The utility of the single-molecule magnet Fe8 as a magnetic resonance imaging contrast agent over a broad range of concentration

Recent reports have compared Fe8(aq) versus Magnevist with seemingly conflicting conclusions: one claims a much greater efficiency of Fe8 for magnetic resonance imaging contrast, and the other claims a much lower efficiency. Our study shows that at concentrations below 1.5 mM Fe8(aq) had a T1 relaxivity, r1, of 5.4 s−1 mM−1 which is comparable to Magnevist. Above 1.5 mM Fe8(aq) had an r1 of 1.1 s−1 mM−1, significantly lower than Magnevist. These results agree with the previous literature over the concentrations they examined. The results for the T2 relaxivity, r2, were similar. Here, we show that the concentration dependence of the relaxivity accounts for these discrepancies. Further, the relaxivity data are correlated with frequency-dependent maxima in χac″ of frozen solutions of Fe8 dissolved in deionized water over the temperature range of 1.8–4 K and the frequency range of 200–1400 Hz. The magnetic properties of the single-molecule magnet Fe8 in room temperature and frozen aqueous solution were found to be highly non-linear when examined over a wide concentration range.

Graphical abstractThe inverse proton longitudinal relaxation time T1-1 of Fe8 and Magnevist aqueous solutions as a function of concentration. While the Magnevist data are monotonic over the entire concentration range, the Fe8 data exhibits two distinct linear regions. These results indicate the magnetic behavior of Fe8 aqueous is highly concentration dependent.Figure optionsDownload full-size imageDownload as PowerPoint slide