Synthesis and Characterization of N-Isopropylacrylamide Microspheres as pH Sensors.

Barry K Lavine,Leah Oxenford,James T Moulton,Nuwan Perera,Kaushalya Sharma Dahal,Rudolf Seitz,Necati Kaval,Mariya Kim,Richard A Bunce

doi:10.3390/s21196493

Abstract

Swellable polymer microspheres that respond to pH were prepared by free radical dispersion polymerization using N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (MBA), 2,2-dimethoxy-2-phenylacetylphenone, N-tert-butylacrylamide (NTBA), and a pH-sensitive functional comonomer (acrylic acid, methacrylic acid, ethacrylic acid, or propacrylic acid). The diameter of the microspheres was between 0.5 and 1.0 μm. These microspheres were cast into hydrogel membranes prepared by mixing the pH-sensitive swellable polymer particles with aqueous polyvinyl alcohol (PVA) solutions followed by crosslinking with glutaric dialdehyde for use as pH sensors. Large changes in the turbidity of the PVA membrane were observed as the pH of the buffer solution in contact with the membrane was varied. These changes were monitored by UV–visible absorbance spectroscopy. Polymer swelling of many NIPA copolymers was reversible and independent of the ionic strength of the buffer solution in contact with the membrane. Both the degree of swelling and the apparent pKa of the polymer microspheres increased with temperature. Furthermore, the apparent pKa of the polymer particles could be tuned to respond sharply to pH in a broad range (pH 4.0–7.0) by varying the amount of crosslinker (MBA) and transition temperature modifier (NTBA), and the amount, pKa, and hydrophobicity of the pH-sensitive functional comonomer (alkyl acrylic acid) used in the formulation. Potential applications of these polymer particles include fiber optic pH sensing where the pH-sensitive material can be immobilized on the distol end of an optical fiber.

Highlights

PH is one of the most common laboratory measurements made due to the fact that many chemical and biological reactions depend on the control of pH
The results of this study unequivocally demonstrate that the apparent pKa of the NIPA polymer particles can be tuned by varying the quantity of the crosslinker (MBA), transition temperature modifier (NTBA), and the amount, pKa, and hydrophobicity of the pH-sensitive functional comonomer in the formulation
The water content of the NK 1-60 particles is less than that of polyvinyl alcohol (PVA), and the turbidity of the PVA membrane is large because the refractive index of the polymer particles is greater than that of PVA

Summary

Introduction

PH is one of the most common laboratory measurements made due to the fact that many chemical and biological reactions depend on the control of pH. Interaction of the indicator with the sample solution leads to a change in its optical properties which is detected through the optical fiber by absorbance or fluorescence of the indicator. These types of measurements are subject to two limitations. As the sensing element becomes smaller, more intensity is required to get a measurable optical signal causing the rate of photodegradation to increase. Another limitation is that the measurement has to be made at the wavelength where the dye absorbs and/or emits which is usually in the visible region of the spectrum. Fiber optic chemical sensors based on absorbance or luminescence cannot take full advantage of technology developed for fiber optic communications which involves measurements in the near infrared region of the spectrum

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Results

Conclusion