在多孔介质中使用电子顺磁共振成像客体分子扩散的原位监测
Summary
A protocol for the in situ monitoring of the diffusion of guest molecules in porous media using electron paramagnetic resonance (EPR) imaging is presented.
Abstract
A method is demonstrated to monitor macroscopic translational diffusion using electron paramagnetic resonance (EPR) imaging. A host-guest system with nitroxide spin probe 3-(2-Iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (IPSL) as a guest inside the periodic mesoporous organosilica (PMO) aerogel UKON1-GEL as a host and ethanol as a solvent is used as an example to describe the protocol. Data is shown from a previous publication, where the protocol has been applied to both IPSL and Tris(8-carboxy-2,2,6,6-perdeutero-tetramethyl-benzo[1,2-d:4,5-d′]bis(1,3)dithiole) methyl (Trityl) as guest molecules and UKON1-GEL and SILICA-GEL as host systems.
A method is shown to prepare aerogel samples that cannot be synthesized directly in the sample tube for measurement due to a size change during synthesis. The aerogel is attached to sample tubes using heat shrink tubing and a pressure cooker to reach the necessary temperature without evaporating the solvent in the process. The method does not assume a clearly defined initial distribution of guest molecules at the start of the measurement. Instead, it requires a reservoir on top of the aerogel and experimentally determines the influx rate during data analysis.
The diffusion is monitored continually over a period of 20 hr by recording the 1d spin density profile within the sample. The spectrometer settings for the imaging experiment are described quantitatively. Data analysis software is provided to take the resonator sensitivity profile into account and to numerically solve the diffusion equation. The software determines the macroscopic translational diffusion coefficient by least square minimization of the difference between the experiment and the numerical solution of the diffusion equation.
Introduction
多孔材料发挥在实际应用中，如催化和层析1的主要作用。通过添加表面基团以及调整孔尺寸和表面特性，所述材料可以根据所需的应用程序2,3。多孔材料的功能关键取决于细孔内客体分子的扩散性能。在多孔材料，必须区分微观平移扩散常数D 微 ，它描述在一方面分子长度规模和宏观平移扩散常数D 宏扩散之间进行 另一方面，这是由扩散通过多个细孔，晶界，扭曲和材料的不均匀性的影响。
有几种磁共振方法可用来研究扩散，每个适合的一部分icular尺度。在毫米刻度，核磁共振（NMR）成像4和电子顺磁共振（EPR）成像（如在此协议提交）都可以使用。较小尺度成为通过使用核磁共振脉冲磁场梯度以及EPR实验5,6的访问。在纳米尺度，EPR谱可以通过观察自旋探针7,8之间海森堡交换作用的变化来使用。使用EPR成像范围从工业催化剂支持平移扩散， 例如，氧化铝9，研究各向异性取得高分子凝胶12流体10,11，药物释放系统– 14和膜模型15。
该协议使用EPR成像监测圆柱形spin探测器的宏观动扩散，多孔介质在原地的做法提出了一个。它展示了由日的主机 – 客户系统Ë氮氧spin探测器3-（2-Iodoacetamido）-2,2,5,5-四甲基-1-吡咯烷（IPSL）作为周期性中孔有机二氧化矽内客（PMO）UKON1-GEL气凝胶作为主机和乙醇作为溶剂。该协议已成功以前使用16比较维宏 作为测定用的EPR成像以D 微主机材料UKON1 -凝胶和硅胶和旅客的物种IPSL和三（8-羧基-2,2,6,6- perdeutero四甲基苯并〔1,2-D ：4,5-D']双（1,3）二硫）甲基（三苯甲基），参见图1。
在基于连续波（CW）的EPR成像17的其它方法，扩散需要分光计以外的地方。与此相反，在这里提出的方法在原位方法使用。一系列的一维自旋密度分布ρ1d的快照（T，γ）是记录一段数小时。在此期间，一个快照是在其他之后拍摄并提供具有约5分钟的时间分辨率的实时扩散图案。
UKON1-GEL和硅胶已在3毫米内径的样品管被合成在文献中描述。16,18,19的UKON1 -凝胶和硅胶合成导致样品的收缩。将样品置于一个热缩管，以防止客体分子从气凝胶和样品管的壁之间移动的内部。这一附加步骤是没有必要的，可以直接在样品管进行合成而不改变它们的大小的样品。气凝胶样品崩溃时，他们干出来的，所以他们必须在溶剂中在任何时候都被淹没。所需要的热缩管的温度比在环境压力的乙醇的沸点。因此该协议描述了使用高压锅的提高沸腾的乙醇点。
该协议包括预先为EPR成像实验和用于监视IPSL自旋探头扩散分光计设置合成UKON1-GEL的样品制备。进行数据分析，在本地编写的软件提供并且描述其使用。从分光计的原始数据可以直接装载。软件计算的空间1d中的自旋密度分布ρ1D（T，γ），并考虑到谐振器灵敏度分布。用户可以选择的气凝胶和一个时间窗口，在其上扩散常数将被确定的区域。然后，该软件确定基于选择扩散方程的边界条件，解决了扩散方程。它支持最小二乘拟合找到维宏其中数值解的实验数据的最佳匹配的值。
<p cl屁股=“jove_content”>该协议可以用不同的宾主材料调整用作只要样品整个样品不改变的横截面面积，即ρ1D（T，γ）给出直接访问的浓度，并且不通过样品的横截面的变化的影响。对于维宏访问值的范围 估计16 10 7 /秒之间-12米2 /秒和·10 -9 m 2以下。Protocol
Representative Results
Discussion
该协议允许顺位分子的扩散的监控。一维成像方法已被选中，是因为它允许较高的时间分辨率相比，2D或3D成像。一维方式需要样品的恒定横截面面积，因为所获得的一维图像的强度不仅取决于浓度，而且在样品上的横截面面积。该方法还要求在样品内的自旋探头的EPR谱只在强度上而不是在形状改变。否则更耗时的谱空间成像，必须使用，这是本方案的范围之外。该方法也只限于系统，其中…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors thank Prof. Peter Imming and Diana Müller for synthesis of the Trityl spin probe and Prof. Sebastian Polarz, Martin Wessig and Andreas Schachtschneider for the synthesis of the porous materials. Financial support by the DFG (DR 743/7-1) and within the SPP 1570 is gratefully acknowledged.
Materials
| X-Band spectrometer | Bruker | E580 | |
| Spectrometer software | Bruker | Xepr 2.6b.108 | |
| gradient coil system | Bruker | E540 GCX2 | |
| imaging resonator | Bruker | TMHS 1007 | |
| micro-classic pipette controller | Brand | 25900 | |
| microcapillary ringcaps 50 µl | Hirschmann | 9600150 | inner diameter 0.5 mm |
| EPR sample tube 2 mm inner diameter | Bruker | ER 221TUB/2 | |
| EPR sample tube 4 mm inner diameter | Bruker | ER 221TUB/4 | |
| heat-shrink tubing DERAY-IB | DSG-Canusa | 2210048952 | 4.8 mm/2.4 mm, 2:1, 95 °C – 200 °C |
| heat gun | Bosch | PHG 600-3 | |
| PTFE band | VWR | 332362S | width 12 mm |
| test tube | length 16 cm, diameter 1.5 cm | ||
| beaker | 250 ml, height 9 cm, diameter 7 cm | ||
| capillary tube sealing | Fisher Scientific | 02-678 | |
| pressure cooker, 3l with trivet | Beem | Vital-X-Press V2, F1000675 | |
| magnetic stirrer with heating element | |||
| ethanol (p.a.) | |||
| ethanol (techn.) | |||
| syringe | Hamilton | 1705 | 0.05 ml, custom length: 20 cm, |
| Pasteur capillary pipette | length 23 cm | ||
| data analysis software | homemade | Available for download at http://www.uni-konstanz.de/drescher/software. Requires Matlab. | |
| UKON1-GEL | kindly provided by Prof. Sebastian Polarz, Martin Wessig and Andreas Schachtschneider | See references 16, 18, 19 for the synthesis |
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