Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 30

3.1 Introduction
The development of simple, rapid, sensitive, selective, and cost-
effective approaches to detection of biomolecules is important
because of their potential applications in the diagnosis of diseases. 1-3
Radioactive, luorescent, chemiluminescent, electrochemical,
and colorimetric probes are common. 4-6 Colorimetric assays are
especially attractive because they allow the observation of results
by the naked eye. For example, pH indicators are widely used to
determine the concentrations of analytes such as urea and ammonia.
However, when compared to other detection formats, colorimetric
detection possesses relatively poor sensitivity. Thus, probes with
extremely high molar extinction coeficients are highly desirable for
the detection of trace amounts of analytes of interest.
The past few years have witnessed signiicant progress in
bioassays using nanomaterials. Gold nanoparticles (Au NPs) are
the most widely used nanomaterials in developing colorimetric
bioassays, mainly because of their unique size-, shape-, composition-,
and distance-dependent optical properties. 7 In addition, Au NPs are
stable, simple to prepare and to bioconjugate, and biocompatibile. 8-10
Researchers have realized a variety of colorimetric sensors using
Au NPs for different analytes, including DNA, proteins, and small
molecules such as metal ions and carbohydrates. Most of the assays
take advantage of the localized surface plasmon resonance (LSPR)
of Au NPs as sensing elements; the dispersed Au NP solution is red
whereas the aggregated Au NP solution is purple. 8,10 Au NPs, with an
interparticle distance greater than their average particle diameter,
appear red as a consequence of their SPR absorption band centered
around 516 nm (for 13 nm Au NPs). As the interparticle distance
decreases to a length shorter than the diameter of the particles,
coupling interactions result in a broadening and a shift to longer
wavelengths of their SPR absorption band, leading to the change in
the solution color from red to purple.
Colorimetric NP-based assays for the detection of DNA using Au
NPs have created much attention for their practical applications. 11,12
Subsequently, researchers have prepared other Au NP probes for
the detection of lectins, 13,14 metal ions, 15-19 antibodies, 20 and other
analytes. 21-23 In each instance, the self assembly of a monolayer of
a molecular recognition molecule on the NP surface is responsible
for the colorimetric assays; controlled aggregation of the NPs occurs
with speciic recognition and binding of the target species.