H3po4

All reagents used were analytical-grade and HPLC-grade purity. Methanol, water, acetonitrile, and H₃PO₄ were obtained from Chem-Lab NV (Zeldegem, Belgium). Folin–Denis reagent, DPPH, and caffeic acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). Oleuropein (CAS: 32619-42-4) and 3-hydroxytyrosol (CAS: 10597-60-1) standards were obtained from Tokyo Chemical Industry Co. (TCI, Tokyo, Japan). Oleocanthal (CAS: 289030-99-5) and oleacein (CAS: 149183-75-5) standards were purchased from Carbosynth (Compton, UK) and PhytoLab (Vestenbergsgreuth, Germany), respectively.

Graphite oxide (GtO) was produced by Gt oxidation (size ≤ 20 µm, Sigma Aldrich, St. Louis, MO, USA) using the modified Hummers method, as described elsewhere [18 (link),50 (link)]. Briefly, 4 g of graphite was added to a mixture of 40 mL of phosphoric acid (H₃PO₄, Chem-Lab, Zedelgem, Belgium) and 160 mL of sulfuric acid (H₂SO₄, VWR, Frankfurt, Germany) under stirring, and cooled using an ice bath. Then, 24 g of potassium permanganate (KMnO₄, JMGS, Odivelas, Portugal) were added gently under stirring. Subsequently, 600 mL of H₂O was slowly added, controlling temperature using an ice bath. Finally, hydrogen peroxide (H₂O₂, 26.5 mL, VWR, Frankfurt, Germany) was added and the mixture was left to rest overnight. Afterwards, the solution was decanted to separate the solid phase from the acidic solution, centrifuged at 4000 rpm for 20 min and redispersed in distilled water. The process was repeated until water pH was achieved in the supernatant. The pellet was recovered, redispersed in distilled water and sonicated for 8 h using a high-power ultrasonic probe (UIP1000hd, Hielscher Ultrasonics GmbH, Teltow, German) to simultaneously exfoliate GtO and breakup the sheets to lateral sizes close to a hundred nanometers, yielding the final product, nanographene oxide (GOn) at a concentration of 7 mg mL⁻¹, which was further diluted for testing.