Proton-enabled activation of peptide materials for biological bimodal memory

The process of memory and learning in biological systems is multimodal, as several kinds of input signals cooperatively determine the weight of information transfer and storage. This study describes a peptide-based platform of materials and devices that can control the coupled conduction of protons and electrons and thus create distinct regions of synapse-like performance depending on the proton activity. We utilized tyrosine-rich peptide-based films and generalized our principles by demonstrating both memristor and synaptic devices. Interestingly, even memristive behavior can be controlled by both voltage and humidity inputs, learning and forgetting process in the device can be initiated and terminated by protons alone in peptide films. We believe that this work can help to understand the mechanism of biological memory and lay a foundation to realize a brain-like device based on ions and electrons.


Supplementary Note 1. Proton conductivity calculation
The proton conductivity of the peptide film was calculated by fitting the data of the Nyquist plot acquired from the impedance spectroscopy measurement. The Nyquist plot showed a semicircle in the high-frequency region and an inclined tail in the low-frequency region which agrees with the typical behavior of proton conductors ( Fig. 1e and Supplementary   Fig. 2, 3) 1-3 . The semicircle represents the bulk impedance and the tail represents the accumulation of protons at the film/electrode interface. Estimated values of bulk resistance were derived from the diameters of the semicircles by fitting the plot to an RC equivalent circuit that has been to model proton exchange membranes ( Supplementary Fig. 1) [4][5][6] . Thus, the proton conductivity of the films can be calculated from the following equation.
where σ, l, Rb and A are conductivity, film thickness, bulk resistance and area of the top electrode, respectively.

Supplementary Note 2. Resistive switching characteristics study
The role of tyrosine in the resistive switching of Y7C is depicted in Fig. 2a. Tyrosine in the spin-coated Y7C solution in trifluoroacetic acid remains protonated because the pKa of the phenol group in tyrosine is approximately 10.1 7 . Under a voltage bias, tyrosine actively renders an electron to reduce the Ag ion from the top electrode and a proton, generating a tyrosyl radical.
The reduced Ag atom is unstable due to the negative reduction potential of -1.8 V and is easily oxidized back to an Ag ion 8,9 . Thus, the Ag atom transfers an electron to the tyrosyl radical, forming protonated tyrosine. This reaction continues until the Ag ion contacts the Pt electrode and/or forms an Ag cluster, because the oxidation of Ag becomes exergonic (+0.8 V) when Ag atoms aggregate 9,10 . Through this process, tyrosine in Y7C promotes the Ag redox and migration so that the set voltage of Y7C is much smaller than that of F7C (Fig. 2b).
The Y7C memristor shows not only bipolar resistive switching characteristics but also unipolar resistive switching characteristics so that the polarity of the reset voltage can be either positive or negative ( Supplementary Fig. 10a). As observed in the bipolar case (Fig. 3a, c), the Y7C memristor also exhibits excellent cycle endurance and data stability in the unipolar case ( Supplementary Fig. 10b, c). The set process was conducted with a current compliance level of 10 -4 A to avoid the permanent breakdown of the device. To investigate the conduction mechanism of the Y7C memristor, double-logarithmic fitting of the I-V characteristics was carried out (Supplementary Fig. 10d). The slope of 1 in the HRS and LRS region indicates ohmic conduction behavior, which can be described using the following equation: where JOhm, q, n0, μ, V, and d are the current density due to Ohm's law, intrinsic charge density, charge mobility, applied voltage, and film thickness, respectively. The slope changed from 1 to 2 in the high-voltage region in the HRS. In this region, the charge conduction mechanism can be explained by the Mott-Gurney law, which represents the trap-controlled space charge limited current, known as SCLC, using the following equation 11 : where JMott and ε are the current density due to the Mott-Gurney law and dielectric constant, respectively.
To confirm the origins of the conduction path in the peptide film, we verified the oxidation effect of the top electrode material on resistive switching by introducing other electrodes in place of the Ag electrode. No resistive switching was observed even at a bias voltage of 35 V with the structure of Au/Y7C/Pt ( Supplementary Fig. 5a). This means that the Y7C peptide is not able to oxidize either electrode due to the stable nature of Au and Pt and remains highly insulating even under a high electric field. Thus, Ag redox evidently plays a key role in the resistive switching phenomenon in accordance with the previously reported electrochemical metallization (ECM) [11][12][13][14][15]

Supplementary Note 3. Proton-mediated switching mechanism
How proton is involved in the resistive switching can be explained in following four steps with experimental evidences.

Origin of the resistive switching
Similar to the conventional electrochemical metallization (ECM) memories 16 , oxidation of the metal electrode causes the resistive switching of the Y7C peptide memristor. The involvement of silver redox was confirmed by the finding that replacing the silver electrode with gold completely nullified the memristor performance ( Supplementary Fig. 5). Under the voltage bias, Ag top electrode is oxidized and dissolved into the peptide layer, and finally forms Ag filament connecting top and bottom electrodes.

Role of tyrosine in the resistive switching
Tyrosine is known as a redox-active amino acid that plays a key role in biological processes 17 . We have shown that the tyrosine rich Y7C peptide has catalytic property owing to low oxidation potential in the previous work 18 . In this regard, we assumed that tyrosine plays a key role in redox reaction with Ag atoms resulting resistive switching in the Y7C film. To understand the role of tyrosine in the resistance transition of the peptide film, the FFACAFF peptide which contains phenylalanine (F) instead of tyrosine was introduced. The only difference between two similar amino acids is that tyrosine has phenolic hydroxyl group while phenylalanine does not. The set voltage of the FFACAFF film was increased to 10.7 V compared with 1.8 V for the Y7C film ( Fig. 2b and Supplementary Fig. 6). This indicates that the hydroxyl group in tyrosine is involved in the resistive switching which results from the redox of Ag atoms and the formation of a conduction path.
The redox chemistry of tyrosine is inherently proton-coupled, indicating that electron transfer is accompanied by proton releasing and accepting 19 . Therefore, it can be assumed that the promotion of Ag filament formation with phenolic hydroxyl group is because reduction of Ag ions is dominated by charge transfer from phenolic hydroxyl group in tyrosine to Ag ions resulting in deprotonation of tyrosyl radical.

Proton involvement in the redox reaction
Proton involvement in charge transfer between tyrosine and Ag atoms is also verified by the experiment on kinetic isotope effect of hydrogen. Deuterium, the isotope of hydrogen and known as heavy hydrogen, has approximately twice mass of hydrogen. To investigate hydrogen ion involvement, water vapor is replaced by deuterium oxide vapor and the kinetics of the phenomena is compared. After removal of water vapor in the chamber, deuterium oxide (D2O) vapor with nitrogen carrier gas was injected to adjusting RH value. The set voltages in which the transition from HRS to LRS occurs were measured for each RH condition ranging from 15 % to 90 %. As shown in Supplementary Fig. 9a, the set voltages in D2O condition is higher than that in H2O condition at the same level of 90 % RH. In addition, Supplementary Fig 9b, presents the linear relationship between the set voltages and the RH. The slope of the fitted curve for the D2O case is 1.25 times higher than that for the H2O case. This suggests that the difference in the atomic mass between hydrogen ion and deuterium ion induces the changes in the kinetics of the redox reaction between Ag atoms and tyrosine. Therefore, the resistive switching phenomenon of the Y7C peptide memristor is enhanced by proton-involved Ag redox by tyrosine.
In addition to the kinetic isotope experiment, the comparative study on the effect of air between tyrosine and phenylalanine was carried out in this revision. Supplementary Fig. 6 shows the I-V characteristics for the Y7C peptide and FFACAFF peptide of the set process both in vacuum and ambient conditions. The set voltage of the Y7C peptide memristor exhibited 57 % reduction after the devices were taken out of vacuum, while that of the FFACAFF peptide memristor showed relatively low change of 5 %. This result is accordance with the assumption that Ag redox is promoted by proton-coupled charge transfer of tyrosine ( Supplementary Fig. 7).

Redox-associated proton conduction in the Y7C film
The Y7C peptide shows the high proton conduction which depends exponentially on the external humidity (Fig. 1f). Comparison of the proton conductivity between YYACAYY (Y7C) and FFACAFF verifies that the existence of phenolic hydroxyl group in tyrosine plays a critical role in the proton conduction ( Supplementary Fig. 3). This corresponds to our previous findings that the tyrosine rich Y7C peptide shows proton coupled electron transfer (PCET) even in thick film that plays important role in the high proton conduction from the measurement of Onsanger coefficient 20 . These results suggest that phenolic hydroxyl group acts as a hopping site for proton. Thus, it can be assumed that the electron transfer is coupled with the protonation and deprotonation of tyrosine during proton hopping. The proposed redox-associated conduction is highly probable due to the low oxidation potential of the Y7C peptide of 0.86 V as shown in the cyclic voltammetry ( Supplementary Fig. 4). This electrochemical reactive property of the Y7C peptide leads the acceleration of reduction of Ag ions as a consequence of the oxidation of the Y7C peptide. As a result, we propose the proton-mediated resistive switching mechanism that increasing humidity induces the increased proton conduction in the Y7C peptide resulting the promotion of the reduction of Ag ions.

Supplementary Note 4. Relative humidity control (steady state)
Each humidity condition was controlled by an injection of either N2 gas to reduce the humidity or H2O humidified air to increase the humidity and detected by a humidity sensor kit (Sensirion, SHT31). To verify the minimum exposure time for proton saturation in the peptide film, the current of the peptide film was measured with various relative humidities (RHs) and incubation times. After 1 hour of exposure to 60 % RH, the current of the peptide film increased approximately 10 times higher than the initial current of the film exposed to ambient conditions at 30 % RH over 1 day. After 2 hours of exposure, the current further increased by a factor of 4. The increase in the current saturated after 2 hours of exposure to 60 % RH, as no increase in the current was observed in the case of exposure for 3 hours. For the 80 % RH condition, the increase in the current saturated even after 2 hours of exposure. Thus, the devices were kept in the chamber for 2 hours at each RH condition, which is considered to be the minimum incubation time to allow moisture to diffuse in the peptide film.

Supplementary Note 5. Relative humidity control (dynamic)
A dynamic measurement of the RH value was conducted at a frequency of 2 Hz by a digital humidity sensor (Sensirion, SHTC3). A dynamic RH sensor was placed adjacent to the device to measure the RH variation on the surface of the peptide film ( Supplementary Fig. 11a Fig. 12a, b, 17a). A reverse RH sweep from 95 % to 5 % was applied by injecting N2 gas. The fall time varies from 60 s to 120 s. At an extremely low RH corresponding to the reset humidity, regardless of the fall rate, abrupt resistive switching from the LRS to the HRS was observed ( Supplementary Fig. 12b, 17b). The scan rates are defined as follows: where τrise, τfall, RHset and RHreset are the rise rate, fall rate, set humidity, and reset humidity, respectively. The set humidity is defined as the RH value when the current exceeds 90 % of the current compliance level only when nonvolatile resistive switching is observed.
The reset humidity is defined as the RH value when the current is measured below 0.1 nA.

Supplementary Note 6. Humidity mode of the Y7C memristor
As the humidity increases, the transient current increases and the Ag redox is promoted due to accelerated PCET 4,17,21 . Therefore, applying humidity without any changes in the voltage bias induces abrupt resistive switching. Regardless of the read voltage from 0.3 V to 1.2 V, resistive switching occurred at approximately 90 % RH ( Supplementary Fig. 18). In hydrated Y7C at a high RH, the major parameter affecting the formation of a conducting filament is humidity, not the voltage bias. The reset humidity is a function of the current compliance level during the measurement ( Supplementary Fig. 19). When the current compliance level was set to 10 -7 A, the reset operation occurred at 16 % RH. On the other hand, the reset humidity became 47 % RH when the current compliance level was set to 10 -8 A. This means that the set/reset window changes according to the current compliance level setting during the RH sweep. The stability of the Ag conduction path might decrease while the current compliance decreases 22 . In this regard, the set/reset window of the humidity mode can be adjusted as needed.
Metal oxidation effects on resistive switching were also studied for the humidity mode of Based on the result, the compliance level of the humidity mode is set to be 10 -7 A while that of bias mode ranges from 10 -4 A to 10 -7 A. For example, if the resistance of LRS set by two different mode should be distinguished, the compliance levels of the bias mode and the humidity mode can be set as 10 -4 A and 10-7 A, respectively. Therefore, we can distinguish between the two modes in which resistive switching from HRS to LRS have done by measuring the resistance of LRS. Supplementary Fig. 22 displays bimodal operation of the Y7C peptide memristor. Current compliance level was set to 10 -4 A and 10 -7 A for bias mode and humidity mode, respectively. The conductance level indicated as the state '0' means the pristine state (or HRS) of the Y7C film which is very low (≈10 -11 S). After the set operation in the bias mode, the conductance level is immediately changed to the state '2' corresponding to the electrically set LRS. The state is returned to the initial state by reset process in the bias mode. Following RH sweep induces the increase in the conductance and the set operation at high RH. In this case, the conductance level is increased to the state '1' corresponding to the LRS set by humidity. Two states of '1' and '2' can be distinguished easily by measuring the conductance of the Y7C film.

Supplementary Note 8. Advantages of In-Ga-Zn-O semiconductor for artificial synapse
We utilized IGZO as a semiconductor material because IGZO has several advantages of physical performance and process compatibility. First, IGZO-based transistors exhibit relatively high on/off current ratio due to the intrinsic high mobility and low carrier concentration 25 . As RH increases, proton conductivity of the Y7C peptide increases significantly. Thus, leakage current from gate electrode would be increased and cover the drain current modulation. To clarify the carrier modulation from the lateral gate, the ratio between on current and off current should be large enough. However, if the transistor operates in depletion mode, the energy dissipation of artificial neural network based on the transistor will be seriously increased. Since IGZO semiconductor has very low intrinsic electron concentration, the transistors based on IGZO operate in enhancement mode. Compared to the other oxide semiconductor materials, IGZO still exhibits very low off current showing high on/off ratio. Therefore, the energy dissipation of the artificial neural network based on the transistors will be potentially decreased. Owing to these electrical properties, there have been several attempts to utilize IGZO as a channel material of synaptic transistors 26,27 . In addition to this electrical advantages, physical properties of IGZO including fair flexibility and good transparency are also preferable to be applied in bio-integrated systems. We previously reported the bio-implantable devices based on the peptide and have developed the photon sensitive oxide-peptide complex for future works. Therefore, the physical properties of IGZO film as well as the electrical properties are adequate for our research scope.
Furthermore, process compatibility of IGZO to the peptide is also considered. One of the process barriers of the peptide film is that the peptide film is easily dissolved by liquid and degraded by thermal stimulus. For that reason, deposition on the peptide film is limited to room-temperature and liquid-free process. To fabricate the lateral gating transistors on the peptide film, semiconductor layer is deposited on the peptide film. IGZO can be deposited by room-temperature sputtering without performance compensation. For that reason, IGZO can be deposited directly on the peptide film. Also, sputtered IGZO shows good uniformity that can be advantageous to be applied in a large-scale crossbar array. IGZO exhibits not only adequate electrical performance for artificial synapse but also process compatibility preventing degradation of the peptide film.

Supplementary Note 9. Proton-activated artificial synaptic plasticity
Synaptic plasticity was emulated by applying presynaptic spikes to the proton-activated artificial synapse at 90 % RH. Triangular-shaped presynaptic spikes were applied to the lateral gate of the device, and the EPSC was measured between two electrodes on the top of the channel layer (Fig. 4a, bottom). Analogous to a biological synapse, the EPSC was modulated by the stimulation time, which corresponds to spike duration-dependent plasticity ( Supplementary Fig. 15a). As the stimulation time increased from 100 ms to 1000 ms, the amplitude and retention time of the EPSC response increased. Two consecutive presynaptic spikes caused a transient synaptic enhancement referred to as paired-pulse facilitation (PPF), which is a function of the time interval between the spikes ( Supplementary Fig. 15b, inset). In this case, the amplitude and duration of the spikes were fixed at 1 V and 1 s, respectively. The PPF index decayed as the time interval of the spikes increased ( Supplementary Fig. 15b). After being stimulated by postsynaptic spikes, the EPSC underwent a decaying process, which is referred to as the relaxation process of the short-term potentiation of the synapse. The data loss of the device can be described as an exponential decay function called the Ebbinghaus forgetting curve 28 : where I0, IA, t and τ are the current offset, fit constant, time after the stimulation, and relaxation time constant, respectively. Data retention as a function of amplitude of presynaptic voltage was measured after single stimulation with duration of 1 s. (Supplementary Fig.15c) Transition from short-term plasticity (STP) to long-term plasticity (LTP) is observed when presynaptic voltage is larger than 2 V. (Supplementary Fig. 15d) To observe the relationship between the stimulation number and relaxation time constant, various numbers of presynaptic spikes with a fixed amplitude of 1 V and a duration of 1 s were applied. The relaxation time constant was increased with a larger number of stimulations ( Supplementary Fig. 15e).
Frequency modulation of the EPSC was observed by applying 10 consecutive spikes with a fixed amplitude of 1 V and a duration of 100 ms (Supplementary Fig. 15f). This indicates that the spike-frequency-dependent plasticity of a biological synapse is emulated by the peptidebased synaptic device.
Spatial variation of humidity effects on the carrier modulation was identified in various RH conditions. (Supplementary Fig. 16a)  Supplementary Fig. 16d does not exhibit significant degradation during 10 cycles. The temporal variation of the maximum conductance was 5.17 %.
The temporal variation of the device is much better than the spatial variation meaning that the device operate stable despite low uniformity.

Supplementary Note 10. Lateral long-range electrostatic effect at high RH.
In the high enough humidity condition, thin and homogeneous layer of water can be formed on the device. Thus, we fabricated IGZO transistors without peptide layer for a comparative study on gating effect of the peptide at high RH. IGZO channel and Au electrodes were deposited directly on the quartz substrate for the device without peptide layer.
Supplementary Fig. 23a shows the transfer curves of the devices with and without peptide layer at 90 % RH. Gating effect in the device with the peptide layer was clearly observed while that in the device without the peptide layer was not observed in the range of Vg = ± 30V. On current of the device with peptide layer is significantly increased only at 90 % RH. In addition, capacitances with and without the peptide layer were measured. Capacitance-voltage (C-V) characteristics is displayed in Supplementary Fig. 23b. Even though some fluctuation was observed, capacitance of the device with the peptide layer was measured to be approximately 2pF at 90 % RH while only noise was observed at 70 % and 80 % RH, not capacitance. In the case of the device without the peptide layer, capacitance was not observed even at 90 % RH.
This indicates that the high humidity does not solely induce the electrostatic coupling on the semiconductor region for carrier generation. Lateral gating effect from electro hydrolysis did not occur probably due to the long distance of 100 µm from the channel to the gate. In addition, typical capacitance-frequency (C-f) characteristics of ion gating that capacitance increased as frequency decreased was measured in the case of the device with the peptide layer at 90 % RH ( Supplementary Fig. 23c) 29 . This suggests that the capacitive effect in high RH is owing to the accumulation of ions, most likely protons.

Supplementary Figures
Supplementary Figure 1