Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 33

to smaller, monodisperse Au and Ag NPs. In contrast, decreasing
the molar ratio leads to slow formation of a few nuclei and results
in larger NPs with a broad size distribution. The nature (reducing
strength) of the reductant is an important parameter for controlling
the size of NPs. For example, sodium citrate and white phosphorus
are used to prepare Au NPs with diameters of 12-100 and 5-12
nm, respectively. 41 Smaller Au NPs (<3 nm in diameter) can be
synthesized by the reduction of HAuCl 4 in aqueous solution in the
presence of NaBH 4 or in an organic solvent in the presence of a thiol-
capping agent. 42 The nature of the capping agent is also important in
determining the size of the NPs. For instance, Ag NPs prepared from
the reduction of silver nitrate by NaBH 4 in the presence of bis( p -
sulfonatophenyl)-phenylphosphine and poly(vinylpyrollidone) have
diameters of 25-33 nm and 15-36 nm, respectively. 40,43
Researchers have devoted substantial efforts to the syntheses
of anisotropic Au and Ag NPs, which display different optical
properties from those of their corresponding spherical NPs. For
example, Au nanorods possess traverse and longitudinal SPR
bands centered at 520 and 700 nm, respectively. Other interesting
anisotropic NPs include Ag nanorods, 44 Ag/Au multimetal
micrometer rods, 45 dumbbe ike and peanu ike Au-Ag
nanomaterials, 46 Ag nanoprisms, 47 Ag and Au nanocubes, 35 Ag
nanolowers, 48 and Au nanowires. 49 Several factors that control
the nuclei and growth rates and the stability of certain sides of the
surfaces are important in determining the growth of certain shapes
and sizes of anisotropic Au and Ag NPs, including the nature and
concentration of the reducing agent, the nature and concentration
of the capping agents, the pH and ionic strength of the solution,
and the temperature. For example, by controlling the pH, the
concentration, and the nature of the amino acids, differently sized
and shaped Au-Ag nanorods can be prepared. 50 By controlling the
concentration of cetyl trimethylammonium bromide and the pH
values, Au nanowires can be prepared from Te nanowires. 51 Two
excellent papers summarize many different synthetic strategies for
the preparation of anisotropic Au and Ag nanomaterials. 44,52

3.2.2 Stabilization

Owing to Brownian diffusion and attraction between NPs, Au and Ag
NPs are usually unstable in aqueous solutions unless their surfaces
are capped with suitable molecules and/or ions. It is extremely