3. Results and discussion
3.1. Tailoring colour changes
Colour changes from yellow to orange or from orange to purple are not easily distinguished by the human eye, especially when they take place gradually. Furthermore, colour changes from yellow to red are less significant for interpretation than colour changes from green to red [14], which is universally interpreted as a change from good to bad.
As a matter of fact, almost no indicator dyes are available that actually show colour changes from green to red upon exposure to analyte ions or molecules, while various dyes show colour changes from yellow to red, for example the pH indicator dye GJM-492, see Fig. 2. In order to provide green–red colour changes, we have therefore combined GJM-492 with the pH-insensitive dye RBBR that exhibits blue colour. While a small amount of blue colour already yields the impression of green (in that wavelength range the eye exhibits higher sensitivity than in the yellow colour range), the red colouration is less affected and still more red than purple. Fig. 3 and Fig. 4 show the colour changes of cellulose layers (and shirts) with increasing amounts of blue dye added to the indicator dye, upon exposure to acids and bases. While the yellow colour gradually changes from bright green to olive and finally blue, the red colour gradually turns purple. By slightly modifying the chemical structure of the phenolic azo dyes, i.e. the substituents ortho to the hydroxyl group, pH indicators with comparable colour change but different pKa value can be obtained (GJM-492: pKa of 6.1, GJM-503: pKa of 7.7). This generic approach of tailoring pKa has been introduced by Hisamoto et al. [15] and has now successfully been adopted by others [16].
3. Results and discussion3.1. Tailoring colour changesColour changes from yellow to orange or from orange to purple are not easily distinguished by the human eye, especially when they take place gradually. Furthermore, colour changes from yellow to red are less significant for interpretation than colour changes from green to red [14], which is universally interpreted as a change from good to bad.As a matter of fact, almost no indicator dyes are available that actually show colour changes from green to red upon exposure to analyte ions or molecules, while various dyes show colour changes from yellow to red, for example the pH indicator dye GJM-492, see Fig. 2. In order to provide green–red colour changes, we have therefore combined GJM-492 with the pH-insensitive dye RBBR that exhibits blue colour. While a small amount of blue colour already yields the impression of green (in that wavelength range the eye exhibits higher sensitivity than in the yellow colour range), the red colouration is less affected and still more red than purple. Fig. 3 and Fig. 4 show the colour changes of cellulose layers (and shirts) with increasing amounts of blue dye added to the indicator dye, upon exposure to acids and bases. While the yellow colour gradually changes from bright green to olive and finally blue, the red colour gradually turns purple. By slightly modifying the chemical structure of the phenolic azo dyes, i.e. the substituents ortho to the hydroxyl group, pH indicators with comparable colour change but different pKa value can be obtained (GJM-492: pKa of 6.1, GJM-503: pKa of 7.7). This generic approach of tailoring pKa has been introduced by Hisamoto et al. [15] and has now successfully been adopted by others [16].
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