Dynamic light scattering is a widely used technique for determining the size distribution of nanoparticles suspended in a liquid medium. The method relies on the analysis of the random motion of particles due to Brownian motion and how this motion affects the scattering of light. The fundamental principle involves illuminating the sample with a coherent laser beam and measuring the fluctuations in the intensity of the scattered light caused by the movement of particles. These fluctuations are then correlated to extract information about particle size.
When a laser beam passes through a colloidal suspension, the particles scatter light in all directions. The intensity of the scattered light fluctuates over time due to the constant random motion of the particles, known as Brownian motion. Smaller particles move more rapidly, causing faster fluctuations, while larger particles move more slowly, leading to slower fluctuations. The key to DLS is measuring these fluctuations and relating them to particle size through time correlation analysis.
The scattered light is detected at a fixed angle, typically 90 degrees or 173 degrees (backscatter), depending on the instrument configuration. The detector converts the scattered light into an electrical signal, which is then processed by a digital correlator. The correlator computes the autocorrelation function, which describes how the scattered light intensity changes over time. The decay rate of this autocorrelation function is directly related to the diffusion coefficient of the particles in the suspension.
The diffusion coefficient (D) is linked to particle size via the Stokes-Einstein equation, which is central to DLS analysis. The equation is given as:
D = kT / (6πηr)
where:
- D is the diffusion coefficient
- k is the Boltzmann constant
- T is the absolute temperature
- η is the viscosity of the solvent
- r is the hydrodynamic radius of the particle
This equation assumes spherical particles and a dilute suspension where particle-particle interactions are negligible. The hydrodynamic radius includes any solvent molecules or surface layers that move with the particle, making it slightly larger than the actual core size.
A DLS instrument consists of three main components: the laser source, the detector, and the correlator. The laser provides a monochromatic, coherent light beam, typically in the visible or near-infrared range, ensuring sufficient scattering intensity. The detector, usually a photomultiplier tube or an avalanche photodiode, captures the scattered light with high sensitivity. The correlator processes the signal in real time, computing the autocorrelation function, which is then fitted to extract the diffusion coefficient and, subsequently, the particle size distribution.
One of the primary advantages of DLS is its sensitivity to small particles, with a typical measurement range of approximately 1 nm to several micrometers. Unlike electron microscopy, which requires sample drying and high vacuum conditions, DLS measures particles in their natural liquid state, providing a more representative size distribution for colloidal systems. Additionally, DLS is non-destructive, requires minimal sample preparation, and delivers results quickly, often within minutes.
The technique is particularly useful for analyzing polydisperse samples, where multiple particle sizes are present. Modern DLS instruments employ advanced algorithms, such as CONTIN or non-negative least squares (NNLS), to deconvolute the autocorrelation function into a size distribution profile. However, DLS has limitations, including difficulty in resolving mixtures of very large and very small particles due to the strong scattering dependence on particle size (intensity scales with the sixth power of diameter for Rayleigh scatterers).
Compared to static light scattering, which measures time-averaged intensity to determine molecular weight and size, DLS provides dynamic information through time-dependent fluctuations. This makes DLS uniquely suited for studying diffusion and hydrodynamic behavior in solution. Furthermore, DLS does not require calibration standards, as the size determination is based on fundamental physical principles rather than empirical comparisons.
Temperature control is critical in DLS measurements because viscosity and diffusion rates are temperature-dependent. Most instruments include a temperature-regulated sample chamber to ensure consistent conditions. The solvent viscosity must also be accurately known or measured independently, as errors in viscosity directly affect the calculated particle size.
DLS finds applications across various fields, including pharmaceuticals, biotechnology, and materials science. It is commonly used to characterize liposomes, proteins, polymer nanoparticles, and metal colloids. In biopharmaceuticals, DLS helps monitor protein aggregation, a critical quality attribute for therapeutic formulations. In nanotechnology, it assists in optimizing synthesis conditions by tracking particle growth and stability over time.
Despite its advantages, DLS has some constraints. The technique assumes spherical particles, so anisotropic shapes may lead to apparent size inaccuracies. Highly concentrated samples can cause multiple scattering, complicating data interpretation. In such cases, dilution or specialized backscatter detection optics may be necessary. Additionally, DLS cannot distinguish between different materials if they have similar sizes, requiring complementary techniques for full characterization.
In summary, dynamic light scattering is a powerful tool for nanoparticle sizing based on the principles of light scattering and Brownian motion. By analyzing fluctuations in scattered light intensity, DLS provides rapid, non-invasive measurements of hydrodynamic size in liquid suspensions. Its reliance on the Stokes-Einstein equation ensures a theoretically grounded approach, while its instrumentation—comprising a laser, detector, and correlator—enables precise and sensitive analysis. While it has limitations, particularly for polydisperse or non-spherical systems, its advantages in speed, sensitivity, and solution-phase measurement make it indispensable in nanoscience and colloidal chemistry.
The continued development of advanced correlation algorithms and multi-angle detection systems further enhances the accuracy and applicability of DLS. As nanotechnology advances, the technique remains a cornerstone for particle characterization, bridging fundamental research and industrial applications where precise size measurement is critical.