b2815 Tissue Engineering and Nanotheranostics “9.61x6.69”162 Tissue Engineering and Nanotheranostics
A higher abundance of small core sizes (<2 nm) is obtained by
quenching the reaction immediately following reduction or by using
sterically bulky ligands.49–53 Subsequently, Brust–Schiffrin procedure,
also called monolayerprotected clusters (MPCs), was applied for the
synthesis of other stable AuNPs,54–59 containing functional thiols
groups.
Rodshaped AuNPs, which transfer the energy of NIR light into
heating efficiently, are synthesized by a seedgrowth method. Murphy
and coworkers^60 firstly used ascorbic acid and silver nitrate to replace
electrochemical reduction and produced AuNRs for various aspect
ratios, including 4.6 ± 1, 13 ± 2, 18 ± 2.5. However, under the high
CTAB concentration and low pH (∼2.5) growth condition, the ascor
bic acid used in this synthesis reduces gold derivatives to Au (I) state^61
instead of metallic state. But addition of small seed particles with
about 3 nm in diameter into the Au(I) solution results in complete
reduction to metallic gold, which is catalyzed by the surface of the
seeds and leads to the gradual change in shape from quasispherical to
rodlike crystal. Subsequently ElSayed^62 improved this method by
utilizing CTABcapped seeds rather than the citratecapped seed par
ticles to produce rodlike morphology in high yield, as shown in
Fig. 1. In brief, small (~1.5 nm diameter) seed nanoparticles are
formed via classical borohydride reduction of Au salt in the presence
Fig. 1. Gold (III) chloride quantitatively displaces Br– counter ions in micelles of
CTAB and subsequent borohydride reduction produces small (~1.5 nm diameter)
seed nanoparticles surface stabilized by a CTABbilayer. Then Au (III) bound to
CTAB micelles is reduced to Au (I) by ascorbic acid. Directional growth of gold
AuNRs occurs via crystallographically preferential reduction of Au (I) onto the seed
nanoparticles.