DETECTION OF PROTEIN CONCENTRATIONS WITH CHRONOPOTENTIOMETRY

The chronopotentiogram of a bare 1 mm(2) Ag/AgCl electrode shows a tra nsition time in the order of 9 s at a current density of -25 A/m(2) at pCl = 2. At this time the concentration of Cl- ions is completely dep leted, a phenomenon that can be explained by using the Nernst-Planck f lux equations. Covering the electrode with an 8 mu m thick membrane of polystyrene beads and repeating the chronopoteniometric experiment re sults 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 m embrane 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 t ransition time as a function of the sample protein concentration. As a n example, it is shown that positively charged lysozyme neutralizes th e initial negative membrane charge, thus increasing the transition tim e. The advantage of this type of protein concentration measurement is its simplicity: only a membrane-covered electrode is needed through wh ich a current is sent, and from the resulting electrode potential chan ge the transition time is determined. (C) 1997 Elsevier Science Limite d.