Biomedical Engineering Reference
In-Depth Information
PAMAM from 1:1 to 1:15 allows both the excitation and emission
energies of the observed luorescent species to be tuned from the
UV to the NIR. Interestingly, the PAMAM scaffold stabilizes Au NCs
of various sizes and yields luorescence quantum yields that are
orders of magnitude higher than that of bulk Au. Mass spectrometric
characterization revealed that the differently colored luorescent
Au NCs in Fig. 9.1 were Au 5 (UV), Au 13 (green), Au 23 (red), and Au 31
(NIR), with each encapsulated within a PAMAM scaffold (i.e., Au
NCs). The photophysical properties of these various water-soluble
Au NCs indicate that they behave not only as quantum-conined
but also as molecular and size-tunable luorophores. Their high
quantum yields are comparable with those of the best water-soluble
emitters currently available, ranging from 70% for UV-emitting Au 5
to 10% for Au 31 in the NIR. In contrast to the preparation of larger
semiconductor NCs, complicated high-temperature syntheses with
toxic precursors are not required when preparing dendrimer-
encapsulated Au NCs.
Because Au NCs are too small to have the continuous density of
states and plasmon absorptions characteristics that occur for larger
Au NPs (>2 nm), the transition energy, rather than the plasmon-
absorption width, should scale inversely with the NC radius. 38-40
The dependence of the emission energy on N in each Au NC may be
it quantitatively for the smallest NCs with no adjustable parameters
when applying the simple scaling relation E Fermi / N 1/3 , where E Fermi
is the Fermi energy of bulk Au. 41-43 For a spherical NC, the radius
( R ) is equal to r s × N 1/3 , where r s is the Wigner-Seitz radius. 44,45
Identical to the electronic absorption for gas-phase alkali-metal NCs,
the transition energy scaling with the inverse NC radius indicates
that the electronic structure of Au NCs is determined solely by
their free electron density and size. These observations suggest
that the free electron shell-illing model corresponds exactly to
the spherical jellium approximation. 21 Observation of this simple
energy scaling ( E Fermi / N 1/3 ) for luorescent Au NCs offers clear
and direct experimental evidence for the discrete nature of the
excited state in noble metal NCs, and of the evolution from discrete
intraband transitions of the free electrons to the plasmon of large
NPs in the condensed phase. 6 The jellium approximation accurately
describes the size-dependent electronic structures and relative
electronic transitions of small NCs. 6,21 These quantum-conined
 
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