A glass microelectrode pH metre (Hanna Instruments) was used to measure the pH of the buffered solutions. A wide pH range buffer was used which was 0.04 mol L−1 in sodium acetate, 0.04 mol L−1 in boric acid, 0.04 mol L−1 in sodium dihydrogen phosphate and 0.1 mol L−1 in sodium sulfate. The pH was adjusted to the desired value by adding 1.0 mol L−1 sodium hydroxide solution or 1.0 mol L−1 hydrochloric acid. The cellulose layers were placed into a self-made flow cell and absorbance spectra were recorded on a Lambda 16 UV–vis spectrometer (Perkin Elmer) at 20 ± 2 °C. Colour images were taken using a Canon Power Shot A 520 or a Nikon D5100 using an AF-S Nikkor 18–105 mm objective lens.