2.2 Thermal properties

magnetization curve in zero-field-cooled, where Co3O4 nanoparticles exhibit weak ferromagnetism. Alrehaily et al. [15] synthesized Co3O4 nanoparticles by gamma irradiation of 0.2–0.3 mM of CoSO4 solutions. Syam Sundar et al. [18] used chemical coprecipitation method for the synthesis of Co3O4 nanoparticles, and they estimated their thermal properties at different particle volume concentrations and temperatures.

Cobalt Compounds and Applications

Co3O4 nanoparticle toxicity in A549 cells: hydrodynamic size distributions of Co3O4 nanoparticles in RPMI 1640 medium with 10% fetal bovine serum (FBS) at t = 0 and after a 24-h exposure without and with A549 cells. In the right panels, the relative Co3O4 NP suspensions used for DLS measurements, showing NP

Co3O4 nanoparticle toxicity in BEAS-2B cells: hydrodynamic size distribution of Co3O4 nanoparticles in

BEGM medium at t = 0 and after a 24-h exposure without and with BEAS-2B cells [16].

sedimentation after a 24-h exposure in the medium without cells [16].

Figure 3.

Figure 4.

34

Co3O4 nanofluids have a potential use in several mechanical engineering applications; in particular, as replacement of low thermal conductivity fluids such as water and ethylene glycol, as a consequence, the thermal properties of nanofluids are of great interest. Mariano et al. [12] prepared Co3O4 nanofluids by dispersing cobalt(II, III) oxide nanopowder in ethylene glycol and determined experimentally their thermal conductivity and viscosity. The thermal conductivity and viscosity of Co3O4/EG nanofluids are shown in Figure 2a and b; it is noticed that the increase of particle volume concentration ð Þ ϕ yields increased values of thermal conductivity and viscosity. They observed thermal conductivity enhancement for 5.7% volume concentration of Co3O4/EG nanofluids is 27% at temperature of 323.15 K (Figure 2a); similarly, the viscosity enhancement for 5.7% volume concentration of Co3O4/EG nanofluids is 40% at a temperature of 303.15 K (Figure 2b).

### 2.3 Toxicity

Knowledge about the toxicity of Co3O4 nanoparticles is very important considering their eventual use in medical applications. Cavallo et al. [16] studied the cytotoxicity of Co3O4 nanoparticles in human alveolar (A549) and bronchial

#### Figure 5.

Co3O4 nanoparticle toxicity in HepG2 cells: representative microphotographs showing Co3O4 NP- and Co2+-induced ROS generation in HepG2 cells. Images were snapped with Nikon phase contrast with a fluorescence microscope. (A) Control, (B) 15 μg/mL of Co2+, (C) 15 μg/mL of Co3O4NPs, and (D) percentage change in ROS generation after 24 and 48 h of exposure to various concentrations of Co3O4NPs and Co2+ in HepG2 cells [17].

(BEAS-2B) cells exposed to 1 40 μg=mL. In A549 cells, they found no cytotoxicity; however, BEAS-2B cells presented viability reduction at 40 μg=mL and early membrane damage at 1, 5, and 40 μg=mL. The results related to toxicity study are presented in Figures 3 and 4. Alarifi et al. [17] investigated the toxicity of Co3O4 nanoparticles in HepG2 cells and observed cytotoxicity and genotoxicity in HepG2 cells through ROS and oxidative stress, and their results are presented in Figure 5.
