Carbon Nanotube-Based Ion Selective Sensors for Wearable

Research Article www.acsami.org

Carbon Nanotube-Based Ion Selective Sensors for Wearable Applications Soumyendu Roy,†,‡ Moshe David-Pur,†,‡ and Yael Hanein*,†,‡ †

School of Electrical Engineering and ‡Tel Aviv University Center for Nanoscience and Nanotechnology, Tel Aviv University, Tel Aviv 69978, Israel S Supporting Information *

ABSTRACT: Wearable electronics offer new opportunities in a wide range of applications, especially sweat analysis using skin sensors. A fundamental challenge in these applications is the formation of sensitive and stable electrodes. In this article we report the development of a wearable sensor based on carbon nanotube (CNT) electrode arrays for sweat sensing. Solid-state ion selective electrodes (ISEs), sensitive to Na+ ions, were prepared by drop coating plasticized poly(vinyl chloride) (PVC) doped with ionophore and ion exchanger on CNT electrodes. The ion selective membrane (ISM) filled the intertubular spaces of the highly porous CNT film and formed an attachment that was stronger than that achieved with flat Au, Pt, or carbon electrodes. Concentration of the ISM solution used influenced the attachment to the CNT film, the ISM surface morphology, and the overall performance of the sensor. Sensitivity of 56 ± 3 mV/decade to Na+ ions was achieved. Optimized solid-state reference electrodes (REs), suitable for wearable applications, were prepared by coating CNT electrodes with colloidal dispersion of Ag/AgCl, agarose hydrogel with 0.5 M NaCl, and a passivation layer of PVC doped with NaCl. The CNT-based REs had low sensitivity (−1.7 ± 1.2 mV/decade) toward the NaCl solution and high repeatability and were superior to bare Ag/ AgCl, metals, carbon, and CNT films, reported previously as REs. CNT-based ISEs were calibrated against CNT-based REs, and the short-term stability of the system was tested. We demonstrate that CNT-based devices implemented on a flexible support are a very attractive platform for future wearable technology devices. KEYWORDS: ion-selective electrode, solid contact, carbon nanotube, solid reference electrode, wearable device, electrochemical sensor

1. INTRODUCTION Wearable devices with real-time and continuous chemical and electrical monitoring hold tremendous potential in the fields of disease management, personal health care, and tracking human physical performances.1,2 Sweat analysis is of particular interest, as sweat contains markers for various diseases and conditions, such as cystic fibrosis, dehydration, hyponatremia, stress disorders, osteoporosis, bone mineral loss, and drug abuse, and is also useful for testing of deodorants.3−8 Na+ ion concentration in sweat is an important marker for many of these diseases. Despite its rich information content, sweat has largely been neglected in medical practice owing to technical difficulties in collection and analysis.9−12 Recent advances in ion-selective electrode (ISE) technology, a type of potentiometric electrochemical sensor, based on host−guest chemistry,13−15 offer an opportunity to overcome these challenges and to replace conventional blood and urine clinical analysis methods.16−19 The main sensing component of an ISE is the ion selective membrane (ISM). Conventional ISEs are liquid contact electrodes; that is, an aqueous solution (of Cl− ions) exists between a metal electrode (usually Ag/AgCl wire) and the ISM. Such devices need containers to store the solution, introducing a constraint in designing wearable systems and in © XXXX American Chemical Society

sensor miniaturization. An additional challenge is solvent evaporation and diffusion of ions and solvent molecules across the ISM into the test solution.13,16,20 Accordingly, solid contact ISEs with ISM-coated metals were studied as an alternative.13 However, in these devices membranes typically have a weak attachment to the metal thin film resulting in poor ion-electron transduction. Moreover, such systems are too rigid to conform to the skin surface or withstand significant bending and stretching. Also, a thin aqueous layer is known to form at the metal−ISM interface, leading to significant instabilities.21 Attempts to improve the metal−ISM adhesion focused on adding an intermediate layer of a conducting polymer.14 An additional fundamental challenge in the development of wearable electrochemical devices is the formation of a reliable solid-state reference electrode (RE).20,22,23 Owing to these challenges, widespread use of wearable/portable devices using solid contact ISEs has not been achieved yet, even though there are several promising technologies that are currently under development. Most notable is a conductive carbon ink-based ISE system in the form of a wearable tattoo.24,25 Received: May 24, 2017 Accepted: September 19, 2017 Published: September 19, 2017 A

DOI: 10.1021/acsami.7b07346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. CNT-based flexible ISEs. (a) Fabrication flow of CNT electrodes: (1) Ni lithography on SiO2/Si substrate, (2) CNT growth by CVD, (3,4) CNT transfer to a flexible adhesive film (5) laser cutting of a double sided adhesive film (6) aligning and bonding the double-sided adhesive film to the CNT electrode array for passivation. (b) ISEs on polyimide (Kapton) substrate. (inset) SEM image of the surface of the CNT electrodes. Scale bar is 200 nm. (c) ISEs on temporary tattoo paper. Some of the electrodes in (c) are CNT-based solid-state REs described in Section 2.2.

Figure 2. (a) Calibration curve of CNT-based ISEs, measured with a voltmeter. (b) Continuous measurement of OCP of an ISE using a potentiostat. Concentration of the test solution was changed from 1 × 10−6 to 1 M in a stepwise manner. (inset) The corresponding calibration curve. All measurements were performed with a std. RE (Ag/AgCl/NaCl(saturated)).

plasticizer bis(2-ethylhexyl) sebacate (DOS). To make this material sensitive toward Na+ ions, it was doped with 4-tertbutylcalix[4] arene-tetraacetic acid tetraethyl ester (sodium ionophore X) and ion exchanger potassium tetrakis((4chlorophenyl)borate24,31 (Figure S1). Sodium ionophore X was chosen owing to its high sensitivity to Na+ ion and high selectivity against similar ions, such as K+ and H+.31−33 CNTbased flexible electrode arrays were prepared following a scheme shown in Figure 1a. The process utilizes standard photolithography and laser-cutting techniques. Patterns of catalyst (Ni) thin film were deposited on oxidized Si wafers, and multiwall CNTs (MWCNTs) were grown via chemical vapor deposition (CVD). The CNT films had low density of nanotubes and high porosity. Surface morphology of the films can be seen in the inset of Figure 1b. CNT films were then transferred onto adhesive flexible substrates (i.e., polyimide tapes or temporary tattoo papers) to prepare flexible electrode arrays. Polydimethylsiloxane (PDMS) film was also used as a substrate. However, it was found that PDMS tends to absorb the solvent used in ISM solutions, leading to mild swelling. Holes were laser-cut into a double-sided adhesive film and was used as a passivation layer for the CNT electrode arrays.34,35 ISMs were finally drop-coated onto the CNT electrodes to form ISEs (Figure 1b,c). To study electrode performances, potentiometric measurements with CNT-based ISEs dipped in NaCl solutions of different concentrations were performed using a voltmeter

In this paper, we address the sensor stability and reliability problem caused by poor ISM−conductor attachment and improve the performance of solid-state RE by utilizing porous carbon nanotube (CNT) electrodes. CNT−polymer composites are known to have good mechanical flexibility and strength.26,27 We show that CNT films with their high porosity allows the ISM (in solution state) to permeate its fibrous network. Upon solidification, the ISM forms a strong attachment to the CNTs, enabling a robust ISE with stable electrical contact to the underlying conducting CNT film. Previous studies involving CNT-based ISEs have indeed yielded encouraging results.28−30 We also present a simple planar solidstate RE based on flexible CNT electrodes, suitable for use with ISEs and other electrochemical devices. Finally, we present a wearable version of these CNT-based ISEs and REs that can measure Na+ ion content in a test solution. The use of the CNT-based devices, as presented here, is a general solution and may be further expanded by changing the ionophores to detect different components in sweat and other bodily fluids.

2. RESULTS AND DISCUSSIONS 2.1. CNT-Based ISE. A key component of contemporary ISEs is the ISM, which is generally a plasticized polymer doped with ionophore. The latter is a hydrophobic compound that can selectively bind to ions of interest via coordinate bonds. In our investigation, a flexible ion-conducting polymeric matrix was prepared by mixing poly(vinyl chloride) (PVC) with a B


Research Article


Figure 3. ISM drop-coated on CNT films. (a) A cross-section SEM image of an ISE prepared by the two-step drop coating process. A strong attachment between the top ISM layer and the CNTs is clearly apparent. ISM also infiltrated and filled the free space in the CNT network. (inset) A typical EDS spectrum obtained from the CNT layer. Cl peak in the spectrum is a confirmation of ISM. Si in the spectrum is from the sample holders. (b) Low-magnification SEM image of an ISE prepared by concentrated ISM solution coated on the CNTs. Poor penetration of ISM into the CNT film and the resultant detachment of ISM from the CNT layer is clearly visible. (c) ISE prepared by drop-coating a dilute solution of ISM on the CNT film showing highly porous surface morphology. (d) ISE prepared by the two-step drop-coating process showing low porosity of the surface.

diffuse throughout the entire solution (∼100 mL). Additionally, the layer of solution at the vicinity of the membrane is generally more stagnant, and diffusion of electrolyte into this layer is more difficult. To aid this diffusion process we increased the convectionary flow of ions by continuously stirring the solution. Other factors affecting the dynamic response of ISEs are kinetics of the electrochemical reaction at the membrane/ solution interface and diffusion of ions within the membrane phase. Response time in the order of 1 min is sufficient for most practical applications. ISM solution concentration used during ISE fabrication was found to have a strong effect on sensor performances. The best sensitivity (56 ± 3 mV/decade) and stability were obtained using a two-step drop-coating method. ISEs were prepared by drop coating the CNT electrodes with a dilute (∼0.008 g/mL) solution of ISM followed by a concentrated (∼0.08 g/mL) solution. The resultant ISM coatings had good penetration into the underlying CNT layer (Figure 3a) and hence a strong attachment to it. Energy-dispersive X-ray spectrum (EDS) obtained from the CNT layer of the ISE is shown in the inset of Figure 3a. The presence of characteristic chlorine signal confirms that PVC (and hence ISM) is present inside the CNT film. ISEs were also prepared by single-step drop-coating methods, using just the dilute or the concentrated ISM solutions. These were found to suffer from poor performances. The ISMs prepared from concentrated solutions had poor attachment to the CNT films, as shown in Figure 3b. This led to poor electrical contact and low sensitivity of 35 ± 5 mV/ decade. Apparently, the solution was too dense to infiltrate into the CNT film and had a rapid solidification rate. ISMs dropcoated from dilute solutions had good attachment and higher sensitivity of 42 ± 3 mV/decade. However, these films had a highly porous morphology (Figure 3c), with an average pore size of 13 ± 9 μm. Pores as large as 30−40 μm could be seen on the surface. During sensing experiments with NaCl

(Figure 2a). Open-circuit potentials (OCPs) at the ISEs were measured with a commercial Ag/AgCl/NaCl (saturated) RE (hereafter referred to as the std. RE). Na+ ion concentration in human sweat is known to lie in the range from 1 to 100 mM; even under extreme conditions it is between 1 × 10−4 and 1 M.36 Average sensitivity of the ISEs in this range (1 × 10−4 to 1 M) was found to be 56 ± 3 mV/decade, which is close to the value expected for detection of monovalent ions using ideal ISMs (59.2 mV/decade, see Section 4.5). Lower limit of detection (LOD) of the ISEs was 1.12 × 10−6 M. The ISEs had a linear response up to the highest concentration tested, that is, 1 M. Testing concentrations higher than 1 M could have been problematic, because the saturation concentration of NaCl in water is ∼5 M at room temperature. Thus, the working range of the ISEs is at least 1.12 × 10−6 to 1 M. The linear range of the ISEs, as estimated from the calibration data, was 6.03 × 10−7 to 1 M. The difference in working and linear ranges stems from the fact that the latter by definition is dependent on error or standard deviations in the measured OCP values. The standard deviations in OCP for a given NaCl concentration varied between 6 and 15 mV. These results indicate that the CNTbased ISEs are a robust sensing platform. In a separate set of experiments, ISEs were kept dipped in NaCl solution, while the concentration of the solution was changed in steps and the OCP was monitored continuously using a potentiostat (Figure 2b). Sensitivity (54 mV/decade) obtained from the corresponding calibration curve (inset of Figure 2b) was similar to that obtained using a voltmeter (Figure 2a). Average response time of the sensor, that is, the interval between the time that the OCP starts to change (in response to a change in concentration of test solution) and the time when it reaches equilibrium value, was found to be 57 ± 23 s. The actual response of ISEs was previously determined to be very fast, with response times in the order of microseconds.13,37,38 The longer duration observed here is due to the finite time needed for the added Na+ ions to C


Research Article


be 59 ± 4 mV/decade of NaCl concentration, which is expected from the Nernst equation.22 To realize miniaturized, wearable electrochemical sensors, it is imperative to use solid-state REs that have simple planar construction and do not need buffer solutions. Possible candidates are bare nonreactive noble metal electrodes, like Au or Pt, or oxides such as indium tin oxide.22,23,46−48 However, as shown in Figures 5b,c and S2 (in Supporting Information), bare metals, CNT films, or carbon REs have significant sensitivity toward NaCl concentration and are therefore bound to give erroneous results in sweat analysis. Additionally, the standard deviation in OCP values of bare Au and CNT at a particular concentration of Na+ ion (Figure 5b,c) can be as high as 77 and 30 mV, respectively. This is generally due to poorly defined and/or slow chemical processes establishing equilibrium at the metal/electrolyte interface. Thus, bare metal and carbon-based REs may also be sensitive to ions other than Na+ and Cl−, making data from the sensors highly unreliable. Hence, ISE devices using these electrodes as REs may suffer from nonrepeatability. Ag/AgCl electrodes dipped in an electrolyte containing Cl− ions rapidly reach a chemical equilibrium via the reaction Ag(s) + Cl−(aq) ⇌ AgCl(s) + e−.22 This reaction yields repeatable OCP values. However, this reaction also makes bare Ag/AgCl highly sensitive to Cl− ion concentration and unsuitable for use as RE for sweat analysis. In commercially available std. REs, Ag/ AgCl is dipped in a solution saturated with Cl− ions and separated from the test solution by a semipermeable, microporous glass or ceramic membrane. The buffer solution ensures that the OCP at the solution-Ag/AgCl electrode interface stays at a fixed value, irrespective of the content and composition of the test solution. Such stability is not possible with a bare Ag/AgCl electrode (Figure 5a). To resolve the challenge outlined above, one can use the Ag/ AgCl electrode while replacing the buffer solution with a solidstate material containing Cl− ions. To this end, we used agarose-based hydrogel prepared in a 0.5 M NaCl solution. Colloidal dispersion of Ag/AgCl particles in Propylene Glycol Monomethyl Ether Acetate (PM acetate) was drop-coated on the CNT electrodes. A thin layer of agarose gel was coated onto the dried Ag/AgCl film, which was further encapsulated by NaCl-doped and plasticized PVC film. Figure 6a shows a crosssection of the RE. As shown in Figure 6b, these electrodes had almost no sensitivity to NaCl solution (−1.7 ± 1.2 mV/ decade), compared to the other REs tested in this study. They also showed low variations in OCP values, with standard deviations in the order of 10 mV. CNT-based ISEs, when

solutions, it is possible that these pores trap a larger volume of the old test solution, making it difficult for the new test solution to replace the old one and thus limit the ISE sensitivity to NaCl ions. In comparison, ISM films prepared by the two-step process had a less porous surface (Figure 3d), with average pore size of 2.6 ± 0.6 μm. Comparing these CNT-based ISE to ISE formed on flat conducting surfaces, we can indeed validate their superior properties. In particular, ISM attachment to metals like gold (Au) and platinum (Pt) or screen-printed carbon electrodes was found to be inferior to ISM attachment to CNT films. ISM films deposited on metals or carbon films can be easily peeled off, leaving the underlying surface clean (Figure 4). These

Figure 4. ISM coated onto (a) Au, (b) Pt, and (c) screen-printed carbon films. Attachment of ISM to underlying surface is poor, and it can be easily peeled off with tweezers.

coatings were prepared by the same methods as described above. Thus, CNT films, with their networklike morphology, are able to form more robust and intimate contact with ISM than carbon or metal films. Additionally, CNT-based flexible films are capable of withstanding mechanical deformations.26,27 Several CNT-based wearable devices have been reported previously.39−41 On the basis of these earlier studies it can be assumed that CNT-based ISEs will have the mechanical strength necessary for functioning as wearable sensors. 2.2. CNT-Based Solid-State RE. RE is a critical component in potentiometric sensors. A basic requirement from REs is the maintenance of a stable potential, independent of variations in test solution composition. Common REs such as Ag/AgCl, saturated calomel, or standard hydrogen electrode require a reservoir of ionic solutions. Several recent articles reported the use of bare Ag/AgCl (without the supporting Cl− ion solution) as RE.42−45 As shown in Figure 5a, the potential of bare Ag/ AgCl electrode dipped in a solution containing Cl− ions depends strongly on Cl− ion concentration, making it poorly suitable as an RE in sweat analysis. The sensitivity was found to

Figure 5. Performance of different materials used as REs. (a) Bare Ag/AgCl, (b) bare Au, and (c) bare CNT electrodes. OCPs in NaCl solutions of different concentrations were measured vs a std. RE (Ag/AgCl/NaCl(saturated)). Sensitivities to NaCl concentration for these electrodes were −59 ± 4, −19 ± 16, and 21 ± 5 mV/decade, respectively. The Y-axis scales in the graphs (i.e., the Δy values per unit length of the axes) are the same to allow easy comparison of the standard deviations in OCP values. D


Research Article


Figure 6. (a) An SEM image of the cross-section of a CNT-based solid-state RE. Ag/AgCl, agarose, and PVC layers can be seen clearly in the image. (b) Calibration curve of CNT-based RE. OCPs in NaCl solutions of different concentrations were measured with respect to an std. RE. The Y axis scale is the same as those in Figure 5. (c) Calibration curve of CNT-based ISE (curve (i)). OCPs in NaCl solutions of different concentrations were measured vs a CNT-based RE. Curve (ii) shows the OCP of CNT-based ISE in 1 × 10−2 M NaCl solution, measured vs CNT-based RE, over a period of 1 h.

Table 1. Characteristics of Solid-State ISEs Previously Reported Compared with CNT-Based Flexible ISE Device of This Study reference

base electrode

RE

target ion Na+

Ag/AgCl

PVC/agarose + NaCl on Ag/AgCl/CNT hydrogel on Ag/AgCl

46

Au

bare Au

Na+

42

Pd

bare Ag/AgCl

Na+

24

C ink + C PVB + NaCl on Ag/ fibres AgCl CNT commercial liquid contact Ag/AgCl CNT on commercial liquid glassy C contact Ag/AgCl C ink + C PVB + NaCl on Ag/ fibres AgCl

this work 3

CNT

28 29 25

Na+

ionophore Na ionophore X calixarene tetramethyl ester

sensitivity (mV/ decade) 58

range (M)

3.16

7.08 × 10−7 to 1

58.5

25

K+

Na ionophore X Na ionophore X valinomycin

K+

valinomycin

52.7

NH4+

nonactin

59.2

Na+

LOD (μM)

57 60

1 × 10−2 to 1 × 10−1 1 × 10−2 to 9 × 10−2 1 × 10−4 to 1 × 10−1

63.75 54.9

response time (s)

10

12.6

2100 30 10 60

1 × 10−5 to 1 × 10−1 1 × 10−4 to 1 × 10−1

5

description wearable tattoo, both ISE and RE are based on CNT electrodes miniature portable solid-state electrode array in vivo studies, textile based wearable sensors wearable sensors with RFID wireless operation wearable screen printed tattoo, wireless operation cotton threads coated with CNTs and ISM CNTs compared with conducting polymers as base electrodes wearable screen printed tattoo

Figure 7. (a) Schematic representation of the flexible CNT-based ISE and RE. (b) A stand-alone wearable sensor for in vivo measurements. The portable device includes a high-input impedance voltmeter connected to a CNT-based solid-state ISEs and REs, fabricated on a temporary tattoo substrate. (c) Comparison of the OCP of a CNT-based ISE vs a CNT-based RE recorded using the portable voltmeter and a traditional benchtop meter.

calibrated with respect to the CNT-based REs (curve (i) of Figure 6c), yielded an average sensitivity of 58 ± 3 mV/decade. Moreover, OCP values of the device in fixed concentration (1 × 10−2 M) showed very little variation when continuously measured over a period of 1 h. (curve (ii) of Figure 6c). The mean value of OCP over that time duration was 73 mV, and the standard deviation was 4 mV indicating a stable electrochemical

system. It is important to note that the large specific capacitance of the CNT electrodes49 may have a stabilizing effect on signals from potentiometric sensors.50 The lower LOD as determined from the calibration curve of Figure 6c is 3.16 × 10−6 M, and the liner range was at least from 7.08 × 10−7 to 1 M. E


Research Article

ACS Applied Materials & Interfaces Hydrogels,51−54 including agarose-based gels,55 coated on Ag/AgCl films have been used previously for making REs. REs for electrocardiography (ECG) and electromyography (EMG) recordings also rely on Ag/AgCl electrodes covered with electrolytic gel containing Cl− ions.35 In the current study, solid-state RE is based on these earlier reports. Sophisticated polymer composites, polymeric membranes containing colloidal dispersion of Ag, AgCl, and KCl (without any thin-film Ag/ AgCl electrode) and membranes containing dispersion of polymeric microcapsules loaded with Ag/AgCl/KCl aggregates have also been used to make solid REs.20,56−58 Compared to these, the REs used here are much simpler to implement and control. Such REs can also be used with other electrochemical potentiometric sensors. A brief comparison of the main device parameters of our flexible CNT-based ISE + RE system with similar solid-state devices, reported in previous studies, is presented in Table 1. Finally, a schematic diagram depicting the CNT-based ISE and the RE is shown in Figure 7a. For in vivo sweat testing, one needs to connect these devices to appropriate measuring equipment. To avoid bulky benchtop instruments like voltmeters and potentiostats,25,28,50 the use of a miniaturized fully portable system is of great interest.42,59,60 Here we show the use of a simple portable meter built using the integrated circuit chip ICL 7106. Such systems have been around since 1980s and were used for a wide variety of applications.61−63 The meter developed is small, lightweight, uses a minimal number of electronic components, and is capable of handling high-impedance electrochemical systems. CNT-based ISEs and REs on a flexible tattoo or tape can be pasted on the skin and plugged to the voltmeter, as shown in Figure 7b. A set of CNTbased ISEs were calibrated against CNT-based REs using these portable voltmeters, as well as the traditional benchtop voltmeters (Figure 7c). Sensitivities obtained were similar (54 and 55 mV/decade, respectively), confirming the compatibility of the CNT-based sensing platform and the portable measuring unit. All measurements reported in this article were conducted in vitro. Further investigation of the in vivo performance of these sensors is beyond the scope of the current report and will be performed in the future. Nanomaterial toxicology is an important concern when developing CNT-based devices to be used directly on the body. There are several contradicting reports on toxicity and biocompatibility of CNTs.64,65 Conflicting results were also reported for CNT toxicity on the skin.66,67 CNT toxicity does not pose a challenge for the device presented in this report, as CNTs do not come into contact with the skin. At the sensing electrode the CNTs are coated with ISM, and at the RE there are films of three different materials, namely, Ag/AgCl, agarose, and PVC, on top of the CNT film. The CNT traces in the electrode arrays were also coated with insulating transparent tape, which acted as a passivation layer. Continuous long-term usage of electrochemical sensors worn on skin has several issues. Foremost, sweat may dry and leave deposits under the sensor surface. This dry sweat may redissolve in fresh sweat and corrupt the measurements. Absorbent patches, microfluidic channels, and per-use washing may be considered as possible solutions. Second, selectivity of the ISEs against simple nontarget ions like K+, H+, and NH4+ is likely to be high, as has been reported earlier.31−33 However, selectivity against other constituents of sweat, especially lipophilic substances that can diffuse into the ISM, needs to be studied. Artificial sweat with controlled compositions can be

used in such studies. Mechanical properties of the device need to be characterized to understand how its sensitivity and performance changes with mechanical stress and deformation. The current study is a proof of concept, illustrating how CNTs can be used to build flexible point-of-care ion-sensing devices. It is meant to be a stepping stone toward more detailed future studies, along the lines mentioned above.

3. CONCLUSION Wearable solid contact ISEs and REs were fabricated using CNT electrode arrays. Flexible temporary tattoo paper and Kapton tapes were demonstrated as effective substrates. The ISM consisted of plasticized PVC doped with Na ionophore X and ion exchanger salt. The ISM attachment to CNT films was stronger than Au, Pt, or carbon films. Sensor transduction was found to depend on the ISM solution concentration. Surface porosity and the ISM attachment strength varied depending on fabrication process. Best sensitivity of 56 ± 3 mV/decade was obtained when the ISE was prepared by a two-step dropcoating process involving a dilute (0.008 g/mL) ISM solution followed by a concentrated (0.08 g/mL) solution. Response time was on the order of 1 min. Layers of Ag/AgCl ink, agarose gel with NaCl, and PVC doped with NaCl were coated onto the CNT electrodes to make solid-state REs. The sensitivity of these REs toward NaCl was very low. Bare Ag/AgCl, metals, CNTs, and carbon, when tested as REs, were found to have high sensitivity and/or low repeatability. Sensitivity of the CNT-based ISE and RE system was 58 ± 3 mV/decade. The OCP of the system was also stable for at least 1 h. A portable voltmeter for future in vivo studies with CNT-based wearable ISEs was realized demonstrating the suitability of the technique for future applications. The skin compatibility of CNT-based electrodes was previously addressed, further establishing the potential use of the materials for skin-wearable applications.68 Overall, CNT electrodes can provide a powerful means to overcome three major challenges in sweat analysis using wearable sensors: Foremost, is the stability of the interaction between the ISM film and the electrodes. Second, is the reliability of solid-state REs. Third, is the ability to form a flexible platform stable against bending. All these features make CNT-based devices, as presented here, a very attractive platform for future wearable technology devices. 4. EXPERIMENTAL SECTION 4.1. CNT Electrodes. MWCNTs were synthesized directly on SiO2/Si substrates by CVD using ethylene (20 sccm) mixed with H2 (1000 sccm) at 900 °C under atmospheric conditions (Lindberg Blue reactor). A 2.5 nm Ni film, deposited on the substrates by e-beam evaporation (VST), was used as the catalyst. Ni films were patterned on SiO2/Si substrates by optical lithography. CNTs were transferred to flexible adhesive substrates, such as polyimide (Kapton, 3M) and temporary tattoo paper (Papilio), using techniques described previously.34 PDMS (Sylgard 184, Dow Corning) was also tested as a flexible substrate. Electrical sheet resistance of the CNT films after transfer to flexible substrates was in the range of 100−1800 Ω/□.26,34 A laser cutter (ELAS Ltd.) was used to define holes in a double-sided adhesive film (Papilio). Details of the process can be found elsewhere.35 The adhesive layer with predefined holes and the CNT electrode array were aligned and pressed together. This formed a passivation layer exposing only those regions of the CNT electrodes that had ISM or Ag/AgCl coatings. 4.2. ISM Solutions. 1320 mg (33% by weight) of PVC (high molecular weight, Selectophore grade, Sigma-Aldrich), 2644 mg (66.1%) of DOS (purity ≥97.0%, Selectophore grade, Sigma-Aldrich), 28 mg (0.7%) of sodium ionophore X (Selectophore grade, SigmaF


Research Article

ACS Applied Materials & Interfaces Aldrich), and 8 mg (0.2%) of potassium tetrakis((4-chlorophenyl)borate (purity ≥98.0%, Selectophore grade, Sigma-Aldrich) were added to 50 mL of tetrahydrofuran (THF; purity ≥99.9%, anhydrous, inhibitor-free, Sigma-Aldrich).24,31 The mixture was allowed to sit undisturbed for ∼5 min and then stirred until a clear solution was obtained. Concentration of this ISM solution was 0.08 g/mL (i.e., 0.08 g of all the different solutes per milliliter of the solvent) and is referred to as the concentrated ISM solution in the manuscript. Additional THF was added to this solution in the ratio of 9:1 (by volume) to produce a solution of concentration 0.008 g/mL, which is referred to as the dilute ISM solution in the manuscript. 4.3. ISEs. ISM coatings on CNT electrodes were prepared by three different methods. (1) Drop coating 200 μL of concentrated (0.08 g/ mL) ISM solution in two coats of 100 μL each, (2) drop coating 2 mL of dilute (0.008 g/mL) ISM solution in 20 coats of 100 μL each, (3) drop coating 1 mL of the dilute ISM solution in 10 coats of 100 μL each, followed by dropping a coat of 100 μL of the concentrated ISM solution. A gap of 40−60 s is allowed between two consecutive coats. Amounts of solutions used were suitable for electrodes with sizes on the order of 1 cm. All the ISM films were allowed to dry slowly for ∼14 h before characterization. The ISMs were also coated on metal and carbon electrodes using the same methods for comparison with the CNTs. For this purpose 50 nm Au and Pt films, along with a 5 nm Cr attachment layer, were deposited on SiO2/Si substrates by e-beam evaporation (VST). Screen printing of carbon conducting ink (C220, Conductive Compounds) on plastic substrates was performed by following previously published methods.35 4.4. Solid-State RE. Ag/AgCl ink (AGCL-675, Conductive Compounds) was dispersed in PM acetate (SolvChem) to produce a colloidal solution of 0.3 g/mL concentration. A 100 μL solution was drop-coated onto the CNT electrodes (1 cm) and kept at an elevated temperature of 60 °C for ∼15 min until the solvent evaporated, leaving behind a thin coating of Ag/AgCl. Three additional coats of Ag/AgCl solution of 100 μL each were applied in the same manner. The electrodes were left on a hot plate for an additional 40 min to ensure that the Ag/AgCl dried thoroughly. To make the surface of Ag/AgCl film hydrophilic, it was exposed to O2 plasma for 2 min in a radio frequency (RF) system (Femto, Diener), operated at 100 W power. Agarose solution (2%; 2 g of agarose (Type I−B, Sigma-Aldrich) per 100 mL of solvent) was prepared in a 0.5 M NaCl (purity ≥99%, anhydrous, Sigma-Aldrich) solution. Appropriate amounts of agarose and NaCl powders were added to deionized water, stirred for a minute, and heated to ∼130 °C, until the agarose melted and formed a clear solution with water. Agarose solution (200 μL) was drop-coated on the Ag/AgCl/CNT electrodes, while the latter was kept at an elevated temperature (∼80 °C). The agarose layer was then allowed to gelate at room temperature for ∼40 min. A hydrophilic surface of Ag/ AgCl enables proper wetting by the agarose solution and helps in binding the two layers. A passivation layer of NaCl-doped PVC was coated on top of the agarose layer. 1320 mg (33% by weight) of PVC, 2668 mg (66.7%) of DOS, and 12 mg of NaCl (0.3%) were dissolved in 50 mL of THF. This is referred to as the concentrated (0.08 g/mL) PVC solution. It was further diluted by THF to produce a solution of 0.008 g/mL concentration. The PVC passivation layer was prepared in the same manner as the ISEs (method No. 3 in Section 4.3). Dilute PVC solution (1 mL) was drop-coated on the agarose gel, followed by 100 μL of the concentrated solution. The films were allowed to dry overnight for ∼14 h. 4.5. ISE Characterization. The ISEs were activated by dipping in 0.1 M NaCl solution for ∼30 min, followed by dipping in deionized water for ∼15 min before studying their sensing performance. The ISE and RE were dipped in solutions with different NaCl concentrations, and the OCPs were recorded using a high-input impedance (>10 GΩ) voltmeter (HP 34401A). Equilibration time (1 min) was allowed before recording the OCPs. Only minor changes in the OCPs were observed beyond 30 s. The RE used in most of these measurements was the std. RE (commercial Ag/AgCl/NaCl(saturated) RE, Princeton Applied Research). The solid-state RE described in Section 4.4 was also used in some of the measurements. Continuous OCP measure-

ments were performed using a potentiostat (263A, Princeton Applied Research) in a two-electrode configuration, which gave us the potential at ISE in the absence of a current through it. The ISE and RE were kept dipped in the test solution, and the OCPs were recorded while changing solution concentration in steps of ×10 M by taking out a precalculated volume of the solution and adding the same volume of a higher concentration solution. This ensured that the total volume of the test solution did not change during the course of the experiment. For the last step, that is, the jump from 1 × 10−1 to 1 M concentration, an appropriate amount of NaCl particles was added. The solution was continuously stirred throughout the measurement process, using a magnetic stirrer. The OCP (E) of an ISE is given by the Nikolskii−Eisenman equation:

E = constant +

2.303RT log C zF

(1)

Here (R) is the universal gas constant, (T) is the absolute temperature, (F) is the Faraday’s constant, (z) is the charge on the ion being detected, and (C) is its concentration in the test solution.13,14,37 For simplification, it has been assumed that there are no other interfering ions. The plot of the OCP versus the log of the concentration (referred to as the calibration curve in the manuscript) is expected to be a straight line, with a slope = 2.303RT . On substituting the values of zF

the constants, the slope becomes 59.2 mV for a monovalent ion at 25 °C. The slope obtained from experimental data is treated as a measure of “sensitivity” of the ISE. The closer it is to the expected value, the better is the sensor. The sensitivities of the solid-state REs were determined in the same manner. For RE evaluation, lower sensitivity values indicate better performance. Lower LOD of the ISEs was determined using the IUPAC convention, which states that LOD is the concentration of the target ion where the error of the analysis is 100%.69 In other words at LOD the concentration is twice or half of that predicted from the linear fit based on eq 1. With the same equation it can be shown that at LOD, the difference between measured and predicted OCP is 2.303RT ΔE = ± zF log 2 = ±18mV , at room temperature, for univalent ion. The linear fit of the ISE calibration curves in the range from 1 × 10−4 to 1 M was extrapolated to lower concentrations, and its difference from the calibration data was calculated. The concentration at which the difference in OCP exceeds ±18 mV is considered as the LOD of the ISE. The linear range of an ISE is defined as that part of the calibration curve where the deviations from the linear fit do not exceed the measurement error or standard deviations in the measured OCP values.69 4.6. Microscopy. Environmental scanning electron microscopy (ESEM) and EDS inspections of the CNT films and ISEs were performed using FEI Quanta 200 FEG ESEM. Cross-sectional samples were prepared by cutting the CNT-based ISEs and REs using a surgical blade. Pore sizes on the surface of the ISM film were determined from the scanning electron microscopy (SEM) images. 4.7. Portable Voltmeter. To demonstrate portable measurements, a high-input impedance, 31/2 digit voltmeter was fabricated using an ICL7106 integrated circuit chip. It is a dual slope analog-todigital converter, with all the active circuitry needed for a voltmeter, including drivers for LCD display. Details of the external circuit used can be found elsewhere.70 A ZIF connector (Omnetics) was used to connect the flexible substrate with the ISEs and REs to the voltmeter. The customized printed circuit board was fabricated by Quasar Electronics.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07346. Molecular structures of the components of ISM and OCP data of bare carbon, Al, and Pt electrodes (PDF) G


Research Article


■

(15) Bakker, E.; Pretsch, E. Peer Reviewed: The New Wave of IonSelective Electrodes. Anal. Chem. 2002, 74, 420 A−426 A. (16) Bakker, E.; Diamond, D.; Lewenstam, A.; Pretsch, E. Ion Sensors: Current Limits and New Trends. Anal. Chim. Acta 1999, 393, 11−18. (17) Lewenstam, A.; Maj-Zurawska, M.; Hulanicki, A. Application of Ion-Selective Electrodes in Clinical Analysis. Electroanalysis 1991, 3, 727−734. (18) Oesch, U.; Ammann, D.; Simon, W. Ion-Selective Membrane Electrodes for Clinical Use. Clin. Chem. 1986, 32, 1448. (19) Young, C. C. Evolution of Blood Chemistry Analyzers Based on Ion Selective Electrodes. J. Chem. Educ. 1997, 74, 177−182. (20) Inzelt, G.; Lewenstam, A.; Scholz, F. Handbook of Reference Electrodes; Springer-Verlag: Heidelberg, Germany, 2013. (21) Fibbioli, M.; Morf, W. E.; Badertscher, M.; de Rooij, N. F.; Pretsch, E. Potential Drifts of Solid-Contacted Ion-Selective Electrodes Due to Zero-Current Ion Fluxes through the Sensor Membrane. Electroanalysis 2000, 12, 1286−1292. (22) Shinwari, M. W.; Zhitomirsky, D.; Deen, I. A.; Selvaganapathy, P.; Deen, M. J.; Landheer, D. Microfabricated Reference Electrodes and Their Biosensing Applications. Sensors 2010, 10, 1679−1715. (23) Guth, U.; Gerlach, F.; Decker, M.; Oelßner, W.; Vonau, W. Solid-State Reference Electrodes for Potentiometric Sensors. J. Solid State Electrochem. 2009, 13, 27−39. (24) Bandodkar, A. J.; Molinnus, D.; Mirza, O.; Guinovart, T.; Windmiller, J. R.; Valdés-Ramírez, G.; Andrade, F. J.; Schöning, M. J.; Wang, J. Epidermal Tattoo Potentiometric Sodium Sensors with Wireless Signal Transduction for Continuous Non-Invasive Sweat Monitoring. Biosens. Bioelectron. 2014, 54, 603−609. (25) Guinovart, T.; Bandodkar, A. J.; Windmiller, J. R.; Andrade, F. J.; Wang, J. A Potentiometric Tattoo Sensor for Monitoring Ammonium in Sweat. Analyst 2013, 138, 7031−7038. (26) Atar, N.; Grossman, E.; Gouzman, I.; Bolker, A.; Hanein, Y. Reinforced Carbon Nanotubes as Electrically Conducting and Flexible Films for Space Applications. ACS Appl. Mater. Interfaces 2014, 6, 20400−20407. (27) Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Carbon Nanotube−Polymer Composites: Chemistry, Processing, Mechanical and Electrical Properties. Prog. Polym. Sci. 2010, 35, 357−401. (28) Guinovart, T.; Parrilla, M.; Crespo, G. A.; Rius, F. X.; Andrade, F. J. Potentiometric Sensors Using Cotton Yarns, Carbon Nanotubes and Polymeric Membranes. Analyst 2013, 138, 5208−5215. (29) Mousavi, Z.; Teter, A.; Lewenstam, A.; Maj-Zurawska, M.; Ivaska, A.; Bobacka, J. Comparison of Multi-Walled Carbon Nanotubes and Poly(3-Octylthiophene) as Ion-to-Electron Transducers in All-Solid-State Potassium Ion-Selective Electrodes. Electroanalysis 2011, 23, 1352−1358. (30) Crespo, G. A.; Macho, S.; Rius, F. X. Ion-Selective Electrodes Using Carbon Nanotubes as Ion-to-Electron Transducers. Anal. Chem. 2008, 80, 1316−1322. (31) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. All-Solid-State Sodium-Selective Electrode Based on a Calixarene Ionophore in a Poly(Viny1 Chloride) Membrane with a Polypyrrole Solid Contact. Anal. Chem. 1992, 64, 2496−2501. (32) Shibutani, Y.; Yoshinaga, H.; Yakabe, K.; Shono, T.; Tanaka, M. Polymeric Membrane Sodium-Selective Electrodes Based on Calix[4]Arene Ionophores. J. Inclusion Phenom. Mol. Recognit. Chem. 1994, 19, 333−342. (33) Bühlmann, P.; Pretsch, E.; Bakker, E. Carrier-Based IonSelective Electrodes and Bulk Optodes. 2. Ionophores for Potentiometric and Optical Sensors. Chem. Rev. 1998, 98, 1593−1688. (34) David-Pur, M.; Bareket-Keren, L.; Beit-Yaakov, G.; Raz-Prag, D.; Hanein, Y. All-Carbon-Nanotube Flexible Multi-Electrode Array for Neuronal Recording and Stimulation. Biomed. Microdevices 2014, 16, 43−53. (35) Bareket, L.; Inzelberg, L.; Rand, D.; David-Pur, M.; Rabinovich, D.; Brandes, B.; Hanein, Y. Temporary-Tattoo for Long-Term High Fidelity Biopotential Recordings. Sci. Rep. 2016, 6, 25727.

AUTHOR INFORMATION

Corresponding Author

*Phone: +972-3-6407698. E-mail: [email protected]. ORCID

Yael Hanein: 0000-0002-4213-9575 Present Address

S.R. is currently affiliated with the Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, Kharagpur, India, 721302 and is supported by the DST INSPIRE Faculty Award (DST/INSPIRE/04/2015/002287). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.R. thanks the Council of Higher Education, Israel, for funds via “The PBC Program for Fellowships for Outstanding Postdoctoral Fellows from China and India”. The work was partially supported by the European Research Council funding under the European Community’s Seventh Framework Program (FP7/2007−2013)/ERC Grant Agreement No. FUNMANIA306707 (Y.H.) and the FTA-INNI Project on Functional Coatings and Printed Devices.

■

REFERENCES

(1) Fraser, K. J.; Curto, V. F.; Coyle, S.; Schazmann, B.; Byrne, R.; Benito-Lopez, F.; Owens, R. M.; Malliaras, G. G.; Diamond, D. Wearable Electrochemical Sensors for Monitoring Performance Athletes, SPIE Photonic Devices+ Applications. Proc. SPIE 2011, 8118, 81180C. (2) Bandodkar, A. J.; Wang, J. Non-Invasive Wearable Electrochemical Sensors: A Review. Trends Biotechnol. 2014, 32, 363−371. (3) Lynch, A.; Diamond, D.; Leader, M. Point-of-Need Diagnosis of Cystic Fibrosis Using a Potentiometric Ion-Selective Electrode Array. Analyst 2000, 125, 2264−2267. (4) De Giovanni, N.; Fucci, N. The Current Status of Sweat Testing for Drugs of Abuse: A Review. Curr. Med. Chem. 2013, 20, 545−561. (5) Mitsubayashi, K.; Suzuki, M.; Tamiya, E.; Karube, I. Analysis of Metabolites in Sweat as a Measure of Physical Condition. Anal. Chim. Acta 1994, 289, 27−34. (6) Rosner, M. H.; Kirven, J. Exercise-Associated Hyponatremia. Clin. J. Am. Soc. Nephrol. 2006, 2, 151−161. (7) Marques, A. H.; Silverman, M. N.; Sternberg, E. M. Evaluation of Stress Systems by Applying Noninvasive Methodologies: Measurements of Neuroimmune Biomarkers in the Sweat, Heart Rate Variability and Salivary Cortisol. NeuroImmunoModulation 2010, 17, 205−208. (8) Bergeron, M. F. Heat Cramps: Fluid and Electrolyte Challenges During Tennis in the Heat. J. Sci. Med. Sport. 2003, 6, 19−27. (9) Beauchamp, M.; Lands, L. Sweat-Testing: A Review of Current Technical Requirements. Pediatr. Pulmonol. 2005, 39, 507−511. (10) Shirreffs, S.; Maughan, R. Whole Body Sweat Collection in Humans: An Improved Method with Preliminary Data on Electrolyte Content. J. Appl. Physiol. 1997, 82, 336−341. (11) LeGrys, V. A. Assessment of Sweat-Testing Practices for the Diagnosis of Cystic Fibrosis. Arch. Pathol. Lab. Med. 2001, 125, 1420− 1424. (12) Mishra, A.; Greaves, R.; Massie, J. The Relevance of Sweat Testing for the Diagnosis of Cystic Fibrosis in the Genomic Era. Clin. Biochem. Rev. 2005, 26, 135. (13) Buhlmann, P.; Chen, L. D. Ion-Selective Electrodes with Ionophore-Doped Sensing Membranes. In Supramolecular Chemistry: From Molecules to Nanomaterials; Gale, P. A., Steed, J. W., Eds.; John Wiley & Sons, 2012. (14) Bobacka, J.; Ivaska, A.; Lewenstam, A. Potentiometric Ion Sensors. Chem. Rev. 2008, 108, 329−351. H


Research Article

ACS Applied Materials & Interfaces (36) Harvey, C. J.; LeBouf, R. F.; Stefaniak, A. B. Formulation and Stability of a Novel Artificial Human Sweat under Conditions of Storage and Use. Toxicol. In Vitro 2010, 24, 1790−1796. (37) Bakker, E.; Bühlmann, P.; Pretsch, E. Carrier-Based IonSelective Electrodes and Bulk Optodes. 1. General Characteristics. Chem. Rev. 1997, 97, 3083−3132. (38) Lindner, E.; Toth, K.; Pungor, E.; Berube, T. R.; Buck, R. P. Switched Wall Jet for Dynamic Response Measurements. Anal. Chem. 1987, 59, 2213−2216. (39) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; IzadiNajafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6, 296−301. (40) Shim, B. S.; Chen, W.; Doty, C.; Xu, C.; Kotov, N. A. Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring Made by Carbon Nanotube Coating with Polyelectrolytes. Nano Lett. 2008, 8, 4151−4157. (41) Han, J.-W.; Kim, B.; Li, J.; Meyyappan, M. A Carbon Nanotube Based Ammonia Sensor on Cotton Textile. Appl. Phys. Lett. 2013, 102, 193104. (42) Rose, D. P.; Ratterman, M. E.; Griffin, D. K.; Hou, L.; KelleyLoughnane, N.; Naik, R. R.; Hagen, J. A.; Papautsky, I.; Heikenfeld, J. C. Adhesive Rfid Sensor Patch for Monitoring of Sweat Electrolytes. IEEE Trans. Biomed. Eng. 2015, 62, 1457−1465. (43) Musa, A. E.; del Campo, F. J.; Abramova, N.; Alonso-Lomillo, M. A.; Dominguez-Renedo, O.; Arcos-Martinez, M. J.; Brivio, M.; Snakenborg, D.; Geschke, O.; Kutter, J. P. Disposable Miniaturized Screen-Printed Ph and Reference Electrodes for Potentiometric Systems. Electroanalysis 2011, 23, 115−121. (44) da Silva, E. T.; Miserere, S.; Kubota, L. T.; Merkoci, A. Simple on-Plastic/Paper Inkjet-Printed Solid-State Ag/Agcl Pseudoreference Electrode. Anal. Chem. 2014, 86, 10531−10534. (45) Polk, B. J.; Stelzenmuller, A.; Mijares, G.; MacCrehan, W.; Gaitan, M. Ag/Agcl Microelectrodes with Improved Stability for Microfluidics. Sens. Actuators, B 2006, 114, 239−247. (46) Coyle, S.; Lau, K.-T.; Moyna, N.; O'Gorman, D.; Diamond, D.; Di Francesco, F.; Costanzo, D.; Salvo, P.; Trivella, M. G.; De Rossi, D. E.; et al. BiotexBiosensing Textiles for Personalised Healthcare Management. IEEE Trans. Inf. Technol. Biomed. 2010, 14, 364−370. (47) Kasem, K. K.; Jones, S. Platinum as a Reference Electrode in Electrochemical Measurements. Platinum Met. Rev. 2008, 52, 100− 106. (48) Jenkins, D. M.; Song, C.; Fares, S.; Cheng, H.; Barrettino, D. Disposable Thermostated Electrode System for Temperature Dependent Electrochemical Measurements. Sens. Actuators, B 2009, 137, 222− 229. (49) Roy, S.; Bajpai, R.; Soin, N.; Bajpai, P.; Hazra, K. S.; Kulshrestha, N.; Sinha Roy, S.; McLaughlin, J. A.; Misra, D. S. Enhanced Field Emission and Improved Supercapacitor Obtained from PlasmaModified Bucky Paper. Small 2011, 7, 688−693. (50) Fierke, M. A.; Lai, C.-Z.; Buhlmann, P.; Stein, A. Effects of Architecture and Surface Chemistry of Three-Dimensionally Ordered Macroporous Carbon Solid Contacts on Performance of Ion-Selective Electrodes. Anal. Chem. 2010, 82, 680−688. (51) Simonis, A.; Lüth, H.; Wang, J.; Schöning, J. Strategies of Miniaturised Reference Electrodes Integrated in a Silicon Based “One Chip” Ph Sensor. Sensors 2003, 3, 330−339. (52) Liao, W.-Y.; Chou, T.-C. Fabrication of a Planar-Form ScreenPrinted Solid Electrolyte Modified Ag/Agcl Reference Electrode for Application in a Potentiometric Biosensor. Anal. Chem. 2006, 78, 4219−4223. (53) Ha, J.; Martin, S. M.; Jeon, Y.; Yoon, I. J.; Brown, R. B.; Nam, H.; Cha, G. S. A Polymeric Junction Membrane for Solid-State Reference Electrodes. Anal. Chim. Acta 2005, 549, 59−66. (54) Desmond, D.; Lane, B.; Alderman, J.; Glennon, J. D.; Diamond, D.; Arrigan, D. W. M. Evaluation of Miniaturised Solid State Reference Electrodes on a Silicon Based Component. Sens. Actuators, B 1997, 44, 389−396.

(55) Huang, I.-Y.; Huang, R.-S.; Lo, L.-H. Improvement of Integrated Ag/Agcl Thin-Film Electrodes by Kcl-Gel Coating for Isfet Applications. Sens. Actuators, B 2003, 94, 53−64. (56) Kisiel, A.; Kijewska, K.; Mazur, M.; Maksymiuk, K.; Michalska, A. Polypyrrole Microcapsules in All-Solid-State Reference Electrodes. Electroanalysis 2012, 24, 165−172. (57) Guinovart, T.; Crespo, G. A.; Rius, F. X.; Andrade, F. J. A Reference Electrode Based on Polyvinyl Butyral (Pvb) Polymer for Decentralized Chemical Measurements. Anal. Chim. Acta 2014, 821, 72−80. (58) Kisiel, A.; Michalska, A.; Maksymiuk, K.; Hall, E. A. H. All-SolidState Reference Electrodes with Poly(N-Butyl Acrylate) Based Membranes. Electroanalysis 2008, 20, 318−323. (59) Novell, M.; Guinovart, T.; Steinberg, I. M.; Steinberg, M.; Rius, F. X.; Andrade, F. J. A Novel Miniaturized Radiofrequency Potentiometer Tag Using Ion-Selective Electrodes for Wireless Ion Sensing. Analyst 2013, 138, 5250−5257. (60) Fay, C.; Anastasova, S.; Slater, C.; Buda, S. T.; Shepherd, R.; Corcoran, B.; O'Connor, N. E.; Wallace, G. G.; Radu, A.; Diamond, D. Wireless Ion-Selective Electrode Autonomous Sensing System. IEEE Sens. J. 2011, 11, 2374−2382. (61) Gruber, J.; de Santana, H.; Gruber, I.; Gazotti, W. Easily Built Low-Cost Apparatus to Measure Polymer Conductivity. J. Chem. Educ. 1997, 74, 418. (62) Warner, B. D.; Boehme, G.; Pool, K. L. The Construction of Inexpensive Digital Ph Meters. J. Chem. Educ. 1982, 59, 65. (63) Barnett, B. E.; Pollock, J. A.; Battino, R. An Overhead Projector Voltmeter and Thermometer. J. Chem. Educ. 1986, 63, 460. (64) Zhao, X.; Liu, R. Recent Progress and Perspectives on the Toxicity of Carbon Nanotubes at Organism, Organ, Cell, and Biomacromolecule Levels. Environ. Int. 2012, 40, 244−255. (65) Cui, H.-F.; Vashist, S. K.; Al-Rubeaan, K.; Luong, J. H. T.; Sheu, F.-S. Interfacing Carbon Nanotubes with Living Mammalian Cells and Cytotoxicity Issues. Chem. Res. Toxicol. 2010, 23, 1131−1147. (66) Murray, A. R.; Kisin, E.; Leonard, S. S.; Young, S. H.; Kommineni, C.; Kagan, V. E.; Castranova, V.; Shvedova, A. A. Oxidative Stress and Inflammatory Response in Dermal Toxicity of Single-Walled Carbon Nanotubes. Toxicology 2009, 257, 161−171. (67) Monteiro-Riviere, N. A.; Inman, A. O. Challenges for Assessing Carbon Nanomaterial Toxicity to the Skin. Carbon 2006, 44, 1070− 1078. (68) Jung, H. C.; Moon, J. H.; Baek, D. H.; Lee, J. H.; Choi, Y. Y.; Hong, J. S.; Lee, S. H. Cnt/Pdms Composite Flexible Dry Electrodesfor Long-Term Ecg Monitoring. IEEE Trans. Biomed. Eng. 2012, 59, 1472−1479. (69) Mikhelson, K. N. Ion-Selective Electrodes; Springer: Berlin, Germany, 2013. (70) Icl 7106/7107 Application Note from Intersil Website: Http:// Www.Intersil.Com/Content/Dam/Intersil/Documents/An02/An023. Pdf.

I


Carbon Nanotube-Based Ion Selective Sensors for Wearable

Recommend Documents