A Scanning Ion-Selective Electrode Technique (SIET) platform

The SIET is a non-invasive, ultra-sensitive technique for measuring the free-ion concentration gradient of a specific ion (e.g., Ca2+, H+, K+, or Cl-) by means of a single ion-sensitive microelectrode (ISM) repeatedly moved between two measurement points in the bulk aquatic media surrounding a specimen. This is done using a system of stepper motors set to move the ISM tip a selected distance (usually between 5 to 50 μm excursion) and at programmed repetition rates lower than 1 Hz (typically 0.3 to 0.5 Hz range) to minimize media mixing. With appropriate shielding and grounding, the SIET can resolve ion flux values in the low to sub pmol/cm2/sec range, and with a spatial resolution that is determined by the ISM tip size, which can be as small as 3 to 5 µm. Positioning of the ISM around the sample and at reference positions, excursion distances, and sampling times for differential measurements, are software programmable and thus computer controlled. The ISM records a voltage at each measuring point representing the local concentration of the ion being measured. Reference measurements with the ISM are taken a few mm away from the specimen where no ion gradient should be detected.