Magnetic
resonance imaging has emerged as an indispensable imaging
modality for the early-stage diagnosis of many diseases. The imaging
in the presence of a contrast agent is always advantageous, as it
mitigates the low-sensitivity issue of the measurements and provides
excellent contrast in the acquired images even in a short acquisition
time. However, the stability and high relaxivity of the contrast agents
remained a challenge. Here, molecules of a mononuclear, mono(aquated),
thermodynamically stable [log K
MnL = 14.80(7)
and pMn = 8.97] Mn(II)-complex (1), based on a hexadentate
pyridine-picolinate unit-containing ligand (H2PyDPA), were
confined within a porous silica nanosphere in a noncovalent fashion
to render a stable nanosystem, complex 1@SiO2NP. The entrapped complex 1 (complex 1@SiO2) exhibited r
1 = 8.46 mM–1 s–1 and r
2 = 33.15
mM–1 s–1 at pH = 7.4, 25 °C,
and 1.41 T in N-(2-hydroxyethyl)piperazine-N′-ethanesulfonic acid buffer. The values were about
2.9 times higher compared to the free (unentrapped)-complex 1 molecules. The synthesized complex 1@SiO2NP interacted significantly with albumin protein and consequently
boosted both the relaxivity values to r
1 = 24.76 mM–1 s–1 and r
2 = 63.96 mM–1 s–1 at pH = 7.4, 37 °C, and 1.41 T. The kinetic inertness of the
entrapped molecules was established by recognizing no appreciable
change in the r
1 value upon challenging
complex 1@SiO2NP with 30 and 40 times excess
of Zn(II) ions at pH 6 and 25 °C. The water molecule coordinated
to the Mn(II) ion in complex 1@SiO2 was also
impervious to the physiologically relevant anions (bicarbonate, biphosphate,
and citrate) and pH of the medium. Thus, it ensured the availability
of the inner-coordination site of complex 1 for the coordination
of water molecules in the biological media. The concentration-dependent
changes in image intensities in T
1- and T
2-weighted phantom images and uptake of the
nanoparticles by the HeLa cell put forward the biocompatible complex 1@SiO2NP as a potential dual-mode MRI contrast
agent, an alternative to Gd(III)-containing contrast agents.
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