Comprehensive Nanoparticle Size Analysis for Polydisperse Samples

The size of nanoparticles greatly affects their properties and functionality. Accurate size measurements are, therefore, crucial in many different areas, such as research, development and manufacturing.

Furthermore, it is important for many applications to measure the concentration and particle size distribution of nanoparticles. Yet, with current characterisation methods, such as conventional nanoparticle tracking analysis, dynamic light  scattering and electron microscopy, it is difficult to determine accurate particle sizes in polydisperse samples.

ViewSizer™ 3000 from MANTA Instruments

This gap is now being filled by the ViewSizer™ 3000 from MANTA Instruments, which enables users to measure precise particle size distributions in liquid samples containing nanoparticles of varying sizes.  

Scientists and engineers who work on any of the wide-ranging fields of applications of nanoparticles, such as catalysis, drug delivery, biomedical imaging or solar cells, depend on correct size and concentration measurements of the nanoparticles in use.

This is because  varying the dimensions of nanoparticles can significantly alter their specific properties, for example their chemical, optical, electronic, magnetic or catalytic characteristics or their physiological effects in humans [1].

For instance, a biomedical imaging agent based on nanoparticles is only functional if its size-dependent absorption properties match the  wavelengths of the probe and detection system in place [2].

When synthesised, it is often desired that the batches contain nanoparticles of the same size. In other words, the samples should be monodisperse. However, in most cases, monodisperse nanoparticle synthesis is not possible.

When real-world samples are investigated, from e.g. biologic, aquatic or industrial applications, many different particle sizes are typically present. These samples are called polydisperse. Hence, in that case not only one particle size but a particle size distribution (PSD) has to be determined.

Conventional characterisation methods fail for real-world samples

There exist two different kinds of particle size characterisation measurements: ensemble methods and individual-particle methods.

In ensemble methods, a large number of particles are measured simultaneously. Prominent examples are light scattering techniques, such as static light scattering (SLS) and dynamic light scattering (DLS), as well as fractionation methods, such as field-flow fractionation (FFF), which is often combined with SLS.

In individual-particle methods, particles are measured one by one. The most common methods in this category are electron microscopy techniques, such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM), as well as nanoparticle tracking analysis (NTA).

All traditionally applied techniques, however, exhibit significant drawbacks, which can result in imprecise determinations of the particle size and other sought-after parameters. In the following paragraphs, the specifics of some of the most popular characterisation techniques — DLS, TEM and NTA — are described, highlighting their constraints for polydisperse samples:

  • With DLS, the particle size can be derived from the intensities of light scattered from the particles, which fluctuates owing to Brownian motion of the particles [3]. A major shortcoming of DLS is that the scattered light intensity of larger particles results in a disproportionately high scattering intensity, which can obscure the existence of other, smaller particles in the sample. Therefore, DLS is not well suited for polydisperse samples.

  • TEM is a frequently used method, with which individual nanoparticles can be visualised by capturing the interaction between the particles and the electron beams that are transmitted through the sample [4]. The PSD can be constructed on the basis of high-resolution TEM images. Although useful for many problems, this method entails expensive and bulky equipment as well as the risk of damaging the sample with the required  ultra-high vacuum and electron beams. Moreover, owing to the restricted field of view in TEM, the obtained image might be unrepresentative of the whole sample.

  • A decade ago, the conventional NTA technology laid the groundwork for a new field of individual-particle visualisation and analysis [5]. Brownian motion is utilised to examine the particle sizes, but in contrast to DLS, the scattered light is captured by a CCD camera that visualises and tracks individual particles over time. The particle size can be derived from the rate of particle movement. The conventional approach, however is, inadequate for measuring co-existing nanoparticle sizes that produce very different scattered light intensities (related to the issues of DLS). Also, the conventional NTA cannot be used for measuring kinetic processes that involve a large change of particle size over time.

Filling the gap: The ViewSizer™ 3000 from MANTA Instruments

In order to overcome the limitations of other techniques, it is common practise to use several methods for each sample. This can be time- and resource-consuming, but leads to a more precise determination of the particle sizes. Still, for real-world samples, this is often not enough.

Motivated by the lack of an instrument that can characterise the size distribution of nanoparticles in polydisperse samples, scientists from the Ocean Optics Research Lab at the University of California San Diego (UCSD) have invented the so far most advanced nanoparticle tracking analysis (MANTA) technology. With the ViewSizer 3000, individual-particle analysis can be performed simultaneously on nanoparticles of widely varying sizes co-existing in a liquid sample [6].

This is realised using an NTA approach with the addition of:

  • An innovative illumination by a plurality of lasers each with different wavelengths
  • A detection system that simultaneously detects the scattered light in each of the illumination wavelengths from individual particles
  • Software that enables most accurate particle-by-particle tracking analysis

With the novel MANTA system, the customer can measure accurate and reproducible particle number concentrations and size distributions even in highly complex polydisperse samples. In addition, the nanoparticles can be visualised over time in form of a video. Owing to the benefits of the MANTA system, it is also possible to measure kinetic processes in real-time. The measurements can be routinely performed in a bench-top instrument, which is time- and cost-effective and easy to run.

Request more information about the ViewSizer™ 3000 from MANTA Instruments

Conclusion

The MANTA ViewSizer™ 3000 makes it easy to perform routine particle size and concentration measurements on polydisperse samples with accurate and reproducible results. Furthermore, kinetic processes can be followed visually, even if particle sizes vary and change over time. With these advantages, the ViewSizer™ 3000 from MANTA Instruments can overcome many constraints of the most common  characterisation techniques, such as DLS, TEM and conventional NTA.

References

  1. Nanoparticles, G. Schmid (Ed.), 2010, Wiley-VCH, 2nd Edition
  2. Gold nanoparticles: interesting optical properties and recent applications in cancer diagnostics and therapy, Huang, X. et al, Nanomedicine, 2007, DOI:10.1016/j.jare.2010.02.002
  3. Dynamic Light Scattering, Berne, B. J. and Pecora, R. (Ed.), 2000Dover Publications
  4. Transmission Electron Microscopy,  Williams, D. B. and Carter, C. B. (Ed.), 1996, Springer Science + Business Media
  5. Nanoparticle Tracking Analysis – The Halo™ System, Particle, 2006, DOI:10.1002/ppsc.200601031
  6. "There must be a better way… to characterize nanoparticles" - Manta Instruments, 2016. [PDF]

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit