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© 2011 HORIBA, Ltd. All rights reserved.
Nanoparticle Analysis: Dynamic Light Scattering for Particle Size Determination
Jeffrey Bodycomb, Ph.D.HORIBA Scientific
www.horiba.com/us/particle
© 2011 HORIBA, Ltd. All rights reserved.
What is Dynamic Light Scattering?
Dynamic light scattering refers to measurement and interpretation of light scattering data on a microsecond time scale.Dynamic light scattering can be used to
determine Particle/molecular sizeSize distributionRelaxations in complex fluids
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Other Light Scattering Techniques
Static Light Scattering: over a duration of ~1 second. Used for determining particle size (diameters greater than 10 nm), polymer molecular weight, 2nd virial coefficient, Rg.
Electrophoretic Light Scattering: use Doppler shift in scattered light to probe motion of particles due to an applied electric field. Used for determining electrophoretic mobility, zeta potential.
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Particle Diameter (m)0.01 0.1 1 10 100 1000
Colloidal
Suspensions and Slurries
DLS – SZ-100
Electron Microscope
Powders
Fine Coarse
Microscopy PSA300
Laser Diffraction – LA-950
Acoustic Spectroscopy – DT-1201 and DT-300(Zeta)
Electrozone Sensing
Disc-Centrifuge (CPS)
Light Obscuration
0.001
Macromolecules
Nano-Metric
Met
hods
Met
hods
App
sA
pps
Size
sSi
zes
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Particles in suspension undergo Brownian motion due to solventmolecule bombardment in random thermal motion.
Brownian MotionRandomRelated to SizeRelated to viscosityRelated to temperature
Brownian Motion
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Particles
Zeta potential
Backscatter (173°)(High conc.)
90° for size and MW, A2
Laser
Modulator
PDFor T%532nm, 10mW
Attenuator
Attenuator
Particles movingdue to Brownianmotion
DLS Optics
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DLS signalRandom motion of particles leads to random
fluctuations in signal (due to changing constructive/destructive interference of scattered light.
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Correlation Function
Random fluctuations are interpreted in terms of the autocorrelation function (ACF).
)()(
)()()( 0
tItI
dttItIC
T
)2exp(1)( C
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Gamma to Size
2qDm 2sin4
nq
m
Bh DT
TkD)(3
decay constantDm diffusion coefficientq scattering vectorn refractive index wavelength scattering angleDh hydrodynamic diameter viscositykB Boltzman’s constant
Note effect of temperature!
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What is Hydrodynamic Size?
DLS gives the diameter of a sphere that moves (diffuses) the same way as your sample.
Dh DhDh
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Hydrodynamic Size The instrument reports the size of sphere that
moves (diffuses) like your particle. This size will include any stabilizers bound to the
molecule (even if they are not seen by TEM).
SEM (above) and TEM (below) images for RM 8011
Technique Size nmAtomic Force Microscopy 8.5 ± 0.3Scanning Electron Microscopy 9.9 ± 0.1Transmission Electron Microscopy 8.9 ± 0.1Dynamic Light Scattering 13.5 ± 0.1
Gold Colloids
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Polystyrene Latex Sample
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Polydisperse Sample Cumulants
For a mixture of sizes, the autocorrelation function can be interpreted in terms of cumulants. This is the most robust method of analyzing DLS data.
!22exp1)( 2C
2qDm
m
Bhz DT
TkD)(3,
22
sityPolydisper
“z-average size”
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SiO2
0.077479.8Avg.0.039487.740.077478.830.066479.520.127473.21
Polydispersity Index
Z-average Diameter (nm)
Run
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Polydisperse Sample (ILT) A more general relationship can be given between the
autocorrelation function and the size distribution. Let each size have a relation constant . The scattering from each population is then given by S(). Now we have an integral equation. Solving for S() gives us size distribution.
dSg )exp()()()1(
2)1( )(1)( gC
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Bimodal SampleNominal 20 nm and 500 nm latex run
individually
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Bimodal Sample
Mixed sample (in black)
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Dynamic Light Scattering Practical Tips
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Dust
Dust: large, rare particles in the sampleGenerally not really part of the sampleSince they are rare cannot get good
statistics
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Filter to removedust. If particles are too large (D >50 nm for 0.1 m filter), at least filter diluent.
Filters available in sizes 20nm to 2m
We can also centrifuge the sample and extract the supernatant.
Filtering
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The Natural limit for Dynamic Light Scattering: Gravitational Settling
Gravitational Settling occurs at about 1-3m
Particle Diameter(m)
Movement due toBrownian Motion
Movement due toGravitational Settling
0.01 2.36 >> 0.0050.25 1.49 > 0.03460.50 1.052 > 0.13841.0 0.745 ~ 0.5542.5 0.334 < 13.84
10.0 0.236 << 55.4
Settling and DLS
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Why DLS?
Non-invasive measurementRequires only small quantities of
sampleGood for detecting trace amounts of
aggregateGood technique for macro-molecular
sizing
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New Nanoparticle Analyzer
Single compact unit that performs size, zeta potential, and molecular weight measurements.
© 2011 HORIBA, Ltd. All rights reserved.
Q&A
Ask a question at labinfo@horiba.com
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Jeff Bodycomb, Ph.D. P: 866-562-4698E: jeff.bodycomb@horiba.com