Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 221

6.2.3.1 Dynamic Light Scattering

In dynamic light scattering (DLS), also called photon correlation spectroscopy or
quasielastic light scattering, the fl uctuations in the intensity of the scattered light,
due to constructive or destructive interferences from point scatterers in the focal
volume, are correlated to the diffusive motion of the NP. The light intensity is mea-
sured at very short time increments (nano- to microseconds) and autocorrelated to
the initial reading. For the initial intensity readings the correlation is unity since
the particles have hardly had any time to diffuse, but as time progress and particles
move the intensity correlation decrease (Figure 6.2). The decay of the autocorrela-
tion function is proportional to the diffusion coeffi cient, D, according to Equation
(6.1), where A is an instrument constant and q is the scattering form factor and t
is the time:

( )

( )

( ) −≈ gt

2

1 A exp − 2 qDt

2

(6.1)

1.0

0.9

0.8

2

( 2 ) ( − 2 q Dt )

g ( t ) − 1 ≈ Ae

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.1 1 10 100 1000 10000 100000 1000000 10000000

time (µs)

Figure 6.2 Dynamic light scattering autocorrelation function over the decay time for a CuO
nanopowder dispersed in distilled water. Since Equation (6.1) holds for small t, then the
cumulant analysis to extract the z-average D or d H , Equation (6.2), is done in the fi rst part
of the autocorrelation function (dotted square). The obtained z-average diameter was 190 nm.
The hump in the autocorrelation with a decay on much longer time scales indicate very large
aggregates or dust contamination.