Logo of the Physikalisch-Technische Bundesanstalt
symbolic picture: "magazines"

Single-particle counter for microand nanoparticles

Simultaneous characterization and counting of particles with large dynamic measuring range

PTBnews 1.2020
Especially interesting for

nanoparticle research

particle analysis

manufacturers in the pharmaceutical industry

semiconductor manufacturers

Within the framework of a technology transfer project, an optical singleparticle counter has been developed at PTB to detect and determine the size of micro- and nanoparticles in suspension. The resulting prototype covers a dynamic measuring range that allows polystyrene particles from 80 nm to 8 μm in diameter to be detected.

In the single-particle counter, the suspension is injected with a cannula into the conical area of the flow-through cell and then accelerated through the taper and the surrounding sheath flow. In this way, the particles are separated along the direction of flow and mostly pass through the laser focus one by one. For each particle, the stray light is measured in the direction of the laser beam (forward scattering) and perpendicular to it (sideward scattering).

Suspensions and emulsions are used in very different areas of industry and in everyday life. Examples range from abrasives and polishes to skincare products such as sunscreen. To ensure the quality and safe use of products, the particle size, the size distribution and the concentration of the particles must be accurately determined. Conventional measurement methods such as dynamic light scattering applied to characterize particles can be used to determine the particle size distribution, but not to measure the concentration.

Building on the experience gained in flow cytometry, an optical singleparticle counter based on light scattering has been designed and set up at PTB. Technically speaking, this counter is a specialized enhanced optical flow cytometer. A flowthrough cell with hydrodynamic focusing is used for the stable positioning of the sample flow in the detection focus. The light source used is a diode laser. The stray light is collected in the direction of the laser beam and perpendicular to it by means of aspherical lenses and is detected with two photomultiplier tubes (PMTs) simultaneously.

In addition to improving the detection optics, it was necessary to re-develop parts of the detection electronics, since due to the physical properties of light scattering, the stray light signals of the particles have a very large dynamic range with regard to their intensity. A difference in the signal intensity of forward scattering by a factor of 108 is expected for polystyrene particles with a diameter from 50 nm and 10 μm. To solve this problem, a transimpedance amplifier, for which a patent is pending, was developed in collaboration with an external electronics developer to convert the current signals of the PMTs. This transimpedance amplifier can detect and process input currents within a dynamic range of seven decades. This allows the simultaneous counting and characterization of micro- and nanoparticles within a wide size range.

The project results were transferred to the LUM GmbH company and then used to build a prototype close to series production.


Martin Hussels
Department 8.3
Biomedical Optics
Phone: +49 30 3481-7628
Opens window for sending emailmartin.hussels(at)ptb.de