The uncapped and capped CuO nanocrystals were synthesized by sol–gel method. Tetra Ethyl Ammonium Bromide (TEABr) used as a capping agent to control the size and morphology of the CuO nanocrystals. Control the nanostructure and optical properties of CuO nanocrystals are premeditated by using capping agent. The obtained samples were characterized by means of UV-Visible spectroscopy (UV-Vis), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), Field Emission Scanning electron microscopy (FESEM) along with Energy dispersive X-ray spectroscopy (EDS). UV-Visible technique is used to measure the band gap of the synthesized samples. The XRD spectra of the synthesized samples established the CuO nanocrystals with monoclinic structure. The occurrences of capping agent have been analyzed by Fourier transform-infrared spectroscopy (FT-IR). The FT-IR study discovered that the transmittance peaks at 501 cm-1 and 592 cm-1 were Cu-O stretching mode, and 677 cm-1 was Cu-O monoclinic phase of the uncapped CuO nanocrystals. Further, FESEM study find out the morphology of the synthesized samples, where the surface morphology changes occurred with an addition of capping agent. Antioxidant of CuO nanocrystals evaluated using L-ascorbic acid and BHA as standards. The antioxidant capacity was expressed as ascorbic acid equivalent. Cyclic voltammetry applications investigated of uncapped and TEABr capped CuO nanocrystals.