Biomedical Engineering Reference
In-Depth Information
sequences are modii ed (such as enzymes, electroactive molecules or metal
nanoparticles [40]) or as dsDNA intercalators (such as cationic metal com-
plexes [11], anticancer drugs [8], or organic dyes [41]) which can form
complexes with specii c fragments of DNA sequences. h e magnitude of
their electrochemical signal, e.g., current, as given by these labels is then
correlated to DNA hybridization yield.
A dif erent approach is represented by label-free protocols, in which
the direct electrochemical oxidation/reduction peaks of DNA nitrogenous
bases (namely, guanine) are measured. In this scheme, the dif erence in the
signal intensity before and at er DNA hybridization reveals the presence of
the target sequence [9, 42-45]. Essentially, label-free approaches are SPR
or piezoelectric genosensors. Another very rapidly developing electro-
chemical technique employed in recent years for biosensing events is elec-
trochemical impedance spectroscopy (EIS). h is technique also allows the
label-free detection of DNA hybridization. Following is a detailed descirp-
tion of EIS fundamentals and applications for genosensing.
4.2
Electrochemical Impedance Spectroscopy for
Genosensing
h e history of impedance spectroscopy began in 1886, when the mathema-
tician and physicist Oliver Heaviside introduced impedance into electrical
engineering [46]. His work was soon extended to include vector diagrams,
representations in the complex plane and use of equivalent circuits to rep-
resent the impedimetric response [47].
Impedance spectroscopy (IS) is a general term that involves the small-signal
measurement of the linear electrical response of the material of interest, and
the consequent analysis of the response to yield useful information about the
physicochemical properties of the system. Electrochemical impedance spec-
troscopy (EIS) is one main category of IS which has been widely used in many
i elds of electrochemistry, e.g., electrode kinetics, double-layer studies, batter-
ies, corrosion, solid-state electrochemistry, and bioelectrochemistry [48].
Electrochemical impedance spectroscopy can be dei ned as a characteriza-
tion technique which provides electric information in the frequency domain
[49, 50]. With this technique, a process occurring in an electrochemical
cell can be modeled using a combination of electrical circuits that give the
same current response provided by the electrochemical system. By the use
of equivalent circuits [51], the experimental spectra can be i tted with the
theoretical curve corresponding to the selected circuit model, thus obtaining
the values of the electrical parameter (i.e., resistance, capacitance, etc.) which
