A polymer brush is the coating obtained by the tethering of one extremity of polymer chains to a substrate. The macromolecules behavior in such a configuration differs strikingly from the one in solution. The particular case of polyelectrolyte brushes adds electrostatic interactions as well as the crucial role played by the counterions compensating the charge borne by the macroions in the formed film. Indeed, under the right conditions, the osmotic pressure due to the high counterions concentration in the brush causes extreme stretching of the chains. The counterions valency is one of the key parameter to control this particular behavior; replacing monovalent by multivalent ions leads to lower concentration and osmotic pressure, hence to a collapse of the brush thickness.
In this study, we use an electrochemical probe as charge compensating ion. Oxidoreduction reactions then leads to a reversible valency change of the brush counterions, while minimizing changes in their size or solvatation, unavoidable if one uses two different ionic species.
Cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy are used to get insight about the redox probe in the polyelectrolyte brush. Its concentration and mobility, as well as the brush permeability to ionic species are studied.

Figure below represents the oxidoreduction reaction of an electrochemical probe confined in a polyelectrolyte brush. Small ions are extracted from or expelled to the solution to maintain neutral overall charge in the coating. Osmotic pressure due to counter ions concentration changes cause the brush to swell or collapse.