In nature, electrical signalling occurs with ions and protons, rather than electrons. Artificial devices that can control and monitor ionic and protonic currents are thus an ideal means for interfacing with biological systems. Here we report the first demonstration of a biopolymer protonic field-effect transistor with proton-transparent PdHx contacts. In maleic-chitosan nanofibres, the flow of protonic current is turned on or off by an electrostatic potential applied to a gate electrode. The protons move along the hydrated maleic–chitosan hydrogen-bond network with a mobility of ~4.9×10−3 cm2 V−1 s−1. This study introduces a new class of biocompatible solid-state devices, which can control and monitor the flow of protonic current. This represents a step towards bionanoprotonics.
Proton (H+) transport is important in many natural phenomena1. Preeminent examples include ATP oxidative phosphorylation in mithochondria2, the HCVN1 voltage-gated proton channel3, light-activated proton pumping in bacteriorhodopsin4, and the proton-conducting single water file in the antibiotic gramicidin5. In living systems, electrical signals are communicated and processed by modulating ionic6 and protonic currents7. In contrast, the development of computing has mainly focused on devices that control electronic currents such as vacuum tubes, solid-state field-effect transistors (FET), and nanoscale molecular structures8,9,10,11. Few examples of protonic-based devices exist, and include an ice FET working with AC current12, and a water bipolar junction transistor13.
At the nanoscale, ionic (and protonic) conductivity has attracted increasing interest with the advent of resistive ionic memories14, memristors15,16, synaptic transistors17, and nanofluidics18,19,20. In hybrid bionanodevices, biological multifunctionality has been added to carbon nanotubes21 or silicon nanowires22 with transmembrane proton conductive proteins. Bionanoelectronic devices23 that can control the current of ions and protons—a more appropriate language than electrons in nature24—are uniquely positioned. In this regard, nanofluidics devices are particularly attractive. However, these require microscopic liquid reservoirs, and current control at physiological concentration is limited to difficult-to-fabricate nanometer channels20.
Recently, conducting polymer ion bipolar junction transistor devices have been demonstrated25,26. These exploit ion selective membranes as contacts to extract and deliver ions (Na+, K,+ Ca2+) and neurotransmitters in solution. To further this exciting route, biocompatible field-effect devices with H+-selective solid-state contacts would allow exclusive interfacing with biological proton-conducting channels.
Here we demonstrate a new class of solid-state H+-FET devices that afford electrostatic control of the protonic current (Fig. 1). In our maleic–chitosan H+-FET, the flow of protonic current—measured with proton transparent palladium hydride (PdHx) (ref. 27) contacts—is turned on or off by an electrostatic potential applied to the gate electrode. Protons dissociated from the maleic acid groups move along the hydrated molecule hydrogen-bond network following the Grotthus mechanism2 (mobility, μH+≈4.9×10−3 cm2 V−1 s−1).
These H+-FET devices comprise a maleic–chitosan nanofibre protonic channel bridging PdHx contacts (source and drain) on the top of SiO2 (gate dielectric). Maleic–chitosan (poly (β-(1,4)-N-Maleoyl-D-glucosamine)) (Fig. 1b,c) is a polysaccharide chitin derivative of particular interest for the prospect of bionanoprotonics. Most chitin derivatives are biodegradable, nontoxic, and physiologically inert28. Palladium is chosen as the contact material for its ability to form proton-conducting PdHx on exposure to hydrogen27. PdHx affords proton exchange between the contacts and the maleic–chitosan channel without electrolysis. The SiO2 (100-nm) gate dielectric insulates the maleic–chitosan and the contacts from the Si electrostatic back gate.
We first probe the maleic–chitosan conductivity in thicker films sandwiched by PdHx contacts, to verify that this H+-FET architecture indeed measures protonic current. When a bias is applied between the contacts, the current increases with relative humidity (and polysaccharide hydration level) (Fig. 2a). Hysteresis also increases with humidity, potentially due to increased charge accumulation/depletion at the contacts. Measurements using hydrogen-depleted Pd or Au contacts—both proton blocking—record a considerably smaller current (Supplementary Fig. S1). It is important to note that both Au and Pd are significantly better electronic conductors than PdHx. We thus confirm that PdHx contacts effectively measure the protonic current in this material. Increase in protonic conductivity with hydration level is common in other biological macromolecules such as collagen29, cellulose30 and keratin31. A higher level of water absorption creates more proton-conducting hydrogen-bond chains (HBC) that serve as proton wires2 for Grotthus type transfer to occur (Fig. 2b). In maleic–chitosan, we propose that the protons responsible for the current (as hydronium ions in the HBC) originate from the polysaccharide maleic acid groups, some of which are deprotonated (Fig. 2c). The protonic conductivity in chitin (Supplementary Fig. S1), which has no maleic groups, is significantly lower than in the maleic–chitosan derivative.
In a H+-FET type device, the source-drain protonic current, Ids, recorded as a function of drain-source bias, Vds, is modulated by changing the potential of the back gate electrode, Vgs (Fig. 3). As expected for a semiconducting FET with predominantly positive charge carriers, a negative Vgs results in a higher source-drain current for the same Vds, whereas a positive Vgs almost turns Ids off. In most materials, protons have to overcome activation energy in the range of 0.1 to 1 eV for current to flow29,31,32. This activation energy may be associated with producing a H+ OH+− pair or with transport of H+ along the HBC. This behaviour is often referred to as protonic semi-conductivity. However, in highly conducting proton wires, such as in gramicidin, the activation energy associated with transport is very small5. In the maleic–chitosan pro-FET, the linear Ids dependence on Vds for low Vds (and Vgs=0 V) indicates that protons do not need to overcome any appreciable energy barrier for Ids to flow (Fig. 3a). This suggests that highly efficient proton wires are formed in the material.
A simple model can be used to describe the H+-FET. Ids is proportional to Vds through the conductivity of the channel σ=e nH+μH+ (e=proton charge, nH+=number of free protons per unit volume, and μH+=proton mobility). For Vgs=0 V, we assume that all the protons available for conduction originate from the maleic acid groups in the polysaccharide (pKa=3.2) (Supplementary Methods). From this assumption, we estimate . Using , the proton mobility can be calculated from the slope of Ids at Vgs=0 V (Supplementary Methods). The calculated value μH+≈4.9×10−3 cm2 V−1 s−1 is consistent with the mobility of H+ in diluted acidic solutions.33 For a negative Vgs, a positive charge is induced onto the channel because the maleic–chitosan and the gate create a capacitor with the SiO2 as the dielectric. The charge per unit area induced onto the channel is directly proportional to Vgs through the gate capacitance per unit area, Cg. To create that charge, excess protons are injected into the maleic–chitosan via the proton transparent contacts. The contribution of these excess protons to the conductivity can then be estimated as an increase in nH+ (higher Ids) by a factor that is equal to −(VgsCg/et) (t=device thickness) (Fig. 3c). At the same time, a decrease in nH+ (smaller Ids) is expected for a positive Vgs (Fig. 3d).
The data in Figure 3b corroborates this description. This plot depicts nH+ estimated from the Ids–Vds experimental data (Fig. 3a) as a function of Vgs. These nH+ values are compared with , which is the contribution to nH+ expected from the excess charge induced onto the channel by Vgs. Given that this model involves several assumptions and approximate estimates of the maleic–chitosan channel volume, this data is in good, even fortuitous, agreement. Measurements performed on a thicker device show a significantly smaller Ids modulation from Vgs, as predicted by our description (Supplementary Fig. S2). It is important to note that this kind of electrostatic gating of a protonic (ionic) solution, unlike in nanofluidic devices, can occur for devices thicker than the Debye length (k−1). From n0H+, the maleic–chitosan ionic strength is ~1 mM, which results in k−1~ 10 nm. Even for the Figure 1a H+-FET, that is significantly thicker than the Debye length (t ~8 k−1), Vgs modulates Ids by changing the proton density of the channel. This is a unique property of having proton transparent contacts and reduces fabrication constraints.
We produce two-dimensional plots of the proton charge density nH+ at different Vgs (Fig. 4a–c) by solving drift diffusion and Poisson equations in the context of a semiconductor device simulator, where the hole mobility is replaced by proton mobility. From these plots, it is clear that, for a negative Vgs, the proton density is drastically increased at the polysaccharide–dielectric interface to form a highly conducting region. The thickness of this region (~10 nm) is in agreement with the estimated Debye length (Fig. 4a). Plots of current density show that the majority of Ids flows in the same area (Supplementary Fig. S3). At the same time, a positive Vgs considerably depletes the channel from protons reducing nH+ and consequently Ids (Fig. 4c). An analogous Vgs effect on the charge density and current density distribution has been observed in thin-film organic transistors34. These transistors have geometry and electrostatics similar to our H+-FET. From these plots, it is also clear that in significantly thinner devices the expected charge modulation will be higher. Such devices may be fabricated with individual maleic–chitosan nanofibres and should offer a greatly improved on-off ratio.
We also estimate Ids as a function of Vds for different Vgs (Fig. 4d). The simulations are in good agreement with the experimental data at low Vds, while the nonlinear dependence of Ids on Vds for higher Vds is not reproduced. We have assumed the contacts to be completely proton transparent. A small contact barrier (neglected in our model) may cause charge accumulation or depletion at the maleic–chitosan PdHx interface. This charge, in turn, may reduce the strength of the electric field at the contacts, thus effectively limiting the current. When the bias is reversed, the charge should be released and cause hysteresis in the Ids–Vds dependence. Hysteresis is indeed observed when Vds is ramped from 0 to 1 V and then back in the same measurement (Supplementary Fig. S4). The causes for hysteresis and the non-linear behaviour of Ids for high Vds are still not fully understood and merit further investigation.
These experiments demonstrate, for the first time, electrostatic gate modulation of source-drain current in a solid-state biopolymer based thin-film protonic (H+) field-effect transistor. PdHx contacts allow for indiscriminate measurement of the protonic current. A voltage applied to the electrostatic gate controls the maleic–chitosan channel proton density and the channel conductivity. Current modulation occurs even for devices thicker than the Debye length. This allows for current control in highly concentrated electrolytes at any length scale. In the future, several nanostructured biological and organic materials can be measured in these devices. The demonstrated ability to control protonic currents in nanostructured biocompatible solid-state devices (bionanoprotonics) may open exciting opportunities for interfacing with living systems. These opportunities include biomedical applications where protonic currents are important such as the in-vivo study and stimulation of proton selective ion-channels. To this end, H+-FETs architectures with optimized performance in physiological solutions will be required.
Maleic–chitosan nanofibres were prepared following previously published procedures35. The maleic–chitosan hydration level was determined with a thermogravimetric analyser (TA Instruments, model 2050).
Cu contacts (1 cm2×1 cm2) coated with 50 nm e-beam evaporated (Balzers PLS 500) Pd (5 nm Cr adhesion layer) were used to sandwich 300 μm films for relative humidity (RH)-dependent measurements. Maleic–chitosan films were prepared from a 5 ml of 3.0 wt% maleic–chitosan aqueous solution after drying for 5 h. Chitin films were prepared from a 5 ml of 0.5 wt% chitin/HFIP solution after drying for 1 h.
Devices were fabricated on p-type Si (Addison Engineering, B, ρ=0.001 ohm cm−1) with thermally grown silicon oxide (100 nm). Photolithography and lift-off was used to define the contacts. Pd metal (50 nm) with a 5-nm Cr adhesion layer was deposited via e-beam evaporation (Balzers PLS 500). After dialysis and freeze drying, maleic–chitosan was prepared in a DI water solution. This procedure eliminates any salt and thus potential salt effects on the conductivity. To make a polysaccharide-based device, 2 μl polysaccharide solutions of concentration ranging from 0.02 mg ml−1 to 0.2 mg ml−1 were carefully drop cast on top of the patterned silicon wafer and blown under N2 flow. Devices were mounted on a chip, and wire bonded.
Measurements were performed with a semiconductor parameter analyser (Agilent 4155C). An environmental chamber was used (5% H2 or 100% N2) with controlled RH monitored with a traceable hygrometer (Fisher Scientific, ±0.1% error). During pro-FET measurements at 75% RH and 5% H2, devices with no connections were monitored to have at most noise current. This was done to ensure that the measured device current was from the maleic–chitosan channel and not from water condensed on the top of the SiO2.
Atomic force microscopy
Atomic force microscopy (AFM) was employed to measure device dimensions. Tapping mode AFM was performed on a Veeco Multimode V with a Nanoscope IV controller using Veeco Probes Sb-doped Si cantilevers (ρ=0.01–0.025 Ω-cm, k=40 N/m, ν~300 kHz).
Electrical properties of the structure were obtained by solving Poisson's equation together with the electron and hole (in this case, proton) continuity equations throughout a CAD tool (ATLAS, Silvaco). We defined a silicon (Si) based metal–oxide–semiconductor transistor with the same dimensions as the experiment. We used a 8.6-μm long, 3.5-μm wide, 82-nm channel with a rectangular cross-section sitting on the top of 100-nm SiO2 gate dielectric (ɛsio2=3.9). Pd (Φ=5.1 eV) source and drain were modelled as ohmic protonic contacts (no barrier) to the material. We replaced the properties of silicon with those of the channel material. Maleic–chitosan ɛmc=10, and n0=8.9×1017 cm−3 from our estimate. Mobility was varied to fit the data. In addition, we assigned the mobility of electrons to a value that is orders of magnitude smaller than that of protons. This minimizes the role of electrons in the electrical response of the device. To calculate the gate capacitance, we first obtained the interface charge at different gate biases; whereas the two other contacts were kept at zero bias. The capacitance was then estimated by the simple equation of ΔQ/ΔV, where ΔQ and ΔV represent the variations of the interface charge and gate, respectively.
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This work was supported by the University of Washington (Royalty Research Fund and New Faculty Seed Funding), and a 3 M Untenured Faculty Award (M.R.). A.K. thanks a fellowship from the grant T32CA138312 from the National Cancer Institute and the Center of Nanotechnology at the University of Washington. Part of the work was performed at the University of Washington Center for Nanotechnology, which is part of the NSF-Funded NNIN. The authors would like to thank Michael Brasino for graphic consulting.
The authors declare no competing financial interests.
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Zhong, C., Deng, Y., Roudsari, A. et al. A polysaccharide bioprotonic field-effect transistor. Nat Commun 2, 476 (2011). https://doi.org/10.1038/ncomms1489
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