The Applications of Metallic Nanowires for Live Cell Studies

Recently, there has been increasing research attentions focused on the development of nanomaterials for solving complicated biological problems. Nanoparticles, such as quantum dots or metallic nanoparticles, have been shown to exhibit superior performance to the conventional techniques in biosensing (Nam et al., 2003; Xiao et al., 2003) and biolabeling (Michalet et al., 2005; Medintz et al., 2005). However, the applications of nanoparticles for the studies of living cells are less explored due to the issues of biocompatibility and cytotoxicity (Derfus et al., 2004; Goodman et al., 2004; Chithrani et al., 2006). Noble metals, such as gold, have been used in the biological studies for a long time because of their stability and low toxicity. The use of metallic nanoparticles may offer several advantages in biomedical applications including simple preparation, well-defined size, various available surface modification schemes, and high sensitivity detection. Therefore, there are renewed research efforts in developing metallic nanoparticle based techniques for labeling (Katz & Willner, 2004), drug delivery (Shen et al., 2004; Salem et al., 2003; Sandhu et al., 2002) and gene regulation (Rosi et al., 2006). A common approach to use nanomaterials for biomedical applications is to chemically modify the surfaces of the nanoparticles such that the nanoparticles can recognize a specific molecule or receptor on the cell surfaces or the nanoparticles can form complexes with drugs or genetic materials to enter the cells. However, in the complicated cellular environments, it often requires individual nanoparticles to possess several functionalities to achieve multiple tasks. The surfaces of nanoparticles may have to be decorated with biomolecules to recognize specific cells or to enhance the uptake efficiency. When the nanoparticles are inside the cells, additional molecules may be needed to help the nanoparticles to escape the endosomes or to reach specific organelles. In addition, the optical properties of nanoparticles may allow monitoring the cellular uptake process and their spatial distribution by optical microscope whereas the magnetic nanoparticles may be used for separation or contrast agent. To engineer the nanoparticles with multiple functionalities, two peptides have been attached to the same gold nanoparticles allowing traversing cell membrane by receptor-mediated endocytosis pathway and endosomal escape (Tkachenko et al., 2003). However, these two peptides were randomly distributed on the surfaces of nanoparticles. It is very difficult to control the spatial distribution of molecules or functionalities on the spherical nanoparticles. This problem can be solved by using non-