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. 2016 Oct 7;7:12981. doi: 10.1038/ncomms12981

Electronic control of H+ current in a bioprotonic device with Gramicidin A and Alamethicin

Zahra Hemmatian 1,2, Scott Keene 2, Erik Josberger 2,3, Takeo Miyake 1,2, Carina Arboleda 2, Jessica Soto-Rodríguez 4, François Baneyx 4, Marco Rolandi 1,2,a
PMCID: PMC5059763  PMID: 27713411

Abstract

In biological systems, intercellular communication is mediated by membrane proteins and ion channels that regulate traffic of ions and small molecules across cell membranes. A bioelectronic device with ion channels that control ionic flow across a supported lipid bilayer (SLB) should therefore be ideal for interfacing with biological systems. Here, we demonstrate a biotic–abiotic bioprotonic device with Pd contacts that regulates proton (H+) flow across an SLB incorporating the ion channels Gramicidin A (gA) and Alamethicin (ALM). We model the device characteristics using the Goldman–Hodgkin–Katz (GHK) solution to the Nernst–Planck equation for transport across the membrane. We derive the permeability for an SLB integrating gA and ALM and demonstrate pH control as a function of applied voltage and membrane permeability. This work opens the door to integrating more complex H+ channels at the Pd contact interface to produce responsive biotic–abiotic devices with increased functionality.

Inline graphicConventional electronics use electrons as charge carriers whereas biological systems use ions, confounding integration of electronics with biology. Here the authors show voltage-regulated flow of protons across a supported lipid bilayer using the ion channel Gramicidin A and the voltage-gated ion channel Alamethicin.

From electroceuticals1 to wearable devices2, and from electronic plants3 to edible electronics4, interfacing electronic devices with biological systems promises new therapies and device functionalities beyond silicon5,6. In biological systems, most of the communication between cells is mediated by membrane proteins and ion channels that passively allow or actively control the flow of ions and small molecules across the cell membrane7. In this fashion, complex functions such as muscle contraction, neuronal signalling and metabolism are achieved. Membrane proteins are studied using patch clamps8, micropore arrays9 and electrode-supported lipid bilayers10,11,12,13, and passive transmembrane ionic transport is controlled by local electrical and chemical potential gradients according to the Nernst–Planck equation14,15. While most common ions are Na+, K+ and Cl−, proton (H+) currents and concentration [H+] gradients play important physiological roles16. Examples include oxidative phosphorylation in mitochondria17, light-activated H+ pumping by archaeal bacteriorhodopsins18, H+-activated bioluminescence in dinoflagellates19, flagellar propulsion in bacteria20, voltage gated H+ channels21 and antibiotic action by polypeptides such as Gramicidin22.

Ionic currents and membrane proteins are the most intimate interface for electronics to communicate with cells and biological systems23. However, conventional electronics typically use electrons as charge carriers instead of ions. To address this issue, gramicidin and bacteriorhodopsin have been integrated as gating elements with carbon nanotubes24, silicon nanowires25 and organic field effect transistors26 to develop biosensors with increased functionality. Organic polymers27,28,29 with ionic30,31 and mixed conductivity have been used to record and stimulate physiological functions and even assembled into logic circuits32. We have recently demonstrated control of H+ flow in bioprotonic field effect transistors (H+-FETs)33,34,35 and memories36, and integrated these devices with enzymatic logic gates37. H+-FETs built with squid reflectin proteins have also been described38,39,40. At the heart of these devices is the use of the Pd/PdHx couple as a contact that enables the translation of an H+ current into an electronic current and thus serves as a transducer between biological systems and electronics35,41. Here, we fabricate and characterize bioprotonic devices incorporating ion channels and Pd/PdHx contacts to control H+ currents and modulate pH gradients across phospholipid membranes (Fig. 1). These devices comprise a supported lipid bilayer (SLB) that mimics the function of a cell membrane at the Pd/solution interface and acts as a self-sealing support for the insertion of the ion channels Gramicidin A (gA) and Alamethicin (ALM). We show that gA can be used to linearly control H+ currents as function of voltage while ALM functions as a voltage-gated channel analogous to an ON-OFF switch. This is a unique and novel architecture compared to prior work with electron conducting Au42 and Pt43 electrodes that allows for direct interfacing of H+ current from the ion channels.

Figure 1. Schematic depiction of the ion channel bioprotonic device.

Figure 1

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(Left) A bioprotonic device with integrated Gramicidin (gA) supports the flow of H+ across the SLB upon application of a negative voltage (−V) to the Pd contact. When H+ reach the surface of the Pd contact, they are reduced to H by an incoming electron and diffuse into Pd to form a hydride (PdHx). A reduction current at the Pd contact is measured as current density (−iH+). (Right) In a bioprotonic device with integrated Alamethicin (ALM), applying a negative or positive voltage (V) above a threshold value at the Pd contact opens the gate and allows H+ flow across the SLB, turning the device ON. At V=0 mV, no H+ flows across ALM and the device switches OFF.

Results

Device architecture

Pd contacts are evaporated in areas ranging from 10 μm2 (2 × 5 μm) to 200 μm2 (2 × 100 μm) onto a Si wafer with a 100 nm SiO2 insulating layer. Microfluidic channels are defined by lithographically patterned SU-8 photoresist and sealed with a polydimethylsiloxane layer comprising of inlet and outlet ports (Supplementary Fig. 1). The channels are filled with a standard buffered solution (5 mM PBS and 100 mM KCl, pH=7.0) and an Ag/AgCl electrode is inserted into the solution to serve both as a reference electrode and as a counter electrode in a two terminal setup.

We apply a potential difference between the Pd contact and the Ag/AgCl electrode (V) and measure the resulting H/H+ current density (iH+) where oxidation of H corresponds to a positive iH+ and reduction of H+ corresponds to a negative iH+ (Fig. 1). For −V applied to the Pd contact, electrons flow from the Pd contact and reduce H+ to H at the Pd/solution interface. H then absorbs into the Pd to form PdHx with x up to 0.6. Conversely, for +V, H oxidizes into H+ at the Pd/solution interface, and is released into solution. This results in the collection of electrons by the Pd contact, which in turn yields a positive iH+ value. In short, by measuring iH+, we effectively monitor the flow of H+ between the solution and the Pd contact mediated by H++e−⇔H, and subsequent absorption of H into Pd to form PdHx or desorption of H from the contact to form H+ (refs 37, 44).

To electrically isolate the Pd contact from the solution and provide a template for ion channel insertion, we deposit a 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC, Sigma-Aldrich Lipids) SLB onto the Pd contact using vesicle fusion45. Force rupture measurements by Atomic Force Microscopy (AFM)46 show that the thickness of the SLB membrane is 4.8±0.7 nm (n=20, for a device with dimension of 2 × 50 μm), which is close to the expected value of 5.5 nm for a DOPC bilayer47,48 (Supplementary Fig. 2 and Supplementary Note 1). In essence, the SLB mimics a cell membrane that electrically insulates the Pd contact (ρ∼3 G Ω cm−1; see Supplementary Fig. 3 and Supplementary Table 1) and divides the solution into two volumes. We refer to the larger volume containing the Ag/AgCl electrode as the bulk solution (B). We refer the small volume between the SLB and the Pd contact, as the isolation layer (IL). The IL provides lubrication and mobility to the SLB49,50,51,52, creating an excellent platform for the investigation of cellular processes involving small size molecules such as gA and ALM.

Voltage control of H+ flow with Gramicidin A

Gramicidin A (gA) is a short helical polypeptide from Bacillus brevis that dimerizes in lipid bilayers to form a transmembrane channel that allows the passage of small cations (including H+) while remaining impermeable to anions53. To control the flow of H+ as a function of V, we integrate gA in the SLB of our devices. In the absence of gA and at an applied V=−200 mV, we measure iH+=−1.2 mA cm−2 (Fig. 2a,d). This small iH+ indicates that few H+ diffuse across the bilayer to become reduced at the Pd surface. To confirm this result, we set V=0 mV after applying V=−200 mV for 10 min. If any H+ flow were to occur across the SLB during the V=−200 mV step, a significant amount of PdHx should form at the Pd/solution interface37. This PdHx has a higher protochemical potential (μH+) than the solution of pH=7.0 at V=0 mV, leading to H oxidation at the PdHx contact, H+ flow from the PdHx into the IL, and giving rise to a measurable positive iH+ (ref. 34). The black trace in Fig. 2d (right hand side panel) shows that this is clearly not the case.

Figure 2. Schematics of bioprotonic gA devices.

Figure 2

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(a) Pd contact coated with a SLB. The SLB inhibits the flux of H+ from the bulk solution to the Pd/solution interface. (b) Pd contact with SLB incorporating gA is semipermeable to H+, with gA channels facilitating the rapid flow of H+ to the Pd/solution interface. (c) Addition of 1 mM Ca2+ to the bulk solution, blocks gA and prevents the flow of H+ to the Pd/solution interface. (d) iH+ versus time plot for V=−200 mV and V=0 mV. Black trace SLB, green trace SLB+gA, blue trace SLB+gA blocked by Ca2+. (The data are collected from 3 different devices with different dimensions: Pd / SLB: 3 different devices of 2 × 50 μm, Pd/SLB+gA: 2 × 20 μm, 2 × 50 μm, 2 × 70 μm, Pd/SLB+gA+Ca2+: 2 different devices of 2 × 50 μm and 1 device of 2 × 70 μm. The error bars are the root mean square of the displacement of the data from the average value).

In contrast, the bioprotonic device with gA and subjected to V=−200 mV experiences a large iH+ that continually increases to a maximum of iH+=−4.5 mA cm−2 at t=10 min (Fig. 2b,d), because the gA inserted within the SLB create pathways for H+ flow. The dependence of iH+ as a function of time will be discussed in the modelling section. Setting V=0 mV after 10 min causes H+ to transfer from the PdHx to the solution with a maximum iH+=0.5 mA cm−2 (Fig. 2d, green trace). The presence of a large oxidation peak for H+ at 50 mV in the I–V sweep confirms that this transfer is indeed occurring (Supplementary Fig. 4). To verify that gA was responsible for H+ flow across the SLB, we added 1 mM Ca+2 to block H+ transfer across the gA channel (Fig. 2c)25. Under these conditions, for V=−200 mV we measure iH+=−0.7 mA cm−2 and subsequently measure iH+=−0.5 mA cm−2 when setting V=0 mV after 10 min (Fig. 2d, blue trace). These values of iH+ are comparable to those measured when no gA is incorporated in SLB and are consistent with Ca2+ blocking gA channels.

Alamethicin voltage-gated H+ switches

To demonstrate that our device architecture can be used to create a voltage-dependent switch that turns the H+ flow on and off between bulk solution and isolation layer (Fig. 3), we integrate alamethicin (ALM) into the SLB. Alamethicin is a 20-amino-acids long peptide from the fungus Trichoderma viride. ALM undergoes spontaneous insertion into lipid bilayers and forms a voltage-gated channel when 4-to-6 molecules associate to form an α-helical bundle54,55,56. An asymmetric threshold voltage above ∼60 mV induces reorientation of the helices, thereby opening the channel to selective transport of cations, including H+, in the direction of V (ref. 57). For V values inferior to this threshold, ALM is closed and there is no H+ flow across the membrane in spite of differences in [H+], which typically creates a driving force for H+ flow (Fig. 3).

Figure 3. Schematics of bioprotonic voltage-gated devices.

Figure 3

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(a) Pd contact with SLB and ALM. At V=−200 mV, ALM channels open and H+ flows to the Pd/solution interface. (b) PdHx contact with SLB and ALM remains closed under V=0 mV and there is no H+ flow. (c) PdHx contact coated with SLB and ALM can only oxidize H under a small positive voltage of V=100 mV which is also required to open ALM channels and allow for flow of H+ from the PdHx contact to bulk solution. (d) iH+versus time plot for V=−200, 0 and 100 mV, SLB black trace and SLB integrating ALM red trace. (The data is collected from 3 different devices with the different dimensions, Pd/SLB: 3 different devices of 2 × 50 μm, Pd/SLB+ALM: 2 × 20 μm, 2 × 40 μm, 2 × 50 μm. The error bars are the root mean square of the displacement of the data from the average value).

Similar to the gA device, we measure iH+=−5.5 mA cm−2 at V=−200 mV for an ALM integrated bioprotonic device; at V=−200 mV the ALM channel is open, allowing flow of H+ (Fig. 3a,d). When the voltage is set at V=0 mV, iH+=0 mA cm−2. This is because the ALM channel is closed, and no H+ flow occurs between IL and B, even after PdHx formation. For V=100 mV, the ALM channel opens and we measure iH+=1.9 mA cm−2, indicating that H+ flows from the PdHx contact and across the SLB. As a control, we added 40 mM urea to the bulk solution to irreversibly disrupt the ALM channel structure and stop H+ flow across the SLB58. Under these conditions, we find that iH+=−1.2 mA cm−2 for V=−200 mV and iH+=0 mA cm−2 for both V=0 mV and V=100 mV (Supplementary Fig. 5), confirming that ALM is responsible for the bioprotonic device behaviour. Moreover, as is the case with the gA device, the H+ oxidation peak at 50 mV dominates the I–V sweep. However, for V>0 mV, iH+ in the ALM device has a rectifying diode-like behaviour (Supplementary Fig. 6). We conclude that the voltage-gated functionality of the ALM channel is maintained on its integration into the device's SLB.

To summarize, the device switches ON when ALM voltage-gated channels are open, allowing H+ flow across the membrane and in the direction of V. The device is OFF when V=0 mV, a voltage below the threshold value needed for ALM opening. Thus, an ALM device can be used to modulate H+ flow and [H+] between IL and B, much like cells control [H+] differences between cytosol and extracellular space.

A model for H+ transfer across gA and ALM channels

To better understand the dynamics of H+ flow in our biotic–abiotic devices, we modelled H+ transport characteristics using the Nernst–Plank equation to fit our device current density data, iH+ (Fig. 4). To this end, we divide the device into four distinct layers: the bulk solution (B); a Supported Lipid Bilayer membrane (SLB) with variable permeability (PSLB) that encompasses the SLB with integrated gA or ALM; the isolation layer (IL); and the Pd/PdHx contact (Fig. 4a). When a negative voltage −V is applied to the Pd contact, H+ flow from B across the SLB and into the IL. We quantify this transport as the H+ current density across membrane, jH+. A portion of the H+ from jH+ contribute to an increase in [H+] in the IL, [H+]IL, while the remaining H+ are reduced to H at the Pd contact and form PdHx. We assume that iH+ measured between the Pd contact and the Ag/AgCl electrode arises entirely from electrons participating in H+ oxidation (positive iH+) or reduction processes (negative iH+), and we quantify it as current density, iH+.

Figure 4. Analytical description of channel permeability.

Figure 4

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(a) Schematic of membrane modelling parameters. (B, bulk solution; SLB, Supported Lipid Bilayer membrane; IL, isolation layer; PSLB, SLB permeability; [H+]IL, H+ concentration in isolation layer; jH+, H+ flux across the membrane; iH+, H+ redox current density; V=−200 mV; jH+ and iH+ are computed with equations 2 and 3, respectively. A rise in [H+]IL increases the d[H+] between Ag/AgCl electrode and Pd contact for H+ reduction, according to the Nernst equation (equation 4). (b) Iterative simulation of iH+ at the Pd/solution interface for SLB (black trace), SLB incorporating gA (green trace), and SLB incorporating ALM (red trace) bioprotonic device. A combination of GHK and Tafel equations was used to describe the behaviours of membrane and Pd contact. (c) Change in pHIL using the parameters from the simulation.

From conservation of mass (equation 1)59

graphic file with name ncomms12981-m1.jpg

where, e is elementary charge=1.6 × 10−19 C, and d[H+]IL is the incremental change in molar concentration of H+ in the IL in moles m−3. jH+ is obtained by inserting the Goldman–Hodgkin–Katz (GHK) solution60 into the Nernst–Planck equation (equation 2)59:

graphic file with name ncomms12981-m2.jpg

where PSLB is the permeability of H+ through the SLB, F is Faraday's constant, f is F/RT, or 38.66 V−1, Vm is the potential difference across the membrane and [H+]B is the H+ concentration in the bulk layer in moles m−3 (10−4 mol m−3). We set Vm=V and assume that the potential drop across B is negligible relative to the potential drop across the membrane. When no gA or ALM channels are present, the resistance of the SLB, RSLB≈GΩ (Figs 2 and 3), is six orders of magnitude higher than the resistance of B (RB=0.6 kΩ).

We derive iH+ using a modified version of the Tafel equation61 (equation 3):

graphic file with name ncomms12981-m3.jpg

where i0 is the Pd exchange current density (a constant describing forward and reverse iH+ at the Pd/solution interface when they are equal to each other and the net iH+ is 0 mA cm−2), α is the transfer coefficient, V0 is the redox potential between Pd and Ag/AgCl (−0.22233 V), and VN is the Nernstian potential created by the change in [H+] across the SLB. VN is derived from (equation 4):

graphic file with name ncomms12981-m4.jpg

We assume that [H+]B remains constant and calculate the change in [H+]IL from the change of pH in the IL, dpHIL, assuming a buffering capacity for the solution, β (Supplementary Note 2)62:

graphic file with name ncomms12981-m5.jpg

where DIL is the thickness of the isolation layer. The resulting d[H+]IL is (ref. 59):

graphic file with name ncomms12981-m6.jpg

To fit iH+ to the experimental data, we programed an iterative solution to the above model using the membrane permeability, PM, as a fitting parameter. For V=−200 mV and DIL=0.5 nm the model fits the experimental data well using membrane permeabilities of PSLB=0.006 s−1 (unmodified SLB), PSLB+gA=0.58 s−1 (SLB with integrated gA) and PSLB+ALM=0.74 s−1 (SLB with integrated ALM). Consistent with our expectation, insertion of either gA or ALM into the SLB increases membrane permeabilities by three orders of magnitude. We did not control or accurately measure the density of the channels and these permeabilities are characteristic of the device and do not represent the absolute conductivities of individual gA or ALM.

We next calculate the change in pHIL using the above permeabilities (Fig. 4c). Qualitatively, if the rate at which H+ are reduced at the Pd contact is higher than the rate at which H+ permeate the membrane (|iH+|>|jH+|), pHIL will increase with respect to the pHB. Considering protochemical potential differences among B, IL and the Pd/PdHx contact, an acidic pHIL will favour transfer of H+ from the IL into Pd (|iH+|>|jH+|), while a basic pHIL will favour transfer of H+ from B to the IL through the gA and ALM channels. For a naked SLB, our model predicts that some H+ reduction will occur at the Pd surface. Very few H+ leak through the membrane and pHIL increases slowly (Fig. 4c, black trace). For an SLB integrating gA, the model predicts an initial increase in pHIL followed by a progressive decrease as more H+ flow through the gA channel over time. These results can be rationalized as follows: at the initial pHIL=7, reduction at the Pd contact interface (|iH+|) is favoured with respect to flow through gA channels (|jH+|) because pHB=pHIL and the only driving force for flow along the gA channel is V. As pHIL increases, |jH+| increases and |iH+| decreases (Fig. 4b) due to the [H+] difference between B and IL driving H+ flow across gA. In the case of an SLB integrating ALM, the dynamics for iH+ and pHIL are essentially the same as for gA but with a shorter time constant and a lower pHIL plateau value. This is likely due to the fact that PALM>PgA: the higher membrane permeability of ALM SLBs favours H+ transfer (|jH+|) over H+ reduction at the Pd/PdHx interface (|iH+|). In summary, the model developed with our bioprotonic devices provides information on the H+ permeability of biomimetic membranes. It should prove valuable to characterize the transport behaviour of other ion channels or H+ pumps in biotic–abiotic devices.

Discussion

In this communication, we demonstrate a biotic–abiotic bioprotonic device with proton (H+) conducting Pd/PdHx contacts that measures and controls the flow of H+ across a lipid bilayer with the ion channels gramicidin A (gA) and alamethicin (ALM). We use gA to increase membrane permeability and control the H+ flow across the lipid bilayer with a voltage applied on the Pd/PdHx contact. We also demonstrate bidirectional voltage gated switching of the H+ flow across the SLB with ALM. Finally, we model H+ transport in this system using the Goldman solution and the Nernst equation and use the model to derive the permeability parameters of the lipid bilayer with integrated gA and ALM ion channels and to predict change in pH of the solution at the lipid bilayer/Pd contact interface. Our results indicate that ion channels increase SLB permeability by three orders of magnitude. This is the first time that H+ conducting channels have been integrated with Pd/PdHx H+-conducting contacts and that the H+ current flowing through these channels has been directly measured and controlled. Future integration of more complex proteins may require Pd contact functionalization with a self-assembled monolayer63 or cushioning lipopolymer64,65,66 to passivate the Pd surface and overcome protein denaturation66. To this end, we demonstrate that passivation of the Pd contact with self-assembled 3-aminopropyl-triethoxy-silane63 retains gramicidin A device functionality (Supplementary Figs 7–9). On the other hand, passivating the Pd contact with poly (ethylene glycol)67,68 insulates the Pd surface and hampers H+ transfer at the Pd contact solution interface. Alternatively, larger proteins such as H. turkmenica detarhodhopsin can be engineered to bind to the Pd contact surface without experiencing denaturation by making use of fused Pd-binding peptides69. This work opens the door to integrating more complex active H+ channels at the Pd contact interface to produce biotic–abiotic devices with increased functionality.

Methods

Materials

1,2-dioleoyl-sn-glycerol-3-phosphocholine (DOPC; Sigma-Aldrich), Bacillus brevis Gramicidin A (90% purity; BioChemika), Trichoderma viride Alamethicin (90% purity; Sigma-Aldrich) were used as received. The heterofunctional PEG, 1,2-distearoyl-snglycero-3-phosphoethanolamine-N-poly (ethylene glycol)-2000-N- (3-(2-(pyridyldithio) propionate) (DSPE-PEG-PDP) and lipids, L-R-phosphatidylcholine from egg (egg-PC), were purchased from Avanti Polar Lipids (Alabaster, AL). 3-aminopropyl-triethoxy-silane was purchased from, Sigma-Aldrich. Potassium ferrocyanide (K4(Fe(CN)6), 99.9% purity) K2HPO4, KH2PO4, KCl, HCl and KOH (all with 99.9% purity) were purchased from Sigma-Aldrich. K-PBS buffer consisted of 100 mM KCl in 5 mM potassium phosphate (K2HPO4/KH2PO4 at a molar ratio of 3:2), pH=7.0. Buffer solutions were prepared with degassed deionized water (Millipore) and the pH was adjusted to 7.0 with HCl or KOH if necessary. The reference and counter electrode (1-mm diameter 3 M KCl Ag/AgCl) was from Warner Instruments. Silicon wafers (100), 4-in. diameter were acquired from International Wafer Service. Ethanol, (200 proof), acetone and chloroform had 99.9% purity.

Device fabrication and characterization

Bioprotonic devices were fabricated with conventional soft- and photo-lithography on a 100 nm thick layer of silicon dioxide. Each device has a microfluidic SU-8 channel of 4 μm thick. The Pd contacts have varied contact area of 10 μm2 (2 × 5 μm), 20 μm2 (2 × 10 μm), 40 μm2 (2 × 20 μm), 80 μm2 (2 × 40 μm), 100 μm2 (2 × 50 μm), 120 μm2 (2 × 60 μm), 140 μm2 (2 × 70 μm), 160 μm2 (2 × 80 μm), 180 μm2 (2 × 90 μm) and 200 μm2 (2 × 100 μm) with a thickness of 50 nm. Pd is deposited on top of 5 nm Cr adhesion layer. Each device has an electrical probe pad out of the microfluidic channel that provides the electrical contact. The microfluidic channel confines the flow of liquid to the top of the Pd contact, while a polydimethylsiloxane well on top of the channel provides space to insert the counter electrode (Supplementary Fig. 1).

SLB formation and characterization

DOPC vesicles (davg=100±23 nm, n=3) were prepared by tip sonication of a solution of 0.5 mg ml−1 DOPC, in K-PBS buffer solution (pH=7.0). Dynamic light scattering was used to characterize vesicle size. Before deposition, the Pd surface was hydrophilized by oxygen plasma70. The vesicle solution was dispensed in the microfluidic channel and the device was gently agitated for 12 h at 100% relative humidity to ensure vesicle fusion, followed by rinsing with K-PBS buffer solution to wash away unfused DOPC vesicles. The resulting bilayers were characterized by AFM, with rupture depth measurements showing a thickness of ca. 4.8±0.7 nm (Supplementary Fig. 2 and Supplementary Note 1)46. Home-built systems were used to conduct cyclic voltammetry experiments in the presence of a redox probe and I–V measurements in the presence of buffer solution. Throughout this process, the pH was monitored using a calibrated pH metre.

Ion channel incorporation into supported lipid bilayers

Gramicidin A (5 mg ml−1) in 200-proof ethanol was mixed with DOPC in chloroform followed by solvent evaporation with N2 the rehydration with aqueous buffer solution. DOPC/gramicidin vesicles at a molar ratio of 200:1 were formed by tip sonication and vesicle size determined by dynamic light scattering (davg=105±26 nm, n=3). These vesicles were then fused onto Pd microfluidic devices. ALM was incorporated into bilayers by incubating 5 mg ml−1 solution of ALM peptide for 30 min on freshly deposited Pd-bilayer structures followed by rinsing with K-PBS buffer solution.

Electrical measurements

All electrical measurements were performed using a Signatone S probe station with a custom built environmental chamber. The probe station was connected to an Agilent 4155C semiconductor parameter analyser.

Simulations

Iterative simulations were executed in the MATLAB software package. Pd contact Tafel parameters i0 and α were measured with low-voltage I–V sweeps at a scan rate of 5 mV s−1 (Supplementary Fig. 10). The permeability of the three membranes were estimated using a composite model of the permeability of individual cationic channels and the permeability of the DOPC matrix, normalized to their respective occupied area. The length of the isolation layer (ca. 1 nm) was estimated using AFM force versus displacement measurements.

Code availability

MATLAB code for simulations is available at https://dx.doi.org/10.6084/m9.figshare.3509846.v1.

Data availability

The data that support the findings of this study are available from the corresponding author on request.

Additional information

How to cite this article: Hemmatian, Z. et al. Electronic control of H+ current in a bioprotonic device with Gramicidin A and Alamethicin. Nat. Commun. 7, 12981 doi: 10.1038/ncomms12981 (2016).

Supplementary Material

Supplementary Information

Supplementary Figures 1-10, Supplementary Table 1, Supplementary Notes 1-2, Supplementary References

ncomms12981-s1.pdf (1.5MB, pdf)

Acknowledgments

This work is supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under award no. <DE-SC0010441>, device integration and device measurement and the Air Force Office of Sponsored Research under award no. <FA9550-15-1-0273>, device fabrication. Part of this work was conducted at the Washington Nanofabrication/Molecular & Nanotechnology User Facility, a member of the NSF National Nanotechnology Infrastructure Network and also at the Molecular Analysis Facility, which is supported in part by funds from the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, the National Science Foundation and the National Institutes of Health.

Footnotes

Author contributions M.R. conceived the research. M.R. and Z.H. designed the experiments. Z.H. and S.K. performed the experiments. S.K. designed and implemented the simulations. M.R, Z.H. and S.K. analysed the data. E.J. and Z.H. fabricated the devices. J.R., C.A. and F.B. helped with SLB device integration. Z.H. and M.R. wrote the manuscript with input from all authors. All authors revised the manuscript.

References

  1. Jonsson A. et al. Therapy using implanted organic bioelectronics. Sci. Adv. 1, e1500039 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Pang C. et al. Highly skin-conformal microhairy sensor for pulse signal amplification. Adv. Mater. 27, 634–640 (2015). [DOI] [PubMed] [Google Scholar]
  3. Stavrinidou E. et al. Electronic plants. Sci. Adv. 1, e1501136 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Kim Y. J., Chun S.-E., Whitacre J. & Bettinger C. J. Self-deployable current sources fabricated from edible materials. J. Mater. Chem. B 1, 3781–3788 (2013). [DOI] [PubMed] [Google Scholar]
  5. Malliaras G. & Abidian M. R. Organic bioelectronic materials and devices. Adv. Mater. 27, 7492–7492 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Noy A. Mimicking biology with nanomaterials: carbon nanotube porins in lipid membranes. Biophys. J. 108, 443a (2015). [Google Scholar]
  7. Shen Y.-x., Saboe P. O., Sines I. T., Erbakan M. & Kumar M. Biomimetic membranes: a review. J. Membr. Sci. 454, 359–381 (2014). [Google Scholar]
  8. Cristof Grewer A. G., Thomas M. & Klaus F. Electrophysiological characterization of membrane transport proteins. Annu. Rev. Biophys. 42, 95–120 (2013). [DOI] [PubMed] [Google Scholar]
  9. Watanabe R. et al. Arrayed lipid bilayer chambers allow single-molecule analysis of membrane transporter activity. Nat. Commun. 5, 4519 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Oscar Gutierrez-Sanz C. T. et al. Induction of a proton gradient across a gold-supported biomimetic membrane by electroenzymatic H2 oxidation. Angew. Commun. 54, 2684–2687 (2015). [DOI] [PubMed] [Google Scholar]
  11. Joanna Juhaniewicz S. S. Atomic force microscopy and electrochemical studies of melittin action on lipid bilayers supported on gold electrodes. Electrochim. Acta 162, 63–61 (2015). [Google Scholar]
  12. Jahnke J. P., Bazan G. C. & Sumner J. J. Effect of modified phospholipid bilayers on electrochemical activity of a membrane-spanning conjugated oligoelectrolyte. Langmuir 31, 11613–11620 (2015). [DOI] [PubMed] [Google Scholar]
  13. Tse E. C. et al. Anion transport through lipids in a hybrid bilayer membrane. Anal. Chem. 87, 2403–2409 (2015). [DOI] [PubMed] [Google Scholar]
  14. Buck R. P. Kinetics of bulk and interfacial ionic motion: microscopic basis and limits for the Nernst-Planck equation applied to membrane systems. J. Membr. Sci. 17, 1–62 (1984). [Google Scholar]
  15. Robertson J. W. Modeling ion transport in tethered bilayer lipid membranes. 1. Passive ion permeation. J. Phys. Chem. B 112, 10475–10482 (2008). [DOI] [PubMed] [Google Scholar]
  16. DeCoursey T. E. The voltage-gated proton channel: a riddle, wrapped in a mystery, inside an enigma. Biochemistry 54, 3250–3268 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Morowitz H. J. Proton semiconductors and energy transduction in biological systems. Am. J. Physiol.-Regul. Integr. Comp. Physiol. 235, R99–R114 (1978). [DOI] [PubMed] [Google Scholar]
  18. Lanyi J. K. Bacteriorhodopsin. Annu. Rev. Physiol. 66, 665–688 (2004). [DOI] [PubMed] [Google Scholar]
  19. Smith S. M. et al. Voltage-gated proton channel in a dinoflagellate. Proc. Natl Acad. Sci. USA 108, 18162–18167 (2011). [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Walz D. & Caplan S. R. Bacterial flagellar motor and H+/ATP synthase: two proton-driven rotary molecular devices with different functions. Bioelectrochemistry 55, 89–92 (2002). [DOI] [PubMed] [Google Scholar]
  21. Capasso M., DeCoursey T. E. & Dyer M. J. pH regulation and beyond: unanticipated functions for the voltage-gated proton channel, HVCN1. Trends Cell Biol. 21, 20–28 (2011). [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Busath D. & Szabo G. Gramicidin forms multi-state rectifying channels. Nature 294, 371–373 (1981). [DOI] [PubMed] [Google Scholar]
  23. Ajo-Franklin C. M. & Noy A. Crossing over: nanostructures that move electrons and ions across cellular membranes. Adv. Mater. 27, 5797–5804 (2015). [DOI] [PubMed] [Google Scholar]
  24. Huang S.-C. J. et al. Carbon nanotube transistor controlled by a biological ion pump gate. Nano Lett. 10, 1812–1816 (2010). [DOI] [PubMed] [Google Scholar]
  25. Misra N. et al. Bioelectronic silicon nanowire devices using functional membrane proteins. Proc. Natl Acad. Sci. 106, 13780–13784 (2009). [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Angione M. D. et al. Interfacial electronic effects in functional biolayers integrated into organic field-effect transistors. Proc. Natl Acad. Sci. USA 109, 6429–6434 (2012). [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Johnson N., Kim Y. J., Dingh H., LeDuc P. & Bettinger C. Bio-Inspired pH responsive hydrogels for programmed activation of electrochemical storage systems in biology. Biophys. J. 108, 485a (2015). [Google Scholar]
  28. Mitraka E. et al. Solution processed liquid metal-conducting polymer hybrid thin films as electrochemical pH-threshold indicators. J. Mater. Chem. C 3, 7604–7611 (2015). [Google Scholar]
  29. Owens R. M. & Malliaras G. G. Organic electronics at the interface with biology. MRS Bull. 35, 449–456 (2010). [Google Scholar]
  30. Tybrandt K., Larsson K. C., Richter-Dahlfors A. & Berggren M. Ion bipolar junction transistors. Proc. Natl Acad. Sci. 107, 9929–9932 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Williamson A. et al. Epilepsy treatment: controlling epileptiform activity with organic electronic ion pumps. Adv. Mater. 27, 3097–3097 (2015). [DOI] [PubMed] [Google Scholar]
  32. Tybrandt K., Forchheimer R. & Berggren M. Logic gates based on ion transistors. Nat. Commun. 3, 871 (2012). [DOI] [PubMed] [Google Scholar]
  33. Deng Y. et al. H+-type and OH−-type biological protonic semiconductors and complementary devices. Sci. Rep. 3, 12118 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Miyake T. & Rolandi M. Grotthuss mechanisms: from proton transport in proton wires to bioprotonic devices. J. Phys.: Condens. Matter 28, 023001 (2015). [DOI] [PubMed] [Google Scholar]
  35. Zhong C. et al. A polysaccharide bioprotonic field-effect transistor. Nat. Commun. 2, 476 (2011). [DOI] [PubMed] [Google Scholar]
  36. Josberger E. E., Deng Y., Sun W., Kautz R. & Rolandi M. Two-terminal protonic devices with synaptic-like short-term depression and device memory. Adv. Mater. 26, 4986–4990 (2014). [DOI] [PubMed] [Google Scholar]
  37. Miyake T., Josberger E. E., Keene S., Deng Y. & Rolandi M. An enzyme logic bioprotonic transducer. APL Mater. 3, 014906 (2015). [Google Scholar]
  38. Ordinario D. D. et al. Bulk protonic conductivity in a cephalopod structural protein. Nat. Chem. 6, 596–602 (2014). [DOI] [PubMed] [Google Scholar]
  39. Rolandi M. Bioelectronics: a positive future for squid proteins. Nat. Chem. 6, 563–564 (2014). [DOI] [PubMed] [Google Scholar]
  40. Ordinario D. D., Phan L., Jocson J.-M., Nguyen T. & Gorodetsky A. A. Protonic transistors from thin reflectin films. APL Mater. 3, 014907 (2015). [Google Scholar]
  41. Glasser L. Proton conduction and injection in solids. Chem. Rev. 75, 21–65 (1975). [Google Scholar]
  42. Kowal J. Ł. et al. Functional surface engineering by nucleotide-modulated potassium channel insertion into polymer membranes attached to solid supports. Biomaterials 35, 7286–7294 (2014). [DOI] [PubMed] [Google Scholar]
  43. Aghdaei S., Sandison M. E., Zagnoni M., Green N. G. & Morgan H. Formation of artificial lipid bilayers using droplet dielectrophoresis. Lab Chip 8, 1617–1620 (2008). [DOI] [PubMed] [Google Scholar]
  44. Hemmatian Z. et al. Taking electrons out of bioelectronics: bioprotonic memories, transistors, and enzyme logic. J. Mater. Chem. C 3, 6407–6412 (2015). [Google Scholar]
  45. Stroumpoulis D., Parra A. & Tirrell M. A kinetic study of vesicle fusion on silicon dioxide surfaces by ellipsometry. Aiche J. 52, 2931–2937 (2006). [Google Scholar]
  46. Attwood S. J., Choi Y. & Leonenko Z. Preparation of DOPC and DPPC supported planar lipid bilayers for atomic force microscopy and atomic force spectroscopy. Int. J. Mol. Sci. 14, 3514–3539 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Huang S.-C. J. et al. Formation, stability, and mobility of one-dimensional lipid bilayers on polysilicon nanowires. Nano Lett. 7, 3355–3359 (2007). [DOI] [PubMed] [Google Scholar]
  48. Leonenko Z., Finot E., Ma H., Dahms T. & Cramb D. Investigation of temperature-induced phase transitions in DOPC and DPPC phospholipid bilayers using temperature-controlled scanning force microscopy. Biophys. J. 86, 3783–3793 (2004). [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Boxer S. G. Molecular transport and organization in supported lipid membranes. Curr. Opin. Chem. Biol. 4, 704–709 (2000). [DOI] [PubMed] [Google Scholar]
  50. Richter R. P. & Brisson A. R. Following the formation of supported lipid bilayers on mica: a study combining AFM, QCM-D, and ellipsometry. Biophys. J. 88, 3422–3433 (2005). [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Tamm L. K. & McConnell H. M. Supported phospholipid bilayers. Biophys. J. 47, 105 (1985). [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Johnson S. et al. Structure of an adsorbed dimyristoylphosphatidylcholine bilayer measured with specular reflection of neutrons. Biophys. J. 59, 289 (1991). [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Woolley G. A. & Wallace B. Model ion channels: gramicidin and alamethicin. J. Membr. Biol. 129, 109–136 (1992). [DOI] [PubMed] [Google Scholar]
  54. Boheim G., Hanke W. & Jung G. Alamethicin pore formation: voltage-dependent flip-flop of α-helix dipoles. Biophys. Struct. Mech. 9, 181–191 (1983). [Google Scholar]
  55. Cafiso D. Alamethicin: a peptide model for voltage gating and protein-membrane interactions. Annu. Rev. Biophys. Biomol. Struct. 23, 141–165 (1994). [DOI] [PubMed] [Google Scholar]
  56. Mottamal M. & Lazaridis T. Voltage-dependent energetics of alamethicin monomers in the membrane. Biophys. Chem. 122, 50–57 (2006). [DOI] [PubMed] [Google Scholar]
  57. Bechinger B. Structure and functions of channel-forming peptides: magainins, cecropins, melittin and alamethicin. J. Membr. Biol. 156, 197–211 (1997). [DOI] [PubMed] [Google Scholar]
  58. Mathew M. K., Nagaraj R. & Balaram P. Membrane channel-forming polypeptides. Aqueous phase aggregation and membrane-modifying activity of synthetic fluorescent alamethicin fragments. J. Biol. Chem. 257, 2170–2176 (1982). [PubMed] [Google Scholar]
  59. Dencher N. A. H. J., Büldt G., Höltje H. D. & Höltje M. in Membrane Proteins: Structures Interactions and Models, 69 (eds Pullman, A. et al.) (Kluwer Academic Publisher, 1992).
  60. Hodgkin A. L. & Katz B. The effect of sodium ions on the electrical activity of the giant axon of the squid. J. Physiol. 108, 37–77 (1949). [DOI] [PMC free article] [PubMed] [Google Scholar]
  61. Kulikovsky A. Analytical model of the anode side of DMFC: the effect of non-Tafel kinetics on cell performance. Electrochem. Commun. 5, 530–538 (2003). [Google Scholar]
  62. Potvin P. G. Modelling complex solution equilibria. I. Fast, worry-free least-squares refinement of equilibrium constants. Can. J. Chem. 68, 2198–2207 (1990). [Google Scholar]
  63. Leonenko Z., Carnini A. & Cramb D. Supported planar bilayer formation by vesicle fusion: the interaction of phospholipid vesicles with surfaces and the effect of gramicidin on bilayer properties using atomic force microscopy. Biochim. Biophys. Acta (BBA)-Biomembr. 1509, 131–147 (2000). [DOI] [PubMed] [Google Scholar]
  64. Diaz A. J., Albertorio F., Daniel S. & Cremer P. S. Double cushions preserve transmembrane protein mobility in supported bilayer systems. Langmuir 24, 6820–6826 (2008). [DOI] [PMC free article] [PubMed] [Google Scholar]
  65. Wong J. Y. et al. Polymer-cushioned bilayers. I. A structural study of various preparation methods using neutron reflectometry. Biophys. J. 77, 1445–1457 (1999). [DOI] [PMC free article] [PubMed] [Google Scholar]
  66. Castellana E. T. & Cremer P. S. Solid supported lipid bilayers: from biophysical studies to sensor design. Surface Sci. Rep. 61, 429–444 (2006). [DOI] [PMC free article] [PubMed] [Google Scholar]
  67. Jin J. et al. A biomimetic composite from solution self-assembly of chitin nanofibers in a silk fibroin matrix. Adv. Mater. 25, 4482–4487 (2013). [DOI] [PubMed] [Google Scholar]
  68. Munro J. C. & Frank C. W. In situ formation and characterization of poly (ethylene glycol)-supported lipid bilayers on gold surfaces. Langmuir 20, 10567–10575 (2004). [DOI] [PubMed] [Google Scholar]
  69. Soto-Rodríguez J., Hemmatian Z., Josberger E. E., Rolandi M. & Baneyx F. A palladium-binding deltarhodopsin for light-activated conversion of protonic to electronic currents. Adv. Mater. 28, 6581–6585 (2016). [DOI] [PubMed] [Google Scholar]
  70. Weber M. J. et al. Atomic layer deposition of high-purity palladium films from Pd (hfac)2 and H2 and O2 plasmas. J. Phys. Chem. C 118, 8702–8711 (2014). [Google Scholar]

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Supplementary Information

Supplementary Figures 1-10, Supplementary Table 1, Supplementary Notes 1-2, Supplementary References

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Data Availability Statement

The data that support the findings of this study are available from the corresponding author on request.

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