H+-type and OH- -type biological protonic semiconductors and complementary devices

Abstract

Proton conduction is essential in biological systems. Oxidative phosphorylation in mitochondria, proton pumping in bacteriorhodopsin, and uncoupling membrane potentials by the antibiotic Gramicidin are examples. In these systems, H(+) hop along chains of hydrogen bonds between water molecules and hydrophilic residues - proton wires. These wires also support the transport of OH(-) as proton holes. Discriminating between H(+) and OH(-) transport has been elusive. Here, H(+) and OH(-) transport is achieved in polysaccharide- based proton wires and devices. A H(+)- OH(-) junction with rectifying behaviour and H(+)-type and OH(-)-type complementary field effect transistors are demonstrated. We describe these devices with a model that relates H(+) and OH(-) to electron and hole transport in semiconductors. In turn, the model developed for these devices may provide additional insights into proton conduction in biological systems.

Figure 1. Protonic device architecture and proton conductivity mechanism.

(a) Two and three terminal devices with PdHx source and drain. PdH_x is created by exposing Pd metal to 5% H₂ atmosphere. At this H₂ concentration, the Pd metal absorbs H₂ to form PdH_x with x ≈ 0.5. PdH_x is kept under 5% H₂ atmosphere throughout the measurements and acts as a H⁺ reservoir. The PdHx source and drain inject and sink protons into and from the proton wire according to the reversible reaction going from left to right at the source and from right to left at the drain. The PdH_x source and drain are connected to outside measurement electronics that measure the electronic current and complete the circuit. (b) Molecular structure of the H⁺-type proton conductor maleic chitosan, (c) Molecular structure of the OH⁻ -type proton conductor proline chitosan. Degree of substitution defined as q/n + m determines the doping level. (d) Hop and turn Grotthuss mechanism for conductivity of H⁺ as hydronium ion along a proton wire. (e) Equivalent mechanism for OH⁻ conductivity as proton hole along proton wire.

Figure 2. Energy diagram representation of conduction in hydrogen bonded proton wire.

(a) A wire with no H⁺ or OH⁻ defect does not conduct. The band gap is defined as the energy required to create a H⁺ OH⁻ pair (proton-proton hole) and is derived from the E_gap = ΔG⁰′ = −k_BT ln K_w = 0.83 eV (Gibbs-Helmholtz equation). (b) For an intrinsic proton wire, the protochemical potential u_H+ is in the middle of the band-gap. The H⁺ is not completely delocalized along the conduction quasi band. Hopping barriers of approximately 100 meV (need to be overcome for conduction to occur. (c) An acid donates a H⁺ into the conduction band of a proton wire to yield a H⁺-type protonic conductor. E_d = ΔG⁰_a = −k_BT ln K_a, K_a is the acid dissociation constant. The maleic acid group pK_a (-log K_a) = 3.2, which corresponds to E_d = 0.18 eV. (d) A base accepts a H⁺ to create a OH⁻ (proton hole) in the valence band of a proton wire to yield a OH⁻-type protonic conductor. E_a = ΔG⁰_b = −k_BT ln K_b, K_b is the base dissociation constant. The proline base pk_b (-log K_b = 3.4), which corresponds to E_a = 0.20 eV. For both H⁺ type and OH⁻ type the protochemical potential is μ_C^H+ = eV₀ + µ₀+ k_BT ln a_H+ where a_H+ is the activity of H⁺.

Figure 3. H⁺- OH⁻ junction.

(a) Red trace- Experimental data for IV characteristics of a H⁺ OH⁻ junction formed by maleic chitosan and proline chitosan. The curve shows the expected nonlinearity. Black dots - data from simulations for the same junction using the semiconductor model. (b) When a H⁺ doped and OH⁻ doped material are placed into contact OH⁻ diffuse into the H⁺ region and H⁺ diffuse into the OH⁻ region until the μ_H+ on both sides is the same. H⁺ and OH⁻ recombine in the depletion region. A contact potential V₀ occurs across the junction and is dependent of the difference in μ_H+ of both sides. (c) A forward bias (+ive on H⁺ side) applied between source and drain reduces the contact barrier e(V₀-V_MP) and thermionic emission of H⁺ into OH⁻ side and vice versa occurs.

Figure 4. H⁺ and OH- transistors.

(a) (b) Plots of I_DS as a function of V_GS for different V_DS (RH 75%) for a maleic chitosan H⁺-FET and a proline chitosan OH⁻-FET with PdH_x contacts. Device dimensions: length 8.6 μm, width 3.5 μm, height 82 nm for (a) and 9.6 μm, width 28 μm, height 200 nm for (b). The small deviation of I_DS from zero at V_DS = 0 is likely due to hysteresis as previously observed for these types of devices, (c) (d) Schematics of H⁺ and OH⁻ transistor capacitative charge carrier n^H+ and n^OH− modulation. (c) (C_G = gate capacitance per unit area, t = device thickness) (d) and . From simulations of dQ/dV_gs, C_g = 3.85 × 10⁻⁴ F m⁻². (e) (f) Plots of as function of V_GS and linear fit for the device in (a) and (b) respectively. For cross σ and charge density calculations the cross sectional area of the devices was derived from AFM and the cross sections were approximated to a rectangle with t = 66 nm for (a) and t = 160 nm (b) with the same widths as the actual devices. From the fit, and .