Combining techniques to comprehensively characterise silver nanoparticles
Silver nanoparticles provide environmentally safe anti-bacterial properties in consumer goods and medical products such as wound dressings. The large surface area to volume of nanoparticles dramatically increases the anti-bacterial efficacy of silver. However, nanoparticle performance is critically dependant on particle size, size distribution, shape and morphology. 
Comprehensive analysis of nanoparticles requires multiple techniques. Transmission Electron Microscopy (TEM) has sub-nanometre resolution and accurately determines the size and shape of silver nanoparticles. The image on the right (copyright NPL) reveals a relatively homogeneous distribution of silver nanoparticles when laid down on a copper TEM grid. Average particle diameter is 28 nm. Simultaneous elemental analysis (Energy Dispersive X-ray Analysis) can be performed to identify contaminants and fingerprint individual particle composition in mixtures. However, TEM has a limited field of view and hence does not necessarily visualise nanoparticles truly representative of the complete sample batch.
Airborne or fluid based particle sizing techniques determine the overall size and volume distribution. Dynamic Light Scattering (DLS) determines the size distribution, volume distribution and zeta potential of nanoparticles dispersed in a fluid through their Brownian motion.
The image above (copyright NPL) shows the particle size distribution by intensity of the as received silver nanoparticles in a dense ethylene glycol solvent. The intensity profile reveals a sharp peak indicating that the size distribution is relatively homogenous through the batch. A slight increase in the measured particle size is attributed to their lower velocity in the viscous solvent.
Our combination of TEM and particle sizing techniques enables the high resolution morphology, structure and shape to be visualised and the variation of nanoparticle size distribution to be determined within batches.
