Graphene-derivatized silica as an efficient solid-phase extraction sorbent for pre-concentration of fluoroquinolones from water followed by liquid-chromatography fluorescence detection

• Graphene flakes bonding onto silica microparticles is simple and reproducible.
• The material is for the first time used for mixed-mode SPE of fluoroquinolones.
• Better performance is observed in comparison with commercial sorbents.
• Quantitative absolute recovery is gained in raw river water (spike 5–20 ng L−1).
• Good precision, selectivity and sensitivity are observed.

This work presents a novel analytical method based on graphene for the determination of five widely used fluoroquinolones (FQs) in aqueous matrices. The procedure entails solid-phase extraction (SPE) on graphene-derivatized silica (200 mg), followed by liquid chromatography with fluorescence detection. Monolayer graphene oxide (GO) flakes were covalently bonded onto aminopropyl silica microparticles, and then treated with aqueous hydrazine to obtain the reduced GO (RGO). The final material (RGO–silica) was characterized by thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and BET analysis, and for the first time evaluated as mixed-mode sorbent for the SPE of FQs from natural waters. Accuracy was studied on tap/raw river water in the concentration range 5–10,000 ng L−1, obtaining mean absolute recoveries from 72 to 118%. The inter-day precision was good, showing relative standard deviations (RSDs) in the range 5–15%. Sample volumes up to 1 L provided enrichment factors up to 1000, achieving accurate quantification of concentrations as low as 5 ng L−1. The analytes were simultaneously and quantitatively eluted from the RGO–silica cartridge in a single fraction by using acetonitrile combined with aqueous tetrabutyl ammonium hydroxide. The batch-to-batch reproducibility was verified on three independently prepared RGO–silica samples. RGO–silica was advantageous in terms of adsorption capacity and reusability with respect to commercial sorbents; the cartridge proved to be reusable for at least 10 consecutive extractions, with no significant loss of efficiency (recovery >70%). The analytical procedure was applied to the determination of FQs in actual environmental waters.