Untangling Photofaradaic and Photocapacitive Effects in Organic Optoelectronic Stimulation Devices.

Vedran Ðerek,Ludovico Migliaccio,David Rand,Eric Daniel Głowacki,Yael Hanein

doi:10.3389/fbioe.2020.00284

Abstract

Light, as a versatile and non-invasive means to elicit a physiological response, offers solutions to problems in basic research as well as in biomedical technologies. The complexity and limitations of optogenetic methods motivate research and development of optoelectronic alternatives. A recently growing subset of approaches relies on organic semiconductors as the active light absorber. Organic semiconductors stand out due to their high optical absorbance coefficients, mechanical flexibility, ability to operate in a wet environment, and potential biocompatibility. They could enable ultrathin and minimally invasive form factors not accessible with traditional inorganic materials. Organic semiconductors, upon photoexcitation in an aqueous medium, can transduce light into (1) photothermal heating, (2) photochemical/photocatalytic redox reactions, (3) photocapacitive charging of electrolytic double layers, and (4) photofaradaic reactions. In realistic conditions, different effects may coexist, and understanding their role in observed physiological phenomena is an area of critical interest. This article serves to evaluate the emerging picture of photofaradaic vs. photocapacitive effects in the context of our group’s research efforts and that of others over the past few years. We present simple experiments which can be used to benchmark organic optoelectronic stimulation devices.

Highlights

Light illumination of a semiconducting material immersed in an electrolyte solution, with an energy greater than its band gap energy, can induce several different processes
Human embryonic kidney (HEK) cells transfected with TRPV1 ion channels could be locally photothermally stimulated by irradiating thin films of the polymeric semiconductor poly(3hexylthiophene), P3HT (Lodola et al, 2017)
Temporally-resolved currents generated by the organic electrolytic photocapacitor (OEPC) during the light pulse and delivered to the conductor back-electrode were measured to help resolve the nature of the processes taking place

Summary

Introduction

Light illumination of a semiconducting material (or heterostructure thereof) immersed in an electrolyte solution, with an energy greater than its band gap energy, can induce several different processes. Such optoelectronic devices with a twoelectrode architecture can be either capacitive, where the photocarriers generated by the semiconductor component lead to the build-up to two oppositely-charged electrical double layers or photofaradaic where both electrodes of the device catalytically support faradaic reactions (Figure 1, bottom).

Results

Conclusion