A versatile and robust surface-poison-resisting Scanning Amperometric Proton Microscopy

Enzyme-Modified Microelectrode for Simultaneous Local Measurements of O2 and pH

The use of miniaturized probes opens a new dimension in the analysis of (bio)chemical processes, enabling the possibility to perform measurements with local resolution. In addition, multiparametric measurements are highly valuable for a holistic understanding of the investigated process. Therefore, different strategies have been suggested for simultaneous local measurements of various parameters. Electroanalytical methods are a powerful strategy in this direction. However, they have been mainly restricted to coupling concurrent independent measurements with different miniaturized probes. Here, we present an enzymatic microbiosensor for the simultaneous detection of O2 and pH. The sensing strategy is based on the pH-dependent bioelectrocatalytic process associated with O2 reduction at a gold microelectrode modified with a multicopper oxidase. After initial investigations of the bioelectrocatalytic reaction over gold macroelectrodes, the fabrication and characterization of micrometer-sized probes are presented. The microbioelectrode exhibits a linear current increase with O2 concentration extending to 17.2 mg L-1, with a sensitivity of (5.56 ± 0.13) nA L mg-1 and a limit of detection of (0.5 ± 0.3) mg L-1. Moreover, a linear response allowing pH detection is obtained between pH 5.2 and 7.5 with a slope of -(47 ± 8) mV per pH unit. In addition, two proof-of-concept analytical examples are shown, demonstrating the capability of the developed sensing system for simultaneous local measurements of O2 and pH. Compared with other miniaturized probes reported before for simultaneous detection, our strategy stands out as the two investigated parameters are acquired from the very same measurement. This strategy greatly simplifies the analytical setup and for the first time provides truly simultaneous local detection in the micrometer scale.

Electroanalytical Strategies for Local pH Sensing at Solid-Liquid Interfaces and Biointerfaces

Obtaining analytical information about chemical species at interfaces is fundamentally important to improving our understanding of chemical reactions and biological processes. pH at solid-liquid interfaces is found to deviate from the bulk solution value, for example, in electrocatalytic reactions at surfaces or during the corrosion of metals. Also, in the vicinity of living cells, metabolic reactions or cellular responses cause changes in pH at the extracellular interface. In this review, we collect recent progress in the development of sensors with the capability to detect pH at or close to solid-liquid and bio interfaces, with spatial and time resolution. After the two main principles of pH detection are presented, the different classes of molecules and materials that are used as active components in these sensors are described. The review then focuses on the reported electroanalytical techniques for local pH sensing. As application examples, we discuss model studies that exploit local pH sensing in the area of electrocatalysis, corrosion, and cellular interfaces. We conclude with a discussion of key challenges for wider use of this analytical approach, which shows promise to improve the mechanistic understanding of reactions and processes at realistic interfaces.

Unravelling the carbonate issue through the regulation of mass transport and charge transfer in mild acid

The electrochemical CO2 reduction reaction (CO2RR) triggered by renewable electricity provides a promising route to produce chemical feedstocks and fuels with low-carbon footprints. The intrinsic challenge for the current CO2RR electrolyzer is the carbonate issue arising from the reaction between hydroxide and CO2. Acid CO2RR electrolyzers, in principle, can effectively solve the carbonate formation, but it remains inevitable practically. In this work, we thoroughly investigated the electrode processes of the CO2RR on the benchmark Ag catalyst in mild acid. The root of the carbonate issue arises from the imbalanced supply-consumption rate of protons-the electron transfer vs. mass transport. Regulating the hydrodynamics substantially reduces the proton diffusion length by 80%, increasing the single-pass carbon utilization efficiency of CO2-to-CO to 44% at -100 mA cm-2. The fundamental difference between mass transport and electron transfer on the spatial and temporal scale still leads to unavoidable carbonate formation. Future work to design intrinsically active catalysts in strong acid or metal-cation-free media is critical to solving the carbonate issue.