Controlling Photo-Stimulated Biological Proton Pumping with an Electrochromic Window

Electrochromic materials, such as tungsten oxide (WO₃), can undergo voltage-triggered changes in color and optical density, potentially valuable characteristics for applications such as energy control for buildings or window tint for privacy. Using external voltage to control the oxidation state of tungsten in WO₃, the optical absorbance of this material can be reversibly tuned, evidenced by a visible transformation from yellow to blue. This change in absorbance overlaps with the absorbance of the light-activated biological proton pump, bacteriorhodopsin (BR). BR absorbs light at 568 nm and, in turn, pumps protons across a lipid membrane. Here we explore the use of electrochromic WO₃ thin film windows to control the activity of BR proteins incorporated into lipid vesicles. Specifically, we utilize solution-phase chemistry to create WO₃ thin films on transparent conductive substrates. Electrochemical windows were created from these thin film substrates, and using cyclic voltammetry in a range of positive 1 V and -3 V with respect to an Ag/AgCl electrode, the absorbance of the WO₃ windows was varied.  These windows were then used to regulate light delivery to BR-containing vesicles in aqueous solution. BR activity, as a function of tunable WO₃ absorbance was determined by monitoring changes in solution pH induced by proton pumping of the BR proteins. This approach represents a creative integration of electrochromic materials with photo-activated proteins to create a programmable biological response. Continued research in this area may lead to the development of new, innovative material hybrids for emerging technologies.