Abstract
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.