Development of near-infrared light responsive cup-stacked carbon nanofiber/ITO electrodes modified with poly(N-isopropylacrylamide)

Abstract

• Energy conversion among light, heat, and mechanical motion. • Photothermal conversion-based switchable mechano-electrochemical properties. • Control of electrochemical responses upon ON/OFF switching of the NIR irradiation. Reversibly activation and inhibition of electrochemical reactions at phase transition polymer-modified electrodes is of great interest from the viewpoints of application to sensing and energy conversion and storage. We synthesized the thermo-responsive poly( N -isopropylacrylamide) (PNIPA) via electrochemically induced free-radical polymerization at the surface of a cup-stacked carbon nanofiber-modified indium-tin-oxide (CSCNF/ITO) electrode. Here, CSCNFs not only allow fast electron transfer reactions with redox species because of highly ordered electroactive graphene edges at their surface but also absorb the near-infrared (NIR) light and convert to heat. As the PNIPA layer swelled at 25 ˚C, a [Fe(CN) 6 ] 3–/4– ion easily diffused to the CSCNF/ITO electrode surface due to the hydrophobic hydration of PNIPA, leading to the larger current responses. As the photothermal conversion of the CSCNF/ITO electrode occurred upon the NIR light irradiation (>940 nm), the contracted structure of PNIPA was formed due to its phase transition, resulting in the suppression of the electron transfer reaction for [Fe(CN) 6 ] 3–/4– even in the electrolyte solution at temperature below the lower critical solution temperature of PNIPA (ca. 32 ˚C). After turning off the NIR irradiation, the current response immediately turned back to the original values thanks to cooling of the electrode surface by the electrolyte solution (25 ˚C). The electron transfer reaction to dissolved redox species is now reversibly and repeatedly tunable by the phase transition of the grafted PNIPA based on not only temperature switching of the electrolyte solution but also the photothermal conversion of the CSCNF/ITO electrode upon the NIR light irradiation.