Smart Thermometer Style Sensor with Volume Readout and Visualization for pH Detection

Abstract A novel visual volume-type hydrogel sensor like thermometer for pH response was developed. The multifunctional stimuli-responsive fluorescent hydrogel was fabricated by employing 5, 6-dicarboxylic fluorescein crosslinked partially ammoniated polyacrylamide (PAM). The polymer hydrogel was characterized with fluorescent inverted microscope (FIM) and scanning electron microscope (SEM), as well as N2 adsorption–desorption analysis. The polymer hydrogel emitted fluorescence and exhibited volume phase transition (VPT) in response to pH in aqueous solution. The intelligent response hydrogel was put into an elaborately altered shower-like pipette with uniform holes in which the water can pass through without the swelling hydrogel. A visual thermometer style hydrogel sensor combination of chemical reaction, separation and detection was designed. It was accurately measure the volume of hydrogel instead of the pH value when response to different pH solution by reading the graduation with naked eye. Therefore, the challenge for direct measurement of hydrogel volume was overcome. Meanwhile, a scale bar was also designed to indicate pH according to the volume. We can directly read the pH from the bar, similar to a thermometer in daily life. The volume-type sensor paves the way for VPT hydrogel sensors with convenience, visualization, low-cost, portable, smartness and ease of operation.