Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 39

resonance bands, respectively. The anisotropic, cubic Ag NPs with
a mean edge length of 80 nm exhibit three LSPR bands (located at
350, 400, and 470 nm), while the isotropic, spherical Ag NPs with
80 nm diameters display a strong LSPR band around 430 nm. As the
size of Ag nanocubes increases from 80 to 175 nm, these three SPR
peaks undergo redshifts to 370, 440, and 560 nm. Au nanoshells
with hollow interiors have a LSPR peak at 720 nm rather than
520 nm. As more Au atoms are plated onto the surfaces of these
shells, their LSPR peak continuously shifts to the blue side (from
720 to 560 nm). The color of these aqueous suspensions of Au
nanoshells also changes from dark blue through purple to pink.

3.4 Applications

Possessing high extinction coeficients, size-dependent optical
properties, and biocompatibility, Au and Ag NPs have become popular
nanomaterials for bioassays. Most colorimetric bioassays in the
nanoscale are based on analyte-induced aggregation of Au and Ag NPs
through crosslinking or non-crosslinking. Interparticle crosslinking
aggregation assays are those that function via interparticle bond
formation, while non-crosslinking aggregation assays are those that
work through the removal of colloidal stabilization effects, which
we discuss in more detail later in this section. Other interesting
colorimetric assays based on the analyte-induced redispersion of Au
NPs aggregates have also been realized. 65,66 Because of possessing
high extinction coeficients, Au and Ag NPs can act as superquenchers
in luorescence quenching-based assays for the analysis of Hg 2+ ,
platelet-derived growth factor (PDGF), and so on. 23,67

3.4.1 Crosslinking Aggregation-Based Assays

A crosslinking aggregation-based assay is the most common
type of colorimetric bioassays when using Au NPs. Crosslinking
occurs either by using crosslinker molecules that have multiple
binding sites for the receptor molecules on Au NPs or by the direct
interaction (without crosslinkers) between receptors on Au NPs to
which complementary (or antireceptor) molecules are attached.
When interparticle bond formation (e.g., H-bonding, electrostatic
attraction, hydrophobic interaction, and metal-ligand coordination)
associated with interparticle biological recognition overcome the