By Steven J. Oldenburg, Ph.D
Table of ContentsIntroductionSilver
Nanoparticle Optical Properties Silver Nanoparticle
CharacterizationSilver Nanoparticle Surface
ChemistrySilver Nanoparticle ApplicationsSilver Nanoparticles for Nanotoxicology ResearchAbout Sigma Aldrich
Silver nanoparticles have unique optical, electrical, and thermal properties
and are being incorporated into products that range from photovoltaics to
biological and chemical sensors. Examples include conductive inks, pastes and
fillers which utilize silver nanoparticles for their high electrical
conductivity, stability, and low sintering temperatures. Additional applications
include molecular diagnostics and photonic devices, which take advantage of the
novel optical properties of these nanomaterials. An increasingly common
application is the use of silver nanoparticles for antimicrobial coatings, and
many textiles, keyboards, wound dressings, and biomedical devices now contain
silver nanoparticles that continuously release a low level of silver ions to
provide protection against bacteria.
Figure 1. Transmission electron microscopy (TEM) images
of silver nanoparticles with diameters of 20 nm (Aldrich Prod. No. 730793), 60
nm (Aldrich Prod. No. 730815), and 100 nm (Aldrich Prod. No. 730777)
respectively. Scale bars are 50 nm.
Understanding how the size, shape, surface, and aggregation state of the
silver nanoparticles change after integration into a target application is
critical for optimizing performance. Aldrich Materials Science offers precisely
manufactured monodisperse silver nanoparticles that are free from agglomeration,
making them ideal for research, development, and use in a variety of innovative
applications (Figure 1). Each batch of nanoparticles is extensively
characterized using transmission electron microscopy (TEM) images, dynamic light
scattering (DLS, for particle size analysis), Zeta potential measurements, and
UV/Visible spectral analysis to ensure consistent materials. A list of silver
nanoparticle dispersions available from Aldrich Materials Science is presented
in Table 1.
Table 1. Silver Nanoparticle Dispersions
Silver Nanoparticle Optical Properties
There is growing interest in utilizing the optical properties of silver
nanoparticles as the functional component in various products and sensors.
Silver nanoparticles are extraordinarily efficient at absorbing and scattering
light and, unlike many dyes and pigments, have a color that depends upon the
size and the shape of the particle. The strong interaction of the silver
nanoparticles with light occurs because the conduction electrons on the metal
surface undergo a collective oscillation when excited by light at specific
wavelengths (Figure 2, left). Known as a surface plasmon resonance (SPR), this
oscillation results in unusually strong scattering and absorption properties. In
fact, silver nanoparticles can have effective extinction (scattering +
absorption) cross sections up to ten times larger than their physical cross
section. The strong scattering cross section allows for sub 100 nm nanoparticles
to be easily visualized with a conventional microscope. When 60 nm silver
nanoparticles are illuminated with white light they appear as bright blue point
source scatterers under a dark field microscope (Figure 2, right). The bright
blue color is due to an SPR that is peaked at a 450 nm wavelength. A unique
property of spherical silver nanoparticles is that this SPR peak wavelength can
be tuned from 400 nm (violet light) to 530 nm (green light) by changing the
particle size and the local refractive index near the particle surface. Even
larger shifts of the SPR peak wavelength out into the infrared region of the
electromagnetic spectrum can be achieved by producing silver nanoparticles with
rod or plate shapes.
Figure 2. (Left) Surface plasmon resonance where the free
electrons in the metal nanoparticle are driven into oscillation due to a strong
coupling with a specific wavelength of incident light. (Right) Dark field
microscopy image of 60 nm silver nanoparticles (Aldrich Prod. No. 730815).
Silver Nanoparticle Characterization
The size and shape of metal nanoparticles are typically measured by
analytical techniques such as TEM, scanning electron microscopy (SEM) or atomic
force microscopy (AFM). Measuring the aggregation state of the particles
requires a technique to measure the effective size of the particles in solution
such as dynamic light scattering (DLS) or analytical disc centrifugation.
However, due to the unique optical properties of silver nanoparticles, a great
deal of information about the physical state of the nanoparticles can be
obtained by analyzing the spectral properties of silver nanoparticles in
solution. The spectral response of silver nanoparticles as a function of
diameter is shown in Figure 3, left. As the diameter increases, the peak plasmon
resonance shifts to longer wavelengths and broadens. At diameters greater than
80 nm, a second peak becomes visible at a shorter wavelength than the primary
peak. This secondary peak is due to a quadrupole resonance that has a different
electron oscillation pattern than the primary dipole resonance. The peak
wavelength, the peak width, and the effect of secondary resonances yield a
unique spectral fingerprint for a plasmonic nanoparticle with a specific size
and shape. Additionally, UV-Visible spectroscopy provides a mechanism to monitor
how the nanoparticles change over time. When silver nanoparticles aggregate, the
metal particles become electronically coupled and this coupled system has a
different SPR than the individual particles. For the case of a
multi-nanoparticle aggregate, the plasmon resonance will be red-shifted to a
longer wavelength than the resonance of an individual nanoparticle, and
aggregation is observable as an intensity increase in the red/infrared region of
the spectrum. This effect can be observed in Figure 3, right, which displays the
optical response of a silver nanoparticle solution destabilized by the addition
of saline. Carefully monitoring the UV-Visible spectrum of the silver
nanoparticles with time is a sensitive technique used in determining if any
nanoparticle aggregation has occurred.
Figure 3. (Left) Extinction (scattering + absorption)
spectra of silver nanoparticles with diameters ranging from 10-100 nm at mass
concentrations of 0.02 mg/mL. (Right) Extinction spectra of silver nanoparticles
after the addition of a destabilizing salt solution.
For silver nanoparticle solutions that have not agglomerated and have a
spectral shape that is identical to the as- received suspension, the UV/Visible
extinction spectra can be used to quantify the nanoparticle concentration. The
concentration of silver nanoparticle solutions is calculated using the
Beer-Lambert law, which correlates the optical density (OD, a measure of the
amount of light transmitted through a solution) with concentration. Due to the
linear relationship between OD and concentration, these values can be used to
quantify the concentration of nanoparticle solutions.
Silver Nanoparticle Surface Chemistry
When nanoparticles are in solution, molecules associate with the nanoparticle
surface to establish a double layer of charge that stabilizes the particles and
prevents aggregation. Aldrich Materials Science offers several silver
nanoparticles suspended in a dilute aqueous citrate buffer, which weakly
associates with the nanoparticle surface. This citrate-based agent was selected
because the weakly bound capping agent provides long term stability and is
readily displaced by various other molecules including thiols, amines, polymers,
antibodies, and proteins.
Silver Nanoparticle Applications
Silver nanoparticles are being used in numerous technologies and incorporated
into a wide array of consumer products that take advantage of their desirable
optical, conductive, and antibacterial properties.
- Diagnostic Applications: Silver nanoparticles are used in biosensors
and numerous assays where the silver nanoparticle materials can be used as
biological tags for quantitative detection.
- Antibacterial Applications: Silver nanoparticles are incorporated in
apparel, footwear, paints, wound dressings, appliances, cosmetics, and plastics
for their antibacterial properties.
- Conductive Applications: Silver nanoparticles are used in conductive
inks and integrated into composites to enhance thermal and electrical
- Optical Applications: Silver nanoparticles are used to efficiently
harvest light and for enhanced optical spectroscopies including metal-enhanced
fluorescence (MEF) and surface-enhanced Raman scattering (SERS).
Silver Nanoparticles for Nanotoxicology Research
There is growing interest in understanding the relationship between the
physical and chemical properties of nanomaterials and their potential risk to
the environment and human health. The availability of panels of nanoparticles
where the size, shape, and surface of the nanoparticles are precisely controlled
allows for the better correlation of nanoparticle properties to their
toxicological effects. Sets of monodisperse, unaggregated, nanoparticles with
precisely defined physical and chemical characteristics provide researchers with
materials that can be used to understand how nanoparticles interact with
biological systems and the environment.
Due to the increasing prevalence of silver nanoparticles in consumer
products, there is a large international effort underway to verify silver
nanoparticle safety and to understand the mechanism of action for antimicrobial
effects. Colloidal silver has been consumed for decades for its perceived health
benefits1 but detailed studies on its effect on the environment have just begun.
Initial studies have demonstrated that effects on cells and microbes are
primarily due to a low level of silver ion release from the nanoparticle
surface.2 The ion release rate is a function of the nanoparticle size (smaller
particles have a faster release rate), the temperature (higher temperatures
accelerate dissolution), and exposure to oxygen, sulfur, and light. In all
studies to date, silver nanoparticle toxicity is much less than the equivalent
mass loading of silver salts.
About Sigma Aldrich
Sigma-Aldrich® is a leading high-technology company. Through
our Materials Chemistry Centers of Excellence in research and manufacturing we
develop advanced, enabling materials for your micro/nanoelectronics, alternative
energy, display/optoelectronics, nanotechnology and related materials science
and engineering applications. Specialties include ALD precursors, ultra-high
purity inorganic halides, fuel cell materials, electronic grade dyes, specialty
monomers and cGMP grade polymers.
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