What Is Dynamic Light Scattering?
Dynamic light scattering (DLS), also known as photon correlation spectroscopy or quasi-elastic light scattering, is one of the most widely used techniques for measuring the size of nanoparticles and colloids in suspension. The technique works by illuminating a sample with a coherent laser beam and analysing the time-dependent fluctuations in the intensity of scattered light. These fluctuations arise because particles in suspension undergo Brownian motion β the random thermal movement driven by collisions with solvent molecules. Smaller particles move faster and produce rapid intensity fluctuations, while larger particles move more slowly and generate slower fluctuations. By mathematically correlating these fluctuations over time, DLS instruments derive the translational diffusion coefficient of the particles, which is then converted to a hydrodynamic diameter using the Stokes-Einstein equation.
Key Parameters: Z-Average and Polydispersity Index
Two parameters dominate DLS reporting: the Z-average diameter and the polydispersity index (PDI). The Z-average is an intensity-weighted mean hydrodynamic size derived from a cumulants analysis of the correlation function. It is the primary and most stable parameter reported by DLS and is the size metric recommended by ISO 22412 for quality control applications.
The polydispersity index quantifies the breadth of the particle size distribution. A PDI below 0.1 indicates a highly monodisperse sample, values between 0.1 and 0.4 suggest moderate polydispersity, and values above 0.7 indicate a very broad distribution that may not be suitable for cumulants analysis alone. For pharmaceutical nanoparticles such as liposomes and lipid nanoparticles used in mRNA vaccine delivery, regulatory guidelines typically require a PDI below 0.3 to ensure dose uniformity and predictable biodistribution.
Instrumentation: The Zetasizer Advance Range
Malvern Panalytical's Zetasizer Advance range represents the latest evolution in DLS instrumentation. These systems feature non-invasive back-scatter (NIBS) optics at a 173Β° detection angle, which minimises the influence of dust and large aggregates while maximising sensitivity for small particles. Key capabilities include:
- Size measurements from 0.3 nm to 15 Β΅m, covering proteins, nanoparticles, emulsions and aggregates.
- Zeta potential determination using electrophoretic light scattering (ELS), critical for predicting colloidal stability.
- Molecular weight estimation via static light scattering (SLS) for characterising proteins and polymers.
- Automated titration for studying size and zeta potential as a function of pH, ionic strength or additive concentration.
Best Practices for Reliable DLS Measurements
Despite its apparent simplicity, DLS is sensitive to sample preparation and measurement conditions. Following best practices ensures reproducible, high-quality data:
- Sample concentration: DLS requires an appropriate scattering intensity. Samples that are too dilute produce noisy correlation functions, while samples that are too concentrated suffer from multiple scattering and particle-particle interactions. The Zetasizer Advance's automatic attenuation and measurement position optimisation handle a wide concentration range, but users should still verify that results are concentration-independent by measuring a dilution series.
- Filtration: Dust and large contaminants scatter disproportionately and can dominate the correlation function. Filtering solvents through 0.1 or 0.02 Β΅m syringe filters before sample preparation is essential.
- Temperature equilibration: Because Brownian motion is temperature-dependent, the sample must reach thermal equilibrium inside the instrument before measurement. Most modern DLS systems provide active temperature control and automated equilibration timers.
- Cuvette selection: Use cuvettes matched to the measurement type β disposable polystyrene for aqueous samples, quartz for organic solvents, and folded capillary cells for simultaneous size and zeta potential measurement.
Applications Across Industries
DLS serves a remarkably diverse range of applications. In pharmaceutical development, it is the frontline technique for characterising liposomes, protein aggregates, nanocrystals and vaccine adjuvants. In polymer science, DLS monitors micelle formation, emulsion polymerisation kinetics and polymer-surfactant interactions. In environmental science, researchers use DLS to study natural colloids in surface water and track the fate of engineered nanomaterials released into aquatic ecosystems.
Complementary Techniques: NanoSight and Beyond
For samples with complex, multimodal size distributions, nanoparticle tracking analysis (NTA) using the NanoSight Pro provides particle-by-particle sizing and concentration measurement. NTA visualises individual nanoparticles moving under Brownian motion and tracks their trajectories, offering higher resolution than ensemble DLS for polydisperse mixtures. Combining DLS for rapid screening with NTA for detailed characterisation gives researchers a powerful and comprehensive nanoparticle analysis toolkit.
Dynamic light scattering remains an indispensable technique for any laboratory working with nanoscale materials. By understanding its principles, respecting its limitations and following measurement best practices, researchers can extract maximum value from every DLS experiment and drive their nanoparticle research forward with confidence.