The diagram above illustrates the workflow for Work Package¬†2 with partners’ responsibilities colour coded. This diagram is regularly updated as activities are completed (marked with ticks)

The aim of this work package is to establish the sensitivity of laboratory based methods to deviations from ideal, monodisperse, spherical particles, for example: a fractional population of agglomerated particles or rod-like particles. The WP will develop two types of test sample, firstly a stable population of dimeric or tetrameric agglomerates, and secondly elongated quantum rods with a range of aspect ratios. The first reference sample (a stable population of dimeric or tetrameric agglomerates) will be used to develop methods to measure agglomerate fraction with a target relative standard uncertainty of 20 %. With both sample types it will be possible to establish the sensitivity of different methods of concentration measurement to agglomeration and variation from a spherical shape. For the agglomeration fraction activities, spherical particles synthesised or purchased in task 1.1 will be employed by preference.

Deviations from sphericity are relatively common for advanced particles and the most commonly used shapes are rod-like particles. These have excellent potential in improved diagnostics, therapies, solar cell and display applications. The dimensions of nanorods are very hard to determine in liquid suspension, except in special cases such as gold rods where distinct LSPR modes may be employed, and characterisation by electron microscopy on dry samples remains the norm. However, particle number concentration may be more amenable through methods such as spICPMS and TRPS. In this work package we will examine quantum rods with different aspect ratios and assess whether particle number concentrations can be measured using the methods from WP1 for spherical particles and if not, whether straightforward corrections can be applied to the data. Some techniques, such as SAXS, are unsuitable for the traceable measurement of non-spherical particles. Others, such as spICPMS, may be insensitive to shape and should be able to count nanorods, with an expectation that the signal will be the same as spherical particles of the same mass. This will permit spICPMS to be used as a benchmark method for other methods where it is far from clear whether meaningful concentration data can be extracted if the particles deviate from sphericity. For example, in DCS this will rely upon a coincidence in the scaling of sedimentation time and optical extinction coefficient with aspect ratio. However, a simple correction procedure may be envisaged if the particle shape is known, but in any case, it is important to understand errors that may arise from non-sphericity in the sample population.

FCS is a microscopy technique that measures fluorescence bursts emitted by single molecules or particles diffusing through a small observation volume (only few femtoliters), which is formed using a strongly focused laser beam, and recorded with pinhole-based detection. The sizes that can be measured with FCS range from 1 nm (small organic dye molecule) to several 100 nm (fluorescent particles or large dye-biomolecule conjugates), with a high size resolution of a few nanometres, which is ideally suitable to distinguish monomeric particles from aggregates (dimers, trimers or tetramers). As FCS measures only single or at least few molecules/particles at a time, the challenge with FCS is how to achieve sufficient statistics.