Electrostatic deposition of polycations and polyanions onto cysteine monolayers

Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor the deposition of polyelectrolytes onto an interface modified with a monolayer of cysteine. When using an anionic redox probe, Fe(CN)(6)(-4), the impedance of the cysteine modified electrode is a function of the solution pH; the charge-transfer resistance being higher at basic pH, and smaller under acidic conditions relative to the value found when the solution is buffered near the pI for cysteine. This behavior is consistent with the electrostatic interaction between the anionic redox probe and the net charge of the interface. After exposure of the cysteine-modified interface to 0.10 mol L(-1) HCl solutions containing polystyrene sulfonate, the impedance of the modified interface increases. This behavior is consistent with the interface having a net negative charge, and suggests that the polyanion is confined to the interface. Quartz crystal microbalance measurements also show that 700+/-20 ng of the PSS is deposited under the acidic conditions. After exposure to 0.10 mol L(-1) NaOH solutions containing poly(diallyldimethyl)ammonium chloride, the impedance of the modified interface decreases, consistent with the electrostatic assembly of the polycation to the cysteine monolayer under these conditions. Quartz crystal microbalance measurements indicate that 770+/-40 ng of the poly(diallyldimethylammonium chloride) are deposited under the basic conditions.