Biology Reference
In-Depth Information
surface, eventually leached to the sample solution during measure-
ments. However, dry adsorption also promotes hydrophobic bonds
and more stable adsorbed layers on solid surfaces [25]. Classical
strategiessuchasphysicalentrapmentinmembranesandcrosslink-
ing by bifunctional reagents—such as glutaraldehyde—also repre-
sent multisite attachment methods for retaining the biorreceptor in
close contact ofthe transducer.
Single-point attachment
is beneficial for the kinetics of the bio-
logicalreaction,especiallyifaspacerarmisused.Single-point
cova-
lent immobilization
can be performed on different surface-modified
electrochemical transducers, such as glassy carbon [26, 27], carbon
paste [28], gold [29], or platinum [11], or, lately, carbon nanotubes
[30] through the linkage of a -COOH with a -NH
2
group by the use
of thecarbodiimide chemistry. Single-point
a
nity linkage
also pro-
vides an interesting strategy for the oriented and stable immobi-
lizationofbiotinilatedbiomoleculestosolidtransducersthroughout
biotin/strept(avidin) binding [23]. Finally, chemisorption based on
SAMs
has also been extensively used for single-point attachment on
gold-basedtransducers [31, 32].
ElectrochemicaldetectionoftheDNAhybridizationeventshould
also be considered, involving the transduction of the hybridization
event into a useful and easy-to-amplify electrical signal. The DNA
recognition event for electrochemical transducing can be detected
mostly by means of external electrochemical markers such as elec-
troactive indicators [33, 34] or enzymes. Enzyme labeling has been
transferred from non-isotopic DNA classical methods to electro-
chemical genosensing. The enzyme labeling relies on the reaction
between a small tag (usually biotin or digoxigenin modified DNA
probe) with the streptavidin [35] or anti-digoxigenin [36] enzyme
conjugates, respectively. Although a second incubation step is usu-
allyrequiredforlabeling,highersensitivityandspecificityhavebeen
reported for the enzyme labeling method compared with the other
reported methods [37]. The use of metal NPs—especially gold NPs
[39-42]—as labels for biosensing devices are also gaining impor-
tance.ThedirectelectrochemicaldetectionofDNAwasinitiallypro-
posed by Palecek [43, 44] who recognized the capability of both
DNA and RNA to yield reduction and oxidation signals after being
adsorbed. The oxidation of DNA was shown to be strongly depen-
