Abstract
In this study, prior to the growth of ZnO nanorods (NRs), a 100 nm-thick ZnO seed layer was deposited by RF magnetron sputtering on a glass substrate. Subsequently, ZnO NRs were grown on ZnO seed layers via the hydrothermal method at 90°C for 6 h and annealing at 400°C. By using a Pt electrode as reference electrode, the potentiometric method was used to measure the potential difference between the two ends, and an operation amplifier (OPA) was employed as the readout circuit. The sensor and reference electrode were placed in a buffer solution of different pH values (4, 6, 8, and 10) for potentiometric analysis. Results showed that the average sensitivity of the ZnO NR sensor was ∼4.4 × 10−2 V/pH, and linearity was ∼0.98. The Arduino Uno was used to collect data, and the XBee module was utilized for wireless transmission. The computer side used the C# program to present the user interface, display the measurement results, and realize a wireless sensing network.
Export citation and abstract BibTeX RIS
This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org.
In recent years, the development of wireless sensor networks (WSNs) has attracted increasing research attention. WSN is an important part of the Internet of Things. It is divided into three parts: sensor, communication, and computation. Sensor applications include gas sensor, photodetector, and pressure sensor. Communication applications include Wi–Fi, Bluetooth, and ZigBee. Microcomputers collect data for analysis and processing to achieve automation. They allow people to complete their tasks faster and easier.1–3 Nowadays, WSN is used in various civil fields. Some examples include the use of WSN in logistics companies to quickly classify products; control the quantity of products in stores at any time; and monitor water quality in the aquaculture industry,4 human health,5 and traffic control, WSN is inseparable from people's lives, and it has enormous potential for development. The combined use of sensors or biosensors with WSNs to monitor various data or physiological data has been widely used to measure signals (e.g., gas, temperature, and voltage) at nodes distributed throughout a WSN architecture. An air quality monitoring system was developed for temperature, relative humidity, gas, and luminosity; this system can be used in a smartphone.6
Zinc oxide is a metal oxide N-type semiconductor material. ZnO has a wurtzite structure with an energy gap of 3.37 eV and a large exciton binding energy of 60 meV. It is suitable for use in various sensors. pH determination is a necessary condition for many biochemical and biological engineering applications. The demand for inexpensive commercial pH sensors continues to gain interest due to their numerous applications. ZnO is an amphoteric material that interacts with H+ and OH− ions in pH solutions through adsorption by forming surface bonds or oriented dipoles. The rate of this adsorption is proportional to the concentration of free electrons or holes at the NR surface. The large surface area-to-volume ratio shortens the diffusion distance of the analyte to the electrode surface, thereby improving the signal-to-noise ratio and increasing the response time and sensitivity.7–10 ZnO nanostructures have the advantages of non-toxicity, biosafety, biocompatibility, high electron communication properties, suitability for pH sensing of ZnO nanostructures, and miniaturization of pH sensors.
In this study, a ZnO seed layer was deposited on a glass substrate by RF sputtering, and a ZnO nanorods pH sensor was fabricated by the hydrothermal method. The hydrothermal method has the advantages of simple operation, high safety, and large growth area for ZnO NRs. The Arduino board was used for data collection and analysis, and the XBee module was wirelessly transmitted to the computer to complete the WSN architecture. In the production of sensors, ZnO is characterized by low cost, safety, and simple operation. The signal collection hardware used the Arduino board, which has the advantages of simple learning, low cost, and easy development.
Experimental
Prior to the growth of vertically aligned ZnO NRs, a ZnO thin film was first deposited on the glass substrate as seed layer through radio frequency magnetron sputtering by a ZnO target at room temperature. Subsequently, ZnO NRs were grown on these seed layers by the hydrothermal method at 90°C for 6 h, followed by annealing for 30 min at 400°C.14–17 Silver electrodes were prepared on ZnO NRs via electron beam evaporation18 as shown in Figure 1. Physical properties were analyzed by FE–SEM (Hitachi S-4800I) and X-ray diffraction (XRD, Bruker D8 advance diffractometer). Photoluminescence (PL) by using a He–Cd laser (325 nm, 5 mW) as the excitation light source was used to analyze the optical property of the ZnO NRs. OPA (AD623) was used as the readout circuit, the Pt electrode was used as the reference electrode, and the potential method was used for measurement.11–13 The ZnO sensor was treated with buffer solutions of different pH values (4, 6, 8, and 10) for potential measurement and analysis. The response potentials at different pH values were recorded, and sensor repeatability measurements were also made. The pH value sensor data collection node was fabricated using an Arduino board (Arduino Uno), ZnO sensor, and reference electrode. The PC end was connected to the XBee (Series 2, ZigBee Mesh) receiving end to receive data from the USB serial port and present the current sensing data value through the C# program interface.
Figure 1. A schematic photo of ZnO NR pH sensor.
Results and Discussion
Figure 2 shows the cross-section image of the ZnO NRs grown on the ZnO seed layer. The ZnO NRs presented a hexagonal wurtzite structure with a diameter of ∼65 nm and length of ∼1.7 μm.19–21 Figure 3 shows that the structural property of ZnO NRs was measured by XRD. According to the Joint Committee on Powder Diffraction Standards (JCPDS Card: 36–1451), this figure shows that the peaks corresponded to the (002), (102), and (103) diffraction peaks of wurtzite ZnO. The (002) diffraction peak was stronger than the other peaks, indicating that the ZnO NRs crystals preferentially grew along the c-axis direction.22–31
Figure 2. A cross section FESEM image of the ZnO NRs.
Figure 3. XRD of the ZnO NRs.
In this study, AD623 was used as the readout circuit, the Pt electrode was used as the reference electrode, and the potentiometric method was used for pH measurement. The pH sensor was connected to pin 2, the reference electrode was connected to pin 3, pin 4 was connected to ground, pin 5 was connected to the 3.3 V power supply, and pin 6 was connected to the voltage output. Signals were measured using Keithley 2400. The pH sensor and reference electrode were placed into the test solution for measurement.
Figure 4 shows that the ZnO NR pH sensor were placed into buffer solution of pH 4, 6, 8, and 10 for 30 s. The response voltages of the pH sensors could be stabilized by using solutions of different pH values, and the ZnO NRs pH sensor exhibited a linear response voltage.
Figure 4. Response voltage of the ZnO NR pH sensor at different pH values.
Figure 5 shows that the pH sensitivity of the ZnO NR pH sensor was ∼4.4 × 10−2 V/pH with a linearity of ∼0.98. The ZnO NRs pH sensors demonstrated high linearity for pH measurement, indicating that a high surface volume and crystal quality could enhance the sensitivity of pH sensing. The pH sensor is a potential sensor. The potential difference of the electrode junction was analyzed based on the Nernst equation (Eq. 1), which was generated by the ion reaction accumulated on the surface of the sensing electrode. The ideal pH sensitivity is 5.9 × 10−2 V/pH, which is called the Nernst slope. E is the measured potential difference, E0 is the reference electrode potential, R is the gas constant, T is the absolute temperature, and F is the Faraday constant.
![Equation ([1])](https://content.cld.iop.org/journals/1945-7111/166/9/B3047/revision1/d0001.gif)
Figure 5. Shows the sensitivity of ZnO NR pH sensor.
In WSN application, the Arduino board was used to collect the sensing potential signal, the XBee module was used for wireless transmission, and the measured pH was instantly displayed on the computer. A battery was used as the power supply. The output pin of AD623 was connected to pin A0 of the Arduino board. Rx of the XBee module was connected to pin3 and Tx to pin2. The XBee coordinator module was connected to the computer USB serial port as the signal receiving end.
Figures 6 shows the XBee module and USB converter (XBEE Adapter). Figures 7 and 8 show the architecture of the sensing terminal by using the Arduino board to supply circuit power for measurement. After the computer program was opened and connected, the solution pH was detected. The ZnO pH sensor was fabricated by the hydrothermal method. The data collection node was fabricated using the Arduino board, and the XBee module was used for wireless transmission. The computer C# program interface could immediately receive the pH data and display it on the user's monitor to implement a WSN system.
Figure 6. XBee module and USB converter.
Figure 7. Sensing side: sensor, reference electrode, XBee, AD623, power supply, and Arduino board.
Figure 8. Prototype of wireless sensing system for pH potentiometric sensor.
Conclusions
In this study, ZnO NR pH sensors were fabricated by a hydrothermal method. The sensors could be applied to a WSN. The average sensitivity of the ZnO NR pH sensor was ∼4.4 × 10−2 V/pH with a linearity of ∼0.98. ZnO NRs structures of the pH sensors exhibited high linearity for pH measurement. In the analysis of physical properties, ZnO NRs demonstrated strong crystal quality and high surface area, which could improve the sensitivity of pH sensing. The Arduino Uno board was used for data collection, and the XBee module was employed to communicate with the computer for wireless transmission. Thus, the wireless sensing system was successfully completed. This system has the advantages of low cost, rapid processing, and real-time detection.
Acknowledgment
This work was supported by the Ministry of Science and Technology under contract numbers MOST 107-2622-E-150-002 -CC2, 106-2221-E-150-041-MY3, 106-2622-E-150-005-CC3, and 106-2622-E-150-017-CC2. We also acknowledge the assistance of the Common Laboratory for Micro/Nano Science and Technology of the National Formosa University for some of the measurement equipment used in this study, the Center for Micro/Nano Science and Technology of National Cheng Kung University for device characterization, and Dr. Y. H. Liu for device fabrication and equipment support.
ORCID
Sheng-Joue Young 0000-0003-3164-2949