Graphene-supported zinc oxide solid-phase microextraction coating with enhanced selectivity and sensitivity for the determination of sulfur volatiles in Allium species.

A graphene-supported zinc oxide (ZnO) solid-phase microextraction (SPME) fiber was prepared via a sol-gel approach. Graphite oxide (GO), with rich oxygen-containing groups, was selected as the starting material to anchor ZnO on its nucleation center. After being deoxidized by hydrazine, the Zn(OH)2/GO coating was dehydrated at high temperature to give the ZnO/graphene coating. Sol-gel technology could efficiently incorporate ZnO/graphene composites into the sol-gel network and provided strong chemical bonding between sol-gel polymeric SPME coating and silica fiber surface, which enhanced the durability of the fiber and allowed more than 200 replicate extractions. Results indicated that pure ZnO coated fiber did not show adsorption selectivity toward sulfur compounds, which might because the ZnO nanoparticles were enwrapped in the sol-gel network, and the strong coordination action between Zn ion and S ion was therefore blocked. The incorporation of graphene into ZnO based sol-gel network greatly enlarged the BET surface area from 1.2 m2/g to 169.4 m2/g and further increased the adsorption sites. Combining the superior properties of extraordinary surface area of graphene and the strong coordination action of ZnO to sulfur compounds, the ZnO/graphene SPME fiber showed much higher adsorption affinity to 1-octanethiol (enrichment factor, EF, 1087) than other aliphatic compounds without sulfur-containing groups (EFs<200). Also, it showed higher extraction selectivity and sensitivity toward sulfur compounds than commercial polydimethylsiloxane (PDMS) and polydimethylsiloxane/divinylbenzene (PDMS/DVB) SPME fibers. Several most abundant sulfur volatiles in Chinese chive and garlic sprout were analyzed using the ZnO/graphene SPME fiber in combination with gas chromatography-mass spectrometry (GC-MS). Their limits of detection were 0.1-0.7 μg/L. The relative standard deviation (RSD) using one fiber ranged from 3.6% to 9.1%. The fiber-to-fiber reproducibility for three parallel prepared fibers was 4.8-10.8%. The contents were in the range of 1.0-46.4 μg/g with recoveries of 80.1-91.6% for four main sulfides in Chinese chive and 17.1-122.6 μg/g with recoveries of 73.2-80.6% for three main sulfides in garlic sprout.
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