“9.61x6.69” b2815 Tissue Engineering and Nanotheranostics
Plasmonic Nanoparticles Application in Biosensor and Bioimaging 163
of a cationic surfactant stabilizer. The solution turned pink on addi
tion of the reducing agent, icecold sodium borohydrate, indicating
the formation of particles. These seeds are then added to a solution
containing additional surfactant and Au salt that has been mildly
reduced by ascorbic acid, forming dehydroascorbic acid and resulting
in an AuCl 2 — surfactant complex.63,64 The color of the solution
changed from pink to reddish brown, indicating the formation of
AuNRs. Other studies improve controlling of the aspect ratio by vary
ing the amount of silver nitrate, the concentration ratio of Au seed to
Au3+ ions, the reducing agent, capping agent, and reaction tempera
ture and time intervals. In seed grown methods, AuNRs grown using
CTABprotected gold seeds in the presence of Ag+ bear Au^65 faces on
the sides of the rods and {100} faces on the ends.^66 Silver under
potential deposition (UPD) is the reduction of Ag+ to Ag^0 at a metal
substrate with a surface potential less than the standard reduction
potential of Ag+. A greater positive shift in potential for silver UPD
on gold surfaces is observed on {110} surfaces compared to the {100}
or {111} faces,67,68 suggesting that deposition of silver on the sides of
the rods ({110} facets) should be faster than on the ends ({100}
facets). Fast deposition of silver, followed by strong CTAB binding
(via bromide) inhibits gold growth on the sides of the rods and leads
to preferential growth of gold at the ends. It is notable that the bond
ing interaction between Au and cetyltrimethylammonium bromide
(CTAB) is typical of almost all chemical conjugation schemes involv
ing AuNPs. Br– anions are known to form atomic surface layers on Au
and other metals/semiconductors (i.e. adlayers) and also form elec
trostatically coordinated CTA+cations and their micelles. The general
consensus for AuNR–CTAB bonding involves the formation of a Br–
adlayer on the AuNR surfaces, which then coordinates N head groups
on CTA+. The aliphatic tail of surfacebound CTA+ then associates
with another CTA+ cationoriented with its N+ Br– head group facing
outwards.^69
3.1.2. Silver nanoparticles
The most common approach for Ag NPs synthesis is chemical reduc
tion by organic and inorganic reducing agents. In general, silver ions
