Biology Reference
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duplexcanprovideamorestablesignal[4]butdonotalways
provide adequate sensitivity. This has led to interest in using
metal nanoparticles as labels for electrochemical detection of DNA
hybridization(reviewed in Refs. 5-7).
Where the nanoparticles are reacted directly (rather than being
used to catalyze reactions), the achievable sensitivity will depend
largely on the quantity of metal attached to each DNA sequence.
This has led to the development in label construction illustrated in
Fig. 8.1. From the binding of individual particles, researchers have
sought techniques to attach assemblies of nanoparticles to a given
DNA sequence. As will be described in this chapter, latex colloids
provide an ideal base for such assemblies, both as solid supports
and as templates for the construction of hollow capsules which can
takeupnanomaterials.Thepointofimportanceisthatthenecessary
latex modifications have already been intensively researched for
other applications, and so the relevant physical chemistry theory
and experimental details are already available. Despite this fact, the
use of latex in constructing electrochemical DNA labels is relatively
unexplored.
8.2 Synthesis of Metal Nanoparticles
ColloidalgoldwasfirstpreparedandstudiedbyFaradayin1857[8].
Intheearly1950s,thepreparationofcolloidalgoldinhomogeneous
solution was described by Turkevich et al. [9] using sodium citrate
to reduce a dilute solution of HAuCl 4 under heating. This method
has become a standard for gold nanoparticle preparation and has
also been applied for the synthesis of platinum nanoparticles by
the reduction of PtCl 6 2 [10]. Similar homogeneous synthesis can
be performed for silver nanoparticles, using NaBH 4 as a reducing
agent for AgNO 3 [11]. When the nanoparticles are formed there
needs to be a force resisting coagulation present for the particles to
remainstableinsolution.Thisforcecanbeprovidedbyelectrostatic
repulsion due to the adsorption of ions onto the metal surface and
in some cases the adsorption of the reducing agent (e.g., when
AuCl 4 is reduced by citrate the citrate ions remain adsorbed on
the particles and impart a negative charge [12]). The electrostatic
 
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