Hyperpolarized NMR in Single-File Nanotubes C.R. Bowers, C.-Y. Cheng, T.C. Stamatatos and G. Christou Department of Chemistry, University of Florida, Gainesville, Florida 32611 USA Abstract. Continuous-flow hyperpolarized xenon-129 NMR is used to characterize gas exchange and diffusion in two types of polycrystalline solids with one-dimensional channels. Expressions for the hyperpolarized NMR selective- saturation recovery signal are derived for normal and single-file diffusion. Keywords: Hyperpolarization, single-file diffusion, nanotubes, tracer exchange, xenon-129 PACS: 82.56.Lz, 82.60.Hc, 81.07.De, 87.64.kj INTRODUCTION In systems of classical particles confined to 1D channels, Fickian diffusion yields a variance 2 2Dt σ = in displacements, where D is the self-diffusivity. In single-file channels, which are too narrow for particles to pass, diffusion may become anomalous, depending on the particle density and time-scale. As the occupancy θ of the channel increases, a cross-over to the single-file diffusion (SFD) regime is expected, where 2 2 F t σ = and F is the single-file mobility. Such behavior has been validated in macroscopic channel-particle systems, where individual particle trajectories are easily tracked [1, 2]. However, it seems the occurrence SFD on the molecular scale is more difficult to prove, with only a handful of reports appearing in the literature [3-6]. Recent interest in molecular SFD stems from its potential use in catalysis and separations [7, 8]. These applications require open-ended channels, where diffusion and exchange are interdependent. To characterize the accumulation of labeled particles in the channels, which at time 0 τ = contain only unlabelled particles, the tracer exchange function is defined: () () ( ) # particles / # particles γτ τ = ∞ () 0 t dt τ φ = ∫ , where ( ) t φ is the residence time distribution [9]. In NMR tracer exchange, the nuclear spin serves as a label. The xenon-129 atom affords key advantages for NMR tracer exchange: its chemical shift is sensitive to the size, shape and loading of pore spaces. Moreover, the 129 Xe NMR signal can be enhanced by >10 4 by spin exchange optical pumping [10]. In hyperpolarized tracer exchange NMR, the sample is exposed to a continuous flow of hyperpolarized 129 Xe gas. Hyperpolarized atoms diffuse into the pore structure of the solid. After a steady-state nuclear spin polarization distribution is established, a selective saturation-recovery pulse sequence is applied, and the subsequent recovery of the adsorbed phase hyperpolarized NMR signal is recorded as a function of τ , the post-saturation delay [6]. EXPERIMENTAL Continuous-flow hyperpolarized 129 Xe NMR studies were performed at 9.4T on two different polycrystalline nanotube materials: 15mg of L-Alanyl L-Valine (AV, MP Biomedicals) and 40mg of [Ga 10 (OMe) 20 (O 2 CMe) 10 ] (Ga 10 ) [11, 12]. SEM images of the samples are shown in Fig. 2. The powders were packed loosely into a 3mm (outside diameter) cylindrical PEEK (polyetheretherketone) sample holder. The Rb-Xe spin exchange optical pumping system and NMR setup are described in Ref. [10]. AV was evacuated to ~10 -5 mbar at 100 o C for 2-3hr; Ga 10 at 25 o C. The samples were immersed in a mixture of hyperpolarized 129 Xe in 4 He at a flow rate of about 100mL/min. After reaching a steady state, the adsorbed phase 129 Xe polarization was destroyed by a train of frequency selective Gaussian shaped pulses. For each recovery delay, the NMR signal was acquired with a non- selective /2 π pulse. Thermally polarized 129 Xe atoms are not detected under the experimental conditions. Magnetic Resonance in Porous Media AIP Conf. Proc. 1330, 43-46 (2011); doi: 10.1063/1.3562229 © 2011 American Institute of Physics 978-0-7354-0885-2/$30.00 43 Downloaded 15 Apr 2011 to 128.227.164.84. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions