Abstract
Spent coffee grounds (SCGs) are known for containing many organic compounds of interest, including carbohydrates, lipids, phenolic compounds and proteins. Therefore, we investigated them as a potential source to obtain carbon dots (CDs) via a nanotechnology approach. Herein, a comparison was performed between CDs produced by SCGs and classic precursors (e.g., citric acid and urea). The SCG-based CDs were obtained via the one-pot and solvent-free carbonization of solid samples, generating nanosized particles (2.1–3.9 nm). These nanoparticles exhibited a blue fluorescence with moderate quantum yields (2.9–5.8%) and an excitation-dependent emission characteristic of carbon dots. SCG-based CDs showed potential as environmentally relevant fluorescent probes for Fe3+ in water. More importantly, life cycle assessment studies validated the production of CDs from SCG samples as a more environmentally sustainable route, as compared to those using classic reported precursors, when considering either a weight- or a function-based functional unit.
Highlights
Carbon dots (CDs) are fluorescent carbon-based nanomaterials with a near-spherical shape and are typically sized below 10 nm [1,2]
The impact potentials evaluated according to the ReCiPe method were: global warming—human health (GW–HH), global warming—terrestrial ecosystems (GW–TE), global warming—freshwater ecosystems (GW–FE), stratospheric ozone depletion (SO), ionization radiation (IR), ozone formation—human health (OF–HH), fine particulate matter formation (FPM), ozone formation—terrestrial ecosystems (OF–TE), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME), terrestrial ecotoxicity (TE), freshwater ecotoxicity (TET), marine ecotoxicity (MET), human carcinogenic toxicity (HC), human non-carcinogenic toxicity (HNC), land use (LU), mineral resource scarcity (MR), fossil resource scarcity (FR), water consumption—human health (WC–HH), water consumption—terrestrial ecosystem (WC–TE) and water consumption—aquatic ecosystems (WC–AE)
While SCG-based CDs present moderate quantum yields (2.9–5.8%) and quite low reaction yields, CDs made from citric acid and urea showed more a significant quantum yield (22.5%) and higher reaction yields
Summary
Carbon dots (CDs) are fluorescent carbon-based nanomaterials with a near-spherical shape and are typically sized below 10 nm [1,2]. We have compared the fabrication of CDs prepared from different SCG samples and from standard precursors (citric acid and urea), [16,29,30] via a one-pot and solvent-free carbonization method [21] The objective of this comparison is to determine if the use of SCGs as precursors really leads to a more environmentally sustainable synthesis route than the use of standard precursors, even when the functional properties of all CDs are considered (such as synthesis yield and fluorescent quantum yield). A comparative life cycle assessment (LCA) study was performed for all CDs, to determine which is the most environmentally sustainable fabrication method for fluorescent CDs
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