Biology Reference
In-Depth Information
(
λ
max
=
406 nm). Hence, after take-up of silver particles by
the capsules/latex particles, the nanoparticles remaining in the
solution can be separated after centrifugation and the remaining
concentration determined. Knowledge of the initial concentration
used enables us to calculate the number of nanoparticles taken up.
Knowledge of the number of capsules/latex particles allows us to
calculate the capsule/latex loading.
8.6 DNA Measurement
The nanoparticle labels can be used to detect DNA following the
general stages: (1) Attachment of DNA probe or target to the
electrode, (2) attachment of DNA probe or target to the label, (3)
hybridization to form a duplex, (4) dissolution of the metal ions in
the label (50% HNO
3
for Ag dissolution, 1 M HBr/0.1 mM Br
2
for
Au), and(5) detection of themetal ions.
DNA probes are usually in the range 12-40 base pairs. Above 40
base pairs, folding of the probe on the electrode is likely to lower
hybridization e
ciency by steric hindrance. Also, at such lengths
the degree of binding to partial mismatches may be significant. At
below12basepairstheprobeisunlikelytobeuniquetoaparticular
sequence.
8.6.1
DNA Immobilization
DNA can be immobilized on the electrode by either covalent linking
or physical adsorption. DNA modified by a thiol group can be
chemically attached to gold electrodes [115-117] following the
formation of the sulphur-gold bond:
DNA
−
SH
+
Au
→
DNA
−
S
−
Au
+
e
−
+
H
+
Alternatively, the gold electrode can be modified with a thiol-based
self-assembledmonolayer(SAM)bearingfunctionalgroupssuitable
to bind DNA [118]. Often this binding is performed via a coupling
reagent such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC), which enables aminated or carboxylated DNA to bond with
the appropriately carboxylated or aminated functional group on
the electrode [119], or on a polymer deposited on the electrode
