Potassium Ion Dynamics Imaging through Supported Lipid Bilayers with Surface Plasmon Resonance Microscopy

We have developed an optical microscope based on surface plasmon resonance (SPR) to observe the dynamics of potassium ion (K+) concentrations. We formed a polyvinyl chloride (PVC) film containing ionophores that trap K+ on top of a gold film that excites surface plasmon. By measuring the attenuated total reflection spectra when injecting pure water or K+ solution on the fabricated substrate, we confirmed that the ionophore chelation reaction changed the PVC refractive index. Subsequently, we constructed a custom-made optical system for SPR imaging, mounted the substrate, and imaged the two-dimensional diffusion of K+ concentration changes when high concentrations of K+ solution were injected onto the substrate. In addition, we formed supported lipid bilayers on the substrate, mimicking cell membranes. Compared with the movie without lipid bilayer formation, that with supported lipid bilayers was confirmed to have taken longer time to increase the concentration due to its shielding effect of K+. Finally, we added K+ channels into the lipid bilayer membrane and visualized a K+ diffusion using the same procedure as that in the previous imaging experiment. We succeeded in observing the dynamics of K+ concentration change through the K+ channels inserted in the lipid bilayer. The addition of K+ channels weakened the shielding effect and shortened the time required for the K+ concentration to increase.