Nanoparticles are often studied as potential drug delivery carriers, but further understanding of their behavior in the body is necessary prior to successful application. The surface chemistry of a nanoparticle is critical to its aggregation behavior. For targeted drug delivery applications, ligands on nanoparticles are often modified for specific targeting purposes with little known about how the ligands will affect the aggregation of the nanoparticles. In the case of targeted drug delivery carriers, nanoparticles that are injected into the blood stream must successfully circulate for a sufficient duration to find their intended targets. To simulate this particle circulation, researchers typically use animal models such as mice. In these experiments nanoparticles are often shunted to the liver and spleen by the immune system shortly after injection, and following this immune modulation, it is difficult to gain any meaningful knowledge. The use of animals for these experiments is costly and also raises ethical questions. As an alternative, a method of analyzing nanoparticles in biologically relevant fluids such as blood plasma has been developed using nanoparticle tracking analysis (NTA) with fluorescent filters. NTA tracks particles individually, and this particle-by-particle approach makes NTA an accurate method of determining particle concentration and size distribution, even in complex fluids such as blood plasma. Typically dynamic light scattering (DLS) is used to analyze particle size and thus aggregation behavior, but DLS cannot be used in this case because the components of blood scatter light as well. Using NTA with fluorescent filters, the aggregation behavior of the nanoparticles can be observed and quantified and specific ligand formulations can be tested for aggregation behavior in blood plasma before proceeding to in vivo studies.
In this work, NTA was used to analyze the aggregation behavior of fluorescently labeled polystyrene (PS) nanoparticles in blood plasma. The effect of surface chemistry on aggregation behavior was investigated by quantifying percentage of particles in aggregates at a given time point after adding to blood plasma. PS nanoparticles with various surface modifications were tested in blood plasma and the percentage of aggregation was quantified. The use of this characterization method will allow for better understanding of particle behavior once nanoparticles are injected into an animal, and potential problems, specifically aggregation, can be dealt with at this stage before investing heavily in in vivo studies.