Comprehensive relaxometric analysis of Fe(<scp>iii</scp>) coordination polymer nanoparticles for <i>T</i>₁-MRI: unravelling the impact of coating on contrast enhancement

Coordination polymer-based systems, particularly Fe(iii)-based polymers, are attracting increasing interest due to their well-controlled morphology, biocompatibility, and versatile surface functionalization. With five unpaired electrons, Fe(iii) offers a promising and safer alternative to Gd(iii) for MRI applications. While some studies have investigated low molecular weight Fe(iii) chelates for MRI, the exploration of Fe(iii)-based nanosystems as <i>T</i> ₁ MRI probes remains limited. This study focuses on the synthesis of Fe(iii)/gallic acid nanoparticles functionalized with a low molecular weight polyethylene glycol (PEG) shell, designed to enhance the second-sphere water interaction and improve <i>r</i> ₁ relaxivity at clinical magnetic fields. The ¹H NMR relaxometric properties of these nanoparticles were systematically analyzed as a function of proton Larmor frequencies and temperature, and their performance was compared with a similar system stabilized by polyvinylpyrrolidone (PVP). We aimed to determine the frequency dependence of relaxivity in Fe(iii)-based coordination polymers, and to assess the impact of coating modifications on their MRI contrast efficacy. This knowledge is crucial for the rational design of improved Fe(iii)-based nanoprobes, allowing for optimized performance in future MRI applications.