A key challenge in biomedical imaging with nanocrystals is determination of sparticle size. In general conjugation will lead to an increase in the hydrodynamic size of the nanocrystal. This can be used to separate conjugated nanocrystals and to estimate the number of biomolecules bound to the nanocrystal. Sizing of biological molecules by TEM is generally fraught with difficult. Consequently, methods which directly measure the hydrodynamic size are preferable. One of the most promising methods is analytical ultracentrifugation. However, as with many other sizing techniques, the method assumes that the diffusion coefficient determined from sedimentation velocities can be converted into an effective size via Stokes Law.It also requires the exact thickness of the ligands used to colloidally stabilize the nanocrystal to be known or determined. This is an ongoing area of research [1]. In this paper we examine new data on the systematic study of CdSe, Au nanocrystals and fullerene in the presence of different ligands [2,3]. We demonstrate that AUC can resolve differences in size down to &lt; 1Å during sedimentation and can resolve different sized ligands. However we find that Stokes' law breaks down for particle sizes &lt; 4nm. The reasons for this relate to increases in the solvent viscosity near the nanocrystal surface, which lower the sedimentation rate. The result is that the predicted particle sizes are larger than experimentally found. A model to explain these deviations will be presented [4]. We find that calibration curves produced by AUC are as good as those from TEM, are statistically more precise and less prone to operator bias. The method is non-destructive and allows even minor aggregation to be detected. These calibration curves will make it possible to study quantitatively the binding of biological molecules to nanocrystals for medical imaging applications. References: 1. Jeffrey Fagan et al. "Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation", ACS Nano 7, 3373 (2014). 2. Borries Demeler et al. "Characterization of Size, Anisotropy, and Density Heterogeneity of Nanoparticles by Sedimentation Velocity", Analytical Chemistry 86, 7688 (2014). 3. Tich Lam Nguyen et al. "Size Calibration of Nanocrystals using Analytical Centrifugation", submitted to ACS Nano (2015). 4. Xinli Zhang et al. "The Hydrodynamic Drag on diffusing Nanoparticles for Size Determination", submitted (2015).

Determination of nanocrystal size by analytical ultracentrifugation: Limits of Stokes law

251st National Meeting (2016)

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