Photocurrent generation based on a light-driven proton pump in an artificial liquid membrane

Journal name:
Nature Chemistry
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Biological light-driven proton pumps use light to move protons across a cell membrane, creating a proton gradient. Although photochromic compounds such as spiropyrans can reversibly convert between two structures with differing pKa values, spiropyrans have not been used to generate either a light-driven proton pump or an electrical current. Here, we report an artificial light-harvesting system based on a supported liquid membrane doped with a spiropyran. Irradiating the membrane with ultraviolet light induces a ring-opening reaction, converting spiropyran to merocyanine, whereas irradiation with visible light induces the reverse reaction. When the membrane is irradiated with ultraviolet and visible light on opposite sides, H+ is taken up by merocyanine, carried through the polymeric membrane and released on the other side. We show that this system produces a light-induced proton flux, an electrical current with an efficiency of ∼0.12%, an open-circuit voltage of ∼210 mV and a membrane gradient of ∼3.6 ΔpH units. Alternating the sides illuminated with ultraviolet and visible light generates an alternating current.

At a glance


  1. Scheme demonstrating the photocurrent generation principle.
    Figure 1: Scheme demonstrating the photocurrent generation principle.

    The left side of the membrane is illuminated with ultraviolet light and the right with visible light or kept in the dark. The membrane contains 5 mg Sp, 6.5 mg ETH 500 and 3 mg K+R in 50 mg NPOE as solvent. The aqueous solutions both contain 0.1 M HCl. For simplicity, ETH 500 and Cl are not shown. Dashed arrows represent mass transport of the species.

  2. Light-induced voltage and current due to a light-driven proton gradient within the spiropyran-doped polypropylene membrane.
    Figure 2: Light-induced voltage and current due to a light-driven proton gradient within the spiropyran-doped polypropylene membrane.

    a, Representation of the flow cell used to measure the photocurrent generated by the light-harvesting membrane. b, Photocurrents generated when the membrane is placed in 0.1 M HCl and illuminated with ultraviolet light on one side. c, Photocurrent generated with illumination by visible light (482 ± 35 nm) on one side of the membrane. d, Observed open-circuit voltage when the membrane is illuminated with ultraviolet on one side and subsequently with visible on the other.

  3. Experimental and theoretical photocurrent evolution characteristics.
    Figure 3: Experimental and theoretical photocurrent evolution characteristics.

    a, Photocurrent evolution from numerical simulation showing the difference between ultraviolet irradiation on one side only (ring-opening rate constant k1 = 10−1.1 s−1, ring-closing rate constant k2 = 1 s−1) and asymmetric illumination with ultraviolet and visible (k1 = 10−1.1 s−1, k2 = 10−6 s−1). b, Experimental photocurrent generated when the membrane is illuminated with the light sequence indicated in the figure.

  4. Evaluation of conversion efficiency for the light-harvesting system.
    Figure 4: Evaluation of conversion efficiency for the light-harvesting system.

    a, Current density characteristics for the light-harvesting cell under light illumination (10 mW cm−2) and in the dark. b, Corresponding power density–voltage relationship under light irradiation.

  5. Changing the direction of the proton gradient in the membrane.
    Figure 5: Changing the direction of the proton gradient in the membrane.

    The direction reverses when illuminated with ultraviolet or visible light, so an alternating current can be generated using light sequences composed of ultraviolet and visible light. The figure shows alternating photocurrent generation by the membrane in 0.1 M HCl with alternating ultraviolet and visible light (ultraviolet for 2 s + off for 4 s + visible for 1 s + off for 4 s) illuminating one side of the membrane.

  6. The presence of an interfering ion can cause transport of the interfering ion in a direction opposite to the proton flux.
    Figure 6: The presence of an interfering ion can cause transport of the interfering ion in a direction opposite to the proton flux.

    Such a counter flux will reduce the net photocurrent density. a, Scheme demonstrating light-induced counter-diffusion in a direction opposite to the proton flux, which competes with the proton pumping, thereby reducing the current. b, Observed photocurrent density from the membrane under ∼0.2 mW cm−2 illumination as a function of pH in the two identical buffer solutions (2.5 mM boric acid + 2.5 mM citric acid + 2.5 mM NaH2PO4), with the Na+ background fixed at 2 M. The counter-diffusion process becomes more important at high pH and results in a lower current density.


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  1. Department of Inorganic and Analytical Chemistry, University of Geneva, Quai E. Ansermet 30, CH-1211 Geneva, Switzerland

    • Xiaojiang Xie,
    • Gastón A. Crespo,
    • Günter Mistlberger &
    • Eric Bakker


X.X. and E.B. designed the research and wrote the manuscript. X.X. and G.A.C. performed the experiments. G.M. helped with designing the experiment.

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