Gold Nanoparticles in Biosensing Analyses - Nanoparticles' Promises and Risks - page 214

development of sensors in a potpourri of applications [ 13 - 26 ]. The ease of prepar-
ing AuNPs also attributes to the vast application of AuNPs for sensing purpose [ 27 ].
11.2 Preparation of AuNP
The most common method for the preparation of AuNP is by sodium citrate
reduction of hydrogen tetrachloroaurate (HAuCl 4 ), whereby the colour changes
from pale yellow to deep red when synthesis is completed [ 28 , 29 ]. Sodium citrate
acting as the reducing agent that can provide AuNPs with desired sizes. Studies
have shown that different ratios of gold (Au) salt to sodium citrate give rise to
different sizes of AuNPs [ 29 ]. Other synthetic strategy include Brust-Schiffrin
method for the preparation of thiol-protected AuNPs, which involves the
two-phase synthetic strategy that depends on strong thiol-Au interactions to protect
AuNPs with thiol ligands [ 30 ]. In this strategy, the transfer of AuCl 4 from aqueous
phase to toluene using the surfactant tetraoctylammonium bromide (TOAB) is then
reduced by sodium borohydride (NaBH 4 ) in presence of dodecanethiol (colour
change from orange to deep brown). Prior to using AuNPs in sensing application,
types of biomolecules and the method adopted to immobilize the biomolecules on
the surface of AuNPs must be taken into account, which engenders a potpourri of
AuNP-based sensing assays.
11.3 Biosensing Configurations of AuNP
To create an ideal biosensing configurations on AuNP, two potential biomolecules,
namely aptamer and antibody as the probes are very commonly in use [ 10 - 12 ,
26 ]. These molecules can be conjugated with AuNP through thiol-linker (Fig. 11.1 ).
11.3.1 Aptamer-Based Sensing
Isolated by a process known as Systematic Evolutions of Ligands by Exponential
Enrichment (SELEX), aptamers are single stranded DNA or RNA that have high
binding affinity and specificity against the target (Fig. 11.2 ). Due to its specificity
against the target, aptamers are utilized as the detection probe in AuNP-based assay.
The most common form of the assay is the differential aggregation of AuNPs in the
presence or absence of the target. This AuNP-based biosensing system that can
result in the visual transduction of the molecular event without the requirement of
special equipments [ 31 , 32 ]. In this platform, the controlled assembly and disas-
sembly of aptamer on unmodified AuNP is the key strategy and the binding of target
can be evaluated by colorimetric measurements as described below.