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.
Highlights
Polymeric membranes have been widely utilized as immobilization matrices for various types of sensors, including optical chemical sensors [1,2], biosensors [3,4] potentiometric sensors [5,6], and amperometric sensors [7,8]
We report on a novel, simple and cost effective method for the preparation of an optical pH sensor based on polyaniline-coated polysulfone (PANI-coated PSU) membranes with a well-defined pH response and reproducibility in the pH range of 4–12
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): A “ dad ̄b pH
Summary
Polymeric membranes have been widely utilized as immobilization matrices for various types of sensors, including optical chemical sensors [1,2], biosensors [3,4] potentiometric sensors [5,6], and amperometric sensors [7,8]. Optical pH sensors can be prepared by immobilization of pH-sensitive indicator dyes into a hydrophilic supporting polymer [23,25,28] or sol-gel matrix [22,29,30]. Optical pH sensors prepared by the former method suffer from non-linear responses, narrow dynamic range for pH measurement and short lifetimes due to the leaching out of the immobilized indicator dyes. Thin films coated with ECPs represent an interesting alternative to indicator-based pH sensor films due to their inherent optical response properties and wider dynamic range for pH measurement. We report on a novel, simple and cost effective method for the preparation of an optical pH sensor based on polyaniline-coated polysulfone (PANI-coated PSU) membranes with a well-defined pH response and reproducibility in the pH range of 4–12. Polyaniline functions as the pH-responsive conducting polymer which is coated on the PSU support to construct the flexible pH-responsive sensor membrane
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