Electrochemical impedance spectroscopy characterization of mercaptopropionic acid capped ZnS nanocrystal based bioelectrode for the detection of the cardiac biomarker--myoglobin.

3-Mercaptopropionic acid (MPA) capped ZnS nanocrystals (ZnS(MPA)) are covalently attached to a self assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APTES) on an indium-tin-oxide (ITO) coated glass plate. The protein antibody, anti-myoglobin (Ab-Mb), is covalently linked to free carboxyl groups present on ZnS(MPA) nanocrystals via carbodiimide coupling reaction to form a bioelectrode (Ab-Mb(BSA)/ZnS(MPA)/APTES/ITO-glass). This bioelectrode has been characterized using atomic force microscopy (AFM), contact angle measurements, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The optimal equivalent circuit model that matches the impedimetric responses of the bioelectrode describes three distinct regions: the electrolyte solution resistance (R(s)), the double layer capacitance (C(dl)) and the specific charge transfer resistance (R(et)). The EIS measurements revealed that the R(et) increases considerably with no significant change in C(dl) after immunoreaction with protein specific antigen myoglobin, Ag-Mb, so that the prepared bioelectrode can be used for the detection of Ag-Mb. The bioelectrode exhibits an electrochemical impedance response to Ag-Mb, in a linear range from 10ng to 1μgmL(-1) phosphate buffer solution (pH 7.4) with a R(et) sensitivity of 117.36Ωcm(2) per decade.