ABSTRACT
Recent achievements in synthesis and investigation methods allowed preparation of monodisperse nanostructures on under-100 nm scale with precisely controlled shape and size. Nanoparticlebased medicine, diagnostics, and drug delivery opened enormous prospects in different areas where common therapy methods appeared to be lacking mainly due to inefϐicient targeting and poor control over proceeding therapeutic process. Nanoparticles have signiϐicant advantages over both micron-sized particles, molecular drugs. Nanoparticles are much smaller than human cells that are about 10-20 μm. However, nanoparticles have sizes similar to that of the biomolecules — proteins, DNA, etc. — present in biological objects. Particles as drug carriers sized within
10-100 nm can systematically circulate through human body and freely penetrate places where larger particles cannot reach. Control over surface functionalization deϐines stability, solubility, and the physicochemical properties deϐines the safety, dispersion in given media, and physiological activity of nanoparticles toward given cell types. The properties and chemistry of nanoparticle surface can be modiϐied to meet the requirements of a certain application. While usual monoclonal antibody-based drugs injected intravenously are inefϐicient because of inadequate selectivity or low solubility, nanoparticles can be easily functionalized with special ligands for water solubility or selective targeting of certain types of cells via nanoparticle-cell interaction [1]. Another remarkable feature of nanomaterials is the ability to employ multiple agents for therapy, selective targeting, and monitoring, which is hardly achievable for molecular or protein-based drugs. Remarkably, many properties on nanometer scale are unique and unachievable by neither their bulk counterparts nor separate atoms or molecules, owing to greater role of quantum effects and increased surface to volume ratio. For instance, in metal nanoparticles when ratio of surface to volume atoms increases and size of particle becomes comparable to average free path of the electron, surface effects become more apparent or even dominate volumetric ones. Moreover, higher amount of surface atoms in nanoparticles usually results in greatly increased surface energy, reactivity, and catalytic activity of material. In general, this leads to quick aggregation of nanoparticles in solution to minimize the surface energy or chemical degradation. Hence, for biomedical application, nanoparticles are selected with consideration of degradation rate, biocompatibility, nontoxicity, and ability to functionalization. Chemical stability of gold nanoparticles (Au NPs) toward medium found in biological object has made them popular inorganic nanocrystals in the nanomedicine. The various bioapplications of these nanoparticles follow from their unique structural properties at nanometer dimensions. Though bulk gold is chemically inert metal, at the nanoscale, chemical activity of gold is increased. It easily forms stable at the room temperature bonds with thiol and thiolate groups, owing to high afϐinity between gold and sulfur, which is widely used for surface modiϐication of gold nanomaterials. Such modiϐication is necessary to prevent aggregation of nanoparticles and achieve stable dispersion of Au NPs in aqueous medium.
