Odor Sensing Based on the Color Changes of pH Indicator/Polyvinylpyrrolidone Films

Changes in the color of pH indicators (Methyl Orange, Methyl Red, Thymol Blue, Phenol Red, Phenolphthalein and Alizarin Yellow) were observed when polyvinylpyrrolidone films containing each of these six pH indecators were exposed to methanol and typical odor components of citrus fruits. Color images of the polymer films obtained with a scanner were separated into red, green and blue components, and then the three components were converted into gray levels, integers from 0 to 255. Differences between the gray levels before and after exposure to the sample gases were characteristic of each of the gases, and showed that Alizarin Yellow/polymer film exposed to linalyl acetate and linalool gave large color changes, as the radar plots of the differences distinctly illustrated. An optical fiber having the Alizarin Yellow/polymer film at the cladding part and a thin gold film on an end surface was used as a sensing element; a decrease in the signal intensity of reflected halogen-lamp light from the fiber was observed at 635 nm when the fiber was exposed to linalyl acetate. A polymer film of 9.00 μm in thickness gave a significant decrease in the intensity; the response reached a steady state in 20 min.