Immunosensors impedance response

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

Bart, M., Stigter, E.C.A., Stapert, H.R., de Jong, G.J., van Bennekom, W.P. On the response of a label-free interferon-y immunosensor utilizing electrochemical impedance spectroscopy. Biosens. Bioelectron. 21, 49-59 (2005)... 

Impedance immunosensors measure changes in electrical field due to biorecognition phenomena. These changes could be overall electrical conductivity of the solution and capacitance alteration due to the Ab/Ag interaction on the electrode surface, which also can be reflected in impedimetric response. Electric impedance spectroscopy (EIS) is a sensitive technique that detects the electrical response of the system studied after application of a periodic small ampHtude AC signal. In recent years, the electrochemical impedance immunosensors have attracted extensive interest in the sensing formation of Ab/Ag (Darain et al., 2004 Chen et al., 2006 Gonzalez et al., 2007 Varki et al., 1999). 

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