Optimization and characterization of nanomaterials for drug delivery – Q&A

In dynamic light scattering, the diffusion coefficient (and hence size) information is obtained by fitting the correlation function, derived from the measurement, with a suitable algorithm. There are two approaches that can be taken. These are the cumulants analysis and a distribution analysis. The cumulants analysis determines a mean size, which Malvern call the z-average, and polydispersity index. The distribution analysis determines the actual size distribution from suitable data. The z-average size is mainly useful for interpreting samples with low polydispersities and can be used for simply measuring the average particle size in a population or to monitor changes occurring in a sample. The size information obtained from a distribution analysis allows us to obtain more specific information relating to the total number of modes present in a population, and how they relate to one another. Therefore, it is probable that the mean size obtained from the distribution analysis will be different to the z-average mean. However, in combination, the two analyses provide us with a greater understanding of the sample being measured.

There are guidelines to the accuracy and precision expected from zeta potential measurements listed in ISO 13099 Part 2 Colloidal systems — Methods for zeta potential determination — Part 2: Optical methods. It states that both the accuracy and precision of the technique for a suitable sample should be better than 10%. However, we would expect both these values to be much lower for suitable samples measured on a Zetasizer Nano instrument.

NTA can distinguish different size populations in a sample. It can even distinguish populations with the same size but different refractive indices like gold and liposomes. It is also possible to measure only fluorescent particles within a sample with an appropriate experimental protocol. But whole cells are normally above the higher end of NTA’s measuring range of 2000nm.

For NTA it is possible to make measurements in IPA, though you would need to know they viscosity at our measurement temperature. The same is true for other solvents provided they are compatible with the system components. If formation on a particular solvent’s compatibility is required, please get in touch.

Any solvent can be used with the Zetasizer Nano series for measurement of both particle size and zeta potential. For sizing measurements, the solvent needs to compatible with the cuvette. For example, if working with toluene, a glass or quartz cuvette is required. For zeta potential measurements in non-aqueous solvents, the dip cell accessory (ZEN1002) is required.

NTA measurements must be made in solutions. It is possible to use liquids other than water, provided the viscosity is known.
The Zetasizer can only be used for measuring the size and zeta potential of particles or molecules dispersed in a liquid.