Recent Advances in Nanoelectrochemistry at The Interface between Two Immiscible Electrolyte Solutions
Conrad Benjamin Milton, Kerui Xu, Mei Shen*
The nanoscale interface between two immiscible electrolyte solutions (nanoITIES) is an emerging, versatile analytical platform. Analytical advantages of chemical analysis using the nanoITIES include imaging with nanometer spatial resolution, probing fast dynamics with millisecond temporal resolution, and fast response times, selectively detecting analytes, probing fundamental chemical processes (e.g., diffusion profiles), and versatile sensing of metal ions, proteins, neurotransmitters, ionic and neutral species, redox-active and non-redox active analytes, etc. We present here a brief theoretical background of the nanoITIES and experimental advances from the past five years. These advances include imaging of nanopores, probing diffusion profiles, biosensing, a new pH modulation mechanism for sensing neutral species, and studying exocytosis from Aplysia californica neurons.
Milton, C. B.; Xu, K.; Shen, M. Recent advances in nanoelectrochemistry at the interface between two immiscible electrolyte solutions. Current Opinion in Electrochemistry 2022, 34, 101005. https://doi.org/10.1016/j.coelec.2022.101005.
Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes
Henry D. Jetmore, Conrad B.Milton, Edappalil Satheesan Anupriya, Ran Chen, Kerui Xu, and Mei Shen*
The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k0), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i–t amperometry. The E1/2 of ACh transfer at the NPOE/water nanoITIES was −0.342 ± 0.009 V versus the E1/2 of tetrabutylammonium (TBA+).
Jetmore, H. D.; Milton, C. B.; Anupriya, E. S.; Chen, R.; Xu, K.; Shen, M. Detection of acetylcholine at nanoscale NPOE/Water Liquid/Liquid interface electrodes. Analytical Chemistry 2021, 93 (49), 16535–16542. https://doi.org/10.1021/acs.analchem.1c03711.
Avocado oil, coconut oil, walnut oil as true oil phase for ion transfer at nanoscale liquid/liquid interfaces
Ran Chen, Kerui Xu, and Mei Shen*
The interface between two immiscible electrolyte solutions (ITIES), typically formed between an organic (oil) phase and an aqueous phase, is essential for chemical sensing and for studying various electron transfer and ion transfer reactions. Solvent, as part of the ITIES structure, plays a critical role in electrochemical reactions at ITIES. While different kinds of organic phases, including viscous ionic liquids, have been reported, the use of true oils as an organic phase has rarely been explored. In this study, we introduce true oils, including avocado oil, coconut oil, and walnut oil, as new organic solvents for ITIES. We observed well-defined potential windows and sigmoidal cyclic voltammograms for ion transfer. We further measured the ion transfer rate constants at true oil-water interfaces supported at a nanopipette of ∼20–60 nm in radius. Our study offers additional insights on the effect of solvent viscosity on the ion transfer rate at the liquid/liquid interface, with the viscosity of these true oils being ∼ 50–70 times that of 1, 2-dichloroethane. We measured the standard ion transfer rate constants of tetrabutylammonium to be 0.21–0.32 cm / s at these true oil-water interfaces. This work lays the foundation to expand the ITIES platform to explore new reactions, playing critical roles in separation science, chemical sensing, chemical synthesis, catalysis, etc.
Chen, R.; Xu, K.; Shen, M. Avocado oil, coconut oil, walnut oil as true oil phase for ion transfer at nanoscale liquid/liquid interfaces. Electrochimica Acta 2020, 357, 136788. https://doi.org/10.1016/j.electacta.2020.136788.