Membrane-protected covalent organic framework fiber for direct immersion solid-phase microextraction of 17beta-estradiol in milk

17beta-estradiol (E2) could accumulate in human body through milk and cause various diseases by interfering with the endocrine system. Herein, we coated stainless steel wire with covalent organic framework LZU1 (COF-LZU1) and Nafion protected by dialysis membrane for direct immersion solid phase microextraction (DI-SPME) and coupled with gas chromatography-flame ionization detection (GC-FID) for the detection of trace E2 in milk samples. With dialysis membrane protection, the stability of SPME fiber was improved and the extraction efficiency was only reduced by 7% after repeated use of 160 times. The extraction efficiency of E2 with the home-made fiber COF-LZU1 was 22.1, 8.4, 3.6 times higher than that of bare stainless steel wire, PDMS/DVB and PDMS, respectively. The method had been successfully applied to milk samples, and the relative recoveries were between 77.27% and 108.26%. It can provide an effective and general method for the pretreatment of complex matrix samples.