Department of Radiology, Stanford University, Stanford, California 94306, United States.
Department of Electrical Engineering, Stanford University, Stanford, California 94306, United States.
Acc Chem Res. 2020 Apr 21;53(4):719-728. doi: 10.1021/acs.accounts.9b00543. Epub 2020 Jan 28.
Electrochemical measurements conducted in confined volumes provide a powerful and direct means to address scientific questions at the nexus of nanoscience, biotechnology, and chemical analysis. How are electron transfer and ion transport coupled in confined volumes and how does understanding them require moving beyond macroscopic theories? Also, how do these coupled processes impact electrochemical detection and processing? We address these questions by studying a special type of confined-volume architecture, the nanopore electrode array, or NEA, which is designed to be commensurate in size with physical scaling lengths, such as the Debye length, a concordance that offers performance characteristics not available in larger scale structures.The experiments described here depend critically on carefully constructed nanoscale architectures that can usefully control molecular transport and electrochemical reactivity. We begin by considering the experimental constraints that guide the design and fabrication of zero-dimensional nanopore arrays with multiple embedded electrodes. These zero-dimensional structures are nearly ideal for exploring how permselectivity and unscreened ion migration can be combined to amplify signals and improve selectivity by enabling highly efficient redox cycling. Our studies also highlight the benefits of arrays, in that molecules escaping from a single nanopore are efficiently captured by neighboring pores and returned to the population of active redox species being measured, benefits that arise from coupling ion accumulation and migration. These tools for manipulating redox species are well-positioned to explore single molecule and single particle electron transfer events through spectroelectrochemistry, studies which are enabled by the electrochemical zero-mode waveguide (ZMW), a special hybrid nanophotonic/nanoelectronic architecture in which the lower ring electrode of an NEA nanopore functions both as a working electrode to initiate electron transfer reactions and as the optical cladding layer of a ZMW. While the work described here is largely exploratory and fundamental, we believe that the development of NEAs will enable important applications that emerge directly from the unique coupled transport and electron-transfer capabilities of NEAs, including in situ molecular separation and detection with external stimuli, redox-based electrochemical rectification in individually encapsulated nanopores, and coupled sorters and analyzers for nanoparticles.
在受限体积中进行的电化学测量为解决纳米科学、生物技术和化学分析交叉领域的科学问题提供了一种强大而直接的手段。在受限体积中电子转移和离子传输如何耦合，以及如何超越宏观理论来理解它们？此外，这些耦合过程如何影响电化学检测和处理？我们通过研究一种特殊类型的受限体积结构——纳米孔电极阵列（NEA）来解决这些问题，该结构的设计尺寸与物理缩放长度（如德拜长度）相当，这种一致性提供了在更大规模结构中无法获得的性能特征。这里描述的实验严重依赖于精心构建的纳米结构，这些结构可以有效地控制分子传输和电化学反应性。我们首先考虑指导具有多个嵌入式电极的零维纳米孔阵列的设计和制造的实验限制。这些零维结构几乎是探索如何将选择性和未屏蔽的离子迁移结合起来以通过实现高效的氧化还原循环来放大信号和提高选择性的理想选择。我们的研究还突出了阵列的优势，即从单个纳米孔逸出的分子可以被相邻的孔有效地捕获，并返回正在测量的活性氧化还原物种的群体中，这些优势来自于离子积累和迁移的耦合。这些用于操纵氧化还原物种的工具非常适合通过光谱电化学来探索单个分子和单个颗粒的电子转移事件，这是通过电化学零模式波导（ZMW）实现的，ZMW 是一种特殊的混合纳米光子学/纳米电子学结构，其中 NEA 纳米孔的下环形电极既作为引发电子转移反应的工作电极，又作为 ZMW 的光学包层。虽然这里描述的工作主要是探索性和基础性的，但我们相信 NEA 的发展将为直接从 NEA 的独特耦合传输和电子转移能力中出现的重要应用铺平道路，包括具有外部刺激的原位分子分离和检测、单个封装纳米孔中的基于氧化还原的电化学整流，以及用于纳米颗粒的耦合分拣器和分析仪。
