During the production of particles of a known size, the most important factor is the monodispersity of the samples in terms of both shape and size. This factor usually mandates a high-resolution technique such as Transmission Electron Microscopy (TEM) to analyze the sample properties.
For instance, Particle Works, a brand of Blacktrace Holdings Ltd, synthesizes gold nanoparticles which should be highly monodisperse and have high batch-to-batch consistency, with CV values as low as 2.5% for the batch consistency and 5% for the size distribution. At present, Particle Works uses TEM as the principal technique for the characterization of its samples for QC. TEM, however, is a time-intensive and high-cost technique requiring a skilled user to carry out the analysis. Hence. Particle Works was keen to investigate whether it would be possible to use the Zetasizer Ultra to minimize the amount of TEM analysis that is needed.
The Zetasizer Ultra comes with various features that help to reduce the time needed for the measurements to be performed and offers a vast amount of details on sample properties. These include Multi-Angle Dynamic Light Scattering (MADLS), Adaptive Correlation, and Depolarized Dynamic Light Scattering (DDLS).
This article describes the measurement of samples of Particle Works gold nanoparticles using both Multi-Angle Dynamic Light Scattering (MADLS) and TEM with the Zetasizer Ultra to determine whether the quality of the samples was sufficiently high to be sold.
The tested samples were gold nanoparticle samples produced by Particle Works at the time of the early development of the Ultraspherical Gold Nanoparticle product range. Samples with target sizes of 10, 15, 20, and 50 nm placed in DTS0012 plastic cuvettes were measured on the Zetasizer Ultra using MADLS. The samples were also measured with the help of TEM, where at least 100 particles were measured for each sample. Table 1 summarizes the details of the samples measured, including their target size.
Table 1. Sample details including target size
||Target Size (nm)
The Peak by Intensity values for all the samples measured using the Zetasizer are higher compared to those measured by TEM as a result of the effects of ionic surfactants, stabilizing ligands, and hydration layers, which increase the hydrodynamic size of the dispersed gold. The hydrodynamic diameter of the gold is measured by the Zetasizer, while only the core particle diameter is measured by TEM; hence, this is the basic reason for the discrepancy.
When the first sample, Au-2-016-7, was measured using DLS, the size populations were found to be considerably larger than the target size of 10 nm, to the extent that the 10 nm peak does not feature in the distribution illustrated in Figure 1. Hence, this sample has failed to fulfill the specifications. It can be also seen from the TEM image that there are a number of large particles apart from the expected 10 nm particles. It is possible that these have been masked in the MADLS measurement due to the scattering of the larger particles.
Figure 1. Intensity size distribution (left) as measured by MADLS and TEM (right) of Au-2-016-7
When the second sample, Au-2-017-6, was measured using MADLS, it was observed that this sample is on the specification as a narrow distribution is measured (see Figure 2). However, there is a discrepancy between the TEM measurements, since while the majority of the populations fall within the expected range, a tail of particles occurs below the target size, indicating that it does not fulfill specifications. This could not be detected by the MADLS measurements since the scattering of the sample is considerably more skewed toward the larger sizes, masking that of the tail of smaller particles. This suggests that although the MADLS measurements are best suited to detecting particles that are larger compared to the majority of the sample, their ability to identify a sample containing particles that fall under the specification is limited.
Figure 2. Intensity size distribution as measured by MADLS (left), TEM image (center) and the TEM size distribution (right) of Au-2-017-6
As illustrated in Figure 3, for the third sample, the TEM image exhibits a uniform size distribution that falls within specifications for the particles themselves. Yet, there are rings around a majority of the particles, and it is obscure whether these are a true reflection of the particle size when dispersed or whether this is due to the drying process. When the sample is measured using MADLS, it can be observed that the distribution is narrow and within specifications.
In case the rings exist around the particles upon being dispersed, the DLS distribution would be considerably wider and higher in size. Hence, it is possible that at the time of the drying process, some of the surfactant in the dispersion has dried around the particles, resulting in these visible rings. If these rings are identified to be not representative of the sample, it indicates that this sample can be sold.
Figure 3. Intensity size distribution as measured by MADLS (left), TEM image (center) and the TEM size distribution (right) of Au-2-037-4
As illustrated in Figure 4, the fourth sample, Au-2-041-3, was measured with MADLS. Although the distribution was largely uniform, there was an unanticipated small peak below 10 nm.
Figure 4. Intensity size distribution as measured by MADLS of Au-2-041-3
Such an unexpected peak can be useful in exploiting the DDLS capabilities on the Zetasizer Ultra. Removal of the vertically polarized scattered light, leaving behind only the horizontally polarized scattered light enables determining whether this unexpected peak is the result of translational diffusion of particles and hence another population that exists in the sample. Rather, this peak could be the result of the rotational diffusion of a non-spherical particle; if that is the case, then the <10 nm peak would be expected to increase in relative intensity compared to the larger size peak when horizontal polarization is used.
Measurements with both polarizations (a typical backscatter measurement), horizontally polarized light and vertically polarized light, were performed as illustrated in Figure 5. It can be observed that while the vertically polarized backscatter measurement is analogous to the typical measurement, the relative intensity of the horizontally polarized measurement is considerably higher for the smaller size peak than the larger size peak. This suggests that the smaller size peak is not due to the translational diffusion of particle, rather from the rotational diffusion; hence, some of the particles are not spherical.
Figure 5. Intensity size distributions of Au-2-041-3 in backscatter using all polarizations (blue), vertical polarization (green) and horizontal polarization (red)
As illustrated in Figure 6, the sample was also measured using TEM, thus confirming the conclusion from using MADLS. This demonstrates that a number of particles are faceted and not spherical and some are even rod-like. It is highly probable that these rod-like particles cause the <10 nm peak observed while using DLS.
Figure 6. TEM image of Au-2-041-3
As depicted in Figure 7, the results of the measurement of the final sample using MADLS demonstrate that it is a sample with monomodal, narrow distribution. This indicates that this sample fulfills the specification, which is confirmed by the TEM measurements, also suggesting that the sample has a uniform size. This data confirms that this sample is a high-quality sample suitable to be sold to customers.
Figure 7. Intensity size distribution as measured by MADLS (left), TEM image (center) and the TEM size distribution (right) of Au-2-016-13
It has been demonstrated that the Zetasizer Ultra is a useful tool for QC for both manufacturing and R&D. Its ease of use and speed enable multiple samples to be rapidly measured to verify whether they are within specifications. In this article, it has been shown that while using the Zetasizer, only fewer samples have to be measured using TEM since many off-spec samples have been identified with the help of the Zetasizer Ultra alone.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
For more information on this source, please visit Micromeritics Instrument Corporation.