New Trends in Antibody-Based Electrochemical Biosensors

Electrochemical Impedance Spectroscopy─A Tutorial

“…According to equation, C = ε ε o A / d where ε, is the dielectric constant of the medium at the electrode/electrolyte interface, ε o , is the permittivity of free space, A , is the surface area of the electrode, and, d , is the thickness of the attached layers onto the electrode surface, the capacitance of the resulting biosensor (electrode/Ab) is decreasing due to the specific interaction between the immobilized receptor and the soluble target and the formation of the Ab-Ag immunocomplex on the electrode surface (electrode/Ab-Ag) (Figure ). Obviously, the response of a capacitive biosensor is dependent on the size and the concentration of the target analyte and thus this biosensor type is mostly indicated for the sensing of bulky targets, such as proteins or even better, cells. − As regards, the formation of the Ab-Ag immunocomplex results in alteration of the dielectric properties of the medium as water molecules (ε = 80) are replaced by compounds with lower dielectric constants (for example, the dielectric constant of a protein is ca. ε = 20 .…”

“…As a result, faradaic impedance measurements, in the presence of a redox couple in the measuring solution, have been considered as an alternative property for probing biomolecules interactions. − ,, In this case, the impedance spectra can be modeled by a Randles circuit in which the charge-transfer resistance ( R ct ) varies in a concentration dependent fashion, depending on the flux rate of the redox molecules to the electrode surface that is being polarized to the formal potential of the redox couple (an equimolar mixture of [Fe­(CN)­6] 3–/4– is often used for this purpose). The flux rate of the redox probe is controlled by the extent of the formation of the Ab–Ag immunocomplex.…”

Metal Nanoparticle and Quantum Dot Tags for Signal Amplification in Electrochemical Immunosensors for Biomarker Detection

“…Silver nanoparticles (AgNPs) are also quite popular labels for electroche munosensors. The oxidation of AgNPs is performed at a noticeably lower po The metal dissolution approach in acidic medium such as HCl or HNO 3 solutions with the further monitoring of released metal ion concentration can also be effectively utilized with Qdots labels (Figure 6C), which as well as MNPs are typically used in sandwich format immunoassay [123]. Qdots typically have a core-shell structure and consist of heavy metals such as Cd, Pb, and Zn, which can be released from Qdots and quantified by sweeping the potential of the electrode.…”