Molecular Parameters Promoting High Relaxivity in Cluster-Nanocarrier Magnetic Resonance Imaging Contrast Agents.

We have investigated the mechanism of relaxivity for two magnetic resonance imaging contrast agents that both employ a cluster-nanocarrier design. The first system termed Mn 8 Fe 4 -coPS comprises the cluster Mn 8 Fe 4 O 12 (L) 16 (H 2 O) 4 or Mn 8 Fe 4 ( 1 ) (L = carboxylate) co-polymerized with polystyrene to form ∼75 nm nanobeads. The second system termed Mn 3 Bpy-PAm used the cluster Mn 3 (O 2 CCH 3 ) 6 (Bpy) 2 or Mn 3 Bpy ( 2 ) where Bpy = 2,2'-bipyridine, entrapped in ∼180 nm polyacrylamide nanobeads. Here, we investigate the rate of water exchange of the two clusters, and corresponding cluster-nanocarriers, in order to elucidate the mechanism of relaxivity in the cluster-nanocarrier. Swift-Connick analysis of O-17 NMR was used to determine the water exchange rates of the clusters and cluster-nanocarriers. We found distinct differences in the water exchange rate between Mn 8 Fe 4 and Mn 8 Fe 4 -coPS, and we utilized these differences to elucidate the nanobead structure. Using the transverse relaxivity from O-17 NMR line widths, we were able to determine the hydration state of the Mn 3 Bpy ( 2 ) cluster as well as Mn 3 Bpy-PAm. Using these hydration states in the Swift-Connick analysis of O-17 NMR, we found the water exchange rate to be extremely close in value for the cluster Mn 3 Bpy and cluster-nanocarrier Mn 3 Bpy-PAm.