Homochiral zeolite imidazolate framework-8-reinforced hollow-fiber liquid-phase microextraction of chiral amino acids in noninvasive fluids followed by capillary electrophoresis with laser-induced fluorescence detection.

Homochiral zeolite imidazolate framework-8 (ZIF-8) combines the inherent characteristics of ZIF-8 with the chiral recognition function and open chiral channel, becoming one preferred choice for enantiomer separation. In this work, ZIF-8 with D-histidine as the chiral carbon center (D-His-ZIF-8) was introduced into the pores to enhance the extraction performance of hollow-fiber liquid-phase microextraction (HF-LPME). A novel method was developed for the selective and sensitive determination of chiral amino acids related to the nervous system by D-His-ZIF-8@HF-LPME coupled with capillary electrophoresis and laser-induced fluorescence detection. Under the optimal conditions, D-type amino acids could obtain higher enrichment efficiency than L-type ones due to the stronger geometry-dependent hydrogen-bonding interactions between L-type amino acids and the chiral channels of ZIF-8 materials modified with D-His, and the maximum enrichment factor reached 640 times (for D-serine). This developed method has been successfully applied to the analyses of four chiral amino acids (D/L-serine and D/L-aspartic acid) in exhaled breath condensate and saliva samples of healthy volunteers. The limits of detection were 0.030-0.30 nM (S/N = 3) in the real sample matrixes. This developed method meets the principles of green analytical chemistry and achieves a greenness score of 80 assessed by the analytical eco-scale tool. This proposed method provides one green alternative for the sensitive and accurate analyses of chiral amino acids in the non-invasive body fluids.