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Conductive Polymer Coated Carbon Nanotubes to Construct Stretchable and Transparent Electrochemical Sensors Zi-He Jin, Yan-Ling Liu, Jing-Jing Chen, Si-Liang Cai, Jiaquan Xu, and Wei-Hua Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04616 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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Analytical Chemistry

Conductive Polymer Coated Carbon Nanotubes to Construct Stretchable and Transparent Electrochemical Sensors Zi-He Jin, Yan-Ling Liu, Jing-Jing Chen, Si-Liang Cai, Jia-Quan Xu, and Wei-Hua Huang* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China * Address correspondence to [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT

Carbon nanotube (CNT)-based flexible sensors have been intensively developed for physical sensing. However, great challenges remain in fabricating stretchable CNT films with high electrochemical performance for real-time chemical sensing, owing to large sheet resistance of CNT film and further resistance increase caused by separation between each CNT during stretching. Herein, we develop a facile and versatile strategy to construct single walled carbon nanotubes (SWNTs)-based stretchable and transparent electrochemical sensors, by coating and binding each SWNT with conductive polymer. As a polymer with high conductivity, good electrochemical activity, and biocompatibility, poly(3,4-ethylenedioxythiophene) (PEDOT) acting as a superior conductive coating and binder, reduces contact resistance and greatly improve the electrochemical performance of SWNTs film. Furthermore, PEDOT protects the SWNTs junctions from separation during stretching, which endows the sensor with highly mechanical compliance and excellent electrochemical performance during big deformation. These unique features allow real-time monitoring of biochemical signals from mechanically stretched cells. This work represents an important step toward construction of high performance CNTs-based stretchable electrochemical sensor, therefore broadening the way for stretchable sensors in a diversity of biomedical applications.

KEYWORDS: Stretchable Sensor, SWNTs, Conductive Polymer, Electrochemical Detection, Endothelial Cells


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INTRODUCTION Flexible sensors, devised for application on the soft and curved human body, have attracted increasing attention due to their fascinating efficacy.1-5 Considerable progress has hitherto been achieved in flexible electrodes for physical signal sensing such as pressure6,7, temperature8,9 and electrophysiology10,11 benefiting from the development of nanomaterials and electronics. In contrast, for the biomolecules that regulate various life activities, electrochemical sensors can quantitatively monitor them with high sensitivity and fast response.12-16 Further endowing the electrochemical sensors with favorable flexibility/stretchability could therefore provide real-time chemical information during deformation of the soft and curved bodies.17,18 Exemplarily, flexible and wearable electrochemical sensor can in-situ quantitatively analyze sweat metabolites and secretions.19-22 Further, for detection of mechanically sensitive cells and tissues,23-27 stretchable electrochemical sensor is expected

to

be

a

promising

approach

for

real-time

monitoring

of

mechanically-induced biochemical signals during mechanotransduction. In regard to these aspects, despite a few successful progress in wearable sensors for noninvasive analysis of biomarkers19-21 and stretchable devices for monitoring of cells and tissues,28 great challenges still remain in fabricating flexible sensors with excellent electrochemical performance during repeated stretching for biological applications. Among the fascinating materials currently used for stretchable electronics,29,30 CNT and their composites, which are intrinsically flexible, have attracted extensive attention due to their high aspect ratio, excellent electric conductivity, and the 3 ACS Paragon Plus Environment


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tremendous potential to subject to various mechanical deformations.31-34 Recently, there are two main strategies that have been developed to fabricate CNT-based stretchable electrodes. One of them is dispersing CNTs as filler materials into a polymer matrix to form stretchable composites.35-38 Electronic devices with high elasticity could be achieved by this strategy. However, these devices are mainly used for physical sensing, and usually fail to be employed as electrochemical sensors since all the electroactive surface of CNTs are encapsulated inside elastic polymer. The other promising stretchable conductors are fabricated either by creating wavy structures out of otherwise-flat CNT ribbons or by using CNT films onto (or between) stretchable substrates (e.g. highly elastic PDMS, Ecoflex).39-42 Nevertheless, despite their theoretically excellent intrinsic conductivity, CNTs film electrodes usually have large sheet resistance34,43,44 owing to numerous structural defects and grain boundaries of artificial CNTs as well as weak joint connections between individual CNTs. This causes poor electrochemical performance of CNTs film and explains why so far there was very few CNTs film electrode for electrochemical sensing. Moreover, separation between individual CNTs during stretching would dramatically increases the sheet resistances, further restricting CNTs film as stretchable electrochemical sensors. To address this problem, CNTs must be bound together to improve the stretchability and the signal stability of CNTs-based sensors during deformation45. Herein, we develop a facile and versatile strategy to construct CNTs-based stretchable and transparent electrochemical sensors, by binding SWNT with conductive polymer to form composite films. The films of nanometer thickness are 4 ACS Paragon Plus Environment

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produced by coating individual SWNTs with a thin layer of PEDOT followed by vacuum filtration. As a conductive polymer possessing high conductivity, good electrochemical activity and biocompatibility,46-48 PEDOT coating effectively eliminate surface defects and provide favorable joint connections between individual SWNTs. In consequence, the conductivity and electrochemical performance of composite films are greatly enhanced as compared to native CNTs film or PEDOT film. Furthermore, the optoelectronic and electrochemical sensing performance exhibit remarkable stability during stretching and bending process. Proof of concept experiments have been performed by recording of NO release from mechanically sensitive endothelial cells being cultured on the stretchable sensor, indicating its promising potential in real-time monitoring of mechanically-induced biochemical signals from living cells and tissues.

EXPERIMMENT SECTION Fabrication

of

the

SWNTs@PEDOT/PDMS

Film.

High

Purified

Single-walled carbon nanotubes (SWNTs) purchased from Nanjing XFNANO Materials Tech Co., Ltd. was used in this study. Sodium dodecyl sulfate (SDS) was used to disperse SWNTs into deionized water with sonication and centrifuge process. SWNTs with a concentration of 0.3 mg/ml and SDS (Sigma–Aldrich) of 3 mg/ml were dissolved in deionized water and sonicated in a bath type (Kunshan Ultrasonic Instruments Co., Ltd, KQ-300DE) at 100 W for 10 hours. The SWNTs solution was centrifuged at 10,000 g for 10 minutes by high speed centrifuger (Anhui USTC Zonkia Scientific Insruments Co., Ltd., HC-2062) to separate out undissolved SWNT 5 ACS Paragon Plus Environment


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bundles and impurities. The 50% supernatant was carefully decanted and mixed with PEDOT:PSS solution (VSWCNTs/VPEDOT=170:1) (Shanghai Kingtech Chemicals Co., Ltd., Clevios PH1000) with a adequately ultrasonic treatment to obtain PEDOT:PSS coated SWNTs. PDMS film was obtained by spin-coating the degassed prepolymer and cross-linker (w/w = 10:1) on the Si substrate at a spin rate of 600 rpm for 10 s and thermally cured at 75 °C for 2 h. The SWNT@PEDOT/PDMS conductive and stretchable film was prepared by vacuum filtration of nanomaterials mixed solution and subsequently transferring to the PDMS (Figure S1). In the filtration process, as the solvent went through the 0.2 µm porous membrane filter, the PEDOT:PSS coated SWNTs were trapped on the membrane surface, forming a homogeneous gray layer. In addition, the patterned array of SWNT@PEDOT/PDMS was obtained by placing a hollowed-out mask on porous membrane before the filtration process. Characterization

of

SWNTs@PEDOT.

SEM

image

of

SWNT@PEDOT/PDMS was taken by a field emission scanning electron microscope (ZEISS SIGMA) after fabrication. XRD measurements were performed on a Bruker D8-Advance using Cu−Kα radiation at 40 kV and 40 mA. The scan angle covered 10° < 2θ < 50° (2θ is the scattering angle, θ is the Bragg angle) at a speed of 5°/min. FT-IR spectra were acquired in attenuated total reflectance (ATR) mode on a Fourier Transform Infrared Spectrometer (Thermo, IS10). The UV–vis absorption spectra of SWNTs/PDMS and SWNTs@PEDOT/PDMS were recorded on a UV-3600 spectrophotometer with PDMS as reference. Thickness of the conductive films were measured by field emission scanning electron microscope. Sheet resistance was 6 ACS Paragon Plus Environment

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measured by ST-21L portable square resistance meter. Electrochemical measurements were carried out on CHI660A electrochemical workstation (CH instruments Inc.) with Ag/AgCl reference electrode and Pt counter electrode. Zeiss Axiovert 200M and AxioObserver Z1 inverted fluorescent microscope were utilized for observation and fluorescence imaging. LED Integrated Circuit. We constructed a simple circuit consisting with an red LED turned on via wiring, and the two metal probes were contacted directly with the ends of SWNTs@PEDOT/PDMS film. The sensors were fixed on a tensile-testing stage to apply desired strain, which can be defined as: Strain = ( L − L0) / L0, where L0 and L respectively indicate the lengths of the film before and after stretching. Flexibility and Stretchability Test. The ends of SWNTs@PEDOT/PDMS film were contacted with a copper wire via conductive carbon paste, and the joints were protected by PDMS. For flexibility test, the SWNTs@PEDOT/PDMS film was wrapped on various cylindrical with different curvatures (from 1 mm to 26 mm in diameter) and folded in half. For stretchability test, the ends of a rectangular SWNTs@PEDOT/PDMS film were clamped by two fixtures connected to a digital force gauge, and specific lengths was stretched under pre-set parameters. The relative change in the sheet resistance was measured by a square resistance meter. Electrode Fabrication for Electrochemical Detection. To connect with outer workstation, SWNTs@PEDOT/PDMS film was connected to a copper wire via carbon paste. PDMS prepolymer was cast on the film and then thermally cured at 75 °C for 2 hours to insulate the joint and to define a chamber (1.0 × 0.5 × 0.2 cm) for 7 ACS Paragon Plus Environment


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solution reserve. The electrode array (active area 0.5 cm × 1.0 cm) was used for calibration curve. For the monitoring of stretching cells, SWNTs@PEDOT/PDMS film (0.5 cm × 1.0 cm) with HUVECs cultured thereon was mounted on a sliding bracket, which was sliding with a frequency of 0.2 Hz by a control stick. NO Solution Preparation and Calibration of SWNTs@PEDOT/PDMS Film. NO was obtained via disproportionation by dropping the solution of 4 M H2SO4 to that of 2 M NaNO2 at a rate of 0.5 mL/min. The generated gas passed through 2 M NaOH solution to eliminate other NOx. Then, the purified NO bubbling in PBS solution for 30 min was used for stock solution (at 20 °C, c≈1.8 mM) Different concentrations of standard solution was obtained by diluting the stock solution gradually. Calibration for NO was performed by gentle dropping NO standard solution into PBS solution stored in the PDMS chamber on SWNTs@PEDOT/PDMS film. A three electrode system was used in the experiment including SWNTs@PEDOT/PDMS film, Ag/AgCl reference electrode and Pt counter electrode. The potential on the electrode for amperometric oxidation of NO was hold at 850 mV. Human Umbilical Vein Endothelial Cell Culture. Human umbilical vein endothelial cells (HUVECs) were routinely cultured using RPMI 1640 culture medium with 12% fetal bovine serum, 0.292 mg/mL L-glutamine, 4.766 mg/mL HEPES, 0.85 mg/mL NaHCO3, penicillin and streptomycin (100 U) in the culture flask at 37 °C in a humidified incubator (95% air with 5% CO2). For cells detection,


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HUVECs were seeded on SWNTs@PEDOT/PDMS film at a density of ~2×106 cell/cm2. A chamber was built around SWNTs@PEDOT/PDMS film with PDMS to reserve culture medium. It was kept in the incubator for 2 h to allow cells adhesion and loosely bounded HUVECs were washed away by culture medium. Cells cultured for 10 h were chosen for NO release detection.

RESULTS AND DISCUSSION Fabrication and Characterization of SWNTs@PEDOT/PDMS Film. PEDOT coated individual SWNTs were obtained by mixing SWNTs and PEDOT:PSS solution with a adequately ultrasonic treatment. After optimizing the composition of the SWNTs@PEDOT composite (see the ‘Methods’ section for detailed procedures), the conductive and stretchable SWNT@PEDOT film was prepared by vacuum filtration of PEDOT coated SWNTs (Figure S1). After vacuum filtration, dry-transfer approach49 was used to peel the SWNT film onto a PDMS stamp and then annealed at 150 °C for 20 min. The thermal annealing process improves the conductivity and stability of SWNTs@PEDOT/PDMS conducting films by improving the structural perfection of SWNTs50. A schematic illustration of the stretchable and conductive film is presented in Figure 1a. The PEDOT not only wrap the SWNTs, but also act as a superior conductive binder to connect SWNTs, forming a dense hybrid film (Figure 1b). X-ray diffraction (XRD) data of the hybrid film (Figure 1c) show that, the peak related to SWNTs at 2θ ~43° 51 is not observed, whereas two high angle reflections at



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2θ = 17.7° and 26.2° indexed to the amorphous halo of PSS and the interchain planar ring-stacking distance of PEDOT52,53 can be seen. Furthermore, pure SWNTs do not exhibit any characteristic IR bands due to the symmetry of the carbon network, while all of these characteristic PEDOT:PSS bands54 are evident in the IR spectrum of SWNTs@PEDOT

(Figure

1d).

These

results

further

indicate

that

the

SWNTs@PEDOT composite was dominated by the molecular structure of PEDOT on the surface of the SWNTs.

Figure 1. Structural characteristics of hybrid film. (a) Schematic illustration of the SWNTs@PEDOT/PDMS film. (b) SEM image of a SWNTs@PEDOT film on a PDMS substrate. (c, d)XRD patterns (c) and IR spectra (d) of SWNT (black line), PEDOT (red line) and SWNTs@PEDOT (blue line).


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The effects of film thickness on the electrode performance were studied. SWNTs@PEDOT film with different thickness were obtained by vacuum filtration different dosages of mixture solution, here the corresponding thicknesses are about 60

±7 nm, 126±19 nm, 250±25 nm, 518±41 nm and 775±63 nm (mean ± sd, n = 8, Figure S2) from 0.25 mL, 0.5 mL, 1 mL, 2 mL and 3 mL mixture solutions. The results show that the sheet resistance and transmittance in the visible region (350–700 nm) decrease as the thickness of the film increase (Figure 2a). When the thickness of the film reaches to 250 nm, the sheet resistance drops to 82 Ω/sq and transmittance is 81.5%. In fact, these parameters meet the requirements of conductivity and transmittance for most applications in flexible electrodes.41 The flexibility and durability of sensors with different thickness were also verified (Figure 2b, c). Obviously, a higher loading of SWNTs@PEDOT composite makes the sensors more stable during stretching. Furthermore, electrochemical tests indicate that the current increase gradually along with the increasing thickness of the conductive film, and a couple of sensitive and reversible redox peaks are obtained when the film thickness exceeding 250 nm (Figure 2d). Taking the sheet resistance, transmittance and electrochemical performance into consideration, composite film with the thickness of 250 nm was finally chosen to construct the stretchable and transparent electrochemical sensor.



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Figure 2. Optimization of the films thickness. (a) Sheet resistance and transmittance at 550 nm of the sensor as functions of various thicknesses. (b) Resistance comparison of sensors with different thickness before (black line) and after (red line) stretching to 50% for 1000 times. (c) Resistance comparison of sensors with different thickness before (black line) and after (red line) stretching to 100% for 200 times. (d) CVs of SWNTs@PEDOT/PDMS sensors of different thicknesses obtained in K3[Fe(CN)6]. Optoelectronic Properties and Flexibility of SWNTs@PEDOT/PDMS Composite Film. After optimization, the composite film is transparent in the visible region (Figure 3a), and the transmittance at 550 nm are 81.5%, which is comparable to SWNT film (83.01%) with the same thickness. The corresponding sheet resistance are 80 ± 7 Ω/sq (mean ± sd, n = 5), which is much lower than that of SWNT film (130 ± 15 Ω/sq). These results signify that PEDOT effectively reduce sheet resistance by assisting connection among SWNTs to decrease discontinuities and providing extra electrical paths to reduce the contact resistance55 without deteriorating transparency, this facilitates cell or tissue imaging during electrochemical detection. In addition, the composite films present good surface compliance, and patterned electrodes can be easily obtained with a film mask during vacuum filtration (Figure S3). 12 ACS Paragon Plus Environment

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To

test

the

flexibility

of

the

SWNTs@PEDOT/PDMS

film,

the

diameter-dependent normalized sheet resistance was measured by wrapping the film on cylindrical objects with different curvatures (from 1-26 mm). Figure 3b displays modest resistance increase under various bending radius, manifesting the excellent bending tolerance. As for stretchability, we constructed a simple circuit consisting with an LED turned on (Figure 3c). Even when the film was stretched to a strain as large as 50%, the LED showed no noticeable change in illumination intensity. Correspondingly, sheet resistance of SWNTs@PEDOT/PDMS increased from 76.53 to 97.23 Ω/sq (R/R0 ≈ 27%) at a strain of 50%, while the data of SWNTs/PDMS is from 137.33 to 699.53 Ω/sq (R/R0 ≈ 409.38%) at a strain of only 25% (Figure 3d). The results indicate that precoating and binding individual SWNTs with PEDOT greatly improved the stretchability of the conductive film. It should be noted that such performance was achieved under the circumstances without any PDMS prestraining treatments, and the stretch tolerance can be greatly enhanced if SWNTs@PEDOT film is prepared on a prestrained PDMS (Figure 3d). Reciprocating fatigue test shows that the sheet resistance increased from 75.86 to 98.34 Ω/sq when the tensile strain reached to 50% during the first stretching (Figure 3e). Upon release of the strain, the sheet resistance recovered to 79.67 Ω/sq (slightly higher than the initial value) when the PDMS was fully released. More stretching/releasing cycles in the strain range of 0–50% were performed and a stable resistance was achieved by the fourth stretching/releasing cycle and remained unchanged for more cycles (Figure 3f). Stretching/releasing cycles in higher strain 13 ACS Paragon Plus Environment


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range (0–100%) were performed and a stable resistance was achieved by the 9th stretching/releasing cycle (Figure S4). These results indicate the stable electronic pathway inside the films could be established after cyclic stretching.

Figure 3. Optoelectronic performance and flexibility of the SWNTs@PEDOT/PDMS films. (a) Transmittance spectra of the film in the visible wavelength range from 350 to 700 nm. A photograph of the sensor is shown as an inset. (b) Resistance of SWNTs@PEDOT/PDMS film as functions of various bending radius. The insets are digital photograph of the films with different bending diameters. (c)Photograph of the LED integrated circuit under tensile strain of 0% and 50%. The conductive film is clamped on a mechanical testing stage. (d) Resistance comparison of SWNTs film (red line), SWNTs@PEDOT/PDMS film (black line) and SWNTs@PEDOT/PDMS film with a prestrain of 30% (blue line) as functions of tensile strains. (e, f) Resistance as a function of tensile strains (0–50%) for the SWNTs@PEDOT/PDMS stretchable film in the first and second (e), fourth and fiftieth (f) stretching cycles. The inset is SEM image of a composite film after stretching/releasing cycles in the strain range of 30%.


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Possible Mechanism for the Improved Flexibility of the Composite Film. After being recovered from multiple stretching/releasing process, a periodic wavy pattern

(Figure

3f)

appeared

on

the

originally

flat

surface

of

the

SWNTs@PEDOT/PDMS layer. The cross-sectional SEM image (Figure S5c) reveals that the wavy structure is out-of-plane relative to PDMS substrate, which is beneficial for stretch. While, obvious cracks or irregular protrusion are observed when SWNTs or PEDOT films on PDMS are recovered from slightly stretching (Figure S5a-b). Further considering the fact that the apparent length of SWNTs@PEDOT layer has no variation before and after stretching (Figure S6), relative sliding and rotating might occur between SWNTs responding to stretch. We think there are possibly two mechanisms to explain the excellent stretching capacity.

On one hand, the

conductive polymer binder (Figure S7a) coats the individual SWNTs before forming the composite film and protects the SWNTs junctions from separation during sliding and rotating (Figure S7b-c, Figure 3f inset). This makes the SWNTs intrinsically stretchable. On the other hand, conductive polymers possess intrinsic mechanical compliance31, but unprocessed PEDOT is easy to fracture at only 10% strain with an increase in resistivity by four orders of magnitude56. However, PEDOT incorporating with fabrics could drastically improve its stretchability57, which may explain our experiment results from another point. Electrochemical Characterization of SWNTs@PEDOT/PDMS Film. We tested the electrochemical performance of the highly stretchable and flexible film. Compared with the poor electrochemical response of SWNTs/PDMS and 15 ACS Paragon Plus Environment


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PEDOT/PDMS,

SWNTs@PEDOT/PDMS

displays

Page 16 of 24

the

best

electrochemical

performance with the same geometric area responding to ferricyanide solution (Figure 4a). This may be attributed to the outstanding conductivity and synergistically improved electrochemical activity by coating and binding SWNTs with PEDOT. Even when the sensor is stretched, electrochemical performance still remain stable just accompanying with a mild increase of current (Figure 4b). The result may be ascribed to the morphology change of the periodic, wavy structure, which cause mildly geometric area augmentation of the senor. To calibrate this deviation, the reproducibility of the stretchable sensors after repetitive cyclic stretching and bending was testified, and the electrochemical response are kept constant (Figure 4c-d) when the flexible sensor are repetitively stretched or bended to a fixed extent. NO was then chosen as a probe molecule to further evaluate the electrochemical performance of the SWNTs@PEDOT/PDMS films because of its central role in control of physiological processes at very low concentration from pM to µM.58-61 Although

newly developed techniques such

as field-effect transistors or

surface-enhanced Raman spectroscopy allow for the real-time detection of NO released from living cells62-64, it is of great significance to real-time monitor NO level from

mechanically

sensitive

cells

since

NO

generation

from

cells

in

mechanotransduction is dramatically influenced by mechanical forces such as strain, tension, compression, and shear stress.65,66 Amperometric result (Figure 4e) shows that a response of 5 nM NO can be clearly observed, and the detection limit was 16 ACS Paragon Plus Environment

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calculated to be about 0.75 nM (S/N = 3). Obvious increase in current responding to the same concentrations of NO was also detectable when the sensor was stretched (Figure S8), further demonstrating the excellent sensing ability as stretchable electrodes. Figure 4f reveals linear regions of NO concentration from 5 to 200 nM before and after stretching. The slight deviation may be caused by the change of microstructure and active area of the composite film, and this result is in agreement with that in Figure 4b.

Figure 4. Electrochemical characterization of the SWNTs@PEDOT/PDMS sensors. (a) CVs of different sensors obtained in K3[Fe(CN)6]. (b) CVs of the SWNTs@PEDOT/PDMS sensors obtained in K3[Fe(CN)6] after being stretched to different tensile strains. (c, d) CVs of the SWNTs@PEDOT/PDMS sensors obtained in K3[Fe(CN)6] after being stretched to different times (c, stretched to 20%) and after being bended for different times (d, bending radius: 4 mm). (e) Amperometric curves 17 ACS Paragon Plus Environment


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of SWNTs@PEDOT/PDMS sensors to a serial of NO concentration in a stirred deaerated PBS solution. A potential of + 0.80 V (vs. Ag/AgCl) was applied to the electrode. (f) Calibration curves of SWNTs@PEDOT/PDMS films for increasing NO concentration before (black line) and after (red line) being stretched to 20%.

Electrochemical Monitoring of NO Release from Mechanically Sensitive Cells. To test our sensors in real-time monitoring of NO release from mechanically sensitive human umbilical vein endothelial cells (HUVECs), HUVECs were seeded on the SWNTs@PEDOT/PDMS electrode for detection. Results from cell proliferation and viability test (Figure 5a) indicate the excellent biocompatibility of SWNTs@PEDOT. Based on above, the release of NO was evoked by stimulating HUVECs cultured on electrode with L-Arg which can be enzymatically oxidized by nitric oxide synthase (NOS) to produce NO67 and the signal response was recorded as ampere-current curve (Figure 5b). As soon as stimulation, an obvious signal (blue line in Figure 5c) can be observed immediately. To confirm that the amperometric signal was produced by NO release, L-NAME (a specific NOS inhibitor) was used as stimulant and there was no increase in current produced (red line in Figure 5c). Furthermore, when L-Arg was injected onto the sensor without cell, there was also no signal (black line in Figure 5c), excluding the disturbance of L-Arg. To verify its function serving as a stretchable electrochemical sensor, HUVECs were seeded on the surface of SWNTs@PEDOT/PDMS sensor which was fixed on a tensile-testing stage. Then, mechanical stretching were applied on the cells. The morphologic change of HUVECs can be seen clearly in Figure 5b, and 23.6%, 30.1% and 45.5% strain extent were calculated by the change in length between two neighboring cells. 18 ACS Paragon Plus Environment

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Furthermore, as expected, similar result showing NO release from stretched HUVECs was achieved (Figure 5d), indicating the promising potential of this sensor in monitoring of NO under more kinds of mechanical stimuli.

Figure 5. Real-time monitoring of NO release from HUVECs. (a) Microscopic images of HUVECs cultured on SWNTs@PEDOT/PDMS film for different times and stained with Calcein-AM (green) and PI (red) after 72 h. (b) The morphologies and states of HUVECs under increasing strain. (c-d) Monitoring NO release from HUVECs by SWNTs@PEDOT/PDMS sensor (c) without stretching and (d) with stretching. The insets are the microphotographs showing the state of HUVECs in (c) and (d), respectively.

CONCLUSIONS In

summary,

electrochemical

we

sensor

develop with

CNTs-based

highly

mechanical

stretchable compliance

and and

transparent excellent

electrochemical performance, by coating individual CNTs and binding them together with conductive polymer PEDOT. The conductive polymer binder could greatly 19 ACS Paragon Plus Environment


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reduce contact resistance, enhance the electrochemical performance, and maintain stable resistance and electrochemical performance during stretching. This allows real-time monitoring of NO release from mechanically stretched cells, as well as potential studies in quantitative detection of biochemicals under various kinds of mechanical stimuli. This work represents an important step toward construction of CNTs-based stretchable and transparent electrochemical sensor. We believe that further practical application of this facile and versatile strategy may be broadened for wearable and vivo implanted electrochemical sensing.

ASSOCIATED CONTENT Supporting Information. Materials, experimental methods and additional Figure S1-S8. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. 20 ACS Paragon Plus Environment

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ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (21375099, 21675121).

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