## 3 Results and Discussion
### Cytotoxicity Analysis  
The cytotoxicity activity of CuFe\({}_{2}\)O\({}_{4}\) and ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles at various concentrations against the MCF-7 cells (human breast adenocarcinoma cell line) using MTT assay is shown in Figure 6a,b. The results revealed the dose dependent cytotoxicity effect of both ferrite nanoparticles. The highest cell viability (100%) observed at 0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL began to decrease gradually with the increased concentrations and exposure time from 24 h to 72 h. The highest toxicity effect was observed at 500 \(\mathrm{\SIUnitSymbolMicro g}\)/mL where 17% and 20% cell viability was observed after 72 h in CuFe\({}_{2}\)O\({}_{4}\) and ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles, respectively, suggesting high toxicity at higher concentrations. Similarly, previous studies also have shown the dose dependent cytotoxicity of several types of nanoparticles [20; 35; 54; 55; 37]. Interestingly, both nanoparticles were observed to be less toxic at lower concentrations up to 100 \(\mathrm{\SIUnitSymbolMicro g}\)/mL where about 80% of the cells were viable. However, both nanoparticles' concentrations above 125 \(\mathrm{\SIUnitSymbolMicro g}\)/mL exhibited higher toxicity towards MCF-7 cells. For CuFe\({}_{2}\)O\({}_{4}\) nanoparticles, the concentration which inhibits 50% of the cellular growth (IC\({}_{50}\)) value was observed to be 415, 320 and 260 \(\mathrm{\SIUnitSymbolMicro g}\)/mL after 24 h, 48 h and 72 h of incubation, respectively. While, ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles exhibited IC\({}_{50}\) value of 310, 285 and 221 \(\mathrm{\SIUnitSymbolMicro g}\)/mL after 24 h, 48 h and 72 h of incubation, respectively. This clearly suggests that ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles possess more toxicity compared to CuFe\({}_{2}\)O\({}_{4}\) nanoparticles. This differential activity could be because of differences in their size, surface to volume ratio, shape as well as the magnetic properties as discussed in the earlier sections. Similarly, it has been reported that nanoparticles prepared from different bulk materials explicate their actions which mainly depends on the chemical composition as well as their sizes and shapes [56]. Our study is in agreement with the earlier reports where CaFe\({}_{2}\)O\({}_{4}\) nanoparticles were shown to exhibit high toxicity when used above 250 \(\mathrm{\SIUnitSymbolMicro g}\)/mL  
Figure 5: 2,2-diphenyl-l-picrylhydrazyl hydrate (DPPH) free radical scavenging activities of ascorbic acid, CuFe\({}_{2}\)O\({}_{4}\) and ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles. Each value is expressed as a mean of \(\pm\) Standard Deviation (S.D, \(n=3\)). concentration [40], while MnFe\({}_{2}\)O\({}_{4}\), ZnFe\({}_{2}\)O\({}_{4}\), NiFe\({}_{2}\)O\({}_{4}\) nanoparticles at 100 ug/mL concentration have shown less cell viability [57; 58]. [PERSON] et al. (2005) [59] stated that the magnetic nanoparticles size ranging from 10 to 100 nm is considered to be ideal for their application in the biomedical field. This is due to easy infiltration into cells, increased surface area for attachment and high permeability in solution [31]. The high surface area of the nanoparticles means superior physico-chemical and mechanical characteristics compared to their bulk materials. In this study, we obtained ZnFe\({}_{2}\)O\({}_{4}\) and CuFe\({}_{2}\)O\({}_{4}\) nanoparticles with sizes of 56 and 68 nm, respectively, which is well within their capacity. The mechanism of these nanoparticles may be attributed to the generation of reactive oxygen species (ROS) due to increased intracellular Cu\({}^{2+}\) and Zn\({}^{2+}\) resulting in the subsequent failure of cellular redox machinery [60]. Recently, protein corona-ZnFe\({}_{2}\)O\({}_{4}\) nanoparticles were reported to show lower cytotoxic effects with improved therapeutic effects [61]. Also, superparamagnetic ZnFe\({}_{2}\)O\({}_{4}\) based nanostructures were used in magneto-photothermal therapy of prostate cancer cells (in vitro) and human glioblastoma tumors (in vivo) [62]. Moreover, [PERSON] et al.