Biology Reference
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helix,orbeinginsertedselectivelyandreversiblyinthedsDNA.Their
usehasbeenwidelystudiedfromthepioneeringworksofMillanand
Mikkelsen[21]intheearly90s.Mostofthemhavebeenreviewedin
the work of Lucarelli [8]. Compounds that join the hollows of the
double helix have major a nity for dsDNA than for ssDNA, so the
signal due to the indicator oxidation increases when hybridization
takes place.
Other indicators, such as daunomycin or cobalt complexes, act
as intercalators. The changes in the area or peak potential of the
indicator oxidation process are used as analytical signal [22, 23].
Nevertheless, MB is another indicator that joins DNA by means
of intercalation, but generates minor reduction signals when it is
joined to dsADN than when joined to ssADN, because the specific
interaction ofthe MB with guaninebases islower in the dsADN.
The hybridization indicators present the great advantage of
avoiding the processes of DNA labeling. Nevertheless, the discrim-
ination between single and double strand used to be not very good.
In addition, a general problem is the high backgrounds obtained,
due to unspecific adsorptions of indicators. However, if a negative
potential is applied to the electrodic surface once finished the assay,
these adsorptions can be repelled, diminishing the background
signals.
Useoflabels TherearetwotypesoflabelsthatjoinDNAcovalently:
electroactive and non-electroactivelabels.
The electroactive labels most used in genosensing design are
ferroceneanditsderivates[24-27](thereversibleoxidationprocess
of ferrocene can be detected by means of several electrochemical
techniques), osmium complexes [28], platinum complexes [29],
gold complexes [30, 31], and metallic [32-36] or semiconductor
nanoparticles [37]. Among the last ones, gold nanoparticles are
the most used, their detection can be carried out by means of the
measurement of resistance or capacitance changes, usually after
an amplification procedure with silver, or by means of the anodic
stripping voltammetry of Au(III) obtained after the nanoparticle
oxidation Fig. 9.3.
An original approach consists in the use of ssDNA probes
labeled with an electroactive marker, the hybridization inducing
 
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