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.
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.
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.
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).
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+.
From conservation of mass (equation 1)59
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:
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):
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):
where DIL is the thickness of the isolation layer. The resulting d[H+]IL is (ref. 59):
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.
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.
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.
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.
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.
MATLAB code for simulations is available at https://dx.doi.org/10.6084/m9.figshare.3509846.v1.
The data that support the findings of this study are available from the corresponding author on request.
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).
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.
Supplementary Figures 1-10, Supplementary Table 1, Supplementary Notes 1-2, Supplementary References