Microwave-assisted combustion synthesis of silica-coated Eu:Gd₂O₃ nanoparticles for MRI and optical imaging of cancer cells

Development of contrast agent for imaging modalities is very much essential to identify the cancer cells at an early stage. Due to the advances in the field of nanotechnology, nanoparticle-based contrast agents are being developed with better imaging capabilities at minimum concentrations. When the paramagnetic gadolinium oxide (Gd₂O₃) nanoparticles are doped with the luminescent europium (Eu³⁺) ions, it results in superior magneto-optical properties. This property makes the nanoparticles suitable for MRI and optical dual modal cancer imaging. In this article, europium doped Gd₂O₃ (Eu:Gd₂O₃) nanoparticles were synthesized by microwave-assisted combustion method. Citric acid (C₆H₈O₇) which is a biocompatible and less reactive fuel was used to prepare the Eu:Gd₂O₃ nanoparticles for the first time. Silica was coated over the Eu:Gd₂O₃ nanoparticles for improving the biocompatibility. X-ray diffraction studies reveal that after coating, the crystallinity was found to decrease. The synthesized nanoparticles have spherical morphology with the average diameter of 10 nm. The presence of silica was observed in the Fourier transform infrared (FTIR) spectrum and transmission electron microscopy images. In the photoluminescence studies, the emission peak at 612 nm confirms the presence of Eu³⁺ ions. Decay times of the uncoated and silica-coated samples were calculated to be 1.1 and 0.9 ms respectively. The in vitro cytotoxicity analysis suggests that the silica-coated nanoparticles are slightly toxic up to the concentration of 200 µg/mL. The magnetic measurement confirms the paramagnetic behaviour of the synthesized nanoparticles. The nanoparticles shows bright contrast images with the relaxivity value of 5.01/(mM)/s, suggest that the silica-coated nanoparticles have potential applications in MRI.