Detection of protein concentrations with chronopotentiometry

The chronopotentiogram of a bare 1 mm2 Ag/AgCl electrode shows a transition time in the order of 9 s at a current density of − 25 A/m2 at pCl = 2. At this time the concentration of Cl− ions is completely depleted, a phenomenon that can be explained by using the Nernst-Planck flux equations. Covering the electrode with an 8 μm thick membrane of polystyrene beads and repeating the chronopoteniometric experiment results in a decrease of the transition time by a factor of 50. This can be explained by the Donnan effect resulting from the fixed negative membrane charge originating from the acidic groups of the polystyrene. Modulating the fixed charge by adsorbing protein molecules inside the porous membrane from a sample solution results in a variation of the transition time as a function of the sample protein concentration. As an example, it is shown that positively charged lysozyme neutralizes the initial negative membrane charge, thus increasing the transition time. The advantage of this type of protein concentration measurement is its simplicity: only a membrane-covered electrode is needed through which a current is sent, and from the resulting electrode potential change the transition time is determined.