Dynamic light scattering (DLS) is a technique that can be used to determine the size distribution of small particles in suspension or macromelecules in solution by means of mathematical relations between light scattering and diffusion behaviour of particles.
In the basic setup of a DLS instrument, a single frequency laser is directed to the sample contained in a cuvette. When light hits small particles in random Brownian motion, the light diffuses in all directions and the scattering intensity fluctuates over time. The scattered light is detected at a certain angle over time and this fluctuating signal is used to determine the diffusion behaviour of the particles which derives from an autocorrelation of the intensity trace recorded during the experiment and is directly correlated to the size of the particles (or more precisely to the particles' hydrodynamic radii): bigger particles diffuse slower than smaller ones. The output of a DLS size measurement is a light intensity-based distribution that can be converted to a mass or volume one by means of Mie theory. The main advantage of the technique is the detectable particle size that ranges from less than one nanometer to few microns. Moreover, any surface effect (such as organic coating or electrical charge variation) can be analysed by DLS.