The flavonoids are compounds synthesized by plants, and they have properties such as antioxidant, anticancer, anti-inflammatory, and antibacterial, among others. One of the most important bioactive properties of flavonoids is their antioxidant effect. Synthetic antioxidants have side toxic effects whilst natural antioxidants, such as flavonoids from natural sources, have relatively low toxicity. Therefore, it is important to incorporate flavonoids derived from natural sources in several products such as foods, cosmetics, and drugs. For this reason, there is currently a need to extract flavonoids from plant resources. In this review are described the most important parameters involved in the extraction of flavonoids by unconventional methods such as ultrasound, pressurized liquid extraction, mechanochemical, high hydrostatic pressure, supercritical fluid, negative pressure cavitation, intensification of vaporization by decompression to the vacuum, microwave, infrared, pulsed electric field, high-voltage electrical discharges, and enzyme-assisted extraction. There are no unified operation conditions to achieve high yields and purity. Notwithstanding, progress has been achieved in the development of more advanced and environmentally friendly methods of extraction. Although in literature are found important advances, a complete understanding of the extraction process in each of the unconventional techniques is needed to determine the thermodynamic and kinetic mechanisms that govern each of the techniques.

Zinc nanoparticles (ZnNP) and graphene quantum dots (GQD) were synthesized as nanocomposite. We have studied this nanocomposite for the H₂O₂ detection by photoluminescence (PL). ZnNP enhanced the PL emission of GQD and contributed to H₂O₂ detection.

Photoluminescence (PL) spectroscopy is one of the best methods to detect molecules due to its easiness, fast time of analysis and high sensitivity. In addition, zinc oxide (ZnO) possesses good optical properties and particularly PL emission in these materials have been exploited for their potential use as photocatalyst, light harvesting and photosensor. These PL properties enhance when graphene quantum dots (GQD) are added to ZnO. For these reasons, we investigated the PL performance of ZnO-GQD nanocomposites. In one experiment we evaluated the PL emission of solid samples ZnO and ZnO-GQD. In a second experiment, these samples were also evaluated in aqueous phase to investigate the H₂O₂ effect during an experiment lasting 170 minutes. Both experiments displayed six peaks and they were related to the same PL emission source. The PL emission peak around 415 nm was found to be principal source where GQD are interacting. By varying the GQD amount to low, medium, and high concentration, the effect of H₂O₂ acted consequently, altering the PL emission during experiment in aqueous phase. An oxygen rich environment (ORE) occurred due to H₂O₂ which oxides the ZnO surface. Low GQD concentration resulted affected by an ORE weakening the GQD-ZnO contact, decreasing PL emission. In high GQD concentration, H₂O₂ induced GQD to reach the ZnO surface, increasing the PL emission. Only medium GQD concentration prevented oxidation of ZnO and maintained the PL emission intensity constant. When H₂O₂ concentration increased, for the medium GQD concentration, an excess of charge by peroxides inhibited the charge transfer from GQD to ZnO. This inhibition produces a quenching of the PL emission.