A review on deep eutectic solvent-functionalized paper analytical platforms

Paper-based analytical devices constitute one of several complementary approaches to enabling portable, decentralized chemical analysis. Despite their low cost, simplicity, and compatibility with on-site testing, their analytical performance is often limited by uncontrolled wetting, reagent leaching, evaporation-driven signal drift, limited stability of recognition elements, and insufficient selectivity in complex matrices. This review critically examines deep eutectic solvents (DESs) not only as sustainable replacements for conventional solvents but also as functional components of paper analytical platforms. Particular attention is paid to how DESs can transform cellulose-based substrates by providing chemically tunable microenvironments through hydrogen-bonding and ionic interactions with cellulose hydroxyl groups, thereby regulating wettability and capillary transport, improving reagent retention, stabilizing biomolecules and nanomaterials, enabling ion-conductive domains for electrochemical transduction, and serving as immobilized sorptive phases for sample preparation. Current strategies for integrating DESs into paper substrates, sensing zones, and porous supports are discussed across colorimetric, optical, electrochemical, and extraction-based workflows. The review also evaluates key limitations, including high viscosity, incomplete understanding of DES-cellulose interactions, residual DES effects, matrix-dependent responses, leaching, long-term stability, and manufacturability. Concluding recommendations highlight key future directions, such as standardized reporting of DES composition and retention, device-relevant characterization of capillary flow and signal stability, quantitative evaluation of matrix effects, and scalable strategies for designing reproducible DES-functionalized paper analytical platforms.