Development of PVC membrane-based label-free K+ image sensor and imaging extracellular K+ dynamics in brain tissue

Because extracellular potassium ion $([\mathbf{K}^{+}]_{\mathbf{o}})$ plays an important role in the regulation of the physiological and pathophysiological activity of neurons, the imaging of $[\mathbf{K}^{+}]_{\mathbf{O}}$ dynamics and its spatiotemporal analysis is crucial for understanding brain function. Toward the high spatiotemporal imaging of $[\mathbf{K}^{+}]_{\mathbf{o}}$ dynamics in the brain, we fabricated a $23.55-\mu \mathrm{m}$ -pitch and $128\times 128$ -pixel label-free $\mathbf{K}^{+}$ image sensor, in which different thicknesses were deposited by controlling the volume of the polyvinyl chloride (PVC) membrane solution, and the detection performance was investigated. In the investigation of the characteristics of K+ measurement with sensors of different thicknesses, the sensors whose film thickness was decreased 9 $\mu\mathrm{m}$ exhibited superior K+ sensitivity with reasonable selectivity. When acutely sliced mouse hippocampus was stimulated with glutamate on the K+ ionophore-immobilized sensor, the output signal was increased in the hippocampal CAl, CA3, and DG regions, but no signal was observed when the slice was stimulated on a sensor without K+ ionophore. Additionally, the spatiotemporal resolution of output images obtained from the $9-\mu \mathrm{m}$ thick sensor was higher than those from the $108-\mu \mathrm{m}$ thick sensor. Taken together, we succeeded in the real-time imaging of $[\mathbf{K}^{+}]_{\mathbf{o}}$ from the acute mouse hippocampal slices, and demonstrated for the first time that membrane thickness significantly affects the spatial resolution of $[\mathbf{K}^{+}]_{\mathbf{o}}$ dynamics.