Biomedical Engineering Reference
In-Depth Information
protoplasmonic transitions of the free conduction electrons suggest
that the NCs are nearly spherical electronic structures with electrons
bound by an approximately harmonic potential in three dimensions.
Thus, the emission energies of smaller Au NCs (<Au
20
) it well to the
energy scaling law
E
Fermi
/
N
1/3
(Fig. 9.1C).
21
For Au NCs of larger sizes
(Au
23
-Au
38
), however, small anharmonicities distort the potential
well.
6,21
A simple protocol, similar to that for the preparation of
PAMAM-stabilized luorescent Au NCs, can be used for the
synthesis and puriication of luorescent pentaerythritol
tetrakis(3-mercaptopropionate)-terminated poly(methacrylic acid)
(PTMP-pMAA)-capped Au NCs that have an emission band at
700-750 nm.
46
The PTMP-pMAA ligand acts as a “multidentate”
capping agent to control the growth of the Au NCs. By varying
the polymer-to-Au molar ratio, the size of the NCs can be inely
tuned, allowing observation of a transition from non-luorescent
to luorescent NPs that have core diameters ranging from 1.1 to
1.7 nm. Using 4 mM PTMP-pMAA and 0.5 mM HAuCl
4
results in the
preparation of the most highly luorescent Au NCs, which have a
particle size of 1.1 nm and a quantum eficiency of 3%.
Alternatively, highly luorescent, water-soluble Au NCs can
be prepared through ligand-induced etching processes.
47
First, a
modiied Brust-Schiffrin method is applied to prepare Au NPs (ca.
8 nm).
48
Tetraoctylammonium bromide is added to transfer HAuCl
4
from the aqueous phase to the toluene phase, where reduction of Au
ions by NaBH
4
occurs under vigorous stirring. Subsequently, ligand
exchange reactions using multivalent coordinating polymers, such
as hyperbranched polyethyleneimine (PEI;
M
n
= 1800 g/mol), are
performed to etch the thus-prepared Au NPs. As a result, luorescent
Au NCs having quantum yields between 10% and 20% are formed.
The excitation and emission wavelength maxima are located at 421
and 505 nm, respectively. After a reaction with a strong reducing
agent (NaBH
4
), the excitation and emission peaks of the Au NCs shift
to 353 and 445 nm, respectively. Electrospray ionization (ESI) mass
spectrometry data revealed that the NCs comprised only eight Au
atoms (Au
8
). There are two possible chemical pathways that lead to
the formation of atomic Au NCs. In one, Au
8
NCs are liberated in a
single step during ligand exchange and are immediately protected by
a multivalent, hyperbranched PEI ligand, which may also function to
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