Environmental Engineering Reference
In-Depth Information
Fig. 11.3 Aptamer-based
colorimetric assay.
Observation of the
dispersion ( red colour
indication) and aggregation
( blue or purple colour
indication) of AuNPs under
salt induction are shown.
Probe indicates the aptamer.
Colour-based spectral
changes are shown with
blue and red shifts
immobilization of these nucleic acids on the AuNP surface [ 33 , 34 ]. This sponta-
neous adsorption of the ssDNA/RNA onto the surface of AuNPs affects the
interparticle distance of the AuNPs. Based on this feature, many aptamer-based
assays were devised for the sensitive detection of the target molecule.
DNA aptamer against target molecule Hg 2+ was used in the AuNP-based assay.
In this assay, the presence of the target Hg 2+ , T-rich single stranded oligonucleotide
DNA aptamer forms duplex with Hg 2+ and is not able to bind to the AuNP. Under
this condition, NaCl will lessen the electrostatic repulsion between the AuNPs
which results in aggregation and blue or purple colour formation.
In the absence of the Hg 2+ , the ssDNA aptamer is spontaneously absorbed onto
the surface of AuNP, which prevents the action of Na + , stabilizing the nanoparticles
against the salt-induced aggregation and gives red colour. In a different strategy,
colorimetric aptasensor for the detection of Staphylococcus aureus using tyramine
signal amplification (TSA) technology was developed. In this assay, aptamer
specific against S. aureus was functionalized with biotin and immobilized on the
surface of the microtiter plate. Subsequent to the addition of the target, biotinylated-
aptamer-streptavidin-HRP conjugates, biotinylated tyramine, hydrogen peroxide
and avidin-catalase were added consecutively. The addition of the Au (III) chloride
trihydrate follows which changes the colour of the reaction and also the absorbance
at 550 nm. The limit of detection is 9 CFU mL 1 [ 35 ]. In another format, an
adenosine sensor system was designed using aptamer against adenosine. This
construct that consists of two types of ssDNA-modified AuNPs and a linker DNA
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