1. Introduction

A quantitative analysis of alkalinity and acidity in solutions is essential in laboratories and industries to ensure the quality and safety of products and processes[1-3]. The pH of a solution is determined using various capacitive, chemiresistive, luminescence, optical, and potentiometric techniques[4-9]. Among them, potentiometric sensors are widely used because of their simple structure, easy operation, and low-cost fabrication. Potentiometric sensors measure the open-circuit potential between a pH-sensitive (working) electrode and a reference electrode[ 10-14]. pH monitoring of human body fluids (e.g., sweat, tear, saliva, and urine) has a great potential for diagnosis and self-health care because changes in pH are significantly related to various diseases, such as cystic fibrosis, ocular rosacea, gingivitis, and nephrolithiasis[ 15-19]. In addition, wearable sensor systems based on non-invasive, continuous, and real-time monitoring methods are shifting the current hospital and clinical paradigms towards individual health management, also known as smart mobile healthcare[20-22].

In this regard, conventional glass-type and solution-filled pH sensors have been transformed into miniaturized, flexible, and solid-state pH sensors that can be more suitable for wearable sensor applications [23-25]. To modify conventional pH sensors, numerous metal oxides and conducting polymers have been explored as pH-sensitive materials, including IrOx, RuO₂, MnO₂, TiO₂, SnO₂, polyaniline, polypyrrole, and polyethyleneimine[26-32]. As a state-of-the-art electrode material, iridium oxide-based pH sensors show near- or super-Nernstian responses (> 60 mV/pH) over a wide pH range, good stability, and fast response time[26,33-35]. However, the preparation of iridium oxide electrodes requires a thermal oxidation process at a temperature above 500 ℃, and expensive raw materials limit their usage in pH sensor applications[ 26]. Compared to metal oxides, conducting polymers, particularly polyaniline, are easy to synthesize, mechanically flexible, and relatively inexpensive[14,31,32]. Polyaniline-based materials are sensitive to different pH levels due to their oxidation and reduction reactions with hydronium ions[37]. Recent reports describe that nanostructured electrode materials, compared to their counterpart bulk materials, have enhanced electrochemical sensing performance because of their large surface area, which can enhance charge transfer[38].

In this study, we developed a simple and scalable method for the fabrication of 3D porous PANI films with a hierarchical nanoweb morphology. The unique structure of the PANI film was prepared by a combined photo- and soft-lithography technique based on a large-area nanopillar array, followed by controlled dilute polymerization. The deposition of PANI nanofibers onto the nanopillar surface resulted in a 3D hierarchical nanoweb film with a large surface and open porous structure. The resulting PANI film was demonstrated as a flexible pH sensor electrode using electrochemical measurements, showing a high sensitivity of 60.3 mV/pH, a fast response time of 10 s, good repeatability, and good selectivity.

2. Experimental

2.1. Materials

Ammonium persulfate (98%), aniline (99.5%), hydrochloric acid, calcium chloride, magnesium chloride, ammonium chloride, and potassium chloride were purchased from Sigma-Aldrich (USA). Perchloric acid (70%) was purchased from Junsei (Japan). The pH buffer solutions (pH 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, and 13.0) were purchased from Samchun Chemicals (Korea). Deionized water (18.2 M⋅ cm) was employed in all experiment.

2.2. Fabrication of 3D PANI electrode

Prior to preparing the 3D PANI electrode, a Si nanohole master mold was prepared as follows: A Si wafer was placed in a furnace to induce surface oxidation of the SiO₂ layer. The thermally oxidized wafer was then spin-coated with a photoresist to form a dot-patterned array. The obtained array wafer was etched by inductively coupled plasma (ICP, TCP9400SE, Lam Research, United States) with a gas blend of Cl₂, HBr, and O₂ to form a Si nanohole master mold. To prepare the nano-pillar array film, a mixture of polyurethane (MINS-311RN, Munuta Tech) and NOA63 (Norland Optical Adhesives) was spin-coated onto the Si master mold. After UV-induced polymerization and curing treatment, the polyurethane and NOA63 (PUN) film was peeled off from the Si master mold. For the uniform and fine-controlled synthesis of PANI nanofibers onto the nanopillar array, gold/titanium (Au/Ti) was coated onto the PUN film using a photolithography technique. The Au/Ti/PUN nanostructured film was immersed in a solution of 1 M HClO₄, 6 mM ammonium persulfate, aniline, and deionized water, in which the concentration of aniline monomer was 18 mM. Dilute polymerization of the aniline was performed at 3 ℃ while incubating for 24 h. The obtained 3D PANI electrode was rinsed with deionized water and washed with ethanol several times.

2.3. Characterization

Scanning electron microscopy (SEM) images were obtained using a field emission scanning electron microscope (S-4800). Fourier transform infrared (FT-IR) spectra were collected on a JASCO FT-IR 4600. Each spectrum was recorded from 4000 to 400 cm^-1. Electrochemical characterization was measured by using an Iviumstat (USA). All electrochemical measurements were performed at room temperature, and the obtained data were within the error range of ± 1%. The sensitivity, limit of detection (LOD), response time, repeatability, selectivity, and durability test were performed using an open-circuit potential technique.

3. Results and Discussion

Figure 1a shows the experimental procedure of the fabrication of the 3D flexible PANI film via preparation of a polymeric nanopillar array film, followed by chemically dilute polymerization of aniline monomer onto the nanopillar surface. The one-dimensional (1D) ordered nanopillar array film was prepared by a combined photo- and soft-lithography process using a silicon hole mold (Figure 1a). The PUN solution was filled in a silicon mold (Figure 1b), followed by UV-curing, and finally the 1D nanopillar array film was peeled off from the mold. The as-obtained nanopillar array of PUN film shows a highly ordered 1D array structure (Figure 1c). The dilute polymerization method enabled the synthesis and growth of PANI nanofibers onto the nanopillar surface, resulting in a 3D interconnected structure (Figure 1d and 1e). The surface and cross-sectional SEM images (Figure 1f and 1e) revealed that the 3D hierarchical pore structure was formed from the bottom to the top surfaces. This 3D continuous and open pore structure is favorable for the charge transfer process, enhancing electrochemical performance.

The pH sensing performance of the 3D PANI film was evaluated by potentiometric measurements. A two-electrode configuration was employed with the 3D PANI film as the working electrode and Ag/AgCl as the reference electrode. The potentiometric technique measures the change in the open-circuit potential (OCP) between the two electrodes at different pH levels. Figure 2a shows the OCP responses of the 3D PANI electrode with solutions of pH 4~9. The calibration curve in Figure 2b exhibits a linear slope of 60.3 mV/pH with R² = 0.995 in the pH range of 4~9. The OCP responses follow the Nernstian equation[ 39,40]:

where E is the electrode potential (mV), E^o is the standard electrode potential, R is the gas constant (8.314 J/K⋅mol), T is the absolute temperature (K), n is the number of electron transfers, and F is the Faraday constant (96485 C/mol). Based on IUPAC recommendations, the limit of detection was calculated from the intersection of the two lines, and found to be pH 12.1 (Figure 3a). The response time of the pH sensors was evaluated by alternately dipping the 3D PANI electrode in solutions of pH 4 and 6 without the washing and conditioning steps (Figure 3b). The response time was determined when ΔE/Δt reached 1 mV/min, as suggested by the IUPAC method[39, 40]. The 3D PANI electrode showed a fast response (saturated) time of 10 s, and after 10 s, stable OCP signals were observed.

To further demonstrate the electrochemical performance of the 3D PANI electrode, its reversibility was evaluated by measuring its OCP responses in acid-to-base-to-acid solutions (Figure 3c). The 3D PANI electrode exhibited same slopes of 60.1 and 59.8 mV/pH for the forward and reverse directions, respectively. During the titration measurements, the OCP responses were almost maintained in both directions. These results indicate that the 3D PANI electrode has good reversibility. The drift of pH sensors is one of the most important factors for the durability of pH sensors. If the pH electrode has been dried or may not have been properly conditioned, the OCP signals drift, especially during long periods of measurement. To demonstrate its durability, the OCP response of the 3D PANI electrode immersed in pH 7 solution was measured for 15 h (Figure 3d). The 3D PANI electrode exhibited a low potential drift of 0.7 mV/h, which is much lower than those of other previously reported PANI electrodes[9]. The most important characteristic of a potentiometric pH sensor is the ability to selectively measure a primary ion (H⁺) over other interfering ions present in the solution. The selectivity coefficient of the pH sensor was investigated by the separate solution method (SSM) recommended by IUPAC. To evaluate the selectivity coefficients of the pH sensor, the OCP responses were measured for the primary ion (H⁺) in the solution and compared with the responses for interfering ions including Na⁺, K⁺, NH₄⁺, Ca²⁺, and Mg²⁺. The selectivity coefficient (K) can be calculated by SSM using the following equation[39,40]:

where I is the primary ion and J is the interfering ion, E is the experimentally measured EMF value of the pH sensor, R is the gas constant (8.314 J/mol⋅K), T is the absolute temperature, F is the Faraday constant (96485 C/mol), z is the charge number, and C is the concentration of primary ion. Based on the IUPAC recommended SSM, the obtained selectivity coefficients (K) for the pH sensor are shown in Table 1. A K value equal to 1.0 means that the pH sensor responds equally to the primary ion (H⁺) as well as interfering ions. The 3D PANI-based pH sensor shows K values smaller than 1.0, indicating that the pH sensor can accurately measure H₃O⁺ over interfering ions. The superior sensing performance of the 3D PANI film originates from the intrinsic redox reactions of PANI in the presence of hydronium ions in solution. To confirm the successful synthesis of PANI on the Au/Ti pillar, Fourier transform infrared (FT-IR) spectroscopy was used, as shown in Figure 4. The prominent peaks of the PANI electrode at 1534, 1396, 1203, 905, and 768 cm^-1 are attributed to the C=C stretching in the quinoid ring, C=C stretching in the benzenoid ring, C-N stretching in the benzenoid ring, C-N stretching in the quinoid ring, and C-H out of plane bending[9,41].

The flexibility of the 3D PANI electrode for pH sensing was further investigated by measuring the OCP responses of the electrode in a mechanically bent state. Figure 5a shows a photograph of the bent 3D PANI electrode with a bending radius of 0.25 mm. The bent 3D PANI electrode exhibited stable OCP responses at pH levels of 4, 5, 6, 7, 8, and 9 (Figure 5b). The slope of the calibration curve of the bent 3D PANI electrode was calculated to be 60.3 mV/pH. This value is consistent with that of the mechanically normal 3D PANI electrode (60.4 mV/pH) (Figure 5c). This result is attributed to the excellent mechanical stability of the 3D PANI electrode.

4. Conclusion

This study demonstrated the application of a 3D porous film, composed of hierarchically interconnected PANI nanofibers, as a pH sensor electrode. The 3D PANI electrode was fabricated by a combined photo- and soft-lithography technique, followed by controlled dilute polymerization. The PANI material could detect changes in pH, via redox reactions with hydronium ions. The 3D porous architecture of the PANI electrode resulted in a high sensitivity of 60.3 mV/pH, which is close to ideal Nernstian behavior. In addition, the 3D PANI electrode showed a fast response time of 10 s, good repeatability, and good selectivity. When the 3D PANI electrode was measured under a mechanically bent state, the electrode exhibited a high sensitivity of 60.4 mV/pH, demonstrating flexible pH sensor performance. The excellent pH sensing performance and flexibility of the 3D PANI electrode could lead to its use for wearable pH sensor applications.