Characterization of Dynamics at the Solid-Liquid Interface via Dynamic Nuclear Polarization

在环境压力和温度条件下，固体/液体界面通常难以在分子水平上进行光谱研究。核磁共振（NMR）是一种用于评估液体动力学的通用工具，但在处理界面上的分子时，存在灵敏度低和谱线宽的问题。动态核极化（DNP）是一类能够增加几个数量级核磁共振信号的技术。低温下固态的DNP可以表征催化介质、多孔材料和金属有机框架的表面，这是标准核磁共振不可能做到的。然而，液体在界面处的动力学仍然是不可接近的。在室温下的液态DNP被证明是一个有价值的工具，既可以增强核磁共振信号，又可以在皮秒时间尺度上研究液体动力学和分子相互作用。然而，这种方法仍然局限于低磁场（0.34 T）和1H作为靶核。最近的发现显示了该技术更广泛的潜力：在掺有有机自由基作为极化剂（PAs）的小分子13C和31P上，测量到了高达9.4 T的高增强（~10^2-10^3）。机理研究表明，快速分子碰撞和结构取向改变驱动了有效的极化转移，一旦目标分子或极化剂的迁移受到限制，这种转移将变得更加突出。在这个项目中，我们的目标是扩大DNP在液体状态下的能力，以快速和敏感地表征非均质材料和界面。我们将对目标分子或PA与纳米颗粒表面结合时的极化转移进行机理研究。这将使我们对慢动作状态下的DNP机制有必要的了解。DNP- nmr实验将进行到9.4 T，在那里DNP增强可以与化学位移分辨率相结合。概念验证实验将在单层涂覆的金纳米颗粒上进行，这种纳米颗粒可以选择性地结合表面的分子。通过这种方式，利用PA作为表面上的局部探针，我们的目标是解开表面液体的动力学，揭示结合机制和瞬态相互作用。该项目将是证明DNP-NMR在液态下研究固/液界面能力的关键。该项目的成果将使一类新的实验能够解决材料科学、催化和传感方面的具体问题。

Solid/liquid interfaces are usually difficult to investigate spectroscopically at the molecular level under ambient pressure and temperature conditions. Nuclear magnetic resonance (NMR) is a versatile tool for assessing the liquid dynamics but, when dealing with molecules at the interfaces, struggles with low sensitivity and line broadening. Dynamic nuclear polarization (DNP) denotes a class of techniques that are capable of increasing NMR signals of several orders of magnitude. DNP in the solid-state at cryogenic temperatures enabled the characterization of surfaces in catalytic media, porous materials, and metal-organic frameworks that would have been impossible with standard NMR. However, the dynamics of the liquid at the interface remained inaccessible. DNP in the liquid state at room temperature proved to be a valuable tool to both enhance the NMR signal and to investigate liquid dynamics and molecular interactions on the picoseconds timescale. However, this approach is still limited to low magnetic fields (0.34 T) and 1H as a target nucleus. Recent discoveries showed a much wider potential of the technique: high enhancements (~10^2-10^3) at fields up to 9.4 T were measured on 13C and 31P in small molecules doped with organic radicals as polarizing agents (PAs). Mechanistic studies revealed that fast molecular collisions and structural reorientations are driving efficient polarization transfer and could become more prominent once the mobility of the target molecule or the polarizing agent is constrained.With this project, we aim at expanding the capabilities of DNP in the liquid state towards a fast and sensitive characterization of heterogeneous materials and interfaces. We will perform a mechanistic study on the polarization transfer when either the target molecule or the PA is bound to the surface of a nanoparticle. This will give us the necessary understanding of DNP mechanisms in the slow-motion regime. DNP-NMR experiments will be performed up to 9.4 T, where the DNP enhancement can be coupled with chemical shift resolution. Proof-of-concept experiments will be performed on monolayer-coated gold nanoparticles that can selectively bind molecules at the surface. In this way, using the PA as a local probe on the surface, we aim to disentangle the dynamics of the liquid at the surface, revealing the binding mechanisms and the transient interactions. This project will be pivotal for demonstrating the capability of DNP-NMR in the liquid state for studying the solid/liquid interface. The outcome of the project will enable a new class of experiments to address specific problems in material science, catalysis, and sensing.