Formation of copper oxide nanoparticles by ESI microdroplets deposition.

The generation of charged microdroplets by using the electrospray ionization (ESI) source of a mass spectrometer is an innovative approach for promoting accelerated chemical transformations in confined environments [1]. Charged microdroplets behave as highly dynamic microreactors characterized by rapid solvent evaporation, strong interfacial electric fields, and steep pH gradients. These conditions lead to reaction rates acceleration up to six orders of magnitude higher than those of the same reactions in bulk. In the last years, copper II oxide nanoparticle (CuONPs) were employed as interesting materials for catalysis, sensing, antimicrobial coatings and energy storage and the search for alternative synthetic strategies can further contribute to the diffusion of CuONPs applications. Accordingly, this study investigates the feasibility of synthesizing CuO nanoparticles through ESI microdroplet deposition of aqueous Copper (II) acetate solutions under mild conditions, avoiding the highly alkaline environments typical of conventional wet-chemical methods. Slightly acidic Cu(CH₃COO)₂ aqueous solutions were electrosprayed using a Z-spray ESI source to generate charged microdroplets directed onto solid substrates, where their coalescence formed reactive thin films [2]. Ionic species and cluster evolution were monitored by ESI mass spectrometry, while the deposited materials were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The influence of deposition time and desolvation conditions was evaluated to optimize nanoparticle formation. ESI-MS analysis revealed the formation of copper–acetate clusters incorporating CuO units already during microdroplets flight, indicating that oxidative processes are initiated within the confined microdroplets environment. Prolonged deposition times produced solid thin films from which CuO was selectively isolated after removal of soluble precursors. The negative voltage of the ESI source proved crucial in directing the reaction toward oxidation, whereas positive or no voltages conditions favored metallic copper formation. SEM images showed nanoparticles homogeneously distributed on the substrates, with primary particle sizes of approximately 30–40 nm forming aggregates of 100–200 nm. EDX and XPS analyses confirmed the formation of copper (II) oxide. The synthesized CuO nanomaterials were used to functionalize graphite-based screen-printed electrodes (SPEs). Deposition time was optimized by cyclic voltammetry in 0.1 M NaOH to maximize the Cu oxidation signal and activation toward glucose oxidation. Catalytic activity increased with desolvation gas flow rate, reaching a maximum at 600 L/h. The proposed approach integrates reaction, deposition, and material fabrication within a single platform, highlighting the potential of electrospray-based techniques to bridge analytical mass spectrometry and materials science for the development of functional nanomaterials and devices [3].