Piezoelectric Quartz Crystal Impedance Study of the Pb²⁺-induced Precipitation of Bovine Serum Albumin and Its Dissolution with EDTA in an Aqueous Solution

Abstract

The piezoelectric quartz crystal impedance technique (QCI) was employed to monitor in situ the Pb²⁺-induced precipitation of BSA onto a gold electrode and the precipitate dissolution with EDTA in an aqueous solution. The critical precipitation concentration of Pb²⁺, at which the resonant frequency decreased significantly, was estimated to be 4.78 × 10^-4 mol/L. The saturated adherence of the precipitate on the electrode was observed when the concentration of Pb²⁺ was greater than 7.53 × 10^-2 mol/L. The frequency response was mainly caused by the mass effect of the precipitate adherence to the electrode, rather than the changes in the physico-chemical properties of the contacting liquid. An excess addition of Na₂EDTA after the Pb²⁺-BSA dissolution led to new precipitation, probably due to the formation of an EDTA precipitate in this medium (pH≈3). The pH effect on the response of the resonant frequency was analyzed by using the sum of two exponential functions. A larger frequency response occurred at a pH greater than pI. These findings have been reasonably explained. Also, a decrease in the concentration of the background electrolyte increased the frequency response.