Development of NMR probe for the measurement of slow molecular diffusion

Pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR) is one of the most powerful tools used to investigate molecular diffusion in solution. The range of diffusion coefficient that can be measured by the PGSE NMR is determined by the gradient strength, gradient pulse width, and allowed molecular diffusion time. For the measurement of slow diffusion, however, the gradient strength of a general commercial probe makes it difficult to provide enough gradient for the measurement of polymeric micelle diffusion dynamics. Thus, in order to measure slow molecular diffusion of polymers and micelles (ca. 10-11m2/s ∼10-12m2/s), a 400MHz NMR probe with a strong gradient along the z-direction (680Gauss/cm at 10A) has been developed. An RF solenoid coil, which simulated to have little field inhomogeneity, was first made with 0.8mm enamel-coated wire, which gives an advantage of easy fabrication. For the convenience of tube insertion, RF solenoid coil was substituted by a saddle type coil made of copper plate. Despite strict constraints on the maximum geometry of gradient coil, strength of 680Gauss/cm at 10A was achieved by double-set Maxwell type coil. Temperature control has been designed to permit experimentation in high temperature. The stability and accuracy tests were performed at 60 .deg. C. The fluctuation of the temperature right below the NMR sample tube was within ±0.1 .deg. C in about 20 minutes. The performance of the developed probe was confirmed through the diffusion coefficient measurement of water at various temperatures using PGSE NMR. At 24 .deg. C, diffusion coefficient came out to be (2.18±0.02)x10-9m2/s which is within 3% error with known value be 2.242x10-9m2/s. The diffusion of 0.4wt% Pluronic F127 in D2O, whose known critical micelle temperature (CMT) is 27 .deg. C, was measured at various temperatures (25 .deg. C∼40 .deg. C. The resulting diffusion coefficient of F127 at 25 .deg. C was 5.268x10-11m2/s and increased until 27 .deg. C, while the diffusion coefficient decreased and saturated as the temperature increased above 27 .deg. C due to the increase of micellar fraction who diffuses slowly. The measured diffusion coefficients of each monomer and micelle as a function of temperature were organized. To determine the approximate size of the F127 micelle, its hydrodynamic radius was calculated. By Stokes-Einstein equation with diffusion coefficient of 3.266x10-11m2/s at 40 .deg. C measured by PGSE NMR, the hydrodynamic radius was calculated to be 8.9nm, which agrees very well with 8.8nm of the previous experiment. However, the hydrodynamic radius obtained by the dynamic light scattering (DLS) was 12nm and the referred radius obtained by small angle neutron scattering (SANS) was 11.9nm. The inconsistence of radius obtained by PGSE NMR with radius obtained by DLS and SANS is due to the coexistence of the monomer and micelle phases. After extracting only micelle diffusion (of 2.361x10-11m2/s at 40 .deg. C) from total normalized signal, the hydrodynamic radius was obtained to be 12.45nm, which is consistent with values measured by DLS and SANS. Therefore, PGSE NMR, which allows separate calculations of the monomer and micelle diffusion coefficients, can be applied in drug delivery system, where it can differentiate between the bounded and free states of a molecule