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Carbonized silk fabric-based flexible organic electrochemical transistors for highly sensitive and selective dopamine detection
Sensors & Actuators: B. Chemical xxx (xxxx) xxxx
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Carbonized silk fabric-based flexible organic electrochemical transistors for highly sensitive and selective dopamine detection Wei Jia, Dongqing Wub,*, Wei Tanga, Xin Xia, Yuezeng Sua, Xiaojun Guoa,*, Ruili Liua,* a b
Department of Electronic Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonized silk fabric Precious metal-free gate electrode Organic electrochemical transistor Dopamine sensor Flexible sensor
In this work, Nafion and reduced graphene oxide-wrapped carbonized silk fabric (Nafion/rGO/CSF) were fabricated into a precious metal-free gate electrode for organic electrochemical transistor (OECT) sensors. The hierarchical structure of CSF can effectively increase the conductivity of the electrode and avoid the aggregation of rGO and Nafion, giving the Nafion/rGO/CSF-based OECT sensors excellent detection capability towards dopamine (DA) with an ultralow detection limit of 1 nM, a broad detection range from 1 nM – 30 μM and a high selectivity. Moreover, the outstanding electrochemical sensing behaviors of the Nafion/rGO/CSF-based OECT sensor are retainable under bent states and in artificial urine, which will greatly facilitate its application in flexible electronics. The abovemenitoned results indicate that OECT sensors with carbon-based gate electrodes can also obtain high sensitivity comparable to that of precious metal gate electrodes, which, therefore, provides an unprecedented strategy for manufacturing flexible OECT sensors with high performance and low cost.
1. Introduction The ever-growing interest in wearable electronics, artificial skin, and the Internet of Things has led to high consumption demands for flexible sensors to detect various biological, environmental, and motion stimuli [1–3]. In terms of the detection speed, sensitivity and selectivity, the requirements for flexible biosensors are more rigorous than those for environmental sensing or motion detection because the biological chemicals in body fluids are usually mixed with many analogs at low concentrations [4]. Among the various biosensing approaches, including capillary electrophoresis, liquid chromatography, chemiluminescence, and ultraviolet-visible spectroscopy, electrochemical methods have obvious advantages in response time and sensitivity, which, therefore, have been widely used in sensors for biological analytes [5,6]. Dopamine (DA) is one of the most important neurotransmitters in humans, and abnormal DA metabolism is associated with many neurological illnesses, such as Alzheimer’s disease, Parkinson’s disease and schizophrenia [7–9]. Therefore, wearable biosensors with rapid and accurate detection capabilities for DA are of significant importance for the point-of-care diagnosis and treatment of related neurological symptoms [10–12]. However, the amounts of DA in body fluids are very low (μM or nM level) [13]. Moreover, ascorbic acid (AA) and uric acid (UA) often coexist with DA, and the responses of the three compounds ⁎
in traditional electrochemical methods often have severe overlap [14–16], imposing additional obstacles for the construction of flexible DA sensors with high sensitivity and selectivity. In past decades, organic electrochemical transistors (OECTs) have received intensive attention in chemical and biological research due to their low working voltages (< 1 V), high sensitivity, good flexibility and high biocompatibility [17–20]. A typical OECT is composed of three electrolyte-immersed terminals as the source, drain and gate electrodes. The transistor configuration allows the modulation of the channel current between the source and drain electrodes via electrochemical reactions on the gate electrode [21], which can result in a very sensitive response to the stimuli, thus making OECTs appealing sensors for biological analytes with low concentrations, such as DA, lactate, glucose, protein, ions and cells [20,22–28]. Platinum (Pt)- and gold (Au)-based gate electrodes have been often used in OECT sensors due to their high sensitivities due to the outstanding catalytic activity and conductivity of the precious metals [29–32]. However, the electrocatalytic behavior of precious metals usually has no selectivity towards analog analytes such as DA, UA and AA. In addition, the poor poison resistance of precious metals would result in unsatisfactory stability in gate electrodes. Moreover, the low natural abundance and high cost of precious metals are also inevitable obstacles for the practical application of OECT sensors. Therefore, the construction of precious metal-free gate electrodes with high sensitivity
Corresponding authors. E-mail addresses: [email protected] (D. Wu), [email protected] (X. Guo), [email protected] (R. Liu).
https://doi.org/10.1016/j.snb.2019.127414 Received 23 April 2019; Received in revised form 10 November 2019; Accepted 12 November 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wei Ji, et al., Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127414
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2.4. Fabrication of the OECT sensors
and conductivity, as well as mechanical stability, is an urgent and challenging task for the development of flexible OECT sensors. Herein, we report the fabrication of flexible OECT sensors with Nafion and reduced graphene oxide-wrapped carbonized silk fabric (Nafion/rGO/CSF) as a precious metal-free gate electrode. Derived from natural silk fabric, the hierarchical CSF in Nafion/rGO/CSF can effectively increase the conductivity of the electrode and avoid the aggregation of rGO and Nafion, resulting in the excellent DA sensing behaviors of Nafion/rGO/CSF, including high selectivity and high sensitivity. More importantly, the electrochemical properties of Nafion/ rGO/CSF can be magnified in the OECT sensor with it as gate electrode, which delivers an extremely low detection limit of 1 nM towards DA with a broad detection range from 1 nM – 30 μM, comparable to that of OECT sensors with precious metal gate electrodes. The outstanding DA sensing performance of Nafion/rGO/CSF-based OECT sensors is still retainable under bending states and in artificial urine, further demonstrating their potential in flexible electronics.
To fabricate the organic electrochemical transistors (OECTs), polyethylene- naphthalate (PEN) with dimensions of 25 mm × 25 mm was used as the flexible substrate and the Ti/Au film (5 nm Ti, 50 nm Au) based source and drain electrodes were deposited on the PEN substrate by a Denton Electron Beam Evaporator with a shadow mask (channel width = 6 mm and length = 0.2 mm). Prior to spin-coating the PEDOT:PSS layer, the PEN substrate was treated with oxygen plasma for 60 s to improve the film adhesion and then stuck to pieces of glass with the same size to guarantee the flatness. Subsequently, the PEDOT:PSS solution was spin-coated (500 rpm for 10 s and 3000 rpm for 60 s) on top of the channel area, and then, the samples were annealed at 100 °C for 15 min. According to the results of the AFM measurement, the thickness of the PEDOT:PSS film was approximately 150 nm. The device was placed on the hot plate for thermal annealing at 120 °C for 20 min. Nafion/rGO/CSF was directly adhered to the PEN substrate as the integrated gate electrode. A polydimethylsiloxane (PDMS, Sylgard 184) well with dimensions of 4 mm × 10 mm × 3 mm was built over the channel area PEDOT:PSS and the Nafion/rGO/CSF gate electrode to stop the contact between the electrolyte and the source/drain electrodes. Before electrochemical measurements, the Nafion/rGO/CSF electrodes were cleaned with PBS solution (0.1 M, pH = 7.4) to remove the unanchored residues.
2. Experimental section 2.1. Materials Dopamine (DA), ascorbic acid (AA) and uric acid (UA), graphite power, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), Nafion solution (5.0 wt.% in water and 1-propanol) and phosphate buffered saline (PBS, 0.1 M, pH = 7.4) were all purchased from Sigma-Aldrich Co. and refrigerated (∼ 4 °C) before use. The artificial urine was bought from Dongguan Chuangfeng Experimental Sales Co. All reagents were analytical grade and used without any further purification. Composed by Bombyx mori silk, the silk fabrics were bought from Dingjia Experimental Equipment Sales Co. Deionized water was used in all the experiments unless further mentioned. The concentrations of the Nafion solution (5 wt%) were diluted to 2.5, 0.5 or 0.1 wt.% with 2-propanol.
2.5. Electrochemical characterization A CHI 660B electrochemical workstation (CH Instruments Inc) and an electrolytic cell were employed for the electrochemical measurements. The electrochemical measurement of the electrodes was investigated with a three-electrode system by differential pulse voltammetry (DPV) and amperometric i-t curve methods in a stirred PBS (0.1 M, pH = 7.4) solution. A Pt foil served as the counter electrode, and an Ag/AgCl (sat. KCl) electrode was used as the reference electrode. To obtain different concentrations of DA, a calculated amount of DA solution (1 mM) was gradually added to the PBS solution. In the OECT sensors, PEDOT:PSS was used as the semiconductor material for the channel area and Nafion/rGO/CSF as the gate electrode. The PBS solution (100 μL) was surrounded by the PDMS well in the OECT sensor. The source and drain electrodes were connected to a Keithley 6430 instrument, and the gate electrodes of the devices were connected to Keithley source meters (Keithley 2400). The gate voltages (VGS) and drain voltages (VDS) were automatically controlled by a Labview program. To obtain the optimized value of VGS, the transfer characteristic, i.e., the channel current (IDS) as a function of VGS (0–2.0 V at a sweeping rate of 0.01 V s−1) at a fixed VDS (-0.1 V), was measured. Additionally, the electrochemical responses of the Nafion/ rGO/CSF electrode to DA were investigated by recording the amperometric i-t curve of the channel currents (IDS). To change the concentrations of DA in the PBS solution, a fixed amount (1 μL) of PBS solution (0.1 M, pH = 7.4) containing different DA concentrations (100 nM - 3 mM) was gradually added into 100 μL of the blank PBS solution (0.1 M, pH = 7.4). The DA sensor was measured at fixed gate and drain voltages (VGS = 0.2 V and VDS = -0.1 V). The detection limit of each device was determined by the channel current response under the condition of signal/noise (S/N) > 3. All the experiments were performed at room temperature.
2.2. Fabrication of the Nafion/rGO/CSF electrode To prepare the Nafion/rGO/CSF electrodes, the silk fabrics were first treated at 900 °C under a nitrogen atmosphere for 2 h to produce the carbonized silk fabrics (CSFs). The aqueous suspension of graphene oxide (GO) was prepared using a modified Hummers’ method. The obtained CSF (2 mm × 10 mm) was subsequently immersed in the GO suspension (0.2 mg mL−1) for 30 min and thermally treated at 300 °C in a 5 % H2 + 95 % Ar atmosphere for another 30 min to reduce the GO. Thus, the composites of the carbonized silk fabrics wrapped with reduced GO (rGO/CSF) were derived as black sheets. In control experiments, the GO solution was drop-cast on a flat silica template, and the resulting GO film was then thermally reduced at 300 °C in a 5 % H2 + 95 % Ar atmosphere for 30 min to produce the rGO film. Thereafter, the Nafion solution (0.1, 0.5 or 2.5 wt.%, 20 μL) was drop-cast on the surface of rGO/CSF to produce the Nafion/rGO/CSF electrodes.
2.3. Structural characterizations The surface morphology of the samples was acquired using fieldemission scanning electron microscopy (SEM) on a Zeiss Ultra Plus system. Transmission electron microscopy (TEM) measurements were carried out by a JEM-2010 F (JEOL, Japan) instrument. The samples for cross-sectional TEM analysis were embedded in epoxide resin and then cut by an ultracut microtome with a diamond knife. The obtained slices were supported on a carbon-coated copper grid. The Fourier transform infrared spectroscopy (FTIR) was performed with a NicoletiN10 MX system. The thickness of the channel area was measured by an atomic force microscope (Multimode Nanoscope IIIa, tapping mode).
3. Results and discussion 3.1. Fabrication of the Nafion/rGO/CSF electrode The fabrication processes of the precious metal-free Nafion/rGO/ CSF electrode are schematically illustrated in Fig. 1a. First, a piece of commercial silk fabric was thermally treated at 900 °C under a nitrogen atmosphere to produce the CSF substrate. In this process, the good 2
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Fig. 1. a) Schematic illustration for the fabrication processes of the Nafion/rGO/CSF electrode. b) and c) SEM images of Nafion/rGO/CSF at different magnifications. d) and e) the cross-section TEM images of a slice of Nafion/rGO/CSF embedded in epoxide resin.
CSF is ∼ 6 Ω/□. With a layer of Nafion on the surface, Nafion/rGO/ CSF still has a low sheet resistance of approximately 10 Ω/□, indicating that the Nafion film can barely influence the conductivity of the electrode, attributable to the presence of the CSF backbone. In contrast, Nafion/rGO has a much lower conductivity than rGO/CSF and Nafion/ rGO/CSF by deriving a high sheet resistance of ∼160 Ω/□, further confirming the important role of CSF for the electron transportation within the electrodes. In the Nafion/rGO/CSF electrode, the wrapping of membrane-like rGO over the vein-like CSFs provides a biomimetic architecture similar to the plant leaves and the insect wings, which can further help the uniform distribution of Nafion over the surface of the rGO layer.
thermal stability of Bombyx mori silk fibroin allows the CSF to maintain the hierarchically woven structure of its precursor, which helps the electrode acquire both high conductivity and good mechanical stability. Consequently, the CSF was immersed in an aqueous solution of GO. During this step, the noncovalent interactions between them, such as van der Waals forces and π-π interactions, allow GO to wrap over the CSF substrate. The further thermal treatment of the resulting composite at 300 °C in H2 and Ar can partially remove the oxygen containing groups of GO, leading to the formation of rGO/CSF. Although it has been reported that rGO alone could be used as the active component for the electrochemical detection of DA, rGO-based sensors usually suffer from unsatisfactory selectivity [31–34]. In this respect, negatively charged Nafion is an appealing ingredient for the gate electrode because it could effectively improve the selectivity of the DA sensors by impeding the access of UA and AA to the electrode with electrostatic repulsion [31,33,35]. Accordingly, an aqueous Nafion dispersion was drop-cast on the surface of rGO/CSF to produce Nafion/ rGO/CSF as a flexible black sheet. In control experiments, two reference samples without the addition of Nafion and CSF were further prepared, which are denoted as rGO/CSF and Nafion/rGO, respectively. As illustrated by the Fourier transform infrared (FTIR) spectra of pure Nafion (Fig. S1), the distinct transmittance peaks at 1153 and 1233 cm−1 are assignable to the symmetric and asymmetric stretching of -CF2 groups [36]. These characteristic absorptions can also be found in the FTIR profile of Nafion/rGO/CSF, confirming the successful deposition of Nafion on rGO/CSF. The scanning electron microscopy (SEM) characterization discloses that CSF inherits the hierarchical yarn structures from the silk fabric, which consists of alternately arranged parallel carbon fibers with the uniform diameter of ∼8 μm (Fig. S2a and b). In rGO/CSF, membrane-like rGO sheets with a few wrinkles can be found on the surface of the carbon fibers (Fig. S2c and d). As expected, the morphology of Nafion/rGO/CSF is very similar to that of rGO/CSF (Fig. 1b and c), indicating that the loading of Nafion has no obvious influence on the structure of Nafion/rGO/CSF. The cross-section transmission electron microscopy (TEM) images of a slice of Nafion/rGO/CSF show that the thickness of the rGO layer is ∼8 nm (Fig. 1d and e). The membrane-like morphology of rGO in Nafion/rGO/CSF should be attributed to the interlaced carbon fibers of CSF, which create an ordered and oriented scaffold to effectively inhibit the aggregation of rGO sheets. In contrast, the graphene sheets in Nafion/rGO without the CSF skeleton have a highly wrinkled architecture (Fig. S2e and f) due to the severe folding and aggregation of rGO during the deposition and thermal treatment [37], which clearly indicates the importance of CSF in the Nafion/rGO/CSF composites. The differences among rGO/CSF, Nafion/rGO/CSF and Nafion/rGO can also be found in their conductivities. The four-point probe resistivity measurement demonstrates that the sheet resistance of rGO/
3.2. Test of Nafion/rGO/CSF in a three-electrode system To evaluate the electrochemical performances of the obtained electrodes, the differential pulse voltammetry (DPV) profiles of rGO/ CSF, Nafion/rGO and Nafion/rGO/CSF were recorded in phosphate buffered saline (PBS, 0.1 M, pH = 7.4) containing DA (0.05 mM), AA (0.2 mM), and UA (0.2 mM). As shown in Fig. 2a, three oxidation peaks at ∼ 0.05, 0.20 and 0.35 V can be found in the DPV curves of all the three electrodes, which should be derived from the oxidation of AA, DA and UA, respectively [16]. According to the peak intensities, Nafion/ rGO and Nafion/rGO/CSF manifest much weaker responses to AA and UA than rGO/CSF (Fig. 2a). The enhanced selectivity of the Nafionmodified electrodes can be attributed to the electrostatic repulsion between the negatively charged Nafion and the carboxyl group containing UA and AA, which can effectively inhibit the diffusion of these analytes to the electrode surfaces [29]. Moreover, Nafion/rGO/CSF has an even weaker response towards UA than Nafion/rGO, although both electrodes are modified by the Nafion solution with the same concentration (0.5 wt.%). The different sensing behavior of Nafion/rGO/ CSF and Nafion/rGO should be attributed to their distinct morphology. Compared with the flat surface of Nafion/rGO/CSF, the highly wrinkled surface of Nafion/rGO (Fig. S2e and f) cannot ensure the homogeneous distribution of Nafion during the electrode modification process. Additionally, the corrugated Nafion/rGO is also not ideal for the penetration of the PBS solution. In addition to the surface morphology, the concentration of the Nafion solution can also influence the selectivity of the electrode. As indicated in Fig. 2b, the Nafion/rGO/CSF electrodes modified with different Nafion solutions (0.1, 0.5 and 2.5 wt%) barely respond towards AA, attributable to the Nafion film on their surfaces. However, the Nafion/rGO/CSF electrode modified by 0.1 wt.% Nafion solution still shows an oxidation peak of UA in the DPV profile, which disappears in the DPV curves of the other two Nafion/rGO/CSF electrodes. It seems that increasing the Nafion concentration up to 2.5 wt% cannot 3
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Fig. 2. a) The DPV profiles of rGO/CSF, Nafion/rGO, and Nafion/rGO/CSF in a PBS solution containing DA, AA and UA. b) The DPV curves of Nafion/rGO/CSF modified by the Nafion solutions with different concentrations. c) The DPV curves of Nafion/rGO/CSF with the concentrations of DA varying from 0.1–20 μM. d) The linear calibration curve of DPV peak currents from Nafion/rGO/CSF versus the concentrations of DA from 0.1–20 μM. The pulse period and amplitude of DPV are 0.2 s and 50 mV, respectively.
The different sensing behaviors of Nafion/rGO/CSF and Nafion/rGO can also be found in their amperometric I-T responses in response to successive injections of DA. As displayed Fig. S4a, Nafion/rGO/CSF can deliver a significant response current to each injection of DA with an average response time of approximately 10 s at the potential of + 0.2 V with the DA concentrations changing from 1 to 100 μM. In contrast, Nafion/rGO needs a long response time of ∼ 100 s to respond to the addition of DA to the PBS solution (Fig. S4b). The excellent electrochemical performance of Nafion/rGO/CSF could be attributed to the synergistic effects between the CSF substrates and Nafion-modified rGO, in which the woven structure of the former provides a hierarchical scaffold to facilitate the diffusion of electron and analyte, and the smooth surface of the latter offers easily accessible active sites.
bring an obvious selectivity improvement for the resulting Nafion/rGO/ CSF electrode, which may even reduce the response of the electrode to the addition of DA (Fig. 2b). Therefore, the Nafion/rGO/CSF electrode modified with 0.5 wt% Nafion solution was selected for the further electrochemical measurements in this work. To determine the linear detection range of Nafion/rGO/CSF towards DA, its DPV responses with the concentrations of DA varying from 0.1–20 μM are summarized in Fig. 2c. With the gradually increased DA concentration, the intensities of the DA oxidation peaks show an obvious upward trend, suggesting the broad concentration range for DA detection. As calculated in Fig. 2d, the linear regression equation for the DA detection behavior of Nafion/rGO/CSF can be expressed as the following equation: Ip = 1.065C + 3.820 (R2 = 0.995)
(1) 3.3. Test of Nafion/rGO/CSF in OECT
Correspondingly, Nafion/rGO/CSF has a high sensitivity of 1.065 μA cm−2 μM-1. Under the same conditions, the sensitivity of Nafion/rGO is only 0.270 μA cm−2 μM-1 (Fig. S3a and b). When DA is in the low concentration range of 0.1–5 μM, the linear regression equation of Nafion/rGO is as follows: Ip = 0.5600C + 12.660 (R2 = 0.980)
The excellent electrochemical performance of Nafion/rGO/CSF encouraged us to fabricate the OECT-based DA sensor with it as the gate electrode (Fig. S5). As displayed in Fig. 3a, the Nafion/rGO/CSF-based OECT sensors have good flexibility, allowing them to be bent to different degrees. The structure and working mechanism of the OECT sensors are illustrated in Fig. 3b and c, respectively. The bias applied between the drain and the source is defined as the drain-source voltage (VDS), which can give rise to an electrical current (the drain-source current, IDS) within the organic semiconductor layer. However, the
(2)
Based on Eq. (2), Nafion/rGO has an improved sensitivity of 0.560 cm−2 μM-1 in the low concentration range, which is still not comparable to that of Nafion/rGO/CSF (Fig. S3c). 4
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Fig. 3. a) Photographs of the flexible organic electrochemical transistor (OECT) sensors with Nafion/rGO/CSF as the precious metal-free gate electrode. b) The electro-oxidation of DA on the gate electrode of the flexible OECT sensor. c) The schematic illustration for the working mechanism of the OECT sensors.
determine the quantity of the charges in the EDL, as well as the electrocatalytic capability of the gate electrode. To find the optimized value of VGS for the detection of DA, the normalized current responses (NCRs) of the Nafion/rGO/CSF-based OECT with the addition of DA, AA and UA at different VGS values are summarized in Fig. S6. As shown in Fig. S6a, the OECT delivers similar NCRs towards DA in the range of 0.2–0.4 V, and the highest NCR is obtained at 0.3 V. In contrast, the OECT only shows responses to UA when VGS is at 0.3 and 0.4 V (Fig. S6b), and the NCRs with the addition of AA are almost unchanged with the VGS values changing from 0.0 to 0.4 V (Fig. S6c). To avoid the interference from UA, the value of VGS is assigned as 0.2 V for the Nafion/ rGO/CSF-based OECT in the following tests. The transfer characteristics of the Nafion/rGO/CSF-based OECT sensor are presented in Fig. 4a, which exhibit an obvious shift to lower values after the addition of DA (0.1 mM) to the PBS solution, attributable to the electro-oxidation of DA [32]. To acquire the continuous response behavior of the OECT sensor towards DA, its real-time channel currents (IDS) were further recorded by the successive injection of DA to the PBS solution. As a result, the OECT sensor can deliver rapid and significant responses to the addition of DA with the DA concentrations ranging from 1 nM to 30 μM (Fig. 4b and inset). It is noteworthy that the OECT sensor has an extremely low detection limit of 1 nM (10−9 M) (signal/noise ratio > 3), which is three orders of magnitude lower than the detection limit of Nafion/rGO/CSF in the three-electrode electrochemical sensing system (∼ 3 μM) and the recently reported precious metal-free electrochemical DA sensors (Table S1). Attributable to its transistor nature, the ultralow detection limit of the OECT sensor towards DA is comparable to the state-of-the-art DA sensors with Pt (5 nM) or graphene/Au (1 nM) as gate electrodes [32,40], which is enough for the detection of DA in many biological systems. In addition to the high sensitivity, the Nafion/rGO/CSF-based OECT sensor also has excellent selectivity towards DA. As displayed in Fig. 4c, the channel currents of the OECT sensor barely change until the concentration of AA is higher than 10 μM. Similarly, there is no obvious channel current response until the concentration of UA is as high as 100 μM (Fig. S7). Thus, the detection limits (signal/noise > 3) of the OECT sensor towards AA and UA are 10 and 100 μM, respectively, which are approximately four orders of magnitude higher than that of DA (1 nM), attributable to the high selectivity of the Nafion/rGO/CSF gate electrode. [31] The changes in the effective gate voltage (ΔVGeff) caused by the concentration variations across different analytes are compared in Fig. 4d. Generally, the response behaviors of the OECT sensors towards the analytes can be indicated by the slopes of the linear range in the fitting curves. The slopes of the fitting curves for AA and UA are only 8.78 and 13.93 mV decade−1, respectively, with the concentrations ranging from 100 nM to 1 mM, which coincide with their concentrations in human body fluids. These values are much lower than the slope of the fitting curve for DA (60.13 mV decade−1), especially in a similar concentration range, confirming the high selectivity of the OECT sensor
application of a bias to the gate electrode (VGS) creates an electrical double layer (EDL) at the channel/electrolyte interface and the gate/ electrolyte interface. During the detection process, the electro-oxidation of DA on the surface of Nafion/rGO/CSF leads to the formation of dopamine-o-quinone (DQ), which is a typical two-electron transfer process, similar to the electrochemical reaction in three-electrode system [16,32]. The electro-oxidation reaction can influence the quantity of the charges of the EDL at the gate/electrolyte interface, thus causing the decrease of the potential drop at the electrolyte/gate interface, which is expressed as the effective gate voltage (VGeff). The relationship between the molar concentration of DA and VGeff can be described in following equation [30,36]:
V Geff = VGS + (1 + CE−C/ CG−E ) kT /2qln[DA] +A
(3)
where VGS is the gate voltage, k is the Boltzmann constant, T is the Kelvin temperature, q is the electron charge, A is a constant, and [DA] is the concentration of dopamine. CE-C and CG-E are the capacitances of the channel/electrolyte interface and the gate/electrolyte interface, respectively (Fig. 3c). The capacitance variation at the gate/electrolyte interface results in an ionic current in the electrolyte, further changing the EDL at the channel/electrolyte interface. This process is supposed to cause the electrochemical dedoping of the PEDOT:PSS channel by cations from the electrolyte. Principally, the conductance of the channel between the source and drain electrodes can be modulated by the doping and dedoping of the cations [37]:
PEDOT+: PSS− + M+ + e− ⇄ PEDOT+ M+:PSS− +
(4) +
+
where M represents the cations, which are Na and H in a PBSbased electrolyte [38,39]. Therefore, the electrochemical reactions at the gate electrodes of OECT can interplay with the electric current between source and drain. Accordingly, the relationship between VGeff and IDS can be ascribed in the following functions:
IDS =
qμp0 tW V ⎛Vp − VGeff + DS ⎞ VDS , (when |VDS| ≪ |Vp − VGeff |) LVp ⎝ 2 ⎠
(5)
Vp = qp0 t / ci
(6)
VGeff = VGS + Voffset
(7)
where μ is the hole mobility; p0 is the initial hole density; t is the thickness of the PEDOT:PSS channel; W and L are the width and length of the channel [17], respectively; Vp is the pinch-off voltage; ci is the effective gate capacitance per unit area; and Voffset is the offset voltage on the gate surface. With the aforementioned equations, the concentrations of DA can be readily determined by monitoring the channel current responses of IDS in the OECT sensors. It should be noted that VGS has strong influences on the electrochemical sensing behavior of the OECT sensor, since it can 5
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Fig. 4. a) The transfer characteristics (IDS vs VGS, VDS = -0.1 V) of the Nafion/rGO/CSF-based OECT sensor in PBS solution (pH = 7.4, 0.1 M) before and after the addition of DA (0.1 mM). b) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the successive addition of DA. (VDS = -0.1 V, VGS = 0.2 V). c) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the successive addition of AA. (VDS = -0.1 V, VGS = 0.2 V). d) The corresponding effective gate voltage changes (ΔVGeff) of the OECT sensor as functions of the concentrations of DA, AA and UA, respectively.
current (Fig. 5b) obtained under the bent state indicates that the OECT sensor can retain high sensitivity towards DA with a low detection limit of 10 nM, which should be due to the excellent mechanical stability of the Nafion/rGO/CSF gate electrode. To examine the potential applications of the Nafion/rGO/CSF-based
towards DA. The ability to work in deformed states is very important for flexible biosensors. As shown in Fig. 5a, the changes of the transfer characteristics of the OECT sensors are negligible even after continuous deformations for 1000 times. However, the response of the channel
Fig. 5. a) The transfer characteristic curves of the Nafion/rGO/CSF-based OECT sensor during up to 1000 bending tests. b) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the injection of DA at the bent state over 45°. 6
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OECT sensor in practical samples, the DA levels in artificial urine were further detected with the sensor. Based on the channel current responses of the OECT sensor to the successive additions of DA in artificial urine (Fig. S8a), the calibrated relationship between the variation in the effective gate voltage (ΔVGeff) and the DA concentration is displayed in Fig. S8b, which still has a slope value of ∼ 60 mV decade−1, suggesting high sensitivity towards DA. Accordingly, the concentrations of DA added in artificial urine can be easily determined with the calibrated curve. As summarized in Table S2, with the increase in the concentration of DA from 30 nM to 10 μM, the Nafion/rGO/CSF-based OECT sensor manifests an impressive DA detection behavior with the appropriate recoveries of 84.25–107 % and a relative standard deviation (RSD) lower than 8.62 %, suggesting the reliability of the OECT sensors for DA detection in practical samples. The reproducibility of the OECT sensor was further evaluated with five OECT sensors fabricated with the same procedures. As shown in Table S3, the channel currents of the five OECT sensors towards DA (1 μM) in PBS solution have a low RSD of 1.65 %, indicating the high reproducibility of the Nafion/rGO/CSFbased OECT sensors.
[3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14]
4. Conclusion A flexible Nafion/rGO/CSF electrode with a hierarchical woven structure and high conductivity was fabricated in this work by the carbonization of silk fabrics and the following surface modification with rGO and Nafion. The outstanding electrochemical properties of Nafion/rGO/CSF allow the OECT sensors using it as a gate electrode to have excellent DA detection, including high sensitivity, a large detection range and high selectivity, which can still be maintained under deformation. More importantly, the results of this work demonstrate that carbon materials can replace precious metal-based gate electrodes to build OECT sensors with high electrochemical performance, good mechanical stability and low cost. Inspired by this work, it can be envisioned that the manufacture of precious metal-free OECT sensors for various biological analytes, such as glucose, lactic acid and proteins, would be experimentally feasible with the proper surface modification of carbon electrodes, thus providing the opportunity to acquire inexpensive OECT sensors for wearable electronics.
[15]
[16]
[17]
[18]
[19] [20] [21] [22]
Declaration of Competing Interest
[23]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[24]
Acknowledgements
[25]
This work was financially supported by National Natural Science Foundation of China (61974091, 61575121, and 51772189). We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University, Advanced Electronics Materials and Devices (AEMD) of Shanghai Jiao Tong University for the characterization of the materials.
[26]
[27] [28]
Appendix A. Supplementary data
[29]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127414.
[30]
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Dr. Wei Tang is a lecturer in Department of Electronic Engineering of Shanghai Jiao Tong University, China. He received the BS degree from Jilin University, China in 2011, and PhD degree from Shanghai Jiao Tong University, China in 2017. His research topics include printable organic transistors and circuits for lower-power sensing. Xin Xi received his BS and MS degrees in Shanghai University, China in 2010 and 2014, respectively. Now he is a PhD candidate at Department of Electronic Engineering of Shanghai Jiao Tong University, China. His current research interests include biological sensors and soft actuators. Prof. Yuezeng Su is a faculty member in Department of Electronic Engineering of Shanghai Jiao Tong University, China. Prof. Su’s work is mainly about the nanomaterials for biosensors and electronics. Prof. Xiaojun Guo received his BS degree from Jilin University, China, in 2002, and PhD degree in electronic engineering from University of Surrey, UK, in 2007. He is currently a professor with Department of Electronic Engineering, Shanghai Jiao Tong University, China. His research interests include the device technologies on transistors, sensors, displays and energy harvesters.
Wei Ji received the BS degree from Heilongjiang Normal University in 2011 and the MS degree from Shanghai University in 2014. Now he is a PhD candidate at Department of Electronic Engineering of Shanghai Jiao Tong University, China. His research interest is focused on carbon based nanomaterials for environment and human health monitoring devices.
Prof. Ruili Liu joined Department of Electronic Engineering, Shanghai Jiao Tong University, China since 2015. Her scientific interests include the controllable synthesis of carbon based nanomaterials and their applications in flexible biosensors and soft actuators.
Prof. Dongqing Wu is now a faculty member in School of Chemistry and Chemical Engineering of Shanghai Jiao Tong University, China. His research interests include organic and carbonaceous functional materials for flexible electronics and energy storage devices.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Carbonized silk fabric-based flexible organic electrochemical transistors for highly sensitive and selective dopamine detection Wei Jia, Dongqing Wub,*, Wei Tanga, Xin Xia, Yuezeng Sua, Xiaojun Guoa,*, Ruili Liua,* a b
Department of Electronic Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonized silk fabric Precious metal-free gate electrode Organic electrochemical transistor Dopamine sensor Flexible sensor
In this work, Nafion and reduced graphene oxide-wrapped carbonized silk fabric (Nafion/rGO/CSF) were fabricated into a precious metal-free gate electrode for organic electrochemical transistor (OECT) sensors. The hierarchical structure of CSF can effectively increase the conductivity of the electrode and avoid the aggregation of rGO and Nafion, giving the Nafion/rGO/CSF-based OECT sensors excellent detection capability towards dopamine (DA) with an ultralow detection limit of 1 nM, a broad detection range from 1 nM – 30 μM and a high selectivity. Moreover, the outstanding electrochemical sensing behaviors of the Nafion/rGO/CSF-based OECT sensor are retainable under bent states and in artificial urine, which will greatly facilitate its application in flexible electronics. The abovemenitoned results indicate that OECT sensors with carbon-based gate electrodes can also obtain high sensitivity comparable to that of precious metal gate electrodes, which, therefore, provides an unprecedented strategy for manufacturing flexible OECT sensors with high performance and low cost.
1. Introduction The ever-growing interest in wearable electronics, artificial skin, and the Internet of Things has led to high consumption demands for flexible sensors to detect various biological, environmental, and motion stimuli [1–3]. In terms of the detection speed, sensitivity and selectivity, the requirements for flexible biosensors are more rigorous than those for environmental sensing or motion detection because the biological chemicals in body fluids are usually mixed with many analogs at low concentrations [4]. Among the various biosensing approaches, including capillary electrophoresis, liquid chromatography, chemiluminescence, and ultraviolet-visible spectroscopy, electrochemical methods have obvious advantages in response time and sensitivity, which, therefore, have been widely used in sensors for biological analytes [5,6]. Dopamine (DA) is one of the most important neurotransmitters in humans, and abnormal DA metabolism is associated with many neurological illnesses, such as Alzheimer’s disease, Parkinson’s disease and schizophrenia [7–9]. Therefore, wearable biosensors with rapid and accurate detection capabilities for DA are of significant importance for the point-of-care diagnosis and treatment of related neurological symptoms [10–12]. However, the amounts of DA in body fluids are very low (μM or nM level) [13]. Moreover, ascorbic acid (AA) and uric acid (UA) often coexist with DA, and the responses of the three compounds ⁎
in traditional electrochemical methods often have severe overlap [14–16], imposing additional obstacles for the construction of flexible DA sensors with high sensitivity and selectivity. In past decades, organic electrochemical transistors (OECTs) have received intensive attention in chemical and biological research due to their low working voltages (< 1 V), high sensitivity, good flexibility and high biocompatibility [17–20]. A typical OECT is composed of three electrolyte-immersed terminals as the source, drain and gate electrodes. The transistor configuration allows the modulation of the channel current between the source and drain electrodes via electrochemical reactions on the gate electrode [21], which can result in a very sensitive response to the stimuli, thus making OECTs appealing sensors for biological analytes with low concentrations, such as DA, lactate, glucose, protein, ions and cells [20,22–28]. Platinum (Pt)- and gold (Au)-based gate electrodes have been often used in OECT sensors due to their high sensitivities due to the outstanding catalytic activity and conductivity of the precious metals [29–32]. However, the electrocatalytic behavior of precious metals usually has no selectivity towards analog analytes such as DA, UA and AA. In addition, the poor poison resistance of precious metals would result in unsatisfactory stability in gate electrodes. Moreover, the low natural abundance and high cost of precious metals are also inevitable obstacles for the practical application of OECT sensors. Therefore, the construction of precious metal-free gate electrodes with high sensitivity
Corresponding authors. E-mail addresses: [email protected] (D. Wu), [email protected] (X. Guo), [email protected] (R. Liu).
https://doi.org/10.1016/j.snb.2019.127414 Received 23 April 2019; Received in revised form 10 November 2019; Accepted 12 November 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wei Ji, et al., Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127414
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2.4. Fabrication of the OECT sensors
and conductivity, as well as mechanical stability, is an urgent and challenging task for the development of flexible OECT sensors. Herein, we report the fabrication of flexible OECT sensors with Nafion and reduced graphene oxide-wrapped carbonized silk fabric (Nafion/rGO/CSF) as a precious metal-free gate electrode. Derived from natural silk fabric, the hierarchical CSF in Nafion/rGO/CSF can effectively increase the conductivity of the electrode and avoid the aggregation of rGO and Nafion, resulting in the excellent DA sensing behaviors of Nafion/rGO/CSF, including high selectivity and high sensitivity. More importantly, the electrochemical properties of Nafion/ rGO/CSF can be magnified in the OECT sensor with it as gate electrode, which delivers an extremely low detection limit of 1 nM towards DA with a broad detection range from 1 nM – 30 μM, comparable to that of OECT sensors with precious metal gate electrodes. The outstanding DA sensing performance of Nafion/rGO/CSF-based OECT sensors is still retainable under bending states and in artificial urine, further demonstrating their potential in flexible electronics.
To fabricate the organic electrochemical transistors (OECTs), polyethylene- naphthalate (PEN) with dimensions of 25 mm × 25 mm was used as the flexible substrate and the Ti/Au film (5 nm Ti, 50 nm Au) based source and drain electrodes were deposited on the PEN substrate by a Denton Electron Beam Evaporator with a shadow mask (channel width = 6 mm and length = 0.2 mm). Prior to spin-coating the PEDOT:PSS layer, the PEN substrate was treated with oxygen plasma for 60 s to improve the film adhesion and then stuck to pieces of glass with the same size to guarantee the flatness. Subsequently, the PEDOT:PSS solution was spin-coated (500 rpm for 10 s and 3000 rpm for 60 s) on top of the channel area, and then, the samples were annealed at 100 °C for 15 min. According to the results of the AFM measurement, the thickness of the PEDOT:PSS film was approximately 150 nm. The device was placed on the hot plate for thermal annealing at 120 °C for 20 min. Nafion/rGO/CSF was directly adhered to the PEN substrate as the integrated gate electrode. A polydimethylsiloxane (PDMS, Sylgard 184) well with dimensions of 4 mm × 10 mm × 3 mm was built over the channel area PEDOT:PSS and the Nafion/rGO/CSF gate electrode to stop the contact between the electrolyte and the source/drain electrodes. Before electrochemical measurements, the Nafion/rGO/CSF electrodes were cleaned with PBS solution (0.1 M, pH = 7.4) to remove the unanchored residues.
2. Experimental section 2.1. Materials Dopamine (DA), ascorbic acid (AA) and uric acid (UA), graphite power, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), Nafion solution (5.0 wt.% in water and 1-propanol) and phosphate buffered saline (PBS, 0.1 M, pH = 7.4) were all purchased from Sigma-Aldrich Co. and refrigerated (∼ 4 °C) before use. The artificial urine was bought from Dongguan Chuangfeng Experimental Sales Co. All reagents were analytical grade and used without any further purification. Composed by Bombyx mori silk, the silk fabrics were bought from Dingjia Experimental Equipment Sales Co. Deionized water was used in all the experiments unless further mentioned. The concentrations of the Nafion solution (5 wt%) were diluted to 2.5, 0.5 or 0.1 wt.% with 2-propanol.
2.5. Electrochemical characterization A CHI 660B electrochemical workstation (CH Instruments Inc) and an electrolytic cell were employed for the electrochemical measurements. The electrochemical measurement of the electrodes was investigated with a three-electrode system by differential pulse voltammetry (DPV) and amperometric i-t curve methods in a stirred PBS (0.1 M, pH = 7.4) solution. A Pt foil served as the counter electrode, and an Ag/AgCl (sat. KCl) electrode was used as the reference electrode. To obtain different concentrations of DA, a calculated amount of DA solution (1 mM) was gradually added to the PBS solution. In the OECT sensors, PEDOT:PSS was used as the semiconductor material for the channel area and Nafion/rGO/CSF as the gate electrode. The PBS solution (100 μL) was surrounded by the PDMS well in the OECT sensor. The source and drain electrodes were connected to a Keithley 6430 instrument, and the gate electrodes of the devices were connected to Keithley source meters (Keithley 2400). The gate voltages (VGS) and drain voltages (VDS) were automatically controlled by a Labview program. To obtain the optimized value of VGS, the transfer characteristic, i.e., the channel current (IDS) as a function of VGS (0–2.0 V at a sweeping rate of 0.01 V s−1) at a fixed VDS (-0.1 V), was measured. Additionally, the electrochemical responses of the Nafion/ rGO/CSF electrode to DA were investigated by recording the amperometric i-t curve of the channel currents (IDS). To change the concentrations of DA in the PBS solution, a fixed amount (1 μL) of PBS solution (0.1 M, pH = 7.4) containing different DA concentrations (100 nM - 3 mM) was gradually added into 100 μL of the blank PBS solution (0.1 M, pH = 7.4). The DA sensor was measured at fixed gate and drain voltages (VGS = 0.2 V and VDS = -0.1 V). The detection limit of each device was determined by the channel current response under the condition of signal/noise (S/N) > 3. All the experiments were performed at room temperature.
2.2. Fabrication of the Nafion/rGO/CSF electrode To prepare the Nafion/rGO/CSF electrodes, the silk fabrics were first treated at 900 °C under a nitrogen atmosphere for 2 h to produce the carbonized silk fabrics (CSFs). The aqueous suspension of graphene oxide (GO) was prepared using a modified Hummers’ method. The obtained CSF (2 mm × 10 mm) was subsequently immersed in the GO suspension (0.2 mg mL−1) for 30 min and thermally treated at 300 °C in a 5 % H2 + 95 % Ar atmosphere for another 30 min to reduce the GO. Thus, the composites of the carbonized silk fabrics wrapped with reduced GO (rGO/CSF) were derived as black sheets. In control experiments, the GO solution was drop-cast on a flat silica template, and the resulting GO film was then thermally reduced at 300 °C in a 5 % H2 + 95 % Ar atmosphere for 30 min to produce the rGO film. Thereafter, the Nafion solution (0.1, 0.5 or 2.5 wt.%, 20 μL) was drop-cast on the surface of rGO/CSF to produce the Nafion/rGO/CSF electrodes.
2.3. Structural characterizations The surface morphology of the samples was acquired using fieldemission scanning electron microscopy (SEM) on a Zeiss Ultra Plus system. Transmission electron microscopy (TEM) measurements were carried out by a JEM-2010 F (JEOL, Japan) instrument. The samples for cross-sectional TEM analysis were embedded in epoxide resin and then cut by an ultracut microtome with a diamond knife. The obtained slices were supported on a carbon-coated copper grid. The Fourier transform infrared spectroscopy (FTIR) was performed with a NicoletiN10 MX system. The thickness of the channel area was measured by an atomic force microscope (Multimode Nanoscope IIIa, tapping mode).
3. Results and discussion 3.1. Fabrication of the Nafion/rGO/CSF electrode The fabrication processes of the precious metal-free Nafion/rGO/ CSF electrode are schematically illustrated in Fig. 1a. First, a piece of commercial silk fabric was thermally treated at 900 °C under a nitrogen atmosphere to produce the CSF substrate. In this process, the good 2
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Fig. 1. a) Schematic illustration for the fabrication processes of the Nafion/rGO/CSF electrode. b) and c) SEM images of Nafion/rGO/CSF at different magnifications. d) and e) the cross-section TEM images of a slice of Nafion/rGO/CSF embedded in epoxide resin.
CSF is ∼ 6 Ω/□. With a layer of Nafion on the surface, Nafion/rGO/ CSF still has a low sheet resistance of approximately 10 Ω/□, indicating that the Nafion film can barely influence the conductivity of the electrode, attributable to the presence of the CSF backbone. In contrast, Nafion/rGO has a much lower conductivity than rGO/CSF and Nafion/ rGO/CSF by deriving a high sheet resistance of ∼160 Ω/□, further confirming the important role of CSF for the electron transportation within the electrodes. In the Nafion/rGO/CSF electrode, the wrapping of membrane-like rGO over the vein-like CSFs provides a biomimetic architecture similar to the plant leaves and the insect wings, which can further help the uniform distribution of Nafion over the surface of the rGO layer.
thermal stability of Bombyx mori silk fibroin allows the CSF to maintain the hierarchically woven structure of its precursor, which helps the electrode acquire both high conductivity and good mechanical stability. Consequently, the CSF was immersed in an aqueous solution of GO. During this step, the noncovalent interactions between them, such as van der Waals forces and π-π interactions, allow GO to wrap over the CSF substrate. The further thermal treatment of the resulting composite at 300 °C in H2 and Ar can partially remove the oxygen containing groups of GO, leading to the formation of rGO/CSF. Although it has been reported that rGO alone could be used as the active component for the electrochemical detection of DA, rGO-based sensors usually suffer from unsatisfactory selectivity [31–34]. In this respect, negatively charged Nafion is an appealing ingredient for the gate electrode because it could effectively improve the selectivity of the DA sensors by impeding the access of UA and AA to the electrode with electrostatic repulsion [31,33,35]. Accordingly, an aqueous Nafion dispersion was drop-cast on the surface of rGO/CSF to produce Nafion/ rGO/CSF as a flexible black sheet. In control experiments, two reference samples without the addition of Nafion and CSF were further prepared, which are denoted as rGO/CSF and Nafion/rGO, respectively. As illustrated by the Fourier transform infrared (FTIR) spectra of pure Nafion (Fig. S1), the distinct transmittance peaks at 1153 and 1233 cm−1 are assignable to the symmetric and asymmetric stretching of -CF2 groups [36]. These characteristic absorptions can also be found in the FTIR profile of Nafion/rGO/CSF, confirming the successful deposition of Nafion on rGO/CSF. The scanning electron microscopy (SEM) characterization discloses that CSF inherits the hierarchical yarn structures from the silk fabric, which consists of alternately arranged parallel carbon fibers with the uniform diameter of ∼8 μm (Fig. S2a and b). In rGO/CSF, membrane-like rGO sheets with a few wrinkles can be found on the surface of the carbon fibers (Fig. S2c and d). As expected, the morphology of Nafion/rGO/CSF is very similar to that of rGO/CSF (Fig. 1b and c), indicating that the loading of Nafion has no obvious influence on the structure of Nafion/rGO/CSF. The cross-section transmission electron microscopy (TEM) images of a slice of Nafion/rGO/CSF show that the thickness of the rGO layer is ∼8 nm (Fig. 1d and e). The membrane-like morphology of rGO in Nafion/rGO/CSF should be attributed to the interlaced carbon fibers of CSF, which create an ordered and oriented scaffold to effectively inhibit the aggregation of rGO sheets. In contrast, the graphene sheets in Nafion/rGO without the CSF skeleton have a highly wrinkled architecture (Fig. S2e and f) due to the severe folding and aggregation of rGO during the deposition and thermal treatment [37], which clearly indicates the importance of CSF in the Nafion/rGO/CSF composites. The differences among rGO/CSF, Nafion/rGO/CSF and Nafion/rGO can also be found in their conductivities. The four-point probe resistivity measurement demonstrates that the sheet resistance of rGO/
3.2. Test of Nafion/rGO/CSF in a three-electrode system To evaluate the electrochemical performances of the obtained electrodes, the differential pulse voltammetry (DPV) profiles of rGO/ CSF, Nafion/rGO and Nafion/rGO/CSF were recorded in phosphate buffered saline (PBS, 0.1 M, pH = 7.4) containing DA (0.05 mM), AA (0.2 mM), and UA (0.2 mM). As shown in Fig. 2a, three oxidation peaks at ∼ 0.05, 0.20 and 0.35 V can be found in the DPV curves of all the three electrodes, which should be derived from the oxidation of AA, DA and UA, respectively [16]. According to the peak intensities, Nafion/ rGO and Nafion/rGO/CSF manifest much weaker responses to AA and UA than rGO/CSF (Fig. 2a). The enhanced selectivity of the Nafionmodified electrodes can be attributed to the electrostatic repulsion between the negatively charged Nafion and the carboxyl group containing UA and AA, which can effectively inhibit the diffusion of these analytes to the electrode surfaces [29]. Moreover, Nafion/rGO/CSF has an even weaker response towards UA than Nafion/rGO, although both electrodes are modified by the Nafion solution with the same concentration (0.5 wt.%). The different sensing behavior of Nafion/rGO/ CSF and Nafion/rGO should be attributed to their distinct morphology. Compared with the flat surface of Nafion/rGO/CSF, the highly wrinkled surface of Nafion/rGO (Fig. S2e and f) cannot ensure the homogeneous distribution of Nafion during the electrode modification process. Additionally, the corrugated Nafion/rGO is also not ideal for the penetration of the PBS solution. In addition to the surface morphology, the concentration of the Nafion solution can also influence the selectivity of the electrode. As indicated in Fig. 2b, the Nafion/rGO/CSF electrodes modified with different Nafion solutions (0.1, 0.5 and 2.5 wt%) barely respond towards AA, attributable to the Nafion film on their surfaces. However, the Nafion/rGO/CSF electrode modified by 0.1 wt.% Nafion solution still shows an oxidation peak of UA in the DPV profile, which disappears in the DPV curves of the other two Nafion/rGO/CSF electrodes. It seems that increasing the Nafion concentration up to 2.5 wt% cannot 3
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Fig. 2. a) The DPV profiles of rGO/CSF, Nafion/rGO, and Nafion/rGO/CSF in a PBS solution containing DA, AA and UA. b) The DPV curves of Nafion/rGO/CSF modified by the Nafion solutions with different concentrations. c) The DPV curves of Nafion/rGO/CSF with the concentrations of DA varying from 0.1–20 μM. d) The linear calibration curve of DPV peak currents from Nafion/rGO/CSF versus the concentrations of DA from 0.1–20 μM. The pulse period and amplitude of DPV are 0.2 s and 50 mV, respectively.
The different sensing behaviors of Nafion/rGO/CSF and Nafion/rGO can also be found in their amperometric I-T responses in response to successive injections of DA. As displayed Fig. S4a, Nafion/rGO/CSF can deliver a significant response current to each injection of DA with an average response time of approximately 10 s at the potential of + 0.2 V with the DA concentrations changing from 1 to 100 μM. In contrast, Nafion/rGO needs a long response time of ∼ 100 s to respond to the addition of DA to the PBS solution (Fig. S4b). The excellent electrochemical performance of Nafion/rGO/CSF could be attributed to the synergistic effects between the CSF substrates and Nafion-modified rGO, in which the woven structure of the former provides a hierarchical scaffold to facilitate the diffusion of electron and analyte, and the smooth surface of the latter offers easily accessible active sites.
bring an obvious selectivity improvement for the resulting Nafion/rGO/ CSF electrode, which may even reduce the response of the electrode to the addition of DA (Fig. 2b). Therefore, the Nafion/rGO/CSF electrode modified with 0.5 wt% Nafion solution was selected for the further electrochemical measurements in this work. To determine the linear detection range of Nafion/rGO/CSF towards DA, its DPV responses with the concentrations of DA varying from 0.1–20 μM are summarized in Fig. 2c. With the gradually increased DA concentration, the intensities of the DA oxidation peaks show an obvious upward trend, suggesting the broad concentration range for DA detection. As calculated in Fig. 2d, the linear regression equation for the DA detection behavior of Nafion/rGO/CSF can be expressed as the following equation: Ip = 1.065C + 3.820 (R2 = 0.995)
(1) 3.3. Test of Nafion/rGO/CSF in OECT
Correspondingly, Nafion/rGO/CSF has a high sensitivity of 1.065 μA cm−2 μM-1. Under the same conditions, the sensitivity of Nafion/rGO is only 0.270 μA cm−2 μM-1 (Fig. S3a and b). When DA is in the low concentration range of 0.1–5 μM, the linear regression equation of Nafion/rGO is as follows: Ip = 0.5600C + 12.660 (R2 = 0.980)
The excellent electrochemical performance of Nafion/rGO/CSF encouraged us to fabricate the OECT-based DA sensor with it as the gate electrode (Fig. S5). As displayed in Fig. 3a, the Nafion/rGO/CSF-based OECT sensors have good flexibility, allowing them to be bent to different degrees. The structure and working mechanism of the OECT sensors are illustrated in Fig. 3b and c, respectively. The bias applied between the drain and the source is defined as the drain-source voltage (VDS), which can give rise to an electrical current (the drain-source current, IDS) within the organic semiconductor layer. However, the
(2)
Based on Eq. (2), Nafion/rGO has an improved sensitivity of 0.560 cm−2 μM-1 in the low concentration range, which is still not comparable to that of Nafion/rGO/CSF (Fig. S3c). 4
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Fig. 3. a) Photographs of the flexible organic electrochemical transistor (OECT) sensors with Nafion/rGO/CSF as the precious metal-free gate electrode. b) The electro-oxidation of DA on the gate electrode of the flexible OECT sensor. c) The schematic illustration for the working mechanism of the OECT sensors.
determine the quantity of the charges in the EDL, as well as the electrocatalytic capability of the gate electrode. To find the optimized value of VGS for the detection of DA, the normalized current responses (NCRs) of the Nafion/rGO/CSF-based OECT with the addition of DA, AA and UA at different VGS values are summarized in Fig. S6. As shown in Fig. S6a, the OECT delivers similar NCRs towards DA in the range of 0.2–0.4 V, and the highest NCR is obtained at 0.3 V. In contrast, the OECT only shows responses to UA when VGS is at 0.3 and 0.4 V (Fig. S6b), and the NCRs with the addition of AA are almost unchanged with the VGS values changing from 0.0 to 0.4 V (Fig. S6c). To avoid the interference from UA, the value of VGS is assigned as 0.2 V for the Nafion/ rGO/CSF-based OECT in the following tests. The transfer characteristics of the Nafion/rGO/CSF-based OECT sensor are presented in Fig. 4a, which exhibit an obvious shift to lower values after the addition of DA (0.1 mM) to the PBS solution, attributable to the electro-oxidation of DA [32]. To acquire the continuous response behavior of the OECT sensor towards DA, its real-time channel currents (IDS) were further recorded by the successive injection of DA to the PBS solution. As a result, the OECT sensor can deliver rapid and significant responses to the addition of DA with the DA concentrations ranging from 1 nM to 30 μM (Fig. 4b and inset). It is noteworthy that the OECT sensor has an extremely low detection limit of 1 nM (10−9 M) (signal/noise ratio > 3), which is three orders of magnitude lower than the detection limit of Nafion/rGO/CSF in the three-electrode electrochemical sensing system (∼ 3 μM) and the recently reported precious metal-free electrochemical DA sensors (Table S1). Attributable to its transistor nature, the ultralow detection limit of the OECT sensor towards DA is comparable to the state-of-the-art DA sensors with Pt (5 nM) or graphene/Au (1 nM) as gate electrodes [32,40], which is enough for the detection of DA in many biological systems. In addition to the high sensitivity, the Nafion/rGO/CSF-based OECT sensor also has excellent selectivity towards DA. As displayed in Fig. 4c, the channel currents of the OECT sensor barely change until the concentration of AA is higher than 10 μM. Similarly, there is no obvious channel current response until the concentration of UA is as high as 100 μM (Fig. S7). Thus, the detection limits (signal/noise > 3) of the OECT sensor towards AA and UA are 10 and 100 μM, respectively, which are approximately four orders of magnitude higher than that of DA (1 nM), attributable to the high selectivity of the Nafion/rGO/CSF gate electrode. [31] The changes in the effective gate voltage (ΔVGeff) caused by the concentration variations across different analytes are compared in Fig. 4d. Generally, the response behaviors of the OECT sensors towards the analytes can be indicated by the slopes of the linear range in the fitting curves. The slopes of the fitting curves for AA and UA are only 8.78 and 13.93 mV decade−1, respectively, with the concentrations ranging from 100 nM to 1 mM, which coincide with their concentrations in human body fluids. These values are much lower than the slope of the fitting curve for DA (60.13 mV decade−1), especially in a similar concentration range, confirming the high selectivity of the OECT sensor
application of a bias to the gate electrode (VGS) creates an electrical double layer (EDL) at the channel/electrolyte interface and the gate/ electrolyte interface. During the detection process, the electro-oxidation of DA on the surface of Nafion/rGO/CSF leads to the formation of dopamine-o-quinone (DQ), which is a typical two-electron transfer process, similar to the electrochemical reaction in three-electrode system [16,32]. The electro-oxidation reaction can influence the quantity of the charges of the EDL at the gate/electrolyte interface, thus causing the decrease of the potential drop at the electrolyte/gate interface, which is expressed as the effective gate voltage (VGeff). The relationship between the molar concentration of DA and VGeff can be described in following equation [30,36]:
V Geff = VGS + (1 + CE−C/ CG−E ) kT /2qln[DA] +A
(3)
where VGS is the gate voltage, k is the Boltzmann constant, T is the Kelvin temperature, q is the electron charge, A is a constant, and [DA] is the concentration of dopamine. CE-C and CG-E are the capacitances of the channel/electrolyte interface and the gate/electrolyte interface, respectively (Fig. 3c). The capacitance variation at the gate/electrolyte interface results in an ionic current in the electrolyte, further changing the EDL at the channel/electrolyte interface. This process is supposed to cause the electrochemical dedoping of the PEDOT:PSS channel by cations from the electrolyte. Principally, the conductance of the channel between the source and drain electrodes can be modulated by the doping and dedoping of the cations [37]:
PEDOT+: PSS− + M+ + e− ⇄ PEDOT+ M+:PSS− +
(4) +
+
where M represents the cations, which are Na and H in a PBSbased electrolyte [38,39]. Therefore, the electrochemical reactions at the gate electrodes of OECT can interplay with the electric current between source and drain. Accordingly, the relationship between VGeff and IDS can be ascribed in the following functions:
IDS =
qμp0 tW V ⎛Vp − VGeff + DS ⎞ VDS , (when |VDS| ≪ |Vp − VGeff |) LVp ⎝ 2 ⎠
(5)
Vp = qp0 t / ci
(6)
VGeff = VGS + Voffset
(7)
where μ is the hole mobility; p0 is the initial hole density; t is the thickness of the PEDOT:PSS channel; W and L are the width and length of the channel [17], respectively; Vp is the pinch-off voltage; ci is the effective gate capacitance per unit area; and Voffset is the offset voltage on the gate surface. With the aforementioned equations, the concentrations of DA can be readily determined by monitoring the channel current responses of IDS in the OECT sensors. It should be noted that VGS has strong influences on the electrochemical sensing behavior of the OECT sensor, since it can 5
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Fig. 4. a) The transfer characteristics (IDS vs VGS, VDS = -0.1 V) of the Nafion/rGO/CSF-based OECT sensor in PBS solution (pH = 7.4, 0.1 M) before and after the addition of DA (0.1 mM). b) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the successive addition of DA. (VDS = -0.1 V, VGS = 0.2 V). c) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the successive addition of AA. (VDS = -0.1 V, VGS = 0.2 V). d) The corresponding effective gate voltage changes (ΔVGeff) of the OECT sensor as functions of the concentrations of DA, AA and UA, respectively.
current (Fig. 5b) obtained under the bent state indicates that the OECT sensor can retain high sensitivity towards DA with a low detection limit of 10 nM, which should be due to the excellent mechanical stability of the Nafion/rGO/CSF gate electrode. To examine the potential applications of the Nafion/rGO/CSF-based
towards DA. The ability to work in deformed states is very important for flexible biosensors. As shown in Fig. 5a, the changes of the transfer characteristics of the OECT sensors are negligible even after continuous deformations for 1000 times. However, the response of the channel
Fig. 5. a) The transfer characteristic curves of the Nafion/rGO/CSF-based OECT sensor during up to 1000 bending tests. b) The channel current responses of the Nafion/rGO/CSF-based OECT sensor to the injection of DA at the bent state over 45°. 6
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OECT sensor in practical samples, the DA levels in artificial urine were further detected with the sensor. Based on the channel current responses of the OECT sensor to the successive additions of DA in artificial urine (Fig. S8a), the calibrated relationship between the variation in the effective gate voltage (ΔVGeff) and the DA concentration is displayed in Fig. S8b, which still has a slope value of ∼ 60 mV decade−1, suggesting high sensitivity towards DA. Accordingly, the concentrations of DA added in artificial urine can be easily determined with the calibrated curve. As summarized in Table S2, with the increase in the concentration of DA from 30 nM to 10 μM, the Nafion/rGO/CSF-based OECT sensor manifests an impressive DA detection behavior with the appropriate recoveries of 84.25–107 % and a relative standard deviation (RSD) lower than 8.62 %, suggesting the reliability of the OECT sensors for DA detection in practical samples. The reproducibility of the OECT sensor was further evaluated with five OECT sensors fabricated with the same procedures. As shown in Table S3, the channel currents of the five OECT sensors towards DA (1 μM) in PBS solution have a low RSD of 1.65 %, indicating the high reproducibility of the Nafion/rGO/CSFbased OECT sensors.
[3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14]
4. Conclusion A flexible Nafion/rGO/CSF electrode with a hierarchical woven structure and high conductivity was fabricated in this work by the carbonization of silk fabrics and the following surface modification with rGO and Nafion. The outstanding electrochemical properties of Nafion/rGO/CSF allow the OECT sensors using it as a gate electrode to have excellent DA detection, including high sensitivity, a large detection range and high selectivity, which can still be maintained under deformation. More importantly, the results of this work demonstrate that carbon materials can replace precious metal-based gate electrodes to build OECT sensors with high electrochemical performance, good mechanical stability and low cost. Inspired by this work, it can be envisioned that the manufacture of precious metal-free OECT sensors for various biological analytes, such as glucose, lactic acid and proteins, would be experimentally feasible with the proper surface modification of carbon electrodes, thus providing the opportunity to acquire inexpensive OECT sensors for wearable electronics.
[15]
[16]
[17]
[18]
[19] [20] [21] [22]
Declaration of Competing Interest
[23]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[24]
Acknowledgements
[25]
This work was financially supported by National Natural Science Foundation of China (61974091, 61575121, and 51772189). We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University, Advanced Electronics Materials and Devices (AEMD) of Shanghai Jiao Tong University for the characterization of the materials.
[26]
[27] [28]
Appendix A. Supplementary data
[29]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127414.
[30]
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Dr. Wei Tang is a lecturer in Department of Electronic Engineering of Shanghai Jiao Tong University, China. He received the BS degree from Jilin University, China in 2011, and PhD degree from Shanghai Jiao Tong University, China in 2017. His research topics include printable organic transistors and circuits for lower-power sensing. Xin Xi received his BS and MS degrees in Shanghai University, China in 2010 and 2014, respectively. Now he is a PhD candidate at Department of Electronic Engineering of Shanghai Jiao Tong University, China. His current research interests include biological sensors and soft actuators. Prof. Yuezeng Su is a faculty member in Department of Electronic Engineering of Shanghai Jiao Tong University, China. Prof. Su’s work is mainly about the nanomaterials for biosensors and electronics. Prof. Xiaojun Guo received his BS degree from Jilin University, China, in 2002, and PhD degree in electronic engineering from University of Surrey, UK, in 2007. He is currently a professor with Department of Electronic Engineering, Shanghai Jiao Tong University, China. His research interests include the device technologies on transistors, sensors, displays and energy harvesters.
Wei Ji received the BS degree from Heilongjiang Normal University in 2011 and the MS degree from Shanghai University in 2014. Now he is a PhD candidate at Department of Electronic Engineering of Shanghai Jiao Tong University, China. His research interest is focused on carbon based nanomaterials for environment and human health monitoring devices.
Prof. Ruili Liu joined Department of Electronic Engineering, Shanghai Jiao Tong University, China since 2015. Her scientific interests include the controllable synthesis of carbon based nanomaterials and their applications in flexible biosensors and soft actuators.
Prof. Dongqing Wu is now a faculty member in School of Chemistry and Chemical Engineering of Shanghai Jiao Tong University, China. His research interests include organic and carbonaceous functional materials for flexible electronics and energy storage devices.
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