文摘
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 (r1) and transverse (r2) relaxivities were obtained for aqueous suspensions of these materials with different lanthanide ions. The values of r1 are very small and varied only slightly with the effective magnetic moment (μeff) of the lanthanide ions, while r2 values are larger and proportional to the value of μeff2. The dependence of R2 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 R2 initially increases with τCP and then saturates at higher τCP at a value that is about 3 to 5 times lower than R2p*. 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(rp)−1 holds (τD = rp2/D, where D is the diffusion coefficient, rp is the radius of the particle, and Δω(rp) 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 r2 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.