Dynamic Light Scattering (DLS)

Overview

• DLS, also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QLS), is a spectroscopy method used to determine the size distribution of particles in solutions.
• Utilized in chemistry, biochemistry, and physics.
• Involves a laser providing monochromatic light that scatters due to Brownian motion of particles, analyzed through Rayleigh scattering.
  • The scattering pattern relates to particle size distribution.

Theory

Rayleigh Scattering

• Particles smaller than the wavelength of light diffract it in all directions.
• Intensity I determined by several factors including incident light intensity I0, wavelength, distance R, scattering angle, refractive index n, and particle radius r.
• Creates a "speckle" pattern due to constructive and destructive interference.

Brownian Motion

• Particles move randomly, causing a Doppler shift between incident and scattered light.
• Intensity fluctuations indicate particle size.
• Diffusion constant D is related to the hydrodynamic radius a of the particle (Stokes-Einstein equation).

Correlation Functions

• Intensity Correlation Function: Measures rate of change in scattering intensity.
• Electric Field Correlation Function: Correlates motion of particles relative to each other.
• Seigert Relationship: Connects the two functions to calculate particle size.

Experimentation

Instrumentation

• Laser light passes through polarizer, shines on scattering medium, and scattered light is detected.
• Detector measures scattering angle.
• Modern setups use photomultipliers and can employ various techniques (filter, homodyne, heterodyne).

Sample Preparation

• Suitable for particles from nm to µm.
• Ideal samples are monodisperse and not overly concentrated.
• High purity solvents and filtered samples are recommended.

Measurement

• Software assists with setting parameters like laser wavelength, stabilization time, and temperature.
• Multiple measurements ensure reliability.

Data Analysis

Cumulant Method

• For monodisperse systems: Single exponential decay used.
• For polydisperse systems: More complex fitting needed, using cumulant expansion.
• Involves moments and cumulant-generating functions for data analysis.

Size Distribution Indices

• Intensity Weighted: Analyzed directly from DLS data, related to scattering intensity.
• Number Weighted: Equal weight to each particle, useful for counting.
• Volume Weighted: Useful commercially, based on particle volume.

Application and Comparison

Application Example: Gold Nanoparticles

• DLS used in synthesis to control particle size.
• Easier and reliable for size distribution analysis compared to other methods.

Comparison with TEM and AFM

• DLS: Measures dispersed particles, easier, bulk properties, assumes spherical shape.
• TEM/AFM: Measures deposited particles, provides reliable data for irregular samples, requires more effort.
• Best results come from combining techniques.

Conclusion

• DLS is a rapid, effective method for determining particle size distribution, especially for spherical particles.
• When combined with TEM/AFM, it provides comprehensive size distribution analysis.

References

• Published by Pavan M. V. Raja & Andrew R. Barron from Rice University.
• Via OpenStax CNX.