A Flexible Optical pH Sensor Based on Polysulfone Membranes Coated with pH-Responsive Polyaniline Nanofibers

Abstract

A new optical pH sensor based on polysulfone (PSU) and polyaniline (PANI) was developed. A transparent and flexible PSU membrane was employed as a support. The electrically conductive and pH-responsive PANI was deposited onto the membrane surface by in situ chemical oxidative polymerization (COP). The absorption spectra of the PANI-coated PSU membranes exhibited sensitivity to pH changes in the range of 4-12, which allowed for designing a dual wavelength pH optical sensor. The performance of the membranes was assessed by measuring their response starting from high pH and going down to low pH, and vice versa. It was found that it is necessary to precondition the sensor layers before each measurement due to the slight hysteresis observed during forward and backward pH titrations. PSU membranes with polyaniline coating thicknesses in the range of ≈100-200 nm exhibited fast response times of <4 s, which are attributed to the porous, rough and nanofibrillar morphology of the polyaniline coating. The fabricated pH sensor was characterized by a sigmoidal response (R² = 0.997) which allows for pH determination over a wide dynamic range. All membranes were stable for a period of more than six months when stored in 1 M HCl solution. The reproducibility of the fabricated optical pH sensors was found to be <0.02 absorption units after one month storage in 1 M HCl solution. The performance of the optical pH sensor was tested and the obtained pH values were compared with the results obtained using a pH meter device.

Keywords: chemical oxidative polymerization; flexible pH sensor; nanofibers; optical pH sensor membrane; polyaniline; polysulfone.

Scheme 1

Chemical Oxidative Polymerization (COP) of aniline.

Figure 1

FTIR spectra for pure PANI in the acid form, pure PSU and PANI-coated PSU in the acid form.

Figure 2

SEM images for the surface of PANI-PSU-20 (a); cross-section of the same membrane PANI-PSU-20 (c); and polyaniline nanofibers collected from the same reaction (d); Optical images in the dark field mode for PANI-PSU-20 (b).

Figure 3

Digital images for optical membrane sensor immersed in solutions with different pH (right); and the corresponding visible-NIR absorption spectrum (left).

Figure 4

Absorption spectra of the PANI-PSU-20 membranes at different pH.

Figure 5

Graphs of the absorbance change vs. pH for PANI-PSU-20 membranes during forward titrations, pH 4 → pH 12, (a); and backward titrations, pH 12 → pH 4, (c); Dual wavelength calibration curves for the pH dependence of the absorption at 600 and 825 nm (b); and the pH titration curves demonstrating hysteresis at 825 nm (d).

Figure 6

Response time for PANI-PSU-10 membranes (a,b) and PANI-PSU-20 membranes (c,d). Each membrane was placed in a buffer solution of pH 12 and then in buffer solution of pH 2 for the measurement of response time at 825 nm; after that, the same membrane was placed back in a buffer solution of pH 12 for the measurement of response time at 600 nm.

Figure 7

Performance of PANI-PSU-20 membranes after 1 month storage in 1 M HCl. Error bars indicate the standard deviation of absorbance for three replicate samples.