5, 6-diamino-naphthalene-1, 3-disulfonic acid (DANDSA) was obtained from Molekula, UK.
4-pyridinium phenolate (4PP) was synthesised and provided by WuXi App Tec (Hong Kong) Ltd. Structure and purity were confirmed by NMR.
The provenances of Brooker's merocyanine (BM), bis-dimethylaminofuchsone (BDF), phenol blue (PB), and 2, 6-dichloro-4-(2, 4, 6-triphenyl-pyridinium-1-yl)-phenolate (ET33) are as stated previously.
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ROW was used throughout this study and had a resistivity of 15MΩcm (checked daily).
Disposable high purity UV-transparent cuvettes (Brand GmbH) with stoppers were used throughout and are described previously.
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Disposable four-sided optically transparent fluorescence cuvettes (Brand GmbH) made of the same material as the UV cuvettes, with stoppers, were used to record fluorescence spectra.