Direct label-free electrochemical detection of proteins using the polarized oil/water interface

Voltammetric behaviors of various globular proteins, including cytochrome c, ribonuclease A, lysozyme, albumin, myoglobin, and alpha-lactalbumin, were studied at the polarized 1,2-dichloroethane/water (DCE/W) interface in the presence of four different anionic surfactants, that is, dinonylnaphthalenesulfonate (DNNS), bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT; AOT), bis(2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl)sulfosuccinate (BDFHS), and bis(2-ethylhexyl)phosphate (BEHP). When the W phase was acidic (pH = approximately 3.4), the surfactants (except for BEHP) added to DCE facilitated the adsorption of the above proteins to the DCE/W interface and gave a well-developed voltammetric wave due to the adsorption/desorption of the proteins. This voltammetric wave, which we here call "protein wave", is promising for direct label-free electrochemical detection of proteins. The current for the adsorption of a protein to the interface showed a linear dependence on the protein concentration in the presence of excess surfactant. The foot potential at which the protein wave appeared in cyclic voltammetry showed different values depending on the natures of the protein and surfactant. Multivariate analysis for the foot potentials determined for different proteins with different surfactants revealed that the protein selectivity should depend on the charged, polar, and nonpolar surface areas of a protein molecule. On the basis of these voltammetric studies, it was shown in principle that online electrochemical separation/determination of proteins could be performed using a two-step oil/water-type flow-cell system.