The fact that the sensor layers (and T-shirts) show the absorbance of RBBR to be significantly weaker than the absorbance of GJM-492, albeit using RBBR in excess during the immobilisation procedure, is due to the fact that both the fixation rates onto cellulose [17] and the molar extinction coefficients of anthraquinone and azo dyes are different (anthraquinone dyes have ɛ values of around 6.000–9000 M−1 cm−1 while azo dyes have ɛ values of around 20.000–40.000 M−1 cm−1) [18] and [19]. The sensor layers and the textiles both showed fully reversible response to changing pH (results not shown). Since the indicator and the inert dye are covalently linked to the T-shirts, they can be washed minimum fifty times at 40 °C using conventional washing agents without showing any significant decrease in colouration.