Production of carbon dots for the valorization of brewing wastes

The brewing industry produces large amounts of solid organic wastes, of which spent grains, surplus yeast and spent hops represent the majority. For years, despite the low-cost and great availability, their reuse has been scarce and limited to animal feed. More recently, alternative valorisation strategies to transform the brewing residues into value-added products have been investigated. Following this approach, in this study the wastes of two high-fermentation beers, i.e. IPA and stout from local breweries, were collected and used to generate N-doped carbon dots (CDs) by one-step hydrothermal carbonization (HTC), without the employment of any oxidising agents, surface passivation or external doping molecules. HTC has been chosen being a facile, eco-friendly and cost-effective method. Initially, CDs from the sole spent grains and surplus yeasts were produced to evaluate the contribution of each residue separately. Afterwards, in view of a potential technology scale-up, all wastes were blended together and residual beer was used instead of water for the HTC treatment. Optical and structural proprieties of CDs were studied by various techniques including UV-visible and fluorescence spectroscopy, transmission and scanning electron microscopy, dynamic light scattering and nuclear magnetic resonance. The blended CDs exhibit high aqueous solubility with a bright blue emission (λex=420 nm) under UV light and a quantum yield of up to 11.3%. The presence of nitrogen-containing functional groups in the CDs was confirmed by elemental analysis. Lastly, envisaging potential photocatalytic applications, the CDs were successfully dispersed in a polyvinyl alcohol-based matrix, leading to flexible and luminescent hydrogels.