3536 | New J. Chem., 2019, 43, 3536--3544 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 Cite this: New J. Chem., 2019, 43, 3536 Synthesis and structural characterization of Co x Fe 3x C (0 r x r 0.3) magnetic nanoparticles for biomedical applications A. Gangwar, a G. Singh, a S. K. Shaw, a R. K. Mandal, a A. Sharma, b Sher Singh Meena, c C. L. Prajapat de and N. K. Prasad * a Herein, cobalt-substituted iron carbide (Co x Fe 3x C, 0 r x r 0.3) magnetic nanoparticles (15–30 nm) were prepared by a sol–gel assisted route. X-ray diffraction (XRD) and electron diffraction patterns suggested that all the samples had a single phase structure. Furthermore, Mo ¨ ssbauer spectroscopy validated the monophasic nature of all the Co-substituted iron carbides, except for x = 0.3. The results of Mo ¨ ssbauer spectroscopy for this sample indicated the presence of the Fe–Co alloy along with the carbide phase. The saturation magnetization value was found to decrease with an increase in Co concentration (x r 0.16); however, it increased for the x = 0.3 sample due to the presence of a secondary phase (Fe–Co alloy). Ferrofluids of the pure and Co-substituted iron carbides in oleic acid exhibited good heating abilities during the magnetic hyperthermia experiment at different external fields. Introduction Nanometer magnetic materials have been included in various domains of scientific research and applications 1,2 such as in drug delivery, 3 magnetic resonance imaging (MRI), 4 magnetic hyperthermia treatment (MHT), 5 etc. Due to their submicroscopic dimension, magnetic nanoparticles (MNPs) can drastically differ in physical characteristics as compared to their bulk counterparts. 6 MNPs have gained popularity because of their ability to be get functionalized at both cellular and molecular levels. 7,8 Metallic iron nanoparticles have a much higher saturation magnetization value (M S )(e.g., 220 A m 2 kg 1 for bulk Fe) than iron oxides such as Fe 3 O 4 and g-Fe 2 O 3 (e.g., 90 A m 2 kg 1 for bulk Fe 3 O 4 ); 9–11 however, the former are still inappropriate for medical applications due to their toxicity in physiological environments; on the other hand, the latter have been explored extensively for various biomedical applications due to their suitable biocompatibility, size, and magnetic nature. 12 In contrast, iron carbide (Fe 3 C) could be a fascinating candi- date for bioapplications due to its high M S value (B140 A m 2 kg 1 for bulk) than that of iron oxides and high chemical stability. 13 This intermetallic carbide has an orthorhombic lattice where carbon atoms occupy the interstices between close-packed iron atoms. 14 Moreover, carbon present in it provides better mechanical strength and chemical inertness as compared to the case of oxide materials; 15 furthermore, as a ceramic, iron carbide displays excel- lent oxidation as well as corrosion resistance behaviors. 16 The bulk or nanoparticles of Fe 3 C have several advantages over iron oxide nanoparticles. However, the tedious processes conducted to obtain the pure form limit its wide applicability. 17,18 There are a few studies on the synthesis techniques used to obtain Fe 3 C nanoparticles. For example, Wagner and Nelson synthesized nanocrystalline Fe 3 C using a sub-ambient alkaloid reduction technique at an annealing temperature of 950 1C. 19 On the other hand, Narkiewicz et al. initially reduced magnetite with H 2 at 500 1C followed by carburization at 520 1C using a mixture of CH 4 /H 2 . 20 Similarly, Jang et al. initially decomposed Fe(CO) 5 into Fe and carburized it in the presence of CH 4 to generate a carbide phase. 21 In addition, one group prepared Fe 3 C by a sol–gel assisted route followed by calcination at 640 1 C using FeCl 3 , ethylene- diamine, and hexamethylenetetramine (HMTA) as precursors. 18 Furthermore, a significant number of studies have been reported on the synthesis and properties of pure and substituted magnetic iron oxides (M x Fe 3x O 4 or g-M x Fe 2x O 3 ; M = Co, Ni, Mn, Zn, Al, Zr, Hf, Li, etc.;0 r x r 1). 22–28 These studies have reported the huge potential of these MNPs for various applications. However, only a few studies have been reported on the effects of substitutions (e.g., Mn or Ni substitution) on the properties of Fe 3 C. It has been observed that Mn doping enhances, whereas a Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India. E-mail: nandkp.met@iitbhu.ac.in; Fax: +91-5422369478; Tel: +91-5422369346, +91-9956629843 b Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400076, India c Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India e Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India Received 15th October 2018, Accepted 23rd January 2019 DOI: 10.1039/c8nj05240a rsc.li/njc NJC PAPER Published on 24 January 2019. Downloaded by Bhabha Atomic Research Centre on 2/19/2019 9:10:00 AM. 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