Development of a pH Sensor Using Nanoporous Nanostructures of NiO

Article Journal of Nanoscience and Nanotechnology

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Vol. 14, 1–5, 2014 www.aspbs.com/jnn

Development of a pH Sensor Using Nanoporous Nanostructures of NiO Z. H. Ibupoto∗ , K. Khun, and M. Willander Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden Glass is the conventional material used in pH electrodes to monitor pH in various applications. However, the glass-based pH electrode has some limitations for particular applications. The glass sensor is limited in the use of in vivo biomedical, clinical or food applications because of the brittleness of glass, its large size, the difficulty in measuring small volumes and the absence of deformation (inflexibility). Nanostructure-based pH sensors are very sensitive, reliable, fast and applicable towards in vivo measurements. In this study, nanoporous NiO nanostructures are synthesized on a gold-coated glass substrate by a hydrothermal route using poly(vinyl alcohol) (PVA) as a stabilizer. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used for the morphological and crystalline studies. The grown NiO nanostructures are uniform and dense, and they possess good crystallinity. A pH sensor based on these NiO nanostructures was developed by testing the different pH values from 2–12 of phosphate buffered saline solution. The proposed pH sensor showed robust sensitivity of −43
74 ± 0.80 mV/pH and a quick response time of less than 10 s. Moreover, the repeatability, reproducibility and stability of the presented pH sensor were also studied.

Keywords: pH Sensor, Nickel Oxide, Nanoporous, Buffer Solution.

1. INTRODUCTION Conventional glass-based pH electrodes are widely used for various applications, though they face certain disadvantages. The glass rod type sensor is difficult to be employed for in vivo biomedical, clinical or food applications because of the brittleness of glass, its longer size and the absence of deformation; it is also unable to measure small volumes of analyte. The use of pH measuring devices is increasing significantly in laboratories, clinics and industries due to fact that many chemical synthetic routes are pH dependent. Widely applicable pH measuring devices that are fast, accurate, sensitive, stable and reliable are required in the above mentioned fields.1
2 pH measurements are performed by different methods, such as potentiometric pH sensitive layers,3 ion sensitive field effect transistors4 and conductometric/capacitive.5 Among these techniques, the potentiometric method is widely used because of the simple device fabrication, its low cost and stability over long periods of use. pH measurements are essential for several biochemical and biological functions; nanostructures of different metal oxides are largely used, ∗

Author to whom correspondence should be addressed.

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such as ZnO6 and CuO,7 in the development of pH analytical devices. Recently, nanostructures have been used to make pH devices due to their high surface to volume ratio, which further reduces the diffusion distance of the substrate close to the surface of the electrode; these results in an increased signal to noise ratio and a faster response time.8–16 Several porous materials have been synthesized and used for the specific application.17–19 In addition to ZnO and CuO, NiO is also considered an excellent nanomaterial and is widely applied in battery cathodes, catalysts, gas sensors, electrochromic films, magnetic devices and more.20–23 Due to the volume effect, the quantum size effect, and the surface effect, NiO nanostructures exhibit enhanced and versatile properties compared to bulk and micro-sized NiO. The synthetic route of the NiO has a considerable effect on its structural properties, particle size, distribution and morphology; hence, different synthetic routes for NiO have been adapted and different nanostructures, such as nanoparticles and nanoflowers, have been prepared.24–29 Several NiO nanostructures have been synthesized by thermal evaporation,30 RF magnetron sputtering31 and spray pyrolysis.32 These complicated and high temperature

1533-4880/2014/14/001/005

doi:10.1166/jnn.2014.9373

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Development of a pH Sensor Using Nanoporous Nanostructures of NiO

techniques limit their use; easier and simpler synthetic processes of nanostructures are preferable. The low temperature hydrothermal method is simple, inexpensive, environmentally friendly, and gives a high yield of the desired nanomaterial. In this work, we present novel NiO nanostructures synthesized by the environmentally friendly hydrothermal method, and these nanostructures are successfully applied in the development of a pH sensor. The novel fabricated pH sensor based on NiO nanostructures shows an acceptable electrochemical potential response for a wide pH range from 2–12 and good stability. It also possesses good sensitivity and a fast response time.

2. EXPERIMENTAL DETAILS Nickel nitrate hexahydrate, hexamethylenetetramine and phosphate buffered saline solution were purchased from Sigma Aldrich Sweden. The growth of NiO consists of the following steps. Firstly, the glass substrate was cleaned with isopropanol in an ultra-sonic bath and then dried at room temperature. A gold layer of 100 nm thickness was evaporated onto the glass substrate according to our previously published procedure.33 The gold-coated glass substrate was washed with deionized water and dried with a flow of nitrogen gas. Then, an equimolar 0.1 M nickel nitrate hexahydrate and hexamethylenetetramine solution was prepared for the growth of NiO. The gold electrodes were vertically dipped in the growth solution for 4–6 hours at 98  C. After the preparation of the nickel hydroxide phase, it was completely converted into NiO by annealing the nickel hydroxide decorated electrodes at 450  C. The morphological and structural studies of the synthesized NiO nanostructures were performed by field emission scanning electron microscopy (FE-SEM, JEOL, JSM, 6335F) and transmission electron microscopy (TEM). Powder X-ray diffraction (XRD) was also applied with an X-ray diffractometer equipped with Cu K1 radiation.

Figure 1. The schematic diagram of fabricated pH sensor

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The electrochemical cell assembly consisted of a two electrode system using NiO nanostructures as the working electrode and Ag/AgCl as a reference electrode. The phosphate buffered saline solution of 0.01 M was used for the measurement of the output potential response, and the pH of the buffer solution was adjusted by adding some drops of 0.1 M sodium hydroxide and 0.1 M hydrochloric acid that depending on the requirement for the desired pH value. All the experiments were performed at room temperature and schematic diagram of the fabricated pH sensor is shown in Figure 1.

3. RESULTS AND DISCUSSION 3.1. The Structural Characterization of the NiO Nanostructures The surface morphological investigation of the poly(vinyl alcohol) (PVA)-assisted NiO nanostructures was carried out by scanning electron microscopy. The SEM images show that the surface of the NiO nanostructures is porous, with thin, long threads closely related to a honeycomblike structure, as shown in Figure 2(a). The PVA-assisted growth pattern is very dense and uniform and covers the

Figure 2. SEM images of the nickel oxide nanostructures (a) before measurement and (b) after measurement.

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Development of a pH Sensor Using Nanoporous Nanostructures of NiO

the pH of electrolytic solution. During the interaction of ions or molecules with the surface of NiO nanostructures, a Helmholtz layer is formed via the dipoles orientation or surface bonds with the chemically active ions present in the electrolytic solution. Due to the possible amphoteric nature of NiO, therefore it reacts with the both strong acids and bases. In the amphoteric metal oxides such as NiO, a metal atom must be electropositive that gives enough negative charge on the oxygen in order to attract the H+ ion from the H3 O+ ions. Besides this, metal ion should be sufficient electronegative for extracting the electrons from the neighboring OH− ions.36
37 + NiOS + H+ = NiOHS

Figure 3. The XRD pattern study of the NiO nanostructures.

entire surface of the gold-coated glass substrate. The regular growth pattern of the NiO nanostructures may be caused by the long chain nature of the PVA molecular system. During the growth stage, the NiO may be involved in a colloidal phase with the PVA matrix; the long chains of PVA are easily adsorbed on the colloidal surface of NiO,34
35 and during the adsorption of PVA on the surface of the NiO, the growth activity of the colloid is decreased.34 The growth kinetics of the NiO colloid occurs in a certain direction after the adsorption of PVA, thus we obtained a porous nanostructure with thread-like walls, which are interconnected in a network-like morphology. Figure 2(b) shows the SEM image of the NiO nanostructures after the measurement of different pH values; it was found that the nanostructure has the ability to sustain its morphology in both highly acidic and alkaline conditions with slight change. Figure 3 shows the X-ray diffractogram of the synthesized NiO nanostructures on the gold-coated glass substrate. The intense peak for gold appears in the XRD spectrum, along with other peaks that can be assigned to the hexagonal nanostructure of NiO with a definite lattice. The measured peaks (200), (220) and (311) for NiO are in accordance with the standard JCPDS data card no. 47-1049 of NiO. 3.2. The Electrochemical Measurements for the NiO Based pH Sensor The working principle of pH sensors is estimated by measuring the electromotive force (EMF) of the electrodes and particularly for the ion-sensitive layers it is the concentration of surface sites which develop the pH based surface potential. The pH dependent response of the NiO could be based on the activity at the electrolyte-NiO nanoporous interface where H+ ions are found on the surface of NiO that might be protonated during the interaction with electrolytic solution. At the protonation or deprotonating on the surface of NiO, a surface potential is produced and the magnitude of this surface potential is depending on J. Nanosci. Nanotechnol. 14, 1–5, 2014

(1)

In the present case, silver–silver chloride is used as reference electrode and its potential is constant and the potential of NiO nanostructures is measured against the silver–silver chloride electrode. Therefore electrochemical representation of the proposed device is shown as:37 + /NiOs Ag/AgCls /KClaq
1 M H2 O/NiOHs

(2)

The electromotive force (EMF) of the NiO electrode is the potential difference (E) between the determined potential of the NiO redox as a working electrode ENiO/ZnOH+ and the known potential of the silver/silver chloride as a reference electrode EAg/AgCl/Cl− :37 E = ENiO/NiOH+ − EAg/AgCl/Cl−

(3)

By applying the Nernst equation and at the equilibrium, then eletrode potential could be represented as:37  ENiO/NiOH+ = ENiO/NiOH + −R×T /n×F 

× lnaNiOH+ /aNiO ·aH+ 

(4)

 ENiO/NiOH+ = ENiO/NiOH + −R×T /n×F lnaNiOH+ /aNiO 

−RT /nF ln1/aH+   = ENiO/NiOH + −2303×R·T /F pH  = ENiO/NiOH + +m·pH

(5)

Table I. The comparison of the proposed pH sensor based on NiO nanostructures with the published pH sensors.

Material IrOx+TiO2 IrOx/Ir wire IrOx/AgCl ZnO nanotubes CuO NF ITO RuO2 ZnO thin film NiO NS

Sensitivity Detection Times Detection (mV/pH) range (pH) spend (s) limit (pH) Reference −591 589 −517 459 −28 53 58 38 −4374

1–13 1.13 1.5–12 4–12 2–11 2–12 2 + 13 2–12 1–12

– 1 2 – 25 – – – 10 s

1 1 1.5 4 2 2 2 2 1

[39] [40] [41] [42] [43] [44] [45] [46] This work

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Development of a pH Sensor Using Nanoporous Nanostructures of NiO

Figure 4. The calibration curve for the proposed pH sensor.

 Here ENiO/NiOH + is the redox potential of NiO electrode, R is the ideal gas constant (8.314 J/mol · K), T is the absolute temperature (298) K, F is the faraday constant (96487.3415 C mol−1 , n represents the number of electrons per mole, and aH+ is the activity of hydrogen ion, thus the theoretical slope can be found at room temperature (298 K): (6) m = −591 mV pH−1

The nanoporous NiO-based pH sensor showed an excellent linearity for the selected pH range of 2–12 of phosphate buffered saline solution. The calibration curve shows the response of the proposed pH sensor for the phosphate buffered saline solutions of different pH values; it demonstrates excellent linearity, a sensitivity of −4374 ± 080 mV/pH and a correlation coefficient of 0.996, as shown in Figure 4. The observed sensitivity is lower than the theoretical value of −5916 mV/pH, which could be due to the morphological effects and defect states present in the NiO nanostructures. The presented pH sensor showed a good Nernstian response for the investigated range of pH values. The Nernstian response for the porous NiO-based sensor electrode can be described

Figure 5. The reproducibility curve measured at pH 7.

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Figure 6.

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The repeatability curve for the presented pH sensor.

by the following redox reaction of nickel oxide in water:38 NiO + 2H+ + 2e− ↔ Ni + H2 O (7) The working performance parameters of the NiO-based pH sensor, such as response time, reproducibility, repeatability and stability, were also examined by conducting the independent experiments. The reproducibility of eight NiO nanostructures-based pH sensors was checked at different pH values, and the response of each pH sensor differed by a relative standard deviation of less than 4.5%, as shown in Figure 5. This experiment confirmed an acceptable reproducibility and exhibited a higher reliability of the developed pH sensor. The repeatability is defined as the usability of a fabricated device more than once, as shown in Figure 6. During this experiment, a single pH sensor electrode was chosen to examine the repeatability, and it was found that the pH sensor maintained its sensitivity and analytical performance, indicating a good repeatability of the developed pH sensor. The response time was found to be less than 10 s, with improved sensitivity. The response

Figure 7. The response time measured at pH 7 of the phosphate buffer saline solution.

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time was measured by a fast addition method in the pH range of 2–12, as shown in Figure 7. A response time of less than 10 s was observed upon the successive addition of different pH solutions. The fast response time of the NiObased pH sensor can be attributed to the porous nature of the NiO nanostructures, which provides a sensitive surface to different pH values.

4. CONCLUSIONS In this work, a pH sensor was developed using nanoporous NiO nanostructures grown by a hydrothermal technique. The porous nanostructures of NiO were obtained using polyvinyl alcohol as a stabilizer during the growth period. The NiO nanostructures can be potentially used for the development of sensitive, stable and rapid-response pH sensors. The pH sensor was tested over a range of pHs, from 2 to 12. The obtained results indicate the potential applicability of the developed pH sensor for the monitoring of pH levels in biological fluids and other samples.

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Received: 21 July 2013. Accepted: 14 August 2013.

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