The onion is one of the most cultivated and consumed vegetables, and is a very valuable source of antioxidant substances. Every onion variety is rich in flavonols—mainly quercetin derivatives which makes onions the main dietary source of these compounds. This study intends to develop an ultrasound-assisted extraction method (UAE), an economical, green, and efficient technique, that allows us to determine the quality of onion extracts in terms of flavonol composition and antioxidant activity. For this purpose, an ultrasound-assisted extraction method has been optimized to obtain extracts with a high amount of flavonols, as well as with a high antioxidant activity, not only separately using a Box–Behnken design, but also simultaneously, based on multi-response optimization. Multiple response optimization has not only demonstrated an efficacy level similar to that of the individual ones, but it also represents a considerable reduction in terms of cost, time and work. The optimal conditions for simultaneous extractions were determined as follows: 76.8% methanol as an extraction solvent at pH 2 and 58.5 °C temperature, 85% amplitude, 0.9 s cycle, and 0.2:13 g:mL sample mass/solvent volume ratio. Furthermore, the developed method exhibited a high precision level and great recoveries in a rather short extraction time (2 min). These results, together with the development of a fast and simple UHPLC analysis method, lead us to consider the developed UAE method as a suitable technique for the extraction of bioactive compounds from onion matrices.

A Schiff base-based fluorescent chemosensor has been studied for divalent copper detection. The formation of 2-hydroxybenzaldehyde benzoylhydrazone-Cu²⁺ complex induced a fluorescence quenching of this compound in a medium of water/ethanol (53% v/v) and 0.05 M phosphate buffer (pH 7.0). The continuous variations and mole-ratio plots of absorbance suggested a complex formation with a 1:1 metal-ligand stoichiometry. The conditional stability constant for the complex was evaluated to be 6 × 10⁶ M⁻¹. A modified Stern–Volmer relationship was employed to obtain a linear calibration plot, obtaining a dynamic working range up to 157.4 μM. The detection limit of this system was found to be 5.6 μM and the relative standard deviation for five measurements of 78.7 μM concentration was 5.2%. This fluorescent chemosensor also showed a high selectivity for copper ions over other metal ions, such as Al³⁺, Ca²⁺, Cd²⁺, Fe²⁺, K⁺, Mg²⁺, Na⁺, Pb²⁺, or Zn²⁺. The results of this investigation show a simple, rapid, low-cost, and selective method that can operate in neutral solutions and is useful for biological and environmental applications.