Absorption-type optical pH sensitive film based on immobilized purple cabbage pigment

A new absorption-type optical pH sensitive film was prepared by immobilizing purple cabbage pigment (PCP) in sol–gel film. For preparation of the pH sensitive film, PCP was entrapped in sol–gel film and then coated on glass slides by dip-coating method. The sensing properties of the PCP-pH sensitive film were examined by ultraviolet–visible spectrophotometer and p-polarized reflectance method. The results indicate that the film responds rapidly (within 1–2 min) over a wide pH range of 2–11 with linearly calibration. The relative standard deviation (R.S.D.) of its reproducibility is less than 0.3%. In addition, the refractive index n f and the thickness d f of the film are not changed with different pH values. However, the extinction coefficient k f of the film changes regularly as pH changes, tendency of which is consistent with that of the film absorbance at 633 nm. Therefore, PCP-pH sensitive film is proved to be an absorption-type film, which is promising to design absorption-type pH sensor.