Surfaces designed for charge reversal.

We have created surfaces which switch from cationic at pH 5, by attaching aminodicarboxylic acid units to silica and gold substrates. Charge reversal was demonstrated by monitoring the adsorption of cationic dyes (methylene blue and a tetracationic porphyrin) and an anionic sulfonated porphyrin, at a range of pH using UV−vis absorption and reflection spectroscopy. The cationic dyes bind under neutral conditions (pH 5−7) and are released at pH 1−4, whereas the anionic dye binds under acidic conditions (pH 1−4) and is released at pH 5−7. Gold surfaces were functionalized with two different amphoteric disulfides with short (CH2)2 and long (CH2)10CONH(CH2)6 linkers; the longer disulfide gave surfaces exhibiting charge reversal in a narrower pH range. Adsorption is much faster on the functionalized gold (t1/2 = 62 s) than on functionalized silica (t1/2 = 6900 s), but the final extents of coverage on both surface are similar, for a given dye at a given pH, with maximal coverages of around...