Evaluation of [Ln(H2cmp)(H2O)] Metal Organic Framework Materials for Potential Application as Magnetic Resonance Imaging Contrast Agents

Aqueous suspensions of metal organic frameworks (MOF) containing different Ln3+ ions, consisting of a series of layered Ln3+ networks formulated as [Ln(H2cmp)(H2O)] (where H5cmp is (carboxymethyl)iminodi(methylphosphonic acid), with a relatively wide size distribution (400 nm to 1 μm) were studied by relaxometry. The water 1H longitudinal (r 1) and transverse (r 2) relaxivities were obtained for aqueous suspensions of these materials with different lanthanide ions. The values of r 1 are very small and varied only slightly with the effective magnetic moment (μeff) of the lanthanide ions, while r 2 values are larger and proportional to the value of μeff 2. The dependence of R 2 on τCP (the time interval between two consecutive refocusing pulses in the train of 180° pulses applied in a CPMG pulse sequence) was evaluated. The value of R 2 initially increases with τCP and then saturates at higher τCP at a value that is about 3 to 5 times lower than R 2p*. This can be explained by the static dephasing regime (SDR) theory, in which the diffusion effect is taken into account and where the condition τD > Δω(r p)−1 holds (τD = r p 2/D, where D is the diffusion coefficient, r p is the radius of the particle, and Δω(r p) is the Larmor frequency shift at the particle’s surface). Separation of the particles into two fractions with different particle sizes led to a significant enhancement of the r 2 relaxivity of the smaller particles with a narrow size distribution. Magnetometric measurements performed with the particles containing Dy(III), Ho(III), and Gd(III) showed a typical paramagnetic behavior from 4 to 100 K, used to determine the Curie constants.