Carbon Dots (CDs), a fluorescent carbon-based nanomaterial, can be seen as more appealing than molecular fluorophores and metallic nanoparticles due to their properties. These include good water solubility, biocompatibility, high stability, low toxicity, adjustable fluorescence, and easy surface functionalization. These features have led to their increasing use in fields such as sensing, bioimaging, and photocatalysis, among others. In fact, CDs' unique photoluminescence properties show promising potential for sensing applications. CDs have been shown to serve as sensing receptors in nanoprobes, in which their features can be used as a sensing signal. Metal cations, non-metal ions, solvents, pesticides, and organic compounds are examples of analytes for which CDs can be used as sensors.
Relevantly, CDs can be produced from different carbon sources, including various types of organic waste. Thus, CDs could contribute toward a circular economy and upcycling waste. However, waste-based CDs usually have a low fluorescence quantum yield (QYFL<20%), which limits their practical applications (especially in sensing).
Herein, we developed a hydrothermal synthesis strategy for CDs that employs different organic wastes as carbon precursors and consistently generates CDs with appreciable fluorescence. This strategy was validated by testing different types of waste: corn stover—CD@CS; coffee—CD@C; sawdust—CD@S; and cork—CD@CK. These CDs present similar fluorescence profiles, with emission and excitation maximums at ~440 and ~350 nm, respectively. More importantly, the QYFL of these CDs presents significant values (CD@CS: 39.3%; CD@C: 24.6%; CD@S: 35.9%; CD@CK: 21.8%). So, we obtained CDs from different waste materials without compromising their performance in terms of QYFL, which is essential for their future use in sensing applications.
Acknowledgments
“Fundação para a Ciência e Tecnologia” (FCT, Portugal) is acknowledged for the funding of R&D Units CIQUP (UIDB/00081/2020) and Associated Laboratory IMS (LA/P/0056/2020). Sónia Fernandes also acknowledges the FCT for funding her Ph.D. grant (2021.05479.BD).
