Smart deep eutectic solvents in analytical chemistry: From responsive materials to advanced membrane-based sample preparation and sensing.

Smart deep eutectic solvents (smart DES), an emerging class of designer solvents capable of reversible, stimulus-triggered switching of their physicochemical properties in response to external stimuli such as CO2, pH, temperature, and light, offer new capabilities in analytical chemistry. Unlike conventional DES used merely as green alternatives, smart DES provide dynamic control over extraction, separation, and detection, enabling enhanced selectivity in specific applications, reusability, and integration into automated analytical platforms. This review critically discusses the classification, stimulus-response mechanisms, and analytical applications of truly switchable DES, with special emphasis on membrane-based analytical formats, including switchable supported liquid membranes and stir membrane devices. Applications covered include switchable liquid-phase microextraction of organic pollutants, metals, and biomolecules; stimuli-tunable stationary phases and mobile phase additives in liquid chromatography and capillary electrophoresis; and electrochemical and optical sensors where DES responsiveness generates a measurable signal. Key challenges such as viscosity, long-term stability, standardisation, and compatibility with analytical instrumentation are critically evaluated. Their environmental footprint is context-dependent, requiring balanced assessment of toxicity, energy inputs, and end-of-life impacts. Finally, we outline a roadmap for the rational design of multi-stimuli-responsive DES, their integration into lab-on-a-chip and automated sample preparation systems, and the unexplored potential of supramolecular and chiral smart DES. By shifting from static green solvents to dynamic functional materials, smart DES represent a promising direction for next-generation analytical chemistry.