Developing nanopipettes with ion current and potentiometry response for applications in topography and pH measurement in single adenocarcinoma cells

Yusuf Muhammed and Robert A. Lazenby

Florida State University

Iridium oxide (IrO₂) pH sensors fabricated using macro and microscale electrodes have been reported for biological measurements. However, these sensors are only able to perform localized measurements at the tissue level. Scanning ion conductance microscopy (SICM) uses glass nanopipette with ion current response for the non-invasive scanning of the topography of single live cells, with nanometer resolution. Information about morphology of single cells, such as width, height, and changes in shape, can be obtained from the topography mapping. To get high resolution, and biochemical details such as proton concentration on the membrane, and topography of single cancer cells, we have fabricated a dual channel nanoscale sensor, that can spatially resolve different locations on the cell membrane. The sensors were developed from double barrel nanopipettes, where one barrel is filled with electrolyte for topography, while the other barrel was carbon pyrolyzed and functionalized with pH-sensitive IrO₂. This topography-pH sensing probe was then used as probe in SICM for the simultaneous mapping of topography and pH of adenocarcinoma cells (A549). Reported studies by Nadappuram et al., have shown that such sensors have spatially resolved the pH at calcite microcrystals, but did not fully explore the fabrication process for this type of sensor. In our study, we have explored the fabrication to ensure the pH sensors give the optimal Nernstian response. Also, we have shown details such as the topography of single A549 cancer cells, fabrication processes of double barrel pH sensors, pH difference when the sensor is far and closer to the surface of the cell membrane, and simultaneous measurement of the topography and pH of single cancer cells. A better understanding of our technique could aid in the understanding of Warburg effect in cancer cells with high spatiotemporal resolution and would provide better information at single cell level. In which other techniques such as magnetic resonance imaging and positron emission tomography (PET) could not reveal such single cell details.

Reference

Nadappuram, B. P., McKelvey, K., Al Botros, R., Colburn, A. W., & Unwin, P. R. (2013). Fabrication and characterization of dual function nanoscale pH-scanning ion conductance microscopy (SICM) probes for high resolution pH mapping. Analytical chemistry, 85(17), 8070-8074.