Aqueous and Nonaqueous Electrochemical Sensing on Whole-Teflon

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Aqueous and non-aqueous electrochemical sensing on whole-Teflon chip Bo Shen, and Hongkai Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.5b00124 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on December 25, 2015

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Aqueous and non-aqueous electrochemical sensing on whole-Teflon chip Bo Shen1, Hongkai Wu*, 1, 2 1

Department of chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 2

Division of Biomedical Engineering , The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ABSTRACT: Whole-Teflon microfluidic chips integrated with gold microelectrodes were successfully fabricated for the first time. The surface morphology of electroless plated gold electrode was characterized with optical microscopy and scanning electron microscopy, whilst its electrochemical performances were investigated using cyclic voltammetry in both aqueous and non-aqueous solutions. The robustness and stability of this whole-Teflon chip were demonstrated by performing long-term hydrodynamic cyclic voltammetry in dichloromethane (DCM). Quantitative detection of 4-chloroaniline in DCM using square wave voltammetry was conducted to corroborate the potential application of this device for environmental pollutant sensing. Microfluidic devices coupled with various detection techniques, including optical spectroscopy,1,2 mass spectrometry3,4 and electrochemistry,5,6 have been proved to be powerful and promising platforms for chemical sensing and biomedical researches. Among them, the on-chip electrochemical methodologies have attracted much attention because they can offer a number of advantages over other sensing schemes, such as low cost,7 portability,8 fast response time and high sensitivity.9 Typically, on-chip electrochemical sensing appears in two configurations. The first one uses micro fabricated electrodes, which are integrated onto the chips, either in a closed microchannel system10 or on an open array platform,11 thus obviating the tedious work of electrode alignment. The second one employs external electrodes as interchangeable components onto the chips,12 this can avoid electrodes fouling by replacing with new electrodes. Benefiting from the suitable size and minimum apparatus requirements, on-chip electrochemical sensing can also be coupled to a variety of sample pre-treatment procedures, such as electrophoresis,13 polymerase chain reaction14 and enzymelinked immunosorbent assay.15 Nevertheless, all these applications are performed in aqueous solution, whereas there are limited reports describing sensing directly in non-aqueous solvents.16,17 Electrochemistry performed in non-aqueous media allows for investigation of electrochemical properties of substances that are insoluble or unstable in water under unusual experimental conditions (e.g. lower temperature than water freezing point, wider electrochemical window), thus promoting research for exploring chemical reactions that are otherwise difficult or impossible to occur in aqueous solution.18 For example, Hu et al. adopted ionic liquid’s excellent extraction and dissolution capacity to achieve in situ ultratrace detection of 2.4-dinitrotoulene solids.19 However, commonly adopted materials for fabricating microfluidic chips suffer from either poor resistance to organic solvent (such as

polydimethylsiloxane and polystyrene) or high inconvenience due to complicated fabrication processes (such as glass and silicon). These problems greatly impede the development and adaptability of on-chip electrochemical sensing in nonaqueous solution. Therefore, seeking for an alternative chip material that can both be processed conveniently and withstand harsh environments is highly desirable. To address the aforementioned issues, we choose Teflon (registered by DuPont) as the device material. Teflon, including polytetrafluoroethylene (PTFE), polyfluoroalkoxy (PFA) and fluorinated ethylene propylene (FEP), is a class of synthetic fluoropolymers that are well known for their superior chemical inertness and robust mechanical strength, making them popular choices in chemical engineering, biomedical science and aerospace industry. Recently, we developed a novel technique to engineer and construct whole-Teflon chips.20 These chips have already been exploited extensively in diverse fields, such as material patterning,21 petrochemical applications22 and automated peptide synthesizer.23 However, there have been no reports that can perform electrochemical analysis on the whole-Teflon chips because of the extreme difficulty of patterning electrodes in the chips. In this paper, we introduce a novel and convenient technique, polydopamine-assisted electroless plating, to integrate gold electrodes onto whole-Teflon microchips. The surface topography of the plated gold electrode was characterized with optical microscopy and scanning electron microscopy (SEM), which revealed its integrity even after high-temperature bonding of whole-Teflon chips. Static cyclic voltammetry (CV) conducted in both aqueous and acetonitrile solutions showed excellent performance of the microchips. Moreover, long-term hydrodynamic CV in organic solvents such as dichloromethane (DCM) caused no damage to the chip or the gold electrodes, demonstrating the excellent chemical compatibility of the electrochemical microchips. In addition,

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Figure 1. Process flows for the fabrication of whole-Teflon chips integrated with gold electrodes. (a) Electrode layouts are patterned by photolithography with a film mask. (b) After developing, the Teflon plate was dip-coating with dopamine solution twice. (c) The remaining photoresist was lifted off with acetone to create PDA-coated electrode patterns. (d) The Teflon plate was covered with a gold plating solution on a hotplate. (e) The whole-Teflon chip was bonded in an oven. open environment. After modification, the Teflon plate was we used 4-chloroaniline, a key chemical intermediate in immersed in acetone solution with gentle agitation to lift off pesticide manufacture, as our model to test the analytical the remaining photoresist, thus leaving the electrode regions performance of our electrochemical microchip for watercoated with PDA. To fabricate gold electrodes, we adopted a insoluble organic compounds. Our results indicate that the previous protocol24 with slight modification: the Teflon plate detection limits for 4-chloroaniline in DCM is 4 µM under static condition and 2 µM under hydrodynamic condition, was covered with chloroauric acid solution (1% w/w, 200 respectively. It is our belief that this electrode integrated mg/mL KHCO3 and 20 mg/mL glucose) and placed on a whole-Teflon chip will be particularly suitable for digital hotplate (MSH-20D, WiseStir, Germany) at 36 °C for implementing research in the fields of electro-synthesis, 30 min. This plating process was repeated two more times. For chemical detection and environmental monitoring. sputter deposition, Teflon surface was first modified with hydrogen plasma (Plasmalab 80 Plus, Oxford) for 5 min, then Experiment section Ti (5 nm) and Au (100 nm) were sequentially sputtered Materials and Reagents. We purchased PDMS prepolymer (Explorer 14, Denton Vacuum). (RTV 615A and 615B) from GE Silicones; Teflon FEP plates Bonding of whole-Teflon chip. The channels on Teflon from Yuyisong, Inc (Shanghai, China); positive photoresist plate were formed by thermal embossing following our (AZ 4620) from Clariant Corp; potassium ferrocyanide previous work on a hot compressor (TM-101F, Taiming, (K4Fe(CN)6) and potassium ferricyanide (K3Fe(CN)6) from Inc).20 Before bonding, a Teflon plate with gold electrodes TianJin Chemical Factory (TianJin, China); dopamine was thermal annealed in an oven (KSW5-12-A, Zhonghuan hydrochloride, Gold(III) chloride hydrate, potassium chloride Experiment Electric Stove) at 120 °C for 1 h. The chip (KCl), potassium bicarbonate, D-glucose, ferrocene and 4bonding was accomplished by assembling the two plates (one chloroaniline (pestanal®, analytical standard) from Sigmawith channel design and the other one with gold electrodes) Aldrich; tertabutylammonium hexafluorophosphate (NBu4PF6, together using screw clamps and heating in an oven at 257 °C 99% purity) from J&K scientific (Shanghai, China); (~ 1 °C variation) for 1 h with a slow temperature ramping Rhodamine B from Alfa Aesar. The organic solvents rate (~ 0.5 °C/min). The gold electrode maintained its (acetonitrile and dichloromethane) were purified by conductivity after the above high-temperature bonding Innovative® solvent purification system. All other chemicals process. The bonded whole-Teflon chip was connected to were used without further purification. Teflon tubes via a custom-made chip holder. The gold Fabrication of gold electrode on Teflon plate. The electrode pads were electrically connected to an process flows for fabricating gold electrodes on Teflon electrochemical station through copper wires with high purity substrate and constructing whole-Teflon chip are illustrated in silver paint (SPI Supplies) and epoxy glue (Pattex®, Henkel). Figure 1. Teflon plate was sequentially cleaned with acetone Instrument and Measurement. All the electrochemical and water in an ultrasonic bath, and dried by nitrogen blow. experiments were conducted with a multi-channel The electrodes layout were patterned by printed film electrochemical analyzer (CHI 1000B, CH Instruments, photomask with traditional photolithography using positive Shanghai, China) in a three-electrode configuration at room photoresist. The surface that was not covered with photoresist temperature. When performing CV in aqueous solution, we was further coated with polydopamine (PDA) adhesion layer prepared equimolar solution of K3Fe(CN)6/K4Fe(CN)6 (5 mM) by dip-coating into dopamine solution (2 mg/mL, 10 mM redox couple in water with KCl (0.1 M) as electrolyte, both bicine buffer, pH=8.5, 12 h) twice at room temperature in an working electrode (WE) and counter electrode (CE) were gold

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electrodes, and a commercial silver/silver chloride electrode (CH Instruments, Shanghai, China) was inserted into the outlet of the channel (attached with a PDMS well as reservior) to serve as reference electrode (RE) for potential calibration. When conducting CV in organic solvents, ferrocence (5 mM) and NBu4PF6 (100 mM) were added into acetonitrile or dichloromethane as electrochemical indicator and supporting electrolyte, respectively. Both organic solvents were dehydrated and degassed by nitrogen prior to use, the gold electrode here served as pseudo-reference electrode. The electrodes’ sizes in quiescent-state CV were designed as follows: both WE and CE width were 1.0 mm, and RE width was 2.0 mm; all the electrodes were placed perpendicularly to channel, so the electrode length was the same as the channel width (1.0 mm). When conducting hydrodynamic electrochemical sensing and pollutant detection, the CE (0.5 mm width) was placed at the downstream of WE (0.1 mm width) and RE (0.5 mm width) to avoid the contamination of WE. We used a gas-tight syringe (SGE, Sigma) to deliver the solution into microfluidic channel (0.2 mm width, 70 µm height) where solution flow rate ranged from 2.0 µL/min to 20 µL/min using a syringe pump (PicoPlus, Harvard Apparatus). The optimized square wave voltammetry (SWV) parameters were set as follows: amplitude voltage: 45 mV; scan increment: 8 mV; frequency: 25 Hz; initial voltage: 0.4 V and final voltage: 1.1 V. The hydrodynamic SWV was performed at a fixed flow rate of 4.0 µL/min. Before each measurement, we cleaned the working electrodes by applying -0.8V (vs.

gold electrode) for 1 min and flushing with detection solution for 2 min. SEM images were taken with a field emission scanning electron microscopy (JSM-6700F, JEOL). Bright field pictures were obtained with either a digital camera (PowerShot G9, Canon) or a stereoscope (Leica EC3, Leica Microsystems Ltd), fluorescent images were obtained via a microscope (AZ100, Nikon) equipped with a cooled chargedcouple device camera (DS-Fi1, Nikon) with a red filter (excitation: 540-580 nm; emission: 600-640 nm). Results and discussion Fabrication of whole-Teflon electrochemical chips. FEP is chosen as our chip material because of its two characteristics: it is optically transparent, which makes it feasible for observation during experiments; and it has a relatively lower melting point (260 °C) compared with that of PFA (315 °C), which facilitates the final chip-bonding step. Although Teflon shows excellent chemical resistance towards a wide range of solvents, its non-reactive and non-sticky surface also prevents the adhesion between metal electrode and Teflon substrate. Generally, two approaches, wet chemical etch (sodium in liquid ammonia, FluoroEtch®) and high energy treatment (plasma, irradiation and corona), are employed to activate Teflon surface and enhance its surface adhesion ability.25,26 However, these methods are either dangerous to process or requiring bulky and expensive apparatuses, which greatly hinder their utilization in common laboratories. Instead, we used mussel-inspired polydopamine (PDA) surface functionalization strategy, which was pioneered and developed by Lee, et al in recent years,27,28 to modify the chemical and physical properties of Teflon surface. This method is very convenient to carry out, and only involves single dip-coating step where self-polymerization of dopamine takes place under mild conditions. During this coating process, a thin layer of PDA film will be generated. This PDA film contains a wealth of catechol and amine groups,

Figure 2. Characterization of gold electrode. (A) Actual picture of the assembled chip that is integrated with three gold electrodes. No cracks were found on the electrodes surfaces. (B) The enlarged view of gold electrodes on a whole-Teflon chip, the electrodes remained intact after high temperature thermal bonding process without any fractures. (C) SEM image of the electroless plated gold electrode after high temperature heating. The inset image is the enlarged view of the electrode surface. which can serve both as reducing agents for nanoparticle generation29 and stabilizers for nanoparticle anchoring.30 Besides, the strong adhesion between PDA film and underlying substrate allows for adhering of electrodes tightly even under harsh conditions (such as strong acidic solutions and various organic solvents31). Here, we combined PDA coating with electroless plating, a widely adopted method in printed circuit boards and surface coating industries, to fabricate gold electrodes that can firmly adhere to the Teflon surface. This electroless plating method has the advantages of simplicity and low cost compared with conventional high-vacuum film deposition technology. During the deposition, Teflon substrate with electrode patterns was covered by gold plating solution on a hotplate. The surface colour of polydopamine would gradually turn from black to gold, which indicated the formation of gold film. In order to improve gold electrodes stability, we repeated this plating process two more times to obtain larger electrode thickness. One more step of annealing Teflon substrate is necessary for enhancing PDA adhesion layer stability, which is attributed from additional crosslinking and magnification of the supramolecular interactions in polydopamine structures.32 To seal the Teflon microchannels, we carried out normal thermal bonding in a programmable oven. In this step, both final bonding temperature and temperature ramping rate need to be carefully optimized to yield leak-free chips while at the same time maintaining the integrity and function of electrodes. In the method previously developed by our group,20 we utilized the bonding temperature of 260 °C, but this temperature would generate broken electrodes that are unable to work normally. Therefore, we lowered the temperature to 257 °C, and found that gold electrodes could keep intact (Figures 2A and 2B) while assuring that there was no leakage across the whole channel boundary and along the electrode area (we loaded 1 mM Rhodamine B in acetone solution to verify no leakage has happened as shown in Figure S1). This decreased bonding temperature proved to be crucial for the final fabrication

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Figure 3. Electrochemical characterization of electrode performance on a whole-Teflon chip in a static aqueous solution. (A) Cyclic voltammograms of Fe(CN)63-/ Fe(CN)64couple at various scan rates. (B) The plot of cathodic (positive) and anodic (negative) peak currents vs. the square root of scan rate. process because thermal expansion of FEP increases substantially when approaching its melting point and even small temperature variation can result in big expansion difference. In addition to lowering bonding temperature, successful bonding is also attributed from the following two aspects: gentle and even pressures that are applied from screw clamps (for fastening the chip assembly); and slow temperature ramping rate that avoids the rapid expansion of chip material (considering that the thermal expansion of coefficient of gold is much smaller than that of FEP). We can achieve approximately 80 % of chip fabrication success rate (over 30 chips). The surface morphology of plated gold electrode is further characterized with SEM as revealed in Figure 2C. The electrode surface is composed of gold nanoparticle (ca. 100 nm diameter) coherently without any cracks, such nanostructure may provide improved electrochemical signal because of the increased electrode surface area. Besides, this nanoparticle morphology may also be helpful in releasing the mechanical stress during temperature changes, thus no cracks would form in the gold electrode. In comparison, we also tried to form gold electrodes by sputtering deposition on hydrogen plasma modified Teflon surface. However, these electrodes cannot withstand high temperature bonding step, which generated large amounts of

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Figure 4. Electrochemical characterization of electrode performance on a whole-Teflon chip in a static acetonitrile solution. (A) Cyclic voltammograms of ferrocene solution at various scan rates. (B) The plot of cathodic (positive) and anodic (negative) peak currents vs. the square root of scan rate. cracks (Figure S2). Therefore, we expect that this polydopamine-assisted electroless plating protocol can also be applied to integrate micro electrodes into other thermoplastic devices that need high temperature process. Electrochemical characterization of whole-Teflon electrochemical chips. We characterized chip’s electrochemical performance by conducting cyclic voltammetry in both water and organic solvents. First, we tested the chip in water solution. The representative CV results are shown in Figure 3A: the voltammograms recorded at different scan rates all display symmetric shapes; the average ratio of anodic and cathodic peak currents (ipa/ipc) is equal to 1.01; and the average difference in potential between the peaks of oxidation (Epa) and reduction (Epa) curves is measured to be 77.1 mV. All these results demonstrate that the chemical transformation between ferricyanide and ferrocyanide in aqueous solution is reversible and fast, and no other side reactions take place on the planar gold electrode

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over the course of scanning. Figure 3B shows that both the anodic and cathodic

Figure 5. Electrochemical characterization of electrode performance on the whole-Teflon chip in the constant-flowing DCM solution. (A) Cyclic voltammograms of ferrocene solution under various flow rates at a fixed scan rate of 0.05 V/s. (B) The plot of measured anodic steady-state limiting currents ( ▲ ) and the corresponding calculated currents according to the Levich equation (●) vs. the cubic root of volumetric flow rate (R2= 0.995). peak currents are linearly proportional to the square root of scan rate (with R2= 0.998 and 0.999, respectively), which fits to the Randles-Sevcik equation.33 This linearity proves that the mass transfer of electro-active species on the electrode surface of is a diffusion controlled process. To evaluate chip performance in non-aqueous media, we selected acetonitrile as the solvent because many studies have already been carried out in this media for electrochemical mechanism studies. Figure 4A displays the characteristic CV results at scan rates of 0.05 V/s to 0.5 V/s. The

voltammograms exhibit symmetric shapes and the average separation of Epa and Epc is 66.5 mv, indicating reversible electrochemical reaction of Fc/Fc+ redox couple in acetonitrile solution. Figure 4B shows the current peaks are directly proportional to square root of scan rate, demonstrating that the above electrochemical process in acetonitrile is governed by diffusion. Additionally, we verified the electrodes’ stability by comparing the consecutive cyclic voltammograms of the 1st cycle and the 100th cycle in water (Figure S3A) and acetonitrile (Figure S3B), respectively. The results show that the electrodes remain intact without any damage or delaminating problems, while their working capabilities keep almost the same with little variation of peak current values (within 3.9% for aqueous solution and 1.7% for acetonitrile solution). Hydrodynamic electrochemical characterization. Hydrodynamic technique is one of the frequently used electrochemical methods in which electrolyte solution moves with respect to electrode. This technique can offer several advantages over corresponding electrochemical measurements performed in stationary solutions: improved signal outputs which are resulted from increased mass transfer of reactants on electrode surface;9 enhanced reproducibility by reducing the interference of convection caused by natural occurred solution motion;34 and controllable electrochemical reaction which is realized through changing the solution volumetric flow rate.33 Microfluidic technologies are well known for their capability of manipulating the liquids at the micro-scale. In this case, we utilized this technology to carry out hydrodynamic voltammetry with the channel microband electrodes configuration, which was introduced by Compton et al.35 These electrodes are positioned perpendicularly to the microchannels where the solutions are constantly flowing with well-defined laminar flow characteristics. Figure 5A exhibits the results of hydrodynamic cyclic voltammetry on wholeTeflon chip at a fixed scan rate of 0.05 V/s: voltammograms at different volumetric flow rates ranging from 2 µL/min to 20 µL/min have typical sigmoidal shapes, and the steady-state limiting currents increase with larger flow rates. We further studied the dependence of steady-state anodic limiting currents on cubic root of volumetric flow rate (Figure 5B). They possess a linear relation with correlation coefficient of 0.995, which is well fitted to the value predicted by the Levich equation.36 Here, the diffusion coefficient value of ferrocene in DCM is referred from previous work.37 The real working electrode’s sizes were measured to be 0.11 mm (width) and 0.19 mm (length), the channel height was found to be 68 µm. It should be noted that in this experiment we employed dichloromethane as the working solvent to test the resistance of this gold electrode integrated whole-Teflon chip. During the process of hydrodynamic CV, no damage of the chip has been observed. Compared with the chip made from cyclo olefin polymer16 that is unable to withstand chlorinated organic solvents, our device shows superior chemical inertness. The electroless plated gold electrode worked well at anodic potential of ~1.5 V. Additionally, bonding strength was verified by the fact that no leakage occurred with the perfusion rate of 20 µL/min (it can possibly work for larger flow rates, which were not tested here). Detection of 4-chloroaniline in DCM. We demonstrated the utility of this gold electrode integrated whole-Teflon chip for electrochemical detection of 4-chloroaniline directly in organic solvent. 4-chloroaniline is used on a large scale in chemical industry as a building block for manufacturing of pesticides, drugs and pigments. It has been widely present and

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accumulated in underground water and agricultural soils.38 Studies show that this chemical poses long-term toxicity and carcinogenicity, and it also tends to adsorb to several laboratory plastics (such as silicones, rubber and polyvinyl chloride).39 Hence, developing a suitable detection strategy with appropriate material is highly demanded. Apart from spectrophotometric based methods,40,41 electrochemical detection is also a popular option for sensing aniline and its

derivatives.42,43 The motivation for performing detection directly in organic solvent is that it can provide a straightforward means to measure some organic compounds that are unstable or hard to dissolve in water solution. More Figure 6. Electrochemical detection of 4-chloroaniline in DCM on the whole-Teflon chip. (A) Typical SWV of different concentration of 4-chloroaniline in the constant-flowing solution with a perfusion rate of 4 µL/min. The SWV parameters were set as follows: frequency: 25 Hz; amplitude voltage: 50 mV; scan increment: 8 mV; initial voltage: 0.4 V and final voltage: 1.1 V. (B) Calibration curve for concentration of 4-chloroaniline vs. anodic SWV peak currents under static condition ( ● ) and hydrodynamic condition ( ▲ ), respectively. The error bars are standard deviations of replicate experiments performed in three independently fabricated devices.

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importantly, it can be coupled to liquid-liquid extraction (usually from aqueous phase to organic phase) based sample pre-treatment process to improve detection selectivity and sensitivity, thus offering a lot of convenience and avoiding the losses or contamination of sample. Herein, we take advantage of Teflon’s superior resistance against organic solvents and versatility of microfluidic chips to carry out electrochemical sensing in DCM, a commonly used organic extractant. Square wave voltammetry (SWV) is selected as the electrochemical technique because it has the advantages of reduced scanning time and improved resolution compared with other voltammetry approach. The sensing scheme is based on the electro-oxidation of 4-chloroaniline.44 During the measurements, we encountered the problem of electrode fouling due to the formation of polymeric byproducts, which led to decreased peak currents, especially in solution with high concentration of chloroaniline (Figure S4A). Fortunately, this problem can be solved by cleaning the electrode surface in two steps: apply a negative potential (-0.8 V versus pseudo gold reference electrode) on the working electrode42 for one minute, and then flush the channel with fresh solution to strip off the absorbed products. Thus, repeatable SWV results can be obtained (intra-device variation of 2%, Figure S4B). When carrying out electrochemical detection, we first performed the experiments in static solutions. Figure S5 shows the experimental results of voltammograms with different concentrations of 4-chloroaniline. The current responses exhibit well-defined peak shapes, and the anodic peak currents increase proportionally with 4-chloroaniline concentration. By plotting the anodic peak currents versus 4-chloroaniline concentration, we obtained a calibration curve with a linear relation (slope= 0.454 nA/µM, intercept= -0.678, R2= 0.979, Figure 6B) from 0 to 100 µM, which yields limit of detection (LOD) of ca. 4 µM (S/N=3). This LOD is comparable to the results that are obtained in aqueous solution,43 but no additives are needed in our work. The shift of anodic peak might be resulted from the adsorption of analyte compound, which altered the reference electrode potential. Furthermore, we conducted hydrodynamic SWV at a fixed flow rate of 4 µL/min (Figure 6A), the calibration curve of the steady-state peak currents vs. analyte concentration gives a LOD of ca. 2 µM (slope= 0.699 nA/µM, intercept= -0.582, R2= 0.988, 6 % inter-device variation, Figure 6B). This enhancement of sensitivity is resulted from the decrease of diffusion layer near electrode surface under laminar flow condition, which is well explained in a previous report.45 Besides, detection in constant-flowing solution tends to be less prone to fouling electrodes because of the smaller residence time of the reactants at the electrode surface. Conclusions In summary, we successfully integrated gold electrodes onto whole-Teflon chips and characterized their electrochemical performances in both aqueous and organic solutions. The robustness and utility of this platform were proved by carrying out CV under hydrodynamic condition at various flow rates; the results can be well predicted by the Levich equation. As a proof of concept, this whole-Teflon chip was used for quantifying 4-chloroaniline concentration in DCM under both static and constant-flowing conditions. This whole-Teflon chip combines the merits of excellent performance of Teflon material, versatility of microfluidic techniques and applicability of electrochemical methodologies together, making it an ideal platform for proceeding electrochemical

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research in non-aqueous solvents. The above results indicate that our chip has the potential to be coupled to other sample preparation procedure (e.g., liquid-liquid extraction for analyte pre-concentration) for facilitating sensing trace level of chemicals. The relevant research work is being investigated in our group.

ASSOCIATED CONTENT Supporting Information Fluorescent images of Rhodamine solution in Teflon channel; figures of broken electrodes; figures of long-term cyclic voltagramms; figures of gold electrodes cleaning; figures of SWV results in static solution. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

two-electrode system integrated in a glass/PDMS microchip. Lab Chip 2014, 14, 2800-2805. (10) Zhou, J.; Ren, K.; Zheng, Y.; Su, J.; Zhao, Y.; Ryan, D.; Wu, H.Fabrication of a microfluidic Ag/AgCl reference electrode and its application for portable and disposable electrochemical microchips. Electrophoresis 2010, 31, 30833089. (11) Wang, J.; Trouillon, R.; Dunevall, J.; Ewing, A. G.Spatial Resolution of Single-Cell Exocytosis by Microwell-Based Individually Addressable Thin Film Ultramicroelectrode Arrays. Anal. Chem. 2014, 86, 4515-4520. (12) Bishop, G. W.; Satterwhite, J. E.; Bhakta, S.; Kadimisetty, K.; Gillette, K. M.; Chen, E.; Rusling, J. F.3DPrinted Fluidic Devices for Nanoparticle Preparation and Flow-Injection Amperometry Using Integrated Prussian Blue Nanoparticle-Modified Electrodes. Anal. Chem. 2015, 87, 5437-5443.

Corresponding Author * (Hongkai Wu) Tel: +852 23587246. Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the grants from Hong Kong RGC (CUHK4/CRF/12G and 604712).

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