Switchable Hydrogel-Gated Organic Field-Effect Transistors

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Switchable Hydrogel-Gated Organic Field-Effect Transistors Laure Fillaud, Thomas Petenzi, Justine Pallu, Benoit Piro, Giorgio Mattana, and Vincent Noel* Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, Univ. Paris Diderot, 15 rue J-A de Baïf, Cedex 13 75205 Paris, France S Supporting Information *

ABSTRACT: Stimuli-responsive hydrogels represent a class of materials capable of reversibly switching their morphological and physicochemical characteristics. An ultrathin poly(acrylic acid) film (ca. 6 nm) grafted onto the gate of a p-type EGOFET is studied, and the correlation between the swelling state of the hydrogel and the transistor output characteristics is presented. The hydrogel-related swelling process occurring in basic medium causes an increase in threshold voltage due to the abrupt and intense increase of the negative charge density on the gate electrode. The variation of the drain current during the in situ modification of the pH electrolyte allows a quantitative analysis of the hydrogel switching kinetics. This work shows not only the relevance of EGOFET as an analytical tool in the broad sense, i.e., able to follow in real time phase transition processes of stimuliresponsive materials, but also the relevance of using a hydrogel for field-effectbased (bio)detection according to the ability of such material to overcome the well-known Debye length problematics.

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both the sensitivity of the field effect toward the EDLs structure and the selectivity of bioreceptors already available (antibodies and nucleic acids). Smart switchable hydrogels are a class of biocompatible materials that could overcome this problem and might contribute to the development of a new generation of analytical devices based on field-effect transistors. “Smart hydrogels” (SHG) that reversibly change their morphology in response to external stimuli have contributed to attract attention due to their potential use in a large range of applications such as biosensors,4 actuators, and MEMS (Microelectromechanical Systems).5 In such hydrogels, pH or temperature acts respectively on the protonation state or on the conformation of the macromolecules. Any change of these parameters leads to macroscopic modifications of the gel morphology as well as of its dielectric properties.6 The swelling/deswelling processes of hydrogels are typically studied at equilibrium without quantitative measurements of the structural dynamics.7 During the transition, hydrogels show structural and mechanical changes, both processes being complex to be investigated in situ and in real time. Classically, structural information is investigated by direct visualization of submicrometer structures by means of scanning and transmission electron microscopy that require ad hoc procedures of sample preparation and restrictive measurement conditions.8 Other techniques including small-angle X-ray and static light scattering provide deep understanding of hydrogel structures but fail to investigate the related dynamics.9 The conventional approach to investigate the dynamics is to weigh the film during the swelling phases,

INTRODUCTION Organic electronic devices and in particular organic field-effect transistors (OFETs) present many advantages for sensing applications because of their sensitivity, easy implementation, and printability (low-cost production).1 Particularly important for biosensing is a subset of OFETs, the so-called electrolytegated organic field-effect transistors (EGOFETs), in which the organic semiconductor (OSC) channel conductivity is modulated through an electrolyte placed between the gate electrode and the OSC.2 Because of the high mobility of the ionic charge carriers present into the electrolyte, the voltage drop between the gate and the channel is concentrated within the electrochemical double layers (EDLs) at both the OSC/ electrolyte and the electrolyte/gate interfaces. Thanks to the high dielectric constant of electrolytic solutions mimicking physiological fluids (relative permittivity of ca. 80) and the few nanometers thick EDLs, EGOFETs can be switched on at very low voltages ( pH*. The PAA-functionalized EGOFETs were characterized as follows: the gate electrode was immersed for a few minutes in an electrolytic solution at a fixed pH, then an output curve was acquired (VGS = −0.6 V) using the same solution as the transistor electrolyte. The same measurement was repeated on the same devices for three different pH values,

Figure 5. Output characteristics (VGS = −0.6 V) of a PAAfunctionalized EGOFET acquired at different pH, in the following order: (●) pH 4, (▶) pH 7, and (◆) pH 9. The arrow indicates the chronological sequence in which the curves were recorded. Inset: same experiment repeated inverting the pH sequence. D

DOI: 10.1021/acs.langmuir.8b00183 Langmuir XXXX, XXX, XXX−XXX

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Figure 6. (a) Gate/electrolyte total capacitance as a function of pH for (●) BrBD-functionalized electrodes and (▲) PAA-functionalized electrodes. (b) Equivalent circuit used to describe the gate/electrolyte interface when pH < 7. (c) Equivalent circuit used to describe the gate/electrolyte interface when pH > 7.

Figure 7. Transfer curves (VDS = −0.4 V) of a PAA-functionalized EGOFET acquired at (■) pH 4, (●) 5, (▲) 6, (▶) 7, (◀) 8, and (▼) 9 (a). Threshold voltages for BrBD- (●) and PAA- (▲) functionalized EGOFETs as a function of pH.

conditions (VGS < −0.3 V), the drain current may be expressed as follows: ID =

1 W μ C(VGS − Vth)2 2 L

one does not consider the electrolyte/semiconductor capacitance, the remaining gate/electrolyte capacitance may be therefore conveniently studied using EIS measurements where the role of the working electrode is played by the same bare/functionalized electrodes also used for the transistors characterization. EIS spectra (in terms of real and imaginary parts of the overall complex impedance Z measured at the gate/electrolyte interface, Figures S5b and S5c) were acquired as a function of the angular frequency ω = 2πf (with f varying in the range between 0.1 and 100 kHz, for a perturbation amplitude of 10 mV). Fitting such measured impedance using the equivalent circuit shown in Figure S5e allows the estimation of the value of the capacitance in dc conditions needed for transistors characterization, namely Cgate/electrolyte. Impedance spectra can be also used to determine the capacitance dependence on frequency. Indeed, under the hypothesis of a totally blocking electrode (i.e., an electrode where no faradaic reactions occur) the imaginary part Zim of the total impedance is almost purely capacitive, and as a consequence, capacitance can be expressed according to the following formula: C = −1/(ωZim) (Figure S5f). Capacitance values at very low frequencies are consistent with the dc values extracted from the overall impedance spectra fitting. Figure 6a shows the plot of capacitance as a function of pH. It can be noted that while the capacitance of BrBD-

(1)

with C the total capacitance (including the contributions of all the capacitances between the gate and the source electrodes, Figures 6a and 6b), μ the charge carrier mobility in pBTTT, and W and L the width and length of the channel, respectively. According to eq 1, changes in output characteristics may be due to variations of Vth and of the total capacitance C. The total capacitance appearing in eq 1 depends on both the gate/electrolyte and the electrolyte/channel capacitances. The drain currents recorded with a bare gold gate or a BrBDmodified gate are constant on the studied pH range (Figure S4). Hence, variations of transistor output characteristics as a function of pH shown in Figure 5 are due to pH-dependent processes occurring at the gate/electrolyte interface. Capacitance Characterization of the Gate/Electrolyte Interface. The overall capacitance appearing in eq 1 can be described as two series interfacial capacitances: the gate/ electrolyte capacitance and the electrolyte/semiconductor capacitance. Of these two capacitances, only the gate/ electrolyte capacitance is sensitive to pH because of the gate functionalization, while the electrolyte/semiconductor capacitance does not respond to pH variations (see also Figure 8a). If E

DOI: 10.1021/acs.langmuir.8b00183 Langmuir XXXX, XXX, XXX−XXX

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Figure 8. (a) Plots of ID versus time (VGS = VDS = −0.4 V) during a dynamic experiment performed using a (black line) bare gate electrode and (blue line) a PAA-functionalized gate for successive pH jumps (values indicated on the graph); each arrow indicates injection of 75 μL of 10−3 M NaOH. VGS = VDS = −0.4 V. (b) Plots of ID versus time, during a dynamic experiment performed on a PAA-functionalized gate while pH was abruptly varied from 8 to 4 and then back to 8 (twice).

indicates that the ohmic drop across the macromolecule layer is negligible in spite of the dielectric behavior of PAA in its shrunken (dried) state. This is in accordance with the extremely thin thickness of the polymer layer (ca. 6 nm, estimated from XPS measurements) and is one of the advantages of using surface-initiated controlled radical polymerization. For pH > 6, Vth decreases strongly (of ca. 110 mV). The variation of drain current upon pH change is therefore correlated to the voltage threshold variation as a function of the PAA swelling state. This phenomenon may be tentatively explained if one considers that the PAA layer swelling (which occurs for pH > 6) leads to a drastic increase of the negative charges on the gate surface, caused by the deprotonation of carboxylic groups. The presence of these negative charges requires the application of a larger gate potential to switch on the transistor. Combining the capacitance values obtained through EIS measurements with the aforementioned Vth variations, one can estimate the negative charge density at the gate surface at ca. −1.5 μC cm−2. Assuming that these charges come exclusively from deprotonated carboxylic functions, this corresponds to approximately 1.7 × 10−11 mol cm−2, a value which is close to that of a dense monolayer.24 It seems therefore that the sensing area probed by the transistor is extremely thin. This result is consistent with the measured increase of capacitance upon PAA swelling indicating that the EDL is reduced to a few nanometers. This result also makes it possible to emphasize the importance of working with switchable layers as thin as possible or to work with layers having a change in their physicochemical characteristics within their entire volume. One of the limiting factors of the SHG utilization in sensing systems is represented by their response time. In order to characterize the switching rate, we carried out drain current measurements under saturation regime for VDS = VGS = −0.4 V as a function of time when the solution pH is abruptly changed by adding an acid or a base. Figure 8a shows the variation of drain current upon additions of concentrated NaOH aliquots, leading to a pH change from 6 (initial conditions) to 7, 8, and finally 9. As shown in Figure 8a, while ID decreased significantly for the PAA-functionalized EGOFET in correspondence with pH variations, no change occurred if the same device was measured using a bare Au gate electrode. For the PAAfunctionalized gate, the drain current drops sharply during the pH transition from 6 to 7, moderately between 7 and 8, and

functionalized electrodes does not respond to pH variations, the capacitance of PAA-functionalized electrodes shows a 2-fold increase when the pH varies from acidic to basic values, i.e., when the PAA layer switches from its shrunken to swollen state. In addition to that one can also notice that for pH > 7 the PAA-functionalized electrode capacitance becomes very close to that of the BrBD-functionalized gate. These results can be explained if one considers the fact that the overall gate/electrolyte capacitance may be defined as a function of two series capacitors, as described in eq 2: 1 1 1 = + Cgate/electrolyte C BrBD C PAA

(2)

where CBrBD and CPAA are the capacitances associated with the BrBD and PAA layers, respectively. When pH < 7, the PAA layer is in its shrunken form and behaves as a second dielectric layer deposited on the top of the BrBD layer (Figure 6b). When pH > 7, the PAA is in its swollen state, which leads to a massive penetration of water and ions inside its molecular chains. This phenomenon causes a marked increase of the PAA dielectric permittivity and, as a consequence, of the associated capacitance, which then becomes negligible with respect to CBrBD; the system can be therefore described almost as if the electrolyte were directly in contact with the BrBD layer (Figure 6c). However, according to eq 1, the drain current is proportional to the total capacitance. The increase in capacity should therefore theoretically lead to an increase in the current, which is the opposite of what is observed experimentally. The decrease of the current when the pH increases is therefore mainly controlled by another phenomenon capable of exceeding the increase in the total capacity. Threshold Voltage Determination. Transfer curves acquired on a PAA-functionalized EGOFET for pH ranging from 5 to 9 are shown in Figure 7a. The corresponding threshold voltages were extracted from plots of ID = f(VGS) (Figure S7), and their variation with pH for both BrBD- and PAA-functionalized gates is presented in Figure 7b. For BrBD-functionalized EGOFETs, the Vth is constant (ca. 0 V) on the studied pH range. On the contrary, for PAAfunctionalized transistors Vth shows a strong shift toward more negative values for pH > 6. For pH < 6, the small difference of Vth between the BrBD- and PAA-functionalized devices F

DOI: 10.1021/acs.langmuir.8b00183 Langmuir XXXX, XXX, XXX−XXX

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Morphological changes occurring at the gate electrode surface modify the transistor threshold voltage, causing in turn measurable variations of the drain current. It is worth noting that the PAA hydrogel utilized in this work is characterized by the presence of many accessible carboxylic acid groups which can be used via peptide coupling for further biomolecules immobilization, just to mention a few examples. Our study therefore paves the way to the development of a novel class of versatile, low-power, and highly sensitive biosensors for detection in aqueous media.

remains almost constant when pH changes from 8 to 9. Hence, modifications observed in Figure 8a may be exclusively attributed to the conformational changes of the PAA. Moreover, the major variation in current is observed during the transition from pH 6 to pH 7, in agreement with PAA pH* value of ca. 6.5.23 In order to verify the reversibility of the PAA conformational change, the drain current was continuously recorded while pH was abruptly varied from 8 to 4 and then back to 8 (twice) (Figure 8b). The reversible PAA swelling/deswelling processes are evidenced by the measurement of the drain current. Indeed, the current has variations in agreement with the pH changes of the solution. Figure 8b present the results of two swelling/ deswelling cycles. According to the literature, the hydrogel dynamics can be investigated by using two major approaches, namely, the relaxation of the stressed polymer network against the solvent25 and the diffusion of the solvent through the polymer network.26 Here, none of the transitions are compatible with a square root of time (Figure S8), indicating that diffusion is not the limiting process. These results are consistent with the extreme thinness of the PAA layers used in this study compared to conventional SHG layers for which such diffusional limitation is observed. Swelling as well as deswelling dynamics can be modeled in a first attempt by using monoexponential functions from which characteristic times can be extracted (Figure S8). The typical response times of swelling (τswell = 121 s) and deswelling (τswell = 75 s) were estimated. Swelling time is typically higher than deswelling time as the two processes, from a thermodynamic point of view, are not energetically equivalent. Polymer swelling requires indeed the breaking of a very large number of hydrogen bonds before water is able to penetrate within the gel and cause the stretch of polymeric chains. This phenomenon is associated with a relatively high activation energy which is therefore responsible for slower kinetics. On the other hand, hydrogel deswelling is faster as the corresponding formation of hydrogen bonds is an energetically favorable process.27 Here both swelling and deswelling characteristic times are shorter than those reported in the literature thanks to the low SHG thickness used here.27 This result shows that the response time of the gel is compatible with its use in detection. Unfortunately, the nonconstant ionic strength in this last experiment does not allow full comparison between swelling and shrinking dynamics. However, works are in progress in our laboratory to insert the PAA-functionalized EGOFETs in simple microfluidic systems allowing to change abruptly the pH, keeping ionic strength constant. Anyway, the results of Figure 8 show that the device is an interesting analytical tool with time resolution within the milliseconds range (dynamics of electrochemical double layers formation) able to sense in situ any hydrogel morphological change. EGOFETs seem to be an unprecedented tool both for fundamental smart-hydrogel dynamics investigation and for their use as sensing components (for instance, by grafting pH-responsive enzymes involving proton production or consumption or probe-containing hydrogels).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00183. Electrochemical grafting of 4-bromobenzenediazonium; film thickness estimation; transfer curves; electrochemical impedance spectroscopy measurements and fitting; determination of hydrogel relaxation kinetics (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +033157277226 (V.N.). ORCID

Vincent Noel: 0000-0003-3901-8358 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by USPC Grant HydroFET. P. Decorse is gratefully acknowledged for XPS analysis. ABBREVIATIONS BrBD, bromobenzene; EDL, electrochemical double layer; EGOFET, electrolyte-gated field-effect transistor; PAA, poly(acrylic acid); PBTTT-C16, poly([2,5-bis(3-hexadecylthiophen2-yl)thieno[3,2-b]thiophene]; SHG, smart hydrogel; SI-ATRP, surface-initiated atom transfer radical polymerization; VGS, gate voltage; Vth, threshold voltage.

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REFERENCES

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CONCLUSIONS The results presented in this work demonstrate that it is possible to grow a stimuli-responsive hydrogel on a gold surface and that the morphological variations of this SHG can be monitored in real time by using it as gate electrode in an electrolyte-gated organic field-effect transistor (EGOFET). G

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DOI: 10.1021/acs.langmuir.8b00183 Langmuir XXXX, XXX, XXX−XXX