## 1 Introduction  
For some time, magnetic fine nanoparticles have been of major research interest because of their technological importance. Among the whole magnetic family, spinel type magnetic ferrites, MFe\({}_{2}\)O\({}_{4}\) (M = Mn, Ca, Mg, Zn, Ni, Co, Cd. etc.) [1; 2; 3; 4; 5; 6], have great importance in electronic, magnetic and especially biomedical applications as well as in modern technologies. Spinel ferrites are used in a broad range of applications; magnetic refrigeration, Ferro fluids, high density recording, spintronics,drug delivery, bio sensors, magnetic resonance imaging and magnetic hydrothermal [7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Copper ferrite is a ferromagnetic material that crystallizes in tetragonal spinel with space group I41/amd, and forms a crystal system based on the anti-parallel alignment of the magnetic moments of A and B sub lattices. ZnFe\({}_{2}\)O\({}_{4}\) is one of the most commonly studied materials. It crystallizes in the spinel structure with octahedral B sites occupied by Fe\({}^{3+}\) and the tetrahedral A sites preferentially occupied by Zn\({}^{2+}\). Nowadays, nanoparticles have gained more attention because of their extraordinary properties, differing from bulk materials [17; 18; 19; 20]. It is well known fact that material properties majorly depend on the different method of synthesis. For spinel ferrite synthesis, physical and chemical approaches are reported such as sol-gel, flash combustion, citrate gel, co-precipitation, hydrothermal synthesis, sol-gel auto combustion method, and micro-emulsion [17; 18; 19; 21; 22; 23; 24; 25; 26; 27]. Among all the different synthesis roots, co-precipitation [28] and sol-gel combustion [29] allow good control over particle size and uniform distribution, enhancing the extraordinary properties of ferrites. The sol-gel self-combustion technique is an effective method to produce small and uniform particles at low temperatures [30]. More recently, considerable attention has been paid to magnetic nanoparticles due to their prospective applications in the field of biomedicine, biotechnology, material science and engineering. These nanoparticles exhibit good magnetic properties as well as various molecular and cellular level interactions in several biological processes [31]. Magnetic nanoparticles are highly preferable; being biocompatible, they are relatively less toxic and possess magnetic properties [32; 33]. Free radicals are molecules or atoms with an unpaired electron. These are highly reactive species that destabilize other molecules and produce many more free radicals. In biological systems, free radicals are formed as a part of the body's normal metabolic processes when biomolecules interact with oxygen. However, increased levels of free radicals are detrimental to human health as they cause several disorders, including cancer, myocardial infarction, atherosclerosis, and neurodegenerative disorders [34]. Chemical substances, known as antioxidants, can scavenge these free radicals and, thus, decrease the occurrence of oxidative stress induced cell death or damage. Researchers have proved that nanoparticles can act as synthetic antioxidants in the body and possess good antioxidant properties compared to their bulk material counterparts, which is attributed to the increased surface to volume ratio of the nanostructures [35; 36; 37]. Similarly, the increased surface area of nanoparticles can be properly functionalized to bind specifically to cancerous cells and, thus, offer an approach for cancer therapy. Moreover, it has been stated that drug delivery systems using nanoparticles are very efficient in the treatment of cancer because of their superior bio-distribution profile and pharmacokinetics [38]. However, potential applications of nanoparticles can be considered practically only when their toxic effect is understood. Though various ferrite nanoparticles are synthesized and characterized, their biological activities including cytotoxicity are not well explored yet.