Translating ionic currents into measureable electronic signals is essential for the integration of bioelectronic devices with biological systems. We demonstrate the use of a Pd/PdHx electrode as a bioprotonic transducer that connects H+ currents in solution into an electronic signal. This transducer exploits the reversible formation of PdHx in solution according to PdH↔Pd + H+ + e, and the dependence of this formation on solution pH and applied potential. We integrate the protonic transducer with glucose dehydrogenase as an enzymatic and gate for glucose and NAD+. PdHx formation and associated electronic current monitors the output drop in pH, thus transducing a biological function into a measurable electronic output.

Ionic species dominate signaling in natural systems, so the conversion of biochemical ionic signals into electronic signals is an essential part of bioelectronics.1–6 Examples include ionic transistors that deliver neurotransmitters,7 electrochemical transistors,8 membrane protein gated nanowires,1 and squid-protein based protonic transistors.9 We have previously developed bioprotonic devices that control the flow of H+ currents such as complementary transistors, rectifying junctions, and memristive memories.10–12 These protonic devices have a proton conducting polymer-based channel and use PdHx protodes to convert a H+ current in the polymer channel into an electronic current in the contacts according to PdH↔Pd + H+ + e.10–12 Here, we extend the functionality of these devices to the conversion of a H+ current from solution, which is a common media for biological systems. To this end, we exploit the electrochemical properties of the formation of PdHx and its dependence on the voltage applied on the contact (Fig. 1).13–15 As a proof of concept, we measure with these devices the pH of a solution modulated by enzymatic reactions, such as the reaction catalyzed by glucose dehydrogenase (GDH) to produce gluconic acid from oxidized nicotinamide adenine dinucleotide (NAD+) and glucose.

FIG. 1.

Schematic of the proton-electron transducer. A PdHx electrode is controlled by enzyme logic proton modulation. GDH acts as and logic with NAD+ and glucose as inputs and gluconic acid as an output, lowering the electrolyte solution pH. The pH change acts as an “on-off” switch that controls the PdH formation.

FIG. 1.

Schematic of the proton-electron transducer. A PdHx electrode is controlled by enzyme logic proton modulation. GDH acts as and logic with NAD+ and glucose as inputs and gluconic acid as an output, lowering the electrolyte solution pH. The pH change acts as an “on-off” switch that controls the PdH formation.

Close modal

To calibrate the protonic transducer, we first investigate the dependence of the formation and depletion of PdHx on the potential applied to the metal (V) (Fig. 2). For this measurement, we prepare Pd films (100 nm thickness) on glass slides by electron beam deposition, and measure the electrochemical current at the Pd electrode (I) (0.2 cm2 surface area) in a 0.5 M Na2SO4 unbuffered solution (pH 6.7). We use the standard three-electrode system with Ag/AgCl as the reference electrode and platinum as the counter electrode (0.5 cm2 surface area) (Fig. 2(a)). First, we form PdHx with a negative V applied to the PdHx contact, which we refer to as VF. A VF applied to the Pd induces a reduction current of e to flow into the Pd, which we refer to as formation current (IF). These e reduce the H+ from solution, which are absorbed as H into the Pd to form PdHx. Second, once the Pd is fully converted to PdHx with x = 0.6, we switch V to a positive depletion voltage (VD) at which PdHx is expected to return to Pd. For this specific measurement, VD = 0 V. The difference in chemical potential of H+, or protochemical potential (μ) between the PdHxPdH) and the solution (μpH), determines whether the H+ will transfer back to the solution. This difference is defined as16 

μ pH μ PdH = e V + k B T ln ( a H + / ( pH 2 ) 1 / 2 ) ,
(1)

where aH+ = activity of H+ in solution with pH = − logaH+, pH2 = hydrogen partial pressure in the Pd.

FIG. 2.

(a) Voltage-controlled Pd/PdHx behavior. Electrons and protons travel into the Pd/PdHx during formation and out of it during depletion. (b) Temporal characteristics of the PdHx formation (VF = − 1.0 V) and depletion (VD = 0 V) in pH 6.7 Na2SO4 solution. (c)–(e) Optical images of 1 cm × 1 cm samples at different stages of the formation of PdHx. (c) is an image taken at t = 0 with VF = 0 V. The substrate appears metallic indicating the presence of Pd. (d) When a negative voltage VF = − 1.0 V is applied, the Pd absorbs protons from the solution, forming PdHx, which appears white. (e) When the voltage is returned to VD = 0 V, the PdHx depletes releasing the absorbed protons and returning to metallic Pd.

FIG. 2.

(a) Voltage-controlled Pd/PdHx behavior. Electrons and protons travel into the Pd/PdHx during formation and out of it during depletion. (b) Temporal characteristics of the PdHx formation (VF = − 1.0 V) and depletion (VD = 0 V) in pH 6.7 Na2SO4 solution. (c)–(e) Optical images of 1 cm × 1 cm samples at different stages of the formation of PdHx. (c) is an image taken at t = 0 with VF = 0 V. The substrate appears metallic indicating the presence of Pd. (d) When a negative voltage VF = − 1.0 V is applied, the Pd absorbs protons from the solution, forming PdHx, which appears white. (e) When the voltage is returned to VD = 0 V, the PdHx depletes releasing the absorbed protons and returning to metallic Pd.

Close modal

From (1), the protochemical potential of H+ in the PdHxPdH) for x = 0.6 and V = 0 V is the same as the protochemical potential for a solution at pH = 0 (μpH=0), and higher than the protochemical potential for a solution at pH = 6.7 (μpH=6.7).14 While hydrogen transport through the PdHx electrode causes overpotential in protochemical potential, this overpotential is negligibly small.17 As a result, for VD = 0 V, H+ flow from PdHx into the solution and induce an oxidation current of e to flow from the electrode. This current is a product of the oxidation of H from the PdHx into H+ (Ref. 18) (Fig. 2(b)) and we refer to it as depletion current (Id). Since IF is affected by dielectric charging of the solution, we use the magnitude of Id to measure the extent of PdHx formation for a given VF assuming that for a specific VD all of the H+ loaded onto the Pd for VF returns to the solution. During the formation of PdHx, we also observe a color change in the electrode from metallic (Pd, Fig. 2(c)) to white (PdHx, Fig. 2(d)), while after depletion the electrode returns to its original metallic color (Pd, Fig. 2(e)).12 The formation of PdHx does not occur for every value of VF. At pH 6.7, Id at Vd = 0 V drops by an order of magnitude between VF = − 1.0 V and VF = − 0.9 V (see supplementary material Fig. S119) indicating that formation of PdHx did not occur for VF > − 1.0 V. We assume that the threshold voltage for formation (at pH 6.7) is VF = − 1.0 V. Similar to formation, the depletion of PdHx is also governed by differences between μPdH and μpH and exhibits a threshold behavior. For fully loaded PdHx, Id is close to 0 for VD < − 0.3 V (in a pH 6.7 solution, see supplementary material Fig. S219). During depletion of the contact, we calculate the total charge transferred from the PdHx to the solution by integrating Id as function of time.12 As we observed for the PdHx-Nafion interface in memory devices,12 the total charge transferred is constant (45 mC) as a function of VD (for Vd = − 0.3, −0.2, −0.1, and 0 V). This observation suggests that as long as formation of PdHx occurs and is allowed to go to completion, the PdHx is loaded with a constant amount of H each time.

Second, we investigate the dependence of the PdHx formation and depletion on the solution pH (Fig. 3(a)). We determine the threshold values for VF and VD for a range of pH values (0.0, 2.0, 3.0, 4.5, 6.7, and 11.0) (Fig. 3(b)). As solution pH increases, the threshold values for VF and VD decrease, as expected due to the influence of pH on the solution protochemical potential (1).14,20 It is interesting to note that there is a range of pH and V (area in white) in which the solution and the PdHx are in equilibrium and no exchange of H+ across the interface occurs. To further elucidate the depletion dynamics, we measure ID as a function of solution pH by pre-loading the Pd with H2 gas rather than loading electrochemically from solution. When exposed to H2 gas, Pd absorbs H2 to form PdHx with x = 0.6 for 0.1 atm < H2 pressure < 1 atm.11,21,22 As expected, at VD = 0 V, ID for a solution with a low proton concentration (e.g., pH 11) is larger than ID from a solution with a high proton concentration (e.g., pH 2), and no depletion occurs at pH 0 because μpH=0PdH. This is because an increase in proton concentration at the protode surface results in a smaller protochemical potential difference between the loaded PdHx contact and the solution. The opposite is true for the formation of PdHx. As the proton concentration at the protode surface is increased (lower pH), the favorability for the reduction is also increased. Importantly, we find that the threshold potentials for formation (VF) and depletion (V D) vary as the solution pH varies. When the reaction is proceeding very slowly, the calculated formation threshold voltage depends on the reaction rate in addition to activation energy, which probably causes the non-linear results for threshold VF.

FIG. 3.

(a) Dependence of formation and depletion on pH and voltage. Formation was measured by absorbing protons for 60 s at given pH and applied VF, then testing for a significant ID. If the depletion current increased by more than an order of magnitude, formation occurred (e.g., Fig. S1). (b) Current from PdHx protodes held at VD = 0 V during depletion. At high pH, depletion occurs rapidly and at low pH, depletion occurs slowly. Depletion was measured by first absorbing H2 in gas phase for 40 min (pH2 in air =0.2 atm), then immediately reducing the voltage to the value of interest and checking for positive depletion current (e.g., Fig. S2).

FIG. 3.

(a) Dependence of formation and depletion on pH and voltage. Formation was measured by absorbing protons for 60 s at given pH and applied VF, then testing for a significant ID. If the depletion current increased by more than an order of magnitude, formation occurred (e.g., Fig. S1). (b) Current from PdHx protodes held at VD = 0 V during depletion. At high pH, depletion occurs rapidly and at low pH, depletion occurs slowly. Depletion was measured by first absorbing H2 in gas phase for 40 min (pH2 in air =0.2 atm), then immediately reducing the voltage to the value of interest and checking for positive depletion current (e.g., Fig. S2).

Close modal

As proof of concept, we integrate this protonic transducer with enzyme logic (Fig. 4). We use GDH (EC 1.1.1.47, 250 U mg−1) donated from TOYOBO.23,24 Two input signals activate GDH; input A is NAD+ and input B is glucose (Fig. 4(a)). GDH requires the presence of both NAD+ and glucose to function, effectively serving as an enzymatic and logic.25 The absence of glucose or NAD+ is defined as a logic “0,” while their presence is defined as logic “1.” We use the formation of PdHx at VD = − 0.85 V and the resulting ID measured at VD = 0 V as the logic output. ID>0.1 mA corresponds to a logic “1,” while ID<0.05 mA corresponds to a logic “0.” In the absence of either substrate (logic input “0,0,” “0,1,” or “1,0”), GDH is not active and the solution pH remains the same. At pH 6, a VF = − 0.85 V does not form PdHx and the resulting ID<0.05 mA. The presence of both NAD+ and glucose (logic “1,1”) activates the biocatalytic reaction of GDH (Fig. 4(b)), which produces gluconic acid and lowers the solution pH from 6.0 to 4.0. At pH 4.0, VF = − 0.85 V results in the formation of PdHx. The resulting ID = 0.164 mA at VD = 0 V (Fig. 4(c)) corresponds to a logic output 1. These results demonstrate that the Pd/PdHx system can transduce proton signals from biochemical inputs into readable electronic currents.

FIG. 4.

(a) Truth table for GDH enzyme logic. (b) pH change of the solution over time for varying inputs. Only when both inputs are present (1,1), the pH changes from ∼6 to ∼4. (c) Digital readout of ID with the PdHx transducer controlled by enzyme logic. For each possible input (A,B), the device is first loaded at VF = − 0.85 V, then depleted at VD = 0 V. Only when both inputs are 1, ID>0.1 mA, which is defined as logic output 1.

FIG. 4.

(a) Truth table for GDH enzyme logic. (b) pH change of the solution over time for varying inputs. Only when both inputs are present (1,1), the pH changes from ∼6 to ∼4. (c) Digital readout of ID with the PdHx transducer controlled by enzyme logic. For each possible input (A,B), the device is first loaded at VF = − 0.85 V, then depleted at VD = 0 V. Only when both inputs are 1, ID>0.1 mA, which is defined as logic output 1.

Close modal

In conclusion, we have characterized the behavior of a Pd/PdHx bioprotonic transducer in response to electrical and biochemical stimuli by measuring the redox currents for proton absorption (IF) and depletion (ID). These currents are controlled with both voltages applied on the transducer (V) and the solution pH. At the depletion voltage (VD = 0 V), oxidation currents increase as the solution pH increases. In contrast, formation currents increase with decreasing pH. By measuring the current at V in different pH solutions, we have produced a process map for PdHx formation and depletion. This map reflects the differences in protochemical potential between the solution and the PdHx as function of pH and H2 concentration in the PdHx. The ability to prevent either process from occurring may be of interest in proton accumulation based devices.12 Furthermore, we demonstrate the integration of this transducer with an enzyme logic system, which is activated by two biochemical input signals (glucose and NAD+). Our bioprotonic transducer is useful with many other dehydrogenase enzymes which produce H+ as the output signal, including alcohol dehydrogenase and lactate dehydrogenase, and it can serve as the proton source/sink in enzymatic type bioelectronic applications.

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. < DE-SC001044> (PdHx formation and enzyme logic), and the National Science Foundation CAREER Award (DMR 1150630) (PdHx depletion). T.M. acknowledges a fellowship from the Japan Society for the Promotion of Science. Part of this work was conducted at the Washington Nanofabrication Facility/Molecular Analysis Facility, a member of the NSF National Nanotechnology Infrastructure Network.

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Supplementary Material