Determination of particle size and number concentrations in gold nanoparticle mixtures: an inter-method study using four analytical techniques (DLS, PTA, SEM and spICP-MS).

The present study was conducted to determine the size of gold nanoparticles and their respective number concentrations in mono-, bi- and trimodal suspensions using dynamic light scattering (DLS), particle tracking analysis (PTA), scanning electron microscopy (SEM) and single particle inductively coupled plasma mass spectrometry (spICP-MS). This intercomparison approach aimed to test the various analysis methods for their suitability to characterise as well as to distinguish the different nanoparticle fractions in the samples. Thereby, the advantages and limitations of both the employed measurement techniques and the colloidal suspensions containing different concentrations of AuNPs became apparent. Here, DLS and PTA provided reasonably good data on the mean hydrodynamic diameter. In addition, PTA was able to determine the total amount of nanoparticles in all monomodal and in six out of eight multimodal suspensions. However, both methods failed to quantify the number of particles for each individual fraction. Using SEM, the mean sizes of the monomodal suspensions and the sizes of all individual fractions in the multimodal samples could be determined without difficulty. Furthermore, relative particle number ratios of the individual fractions could be obtained, even in the absence of data on the total particle concentration. Finally, despite having its own challenges regarding calibration, spICP-MS was the only method investigated capable of determining not only the mean diameter but also the particle concentration and simultaneously distinguishing and determining the individual fractions in all multimodal samples. This study demonstrated that experimental scenarios exist where individual methods reach their limits even by using well-defined gold nanoparticles of known sizes and concentrations. Therefore, various complementary methods are necessary for nanoparticle analysis to obtain the most comprehensive picture.