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.

Considering the increased anthropogenic emissions of CO₂ into the atmosphere, it is important to develop economic incentives for the use of CO₂ capture methodologies. The conversion of CO₂ into heterocyclic carbonates shows significant potential. However, there is a need for suitable organocatalysts to reach the required efficiency for these reactions. Given this, there has been an increasing focus on the development of organocatalytic systems consisting of a nucleophile and a hydrogen bond donor (HBD) so that CO₂ conversion can occur in ambient conditions. In this work, we evaluated the potential of fluorescent carbon dots (CDs) as catalytic HBDs in the ring-opening reaction of epoxides, which is typically the rate-limiting step of CO₂ conversion reactions into heterocyclic carbonates. The obtained results demonstrated that the CDs had a relevant catalytic effect on the studied model reaction, with a rate constant of 0.2361 ± 0.008 h⁻¹, a percentage of reactant conversion of 70.8%, and a rate constant enhancement of 32.2%. These results were better than the studied alternative molecular HBDs. Thus, this study demonstrated that CDs have the potential to be used as HBDs and employed in organocatalyzed CO₂ conversion into value-added products.

Carbon Dots (CDs) application in biomedicine has been increasing, due to their properties of high photoluminescence, biosafety and low cost, which allow for possible applications in bioimaging and as drug carriers. However, their synthesis strategies are quite flexible, as tuning-reaction precursors and synthesis procedures can lead to an endless number of CDs with distinct properties and applications, which make their practical development difficult. In this work, we performed a systematic evaluation of the effect of three representative bottom-up strategies (hydrothermal, microwave-assisted and thermal heating) on the properties of CDs prepared from the same precursors (glucose and urea). In this way, the CDs were thoroughly evaluated in terms of structure, morphology and photoluminescent properties. To screen their potential as drug carriers, the biosafety of these CDs was tested against the normal breast cell line MCF-10A, as drug carriers need to be compatible with healthy cells to minimize harmful side-effects. The characterization results demonstrated a similar size range and composition for all the CDs. While hydrothermal synthesis generates CDs with lower fluorescence and synthesis yields, presenting also an emission more dependent on surface states, these CDs have the most promising viability profile in MCF-10A when compared with microwave-assisted and thermal-heating CDs, which present better fluorescence properties and. Our results suggest these CDs have the potential to proceed in further investigations in animal models as imaging candidates or biosensing tools, as well as drug carriers for future applications in medicine.

Carbon dots (CDs) are fluorescence carbon-based nanomaterials that possess several properties such as photoluminescence, biocompatibility and good water solubility. They can be fabricated from a large variety of precursors; however, most available organic molecules are still expensive and their use or synthesis can lead to significant challenges to the environment and human health. It has become desirable to use biomass waste as alternative precursors in the synthesis of CDs, given that biomass waste material is ubiquitous, nontoxic, cheap and renewable. Spent coffee grounds (SCGs) are the residues of the treatment of coffee powder can be a potential carbon source to a more environmentally sustainable synthesis route. In this work, we fabricated SCG-based CDs via one-pot and solvent-free carbonization at 200 °C of solid samples generating particles with sizes between 2.1 and 3.9 nm. These carbon nanoparticles exhibited blue fluorescence and excitation-dependent emission of carbon dots with moderate quantum yields (2.9–5.8%). The presence of heavy metals in water resources, such as Fe³⁺, can lead to adverse health effects. SCG-based CDs showed potential for being used as optical Fe³⁺ optical sensors, with Life Cycle Assessment (LCA) studies validating the SCGs as more sustainable precursors than classical precursors, both considering a weight- or function-based functional unit.