Can Single Cell Respiration be Measured by Scanning Electrochemical Microscopy (SECM)?

Ultramicroelectrode (UME), or, equivalently, microelectrode,probesare increasingly used for single-cell measurements of cellular propertiesand processes, including physiological activity, such as metabolicfluxes and respiration rates. Major challenges for the sensitivityof such measurements include: (i) the relative magnitude of cellularand UME fluxes (manifested in the current); and (ii) issues aroundthe stability of the UME response over time. To explore the extentto which these factors impact the precision of electrochemical cellularmeasurements, we undertake a systematic analysis of measurement conditionsand experimental parameters for determining single cell respirationrates via the oxygen consumption rate (OCR) in single HeLa cells.Using scanning electrochemical microscopy (SECM), with a platinumUME as the probe, we employ a self-referencing measurement protocol,rarely employed in SECM, whereby the UME is repeatedly approachedfrom bulk solution to a cell, and a short pulse to oxygen reductionreaction (ORR) potential is performed near the cell and in bulk solution.This approach enables the periodic tracking of the bulk UME responseto which the near-cell response is repeatedly compared (referenced)and also ensures that the ORR near the cell is performed only briefly,minimizing the effect of the electrochemical process on the cell.SECM experiments are combined with a finite element method (FEM) modelingframework to simulate oxygen diffusion and the UME response. Takinga realistic range of single cell OCR to be 1 x 10(-18) to 1 x 10(-16) mol s(-1), resultsfrom the combination of FEM simulations and self-referencing SECMmeasurements show that these OCR values are at, or below, the presentdetection sensitivity of the technique. We provide a set of model-basedsuggestions for improving these measurements in the future but highlightthat extraordinary improvements in the stability and precision ofSECM measurements will be required if single cell OCR measurementsare to be realized.