Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 182

9.1 Introduction
In their bulk forms, noble metals exhibit many interesting properties,
including high conductivity, excellent chemical stability, and beautiful
surface luster. 1 In the form of nanomaterials, however, these noble
metals display features distinct from those found in the bulk.
The chemical and physical properties of noble metal nanomaterials
are highly dependent on their size, shapes, surface functionality, and
the refractive indices of their surrounding media. 1-3 For example,
small-diameter gold nanoparticles (Au NPs) exhibit greater catalytic
activities than their larger congeners. 4 Likewise, spherical Au NPs
feature only one absorption band (near 520 nm), whereas Au
nanorods possess two: one at ca. 520 nm and the other one centered
at a longer wavelength (600-1200 nm), depending on the aspect
ratio (length/width). 5
The study of few-atom metal nanoclusters (NCs), which bridge
the divide between the properties of isolated atoms and those of
NPs or even bulk metals, has attracted great interest because of their
distinct optical and catalytic properties. 6-9 Nanoscale metals are
roughly classiied into three size domains: large NPs, small NPs, and
NCs, according to their length scales ( R ) relative to the wavelength
( λ ) of the external electromagnetic ield. 10,11 The optical responses
of large metal NPs ( R > λ ) toward external electromagnetic ields
depend on their sizes and their free-electron density. Mie theory can
be used to describe the optical properties of large NPs. 10 When the
particle size approaches the electron mean free path (e.g., 50 nm
for Au or silver (Ag)), collective excitations of electrons become
dominant, leading to plasmon resonance. 11 Mie theory also provides
an adequate description if the size effects are suitably incorporated.
Eventually, when the particle size becomes comparable with the third
characteristic length — the Fermi wavelength of an electron (i.e., de
Broglie's wavelength of an electron at the Fermi energy, or 0.5 nm for
Au or Ag) — the optical, electronic, and chemical properties of metal
NCs differ dramatically from those of the other two size regimes. 12,13
In the smallest size regime, metal NCs behave as molecule-like
species, often displaying strong single-electron excitations with
discrete states. 14-16 Among the primary topics of this chapter is a
discussion of such highly polarizable, few-atom Au NCs.
Luminescence (photoluminescence, PL) of metals is normally
extremely weak because of eficient nonradiative decay and the