Surface Characteristics

Surface characteristics contribute to the nanoparticle's solubility, aggregation tendency, ability to traverse biological barriers (such as a cell wall), biocompatibility, and targeting ability. The nanoparticle surface is also responsible for interaction and binding with plasma proteins in vivo, which in turn may alter the nanoparticle's distribution and pharmacokinetics. For multifunctional nanoparticles, modifying agents are often attached to the surface to bind to receptors in target tissues and organs. The presence of charged functionalities on the nanoparticle surface may increase nonspecific uptake, making the preparation less effective in targeting. It has been shown that dendrimer nanoparticles displaying positively charged amine groups on their surface can be significantly more hemolytic and cytotoxic than nanoparticles displaying negatively charged carboxylates.20 The negatively charged nanoparticles were also cleared more slowly from the blood compared to positively charged species, following intravenous administration to rats.20 Another potential effect of surface charge is to alter a nanoparticle's ability to penetrate the blood-brain barrier. Studies have shown that for emulsifying wax nanoparticles, anionic surfaces were superior to neutral or cationic surfaces for penetration of the blood-brain barrier.33

Surface characteristics can be tuned to improve receptor binding, reduce toxicity, or alter biodistribution. For example, when the above-mentioned dendrimers were acetylated to neutralize exposed surface charges, the toxic effects of the nanoparticles were also neutralized.20,34 Surface properties can also lead to toxicity through interaction with molecular oxygen, leading to oxidative stress and inflammation. Electron capture at the surface of the nanoparticle results in the formation of the superoxide radical, which can set off a cascade of reactions (e.g., through Fenton reaction or disumation) to generate reactive oxygen species (ROS). ROS generation has been studied extensively for inhaled nanoparticles,11,35 and has been observed in engineered nanoparticles such as fullerenes, single walled nanotubes (SWNTs), and quantum dots.6,36-44 Studies have shown a direct correlation between nanoparticle surface area and ROS-generating capacity and inflammatory effects.11

The nature and integrity of nanomaterial surfaces must be established through analytical measurements to ensure product quality and account for surface-dependent effects on biodistribution and toxicity. Potentiometric titrations provide crucial information on the net charge of a nanoparticle, and include zeta potential analysis, which provides information on the net charge and distribution under physiological conditions. Polyacrylamide gel electrophoresis (PAGE) analysis of dendrimers and other nanopolymers yields information on the molecular weight and the polydispersity of nanoparticles (such as trailing generations in dendrimer populations) based on their migration through the gel under an electric field. PAGE is also a powerful tool in the qualitative analysis of bioconjugates of nanomaterials with DNA, oligonucleotides, antibodies, and other ligands. Further analysis of the surface charge distribution and polydispersity of nanomaterials can be conducted using CE. MS is also effective in ascertaining the number and distribution of charges, especially for smaller and purer nanoparticles with known molecular weight.

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