Ribbon-shaped supramolecular solvents: Synthesis, characterization and potential for making greener the microextraction of water organic pollutants

Liquid-liquid microextraction (LLME) techniques have experienced a tremendous growth over the last years but still face major challenges related to the use of more efficient and environmentally friendly solvents. Supra-molecular solvents (SUPRASs) have proved outstanding efficiency in LLME, but many of the experimental conditions required for SUPRAS formation and/or application cannot be considered green or experimentally convenient. This paper was intended to make greener both SUPRAS formation and their application to the LLME of low-concentration organic pollutants in environmental waters. For this purpose, a variety of SUPRASs were produced at room temperature by simply mixing alkyl phosphonates (A6-12PO3H- and A6-12PO3-2) and tetra-hexylammonium (He4N+) ions in aqueous media. Among them, the SUPRASs produced from decyl hydrogen phosphonate (DePO3H-) and He4N+ allowed, for the first time, the development of SUPRAS-based LLMEs where the SUPRAS previously synthesized was added to the liquid sample, instead of being formed in situ as usual, which was proved particularly advantageous for analyses involving large sample/SUPRAS volume ratios. At near equimolar amounts of DePO3H- and He4N+, the amphiphile arranged in the SUPRAS as planar ribbons consisting of water (21 +/- 3%, w/v) and DePO3H- and He4N+ in the concentration range 1.0-1.4 M. The application of these SUPRASs to LLMEs was proved by extracting carcinogenic polycyclic aromatic hydrocarbons (CPAHs) from drinking (tap and bottled) and natural (river, reservoir and underground) water (recoveries between 84 and 117% with standard deviations varying between 1 and 14%). The developed method was simple (it only required the addition of 500 mu L of SUPRAS to 75 mL of sample, stirring and centrifugation), sensitive (method quanti-tation limits were below the maximum allowed limits set by the EU; were 0.6-7.1 ng L-1) and selective (SUPRAS extracts were directly analyzed by liquid chromatography-fluorimetry). This research proves that SUPRASs can be operationally used in LLMEs similarly to conventional solvents, which should favor their routine application in high-sample throughput laboratories.

Automated summary

Liquid-liquid microextraction (LLME) techniques have shown great potential in extracting low-concentration organic pollutants from environmental waters. However, the use of more efficient and environmentally friendly solvents remains a challenge. This study aimed to make the formation and application of supra-molecular solvents (SUPRASs) greener for LLME. The developed SUPRAS-based LLME method proved to be simple, sensitive, and selective, making it suitable for routine application in high-throughput laboratories.