Analytical applications and the underlying principles of unique electrochemistry in nanoporous structures are reviewed and discussed. In addition to the conventional concept of enlarged surface area, the structural effects of nanoporous materials can play significant roles such as discriminative electrokinetics, the nano-confinement effect, electrical double layer overlapping, ion-selective impedance, etc. The applications described in this review article include solid-state pH sensors, miniaturized pseudo-reference electrodes, nonenzymatic glucose monitoring, ion diodes, transistors, extracellular neural probes, and a few more. Further intensive research is required to develop creative analytical tools based on nanoporous structures and to unravel the underlying physicochemical principles.

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Recently many microfluidic systems are increasingly equipped with functional units for ionic controls for various applications. In this review article, we define an ion bridge as a structure that controls current or distribution of ions in a microfluidic system, and summarize the ion bridges in the literature in terms of characteristics, fabrication methods, advantages and disadvantages. The ion bridges play two basic roles, namely to ensure electrical contact in a microfluidic network and mechanically separate a liquid phase from another. More interestingly, the charged surfaces of ion bridges, which can be chemically modified, create new characteristics such as permselectivity and concentration polarization. Asymmetric ion transport as well as ionic conductivity through the ion bridges suggests a variety of applications including sample preconcentration, electroosmotic pump, electrospray ionization, electrically driven valve and many others. This review categorizes the ion bridges into several classes and describes the structures, materials, fundamental functions and applications. In Perspectives, new opportunities of microfluidics and nanofluidics provided by the ion bridges are discussed.

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