<p>In this work, Cu-doped ZnSe (ZnSe:Cu) quantum dots were prepared by a photochemical method, and Fe₃O₄/AC/ZnSe:Cu nanocomposites were fabricated by a hydrothermal method. ZnSe:Cu quantum dots were utilized as a fluorescence sensor for the lead ion detection in water. The results showed that ZnSe:Cu quantum dots had an excellent capability for the detection of lead ions. The ZnSe:Cu had good sensitivity with a low limit of detection at about 3.1 nM. The Fe₃O₄/AC/ZnSe:Cu nanocomposites were characterized by various analyses, and the successful formation of Fe₃O₄/AC/ZnSe:Cu nanocomposites was confirmed by VSM, FT-IR, XRD, Zeta potential, EDS, and Raman analysis. Also, FESEM and TEM analysis confirmed successful spherical fabrication of Fe₃O₄ and ZnSe:Cu nanoparticles on activated carbon substrate. The Fe₃O₄/AC/ZnSe:Cu nanocomposites had excellent catalyst activity for the treatment of water and wastewater from methylene blue and crystal violet dyes. The results showed that Fe₃O₄/AC/ZnSe: Cu nanocomposites completely removed methylene blue and crystal violet dyes after 4 min and 6 min from water, respectively. Based on this research, ZnSe:Cu QDs had excellent ability for application as a fluorescence sensor, and Fe₃O₄/AC/ZnSe:Cu nanocomposite had high ability to be used high-performance nanocatalyst for treating water from pollutant dyes.</p>