Regular ArticleHigh performance flexible pH sensor based on polyaniline nanopillar array electrode
Graphical abstract
Flexible and thin pH sensors were fabricated using a two electrode configuration comprised of a polyaniline nanopillar array working electrode and an Ag/AgCl reference electrode, showing excellent sensor performances in terms of pH sensitivity, response time, reversibility, repeatability, selectivity, and stability.
Introduction
pH sensors provide a logarithmic measure of hydrogen ion concentration and are essential analytical tools in laboratories, clinics, and industries [1], [2], [3], [4], [5]. Since many biological and chemical reactions are dependent on pH level, pH sensors are widely used in continuous processes to ensure human health care, water quality, food quality, and to monitor chemical or biological reactions [1], [2], [3], [4], [5]. Recently, flexible/wearable sensors have received a great deal of attention for the continuous monitoring of human health [6], [7], [8], [9], [10], [11], [12]. For instance, Wang et al. devised bandage-based pH sensors for a real-time wound pH monitoring [10]. Diamond et al. developed textile-based pH sensors for measuring the pH of sweat, and suggesting a correlation exists between pH and sweating rate [13]. Clinics and quality control laboratories require point of care testing (POCT) technologies because they provide test results quickly, do not require user-expertise, and are relatively cheap [14], [15], [16], [17]. Davis et al. described the fabrication of POCT pH sensors based on disposable capillary fill system for monitoring wound pH [16]. The most common commercial pH sensors are based on conventional glass-type electrodes due to their sensitivities, stabilities, and longevities. However, these pH sensors cannot fully meet the requirements of the abovementioned applications because of the brittleness of glass, size limitations, and lack of structure transformation. For these reasons, pH sensor developments are focused on advanced sensing materials, miniaturization, mechanical properties, cost, and scalability for mass production.
Numerous pH sensitive materials based on metal oxides, such as IrO2 [18], TiO2 [19], RuO2 [20], ZnO [21], Co3O4 [22], WO3 [11], and CuO [23], have been developed for many applications. These materials showed pH sensitivities between 28 and 69 mV per pH. Yang et al. reported that heat-treated iridium oxide films exhibit high pH sensitivity (59.5 mV/pH), a wide pH range (2.38–11.61), and a rapid response time (∼2 s) [18]. Chou et al. fabricated RuO2 thin films as the pH sensing layer, and reported high pH sensitivity, but with enhanced drift and hysteresis effects [20]. Although these metal oxide based sensors offer promising alternative materials to glass electrodes, their non-guaranteed mechanical properties and high cost limit their usages as flexible/wearable sensors. Conducting polymers are also capable of providing pH sensing ability due to the protonation/deprotonation of functional groups at different pH levels [24], [25], [26]. Moreover, conducting polymer-based sensors have the advantages of simplicity, environmental processability, mechanical flexibility, high electrical conductivity, and low cost. Wang et al. described fabrication methods for preparing polyaniline-based wearable sensors to measure pH values in the physiological range, and demonstrated near-Nernstian response (∼58 mV/pH) [10]. Furthermore, as compared with bulk macroscopic structured materials, nanomaterials offer potential advantages because their large surface-to-volume ratios enhance charge transfer abilities, which improve pH sensitivities and response times.
Herein, we report the development of a polyaniline nanopillar (PAN) array-based disposable and flexible pH sensor. Soft lithography using a polymeric blend was employed to prepare flexible nanopillar backbone film. Polyaniline was used as a working electrode and silver/silver chloride (Ag/AgCl) as a reference electrode and these were deposited on patterned-nanopillar backbone films by stencil lithography. Furthermore, the method used is simple and scalable and has a high reproducibility. The produced PAN pH sensors demonstrated high pH sensitivity, good reversibility, rapid response time, good ion selectivity, and low potential drift.
Section snippets
Reagents and materials
Aniline (99.5%), sulfuric acid, hydrogen chloride, sodium hydroxide, potassium hydrogen phthalate, potassium dihydrogen phosphate, tris(hydroxymethyl)aminomethane, borax, potassium chloride, calcium chloride, magnesium chloride, and ammonium chloride were obtained from Sigma-Aldrich. Sodium chloride was purchased from Junsei. A dielectric ink ESL 242-SB was obtained from ElectroScience.
Fabrication of PAN sensor
Polymeric nanopillar arrays were fabricated using silicon mold and soft lithography process using mixture of
Results and discussion
Fig. 1a and 1b shows a schematic of prototype PAN sensor of size 11.5 × 36 mm. The sensor is flexible and is easily bent without breaking because of the mechanical support provided by the polymeric nanopillar arrays (Fig. 1a). To configure the two-electrode pattern, a gold layer was firstly deposited by stencil lithography. Polyaniline and Ag/AgCl layers were used as working and reference electrodes, respectively. The polyaniline layer was electrochemically deposited on nanopillar array using 30
Conclusions
We developed potentiometric pH sensors based on polyaniline nanopillar arrays fabricated by soft lithography and electrodeposition processes. The use of polymer backbone and polyaniline working electrode allowed us to provide mechanical flexibility of PAN sensors. The PAN sensors showed a high pH sensitivity (∼60.3 mV/pH) in a wide pH range of 2.38–11.61, which was maintained even at a bent state. In addition, PAN sensor exhibited good reversibility, rapid response time, good ion selectivity,
Acknowledgements
This research was supported by research program to solve social issues of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3A9D7067457). This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2015R1C1A1A02036556). This work also supported by the BioNano Health-Guard Research Center funded by MSIP of Korea as the Global Frontier Project (Grant number
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2023, Microchemical JournalCitation Excerpt :Specifically, the two typical characteristic peaks of polyaniline were located at 1580 cm−1 and 1500 cm−1, corresponding to the CN and CC stretching vibration of the quinoid ring and benzene ring, respectively [27,33]. The peak of 1300 cm−1 was assigned to the CN stretching of the benzene ring unit, while the peak around 1140 cm−1 can be explained by the CN stretching vibration of the doped quinonoid unit [32,34]. The broad peak near 3430 cm−1 was attributed to either the NH stretching of polyaniline or the –OH groups in phytic acid [31].
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These authors contributed to the paper equally.