Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 41

Another interesting example is the use of Au nanoshells for
protein assays. Each nanoshell consists of a silica core surrounded
by a thin shell of Au atoms. By tuning the relative dimensions of
the core and shell, the extinction maximum can be systematically
varied from 700 to 1300 nm. One advantage of this property is that
the extinction maximum can be tuned to a wavelength at which
interference from absorbing molecules in blood and other matrices is
minimal. Researchers have used Au nanoshells consisting of a 96 nm
diameter core surrounded by a 22 nm shell for distance-dependent
immunoassays for immunoglobulins in dilute serum. 71

(A) (B) (C)

Figure 3.5 A schematic illustration for the colorimetric detection of
protein-protein interactions. See also Color Insert.

Au NP-based competitive colorimetric assays are useful for
the identiication of proteins and the determination of the binding
constants between recognition molecules and target proteins.
Mannopyranoside-encapsulated Au NPs (Man-Au NPs) are used for
the identiication of Con A and the determination of the interaction
between Con A and thyroglobulin by the naked eye. 72 In the absence
of thyroglobulin, Con A-induced aggregation of the Man-Au NPs
results in a color change from red to purple. In the presence of
thyroglobulin, interaction of Con A and Man decreases as a result
of a competitive reaction between Con A and thyroglobulin, leading
to decreased aggregation. Figure 3.5 illustrates a representative
detection scheme for protein-protein interactions. A protein,
denoted by X, binds the ligands protruding from the Au NP surface
and promotes agglomeration of the particles via multivalent
ligand-protein interactions giving rise to a blue colored solution.
However, the addition of a putative protein, denoted by Y, capable
of interacting with protein X, inluences the binding between X and