Transcript Growth of Gold(Au) Nanoparticles

Gold(Au) Nanoparticles
(and silver)
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Motivation
History
Colloids
Nucleation
Growth
Coagulation
Procedure
Viewing the particles
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• Some quacks still assert profound medicinal
properties
• Spherical nanoparticles can serve as biological
tags for tracking purposes
• The red color in stained glass windows is due to
colloidal gold
• Can be used for a new ultrasensitive and
selective detection scheme for DNA
• Gold nanorods can be bar-coded Au/Pt
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• Alchemists believed Au sols might be the “elixir of life”
• Faraday researched many of the properties of colloidal
gold in the 1850’s.
• Mie’s theory of light scattering was developed to explain
the color of colloidal gold.
• Medical applications were developed that diagnosed
certain diseases based on the interaction of colloidal
gold and spinal fluids
• The first comprehensive investigation using the electron
microscope began in 1948 at Princeton University and
RCA Labs
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• The color of the sol arises from a combination of absorption and
scattering of light and depends on particle size
• More specifically it is due to a resonance of the free electrons in
the metal particle. The light’s electromagnetic field causes them
to slosh back and forth (plasmon oscillations).
• At a characteristic frequency which depends of the size and the
metal, the sloshing is the most intense. This is the frequency
where plasmon oscillations are excited.
• The plasmon resonance is easily seen in the extinction spectrum
of the sol.
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• A colloid is a homogeneous dispersion of particles in a solution
which are so small as to not settle out easily
• A sol is a specific type of colloid characterized as a solid dispersed
in a liquid
• The colloid is stabilized by electric charges on its surface due to
adsorbed ions. The charge causes the particles to repell each other.
The both the Gold and Silver sols used here have a negative
charge.
• The particles experience the constant buffeting of Brownian motion
which also helps to keep them in suspension.
• Formulation of Au nanoparticles is a three step process: nucleation,
growth, and coagulation
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• Nucleation is the creation of nuclei upon which growth
can occur
• This is a redox reaction: oxidation of the citrate ion
produces the necessary reducing reagent for the gold:
acetone dicarboxylic acid
– The acetone dicarboxylic acid is the limiting reagent for
nucleation
• The formation of this molecule in the solution creates an
induction period before which no product can be seen
• The nature of the nucleation curve is evidence of an
autocatalytic reaction
– That is to say it has a rapid growth after the induction period
followed by a linear portion and then decay
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• A type of polymerization (complexation) occurs in which
the gold ions coordinate with acetone dicarboxylic acid
and join together
• When the “polymer,” or complex, reaches a critical mass
that is just greater its thermodynamic stability, reduction
to metallic gold occurs, yielding the nuclei
• Reduction is the rate determining step in the kinetics of
the reaction
• The less citrate in the mixture, the larger the particles will
be in size
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• Growth is the addition of more gold particles to
the existing nuclei
• The process of growth stops when all of the gold
is used
• The rate of growth is a first order in the gold
nuclei size
– Having the equation dD/dt = kD, where k is a
constant whose value is independent of particle
size
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• Creation of the larger gold particles, such as 20 nm,
requires a coagulation of multiple (smaller) twins of
various shapes
• A conglomeration of multiple nuclei into particles can be
large enough to disturb the stability and fall out of the
colloid
• Control of the coagulation process during preparation
determines the size, structure, and size distribution of
the particle
• Once the preparation of the gold nanoparticles is
complete, the absence of coagulation insures its stability
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• Bring to a boil 50 mL of 2.5×10-4 M chlorauric acid solution
• Add 0.16 to 1.0 mL of 34 mM sodium citrate solution to the
boiling solution while stirring
• After a minute will be faint blue and then darkening over 5
min to a brilliant red
• The size of the gold nanoparticles can be controlled by
varying the amount of sodium citrate solution. The above
procedure can grow controlled sizes from 147 nm (0.16 ml)
down to 16 nm (1.0 ml).
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• John Turkevich, Gold Bulletin, 18(3), 86 (1985).
• John Turkevich, Gold Bulletin, 18(4), 125 (1985).
• Turkevich, Stevenson, Hillier, Discussions of the Faraday Society,
11, 58 (1951).
• G. Frens, Nature Physical Science, 241, 20 (1971).
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