9
Synthesis, Assembly and Reactivity of Metallic
Nanorods
C. J. Murphy, N. R. Jana, L. A. Gearheart, S. O. Obare, K. K.
Caswell, S. Mann, C. J. Johnson, S. A. Davis, E. Dujardin, and
K. J. Edler
9.1
Introduction
Metal particles with dimensions on the nanometer scale are of great current interest for their unusual properties [1–3]. Fundamentally, the mean free path of an
electron in a metal at room temperature is @10–100 nm, and one would predict
that as the metallic particle shrinks to this dimension, unusual effects might be
observed [3]. Indeed, gold nanoparticles of diameter @100 nm or less appear red
(not gold) when suspended in transparent media [1–3]; and gold nanoparticles of
diameter @3 nm are no longer ‘‘noble’’ and unreactive, but can catalyze chemical
reactions [4].
The optical properties of silver and gold nanoparticles in the visible region of the
spectrum, specifically, absorption and scattering, are exquisitely sensitive to nanoparticle size, shape, aggregation state, and local environment [2, 5–10]. Additionally, molecules adsorbed to the surface of gold and silver nanoparticles undergo
surface-enhanced Raman scattering (SERS) effects, due to the coupling of the
plasmon band of the irradiated metal (i.e., the collective oscillation of the conduction band electrons upon absorption in the visible for these particular metals,
due to their dielectric constant) with the molecules’ electronic states [11, 12]. Thus,
one emerging application of metallic nanoparticles is optical sensors, and singlemolecule detection via SERS has been reported [8, 9, 11–19].
The aspect ratio of a solid is defined as its length divided by its width; therefore,
spheres have an aspect ratio of 1. We define, somewhat arbitrarily, a ‘‘nanorod’’ to
be an object with an aspect ratio between 1 and 20, with the short dimension on
the 10–100 nm scale, and a ‘‘nanowire’’ to be an object with an aspect ratio greater
than 20 (with the short dimension on the 10–100 nm scale) [20]. The extinction
spectra (the combination of visible absorption and scattering) of silver and gold
nanoparticles are tunable throughout the visible, depending on the aspect ratio
[5–10; Figure 9.1].
For nanorods and nanowires, the plasmon band of the metal is split in two: the
285
Synthesis, Assembly and Reactivity of Metallic
Nanorods
C. J. Murphy, N. R. Jana, L. A. Gearheart, S. O. Obare, K. K.
Caswell, S. Mann, C. J. Johnson, S. A. Davis, E. Dujardin, and
K. J. Edler
9.1
Introduction
Metal particles with dimensions on the nanometer scale are of great current interest for their unusual properties [1–3]. Fundamentally, the mean free path of an
electron in a metal at room temperature is @10–100 nm, and one would predict
that as the metallic particle shrinks to this dimension, unusual effects might be
observed [3]. Indeed, gold nanoparticles of diameter @100 nm or less appear red
(not gold) when suspended in transparent media [1–3]; and gold nanoparticles of
diameter @3 nm are no longer ‘‘noble’’ and unreactive, but can catalyze chemical
reactions [4].
The optical properties of silver and gold nanoparticles in the visible region of the
spectrum, specifically, absorption and scattering, are exquisitely sensitive to nanoparticle size, shape, aggregation state, and local environment [2, 5–10]. Additionally, molecules adsorbed to the surface of gold and silver nanoparticles undergo
surface-enhanced Raman scattering (SERS) effects, due to the coupling of the
plasmon band of the irradiated metal (i.e., the collective oscillation of the conduction band electrons upon absorption in the visible for these particular metals,
due to their dielectric constant) with the molecules’ electronic states [11, 12]. Thus,
one emerging application of metallic nanoparticles is optical sensors, and singlemolecule detection via SERS has been reported [8, 9, 11–19].
The aspect ratio of a solid is defined as its length divided by its width; therefore,
spheres have an aspect ratio of 1. We define, somewhat arbitrarily, a ‘‘nanorod’’ to
be an object with an aspect ratio between 1 and 20, with the short dimension on
the 10–100 nm scale, and a ‘‘nanowire’’ to be an object with an aspect ratio greater
than 20 (with the short dimension on the 10–100 nm scale) [20]. The extinction
spectra (the combination of visible absorption and scattering) of silver and gold
nanoparticles are tunable throughout the visible, depending on the aspect ratio
[5–10; Figure 9.1].
For nanorods and nanowires, the plasmon band of the metal is split in two: the
285