lead to an endless number of CDs with distinct properties and applications. However, this complex variability has made the characterization of the structural and optical properties of the nanomaterials difficult. Herein, we performed a systematic evaluation of the effect of three representative bottom-up strategies (hydrothermal, microwave-assisted, and calcination) on the properties of CDs prepared from the same precursors (citric acid and urea). Our results revealed that these synthesis routes led to nanoparticles with similar sizes, identical excitation-dependent blue-to-green emission, and similar surface-functionalization. However, we have also found that microwave and calcination strategies are more efficient towards nitrogen-doping than hydrothermal synthesis, and thus, the former routes are able to generate CDs with significantly higher fluorescence quantum yields than the latter. Furthermore, the different synthesis strategies appear to have a role in the origin of the photoluminescence of the CDs, as hydrothermal-based nanoparticles present an emission more dependent on surface states, while microwave- and calcination-based CDs present an emission with more contributions from core states. Furthermore, calcination and microwave routes are more suitable for high-yield synthesis (~27–29%), while hydrothermal synthesis present almost negligible synthesis yields (~2%). Finally, life cycle assessment (LCA) was performed to investigate the sustainability of these processes and indicated microwave synthesis as the best choice for future studies.</jats:p>

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“… 10 Solvothermal synthesis, being applied at relatively mild conditions, is one of the simplest facile approaches, allowing one to obtain different fluorescent properties. 11 The majority of CDs emit light in the blue spectral range, and a few CDs were found to emit red light. Nitrogen (N), phosphorus (P), and oxygen (O) dopants cause a spectral shift, pushing emission lines to green and yellow.…”

“…In most of the common synthesis methods, hydrothermal and solvothermal RYs are significantly lower than 2%. 11 …”

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“…The higher flexibility of the "bottom-up" approach by the means of the selection of chemical precursors and applying optimal operating conditions makes "bottom-up" methods more favorable techniques for obtaining different CQDs. Thermal/combustion techniques are the most commonly reported synthetic routes for CQDs preparation due to their simplicity, better variability of using different carbon sources, surface functionalization, environmental friendliness, and lower costs of synthesis [5,12,33]. Another important advantage of the "bottom-up" approach is better control over the particle size, morphology, and chemical composition.…”

“…Another important advantage of the "bottom-up" approach is better control over the particle size, morphology, and chemical composition. In general, this approach generates CQDs from different precursors (smaller molecules or heterogeneous sources) by thermal treatment [33,34]. Very often, this includes rigorous thermal treatment at high temperatures as well as sample treatment with concentrated acids and bases.…”

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“…This is helped by various surface modifications, strict particle size ranges, etc. [ 26 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 ].…”