Nanoparticles frequently agglomerate when dispersed into relevant biological and environmental media, with the resulting change to the effective size distribution dramatically affecting the potential nanotoxicity and the absorbance for biosensor measurements. Herein we demonstrate analytical ultracentrifugation (AUC) as a powerful method for measuring two critical characteristics of nanoparticle (NP) agglomerates in situ in biological media: the NP agglomerate size distribution, and the localized surface plasmon resonance (LSPR) absorbance spectrum of monodisperse gold NP agglomerates. To characterize the size distribution we present a theoretical framework for calculating the hydrodynamic diameter distribution of NP agglomerates from their sedimentation coefficient distribution. We measure sedimentation rates for monomers, dimers, and trimers, as well as for larger agglomerates with up to 600 NPs. The AUC size distributions were found generally to be broader than the size distributions estimated from dynamic light scattering and diffusion-limited colloidal aggregation theory, an alternative bulk measurement method that relies on several assumptions. In addition, the measured sedimentation coefficients can be used in nanotoxicity studies to predict how quickly the agglomerates sediment out of solution under normal gravitational forces, such as in the environment. We also calculated the absorbance spectra for monomers, dimers, trimers, and larger agglomerates up to 600 NPs, which enable a better understanding of LSPR biosensors. Finally, we validated a new method that uses these spectra to deconvolve the net absorbance spectrum of an unknown bulk sample and approximate the proportions of monomers, dimers, and trimers in a polydisperse sample of small agglomerates, so that every sample does not need to be measured by AUC.
Citation: ACS Nano
Pub Type: Journals
gold colloid, aggregate, nanotoxicity, nanoparticle toxicity, biosensors