Near-Infrared pH Sensor Based on a SPEEK– Polyaniline Polyelectrolyte Complex Membrane

: A polyelectrolyte complex (PEC) membrane based on sulfonated poly (ether ether ketone) and polyaniline (SPEEK-PANI) was developed for pH sensing applications. Aniline was polymerized in the presence of the SPEEK membrane by using in situ chemical oxidative polymerization to yield an ionically crosslinked SPEEK-PANI membrane. The fabricated membrane exhibited sensitivity in the physiological pH range of 2–8. The PEC membrane pH sensor showed good absorption properties in the near-infrared region (NIR). The membrane showed fast response during a de-doping process ( ≈ 90 s), while longer response times are essential for doping processes from the alkaline/neutral pH region to the acidic pH region, which is attributed to the presence of highly acidic sulfonic acid groups with a high buffering capacity in the PEC membrane. The SPEEK-PANI membrane exhibited slightly higher water uptake compared to the neat SPEEK membrane. The membrane exhibited good stability, as it was stored in 1M HCl solution for more than 2 years without physical or visual deterioration. A preconditioning step in 1M HCl ensured that the results were reproducible and allows the pH sensor to be used repeatedly. The PEC sensor membranes are suitable for applications that start at low pH values and move upwards to higher pH values in the 2–8 pH range.


Introduction
Polyelectrolytes are charged macromolecules with positive or negative charges, which are classified as either cationic polyelectrolytes or anionic polyelectrolytes. When two oppositely charged polyelectrolytes mix, a polyelectrolyte complex (PEC) forms due to the entropy gained upon the release of the counter ions [1]. PEC can be formed by different techniques such as direct polyelectrolyte titration [2], jet mixing [3], direct mixing with the required substrate [4], layer-bylayer (LbL) assembly (layered PECs) [5][6][7], and through the in situ polymerization of a suitable monomer onto a preformed macromolecule, which is known as "template" or "matrix" polymerization [8,9].
PECs have been employed in a variety of applications such as self-healing coatings [10], sensors [11], biosensors [12] and biomedical applications [13][14][15]. PEC membranes are considered to be a special class of PECs that can be prepared using LbL assembly [16] or by the direct mixing of polycations and polyanions, followed by a casting process [17]. PEC membranes are pH-responsive due to the presence of physical ionic crosslinks that can be altered by a pH change in the surrounding environment [18]. The swelling/de-swelling behavior of pH-responsive PECs enables their use for optical pH sensing applications [11,19,20]. In this paper, we employ in situ chemical oxidative polymerization to prepare an optical pH-responsive PEC membrane by polymerizing an aniline monomer onto a solid SPEEK membrane.

Preparation of the SPEEK-Polyaniline PEC Membrane
The SPEEK membranes were cast from 5 wt. % N,N-dimethylacetamide solutions using glass Petri dishes to yield membranes with a thickness of ≈50 μm. The PEC membranes were prepared using in situ chemical oxidative polymerization as follows: Dried SPEEK membranes were immersed in 500 mL of 1M HCl solution for 30 min. After that, 0.5 mL of aniline monomer was added to the reaction vessel and stirred for 30 min. Finally, 10 mL solution of a pre-dissolved 1.45 g APS/1M HCl solution was added to the reaction mixture. After a certain period of reaction time, the resulting greenish color SPEEK-polyaniline PEC membrane was removed and washed repeatedly with distilled water, in order to ensure the removal of unreacted materials. The membrane was stored in 1M HCl solution for further analysis.

Sensor Characterization and pH Measurements
A double beam spectrophotometer Cintra 2020 (GBC Scientific Equipment (Melbourne, Australia) was used for recording the absorbance in the range of 400-1000 nm. The reference cell holder was filled with the desired pH buffer solution, the sensor membrane (6 cm long × 0.6 cm wide) was placed and fixed in the sample cell holder filled with the same pH buffer solution, and the measurements were performed in a batch mode. All the measurements were carried out as triplicates and the standard deviations were calculated accordingly. To calculate the pH values based on the detected absorbance, the optical pH sensor membrane was calibrated using a four parameter logistic (4PL) nonlinear regression model, with sigmoidal curve fitting, according to Equation (1): where A is the measured absorbance; a is minimum asymptote; d is the maximum asymptote; c is the inflection point; and b is the hill's slope. This enables the measurement of pH over a wide proton concentration range, as the pseudolinear part of the calibration curve is not the only part used. From the constructed calibration curve, the pH can be calculated as shown in Equation (2):

Results and Discussion
Chemical oxidative polymerization is a simple and rapid method for the preparation of polyaniline-coated substrates for a variety of applications [22]. In this study, the PEC membrane was prepared by the chemical oxidative polymerization of aniline in the presence of a solid thin film of SPEEK. The transparent SPEEK membrane acquired a green color upon the completion of the reaction. Polyaniline is a pH-responsive polymer that has a positive charge on its backbone. The presence of strong sulfonic acid groups (pKa < 1) allows SPEEK to: (1) act as anionic polyelectrolyte dopant for the cationic polyaniline, see Figure 1; and (2) swell fast due to the solvation of -SO3H groups by the surrounding water molecules [23]. Table 1 displays the swelling properties of the neat SPEEK and SPEEK-PANI PEC membranes. Both membranes have reasonable water uptake values which ensure the wettability of the membrane's surfaces for a fast ion-exchange process, while switching through different pH values. The slight extra water uptake in SPEEK-PANI membranes could be attributed to the higher hydrophilicity of the polyaniline matrix.  Polyaniline is a conductive polymer which has been utilized for pH sensing applications due to its pH-responsive nature [24][25][26][27]. The pH sensing property of polyaniline arises from the swelling/deswelling equilibrium upon pH change, during the doping/de-doping process. The obvious advantage of using polyaniline is the chemical stability of the macromolecular indicator compared to the leachable indicator molecules. This allows for the preparation of reusable sensors with extended life time and reproducible results [21].
The prepared sensor membranes were tested as an optical pH sensor across the visible and near infra-red (NIR) regions (400-1000 nm). The SPEEK-PANI PEC membranes exhibited well-defined pH sensitivity in the pH range of 2-8, Figure 2. The experiments were conducted from pH 2 to 8, with 1 pH unit increments. After each experiment, the membrane was reconditioned by rinsing with 1M HCl. The absorption maxima was shifted from (λ = 810 nm at pH = 2) to (λ = 840 nm at pH = 8), which is attributed to the strong ionic interactions and hydrogen bonding between the sulfonic acid groups and the imine groups [28]. Hence, the absorption maxima in the NIR were selected in the middle range (λmax = 825 nm). As expected, the absorption decreased when the pH increased, due to the gradual conversion of polyaniline from the conductive emeraldine salt (ES) into the non-conductive emeraldine base (EB) via the de-doping process. Regardless, at basic pH = 8, a strong blue shift was not observed, which means that the pKa of the polymer was above 8. This behavior could be attributed to the fact that the sulfonic acid groups with low pKa (<1) have a strong ionic interaction and cannot be exchanged easily due to the formation of strong hydrogen bonding with polyaniline [29]. In other words, the sulfonic acid groups tethered from the bulky macromolecule chains require more alkaline conditions (higher basicity) in order to be titrated and exchanged with small anions.

pH Measurements
The calibration graph for the pH dependence of the absorptions at 825 nm is shown in Figure  3a. The measurements were done in triplicate with a standard deviation of the absorbance value over the calibration range of less than 0.003 (a.u.). The obtained data were fitted by using the four parametric logistic Equation (1), and the calibration curve (R 2 = 0.996) was obtained for the absorbance change vs. pH, Figure 3b. This calibration graph, which has a semi-sigmoidal curve character, can be used to find out the pH values of unknown solutions using Equation (2), in forward titrations (going from low to high pH). The backward titrations (from high to low pH) could not feasibly be carried out (refer to the discussion in the next section).

Response Time
The sensor response time is defined as the time required for the sensor output to reach 90% of the change from its previous value to the final settled value [30]. Figure 4 displays the response diagrams for the membrane sensor during both the doping and de-doping processes. The response time for the de-doping process, moving from pH 2 upwards to pH 8, was found to be ≈90 s, see Figure  4a. However, moving in the reverse direction from pH 8 downwards to pH 2 (doping process) proved to be troublesome and required long times that are not practical for sensing applications. This observation is evident from Figure 4b, as there was a negligible increase in the sensor reading during the first five minutes. This behavior could be attributed to the presence of highly acidic sulfonic acid groups (pKa < 1), which act as dopants with strong and broad buffering effect in the applied pH range. Regardless of this shortcoming, SPEEK-polyaniline sensor membranes can be conditioned with 1M HCl and used repeatedly for applications that start at low pH and move forward to higher pH values in the optimized range. . The membrane was conditioned initially with 1M HCl, and then placed in the required starting buffer solution until a constant reading was obtained; this was followed by placing the membrane in the new buffer solution to measure the absorbance. The process was repeated many times with the same membrane sensor.

Sensor Stability
The PEC membrane sensor was stored and preserved in 1M HCl solution for more than two years without any observed physical deterioration of disintegration. The readings for the sensor membrane were checked during interval periods of 2 months and the variation in the obtained readings was found to be within the range of machine error (≤0.02 absorption units).

Conclusions
The current study reported on the fabrication of PEC membrane sensors with SPEEK as the anionic polyelectrolyte solid membrane coated with pH-responsive polyaniline as the cationic polyelectrolyte. The pH sensor membrane exhibited well-distinguished sensing in the pH range 2-8 and displayed the maximum absorbance in the near-infrared region at λmax = 825 nm. The calibration curve for pH measurement was constructed using a four parameter logistic nonlinear regression model and resulted in a semi-sigmoidal curve (R 2 = 0.996). The response time for the sensor membrane was found to be about 1.5 min during the movement from low pH to high pH environments (de-doping). The sensor required extended response times in the reverse direction. This phenomenon requires more investigations in the future. Regardless, the sensor membrane is stable, can be reconditioned using 1M HCl, and is suitable for applications where the movement direction is from acidic to alkaline medium in the 2-8 pH range.