Quantitative analysis of nanoparticle interactions and their distributions
Analytical Chemistry
Final Report Abstract
The main objective of the research project was to develop a new methodology to determine size– and shape–dependent distributions of interaction constants, stoichiometry and cooperativity of the reversible aggregation process of nanoparticles (NPs). From isotropic and monodisperse NPs, to eventually anisotropic and polydisperse ones, the methodology would take advantage of combined sedimentation and diffusion coefficient distributions from Analytical Ultracentrifugation (AUC). For anisotropic NPs, face–dependent interactions were to be investigated, as well as interactions between dissimilar NPs. Early stages of the process development showed good results for spherical polystyrene (PS) nanoparticles. Titration of the PS NPs into a NaCl solution destabilized the system and induced aggregation of the NPs. This process could be followed by both Dynamic Light Scattering (DLS) and AUC. Fitting the data points with an adaption of the Hill adsorption model for NP–NP interactions, it was possible to acquire information on the process itself, the dissociation constant (KD), the Hill coefficient (cooperativity factor, identified as n), the shape factor and the maximum number of particles bound to one monomer. The values were then verified by running a complementary Isothermal Titration Calorimetry (ITC) analysis. Since different adaptions of fitting models were considered to best describe the aggregation process, to confirm the goodness of one or more models PS NPs were then destabilized using a different mechanism, employing a dextran solution. Moving on from the PS NPs, other systems investigated were spherical carboxyl silica particles (destabilized with NaCl and dextran), gold spherical particles coated with cetyltrimethylammonium bromide (AuNS@CTAB) and, to account for the shape factor in the NPs aggregation process, gold nanorods coated with CTAB (AuNR@CTAB). Gold nanoparticles were initially destabilized with trisodium citrate (Nacit). The results from the investigation on the PS NPs with AUC were then used to resolve a 2D evaluation, acquiring information on the entirety of the species and their population density in the sample, and, consequently, on the hydrodynamic radius distribution (rH). In all the cases, the data could at least be fitted by one of the adapted models, and the dissociations constants of interacting particles systems could be quantitatively determined. In this regard, this work presents a first step into the investigation of particle–particle interactions parameters, and, in principle, the developed fitting equations can be applied to other reversible systems. However, quite a few limitations or issues were found, both on the systems themselves and the methods chosen for analysis.
