Synthesis and characterizations of water-based ferrofluids of substituted ferrites [Fe 1x B x Fe 2 O 4 ,B ¼ Mn, Co (x ¼ 0–1)] for biomedical applications Jyotsnendu Giri a,1 , Pallab Pradhan b , Vaibhav Somani a , Hitesh Chelawat a , Shreerang Chhatre a , Rinti Banerjee b , Dhirendra Bahadur a,Ã,2 a Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Mumbai 400076, India b School of Biosciences and Bioengineering, Indian Institute of Technology, Mumbai 400076, India Abstract Nanomagnetic particles have great potential in the biomedical applications like MRI contrast enhancement, magnetic separation, targeting delivery and hyperthermia. In this paper, we have explored the possibility of biomedical applications of [Fe 1x B x Fe 2 O 4 , B ¼ Mn, Co] ferrite. Superparamagnetic particles of substituted ferrites [Fe 1x B x Fe 2 O 4 ,B ¼ Mn, Co (x ¼ 0–1)] and their fatty acid coated water base ferrofluids have been successfully prepared by co-precipitation technique using NH4OH/TMAH (Tetramethyl- ammonium hydroxide) as base. In vitro cytocompatibility study of different magnetic fluids was done using HeLa (human cervical carcinoma) cell lines. Co 2+ -substituted ferrite systems (e.g. CoFe 2 O 4 ) is more toxic than Mn 2+ -substituted ferrite systems (e.g. MnFe 2 O 4 , Fe 0.6 Mn 0.4 Fe 2 O 4 ). The later is as cytocompatible as Fe 3 O 4 . Thus, Fe 1x Mn x Fe 2 O 4 could be useful in biomedical applications like MRI contrast agent and hyperthermia treatment of cancer. Keywords: Ferrite nanoparticle; Ferrofluid; Biocompatibility 1. Introduction In recent years, nanomagnetic particles have generated a lot of interest in the biomedical applications like MRI contrast enhancement, magnetic separation, targeted de- livery and hyperthermia [1–3]. Among the different magnetic oxides, Fe 3 O 4 and g-Fe 2 O 3 particles have been extensively studied as they are known to be biocompatible within certain threshold limits. Substituted ferrite systems Fe 1x B x Fe 2 O 4 (B ¼ Mn, Co) where certain degree of substitution of Fe(II) ions by other divalent ions (Mn or Co) can increase the magnetic moment. In MRI, super- parmagnetic nanoparticles are used as contrast agent and the relaxation times (T2, spin–spin) of magnetic nanopar- ticles are determined by their effective anisotropy constant and the mean magnetic volume. Thus, it is expected that these substituted ferrite particles have versatile spin–spin relaxation time depending on their compositions. Another prospective application of magnetic nanoparticles is hyperthermia treatment of cancer [4,5]. Heating of specific tissues or organs inside the body preferentially for treatment of cancer called hyperthermia has been known since long time. The challenging work is the development of heating mediator (magnetic nanoparticles) with high specific absorption rate (SAR), which allows reduction of ferrofluid dose in vivo. SAR depends on several parameters like particles magnetization, size and distributions, AC magnetic field and frequencies [6]. Literature is avai- lable on the maximization of SAR of Fe 3 O 4 /g-Fe 2 O 3 ARTICLE IN PRESS