Brass wires with different surface wettability used for in-tube solid-phase microextraction.

Metal wires have been widely used as substrates for solid-phase microextraction (SPME) fibers instead of commonly fragile silica fibers, but complicated coating modification of their surface is required. Herein, a series of brass wires were soaked in an acidic iron trichloride solution with ultrasonication, which etched the brass surface through a redox reaction. The surface wettability of the pristine brass wire was transformed from hydrophobic to hydrophilic owing to the formation of micro/nanoscale hierarchical structures. After modification with n-octadecanethiol (ODT) and 2-naphthalenethiol (NT), respectively, both wires exhibited superhydrophobicity. Characterization of the resulting wires was conducted using SEM and EDS, and the surface wettability was measured by a contact angle goniometer using identical brass meshes. To build an in-tube SPME-high-performance liquid chromatography (HPLC) online system, the extraction tube was connected with HPLC equipment by replacing the sample loop of a six-port valve. Four types of wires, including the pristine hydrophobic brass wire, the hydrophilic wire after chemical etching, and both superhydrophobic wires, were comparatively applied to the extraction of six estrogens. The optimized extraction conditions were a sample volume of 60 mL, an injection rate of 2 mL/min, and a desorption time of 2 min at a flow rate of 1 mL/min. The results showed that the highest estrogen extraction efficiency was obtained using the superhydrophobic wire modified by NT, with the enrichment factors in the range of 36-350. Furthermore, the superhydrophobic NT wire exhibited a higher extraction efficiency than the ODT wire with identical superhydrophobicity. This demonstrated that the higher extraction efficiency was mainly dependent on  π-π interactions between the sorbent containing naphthalene rings and the target compounds containing benzene rings, rather than surface wettability.