Comprehensive relaxometric analysis of Fe(iii) coordination polymer nanoparticles for T1-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 T₁ 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 r₁ 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.