Switchable hydrophilicity solvents and biosolvents for green microextraction in bioanalysis

Sample preparation is critical in bioanalysis because of complex biological matrices and low analyte concentrations. Conventional liquid–liquid and solid-phase extraction are effective but often consume substantial organic solvents and resources. Green analytical chemistry has therefore promoted miniaturized microextraction approaches that reduce solvent use and waste while maintaining analytical performance. This review critically examines bioanalytical microextraction through two intersecting solvent-design perspectives: stimulus-responsive switchability and renewable/bio-derived solvent design, focusing on switchable-hydrophilicity solvents (SHSs) and bio-derived deep eutectic solvents (DESs)/natural deep eutectic solvents (NADESs). SHSs enable reversible changes in hydrophilicity or ionic character to facilitate extraction and phase separation, whereas bio-derived DES/NADES systems provide tunable alternatives to conventional petroleum-based solvents. Their analytical performance, matrix-related challenges, reproducibility, and greenness are critically assessed. Future developments are expected to emphasize automated and miniaturized platforms, life-cycle assessment, and computationally assisted solvent design. Although molecular simulations are increasingly used to support solvent selection, predictive COSMO-RS and artificial-intelligence approaches remain emerging directions in bioanalysis. Before drugs and other chemicals can be measured in biological samples such as blood, plasma, or urine, the target substances often need to be separated from proteins, salts, and other sample components that may interfere with the analysis. Traditional methods for this sample preparation can use large amounts of organic solvents, generate chemical waste, and require several processing steps. This review examines newer approaches that aim to make this process smaller, simpler, and more environmentally responsible. It focuses on switchable solvents, which can change their ability to mix with water when conditions such as acidity or carbon dioxide are changed, and on alternative solvent systems made partly or fully from renewable materials, including deep eutectic and natural deep eutectic solvents. These solvents can reduce the amount of conventional organic solvent required and can be designed for different target substances. However, being renewable or using a small volume of solvent does not automatically make a method environmentally friendly. Issues such as additional chemicals, energy use, waste generation, interference from biological samples, solvent stability, water uptake, and reproducibility must also be considered. Future developments are expected to combine these solvents with automated and miniaturized extraction systems and use computer-based tools to help identify suitable solvent formulations. More consistent validation and environmental assessment will be important before these approaches can be widely adopted for routine biological analysis.