Sustainable synthesis of cyclodextrin-based polymers by exploiting natural deep eutectic solvents

Natural deep eutectic solvents (NADESs) represent an environmentally friendly replacement for petroleum-based solvents; for this reason, they are a major field of research which aims to reduce the industrial emissions, looking forward to greener processes. Besides, beta cyclodextrin (beta CD)-based polymers are a class of materials widely exploited for the controlled release of drugs and the adsorption of undesired substances in many pharma, food, and environmental applications. However, most syntheses of beta CD-based polymers require the use of organic solvents or toxic reactants; hence, describing a green way to obtain such a class of materials would make the process more sustainable and suitable for eco-friendly scaling-up. In this work, the synthesis of water-soluble beta CD-based polymers characterized by molecular weight ranging from 15 kDa to 19 kDa has been reported, using 1 : 1, 1 : 1.5, and 1 : 2 mol : mol choline chloride/citric acid NADESs as a solvent. The peculiar structure displayed by the resulting polymer allowed the latter to be cured into a beta CD-based nanosponge, turning its structure from a water-soluble to cross-linked one. Eventually, the obtained beta CD-based nanosponges displayed positive zeta-potential values ranging up to +20 mV, related to the presence of quaternary ammonium functions. This positive charge obtained was dependent on the choline chloride/citric acid molar ratio chosen for the NADES preparation, and further confirmed by adsorption and release studies with charged probe molecules.