Organic Electrochemical Transistor Arrays with Integrated Lipid-Sealed Femtolitre Chambers for Simultaneous Electrical and Optical Detection of Membrane Protein Activity
We report a method for producing an array of fifty two ion-sensitive PEDOT:PSS organic electrochemical transistors on a glass coverslip, each featuring an integrated fluoropolymer microwell sealed with lipid bilayer into which membrane proteins can be inserted for simultaneous electrical and fluorescence microscopy studies. To demonstrate capability, we fill the microwells with an `inner' phosphate assay buffer solution containing 20 $\mu$M Alexa-488 dye and 50 mM KCl, seal the microwells with lipid bilayer using an aqueous-organic-aqueous liquid exchange technique, and then fill the common flow-cell volume above the sealed microwells with a dye-free `outer' phosphate assay buffer containing 100 mM KCl. We insert $\alpha$-hemolysin, which embeds into the lipid bilayer forming a heptameric pore with diameter ~ 2.6 nm. The pore allows K$^{+}$ ions to diffuse into the microwell and Alexa-488 dye molecules to diffuse out of the microwell producing a corresponding drop in transistor conductance and microwell fluorescence intensity, respectively. These two signals occur at different timescales, consistent with the known size difference between K$^{+}$ ions and Alexa-488 molecules. Our approach to fabricating microwell arrays with PEDOT:PSS OECTs incorporated into the bottom of selected microwells distributed in the array is both scalable and versatile, opening a path to studies using larger arrays and with other membrane proteins embedded in the lipid bilayer sealing the microwells.