Delivering confidence in the measurement of nanoparticle concentration

Nanoparticles are increasingly used in innovative products manufactured by advanced industries and provide enhanced, unique properties of great commercial and societal value. Measuring the number concentration of particles in colloidal suspension is a major commercial interest for a large range of industries, including personal care, fine chemicals, pharmaceutical and biomedical industries. It enables compliance with EU regulation linked to the EU definition of a nanomaterial ((2011/696/EU) and underpins any claim to reliability, performance and lifetime in the formulation of products containing nanoparticles.

Currently, there are a number of methods which may be capable of measuring nanoparticle number concentration in colloidal suspension, but no standards or primary methods exist and no certified reference materials are available for calibration and validation purposes.

Within the NPL-led European project Innanopart, methods for Small Angle X-ray Scattering and Single Particle Inductively Coupled Mass Spectrometry were developed to measure the colloidal concentration to better than 10% relative accuracy for spherical gold nanoparticles. Both techniques agree to within 5% with UV-visible spectrometry, which makes use of extinction coefficients calculated from Mie theory. A range of other methods were evaluated with well characterised reference materials, including Tunable Resistive Pulse Sensing and Particle Tracking Analysis.

The effect of mixed nanoparticle populations, as well as particle agglomeration, was investigated and strategies for handling these challenging samples proposed. The project has generated two ongoing standardisation efforts which will lead to documentary standards for the measurement of particle concentration and nanoparticle reference material available to the stakeholder community for method validation and calibration:

• The NPL-led Versailles Project on Advanced Materials and Standards (VAMAS) inter-laboratory comparison TWA34 Project 10 “Measurement of number concentration of colloidal nanoparticles”.
• The LGC-led International Bureau of Weights and Measures CCQM-P194 inter-laboratory comparison.