Biophotonics_Concepts_to_Applications

intensity willfluctuate over time because the distances between the molecules in the
solutions are constantly changing due Brownian motion. As a result of this motion,
the scattered light then interferes either constructively or destructively, which
produces a dynamically changing speckle pattern. These sets of speckle patterns
then are analyzed by an autocorrelator that compares the intensity of light at each
spot over time.
Information for evaluating the sizes and the movements of the scattering parti-
cles can be obtained from the time correlations of the intensityfluctuations. As
Fig.9.20b shows, larger particles move slower than smaller ones. Thus the intensity
fluctuations are slower for large particles compared to faster moving small particles
for which the intensity varies more rapidly. In the correlation functions for the two
different sized particles, the onset of the correlation decay (loss of correlation)
indicates the mean size of the particles. The gradient of the curves are related to the
polydispersityof the particles, which is a measure of the distribution of molecular
mass in a given polymer sample.
For a given temperature T, the PCS method only requires knowing the viscosity
of the suspendingfluid in order to estimate the average particle size and the particle
distribution function. To obtain accurate information from the PCS method, the
light can only be scattered once. Thus, PCS requires highly diluted suspensions in
order to avoid multiple scattering that can lead to incorrect results.

Laser

Detector

Sample holder

Signal

analysis

unit

Intensity

Time

Large particles

Intensity

Time

Small particles

Correlationcoefficient Time

Start of decay time

gives mean size

Correlationcoefficient Time

Gradient indicates

the polydiversity

(a)

(b)

Fig. 9.20 aA basic setup for PCS;bSignalfluctuations and the related correlation coefficient for
large and small particles

9.10 Photon Correlation Spectroscopy 283