Magnetic relaxation of various silica-based Mn-contrast agents in relationship with Mn2+ chemical transformations during fabrication

The present work introduces the comparative analysis of different synthetic techniques for the facile one-pot synthesis of silica-based Mn-contrast agents. The growth of silica nanoparticles (SNs) via water-in-oil microemulsion and Stöber techniques occurring under ammonia-rich oxygenated conditions triggers hydrolytic and oxidative transformations of Mn2+ ions. However, high surface activity of in situ generated silica seeds facilitates a coordinative adsorption of both Mn2+ ions and products of their hydrolytic and oxidative transformations, which allows to optimize the synthetic techniques for distinguishing the longitudinal and transverse relaxivitiy r1 and r2 at 13.8 and 16.9 mM−1s−1 respectively. The amino acid-catalyzed growth of silica nanoparticles (SNs) allows their loading with Mn2+ ions with exclusion of their hydrolytic and oxidative transformations, although r1 and r2 are 2.9 and 12.6 mM−1s−1 correspondingly. The electrochemical behavior of the synthesized SNs confirm their efficient doping with Mn2+ even under the synthesis in the ammonia-rich conditions. The r1 and r2 values of the SNs synthesized under different conditions correlate with the symmetry and hydration numbers of the Mn2+ centers incorporated into the SNs. The low cytotoxicity and high hemocompatibility of the nanoparticles with the optimal r1(2)-values makes them promising for further in-vivo application. [Display omitted] •Synthetic modification is aimed to optimize r1(2) relaxivity values of SNs.•Loading of Mn2+ is favored via lysine-catalyzed synthesis, but r1(2) are inconvenient.•Optimal SNs with r1(2) = 13.8 (16.9) mM−1s−1 are synthesized via ammonia-catalyzed method.•Optimal SNs exhibit low cytotoxicity and high hemocompatibility.