Surfaces Designed for Charge Reversal

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Abstract
We have created surfaces which switch from cationic at pH < 3 to anionic 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 2 molecules nm-2. These charge-reversal processes are reversible and can be repeatedly cycled by changing the pH. We have also created surfaces which undergo irreversible proton-triggered charge switching, using a carbamate-linked thiol carboxylic acid which cleaves in acid. These surfaces are versatile new tools for controlling electrostatic self-assembly at surfaces.