Study of protein adsorption on polymer coatings surface by combining quartz crystal microbalance with electrochemical impedance methods

Understanding protein adsorption at electrode surface is important for biosensor development, in particular for piezoelectric quartz crystal (PQC) microbalance (QCM) sensor. Two proteins, bovine serum albumin (BSA) and fibrinogen were monitored in situ for their electrode-surface processes using both QCM and electrochemical impedance method. Conductive polymer films, which are based on de-doping polyaniline (PAN), ions-doped PAN (PAN + ClO4−) or sodium dodecyl sulfate (SDS) doped PAN (PAN + SDS) were prepared by electrochemical methods. The effect of the polymer film thickness and doping agents of PAN on protein adsorption were investigated. The maximum adsorptions of proteins onto PAN, PAN + ClO4− and PAN + SDS films were measured, with results showing 0.71, 0.65 and 0.99 μg/cm2 for BSA and 2.73, 1.85 and 4.07 μg/cm2 for fibrinogen, respectively. The results suggest an end-on type adsorption of BSA to produce a mono-layered structure onto all three PAN films. As for fibrinogen, only the ClO4−-doped PAN film was adsorbed in an end-on type mono-layered structure. For the adsorption of fibrinogen on the other two PAN films, the adsorbed protein molecules formed multi-layered structure. The adsorption of proteins onto the PAN films was found to follow the Langmuir isotherm. Results from adsorption kinetics studies showed two consecutive steps in the adsorption of BSA and fibrinogen on PAN films. BSA and fibrinogen molecules were firstly adsorbed onto the films before undergone rearrangement. The adsorption equilibrium constants were 1.64 × 106, 9.51 × 105 and 2.42 × 106 for BSA adsorption and 2.74 × 108, 2. 43 × 108 and 2.52 × 108 for fibrinogen adsorption onto ClO4−-doped PAN, de-doping PAN and SDS-doped PAN films, respectively.