Label free bio image sensor for real time monitoring of potassium ion released from hippocampal slices

To assess the two dimensional distribution of the extracellular potassium ion concentration ([K+]out) surrounding hippocampal slices, we developed a 128 × 128 pixel type potassium ion-sensitive image sensor (PIS) composed of a charge-coupled-type pH image sensor and a poly(vinyl chloride) (PVC) plasticized membrane containing valinomycin, a potassium ion-sensitive ionophore. The potential slope was 50 mV/decade for a potassium ion concentration between 10−3 and 10−1 M. Although variations in the signal were not recorded in the absence of a brain slice or in areas outside of the slice position, significant changes in [K+]out were detected underneath the hippocampal slices. We successfully detected the glutamate-induced changes in [K+]out associated with gating through potassium ion channels in hippocampal neurons without a labeling process. [K+]out increased from 10 mM to 50 mM in the area between the sensor and slice stimulated with glutamate.