Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 31

Although not frequently reported, Ag NPs can also be used in
bioassays, for example biotinylated Ag NPs for detecting avidin
through high afinity biotin-avidin interactions. 24 In fact, Ag NPs are
the second most useful novel metal nanomaterials for bioassays. 25-29
Monodisperse spherical Ag NPs have LSPR absorption bands around
400 nm. Once aggregation occurs, an Ag NP solution changes its color
from yellow to orange. For colorimetric assays, Ag NPs have some
advantages over Au NPs to a certain degree, including their higher
extinction coeficients and relatively lower costs. 30 However, Ag NPs
are not as stable as Au NPs; oxidation and chemical degradation
during functionalization may occur. In addition, when Ag NPs are
prepared by standard chemical reductions, their size distributions
are usually broad. Thus, little attention has been paid to Ag NP based
colorimetric assays. 31-34
When compared to common organic chromophores, Au and Ag
NPs have extremely large molar coeficients, leading to the high
sensitivity of NP-based colorimetric assays. The molar extinction
coeficients for 30 nm Au NPs and 80 nm Ag NPs are 7.1 × 10 12 M −1
cm −1 at 526 nm and 9.5 × 10 12 M −1 cm −1 at 430 nm, respectively,
which are much larger than those (10 4 to 10 5 M −1 cm −1 ) for more
common chromophores.
To develop sensitive colorimetric assays, the preparation of
monodisperse and stable Au and Ag NPs is important. Thus, this
chapter focuses on strategies for the preparation and stabilization of
Au and Ag NPs. Most Au NPs and Ag NPs that are prepared by chemical
reduction approaches lack selectivity for most biomolecules of
interest; functionalization of the NPs to provide selective recognition
sites on their surfaces is necessary after their preparation. We briely
discuss conjugation approaches to functionalization of Au NPs below.
To understand the sensing strategies of colorimetric assays using
Au and Ag NPs, the optical properties of these nanomaterials are
emphasized. Finally, applications of Au NPs and Ag NPs are provided
to show their potential roles in the detection of biomolecules and in
cell imaging.

3.2 Synthesis of Au and Ag NPs

Because of their potential uses in many ields, such as catalysis,
sensing, and electronics, there are many strategies for the pre-