Effect of choline-tryptophan based ionic liquid on the solubility and scavenging activity of quercetin: thermophysical and spectroscopic approach

The limited aqueous solubility of quercetin severely restricts its bioavailability and functional applications. This study demonstrates the effectiveness of a biocompatible choline-tryptophan ionic liquid ([Ch][Trp]) IL in enhancing the solubility and antioxidant performance of quercetin. Incorporation into the IL medium produced a concentration-dependent modulation of 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radical scavenging activity, with the highest IL concentration (0.18 mM) increasing RSA from 74.76% (quercetin alone) to 78.18%, whereas lower IL concentrations resulted in slightly reduced activity. Physicochemical evaluation of quercetin-[Ch][Trp]-water solutions revealed structure-making behaviour, reflected by a positive Hepler constant and negative S_v values, indicating strengthened water structuring and weak ion-ion interactions conducive to solute stabilization. Spectroscopic analyses supported these findings: UV-Vis absorption exhibited a red shift, fluorescence intensity increased, and FTIR spectra confirmed specific hydrogen-bonding and π-π interactions between quercetin and the IL. Collectively, these results show that [Ch][Trp] creates a favourable micro structured environment that stabilizes quercetin, modulates its radical-scavenging performance, and improves its physicochemical properties. This work highlights the potential of amino-acid-based ILs as green, multifunctional media for improving the performance of poorly water-soluble bioactive compounds.