Controlling PEDOT:PSS electropolymerization for label-free optical recording of bioelectric potentials

Label-free optical recording of bioelectric potentials provides a non-perturbative approach for detecting bioelectric activities. Electrochromic materials, such as PEDOT:PSS, change their optical absorbance under external voltages. The voltage-dependent absorbance change allows recording of bioelectric potentials without using fluorescent reporters, and does not suffer from photobleaching or phototoxicity. Nevertheless, the voltage recording sensitivity of PEDOT:PSS strongly depends on its electrochromic properties. Here, we investigated how electropolymerization conditions of PEDOT:PSS thin films, including counterions, pH, PSS molecular weight, and substrate surface treatment, influence their electrochromic response. We found counterion competition is the dominant factor: introducing Cl^- as a competing counterion in addition to PSS during PEDOT electropolymerization blue-shifts the π-π* absorption peaks by ∼120 nm, alters film morphology and electrochromic response, and enhances optical anisotropy. At the same time, it reduces the voltage-dependent absorbance change at shorter (blue-green) wavelengths while enhancing it at longer (red) wavelengths. Using PEDOT:PSS thin films electropolymerized without Cl^- and a 561 nm laser, we achieved a voltage detection sensitivity of 2.8 µV at 10 kHz recording bandwidth under shot-noise-limited conditions. We then performed prolonged label-free optical recording of isolated embryonic chicken hearts. We recorded both the cardiac field potentials and mechanical contractions, which increased from embryonic day 10 to day 15, while the latency time between them remained unchanged. These results established electropolymerization as a strategy to tune the electrochromic response of PEDOT:PSS thin films across different wavelengths, thereby optimizing their voltage detection sensitivity.