Abstract
A fiber-optic pH sensor based on a tilted fiber Bragg grating (TFBG) with electrostatic self-assembly multilayer sensing film is presented. The pH sensitive polymeric film, poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) was deposited on the circumference of the TFBG with the layer-by-layer (LbL) electrostatic self-assembly technique. The PDDA/PAA film exhibits a reduction in refractive index by swelling in different pH solutions. This effect results in wavelength shifts and transmission changes in the spectrum of the TFBG. The peak amplitude of the dominant spectral fringes over a certain window of the transmission spectrum, obtained by FFT analysis, has a near-linear pH sensitivity of 117 arbitrary unit (a.u.)/pH unit and an accuracy of ±1 a.u. (in the range of pH 4.66 to pH 6.02). The thickness and surface morphology of the sensing multilayer film were characterized to investigate their effects on the sensor's performance. The dynamic response of the sensor also has been studied (10 s rise time and 18 s fall time for a sensor with six bilayers of PDDA/PAA).
Highlights
PH monitoring is widely demanded in biomedical, environmental, and industrial fields such as clinical blood analysis, industrial wastewater treating, food processing, and process control in bioreactors [1]
We propose a new fiber-optic pH sensor based on a tilted fiber Bragg grating (TFBG) with poly(diallyldimethylammonium chloride) (PDDA)/poly(acrylic acid) (PAA) multilayer sensing films
Can be controlled to within a few nanometers. This control is very important for preparing fiber-optic sensors, as some types of sensors have a very short range of penetration of the optical probing field outside of the fiber [4]
Summary
PH monitoring is widely demanded in biomedical, environmental, and industrial fields such as clinical blood analysis, industrial wastewater treating, food processing, and process control in bioreactors [1]. The PDDA/PAA coating has a high refractive index (relative to the material of the fiber, i.e., silica), so the resonance wavelength and the coupling strength change with the pH of the solution. Similar to our work on a TFBG pressure sensor [14], by applying a fast Fourier transform (FFT) analysis of a suitable region of the transmission spectrum, we obtain a direct measure of the visibility and of the pH of the solution. Another advantage of the TFBG pH sensor is its inherent temperature reference that is provided by the core mode reflection wavelength. Dynamic measurements show that fast responses were obtained with a rise time (tr) of 10 s and fall time (tf) of 18 s for the sensor with six bilayers of PDDA/PAA
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