Hydrogel-based piezoresistive pH sensors: investigations using FT-IR attenuated total reflection spectroscopic imaging

The strong swelling ability of the pH-responsive poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) hydrogel makes the development of a new type of sensor possible, which combines piezoresistive-responsive elements as mechanoelectrical transducers and the phase transition behavior of hydrogels as a chemomechanical transducer. The sensor consists of a pH-responsive PAA/PVA hydrogel and a standard pressure sensor chip. However, a time-dependent sensor output voltage mirrors only the physical swelling process of the hydrogel but not the corresponding chemical reactions. Therefore, an investigation of the swelling behavior of this hydrogel is essential for the optimization of sensor design. In this work, Fourier transform infrared (FT-IR) spectroscopic imaging was used to study the swelling of the hydrogel under in situ conditions. In particular, laterally and time-resolved FT-IR images were obtained in the attenuated total reflection mode and the entire data set of more than 80,000 FT-IR spectra was evaluated by principal component analysis (PCA). The first and third principal components (PCs) indicate the swelling process. Molecular changes within the carboxyl groups were observed in the second and fourth PC and identified as key processes for the swelling behavior. It was found that time-dependent molecular changes are similar to the electrical sensor output signal. The results of the FT-IR spectroscopic images render an improved chemical sensor possible and demonstrate that in situ FT-IR imaging is a powerful method for the characterization of molecular processes within chemical-sensitive materials.