Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 32

paration of Au and Ag NPs, including wet chemical, photochemical,
and mechanical (top-down) approaches. 35,36 From the view point
of green chemistry, the synthesis of Au and Ag NPs in aqueous
solutions (wet chemical approaches) is more interesting. In
addition, the as-prepared NPs are usually readily bioconjugated and
more biocompatible. In this section, we discuss preparation and
stabilization of Au and Ag NPs in aqueous solutions. Functionalization
of these NPs for the selective sensing of analytes of interest is
emphasized as well.

3.2.1 Wet Chemical Approaches

A typical synthesis of spherical Au NPs involves the reduction of gold
salts (e.g., NaAuCl 4 ) in solution by reducing agents. Two of the most
common reducing agents are citrate and NaBH 4 . Citrate is usually
used to prepare Au NPs with diameters >3 nm because of its weaker
reducing strength when compared to that of NaBH 4 . On the other
hand, NaBH 4 is used for the preparation of small Au NPs (1-3 nm).
Citrate provides one advantage over NaBH 4 as it also acts as a capping
reagent to stabilize Au NPs. Usually, the growth of Au NPs proceeds
via three stages. 37-39 In the irst stage (typically complete within a
few seconds), a reducing agent reduces metal ions to metal atoms,
which then undergo rapid collisions to form stable icosahedral nuclei
of 1-2 nm in size. The initial concentration of nuclei depends on
the concentration and reducing strength of the reducing agent, the
solvent, the temperature, and the reduction potential of the reaction.
This stage is important because it determines the heterogeneity of
the size and shape of the NPs. Further growth of the NPs occurs,
in the second stage, via the reduction of metal ions on the surface
of the nuclei until all the metal ions are consumed. The third and
inal stage involves the prevention of the aggregation of the Au NPs,
which is typically achieved by the addition of stabilizing agents. For
example, electrostatic repulsions due to the adsorbed citrate ions
on the Au NPs' surfaces, which impart negative charge to the NPs,
stabilize them. Similarly, Ag NPs are prepared by using AgNO 3 as the
precursor and citrate or NaBH 4 as the reducing agents. Ultimately,
polymers such as poly(vinylpyrollidone) are more common capping
reagents, used to stabilize the Ag NPs. 40
Varying the ratio of the reducing agents to the metal salts controls
the size of the NPs. Increasing the molar ratio of reductant to metal
salt causes rapid formation of a large number of nuclei and leads