Pure metallic copper nanoparticles free of any substrate were synthesized by the thermo-chemical reduction of copper acetate using triethanolamine as a reducing-cum-protection agent. The structure and physical and magnetic properties of the Cu NPs were analysed physicochemically. Microscopic analysis reveals the formation of particles of size of 3-5 nm as seen by TEM but present as a large agglomeration as identified by SEM. A structure of Cu is predicted for the Cu NPs on the basis of investigations using XPS, MALDI, EPR, and magnetic measurements and supported by the prediction of DFT calculation from an earlier work. The most important findings come from magnetization studies which prove the existence of giant diamagnetism from the nanomer clusters of copper as well as the formation of two different ferromagnetic transitions at ∼40 K and ∼100 K, the latter two arising from the surface properties possibly due to thin films of CuO and/or the presence of TEOA giving rise to temperature dependent coercivity revealing them to be soft room temperature ferromagnets. The clusters of Cu NPs with the identified structure show temperature and field dependent giant diamagnetism which is about 29-39 times larger than the diamagnetism calculated from known and established atomic values. Though such enhanced diamagnetism has been predicted for noble metal clusters, experimental observation so far has been restricted to Au and Pt and this is probably the first report on substrate-free metallic copper clusters.

Cu clusters on ZnO have been prepared by a simple low-temperature solid-state reaction from their respective acetate precursors. The formation of metallic Cu along with a small quantity of CuO was influenced by the presence of the zinc acetate precursor. Although there is a lack of formation of any metallic Cu in the absence of zinc acetate, increase in the heating duration helps in the formation of increased metallic Cu. A mechanism for formation of the Cu@ZnO nanocomposite has been suggested. The prepared Cu@ZnO nanocomposite, with metallic Cu, was identified by X-ray diffraction studies followed by confirmation of clusters of the kind Cu9 and Cu18 by transmission electron microscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The photoelectron spectroscopy is able to clearly distinguish the Cu from CuO, which is very well complimented by electron spin resonance analysis. The morphological feature of ZnO changes from flakes to rods on increasing the duration of heating, as shown by scanning electron microscopy (SEM) analysis. The observed Cu plasmonic band in UV–vis diffuse reflectance gets blue-shifted to 463 nm from its normally observed position of 550–580 nm possibly due to cluster formation and interaction with ZnO, the band gap of the latter getting red-shifted to 3.2–3.0 eV. The antibacterial activity of the synthesized Cu cluster–ZnO nanocomposites was investigated against Escherichia coli ATCC-25922 for Gram-negative and Bacillus cereus ATCC-10876 for Gram-positive bacteria. Tests were performed on a nutrient agar medium and liquid broth supplemented with different concentrations of nanoparticles. SEM analysis of the native and treated Gram-positive and Gram-negative bacteria established a high efficacy of biocide activity in 24 h, with 200 μg/mL of Cu@ZnO nanocomposites.