Ion channel proteins showed great promise in the field of nanopore sensing and molecular flux imaging applications due to the atomic-level precision of the pore size and high signal-to-noise ratio. More specifically, ion channel probes, where the protein channels are integrated at the end of a solid probe, leverage the sensing capabilities of nanopore sensing to achieve highly localized detection measurement. Metal probe materials such as gold have been developed to support lipid bilayers and enable the use of smaller probes, or nanoneedles, compared to glass micropipette ion channel probes. However, gold nanoneedle exhibited a significant channel current decay due to the double layer charging at the gold surface. To mitigate this problem, we recently developed a novel silver nanoneedle with a AgCl layer formed around the tip during the fabrication process. In contrast to gold, the silver probe showed a sustained DC stable channel current. One of the current challenges in such measurements is maintaining a single-channel recording- multiple protein insertions complicate data analysis and destabilize the bilayer. Herein, we combine the promising probe material (Ag/AgCl) with an approach based on tip-positioning control to maintain long-term single-channel recordings (up to 3 hrs). In this approach, the channel current is used as feedback to control the vertical movement of the silver tip so that we can control the number of protein channels inserted in the lipid membrane. Our findings reveal that the area of the lipid bilayer decreases with moving the silver tip up (i.e., decreasing the displacement in the z-direction). By reducing the bilayer area around the fine silver tip, we minimize the probability of multiple insertions and remove unwanted proteins. In addition, we characterize the effect of different lipids on the behavior of the lipid membrane in terms of the ease of bilayer control and maintaining single-channel recordings. This will provide more precise control over lipid membrane and protein insertion, and thus, we can maintain long-term single-channel recordings suitable for future imaging applications. We believe that the use of silver nanoneedle, which enables DC stable channel current, coupled with the developed tip displacement mechanism will offer more opportunities to employ these probes for chemical imaging and mapping different surfaces.

Silver nanoneedle ion channel probe to maintain single-channel recordings: Probe design and tip positioning control

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ACS Fall 2022

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