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.