(132c) Multiwavelength Analytical Ultracentrifugation – Simultaneous Analysis of Hydrodynamic and Optical Properties

Product design is probably the most important paradigm of modern particle technology. By selectively changing the size, shape and composition of the material at hand, characteristic properties can be tailored. The analytical challenge is to fully characterize property functions in dependency of size, shape and structure of the particles at hand. Techniques that are used for the characterization of particle ensembles such as light scattering, laser diffraction, UV-VIS spectroscopy or ultrasound absorption spectroscopy require deconvolution techniques which are often limited for polydisperse samples.

However, analytical ultracentrifugation equipped with an UV-Vis multiwavelength detector (MWL-AUC) is a powerful tool for multidimensional characterization of polydisperse nanoparticles. AUC allows the simultaneous analysis of hydrodynamic and optical properties without the requirement for purification, as it combines UIV/VIS spectroscopy with fractionation in a centrifugal field [1]. It is thus possible to determine distribution of particles by measuring complete spectra from 240 to 1000 nm using a CCD spectrometer at each radial position with high temporal resolution. From the measured turbidity data, concentration profiles may be deduced by the Beer-Lambert law. Subsequently, sedimentation coefficient distributions and particle size distributions (PSD) can be calculated. Our recent improvements on the instrumentation as well as acquisition software and data evaluation allowed us to significantly enhance the possibilities of MWL-AUC as an outstanding characterization method for nanoparticulate systems [1,2]. Optical properties can be directly correlated with size information in polydisperse dispersions. Therefore extinction spectra of any species present within a mixture can be extracted or information on optical properties can be used to deconvolute PSDs comprising chemically distinct particles.

The possibility to adjust the centrifugal force over a wide range enables to characterize particles with largely varying sizes from below 1 nm up to almost 1 µm using the same instrument. This huge dynamic range is accessible via a single run using gravitational sweep experiments [3]. This procedure allows dealing with strong polydispersity, as large particles are sedimenting at low centrifugal fields while smaller particles are characterized at higher rotor speeds. Based on the size and wavelength dependency of light scattering, high dynamic range particle size evaluation is possible, while simultaneously determining optical properties of the individual fractions in a mixture.

In order to illustrate the new possibilities of nanoparticle characterization with MWL-AUC we will report on several material systems including semiconductor quantum dots, silica nanoparticles, carbon allotropes and gold nanorods. These results demonstrate that MWL-AUC is a powerful and highly accurate technique, which provides the unique capability to correlate size and optical properties in just one experiment.

References

1. J. Walter, K. Löhr, E. Karabudak, et al., Multidimensional Analysis of Nanoparticles with Highly Disperse Properties Using Multiwavelength Analytical Ultracentrifugation.
ACS Nano, 2014, 8, 8871.

2. J. Walter, P. J. Sherwood, W. Lin et al., Simultaneous Analysis of Hydrodynamic and Optical Properties Using Analytical Ultracentrifugation Equipped with Multiwavelength Detection. Analytical Chemistry 2015, 87 (6), 3396-3403.

3. J. Walter; W. Peukert, Multidimensional Analysis of Nanoparticles with Highly Disperse Properties Using Multiwavelength Analytical Ultracentrifugation.
Nanoscale 2016, 8 (14), 7484-7495.