Biology Reference
In-Depth Information
devices with a gold metal gate, mixed self-assembled monolayers of
thiols are usually chosen since they give rise to highly organized,
stable, and reproducible films in which the surface density of
the oligonucleotides can be controlled in order to eliminate
steric hinderance effects and increase the hybridization e
ciency.
To achieve fast hybridization kinetics and a high hybridization
e
ciency, a probe density of
10
12
cm
−
2
is required [17]. To
obtainthegreatestshiftingatepotential(
V
G
)inafield-effectsensor,
there will be a trade-off between greater hybridization e
ciency
and greater counterion screening of the DNA charge as the probe
density is reduced. In addition, if the DNA layer is considered as a
planecharge,thevoltageshiftdependsnon-linearlyuponthecharge
density through the Grahame equation, so that an increase in the
density of probes may lead to a large increase in the charge density
upon hybridization,butonly a smallincrease in thevoltage shift.
Hybridization kinetics can be promoted with a high ionic
strengthbuffer,withspecificityachievedbywashingwithalowionic
strengthbuffer.Alowionicstrengthmeasurementbufferisrequired
for field-effect sensing to give little screening of charge. However,
the stability of the DNA duplex in these low ionic strengths must
beconsidered.Togivegreaterhybridizatione
ciencyandsequence
selectivityandtoincreasestabilityatlowionicstrength,PNAprobes
can beutilized.
≤
3
×
6.3.1.1 Description of the electrochemical system
Whenanelectrolyte isincontactwithanelectrode, anelectrochem-
ical double layer forms. In the Gouy-Chapman-Stern model of the
electrochemical double layer [18], it is assumed that the solvent
providesacontinuousdielectricmediumwithdielectricpermittivity
equalto itsbulkvalue, that charges ofdiscrete ionsare smeared out
into a continuous distribution of net charge density, and that ion-
ion interactions can be neglected so that all ions in solution are free
to contribute to the charge density. Due to their finite size, ions may
not approach the electrode closer than the outer Helmholtz plane
(OHP). Since there is no charge between the electrode and OHP,
the electric field
E
is constant in this region, and the electrostatic
potential
ϕ
varies linearly. Outside the OHP, the potential may be
