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Flexible sensors based on two conductive electrodes and MWCNTs coating for efficient pH value measurement
Journal of Alloys and Compounds 794 (2019) 76e83
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Flexible sensors based on two conductive electrodes and MWCNTs coating for efficient pH value measurement Goran Stojanovi c a, *, Tijana Koji c a, Milan Radovanovi c a, Dragana Vasiljevi c a, c b, Jelena Cveji cc Sanja Pani c b, Vladimir Srdi a b c
Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia Faculty of Technology, University of Novi Sad, Novi Sad, Serbia Faculty of Medicine, University of Novi Sad, Novi Sad, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2019 Received in revised form 21 April 2019 Accepted 23 April 2019 Available online 25 April 2019
The measurement of pH value of solutions is crucial in many fields of modern science. We deposited MWCNTs coating on a planar capacitive structure consisting of interdigitated gold and aluminium electrodes on a flexible Kapton foil substrate, to develop the pH sensors. MWCNTs coating acts as an Hþ ion sensing layer. The complete structural, mechanical and electrical characterization has been performed. The sensors involve supplying gold/aluminium electrodes with an input voltage and measuring the changes in the capacitance, resistance and impedance when the sensor chip is exposed to different pH solutions. The experimental setup was built for dipping the sensors in the sample liquids of different pH values from 4 to 9, being very flexible, reproducible and robust. © 2019 Elsevier B.V. All rights reserved.
Keywords: Flexible electronics pH values Sensors
1. Introduction Measurement of pH value (hydrogen ion activity) is very important in many fields of science and industry sectors, such as medicine, biology, agriculture, environmental monitoring [1e4], etc. For example, pH value of water is significant parameter for aquatic animals and plants which can only survive in the framework of the exact pH values or in acidic or alkaline environment [5]. There are many operational principles of pH sensors such as potentiometric, capacitive and optical. Unlike commercial pH sensors which are usually based on conventional rigid glass electrode or on rigid silicon substrate [6e8], in many fields such as wearable electronics, electronic skin or neuronal implants one of the important requirements is to have sensors realized on flexible substrates. In Ref. [5], authors presented interdigital electrode array, with polyaniline layer, manufactured by conventional MEMS technology for detection pH value in the range from 2 to 11. Specially formulated Pt ink (sintered at 190 C) was used in Ref. [9] to fabricate pH sensor using ink-jet printing process, after electrodepositing iridium oxide film over Pt working electrode. The screen printing process was applied in Ref. [10] for fabrication of Ag
* Corresponding author. E-mail address: [email protected] (G. Stojanovi c). https://doi.org/10.1016/j.jallcom.2019.04.243 0925-8388/© 2019 Elsevier B.V. All rights reserved.
reference electrode as well as complex and costly RF sputtering process for deposition of NiO film for pH sensor, tested for the values between 1 and 13. The application of sol-gel procedure for fabrication of pH sensor based on IrOx film on Kapton substrate and enclosed in PDMS enclosure was presented in Ref. [11]. Costly processes such as photolithography and etching were used in Ref. [12] for fabrication of pH micro-sensors based on iridium oxide (IrOx). Moreover, in Ref. [13] authors used IrOx nanoparticles and polydiallyldimethylammonium (PDDA) polymer layers for manufacturing pH potentiometric sensor by means of a layer-bylayer inkjet printing technique. Additionally, a potentiometric textile-based pH sensors composed of a conductive textile material with an electrodeposited IrOx film were reported in Ref. [14]. A pH sensor assigned to smart dressing material was fabricated on PET film, which was attached on a Si substrate [15]. The screen printing process was used in Ref. [16] for fabrication of CuO nanorods based electrochemical pH sensors which were tested for pH in the range from 5 to 9. The values of pH from 5 to 9 were also measured in Ref. [17], using carbon electrode made by different pencils as a working and the Ag/AgCl paste as reference electrodes printed on chromathography paper. The pH sensor using graphite oxide layer as a working electrode and Ag/AgCl as a reference electrode, which worked over one week in contact with human serum was reported in Ref. [18]. Carbon nanotubes (CNTs) can be used in pH sensors, due to their very dominant electrical and mechanical
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characteristics. The techniques applied up to now have been: unpatterned CNT sheet [19], drop casting [20], aerosol jet printing [21], CNT network deposited on polyaniline [22]. However, reported methods have disadvantages in increasing total dimension, non-reproducibility and commercial dispersion usage, respectively. Bearing in mind a wide range of application in which pH measurement is one of the main part, the necessity for an unconventional and fast fabrication technique for realization of pH sensors is obvious. In this work, we present pH sensor constructed as follows: (a) cost-effective foil as flexible (i. e. mechanically bendable) substrate; (b) interdigitated capacitive (IDC) structure realized using two conductive materials gold (Au) and aluminium (Al); and (c) sensing layer made from carbon nanotubes coating on the top of IDC structure. Impedance spectroscopy characterization was conducted for measuring different pH values in solution, using fabricated sensor. Proposed sensor with IDC structure do not need reference electrode which has as a consequence simple operation mode and low-cost fabrication.
2. Experimental 2.1. Fabrication of sensors Sensors electrodes are designed in AutoCAD© program. The sensor has been designed as an interdigitated capacitor with eight fingers. Dimensions of IDC were presented in Fig. 1. The overall dimension of the sensor was 21.75 mm 9.75 mm. The length of each finger was 6 mm, while the width and the gap (spacing between fingers) were 1.5 mm and 0.75 mm, respectively. The same IDC design has been used to manufacture two types of sensors. The sensors have been fabricated using cutting method with two types of conductive materials for electrodes in IDC structure. The cutting
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(xurographic) technique was conducted with Graphtec ce6000-60 plus device. The Kapton polymide film, with a total thickness of 75 mm, was used as a substrate. The conductive electrodes were laminated on the substrate using a device CardLaminator on the temperature of 150 C. The aim of this work is to present flexible pH sensors, which means that low temperature process have to be implemented. Because of that we used unconventional xurographic technique for manufacturing proposed sensors. This method is based on gluing thin sheet of conductive material on foil and after that mechanical cutting of that thin sheet in designed geometrical shape (pattern) of sensor electrodes and finally lamination of that structure with mechanically flexible, previously cleaned Kapton film substrate. Widely accessible conductive materials in the form of thin sheets are aluminium foil and gold leaf, which have been also used in this study. Both materials, aluminium (Al) and gold (Au) are excellent electrical conductors. The aluminium foil is widespread, robust, and it is usually used when the weight of the component should be decreased. The Au has higher electrical conductivity comparing with Al and gold leaf is more expensive than aluminium foil as well as it more delicate for handling. The price of pH sensors can be reduced by replacing Au with other metals, such as Al in this study. In order to: (1) prevent aluminium instability in acid or alkali solution; (2) to protect gold leaf fragility and (3) to increase sensitivity, we created MWCNTs coating from upper side. 2.2. Synthesis and functionalization of multi-walled carbon nanotubes (MWCNTs) MWCNTs synthesis was carried out for 1 h in a flow of ethylene/ nitrogen mixture (1:1) at 700 C, using an in situ pre-reduced 5% Fe-Co/Al2O3 catalyst in a home-made reactor setup that was described earlier [23,24]. The obtained raw material was boiled under reflux for 6 h in diluted NaOH and 16 h in concentrated HNO3 in order to remove the catalyst remains and functionalize the surface of MWCNTs. The resultant sample was collected on a filter and rinsed with distilled water until a pH neutral followed by drying at 110 C for 24 h. 2.3. Preparation of MWCNTs dispersion The obtained functionalized nanotubes (100 mg) were further dispersed in 2-propanol and a certain amount of polyvinylpyrrolidone (PVP) was added in order to decrease the MWCNTs affinity to agglomerate. The choice of 2-propanol as a solvent was justified in terms of its ability to wet the tubes, low toxicity and low freezing point. The MWCNTs dispersion was prepared by ultrasonication treatment for 15 min at 5 C followed by its storage in a freezer prior to use. 2.4. Deposition of MWCNTs film
Fig. 1. Design and dimensions of IDC structure of pH sensor.
The MWCNTs dispersion was deposited on the previously prepared flexible substrates, with corresponding interdigital electrodes, by dip-coating technique (with withdrawal speed of 0.02 cm/s). Layer-by-layer deposition procedure was used and every deposited layer was dried by flowing air for 10 min. Layer deposition procedure was repeated five times and after that the formed film was dried at 120 C. Interdigitated electrode structure was chosen because it enables planar fabrication and at two terminals capacitance, resistance, etc. can be measured and consequently pH value of a small amount of the solution. Two excellent conductive materials Au and Al were used and thanks to the MWCNTs coating a very good sensitivity can be obtained. The fabricated sensor on Kapton film with gold and aluminium electrodes and MWCNTs film are presented in Fig. 2.
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2.5. Characterization techniques The following instruments have been used for sensors characterization: (1) For structural characterization e 3D Optical Profilometer, Huvitz microscope with Panasis software; (2) For morphological and topographic properties: scanning electron microscope (SEM), JOEL JSM 6460 LV scanning microscope with EDS; (3) X-ray diffraction (XRD) measurement was performed on a Rigaku Miniflex 600 unit (CuKa radiation, l ¼ 0.15406 nm) using a counting step of 0.3 and a counting time per step of 3 s. Raman spectra of the sample was obtained using a DXR Raman Microscope equipped with a diode pumped solid state laser (DPSS) with an excitation wavelength of l ¼ 532 nm. The laser was coupled with a CCD camera as a detector, full range grating (900 lines/mm), 10x microscope objectives and OMNIC software for collecting and analyzing the spectra. The sample was exposed to the radiation of the laser with power of 9 mW, six times for 30 s; (4) For mechanical characterization - nanoindentation, Nanoindenter G200, which uses the Berkovich diamond indenter with a face angle of 65.2 ; and (5) For electrical characterization - HP4194A Impedance analyzer connected to PC.
3. Results and discussion 3.1. Structural characterization The profilometer light microscope has been used to discover
structure and roughness of sensor surface and to display 2D and 3D surface profile of the sample. Profilometar 2D and 3D results of sensor with aluminium electrodes are presented in Fig. 3, whereas for sensor with gold electrodes are displayed in Fig. 4. The presented results are on the sensors before the layer of MWCNTs film was applied. The 2D pictures of sensors are depicted with magnification of 5 and 20 times. From Figs. 3 and 4, it can be seen that a layer of aluminium foil and gold leaf on the Kapton is uniform and the edges of the fingers of the IDC capacitor are straight. From 3D pictures can be seen that the thickness of the aluminium electrode is about 120 mm and the thickness of the gold electrode is around 90 mm. Structural characterization of sensors without layer of MWCNTs film were done with scanning electron microscopy (SEM). SEM micrographs of two types of sensors were presented in Fig. 5. It can be seen from this figure that magnification for SEM measurements were 300 and 250 times, respectively. From Fig. 5a, where the layer of aluminium electrode is depicted, it can be seen that thickness of electrode is around 115 mm which coincides with the result from 3D measurements done using Profilometer. Also from Fig. 5b, where the layer of gold electrode is displayed, it can be seen that thickness of electrode approximately 93.6 mm which is a very good agreement with profilometer measurement. The thickness of the surface layer of the electrode of the sample with aluminium is higher than the thickness of the surface layer of the electrode of the sample with gold electrodes.
Fig. 2. Fabricated sensors with MWCNTs film layer and with gold and aluminium electrodes
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Fig. 3. 2D and 3D Profilometer images of sensor on Kapton film with aluminium electrodes.
Fig. 4. 2D and 3D Profilometer images of sensor on kapton film with gold electrodes
Fig. 5. SEM micrograph of sample with (a) aluminium electrodes and (b) gold electrodes.
XRD patterns of the CNTs film on Kapton and pure CNTs are shown in Fig. 6. It can be seen that the strong XRD peaks of the Kapton are dominant in Fig. 6 and overlap the characteristic XRD peaks of CNTs. Thus, the pure CNTs sample was analysed, which exhibits Bragg's reflections characteristic for crystalline graphite the most intense sharp peak at 2q ¼ 25.7 (002), a broad peak around 2q ¼ 43 formed by the overlapping of (100) and (101) signals, as well as the small intensity peak at 2q ¼ 53.3 (004) (PCPDFWIN database, CAS number: 75-1621). Prior to XRD parameter analysis, the (002) peak profile was fitted using pseudoVoigt function. Based on Debye-Scherrer's equation and Bragg's law, the mean crystalline size along the nanotube diameter, as well as an average inter-layer distance (d002) were calculated, followed by the determination of number of graphene sheets in the tube walls [25]. The obtained mean crystalline size along the tube diameter was 3.5 nm. Based on this value and the calculated average inter-layer distance (d002 ¼ 0.346 nm), it was determined that the used CNTs consist of around 10 graphene sheets inside
their walls. As a prominent tool to characterize sp2 and sp3 hybridized carbon atoms, Raman spectroscopy was used to assess the structural quality of the used CNT sample. The spectrum shows three major peaks around 1340 cm1, 1580 cm1 and 2670 cm1, associated to D, G and 2D bands, respectively (Fig. 7). The common interpretations of these bands are related to structural defects in the graphitic tube walls (D band), high symmetry of ordered CNTs (G band) and their crystallinity degree (2D band) [26]. In order to characterize the structural quality of the CNT sample, Raman quality indicators were calculated as intensity ratios of the corresponding bands. Based on relatively high ID/IG ratio (1.98), it can be suggested that the used CNTs exhibit high density of structural defects. According to the commonly accepted meaning of the crystallinity indicators (I2D/IG, I2D/ID) [27], the examined CNT sample can be characterized by weakly developed graphitic ordering (I2D/IG ¼ 0.16) together with the low overall crystalline quality (I2D/ ID ¼ 0.082). In general, the obtained CNTs quality assessment
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Fig. 8. Load-displacement curves of sensors on Kapton film.
Fig. 6. XRD patterns of CNTs film on Kapton and pure CNTs.
Fig. 7. Raman spectrum of CNTs used for the preparation of pH sensor.
revealed that the tubes have undergone a significant structural modification during their treatment in concentrated HNO3 (introduction of surface functional groups, structural deterioration). However, this is a prerequisite and therefore justified step in order to improve the chemical compatibility of CNTs with the dispersing medium and prepare a high-quality stable suspension. 3.2. Mechanical characterization The nanoindentation technique has been applied for measurement of the mechanical properties of fabricated sensors, bearing in mind that they can be exposed to various mechanical stresses during practical application, especially in curved and conformal shapes. The Nanoindenter G200, equipped with a Berkovich threesided pyramidal diamond tip with the face angle of 65.27, was used for nanoindentation measurements. In all indentation experiments, the same settings were used to perform acceptable
indentations. Examination of samples were performed at room temperature and Poisson's ratio was set to be 0.33 for aluminium foil and 0.42 for gold leaf. The indentation cycle is set, time to load was set to 10 s, while peak hold time was set to 5 s. The indents were located 250 mm apart to avoid the influence from adjacent impressions. A pre-set depth of 5 mm has been applied in contact with the electrodes surface layer. Nanoindentation tests were multiple, at least 8 indentations were made, to ensure measurement repeatability for the mechanical properties of analysed samples. Fig. 8 presents mean value of load-displacement curves measured on aluminium and gold electrodes on the Kapton film as a substrate. Fig. 8 reveals that in loading depth at maximum pre-set depth of 5 mm, penetration load is around 14 mN for aluminium electrodes on Kapton film and 22 mN for gold electrodes on Kapton film. The load depends on the material, in this case, on the material of electrodes and substrate, as well as their thicknesses. It can be noticed from Fig. 8 that the displacement into surface exceeds the pre-set depth of 5 mm by some 500 nm. It happens when the Berkovich tip reaches the given pre-set depth, it holds the 5 s as it is set in the initial conditions, and because of the structure and type of material, the tip additionally enters the material a few nanometers. The curve demonstrates a smooth shape, and no pop-in could be detected. Pre-set depth that is set to 5 mm is located in the aluminium and gold, since the thickness of aluminium foil is about 120 mm and the thickness of gold leaf is about 90 mm, as it shown in measurements by means of Profilometer and SEM. It can be confirmed that Kapton film has not been reached. In both curves it can be seen a viscoelastic return of the samples during the hold load period after unloading. Viscoelastic return is smaller for gold electrodes. It can be also concluded from Fig. 8, that aluminium electrodes on Kapton film has the soften structure than the gold electrodes, because it needed the smallest load to reach the same pre-set depth of 5 mm. The hardness of gold is approximately 2.5e3 and for aluminium is 2e2.9. Therefore, for sensor with gold electrodes higher load is needed to reach the specified penetration depth, while the sensor with Al electrodes requires significantly less load.
3.3. Electrical characterization Electrical measurements were conducted using laboratory experimental setup, shown in Fig. 9. It consists of a sterile test tube
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Fig. 9. Laboratory experimental setup for pH value measurement.
filled with solution of different pH values, test tube holder and HP4194A Impedance analyzer connected to PC. As it can be seen from Fig. 10a and b, sensors on Kapton were dipped inside of solution in test tube and connected to the Impedance analyzer. It can be seen from Fig. 10 that measurements were done with two sensors, one with aluminium electrodes and one with gold electrodes. In order to test the sensors capability to detect different pH values, testing solutions with pH values from 4 to 9 were prepared. The sensors tested in five different pH values 4, 5.5, 7, 8 and 9 (these buffers were prepared and tested with a Standard pH meter) in a wide range of frequencies from 1 MHz to 40 MHz. In this range impedance, capacitance and resistance of sensors dipped in solution were measured. Fig. 11 shows resistance of both sensors as a function of pH value of solution at 1 MHz. The capacitance of IDC structure has variation depending upon the change in the dielectric constant of the material over the electrodes as the field changes according to the different media. When this structure exposed to different concentration of Hþ ions different capacitance and resistance measured using Impedance analyzer has been obtained. The obtained resistance is four times lower when we applied MWCNTs coating. When the sensor was exposed to solutions with different pH value, more Hþ ions penetrate into MWCNTs structure, decreasing normal high conductivity of MWCNT coating. As a consequence the resistance of the sensor increases when pH value increasing. The same behaviour were reported in Refs. [28,29]. It
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Fig. 11. Resistance as a function of pH value of solution for sensors with Al and Au electrodes at 1 MHz, including linear fit (dashed lines).
can be seen from Fig. 11 that resistance is almost double smaller for sensor with gold electrodes, in the same conditions, due to higher conductivity of gold comparing to the aluminium conductive materials. We also analysed the variation of resistance in a time, measuring resistance of sensors which were dipped into solution for 40 min. The results showed that the variation of resistance was less than 1%, which means that fabricated sensors are quite stable. Table 1 presents some important parameters of pH sensors proposed in this study and those already reported in literature with the aim to compare their performances. The sensitivity of the described sensors in this work can be calculated using following equation:
S¼
DR DpH
(1)
where DR is the sensor's change of resistance and DpH is the pH value change. The sensitivity of the sensor with Al electrodes is 16.20 U/pH, whereas this parameter is 17.08 U/pH for the sensor with Au electrodes. The linear fitting curves of the measured results are also depicted in Fig. 11 (dashed lines) for both sensors with Al and Au electrodes. We repeated three measurements in order to demonstrate repeatability of the measured results, with increase of pH values as well as during decreasing the pH of tested solution and
Fig. 10. Sensor (a) completely dipped into tested solution, (b) zoomed both sensors in test tube.
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Table 1 Performances comparison of proposed pH sensors and those from literature. Sensor based on
Size
pH range
Linearity
Sensitivity
Ref. no.
IrOx film WO3/Ag Au-IrOx film SnO2 Al elec þ MWCNT Au elec þ MWCNT
26 mm 3 mm 0.82 mm 0.65 mm 500 mm2 4.4 mm 4.8 mm 21.75 mm 9.75 mm 21.75 mm 9.75 mm
2 ÷ 12 4 ÷ 10 4 ÷ 10 2 ÷ 11 4÷9 4÷9
95% 98.9% N/A N/A 93.82% 94.13%
51.4 mV/pH 24.4 mV/pH 62 mV/pH 1.016*104 S/pH 16.20 U/pH 17.08 U/pH
[30] [31] [12] [32] This work This work
Fig. 12. (a) Capacitance as a function of frequency and pH value as parameter for Al sensor, (b) Impedance as a function of frequency and pH value as parameter for Au sensor.
obtained maximum discrepancy was 3.2% for sensor with Al electrodes and 3.7% for sensor with Au electrodes. Response/recovery time was approximately 1.8s/1.7s for both sensors, which is comparable with the results already reported for flexible pH sensors in literature [31]. From Fig. 12a it can be seen how capacitance changes by increasing frequencies, up to 40 MHz. Capacitance decreases as pH value increases, and for pH 9 the capacitance is smallest. From Fig. 12b it can be seen how impedance changes by increasing frequencies up to 40 MHz. Impedance increases as pH increases, and for pH 9 has the highest value. 4. Conclusion In this study we presented flexible interdigitated capacitive sensors for measurement of pH value based on the variation of electrical parameters. The flexible sensors were fabricated with IDC structure and the MWCNTs layer deposited by dip-coating technique. The following characterization techniques have been applied: structural characterization with Profilometer, morphological and topographic properties with SEM, mechanical characterization with nanoindentation technique and electrical characterization using Impedance analyzer. They revealed that the variations of resistance in an ambient environment during time were less than 1%. Thus, the pH sensor was able to provide a reliable and stable results. Electrical resistance increased with increasing pH value of solutions. We have demonstrated advantages of sensing pH values by means of a cost-effective and easy manufactured IDCelectrode sensor with a MWCNTs layer. After repetition of same measurements, interdigitated electrodes as well as MWCNTs coating were without any degradation, thanks to the applied fabrication method which is flexible and robust. Acknowledgements This work is funded in the framework of the projects III45021
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[23] S. Ratkovic, Dj. Vujicic, E. Kiss, G. Boskovic, O. Geszti, Different degrees of weak metalesupport interaction in Fee(Ni)/Al2O3 catalyst governing activity and selectivity in carbon nanotubes' production using ethylene”, Mater. Chem. Phys. 129 (2011) 398e405. Kukovecz, ny, E. Kiss, G. Boskovic, Z. Ko nya, A. [24] S. Panic, D. Rakic, V. Guzsva Optimization of thiamethoxam adsorption parameters using multi-walled carbon nanotubes by means of fractional factorial design, Chemosphere 141 (2015) 87e93. [25] B. Louis, G. Gulino, R. Vieira, J. Amadou, T. Dintzer, S. Galvagno, G. Centi, M.J. Ledoux, C. Pham-Huu, High yield synthesis of multi-walled carbon nanotubes by catalytic decomposition of ethane over iron supported on alumina catalyst, Catal. Today 102e103 (2005) 23e28. [26] L. Bokobza, J.-L. Bruneel, M. Couzi, Raman spectroscopy as a tool for the analysis of carbon-based materials (highly oriented pyrolitic graphite, multilayer graphene and multiwall carbon nanotubes) and of some of their elastomeric composites, Vib. Spectrosc. 74 (2014) 57e63. [27] S. Santangelo, G. Messina, G. Faggio, M. Lanza, C. Milone, Evaluation of crystalline perfection degree of multi-walled carbon nanotubes: correlations between thermal kinetic analysis and micro-Raman spectroscopy, J. Raman Spectrosc. 42 (2011) 593e602. [28] Y. Chen, S.C. Mun, J. Kim, A wide range conductometric pH sensor made with titanium dioxide/multiwall carbon nanotube/cellulose hybrid nanocomposite, IEEE Sens. J. 13 (2013) 4157e4162. [29] S.K. Su, M.Y. Hua, S.L. Cheng, T.W. Juan, M.Y. Shih, C.M. Yang, C.S. Lai, PEDOT/ PSS membrane on flexible substrate for conductometric pH sensor study, in: Extended Abstracts of the 2012 International Conference on Solid State Devices and Materials, Kyoto, 2012, pp. 1077e1078. [30] Y.M. Fu, J.C. Pan, K.L. Tsou, Y.T. Cheng, A flexible WO3-based pH sensor array for 2-D pH monitoring using CPLoP technique, IEEE Electron. Device Lett. 39 (2018) 881e884, 2018. [31] W.D. Huang, H. Cao, S. Deb, M. Chiao, J.C. Chiao, A flexible pH sensor based on the iridium oxide sensing film, Sens. Actuators A 169 (2011) 1e11. [32] E. Gill, K. Arshak, A. Arshak, O. Korostynska, Mixed metal oxide films as pH sensing materials, Microsyst. Technol. 14 (2008) 499e507.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Flexible sensors based on two conductive electrodes and MWCNTs coating for efficient pH value measurement Goran Stojanovi c a, *, Tijana Koji c a, Milan Radovanovi c a, Dragana Vasiljevi c a, c b, Jelena Cveji cc Sanja Pani c b, Vladimir Srdi a b c
Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia Faculty of Technology, University of Novi Sad, Novi Sad, Serbia Faculty of Medicine, University of Novi Sad, Novi Sad, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2019 Received in revised form 21 April 2019 Accepted 23 April 2019 Available online 25 April 2019
The measurement of pH value of solutions is crucial in many fields of modern science. We deposited MWCNTs coating on a planar capacitive structure consisting of interdigitated gold and aluminium electrodes on a flexible Kapton foil substrate, to develop the pH sensors. MWCNTs coating acts as an Hþ ion sensing layer. The complete structural, mechanical and electrical characterization has been performed. The sensors involve supplying gold/aluminium electrodes with an input voltage and measuring the changes in the capacitance, resistance and impedance when the sensor chip is exposed to different pH solutions. The experimental setup was built for dipping the sensors in the sample liquids of different pH values from 4 to 9, being very flexible, reproducible and robust. © 2019 Elsevier B.V. All rights reserved.
Keywords: Flexible electronics pH values Sensors
1. Introduction Measurement of pH value (hydrogen ion activity) is very important in many fields of science and industry sectors, such as medicine, biology, agriculture, environmental monitoring [1e4], etc. For example, pH value of water is significant parameter for aquatic animals and plants which can only survive in the framework of the exact pH values or in acidic or alkaline environment [5]. There are many operational principles of pH sensors such as potentiometric, capacitive and optical. Unlike commercial pH sensors which are usually based on conventional rigid glass electrode or on rigid silicon substrate [6e8], in many fields such as wearable electronics, electronic skin or neuronal implants one of the important requirements is to have sensors realized on flexible substrates. In Ref. [5], authors presented interdigital electrode array, with polyaniline layer, manufactured by conventional MEMS technology for detection pH value in the range from 2 to 11. Specially formulated Pt ink (sintered at 190 C) was used in Ref. [9] to fabricate pH sensor using ink-jet printing process, after electrodepositing iridium oxide film over Pt working electrode. The screen printing process was applied in Ref. [10] for fabrication of Ag
* Corresponding author. E-mail address: [email protected] (G. Stojanovi c). https://doi.org/10.1016/j.jallcom.2019.04.243 0925-8388/© 2019 Elsevier B.V. All rights reserved.
reference electrode as well as complex and costly RF sputtering process for deposition of NiO film for pH sensor, tested for the values between 1 and 13. The application of sol-gel procedure for fabrication of pH sensor based on IrOx film on Kapton substrate and enclosed in PDMS enclosure was presented in Ref. [11]. Costly processes such as photolithography and etching were used in Ref. [12] for fabrication of pH micro-sensors based on iridium oxide (IrOx). Moreover, in Ref. [13] authors used IrOx nanoparticles and polydiallyldimethylammonium (PDDA) polymer layers for manufacturing pH potentiometric sensor by means of a layer-bylayer inkjet printing technique. Additionally, a potentiometric textile-based pH sensors composed of a conductive textile material with an electrodeposited IrOx film were reported in Ref. [14]. A pH sensor assigned to smart dressing material was fabricated on PET film, which was attached on a Si substrate [15]. The screen printing process was used in Ref. [16] for fabrication of CuO nanorods based electrochemical pH sensors which were tested for pH in the range from 5 to 9. The values of pH from 5 to 9 were also measured in Ref. [17], using carbon electrode made by different pencils as a working and the Ag/AgCl paste as reference electrodes printed on chromathography paper. The pH sensor using graphite oxide layer as a working electrode and Ag/AgCl as a reference electrode, which worked over one week in contact with human serum was reported in Ref. [18]. Carbon nanotubes (CNTs) can be used in pH sensors, due to their very dominant electrical and mechanical
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characteristics. The techniques applied up to now have been: unpatterned CNT sheet [19], drop casting [20], aerosol jet printing [21], CNT network deposited on polyaniline [22]. However, reported methods have disadvantages in increasing total dimension, non-reproducibility and commercial dispersion usage, respectively. Bearing in mind a wide range of application in which pH measurement is one of the main part, the necessity for an unconventional and fast fabrication technique for realization of pH sensors is obvious. In this work, we present pH sensor constructed as follows: (a) cost-effective foil as flexible (i. e. mechanically bendable) substrate; (b) interdigitated capacitive (IDC) structure realized using two conductive materials gold (Au) and aluminium (Al); and (c) sensing layer made from carbon nanotubes coating on the top of IDC structure. Impedance spectroscopy characterization was conducted for measuring different pH values in solution, using fabricated sensor. Proposed sensor with IDC structure do not need reference electrode which has as a consequence simple operation mode and low-cost fabrication.
2. Experimental 2.1. Fabrication of sensors Sensors electrodes are designed in AutoCAD© program. The sensor has been designed as an interdigitated capacitor with eight fingers. Dimensions of IDC were presented in Fig. 1. The overall dimension of the sensor was 21.75 mm 9.75 mm. The length of each finger was 6 mm, while the width and the gap (spacing between fingers) were 1.5 mm and 0.75 mm, respectively. The same IDC design has been used to manufacture two types of sensors. The sensors have been fabricated using cutting method with two types of conductive materials for electrodes in IDC structure. The cutting
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(xurographic) technique was conducted with Graphtec ce6000-60 plus device. The Kapton polymide film, with a total thickness of 75 mm, was used as a substrate. The conductive electrodes were laminated on the substrate using a device CardLaminator on the temperature of 150 C. The aim of this work is to present flexible pH sensors, which means that low temperature process have to be implemented. Because of that we used unconventional xurographic technique for manufacturing proposed sensors. This method is based on gluing thin sheet of conductive material on foil and after that mechanical cutting of that thin sheet in designed geometrical shape (pattern) of sensor electrodes and finally lamination of that structure with mechanically flexible, previously cleaned Kapton film substrate. Widely accessible conductive materials in the form of thin sheets are aluminium foil and gold leaf, which have been also used in this study. Both materials, aluminium (Al) and gold (Au) are excellent electrical conductors. The aluminium foil is widespread, robust, and it is usually used when the weight of the component should be decreased. The Au has higher electrical conductivity comparing with Al and gold leaf is more expensive than aluminium foil as well as it more delicate for handling. The price of pH sensors can be reduced by replacing Au with other metals, such as Al in this study. In order to: (1) prevent aluminium instability in acid or alkali solution; (2) to protect gold leaf fragility and (3) to increase sensitivity, we created MWCNTs coating from upper side. 2.2. Synthesis and functionalization of multi-walled carbon nanotubes (MWCNTs) MWCNTs synthesis was carried out for 1 h in a flow of ethylene/ nitrogen mixture (1:1) at 700 C, using an in situ pre-reduced 5% Fe-Co/Al2O3 catalyst in a home-made reactor setup that was described earlier [23,24]. The obtained raw material was boiled under reflux for 6 h in diluted NaOH and 16 h in concentrated HNO3 in order to remove the catalyst remains and functionalize the surface of MWCNTs. The resultant sample was collected on a filter and rinsed with distilled water until a pH neutral followed by drying at 110 C for 24 h. 2.3. Preparation of MWCNTs dispersion The obtained functionalized nanotubes (100 mg) were further dispersed in 2-propanol and a certain amount of polyvinylpyrrolidone (PVP) was added in order to decrease the MWCNTs affinity to agglomerate. The choice of 2-propanol as a solvent was justified in terms of its ability to wet the tubes, low toxicity and low freezing point. The MWCNTs dispersion was prepared by ultrasonication treatment for 15 min at 5 C followed by its storage in a freezer prior to use. 2.4. Deposition of MWCNTs film
Fig. 1. Design and dimensions of IDC structure of pH sensor.
The MWCNTs dispersion was deposited on the previously prepared flexible substrates, with corresponding interdigital electrodes, by dip-coating technique (with withdrawal speed of 0.02 cm/s). Layer-by-layer deposition procedure was used and every deposited layer was dried by flowing air for 10 min. Layer deposition procedure was repeated five times and after that the formed film was dried at 120 C. Interdigitated electrode structure was chosen because it enables planar fabrication and at two terminals capacitance, resistance, etc. can be measured and consequently pH value of a small amount of the solution. Two excellent conductive materials Au and Al were used and thanks to the MWCNTs coating a very good sensitivity can be obtained. The fabricated sensor on Kapton film with gold and aluminium electrodes and MWCNTs film are presented in Fig. 2.
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2.5. Characterization techniques The following instruments have been used for sensors characterization: (1) For structural characterization e 3D Optical Profilometer, Huvitz microscope with Panasis software; (2) For morphological and topographic properties: scanning electron microscope (SEM), JOEL JSM 6460 LV scanning microscope with EDS; (3) X-ray diffraction (XRD) measurement was performed on a Rigaku Miniflex 600 unit (CuKa radiation, l ¼ 0.15406 nm) using a counting step of 0.3 and a counting time per step of 3 s. Raman spectra of the sample was obtained using a DXR Raman Microscope equipped with a diode pumped solid state laser (DPSS) with an excitation wavelength of l ¼ 532 nm. The laser was coupled with a CCD camera as a detector, full range grating (900 lines/mm), 10x microscope objectives and OMNIC software for collecting and analyzing the spectra. The sample was exposed to the radiation of the laser with power of 9 mW, six times for 30 s; (4) For mechanical characterization - nanoindentation, Nanoindenter G200, which uses the Berkovich diamond indenter with a face angle of 65.2 ; and (5) For electrical characterization - HP4194A Impedance analyzer connected to PC.
3. Results and discussion 3.1. Structural characterization The profilometer light microscope has been used to discover
structure and roughness of sensor surface and to display 2D and 3D surface profile of the sample. Profilometar 2D and 3D results of sensor with aluminium electrodes are presented in Fig. 3, whereas for sensor with gold electrodes are displayed in Fig. 4. The presented results are on the sensors before the layer of MWCNTs film was applied. The 2D pictures of sensors are depicted with magnification of 5 and 20 times. From Figs. 3 and 4, it can be seen that a layer of aluminium foil and gold leaf on the Kapton is uniform and the edges of the fingers of the IDC capacitor are straight. From 3D pictures can be seen that the thickness of the aluminium electrode is about 120 mm and the thickness of the gold electrode is around 90 mm. Structural characterization of sensors without layer of MWCNTs film were done with scanning electron microscopy (SEM). SEM micrographs of two types of sensors were presented in Fig. 5. It can be seen from this figure that magnification for SEM measurements were 300 and 250 times, respectively. From Fig. 5a, where the layer of aluminium electrode is depicted, it can be seen that thickness of electrode is around 115 mm which coincides with the result from 3D measurements done using Profilometer. Also from Fig. 5b, where the layer of gold electrode is displayed, it can be seen that thickness of electrode approximately 93.6 mm which is a very good agreement with profilometer measurement. The thickness of the surface layer of the electrode of the sample with aluminium is higher than the thickness of the surface layer of the electrode of the sample with gold electrodes.
Fig. 2. Fabricated sensors with MWCNTs film layer and with gold and aluminium electrodes
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Fig. 3. 2D and 3D Profilometer images of sensor on Kapton film with aluminium electrodes.
Fig. 4. 2D and 3D Profilometer images of sensor on kapton film with gold electrodes
Fig. 5. SEM micrograph of sample with (a) aluminium electrodes and (b) gold electrodes.
XRD patterns of the CNTs film on Kapton and pure CNTs are shown in Fig. 6. It can be seen that the strong XRD peaks of the Kapton are dominant in Fig. 6 and overlap the characteristic XRD peaks of CNTs. Thus, the pure CNTs sample was analysed, which exhibits Bragg's reflections characteristic for crystalline graphite the most intense sharp peak at 2q ¼ 25.7 (002), a broad peak around 2q ¼ 43 formed by the overlapping of (100) and (101) signals, as well as the small intensity peak at 2q ¼ 53.3 (004) (PCPDFWIN database, CAS number: 75-1621). Prior to XRD parameter analysis, the (002) peak profile was fitted using pseudoVoigt function. Based on Debye-Scherrer's equation and Bragg's law, the mean crystalline size along the nanotube diameter, as well as an average inter-layer distance (d002) were calculated, followed by the determination of number of graphene sheets in the tube walls [25]. The obtained mean crystalline size along the tube diameter was 3.5 nm. Based on this value and the calculated average inter-layer distance (d002 ¼ 0.346 nm), it was determined that the used CNTs consist of around 10 graphene sheets inside
their walls. As a prominent tool to characterize sp2 and sp3 hybridized carbon atoms, Raman spectroscopy was used to assess the structural quality of the used CNT sample. The spectrum shows three major peaks around 1340 cm1, 1580 cm1 and 2670 cm1, associated to D, G and 2D bands, respectively (Fig. 7). The common interpretations of these bands are related to structural defects in the graphitic tube walls (D band), high symmetry of ordered CNTs (G band) and their crystallinity degree (2D band) [26]. In order to characterize the structural quality of the CNT sample, Raman quality indicators were calculated as intensity ratios of the corresponding bands. Based on relatively high ID/IG ratio (1.98), it can be suggested that the used CNTs exhibit high density of structural defects. According to the commonly accepted meaning of the crystallinity indicators (I2D/IG, I2D/ID) [27], the examined CNT sample can be characterized by weakly developed graphitic ordering (I2D/IG ¼ 0.16) together with the low overall crystalline quality (I2D/ ID ¼ 0.082). In general, the obtained CNTs quality assessment
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Fig. 8. Load-displacement curves of sensors on Kapton film.
Fig. 6. XRD patterns of CNTs film on Kapton and pure CNTs.
Fig. 7. Raman spectrum of CNTs used for the preparation of pH sensor.
revealed that the tubes have undergone a significant structural modification during their treatment in concentrated HNO3 (introduction of surface functional groups, structural deterioration). However, this is a prerequisite and therefore justified step in order to improve the chemical compatibility of CNTs with the dispersing medium and prepare a high-quality stable suspension. 3.2. Mechanical characterization The nanoindentation technique has been applied for measurement of the mechanical properties of fabricated sensors, bearing in mind that they can be exposed to various mechanical stresses during practical application, especially in curved and conformal shapes. The Nanoindenter G200, equipped with a Berkovich threesided pyramidal diamond tip with the face angle of 65.27, was used for nanoindentation measurements. In all indentation experiments, the same settings were used to perform acceptable
indentations. Examination of samples were performed at room temperature and Poisson's ratio was set to be 0.33 for aluminium foil and 0.42 for gold leaf. The indentation cycle is set, time to load was set to 10 s, while peak hold time was set to 5 s. The indents were located 250 mm apart to avoid the influence from adjacent impressions. A pre-set depth of 5 mm has been applied in contact with the electrodes surface layer. Nanoindentation tests were multiple, at least 8 indentations were made, to ensure measurement repeatability for the mechanical properties of analysed samples. Fig. 8 presents mean value of load-displacement curves measured on aluminium and gold electrodes on the Kapton film as a substrate. Fig. 8 reveals that in loading depth at maximum pre-set depth of 5 mm, penetration load is around 14 mN for aluminium electrodes on Kapton film and 22 mN for gold electrodes on Kapton film. The load depends on the material, in this case, on the material of electrodes and substrate, as well as their thicknesses. It can be noticed from Fig. 8 that the displacement into surface exceeds the pre-set depth of 5 mm by some 500 nm. It happens when the Berkovich tip reaches the given pre-set depth, it holds the 5 s as it is set in the initial conditions, and because of the structure and type of material, the tip additionally enters the material a few nanometers. The curve demonstrates a smooth shape, and no pop-in could be detected. Pre-set depth that is set to 5 mm is located in the aluminium and gold, since the thickness of aluminium foil is about 120 mm and the thickness of gold leaf is about 90 mm, as it shown in measurements by means of Profilometer and SEM. It can be confirmed that Kapton film has not been reached. In both curves it can be seen a viscoelastic return of the samples during the hold load period after unloading. Viscoelastic return is smaller for gold electrodes. It can be also concluded from Fig. 8, that aluminium electrodes on Kapton film has the soften structure than the gold electrodes, because it needed the smallest load to reach the same pre-set depth of 5 mm. The hardness of gold is approximately 2.5e3 and for aluminium is 2e2.9. Therefore, for sensor with gold electrodes higher load is needed to reach the specified penetration depth, while the sensor with Al electrodes requires significantly less load.
3.3. Electrical characterization Electrical measurements were conducted using laboratory experimental setup, shown in Fig. 9. It consists of a sterile test tube
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Fig. 9. Laboratory experimental setup for pH value measurement.
filled with solution of different pH values, test tube holder and HP4194A Impedance analyzer connected to PC. As it can be seen from Fig. 10a and b, sensors on Kapton were dipped inside of solution in test tube and connected to the Impedance analyzer. It can be seen from Fig. 10 that measurements were done with two sensors, one with aluminium electrodes and one with gold electrodes. In order to test the sensors capability to detect different pH values, testing solutions with pH values from 4 to 9 were prepared. The sensors tested in five different pH values 4, 5.5, 7, 8 and 9 (these buffers were prepared and tested with a Standard pH meter) in a wide range of frequencies from 1 MHz to 40 MHz. In this range impedance, capacitance and resistance of sensors dipped in solution were measured. Fig. 11 shows resistance of both sensors as a function of pH value of solution at 1 MHz. The capacitance of IDC structure has variation depending upon the change in the dielectric constant of the material over the electrodes as the field changes according to the different media. When this structure exposed to different concentration of Hþ ions different capacitance and resistance measured using Impedance analyzer has been obtained. The obtained resistance is four times lower when we applied MWCNTs coating. When the sensor was exposed to solutions with different pH value, more Hþ ions penetrate into MWCNTs structure, decreasing normal high conductivity of MWCNT coating. As a consequence the resistance of the sensor increases when pH value increasing. The same behaviour were reported in Refs. [28,29]. It
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Fig. 11. Resistance as a function of pH value of solution for sensors with Al and Au electrodes at 1 MHz, including linear fit (dashed lines).
can be seen from Fig. 11 that resistance is almost double smaller for sensor with gold electrodes, in the same conditions, due to higher conductivity of gold comparing to the aluminium conductive materials. We also analysed the variation of resistance in a time, measuring resistance of sensors which were dipped into solution for 40 min. The results showed that the variation of resistance was less than 1%, which means that fabricated sensors are quite stable. Table 1 presents some important parameters of pH sensors proposed in this study and those already reported in literature with the aim to compare their performances. The sensitivity of the described sensors in this work can be calculated using following equation:
S¼
DR DpH
(1)
where DR is the sensor's change of resistance and DpH is the pH value change. The sensitivity of the sensor with Al electrodes is 16.20 U/pH, whereas this parameter is 17.08 U/pH for the sensor with Au electrodes. The linear fitting curves of the measured results are also depicted in Fig. 11 (dashed lines) for both sensors with Al and Au electrodes. We repeated three measurements in order to demonstrate repeatability of the measured results, with increase of pH values as well as during decreasing the pH of tested solution and
Fig. 10. Sensor (a) completely dipped into tested solution, (b) zoomed both sensors in test tube.
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Table 1 Performances comparison of proposed pH sensors and those from literature. Sensor based on
Size
pH range
Linearity
Sensitivity
Ref. no.
IrOx film WO3/Ag Au-IrOx film SnO2 Al elec þ MWCNT Au elec þ MWCNT
26 mm 3 mm 0.82 mm 0.65 mm 500 mm2 4.4 mm 4.8 mm 21.75 mm 9.75 mm 21.75 mm 9.75 mm
2 ÷ 12 4 ÷ 10 4 ÷ 10 2 ÷ 11 4÷9 4÷9
95% 98.9% N/A N/A 93.82% 94.13%
51.4 mV/pH 24.4 mV/pH 62 mV/pH 1.016*104 S/pH 16.20 U/pH 17.08 U/pH
[30] [31] [12] [32] This work This work
Fig. 12. (a) Capacitance as a function of frequency and pH value as parameter for Al sensor, (b) Impedance as a function of frequency and pH value as parameter for Au sensor.
obtained maximum discrepancy was 3.2% for sensor with Al electrodes and 3.7% for sensor with Au electrodes. Response/recovery time was approximately 1.8s/1.7s for both sensors, which is comparable with the results already reported for flexible pH sensors in literature [31]. From Fig. 12a it can be seen how capacitance changes by increasing frequencies, up to 40 MHz. Capacitance decreases as pH value increases, and for pH 9 the capacitance is smallest. From Fig. 12b it can be seen how impedance changes by increasing frequencies up to 40 MHz. Impedance increases as pH increases, and for pH 9 has the highest value. 4. Conclusion In this study we presented flexible interdigitated capacitive sensors for measurement of pH value based on the variation of electrical parameters. The flexible sensors were fabricated with IDC structure and the MWCNTs layer deposited by dip-coating technique. The following characterization techniques have been applied: structural characterization with Profilometer, morphological and topographic properties with SEM, mechanical characterization with nanoindentation technique and electrical characterization using Impedance analyzer. They revealed that the variations of resistance in an ambient environment during time were less than 1%. Thus, the pH sensor was able to provide a reliable and stable results. Electrical resistance increased with increasing pH value of solutions. We have demonstrated advantages of sensing pH values by means of a cost-effective and easy manufactured IDCelectrode sensor with a MWCNTs layer. After repetition of same measurements, interdigitated electrodes as well as MWCNTs coating were without any degradation, thanks to the applied fabrication method which is flexible and robust. Acknowledgements This work is funded in the framework of the projects III45021
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