With the rapid advancement in nanotechnology, various techniques have been developed to synthesize nanoparticles with novel optical, magnetic, electrical, mechanical and catalytic properties [1]. By controlling the size, shape and composition of the nanoparticles, their properties can be tailored to fit a specific requirement, which allows great flexibility in designing new experiments or applications. Recently, there has been increasing research attentions focused on the development of nanomaterials for solving complicated biological problems. For example, nanoparticles, such as quantum dots or metallic nanoparticles, have been shown to exhibit superior performance to the conventional techniques in biosensing [2-5]
and biolabeling [6-9]. However, the applications of nanoparticles in the study of living cells are less explored due to the issues of biocompatibility and cytotoxicity [10-17].
Noble metals, such as gold, have been used in the biological studies for a long time because of their stability and low toxicity. Lately, there are renewed research efforts in developing metallic nanoparticle-based techniques for labeling [18-20], drug delivery [21-24] and gene regulation [25]. A common approach used in these applications is to chemically modify the surface 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 a more complicated experiment, it may require the nanoparticles to possess several functionalities so that several tasks can be performed by a
single particle. For example, two peptides have been attached to the same gold nanoparticles to allow both receptor-mediated endocytosis and endosomal escape [26]. Multi-segment nanowires or nanorods are particularly useful for such type of applications due to their symmetry. In a recent study, two types of molecules have been incorporated onto the gold-nickel nanowires where the gold end was used to bind to a plasmid DNA through electrostatic interaction while the nickel surface was engineered to carry a specific polypeptide for site recognition [22]. However, most of the studies on nanorods or nanowires have been limited to the nanometer length. The micrometer long nanowires were less explored. Micrometer long multi-segment nanowires have been used as barcodes for biological multiplexing, which could be easily visualized by an optical microscope [27].
Recently, it has been shown that the micrometer long nickel nanowires could be internalized by cells allowing the manipulation of living cells through magnetic field [28, 29]. However, it is not known whether the micrometer long nanowires can be internalized by the cells without damaging the cells, which is an important issue for the development of nanowire-based living cell probing system. If the nanowires can be internalized by the cells, it would allow us to observe directly the intracellular microenvironment around the individual nanowires through an optical microscope. We are interested in developing nanowires based living cell probing system. Our idea is to modify the surface of nanowires to carry the plasmid DNA or probe molecules into cells where the nanowires and the probe molecules can be monitored by a
microscope. Since molecules could be attached to the surface of the nanowires through electrostatic interaction, it is necessary to control the surface charge density, which can be achieved by various surface modification schemes. To utilize surface modified nanowires for the living cell study, it is very important to first investigate the cytotoxicity of these nanomaterials. As long as the surface modified nanowires could be internalized by the cells without damaging the cells, these nanowires could be used to explore the local environment inside the living cells. However, in some occasion the surface modified nanowires were found to settle on the bottom of the cell culture dishes. Therefore, it is essential to compare the internalization process of nanowires for the adherent cell and the non-adherent cells. In this research, we have studied the cytotoxicity and the internalization process of the functionalized gold with different surface charges for two different cell lines, NIH 3T3 fibroblast cells from normal tissue, which is an adherent cell line and HeLa S3 cells from neoplastic tissue, which can grow in suspension.
Learning that the micrometer long nickel nanowires could be internalized by cells, it is interesting to investigate whether the micrometer long nanowires are capable of delivering DNA molecules or drugs into the cells without damaging the cells, which is an important issue for the development of nanowire based live cell probing system. To investigate the possibility of using nanowires for DNA delivery, it is necessary to engineer the surface of the nanowires in such way that the DNA molecules can bind to the nanowires in a reversible
manner, which can protect the DNA molecules from the digestion of enzymes in the extracellular environment while the DNA molecules can be released inside the cells for effective transfection. Since the DNA molecules are negatively charged, a common practice is to engineer the surface of the nanowires with positive charges. Therefore, the DNA molecules can bind to the surface of the nanowires through the electrostatic interaction. Because of the low cytotoxicity and easy surface modification process of gold nanowires, we have investigated the interactions between the nanowires and the cells through the studies of the DNA transfection efficiency using single segment surface modified gold nanowires.
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