Biology Reference
In-Depth Information
thesensingeventdependsontheconformationalchangeratherthan
on adsorption to the electrode surface.
In addition, it was shown that E-DNA can readily detect single
nucleotide mismatches and can be recycled for the multiple assays.
A “signal-on” type E-DNA sensor based on a pseudo-knot is shown
in Fig. 7.9. In this system, hybridization induced conformational
change brings the redox label close to the electrode surface
and thus enhances electron transfer e
ciency [72]. The system
is characterized by the 5'-end being attached to the transducer
surface while the redox-labeled 3'-end forms a pseudo-knot loop
that hybridizes on top of the hairpin loop. The stability of the
pseudo-knot loop at the 3'-terminus depends on the number of
base pairs, which was found to be 7 bp for maximum stability.
The signal response was found to be enhanced twofold with a
more flexible poly(T) loop as compared to poly(A) loop for all
systemsinvestigated,includingforfully-matched,single,doubleand
triple mismatches. The pseudo-knot-based sensor was found to be
selective in presence of serum and sensitive up to 30 pmoles. More
recently, a method was reported based on unlabeled stem loop
structures. Hybridization to the stem loop and opening of the stem
loop will alter the film structure, generally resulting in an increase
in the film thickness.
The charger transfer resistance
R
ct
for electron transfer from
the anionic redox probe [Fe(CN)
6
]
3
−
/
4
−
through the film will
be greatly influenced by this conformational change. Importantly,
differences in the film caused by the presence of single nucleotide
mismatches are su
ciently large that they cause differences in the
R
ct
. In particular, the addition of Zn
2
+
ions amplifies the resistive
differences allowing the detection of single nucleotide mismatches
at concentrations as low as 10 pM [73] (see Fig. 7.10). The effect of
themetalionsisdiscussedinmoredetailinthesectiononmetal-ion
amplified sensors.
7.3.6.1 Enzyme-mediated sensors
There are a number of reports of mismatch detection strategies, in
whichenzymaticreactionsareexploitedtoamplifytheelectrochem-
ical signal.
