Gadolinium-Doped Carbon Nanoparticles with Red Fluorescence and Enhanced Proton Relaxivity as Bimodal Nanoprobes for Bioimaging Applications

Carbon-based nanoparticles (CNPs) have demonstrated great potential in biomedical applications because of their unique physical and chemical properties, and excellent biocompatibility. Herein, we have studied two types of CNPs with gadolinium (Gd) impurities (Gd-CNPs), which were prepared by microwave synthesis (MWS) and hydrothermal synthesis (HTS), for potential applications as photoluminescent (PL) labels and contrast agents in magnetic resonance imaging (MRI). The prepared Gd-CNPs were investigated by means of transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy, UV-visible absorption spectroscopy, and magnetic resonance relaxometry, which allowed us to reveal specific features and functional properties of the prepared samples. While the TEM data showed similar size distributions of both types of Gd-CNPs with mean sizes of 4-5 nm, the optical absorption spectroscopy showed higher absorption in the visible spectral region and stronger PL in the red and near-infrared (NIR) spectral regions for the MWS samples in comparison with those prepared by HTS. Under green light excitation the former samples exhibited the bright red-NIR PL with quantum efficiency of the order of 10%. The proton relaxometry measurements demonstrated that the HTS samples possessed longitudinal and transverse relaxivities of about 42 and 70 mM(-1)s(-1), whereas the corresponding values for the MWS samples were about 8 and 13 mM(-1)s(-1), respectively. The obtained results can be useful for the selection of appropriate synthesis conditions for carbon-based nanoparticles for bimodal bioimaging applications.