Flexible Electrochemical Urea Sensor Based on Surface Molecularly

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Flexible Electrochemical Urea Sensor Based on Surface Molecularly Imprinted Nanotubes for Detection of Human Sweat Yan-Ling Liu, Rong Liu, Yu Qin, Quan-Fa Qiu, Zhen Chen, Shi-Bo Cheng, and Wei-Hua Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04223 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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Analytical Chemistry

Flexible Electrochemical Urea Sensor Based on Surface Molecularly Imprinted Nanotubes for Detection of Human Sweat Yan-Ling Liu, Rong Liu, Yu Qin, Quan-Fa Qiu, Zhen Chen, Shi-Bo Cheng 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 ABSTRACT: Flexible electrochemical (EC) sensors have shown great prospect in epidermal detection for personal healthcare and disease diagnosis. However, no reports have been seen in flexible device for urea analysis in body fluids. Herein, we developed a flexible wearable EC sensor based on surface molecularly imprinted nanotubes for non-invasive urea monitoring with high selectivity in human sweat. The flexible EC sensor was prepared by electropolymerization of 3,4-ethylenedioxythiophene (EDOT) monomer on the hierarchical network of carbon nanotubes (CNTs) and gold nanotubes (Au NTs) to imprint template molecule urea. This sensor exhibited a good linear response toward physiologically relevant urea levels with negligible interferences from common coexisting species. Bending test revealed that this sensor possessed excellent mechanical toler-ance and its EC performance was almost not affected by bending deformation. On-body results of human subjects showed that the flexible platform could distinctly response to the urea levels in volunteer’s sweat after aerobic exercise. The new flexible epidermal EC sensor can provide useful insights into non-invasive monitoring of urea levels in various biofluids, which is promising in the clinical diagnosis of diverse biomedical applications. Recently, wearable devices have attracted extensive attention in the fields of health care, owing to their conformal contact with soft skins or organs and continuous monitoring of physiological state.1-5 Hitherto, considerable flexible sensors have been developed for measuring physical signals, such as temperature6, blood pressure7 and motion frequency8. In order to obtain more comprehensive information about individual’s health, the research on flexible chemical sensors has been carried out in recent years to monitor human vital signs,9-11 in which the flexible EC devices have gained more attention because of the fast response and high sensitivity. Especially, with increasing recognition of the significance of body fluids, which are rich in physiological information, wearable epidermal sensors for human sweat analysis have sprung up.12-20 At present, a series of tattoo biosensors have been developed for the detection of lactate15, glucose16, alcohol17, ammonium18 and zinc19 levels in the human perspiration.20 To achieve screening of complex sweat secretion, the fully integrated sensor array was fabricated for multiplexed perspiration analysis of glucose, lactate, sodium and potassium ions.13 An alternative approach based on wearable textile organic EC transistors with intrinsically amplifying faradic signals and simplifying the readout electronics, has made advances in the detection of ascorbic acid, adrenaline and dopamine.21,22 Moreover, urea is an important nitrogen metabolite in body fluids (e.g., serum, urine and sweat) and also one of the markers of renal function and heart metabolic disorders clinically, and people with hepatic or renal failure, or uremia have significantly higher levels of the toxic ingredient urea in their sweat than the normal.23, 24 However, despite the above great achievement of wearable sensor, no device has been witnessed for the measurement of urea levels in body fluids. Therefore, developing wearable

device for urea analysis is of great importance for personalized healthcare monitoring.25, 26 As regards the detection of non-electroactive urea, EC sensors based on enzyme catalysis27, 28 and molecular imprinting2931 are commonly constructed on rigid electrodes. The enzymatic sensors usually involved two enzymes (e.g., urease and glutamate dehydrogenase) and a mediator (e.g., NADH, αketoglutarate and 2-L-glutamate) to achieve the detection of the urease-catalysed hydrolysis products, which lead to the complexity and instability of these sensor. In contrast, the molecularly imprinted technique based on selective recognition of specific molecule has the advantages of low cost, good reproducibility and high selectivity.32, 33 With respect to the molecularly imprinted polymer (MIP), chitson29, 31 and methacrylic acid34 were used in EC sensors because they can form binding sites with urea template molecule via non-covalent interaction. In recent years, conductive polymers have been widely applied in imprinting films,32 such as polypyrrole, polyaniline and polythiophene. Among these polymers, poly(3,4ethylenedioxythiophene) (PEDOT) is the most promising matrix for its facile preparation and extraordinary stability.33, 35 However, despite the progress of EC sensors, there has been no report about flexible device for urea determination so far. Herein, in view of the excellent flexibility and EC performance of CNTs and Au NTs based sensor revealed by our previous work,36-39 we developed a facile strategy to construct urea-PEDOT/CNTs/Au NTs (urea-PEDOT/C-Au NTs) based flexible wearable EC sensor by electropolymerization on the C-Au NTs electrode in the solution of urea and EDOT to form molecularly imprinted polymer membranes, during which urea as a target molecule could be bonded with PEDOT via hydrogen bonds (Figure 1A, B). Then, the urea template molecules

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Figure 1. Schematic illustration of the flexible urea-PEDOT/C-Au NTs/PDMS EC sensor fabrication. A) C-Au NTs nanocomposite on PDMS substrate. B) UreaPEDOT/C-Au NTs MIP electrode. C) Urea imprinted recognition sites of urea-PEDOT/C-Au NTs EC sensor. D) Urea sensing with some imprinted recognition sites recombined by urea in solution or sweat.

in the MIP were removed by thorough elution, and the molecularly imprinted recognition sites were acquired as shown in Figure 1C. Next, the urea adsorption detection was proceeded, in which the urea molecules in solution or sweat could rebind with molecularly imprinted sites as displayed in Figure 1D. The recombination of urea moleccules with recognition sites leads to the electron transfer resistance of potassium ferricyanide probe, realizing the selective urea determination. This EC sensor showed excellent flexibility and selectivity, and satisfactory linear sensitivity to urea in physiological concentration range. The epidermal urea detection was performed by being attached on the wrist of volunteers and sampling their sweat, demonstrating the promising application of the device in the fields of non-invasive detection and physiological monitoring. EXPERIMENTAL SECTION Reagents and Instruments. Urea and potassium hexacyanoferrate(III) (K3[Fe(CN)6]) were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Dopamine (DA) was provided by Sigma-Aldrich (St. Louis, USA). PDMS liquid prepolymer and cross-linker were purchased from Momentive Performance Materials (Waterford, NY, USA). D (+)-glucose and L (+)-lactic acid were obtained from Aladdin (Shanghai, China). Uric acid (UA) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 3,4ethylenedioxythiophene (EDOT) was obtained from Nine Ding Chemistry Co., Ltd. (Shanghai, China). Poly(styrenesulfonate) (PSS, MW = 70,000) was procured from Alfa Aesar (Ward Hill, USA). Single-walled carbon nanotubes (SWNTs, OD < 2 nm, 5~30 µm in length, purity > 95 wt%) were purchased from

Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). Silver nanowires (35~45 nm in diameter, 10~20 µm in length) were acquired from Zhejiang Kechuang Advanced Materials Tech Co., Ltd. (Hangzhou, China). Artificial sweat (ionized water: 1L, sodium chloride: 20 g/L, ammonium chloride: 17.5 g/L, urea: 5 g/L, lactic acid: 15 g/L, acetic acid: 2.5 g/L, pH was adjusted to 4.7 by sodium hydroxide) was purchased from Fanying precision instrument Co., Ltd. And other chemical reagents were reagent grade and used without further purification. Deionized (DI) water (resistivity >18.2 MΩ·cm, Millipore Inc., USA) was used for rinsing and preparing all aqueous solutions. SEM images were obtained by a field-emission scanning electron microscope (ZEISS SIGMA). FT-IR spectra were acquired in attenuated total reflectance (ATR) mode on a Fourier transform infrared spectrometer (Thermo, IS10). All EC measurements were carried out on a CH Instruments EC analyzer (Model 660A, Shanghai, China) at room temperature. A three-electrode system was used in the experiment including working electrode, Ag/AgCl reference electrode and Pt counter electrode. The applied potentials in all measurements were versus the Ag/AgCl reference electrode. Fabrication of the Flexible C-Au NTs Substrate Electrode. CNTs/Au NTs/PDMS electrode was fabricated according to our recently published work37, 38. Briefly, PDMS film (~200 µm in thickness) was soaked in 1 mg·mL-1 dopamine hydrochloride solution (Tris-HCl buffer solution, 10 mM, pH~8.5) for 24 h to improve the surface hydrophilicity, and then silver nanowires (Ag NWs) solution was spin-coated on the surface of dopamine-coated PDMS to acquire the uniform Ag


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Analytical Chemistry NWs/PDMS film. The Au NTs/PDMS films were obtained by in situ galvanic displacement of sacrificial Ag NWs at 90 ℃ for 2 h. Then, the CNTs membrane was loaded on Au NTs skeleton by means of membrane transfer. The CNTs/Au NTs (C-Au NTs) was contacted with a copper wire via conductive silver adhesives, and the joints were insulated and fixed by PDMS. The fabricated electrode (active area 0.5 cm × 0.5 cm) based on C-Au NTs/PDMS film was used in the subsequent electropolymerization process. Fabrication of Flexible Urea-PEDOT/C-Au NTs EC Sensor. The solution containing 10 mmol·L-1 EDOT, 1 mg·mL-1 PSS and 1.0 mol·L-1 urea was first prepared.40, 41 The electropolymerization process of molecularly imprinted membrane was performed in this solution with C-Au NTs/PDMS electrode as working electrode. The urea-PEDOT composite was deposited onto the flexible C-Au NTs/PDMS electrode under a constant potential of +1.0 V for 60 s. Then, the obtained flexible ureaPEDOT/C-Au NTs EC sensor was rinsed with water and allowed to dry at room temperature. Finally, the urea template molecules in the imprinted membrane were removed by eluting in DI water for 1 h to get the eluted PEDOT/C-Au NTs sensor. Artificial Sweat Analysis. The EC performance of the flexible urea MIP sensor was evaluated in 1.0 mmol·L-1 K3[Fe(CN)6] solution containing 1 mol·L-1 KCl, and differential pulse voltammetry (DPV) was employed to characterize the urea levels at an applied potential range from -0.2 V to +0.6 V (vs Ag/AgCl), with a amplitude of 0.05 V, plus width of 0.05 s, and sampling width of 0.0167 s. The current responses of the freshly eluted flexible PEDOT/C-Au NTs EC sensor were firstly recorded as control with a peak current I0. Then the senor was immersed in the test solution to allow full urea adsorption, and the current response in K3[Fe(CN)6 solution was measured again with a peak current I. The content of urea in the test solution was indirectly determined by measuring the ratio of peak current change value between the above twice responses, that is ∆I/I0 = (I0 - I)/I0. The non-molecularly imprinted polymer (NIP) flexible EC sensor control experiment was performed as the MIP flexible EC sensor except the electropolymerization solution without urea. Interference experiment of flexible urea-PEDOT/C-Au NTs EC sensor was also operated. The concentration of each interferent in the experiment is chosen depending on their physiological normal levels, and in this work the content of glucose, lactate, riboflavin, UA, NaCl, KCl, CaCl2 and NH3 in sweat was 1.7 × 10-4 mol·L-1, 1.4 × 10-2 mol·L-1, 2.0 × 10-2 mol·L-1, 5.9 × 10-5 mol·L-1, 3.1 × 10-2 mol·L-1, 6.1 × 10-3 mol·L-1, 5.2 × 10-3 mol·L-1 and 5.2 × 10-3 mol·L-1, respectively.42 The current responses of the eluted flexible urea-PEDOT/C-Au NTs EC sensor were recorded before the interference detection. Subsequently, the DPV responses of the flexible EC sensor were measured after adsorption of different interferent in solution. Flexibility Test. In order to investigate the flexible performance of the urea-PEDOT/C-Au NTs EC sensor, bending tests were conducted with the response of 10 mM urea determination as the evaluation standard. Firstly, the peak current responses of the initial unbent urea-PEDOT/C-Au NTs EC sensor before (I0) and after (Iunbent) 10 mM urea determination were recorded, and the ratio of peak current change was calculated to be (I0 - Iunbent)/I0. Next, the flexible EC sensor was at-

tached onto the surface of various cylinders with different radii (0.5 - 3.0 cm) to mimic the curvature of curved skin surfaces (from the finger to the wrist). After bending with each curvature for 10 times, the above urea-PEDOT/C-Au NTs EC sensor was immersed in 10 mM urea and then the EC response was recorded again, and the peak current response was marked as Ibent. In this way, the influence of mechanical deformation on EC performance was evaluated by comparing peak current change value of the sensor before and after bending, defined as flexible factor Ff = (I0 - Ibent)/(I0 - Iunbent). The stability experiment of the urea-PEDOT/C-Au NTs EC sensor was conducted by recording the flexible factor Ff after repeated bending under 1.0 cm radius for 200 times. Epidermal Urea Sensing. The epidermal EC sensor evaluation was performed on three consenting healthy volunteers (2 males and 1 female between the ages of 25 and 28), with no prior medical history of heart conditions, uremia or kidney failure. The EC responses were evaluated by different subjects since urea levels varies among individuals. The current response (I0) of the eluted flexible urea-PEDOT/C-Au NTs EC sensor was recorded before the epidermal urea sensing. Subsequently, the on-body urea studies was conducted, and the concrete experiment process was that three participants had played badminton for half an hour for sweating, then flexible EC sensor was affixed to the wrist of volunteers to adsorb urea in sweat. After 15 min adsorption, the DPV response (I) of flexible EC sensor was recorded on an EC analyzer. The content of urea in sweat was indirectly assayed by calculating the ratio of the above twice peak currents, that is ∆I/I0 = (I0 - I)/I0. The sweat sample of each volunteer was tested 10 times for replicate variability. RESULTS AND DISCUSSION Characterization of Urea-PEDOT/C-Au NTs Flexible Sensor. The surface morphologies of C-Au NTs/PDMS electrode and flexible urea-PEDOT/C-Au NTs EC sensor have been characterized by SEM (Figure 2A, B). It was clearly observed that the Au NTs and CNT ultrathin-film network distributed randomly and uniformly on PDMS substrate, which ensured excellent flexibility and outstanding electrical property of wearable EC sensor. The electropolymerization time of MIP film influences the electrochemical performance of the flexible urea-PEDOT/C-Au NTs EC sensor (Figure S1), and it was determined to be 60 s after optimization. After the process of electropolymerization, the urea-PEDOT MIP film was coated on the surface of the CNTs and the Au NTs skeleton, but their wire-like morphologies still remained clearly, which just made the nanomaterial fibers appear slightly rougher. In other words, the thickness of the wrapped urea-PEDOT MIP on the C-Au NTs/PDMS electrode was at the nanoscale and had negligible effect on its network shape and flexible property. The FT-IR spectra of sensor samples were shown in Figure 2C, and these three IR curves belong to the C-Au NTs/PDMS electrode (a, black line), the flexible urea-PEDOT/C-Au NTs EC sensor after the polymerization (b, red line), and the eluted flexible urea-PEDOT/C-Au NTs EC sensor (c, blue line). It was obvious that the spectrum (Figure 2C, red line) of flexible urea-PEDOT/C-Au NTs EC sensor existed two obvious peaks at 1622 cm-1 and 1667 cm-1 assigning to NH2 deformation vibration at amide II band and C=O stretching vibration at amide



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and 55.79%, respectively. Figure 3B displayed that the flexible urea-PEDOT/C-Au NTs EC sensor exhibited good linearity in the concentration range of 1-100 mM, with the regression coefficient of 0.9962, and the detection limit was as low as 0.1 mM (S/N = 3). Control experiment based on the NIP EC sensor (PEDOT/C-Au NTs electrode without urea recognition sites) revealed that the PEDOT/C-Au NTs layer had very weak binding affinity with urea molecule and did not affect the experiment results (Figure S3). Additionally, we investigated the stability of the urea-PEDOT/C-Au NTs EC sensor as shown in Figure S4. The flexible MIP EC sensor still maintained good detection performance after one week, indicating that this flexible senor could completely meet the requirement of urea detection in human sweat.

Figure 2. A) SEM image of C-Au NTs nanocomposite on PDMS. B) SEM image of flexible urea-PEDOT/C-Au NTs EC sensor. C) IR spectra of C-Au NTs base electrode (curve a, black line), flexible urea-PEDOT/C-Au NTs EC sensor after electropolymerization (curve c, red line) and flexible ureaPEDOT/C-Au NTs EC sensor after elution (curve b, blue line), respectively. D) Typical DPV responses of urea-PEDOT/C-Au NTs sensor after electropolymerization (black line), elution (red line) and urea adsorption (blue line) in 1 mM K3[Fe(CN)6] solution, respectively.

I band, respectively. However, none of these peaks were seen in the case of C-Au NTs/PDMS electrode (Figure 2C, black line) and eluted flexible urea-PEDOT/C-Au NTs EC sensor (Figure 2C, blue line), which indicated that urea molecule could be bonded with PEDOT in the electropolymerization process to form the urea-PEDOT MIP film, and the urea template molecule could be washed off to generate imprinted recognition sites. Figure 2D demonstrated the typical DPV responses of the urea-PEDOT/C-Au NTs EC sensor after electropolymerization, elution and adsorption, respectively, When the electropolymerized flexible urea-PEDOT/C-Au NTs EC sensor was synthesized, the initial current response was recorded (Figure 2D, black line). Then, a large increased current response was obtained for eluted flexible urea-PEDOT/C-Au NTs EC sensor (Figure 2D, red line) compared with original DPV response. However, the following urea sensing of the urea-PEDOT/C-Au NTs EC sensor led to a current response decrease (Figure 2D, blue line), which was attributed to the hindrance of potassium ferricyanide transfer caused by urea rebinding on the imprinted recognition sites, which proved the feasibility of the molecularly imprinted method to construct an EC sensor. Besides, the effects of different urea adsorption time on the EC response were studied, and the result revealed that 15 min was the optimal adsorption time (Figure S2). EC Characterization of Flexible Urea-PEDOT/C-Au NTs Sensor. The urea levels in the sweat has been reported in the concentration ranging from 1.8 mM to 46 mM.42 Keeping this in view, the response of the flexible EC sensor was evaluated in the 1 mM to 100 mM urea concentration range. The EC responses of the urea-PEDOT/C-Au NTs sensor were carried out using the DPV technique in redox probe electrolyte solution as shown in Figure 3A. The current response results exhibited that the ratio of current change value for ureaPEDOT/C-Au NTs EC sensor to the initial eluted current (∆I/I0) increased as the concentration (1, 10, 20, 50, 100 mM) of urea in solution increased. And the corresponding change of peak current response was 22.84%, 26.78%, 29.76%, 40.85%

There are a variety of metabolic substances in human sweat, some of which have EC activity or are similar to urea in structure, and these coexisting substances may interfere with the actual detection of urea in sweat. Therefore, interference test of urea-PEDOT/C-Au NTs sensor was evaluated by comparing the response of the sensor to 10 mM urea with that of physiological concentration of different interferent (glucose (0.2 mM), lactate (15 mM), riboflavin (20 mM), UA (0.05 mM), NaCl (30 mM), KCl (5 mM), CaCl2 (5 mM) and NH3 (5 mM). As shown in Figure 3C, the calculated ratio of current change value for glucose, lactate, riboflavin, UA, NaCl, KCl, CaCl2 and NH3 was 0.35%, 0.12%, 0.80%, 0.17%, 0.78%, 0.0%, 0.0% and 0.52%, respectively, whereas the current change value of urea adsorption detection reached up to 27.38%. Obviously, there was no obvious current change with adding interfering substances, demonstrating the admirable selectivity of the flexible MIP EC sensor. In order to further prove the high specific recognition of the prepared sensor, the 10 mM urea standard solution was detected after the interference test. As shown in Figure 3D, the current response of this sensor to the interference (UA as an example) was rarely changed, but it decreased by 25.31% after 10 mM urea adsorption. Summarily, this flexible urea-PEDOT/C-Au NTs EC sensor has the advantage of

Figure 3. A) DPV responses of the urea-PEDOT/C-Au NTs sensor in 1 mM K3[Fe(CN)6] solution after the adsorption of different concentration of urea for 15 min. B) Calibration curve of the MIP sensor to increasing urea concentration. C) Comparison diagram of interference detection (glucose, lactate, riboflavin, UA, sodium chloride, potassium chloride, calcium chloride and ammonia). D) DPV responses of flexible urea-PEDOT/C-Au NTs EC sensor after elution (black line), interferences adsorption (red line) and 10 mM urea adsorption (blue line), respectively.


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excellent selectivity of molecular imprinting and high sensitivity of EC analysis, which can be applied for the efficient and fast urea determination in the real sweat samples and has potential application in clinical diagnosis. Flexibility and Stability of Urea-PEDOT/C-Au NTs Sensor. To test the flexibility, the urea-PEDOT/C-Au NTs EC sensor was wrapped on cylindrical objects with different radii of curvature and folded in half for 10 times (Figure 4A, inset). The radii of curvature ranged from 0.5 cm to 3.0 cm, which could almost satisfy general curvature of the skin. The flexible factor, Ff = (I0 - Ibent)/(I0 - Iunbent), was defined as an index of bending performance, and it was nearly 100% in value after being subjected to different bending stresses (Figure 4A). The result indicated that the EC performance of the flexible urea-PEDOT/CAu NTs EC sensor was basically unchanged after bending, which proved that the sensor has good flexural performance. And the photographs of the MIP EC sensor in bending and torsion state shown in Figure S5 also revealed the excellent flexible and conformal properties of the sensor. Moreover, the stability of flexible EC sensor was further verified by repeated bending under 1.0 cm radius. The bending test in Figure 4B exhibited that the values of flexible factor (Ff) for different cycles (10, 20, 50, 100, 200 times) were also nearly maintained at 100%. These results indicated the stable electronic pathway inside the films even after cyclic bending, manifesting the excellent mechanical tolerance.

Figure 4. A) The flexible factor (Ff) of urea-PEDOT/C-Au NTs EC sensor in bending test with different bending radii (0.5, 1.0, 1.5, 2.0, 2.5, 3.0 cm). The inset is digital photograph of the sensor bent in 0.5 cm and 1.5 cm radii of curvature. B) The flexible factor (Ff) of urea-PEDOT/C-Au NTs EC sensor in stability test with different bending cycle numbers (0, 10, 20, 50, 100, 200 times).

Artificial Sweat Analysis. In order to evaluate the feasibility of the flexible urea-PEDOT/C-Au NTs EC sensor for clinical application, the standard addition method was applied here to test the recovery. The eluted urea-PEDOT/C-Au NTs sensor was immersed in artificial perspiration samples, containing different concentrations of additional standard urea (0 mM, 10 mM, 20 mM, 30 mM) solution. Then, the corresponding DPV response was recorded (Figure 5A), and the current change value (∆I/I0) of urea at four different concentrations in artificial perspiration samples was 28.61%, 31.94%, 35.23% and 38.64%, respectively. Figure 5B displayed that the current change ratio increased as the concentration of the added urea increased, indicating that this flexible urea-PEDOT/C-Au NTs EC sensor could effectively detect the urea levels in artificial sweat. With different additions of urea to the artificial perspiration samples, the calculated recovery ranged from 99.97% to 100.57% (Table S1), which demonstrated that a reliable urea MIP sensor has been acquired for urea determination in artificial sweat samples. Epidermal Evaluation. To further verify the reliability of the electrodes to detect urea in sweat, we performed an EC test with the actual human sweat. As described before, the peak current response of the freshly eluted flexible PEDOT/C-Au NTs EC sensor was firstly recorded with a peak current I0. In the epidermal test, the flexible EC sensor was attached to the

Figure 5. A) SEM image of C-Au NTs nanocomposite on PDMS. B) SEM image of flexible urea-PEDOT/C-Au NTs EC sensor. C) IR spectra of C-Au NTs base electrode (curve a, black line), flexible urea-PEDOT/C-Au NTs EC sensor after electropolymerization (curve c, red line) and flexible ureaPEDOT/C-Au NTs EC sensor after elution (curve b, blue line), respectively. D) Typical DPV responses of urea-PEDOT/C-Au NTs sensor after electropolymerization (black line), elution (red line) and urea adsorption (blue line) in 1 mM K3[Fe(CN)6] solution, respectively.

surface of volunteers’ wrist for urea absorption in sweat as shown in Figure 5C and Figure S6 , and then the current response of potassium ferricyanide was determined again with a peak current I. The current change value (∆I/I0) was corresponded to the amount of urea, thus achieving an assessment and analysis of urea content in sweat. Figure 5D exhibited that the experimental results of three participants after playing badminton for half an hour. The value of average current change were 27.99%, 27.34% and 33.26%, and the corresponding average urea concentration were 14.41 mM, 12.46 mM and 30.33 mM, respectively. Although there are individual differences, the detected urea levels were within the normal range. These results indicate that the molecularly imprinted conformal EC sensor can be used in the analysis of urea content in human sweat, which is expected to be applied in clinical analysis. CONCLUSIONS In summary, we developed a flexible and conformal urea sensor by electropolymerizing urea-PEDOT MIP on the CNTs and Au NTs hybrid nano-structure. The percolation network of C-Au NTs provided excellent EC sensing performance, mechanical compliance and stability, and MIP layer on the surface of nanotubes endowed this flexible senor with high specific recognition for urea template molecule. This allowed successful analysis of urea level both in vitro solution and human sweat with satisfactory sensitivity and selectivity, providing an efficient and promising approach for non-invasive monitoring of urea levels in human body fluid. Importantly, the presented strategy of constructing flexible wearable EC sensor can be applied to the epidermal non-invasive detection of other components in human sweat, which broaden the way for flexible sensors in the applications of physiological monitoring, health care and clinical diagnosis.

ASSOCIATED CONTENT Supporting Information



The Supporting Information is available free of charge on the ACS Publications website. SEM images and EC responses of the flexible ureaPEDOT/C-Au NTs EC sensor with different electropolymerization time (Figure S1). The effect of urea adsorption time on EC response (Figure S2). Typical DPV responses of NIP flexible EC sensor as a control (Figure S3). Stability of the flexible urea-PEDOT/C-Au NTs EC sensor for a period of time interval (Figure S4). Photographs of the flexible ureaPEDOT/C-Au NTs EC sensor with mechanical deformation and attached on wrist for perspiration adsorption and detection (Figure S5, S6). Recovery test of the flexible urea-PEDOT/CAu NTs EC sensor for the determination of urea in artificial sweat serum (Table S1).

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. Y.-L. L. and R. L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 21725504, 21675121, and 21721005), the Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201706), and the China Postdoctoral Science Foundation (Grant 2017M620329, 2018T110790).

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Flexible Electrochemical Urea Sensor Based on Surface Molecularly

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