There have been an increasing number of investigations on various aspects of magnetic nanomaterials in the past two decades (Jasmin de Souza et al.
2017; Lee et al.
2013; Pankhurst et al.
2009; Shubayev et al.
2009; Sun et al.
2008; Vallabani et al.
2019; Wang et al.
2001). Among the different magnetic nanoparticles studied to date, iron oxide nanoparticles have received much attention as contrast agents for use in magnetic resonance imaging (MRI) (Chung et al.
2011; Maurea et al.
2014; Vasanawala et al.
2016; Wang
2011; Wang and Idée
2017). According to the hydrodynamic size of iron oxide nanoparticles, these iron species are classified into three categories of particles: micro-size iron oxide (MPIO; > 1000 nm), superparamagnetic iron oxide (SPIO; 60–180 nm), and ultra-small superparamagnetic iron oxide (USPIO; < 50 nm). Subsets of USPIO are also identified as very small nanoparticles of iron oxide (VSOP; 7–9 nm) and monocrystalline iron oxide nanoparticles (MION; 10–30 nm). Most of the iron oxide nanoparticles used as contrast agents in MRI are composed of both Fe(II) and/or Fe(III) cations and O
2− and/or OH
− anions forming a group of oxides, hydroxides, and oxy-hydroxides (Laurent et al.
2008). In the presence of an external magnetic field, these magnetic particles tend to align with the field and exhibit net magnetization in the direction of the magnetic field (Di Marco et al.
2007). It should be emphasized that a major disadvantage of iron oxide nanoparticles is related to their relatively moderate magnetic performance (e.g., in comparison to bare Fe nanoparticles), which is considered to be an important parameter in molecular MRI. To elucidate this in a better way, one may compare the saturation magnetization of the magnetite (Fe
3O
4) and alpha iron (Fe) nanoparticles. The maximum achievable magnetization for the first and second phases is estimated to be ca. 80 and 224 emu/g (both values refer to room temperature), respectively (Pankhurst et al.
2009; Tartj
2006). In other words, one can easily predict that the mitigation of signal intensity observed on postcontrast T2- and T2*-weighted MR images can be achieved at the lower concentration, when pure Fe nanoparticles are used to obtain data. Hence, the nanoparticles comprising pure iron (Fe) phases have substantially better magnetic characteristics for application in cancer diagnostics based on molecular MRI. It should be noted that magnetic nanoparticles composed of different bare metals (i.e., Fe, Co, Ni, or their alloys) have been found to have some limited resistance properties to oxidation and agglomeration/aggregation processes (El-Gendy et al.
2009). These drawbacks can be substantially overcome by encapsulation of pure metallic nanoparticles in carbon layers, leading to production of the so-called carbon-encapsulated iron nanoparticles (CEINS) (Bystrzejewski et al.
2007). Such heterogeneous nanoparticles are composed of the metallic core (generally of the spherical shape), which is covered by few layers of graphene coating, leading to the “core-shell”–type magnetic nanostructures (Bystrzejewski et al.
2007). The carbon coating is mainly formulated of highly crystalline and curved graphene monolayers, which protect the core against any oxidization processes, preserve its specific magnetic properties, and further endow the encapsulated metal nanoparticles with the features of biocompatibility and stability in many organic and inorganic media, including animal and human fluids (Bystrzejewski et al.
2011). The encapsulation of iron nanoparticles in graphene-like shells was found to be a well-accepted method to increase their stability and preserve specific magnetic properties. The carbon coating is also considered as the optimal encapsulating material because it is light and tightly covers the magnetic core in nanoparticles (Bystrzejewski et al.
2011). Our previous studies showed that CEINS have a high corrosion resistance (Bystrzejewski et al.
2006) and are readily susceptible to various functionalization processes (Kasprzak et al.
2015). It should also be noted that the mean diameter distribution of CEINS can be controlled by changing a very simple macroscopic parameter, namely grain size of the starting Fe powder used in synthesis processes or the chemical composition of the anode (Bystrzejewski et al.
2013). This phenomenon opens up new possibilities to control the magnetic characteristic of CEINS, leading to increase in the magnetic performance. More recently, studies have shown that CEINS can be precisely functionalized with a large number of functional groups that can be successfully linked with some biological molecules (Kasprzak et al.
2016; Poplawska et al.
2014). Interestingly, carbon-encapsulated iron oxide nanoparticles (Fe
3O
4) and carbon-coated gadolinium particles have been also tested as novel carbon-shell–type contrast drug candidates in MRI (Atabaev et al.
2015; Bae et al.
2012). Some gadolinium Gd(III) chelates such as Primovist™, Omniscan™, and MultiHance™ as well as Fe
3O
4 nanoparticles coated with polyethylene glycol, such as Feridex™ (SPIO), Resovist™ (SPIO), and Combidex™ (USPIO), are already available for clinical trials as positive (gadolinium) and negative (iron oxide) MRI contrast agents. Because carbon-based shell containing few layers of graphene is now recognized as a novel platform for both drug and gene delivery systems (DGDS) in nanomedicine (Bamburowicz-Klimkowska et al.
2019), the CEINS nanomaterials composed of the iron (Fe) core covered with carbon shells are now expected to be the perfect targeting magnetic core-shell–type negative contrast candidate when decorated with surface ligands in MRI. This feature has opened up some new exciting avenues for bioengineering carbon-coated magnetic nanomaterials as potent theranostic agents in preclinical MRI studies (Bae et al.
2012; Lee et al.
2014; Khramtsov et al.
2019; Park et al.
2011).
In the present study, we report the comprehensive and systematic characteristics of carbon-encapsulated iron nanoparticles examined as a potent contrast drug candidate for MRI by testing in the phantom gelatin models (in vitro). The CEINS were synthesized using a carbon arc discharge route, and the as-synthesized CEINS were purified and functionalized with acidic groups, including carboxylic moieties. The size and hydrodynamic properties of the CEIN samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scattering (DLS). Further structural details were evaluated from powder X-ray diffraction (XRD), thermogravimetry (TGA), Raman spectroscopy, and Fourier transform-infrared (FT-IR) spectroscopy. The global surface charge density and zeta potential measurements were also performed to obtain data. The magnetic properties of CEINS were measured using a vibrating sample magnetometer. Finally, the spin-lattice relaxation (T1) and spin-spin (T2) processes were examined in the aqueous gelatin media of CEINS by using a magnetic resonance scanner operating at 1.5 T. Our results revealed that CEINS can function as a new T2 (negative) contrast agent in MRI.