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Carbon nanotube thread electrochemical cell: Detection of heavy metals Daoli Zhao, David Siebold, Noe T. Alvarez, Vesselin N. Shanov, and William R. Heineman Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04724 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Analytical Chemistry

Carbon nanotube thread electrochemical cell: Detection of heavy metals Daoli Zhao,† David Siebold,‡ Noe T. Alvarez,‡ Vesselin N. Shanov,‡ and William R. Heineman†,*

†

Department of Chemistry, University of Cincinnati, Cincinnati, Ohio, 45221-0172, United States

‡

Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221-0072, United States

Corresponding author: Email: [email protected]. Phone: 01-513-556-9210. Fax: 01-513-556-9239.

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ABSTRACT:

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In this work, all three electrodes in an electrochemical cell were

fabricated based on carbon nanotube (CNT) thread. CNT thread partially insulated with a thin polystyrene coating to define the microelectrode area was used as the working electrode; bare CNT thread was used as the auxiliary electrode; and a micro quasireference electrode was fabricated by electroplating CNT thread with Ag and then anodizing it in chloride solution to form a layer of AgCl. The AgǀAgCl coated CNT thread electrode provided a stable potential comparable to the conventional liquidjunction type AgǀAgCl reference electrode. The CNT thread auxiliary electrode provided a stable current, which is comparable to a Pt wire auxiliary electrode. This all-CNT thread three electrode cell has been evaluated as a microsensor for the simultaneous determination of trace levels of heavy metal ions by anodic stripping voltammetry (ASV). Hg2+, Cu2+ and Pb2+ were used as a representative system for this study. The calculated detection limits (based on the 3σ method) with a 120 s deposition time are 1.05 nM, 0.53 nM and 0.57 nM for Hg2+, Cu2+ and Pb2+, respectively. These electrodes significantly reduce the dimension of the conventional three electrode electrochemical cell to the microscale.


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INTRODUCTION Carbon nanotubes (CNTs) have attracted much attention due to their outstanding features, such as fast electron transfer rate, high surface area, excellent electrical and thermal conductivities, and corrosion resistance.1-8 Highly ordered CNT arrays take advantage of the bulk properties of CNTs. For example, CNT electrode arrays show enhanced mass transport and decreased influence from solution resistance, which make them an excellent electrochemical transducer for various applications.9,10 The aligned CNT electrode has high mass sensitivity, decreased influence from solution resistance, and high signal-to-noise ratio, leading to much lower background current and lower detection limits compared to randomly ordered CNTs as the electrode.4,9,11-18 The edgeplane-like nanotubes ends are known to be responsible for fast heterogeneous electron transfer rate for redox couples that undergo outer-sphere electron transfer such as Fe(CN)64- /Fe(CN)63-, giving

enhanced electrochemical response.10,15,19 We have

explored a number of CNT electrode structures for the purpose of determining heavy metals by stripping voltammetry and obtained low limits of detection.20-24 CNT threads, which are spun from shorter CNTs, are especially interesting because they possess the advantages of CNTs, but avoid the toxicity issues of individual CNTs. Aligning nanotubes in fibers is an effective way to exploit the anisotropic properties of individual CNTs for micro/macro scale uses such as electrodes for ASV. CNT threads have the advantages of ease of handling, longer length and smaller diameter compared to CNT towers and arrays, high electrical conductivity, and good tensile strength.20 CNT



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threads also have a high surface area and retain the electrocatalytic properties of CNTs, making them potentially useful for the development of electrochemical sensors. In a previous paper we demonstrated the ability to do ASV of heavy metals at CNT thread working electrodes, achieving a detection limit of 1.4 nM with zinc as a representative heavy metal.22 We then expanded our ASV research with CNT thread electrodes to include the simultaneous determination of multiple metals as demonstrated for Cu2+, Pb2+, Cd2+ and Zn2+ with detection limits also in the low nM range.20 We also observed that CNT thread is potentially suitable for applications that require inexpensive, easily fabricated, disposable electrochemical cells because it is easily made with the appropriate equipment. CNT thread also lends itself to miniaturization below the ca. 35 µm diameter threads that we used since it can be easily made in much smaller diameters. The suitability of CNT thread to ASV coupled with its good physical properties for making electrodes caused us to continue our characterization of this especially interesting material for the long-term goals of a disposable cell which can also be easily miniaturized. Here we report the first use of a three electrode electrochemical cell for electroanalysis where all three electrodes are based on CNT thread. The goal was to make an electrochemical cell in which all electrodes are based on the same material, which can be advantageous in fabrication, and to take advantage of CNT thread as a means of miniaturizing the cell. The dimensions of the three electrodes (working, reference, and auxiliary) were in the micrometer range for this initial work, however, the dimensions of the electrochemical cell could be significantly reduced by using CNT thread with a smaller diameter. The general characteristics of the working electrode and the complete electrochemical cell were evaluated by cyclic voltammetry of ferricyanide. The 4 ACS Paragon Plus Environment

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performance of the AgǀAgCl coated CNT thread quasi-reference electrode was evaluated and compared with a conventional reference electrode with a filling solution of fixed chloride concentration. The performance of a traditional electrochemical cell with all three electrodes based on CNT thread was then evaluated for the detection of heavy metals ions by ASV using a mixture of Hg2+ (for the first time on CNT thread) and Cu2+ and Pb2+ (which we reported in our previous paper)20 as representative metals.

EXPERIMENTAL SECTION Materials and Instrumentation. All chemicals Hg(NO3)2·H2O (≥ 99.99% ) Pb(NO3)2 (≥99.99%), Cu(NO3)2·2.5H2O (≥ 99.99%) and Cu wire (99.9%, diameter 0.25 mm) were from Sigma-Aldrich, Na2OAc·H2O (≥ 99.0%) and CH3COOH ( ≥ 99.0%) were from Fisher Scientific. All chemicals were used as received without further purification. Silver conductive epoxy and 5-min epoxy were from MG Chemicals and Loctite, respectively. Deionized water was used for preparing all solutions (Nanopure water purification system). Stock solutions of Hg2+, Pb2+ and Cu2+ were prepared by dissolving the appropriate amounts of their salts in deionized water and then diluting to various concentrations in 0.1 M acetate buffer pH 4.5 to serve as working solutions. CNT thread was prepared as previously described.22,25,26 Electrochemistry was done with a single-compartment three electrode glass cell containing 15 mL of solution. Bare CNT thread and AgǀAgCl coated CNT thread were used as the auxiliary electrode and reference electrode, respectively. For comparison, platinum wire and AgǀAgCl (filled with 3 M KCl) were used as auxiliary electrode and reference electrode, respectively. Supporting electrolyte was 0.1 M acetate buffer (pH 5 ACS Paragon Plus Environment


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4.5). The stripping voltammetry and cyclic voltammetry were done with a BASi 100B Electrochemical Analyzer from BASi. Scanning electron micrographs (SEMs) were obtained on an FEI XL30 ESEM (Philips) operated at an accelerating voltage of 15 and 10 kV. Raman spectra were acquired with a Renishaw inVia Raman microscope system, excited by a 514 nm Ar-ion laser. The diameter of the area analyzed by the microRaman is about 0.75-1 µm (or an area of about 0.42 to 0.79 µm2). The optical images were obtained on the digital microscope (Keyence, VHX-2000). Working electrode preparation. The procedure for preparing electrodes from the CNT thread is similar to our earlier report.20,21 A CNT thread was connected to a copper wire using silver conductive epoxy. Then, the CNT thread was totally coated with polystyrene solution (15 wt% in toluene) and air-dried at 50 °C. Following, the CNT thread was aspirated into a glass capillary and the end of glass capillary was sealed with a hot glue gun. Last, the polystyrene coated CNT thread electrode was cut off at the end with a sharp blade. In this way, only the end of the CNT thread was exposed to the solution. AgǀAgCl coated CNT thread quasi-reference electrode. A bare CNT thread electrode connected to a copper wire by silver conductive epoxy and sealed in the glass capillary tube was plated with Ag from 15 mL of 0.3 M AgNO3 in 1 M NH3 solution in a conventional three-electrode cell at room temperature where platinum wire and AgǀAgCl (filled with 3 M KCl) were used as auxiliary electrode and reference electrode, respectively and the CNT thread electrode to be plated was the working electrode. First, an oxidative pretreatment at 600 mV was applied for 30 s. Second, the plating was done for 15 min with the reduction potential at -100 mV. Following, the Ag plated CNT thread 6 ACS Paragon Plus Environment

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was rinsed with DI water and acetone several times, then dried under N2 atmosphere. Third, Ag plated CNT thread electrode was treated with 50 mM FeCl3 for 60 s to form a thin layer of AgCl on the surface. Last, the electrode was rinsed with DI water several times and dried with compressed air. Auxiliary electrode. A CNT thread connected to a metal wire by silver conductive epoxy was used as the auxiliary electrode. Analysis Procedure. The analyses were performed after removal of oxygen by bubbling N2 for 10 min unless otherwise stated. Hg2+, Cu2+ and Pb2+ were deposited at 1.20 V for 120 s unless specified otherwise in 0.1 M acetate buffer (pH 4.5). After the preconcentration step, the reduced metals were stripped off using Osteryoung squarewave stripping voltammetry (OSWSV) with step potential of 4 mV, amplitude of 25 mV, and frequency of 15 Hz. Between measurements, the electrode was cleaned at 800 mV for 150 s to remove any metal deposit from the previous measurement. Statistical Analysis. All data are expressed as the mean ± SD (standard deviation) unless indicated otherwise. For statistical analysis of Hg2+, Cu2+ and Pb2+, measurements were made 5 times for the individual metal ion detections and the simultaneous detections (N=5). The statistical analysis was performed using GraphPad Prism 3 software (GraphPad Prism Software, Inc.). Statistical significance of difference between groups was assessed by one-way analysis of variance (ANOVA). The significance of differences was evaluated by the Student t-test. P values less than 0.05 were considered statistically significant.



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RESULTS AND DISCUSSION Electrode Surface Characterization. The SEM image of the CNT thread in Figure 1a shows that CNT strands of about 6-10 nm in diameter are twisted together to form the CNT thread, which has a diameter of about 34 µm on average. The appearance of the CNT thread is similar to ordinary thread made of natural cotton fibers. This CNT thread was used as the base material for all three electrodes in the electrochemical cell. The Raman spectrum (Figure 1b) of the CNT thread shows three characteristic peaks at 1362 cm-1 (D band), 1594 cm-1 (G band ) and 2708 cm-1 (G’ band). The D band at 1362 cm-1 represents the amorphous carbon, disorder, and other defective graphite structures. Its intensity is proportional to the amount of disorder in the sample. The G band at 1594 cm-1 is due to the E2g2 C=C stretching vibration, which is the characteristic feature of an ordered graphitic structures. The G’ band at 2708 cm-1 is indicative of defect free sp2 carbons in the long-range order in the sample.27 The ratio of Raman intensities of the D band (ID) and the G band (IG) is proportional to the number of the scattering disordered and ordered sp2 bonding carbon atoms in the illuminated area. Higher ratios mean more defects exist in the material. In our case the ratio of ID/IG is 0.59, which is smaller than the reported CNT yarns (0.81-1.42).28-30 This also shows that our CNT threads have a higher ratio of ordered graphite structures. CNT thread used for the working electrode was first coated with nonconducting polymer polystyrene that served as an insulator. The SEM in Figure 1b shows that the polystyrene completely coats the CNT thread electrode, although the surface is rough. After coating with polymer, the overall diameter is about 35 µm, making the thickness of polystyrene about 0.5 µm. To prepare a working electrode with a well-defined area that


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consisted primarily of the edge planes of the CNTs where fast electron transfer occurs, the end of a polymer coated CNT thread was sliced with a razor blade. A typical working electrode end is shown in Figure 1c. A “clean slice” that gives a sharp cut with the polymer adhering tightly to the edges is important to give good working electrode behavior (vide infra). The nonconducting polymer coating on the side wall of the thread eliminates its contribution to the electrochemical signal, preventing current leakage.15,31 Moreover, the polymer coating also protects the fragile CNT thread from mechanical tearing during cell assembly and use and facilitates assembling the miniature three electrode cell without a short-circuit to the other electrodes. A representative electrode with a poorly cut tip is shown in Figure 1e.

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d

20 µM

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e

20 µM

Figure 1. SEM image of (a) as-prepared CNT thread electrode and its Raman spectrum (b), SEM images of CNT thread after coating with polystyrene (c). Image of working electrode cross section after cutting (d). (e) Image of working electrode cross section that has been squeezed by poor cutting. The operating voltages of the electron beam are 15 kV for a, d and e, and 10 kV for c.

CNT thread working electrode. To evaluate the electrochemical properties of the CNT thread working electrode, cyclic voltammetry (CV) in 5 mM K3[Fe(CN)6] was performed since the electron transfer of Fe(CN)63-/4- is especially sensitive to the nature of the electrode surface chemistry and has been commonly used to study the surface of carbon electrodes.6,21,22,32 A supporting electrolyte of 0.1 M acetate buffer pH 4.5 was chosen for these experiments because it is a medium commonly used for ASV of heavy metals, which is the application in this paper (vide infra). Also, we could compare the behavior with earlier work on uncoated CNT thread working electrodes in this supporting electrolyte.22 Figure 2 shows CVs at the polystyrene coated CNT thread working electrode with a commercial conventional liquid-junction type AgǀAgCl reference electrode and a Pt wire as the auxiliary electrode. The effect of scan rate on the CV 10 ACS Paragon Plus Environment

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behavior (10-900 mV/s) of the CNT thread electrode was investigated to understand the electrode properties. The CV feature is a sigmoidal steady state curve at the slow scan rates (Figure 2a) that is characteristic of hemispherical diffusion at microelectrodes compared to the usual peak shaped voltammogram that is characteristic of planar diffusion (Figure 2b). This behavior is due to the dimension of the diffusion layer exceeding the dimension of the electrode at the slow scan rates.33,34 For a disk-shaped microelectrode, the steady state current Ilim is given by the following equation 35: Ilim = 4nFDrC

(1)

Using a diffusion coefficient (D) of 7.62 x10-6 cm2/s and a concentration 5x10-6 moles/cm3 for ferricyanide,36 an estimated radius (r) for the tip of the CNT electrode of 17 µm, and an n value of 1.00 for reduction to ferrocyanide, we calculate the limiting cathodic current to be 250 µA. This is about 10x larger than the limiting cathodic current of ca. 25 µA in Figure 2A. We attribute the smaller observed current to partial blocking of the electrode surface by polystyrene that smears over the end of the thread during the cutting process and irreproducible differences in geometry of the end as shown in Figure 1c and 1e. An attempt was made to clean the electrode surface by placing the polystyrene coated CNT electrode into toluene at room temperature and then drying. The limiting cathodic current increased by 5-6 times after this treatment, suggesting that an organicetching solution could be used for electrode cleaning. However, the organic solution also etched the coating on the whole CNT thread, rather than just the electrode end. This made it harder to clearly define the electrode surface, so we did not use this procedure. Figure 2 also shows that the transition from microelectrode behavior to macroelectrode semi11 ACS Paragon Plus Environment


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infinite diffusion behavior occurs primarily over the scan rate range of 40 mV/s, where a peak begins to appear on the cathodic wave, to 200 mV/s where the usual CV is obtained. This range is in general agreement with what is expected for an electrode of this size where r≪(Dtc)1/2 (tc, electroanalysis time of a cyclic voltammetry experiment, expressed by the reciprocal of the scan rate tc= RT/nFν, where, 0.04 V/s was used for the scan rate ν in the calculation) varies from 1 to 10 ((Dtc)1/2/r) in the transition region.37 The voltammograms display well-defined voltammetric peak shaped cathodic and anodic waves at the higher scan rates (Figure 2b), which indicate a shift to planar diffusion as the diffusion layer becomes smaller. When the scan rate increases from 50 to 900 mV/s, the oxidation peak potential gradually shifts to a more positive potential, whereas, the reduction peak potential shifts to a negative potential, and correspondingly, ∆Ep increases from 76 mV to 130 mV. The ∆Ep increase is primarily attributed to quasireversibility caused by slow electron transfer and some IR drop in the CNT thread. The peak current increases linearly with square root of scan rate (50-900 mV/s). This indicates that the electron reaction is a diffusion controlled process at the higher scan rate. The correlation equations for one set of measurements on a single electrode are: Ipc = (67.3 ± 1.1)x + (2.4 ± 0.6) (R2= 0.996) and Ipa = (-64.1 ± 0.6)x - (3.6 ± 0.4) (R2 = 0.998), where x is the square root of scan rate (V/s)1/2 and uncertainties are standard deviation, hereinafter (Figure 2c).


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b

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c

Figure 2. Cyclic voltammetry of 5 mM K3[Fe(CN)6], in 0.1 M acetate buffer, pH 4.5 with increasing scan rates at CNT thread electrode. Cyclic voltammograms at varying scan rates of (a) 10, 15, 20, 25, and 40 mV/s and (b) 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, and 900 mV/s from the lowest to the highest peak current. (c) Relationship of peak current with square root of scan rate at 50-900 mV/s.

Preparation of the working electrode is important for obtaining good cyclic voltammograms.

Distorted

voltammograms

with

a

sharper

reduction

peak

superimposed on a broader wave for Fe(CN)63- were sometimes observed (vide infra). To better understand this observation, the cross section of several electrodes was evaluated by SEM. The image in Figure 1e shows the cross section of a polystyrene coated CNT thread working electrode cut by razor blade where the cross section is squeezed and no longer a uniform circular fiber. Here the exposed surface of the CNT 13 ACS Paragon Plus Environment


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thread electrode consists of an irregular surface and the thickness of the coating at the cross section was not uniform any more. In some electrodes a clear gap existed between the polymer coating and the CNT thread. This gap forms a thin layer of solution adjacent to the electrode resulting in restricted thin-layer diffusion in this region combined with semi-infinite diffusion to the end region. Electrodes that were not sealed well exhibited increased charging current, which could also be used to screen electrodes for good performance. It was also noted that the method of cutting the end of the electrode affects the behavior. When the electrode end was cut by a laser, the polymer coating was burned and some of the coating on the sidewalls was removed resulting in a larger exposed surface area and a larger current obtained by cyclic voltammetry under the same conditions. This behavior suggests that CNT thread sides contribute substantially to the current when they are exposed. Therefore, we used razor blades to carefully cut the ends manually. Electrodes with circular shapes (Figure 1d) that give good cyclic voltammograms were usually obtained by cutting the ends with a razor blade while slowly rotating the electrode, rather than with a single downward stroke onto a fixed electrode, which often resulted in an oval-shaped/squished end with odd-looking cyclic voltammograms. We observed a sufficiently strong correlation between the SEM images and the cyclic voltammograms that voltammetry alone could be used to confirm a “good cut”.

AgǀAgCl coated CNT thread quasi-reference and CNT auxiliary electrodes. Reference electrodes based on AgǀAgCl are widely used because of their easy fabrication, miniaturization, and minimal toxicity. A substantial drawback of conventional reference 14 ACS Paragon Plus Environment

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electrodes for many applications is their liquid electrolyte filling. Although very stable and reliable, such reference electrodes require maintenance such as electrolyte refill, are generally mechanically fragile, and are suitable to only a limited extent for miniaturization or application at high pressure and temperature. The solid-state quasireference electrode based on AgǀAgCl provides advantages over the conventional liquidjunction type reference electrode. It does not have the drawbacks such as internal filling solution leakage and the storage problem of the internal filling solution because no internal filling solution is used. It is also easily fabricated in miniature, which makes it useful for in vivo studies.38 They are even heat and pressure resistant up to 140 °C and 16 bar overpressure, respectively.39 Because of these features, solid-state quasi-reference electrodes based on AgǀAgCl are commonly used for microfabrication and for disposable sensors such as for the glucose biosensor for self-testing.40-42 So far, the commonly used substrates for AgǀAgCl film are platinum, glassy carbon, graphite paste, copper and silver wires and silicon plates. There is no report on CNTs as the substrate for a reference electrode yet. Using as the reference electrode based on CNTs easily reduces the dimensions into the micrometer range and does not require the internal filling solution. The AgǀAgCl coated CNT thread quasi-reference electrode was fabricated by electroplating in 0.3 M AgNO3, 1 M NH3 solution, followed by treatment with FeCl3. FeCl3 was chosen as the oxidation reagent to produce AgCl because the procedure was much faster to perform than electrochemical anodization in the presence of chloride and produced a uniform coating.43 Figure 3 shows microscope images of an AgǀAgCl plated CNT thread electrode. It is clearly seen that the CNT thread surface was coated with silver particles/beads, and the surface is not perfectly smooth (Figure 3a). Higher



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magnification images reveal that the coating on a CNT thread electrode comprises both bigger particles on the 10 µm scale (Figure 3b) and smaller particles on the 100-200 nm scale (Figure 3c). Although the sizes of the silver nanoparticles cover a wide range, the surface of the CNT thread is heavily coated. Initial attempts to form films of AgǀAgCl with a shorter deposition time of 2 min or less gave irreproducible electrochemical results, which is probably due to incomplete coating of the CNT thread. The potential for the reduction/oxidation of 5 mM Fe(CN)63-/4in acetate buffer solution determined by cyclic voltammetry was not stable, shifting about 0.2 V in 10 min. With a longer deposition time, more Ag was coated on the CNT thread electrode, which greatly improved the potential stability in the test condition. Figure S1a-c (5, 10 and 20 min) shows how the deposition time influences the thickness and coverage of Ag coating on the electrode. Longer deposition times give thicker coatings with better coverage of the underlying CNT thread. The most

uniform coating was

obtained with 15 min deposition whereas a 20 min deposition time gave larger, more uneven Ag particles. The cyclic voltammetry measurements showed that the electrode performance is stable for a quasi-reference electrode that had been electroplated for 15 min.

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Figure 3. Optical image (a) and SEM images at (b) lower magnification and (c) higher magnification of Ag electroplated from silver nitrate in aqueous ammonia solution on a CNT thread for 15 min and then treated with FeCl3 for 60 s.

The AgǀAgCl coated CNT thread electrode was evaluated as the quasi-reference electrode compared to the commercial liquid-junction type AgǀAgCl reference electrode in 0.1 M acetate buffer, pH 4.5 because this is the medium that has been proven to work well for stripping voltammetry of heavy metals. The redox potential of the voltammograms shifted negatively by around 200 mV (Figure 4) due to the significant difference in Cl- concentration between the internal filling solution of the commercial electrode (3 M) and the absence of Cl- in the buffer. The unusually sharp cathodic peak for the two voltammograms on the right is a result of separation of the coating from the working electrode during the cutting process (vide supra). This allows solution to seep into a thin layer between the coating and the electrode where it undergoes thin-layer electrolysis that gives sharp voltammetric peaks due to the constrained diffusion. Formation of Prussian blue on the electrode surface could also explain this anomalous 17 ACS Paragon Plus Environment


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behavior in the voltammogram. However, we did not observe any color, even after 1000 cycles by CV.

Figure 4. Cyclic voltammograms at CNT thread working electrode with different combinations of auxiliary electrode and reference electrode. The two voltammograms on the left compare the CNT thread auxiliary electrode (blue) with a Pt wire auxiliary electrode (red), and background scan (dashed line) with Pt wire auxiliary electrode using the commercial AgǀAgCl reference electrode for these three. The two voltammograms on the right were recorded with the AgǀAgCl coated CNT thread quasi-reference electrode, one with the Pt wire auxiliary electrode (green) and the other with the CNT auxiliary electrode (black), 5 mM K3[Fe(CN)6], in 0.1 M acetate buffer with scan rate of 200 mV/s. To test the stability of the AgǀAgCl coated CNT quasi-reference electrode in this supporting electrolyte, its open circuit potential was monitored versus the commercial liquid junction AgǀAgCl (3 M KCl) electrode in the acetate buffer. The electrode drifts slowly during the first couple of hours with a drift rate of 0.32 mV/h (Figure 5a). Then, the potential stabilizes with a lower drift rate of 0.05 mV/h from 20 to 89 h. The drift is attributed to slow dissolution of AgCl that forms Cl- in solution. Cl- concentration affects


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the half-cell potential of the Ag/AgCl redox couple, which is Cl- dependent. Similar behavior was observed for a Cu/CuCl2 quasi-reference electrode with faster drift rate at the beginning and then slower drift rate with longer exposure, although over a shorter testing time (1 h) under similar conditions.44 No morphological changes in the AgǀAgCl particles were observed over the time period of a typical experiment. However, we did notice fewer AgǀAgCl particles on the CNT thread after a month of use. Part of the silver chloride dissolves in solution with continuing use. However, the amount of silver coming off the quasi-reference electrode that could redeposit on the working and auxiliary electrode is small because dilution in the sample makes the concentration very small. We saw no evidence for deposition on the other electrodes happening in our experiments. The drift of the AgǀAgCl coated CNT thread quasi-reference electrode is larger compared to a solid state AgǀAgCl quasi-reference used for a glucose sensor which exhibits the extremely low drift rate of 0.004 mV/h in 150 mM KCl (100 mM TES(Ntris(hydroxymethyl)methyl-2-aminoethanesulfonic), pH 7.0) 40 and 0.01 mV/h for a solid AgǀAgCl quasi-reference electrode in 50 mM KH2PO4/NaOH buffer solution containing 0.1 M KCl (pH 7.0).45,46 Their smaller drift rates are expected since their AgǀAgCl quasireference electrodes were protected by multiple layers of membrane. In our case, the AgǀAgCl coated CNT thread quasi-reference electrode is directly exposed to the solution and without chloride for stabilization. Compared to other AgǀAgCl quasi-reference electrodes in the presence of chloride without a protective layer, the drift rate for our quasi-reference electrode is smaller. For example, the drift rate of a microfluidic AgǀAgCl quasi-reference electrode was 0.625 mV/h for 8 h test in saturated KCl solution,47 and it is 0.12 mV/h for a AgǀAgCl micro quasi-reference electrode in 1 mM KCl solution tested



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for 1000 minutes.43 Although not as stable as a commercial reference electrode, the drift rate of the bare CNT AgǀAgCl quasi-reference electrode is acceptable for stripping voltammetry.44

b

a

Figure 5. (a) Stability of AgǀAgCl coated CNT thread quasi-reference electrode vs. commercial AgǀAgCl (double junction) in 0.1 M acetate buffer. (b) Cyclic voltammograms in 5 mM K3[Fe(CN)6], 0.1 M acetate buffer with CNT thread for all three electrodes, 1 to 1100 cycles at 200 mV/s. The auxiliary electrode is another important component in a three electrode electrochemical cell. Conventionally, Pt wire is used for the auxiliary electrode. Carbon are attractive for use as an auxiliary electrode material instead of Pt owing to their high conductivity and corrosion resistance to iodide ions in dye-sensitized solar cells (DSSCs). So far, the application of the CNT based auxiliary electrode is mainly on the DSSCs.48 The use of CNT as the auxiliary electrode is not yet reported for electrochemical analysis. As shown above, when the Pt wire was replaced by uncoated CNT thread as the auxiliary electrode, the voltammograms were essentially unchanged, and the uncoated CNT thread functioned as a good auxiliary electrode.


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All CNT electrode cell. To further test the performance of the complete CNT thread electrode system - CNT thread working electrode, AgǀAgCl coated CNT thread quasireference electrode and CNT thread auxiliary electrode - CVs were recorded for 1100 cycles at 200 mV/s. Compared to the voltammogram in Figure 2b at this scan rate and the left two voltammograms in Figure 4, the voltammograms in Figure 5b have somewhat rounded peaks for both cathodic and anodic waves. We attribute this to a change in the nature of the surface over the longer time period of this experiment with its many repetitive scans in the presence of dissolved oxygen, such as the formation of oxygen functional groups on the CNT surface that affect the rate of electron transfer. The ∆Ep is 150 mV, with Ipc/Ipa = 1.50 and Eo =2.5 mV a scan rate of 200 mV/s. It is clearly seen that this electrode system exhibits a stable behavior by providing a relatively constant potential and current even after 1100 cycles (Figure 5b). The cathodic peak current varied by 5.7 % over the 1100 cycles (2 hours and 10 minutes of cycling). These results show that the AgǀAgCl coated CNT thread electrode as the quasi-reference electrode and bare CNT thread as the auxiliary electrode are stable and suitable for electroanalytical applications. Although the drift rate of 0.32 mV/h in the first couple hours followed by a lower drift rate of 0.05 mV/h from 20 to 89 h reported here is greater than that observed with a commercial AgǀAgCl reference electrode, it should be tolerable for stripping voltammetry. Even at the higher initial drift rate, the potential would drift less than 5 mV over a period of 10 hours. In ASV one generally measures the peak current making accurate identification of the peak potential important. This can be done by software that measures the change in slope so that the peak can be easily identified even with some drift. Thus, 21 ACS Paragon Plus Environment


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one would always measure current at the soft-ware identified peak rather than at a fixed potential that could drift off of the peak and cause error in the measurement of peak current.

Individual Metal Ion Detection by ASV. A sensor set consisting of a polystyrene coated CNT thread electrode working electrode, AgǀAgCl coated CNT thread quasireference electrode and uncoated CNT thread auxiliary electrode was applied to the determination of Hg2+, Cu2+ and Pb2+ by ASV. The deposition potential is at -1.20 V, which is negative enough to reduce Hg2+, Cu2+ and Pb2+, the accumulation time is 120 s for this study (vide infra). To balance low detection limit versus wide response range, 120 s was chosen as the accumulation time. Well-defined stripping peaks were obtained for all three metal ions when determined individually (Figure 6). Hg2+ and Pb2+ have sharper stripping peaks compared to Cu2+. The peak currents increase linearly versus metal ion concentration; the correlation equations are Ip = (21.3 ± 1.1)C - (26.6 ± 2.1) (R2 = 0.992), Ip = (8.9 ± 0.2)C - (35.4 ± 0.5) (R2 = 0.999) and Ip = (11.7 ± 0.6)C - (8.9 ± 1.0) (R2 = 0.990) for Hg2+, Cu2+ and Pb2+, respectively, where Ip is the stripping peak current (nA) and C is the concentration of Hg2+, Cu2+ and Pb2+ (µM). The stripping peaks are at 84 mV, -88 mV and -676 mV for Hg2+, Cu2+ and Pb2+, respectively. The small peak at about -600 mV in Figure 6c is attributed to some underpotential stripping of Pb or Pb stripping off of a small fraction of the electrode surface that is different in composition from the primary CNT thread surface. Compared to an uncoated CNT thread working electrode, commercial liquid-junction AgǀAgCl (3 M KCl) reference electrode and Pt wire auxiliary electrode, the stripping peaks show a negative shift in potential of 112 mV and 188 mV for Cu2+ and Pb2+, respectively.20 This potential shift is consistent with the CV results for 22 ACS Paragon Plus Environment

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K3[Fe(CN)6] with AgǀAgCl coated CNT thread as quasi-reference electrode mentioned above. The calculated detection limits (based on the 3σ method) are 1.05 nM, 0.53 nM and 0.57 nM for Hg2+, Cu2+ and Pb2+, respectively. The detection limits of these three metal ions are well below the allowable limits instituted by United States Environmental Protection Agency (EPA) (Hg, 9.97 nM, Cu: 20.45 µM and Pb, 72.39 nM )49 and by World Health Organization (WHO) (Hg, 29.90 nM, Cu: 31.47 µM and Pb, 48.26 nM).50 From the slopes of the calibration plots, the sensitivity is highest for Hg, followed by Pb, and least for Cu. The NH3 plasma treated CNT modified glassy carbon electrode shows higher sensitivity for Hg, Cd and Cu compared to the untreated CNT modified glassy carbon electrode.51 At the same time, the sensitivity for Hg is higher than Cu after modification in this plasma treated electrode. However, several research groups reported the reverse observation. A hollow fiber-supported sol-gel combination with multiwall carbon nanotubes (MWCNTs) demonstrates higher sensitivity for Cu than Pb, under similar conditions.52 A gold nanoparticles film modified CNT electrode exhibits higher sensitivity for Cu than Pb.53 Morton et al. reported higher sensitivity for Pb than Cu using a functionalized CNT modified electrode.54 A Schiff base treated CNT modified electrode shows higher sensitivity for Pb than Hg.55 The detection limits of Hg, Cu and Pb are lower compared to reported CNT electrodes. For example, the detection limit of Bi-modified screen printed CNT electrode for Pb is 6.3 nM.56 The detection limit of Hg on chitosan modified CNT electrode is 3 nM.57 Bui et al reported that the detection limit for Pb and Cu is 2.7 and 9.6 nM on gold nanoparticle patterned carbon nanotube thin film electrode, respectively.53 Morton et al has used cysteine-modified CNT electrode for Pb and Cu detection, with the detection limit is 4.8 nM for Pb and 236 nM.54 The detection



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limit for Pb Nafion/bismuth modified CNT electrode is 25 nM.58 We explored the ability to detect low levels of Pb by increasing the deposition time. We were able to obtain well defined stripping voltammograms at 10.0 nM with 15 min deposition time and at 7.0 nM with 30 min deposition time. The repeatability of the CNT electrochemical cell is demonstrated in the 1.0 µM Pb2+ solution under the same condition as the individual metal ions detection. Figure S2 shows 10 continuous measurement of corresponding peak height for Pb detection at different CNT thread working electrodes. The relative standard deviation (RSD) of the peak measurements is 2.9%.

a

b

c

Figure 6. OSWV Stripping voltammograms for Hg2+, Cu2+ and Pb2+ in 0.1 M acetate buffer pH 4.5 at CNT thread electrode: (a) 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0 24 ACS Paragon Plus Environment

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µM of Hg2+; (b) 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5 µM of Cu2+; (c) 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5 µM of Pb2+. Accumulation time: 120 s; deposition potential: -1.2 V.

Simultaneous Detection of Hg, Cu and Pb. Stripping voltammograms for simultaneous detection of Hg2+, Cu2+and Pb2+ at different concentrations are shown in Figure 7. The three peaks are well resolved and the peak current increases linearly with the increase in concentration of Hg2+, Cu2+ and Pb2+. The correlation equations are: Ip = (29.0 ± 2.8)C - (29.8± 3.9) (R2 = 0.959), Ip = (9.9 ± 1.7)C - (21.7± 1.9) (R2 = 0.960) and Ip = (17.8 ± 0.7)C - (16.4 ± 1.3) (R2 = 0.990), respectively. Compared to the individual metal ions detection, the presence of Hg increases the sensitivity for Pb significantly (P (0.0001) < 0.05, N=5). Although the sensitivity increases for Cu, the differences are too small to be statistically significant (P (0.227) > 0.05, N=5). The increased sensitivity for Cu and Pb is due to the formation of a thin mercury film and its amalgamation of the other metals. It is known that the formation of an amalgam in mercury electrodes lowers their detection limit because of its ability to minimize background interferences.21,59-62



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Figure 7. OSWV Stripping voltammograms for simultaneous detection of Hg2+, Cu2+ and Pb2+ at CNTs thread electrode. Concentrations: 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 µM of Hg2+ and Pb2+, 3.0, 4.5, 6.0, 7.5, 9.0, 10.5, 12.0, 13.5, 15.0 µM of Cu2+ in 0.1 M acetate buffer pH 4.5. CONCLUSIONS The first microsensor based on a three electrode cell system where all three electrodes consist of CNT thread has been developed: polystyrene coated CNT thread with exposed tip working electrode, AgǀAgCl coated CNT thread quasi-reference electrode, and bare CNT thread auxiliary electrode. Good performance was obtained by CV on ferricyanide and ASV on three metal ions that are commonly determined by electrochemistry. Several features make this sensor important for applications such as ASV. The CV measurements demonstrated that AgǀAgCl coated CNT thread as the quasi-reference electrode and bare CNT thread as the auxiliary electrode were sufficiently stable. Therefore, the CNT thread based sensors are suitable for heavy metal ion detection. Moreover, the fabrication procedure for AgǀAgCl coated CNT thread quasi-reference electrode is quite simple. The three electrodes reduce the dimension of the conventional three electrode system. The three electrode cell system based on CNT thread has the potential to reduce cell dimensions and thereby reduce sample volume compared to the conventional three electrode system. The diameter of the CNT thread used for the three electrodes (working, reference, and auxiliary) was about 35 µm for this initial work as a compromise between small size and ease of working with. However, the threads can be made much smaller. We have used threads with the diameter as small as 8 µm, although they are more difficult to work with and require a microscope to see. The CNT thread based three


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electrode microsensor system offers competitive performance for electrochemical detection by ASV as demonstrated for single and simultaneous detection of Hg2+, Cu2+ and Pb2+. This microsensor shows comparable limit of detection with other mercury free electrodes such as CNT modified electrodes. The present microsensor system has the potential for other applications such as in vivo or in vitro detection of metals with small samples due to the small dimensions of the electrodes. ASSOCIATED CONTENT Supporting Information Optical images of electroplated Ag from silver nitrate in aqueous ammonia solution on the CNT thread for different times; peak current response for Pb2+ at 10 different CNT thread working electrodes with CNT thread electrochemical cell. AUTHOR INFORMATION Corresponding author: Email: [email protected]. Phone: 01-513-556-9210. Fax: 01-513-556-9239. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge National Science Foundation (NSF ERC 0812348), NIH through UC Center for Environmental Genetics (NIEHS P30-ES006096) and NSF Grand Challenge grant (NSF IIP-1134684) for financial support. The authors thank



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Shirmir B. Branch for helpful discussions about AgǀAgCl coating. The authors also thank reviewers for many very helpful suggestions.

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