Nanoparticles are widely used in many applications. Accurate characterization of nanoparticles is important in research, production and applications in several fields, including industry, health, safety and related regulation. At VTT MIKES metrology, particles can be characterized using two different methods: Dynamic Light Scattering (DLS) and atomic force microscopy (AFM). Measurements are traceable to the definition of the metre via MIKES interferometrically traceable metrological atomic force microscope (IT-MAFM). Both methods have advantages and limitations. DLS is a fast method and the results are statistically representative. In DSL measurements even a small number of large particles can prevent detection of small particles. AFM can be used to measure both the size and shape of single particles. The disadvantage of AFM measurements is that only a limited number of particles can be measured, which leads to poor statistics. Tip sample interaction is important especially when measuring small particles. Also sample preparation might be challenging.
Figure 1. DLS results of 100 nm nanoparticles.
Dynamic Light Scattering is used to measure particle and molecule sizes. This technique measures the diffusion of particles moving under Brownian motion, and converts this to size and a size distribution using the Stokes-Einstein relationship. A typical result graph for DLS is shown in Figure1.
Laser Doppler Micro-electrophoresis is used to measure the zeta potential. An electric field is applied to a solution of molecules or a dispersion of particles, which then move with a velocity related to their zeta potential.
Figure 2. AFM image of 100 nm nanoparticles.
Atomic force microscopy (AFM)
An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, an increasingly repulsive force takes over and causes the cantilever to deflect away from the surface. AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by a position-sensitive photo diode (PSPD). By using a feedback loop to control the height of the tip above the surface, the AFM can generate an accurate topographic map of the surface features. Nanoparticle sizes can be analyzed from measured particle height or lateral dimensions of closely packed particle layers. A typical nanoparticle measurement image is shown in Figure 2.
Services for nanoparticle characterisation at MIKES
- Nanoparticle size and shape measurements using AFM
- Nanoparticle size distribution in solution using DLS
- Nanoparticle surface charge (Zeta-potential) measurements in solution