(122f) Analytical Ultracentrifugation As a Tool for Multidimensional Characterization of Nanoparticles

The analysis of size, shape and density is essential for nanoparticles (NP), especially as they exhibit a large variety of properties. A combined approach to assess and correlate these parameters is challenging. Conventional particle measurement techniques like scattering, field flow fractionation or microscopy techniques are often limited by statistics, resolution and applicability to broad particle size distributions (PSD).

Analytical Ultracentrifugation equipped with an UV/VIS multiwavelength spectrometer (MWL-AUC) can serve as a powerful tool for the multidimensional characterization of nanoparticles. It combines the detection of optical spectra in the range from 240 to 1000 nm with fractionation by a centrifugal field. Our recent developments regarding instrumentation, data acquisition and evaluation allow us to largely extent the possibilities of MWL-AUC in the context of nanoparticle characterization.

Graphene oxide (GO) is a representative for the emerging class of 2D materials and originates from the exfoliation of chemically modified graphite as a first step in the top-down approach to produce graphene. However, statistically relevant characterization techniques of shape and size distributions with reasonable temporal time effort were not available for these nanoplatelets. Therefore, a new methodology was developed to determine the lateral size distribution directly in solution within a few hours [1]. This unique approach allows to investigate processes like ultrasonication or ball milling of GO and determine the size evolution of nanoplatelets in detail [2]. The thereby determined size reduction rate could be correlated with material properties originating from raman-, solid-state nuclear magnetic resonance and thermal gravimetric analysis measurements, enabling us to identify relevant process parameters for processing of 2D materials. By changing type and amount of functionalization, controlled and scalable production of nanoplatelets becomes possible.

MWL-AUC can be combined with other techniques like the scanning mobility particle sizer (SMPS), where particles are classified according to their mobility diameter in the gas phase [3]. This is especially interesting as the two methods give information on the same particles in two distinct environments allowing for quantitative shape analysis and for validating colloidal stability of e.g. ionomer particles in solution.

Finally, core shell NPs (e.g. ZnO, CuInS₂) were addressed as these particle systems cause special challenges to conventional measurement techniques. This is mainly due to the pronounced polydispersity of many nanoparticle systems. Therefore, we developed a new methodology for the simultaneous determination of size and density for polydisperse PSDs based on the parametrically constrained spectrum analysis implemented in the free software tool UltraScan3 [4]. The high resolution both in density and size allows characterizing the core-shell properties of NPs within one single experiment.

MWL-AUC has proven to be a powerful and indispensable method for the multidimensional characterization of nanoparticles directly in solution. The direct correlation of size, shape and optical properties of NPs is possible in a range from 1 nm to 1 µm making it highly relevant for numerous applications in industry and science.

References

1. J. Walter, T. J. Nacken, C. Damm, et al., Determination of the Lateral Dimension of Graphene Oxide Nanosheets Using Analytical Ultracentrifugation. Small, 2015, 11, 814.

2. C. E. Halbig, T. J. Nacken, J. Walter, et al., Quantitative investigation of the fragmentation process and defect density evolution of oxo-functionalized graphene due to ultrasonication and milling. Carbon, 2016, 96, 897.

3. T. Thajudeen, J. Walter, R. Srikantharajah et. al., Determination of the length and diameter
of nanorods by a combination of analytical ultracentrifugation and scanning mobility

particle sizer. Nanoscale Horiz., 2017.

4. J. Walter, G. Gorbet, T. Akdas, et al., 2D analysis of polydisperse core–shell nanoparticles using analytical ultracentrifugation. Analyst, 2017, 142.