Deep Eutectic Solvent Applications in Liquid-Liquid Microextraction

Deep eutectic solvents (DESs) have emerged as versatile, environmentally benign extractants for liquid–liquid microextraction (LLME), owing to their low volatility, tunable polarity and straightforward preparation from readily available components. By combining a hydrogen‐bond acceptor (for example choline chloride) with a hydrogen‐bond donor (such as a carboxylic acid, alcohol or amide), DESs attain melting points substantially below those of their individual constituents. This depression in melting point grants a liquid extractant at ambient temperature that can be tailored to be hydrophilic or hydrophobic. In LLME applications, DESs serve to concentrate trace analytes—from pharmaceuticals and endocrine disruptors to food contaminants—into microlitre‐scale phases that are readily recovered for subsequent analysis. Hybrid approaches, including dispersive LLME, solidification of the floating organic droplet and back‐extraction, exploit the unique viscosity and solidification behaviour of selected DESs to enhance phase separation and enrichment factors. These techniques minimise organic solvent consumption, afford high selectivity through component choice and permit direct hyphenation to chromatographic or spectrometric detectors. The global significance of DES‐based LLME is underscored by its adoption in food safety, environmental monitoring and clinical assays, where rapid, green and cost‐effective sample preparation is paramount.

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Recent studies have demonstrated the efficacy of hydrophobic DESs in antibiotic residue analysis. One work developed a liquid–liquid microextraction method using thymol–decanoic acid and related DES mixtures to isolate tigecycline from bovine milk. By optimising the DES molar ratio, pH and extraction conditions, limits of detection down to 0.01 μg kg⁻¹ were achieved, with precision under 8% RSD and compatibility with LC-MS/MS quantification. This approach exemplifies how DES composition can be fine‐tuned to target polar drug molecules in complex matrices.

Another investigation explored dispersive LLME coupled with solidification of the floating organic droplet (DLLME-SFO) using fenchol–acetic acid DESs for the extraction of emerging contaminants—bisphenols, hormones and phthalates—from water and urine. By selecting optimal molar ratios (2:1 and 1:1), the method delivered recoveries of 49–100% and enrichment factors up to 100, with simple centrifugation and minimal equipment. The greenness of the workflow was validated using established metrics, highlighting its potential for routine environmental screening.

A comprehensive review of green solvent extraction techniques for antibiotic residue determination emphasised the rise of DES-based LLME as a sustainable alternative to conventional organic solvents. The survey outlined successes in extracting sulfonamides, tetracyclines and macrolides across food and water matrices, underscoring the role of DES viscosity, hydrogen‐bonding interactions and component biodegradability in achieving high selectivity and throughput.

Deep Eutectic Solvent Applications in Liquid-Liquid Microextraction publication trend

The graph below shows the total number of articles in deep eutectic solvent applications in liquid-liquid microextraction across all publications each year (not limited to Nature Index journals).

Technical terms

Deep eutectic solvent (DES): A liquid solvent formed by mixing two or more solid components, usually a hydrogen‐bond acceptor and donor, whose eutectic melting point is lower than that of each component.

Liquid–liquid microextraction (LLME): A sample preparation technique where a small volume of extractant is dispersed in an aqueous sample to concentrate target analytes into the extractant phase.

Hydrophobic DES: A deep eutectic solvent formulated to have low miscibility with water, enhancing extraction of non‐polar or moderately polar compounds.

Dispersive LLME: A variation of LLME in which the extractant is rapidly dispersed throughout the sample, often by vortexing or injection with a dispersive solvent, to speed analyte transfer.

Solidification of floating organic droplet (SFO): A LLME approach that exploits the solidification of a hydrophobic extractant at low temperature to facilitate phase separation and recovery.

Enrichment factor: The ratio of analyte concentration in the extractant phase to its initial concentration in the sample, indicating extraction efficiency.

References

1. Microextraction of Tigecycline Using Deep Eutectic Solvents and Its Determination in Milk by LC-MS/MS Method. Journal of Agricultural and Food Chemistry (2023).
2. Extraction of Emerging Contaminants from Environmental Waters and Urine by Dispersive Liquid–Liquid Microextraction with Solidification of the Floating Organic Droplet Using Fenchol:Acetic Acid Deep Eutectic Mixtures. ACS Sustainable Chemistry & Engineering (2022).
3. A review of green solvent extraction techniques and their use in antibiotic residue analysis. Journal of Pharmaceutical and Biomedical Analysis (2021).