1 Introduction
Magnetic resonance imaging (MRI) is one of the most potent techniques for diagnosis and detection in biological molecular imaging. MRI provides the visibility to demarcate most of the tissues and acquire three-dimensional images of whole tissue along with high spatial resolution without using radiotracers. The image contrast can be enhanced by a negative-positive contrast agent that sharpens the MRI image by affecting the spin moment of the protons. The introduction of these agents advances the characterization possibilities by giving heightened signals of specified cells/tissue when compared with the other cells or diseased ones [1-3]. MRI uses the property of protons of hydrogen nuclei in water molecules to align with and precess about an applied static magnetic field. The precessing protons can be disturbed by radiofrequency pulses “RF-pulses”, and the mechanism through which they realigned to their original state can be recorded to get an image. The contrast originates from the differences in spin relaxation kinetics of hydrogen protons along with the transverse (spin-spin relaxation time T2) and longitudinal (spin-lattice relaxation time T1) planes of the externally applied magnetic field to the sample. Negative contrast agents may shorten the transverse relaxation giving darkening of contrast images whereas positive contrast agents interacting with water protons may give a brighter image with shortening of longitudinal relaxation time. Since, contrast generated under MRI is dependent on the physicochemical nature of the targeting tissues, relaxation times of protons, and proton density of water molecules [3-8]. Contrast agents employed for MRI analysis are classified according to the relaxation mechanism they followed when the RF-pulse is removed. To date, T1 weighted contrast agents (gadolinium), and T2 weighted contrast agents (superparamagnetic iron oxide-based materials) are commonly synthesized these days to be used in testing. Superparamagnetic materials can produce promptly enhanced proton nuclei relaxation than paramagnetic materials. Consequently, even at a lower dosage, these can target large surface areas. T2 contrast agents based on magnetic colloidal suspensions are frequently termed as USPIO (ultrasmall superparamagnetic iron oxide) [9-11]. The magnetism, magnetic behavior, and magnetic properties of fine colloidal suspensions have been a dynamic field of study for over fifty years. Generally, magnetic colloids or ferrofluids are synthesized by a one-step process involving co-precipitation of Fe+2 and Fe+3precursors in aqueous solutions [12]. Key parameters for the communications with biological tissues are their magnetic properties, surface functionality, and solubility. Depending on the applications, the efficacy of magnetic colloids can thus be altered to vary their contrast capability [13-16].
In the present study, core-shell magnetic polymer colloids (CSMPC) with average particle size 527 nm have been prepared using a simple seeded emulsion polymerization technique, followed by decoration of thin films via LBL self-assembly. The final prepared colloidal particles were then incorporated and tested as an MR contrast agent by measuring the transverse and longitudinal relaxation times T2 and T1 of protons present in the colloidal suspension using a 1.5T clinical scanner. Furthermore, in this work, we present a novel technique for the fabrication of contrast agents based thin films of colloids to obtain in vitro MRI images to open the ways for lab-on-chip technology for diagnostics. Fabrication of films by electrostatic attraction of oppositely charged ions via layer by layer self-assembly (LBL-SAMU) using eco-friendly colloids suspensions has been broadly explored numerous biomedical applications. LBL-SAMU technique is a highly efficient, versatile, cost-effective, and facile technique to fabricate robust thin films of colloids by immersing the substrate in colloidal suspensions [17-19]. Each immersion cycle in oppositely charged species is capable of uniform particle distribution and can induce reproducible growth of thin films [20]. The salient characteristics of such thin-film development include their future potential in various applications, no pre-injection requirement, efficient contrast, biomolecule capturing, proficient cell labeling, and assistance to study dynamics of drug delivery and distribution within the body.