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Preparation of the pH-indicators 1–3 and the two types of pH-sensor beads. Reagents and conditions: (a) pentafluorobenzaldehyde, CH3SO3H, 210 °C (28%); (b) 3-mercaptopropionic acid, Et3N, N,N′-dimethylacetamide, 50 °C (39%); (c) mercapto-functionalised silica gel beads, Et3N, N,N′-dimethylacetamide, 60 °C; (d) amino-functionalised poly(HEMA) beads, DCC, NHS, DMAP, N,N′-dimethylformamide; and (e) trimellitic anhydride, CH3SO3H, 165 °C (21%).
