Nanoelectrodes, of which critical dimension governing their electrochemical/electrical characteristics are in submicron-scale, have attracted great interests since they can provide unique and important applications which are hardly realizable by using macroscopic electrodes. Particularly, they are highly useful in biological and medical applications since their dimension is comparable or even smaller than that of biological materials such as cells, which renders excellent spatial precision and biological compatibility to nanoelectrode-based cell stimulation or analyses.
The bottleneck lying in the researches using nanoelectrodes is the fabrication of high-quality nanoelec-trodes. Au nanowires, grown by vapor transport method, have defect-free single-crystalline surface without metallic catalysts or organic ligands and their width and length are easily controlled, which are excellently competent as high-quality nanoelectrodes with reproducible electrode characteristics. In this study, the fabri-cation method and various biological applications of single-crystalline Au nanowire electrode are reported.
The first biological application of Au nanowire electrodes is electrical recording of biological signal generated in a living organism. In this case, it is inevitable to insert or contact an electrode to biological ele-ments (cells, tissues), so that the dimension and mechanical and chemical properties of the electrodes critical-ly affect the physiological long-term stabilities. Especially, dimension of an electrode is highly important since it is directly related to the spatial resolution of the recording. Extremely thin, mechanically flexible and strong, chemically inert and biocompatible Au nanowire electrodes allow excellent recording of biological signals. To verity this, Au nanowire electrodes were implanted in brain tissue of a living mouse and recorded electrical signals with high sensitivity and spatial resolution. It is expected that Au nanowire electrodes can monitor the signal in muscle tissues as well as neural ones with excellent quality and consequently play an important role in understanding of biological phenomena and further developing the therapeutic technologies.
The second biological application of Au nanowire electrodes is electrochemical delivery of exogenous materials into a living biological element. Au nanoinjector was developed by attaching exogenous materials to the surface of a Au nanowire electrode, inserting the electrode into a specific site of a biological element and then electrochemically releasing the exogenous materials quantitatively. Au nanoinjector is an excellent device since it enables precise delivery at subcellular level, maximizing the efficacy of delivered materials. In addition, it is desirable to load various materials to the Au nanoinjector via facile surface modification and to deliver the materials in fast and timed manner by using electrochemical trigger. In order to demonstrate the superior competence of a Au nanowire as a nanoinjector, gene delivery by a Au nanowire electrode was per-formed into a nucleus of a living cell with high spatial precision and subsequent gene expression was success-fully observed. Nanoinjectors based on Au nanowires are expected to delivery diverse useful materials (pro-teins, drugs, etc) as well as gene into various cells or tissues in an electrochemical manner which guarantees fast and timed delivery. This would lead the investigations of biological responses to the delivered materials and the development of new drugs.
The third biological application of Au nanowire electrodes is electrically stimulating the biological components and electrochemically monitoring the subsequent responses. Externally applied electrical stimulation can control the biological phenomena which could be the onset of the development of electrical therapies. It is necessary to observe the effects of electrical stimulation at cellular level in order to understand the mechanism of electrical therapy and consequently maximize the therapeutic efficacy. In this sense, a research platform was designed for electrical cell stimulation and monitoring of the resultant cellular responses with high spatial resolution using a set of Au nanowire electrodes. To examine this system, a neuro-secretory cell was electrically stimulated by a stimulating Au nanowire electrode and the consequent exocytosis was electrochemically monitored by a recording Au nanowire electrode. A Au nanowire electrode allowed the precise observation of exocytosis which occurs at highly localized site of cell surface, accurately revealing the effects of a certain parameter of electrical stimulation on exocytosis. It is expected that this research platform would enable accurate understanding of physiological responses in electrical therapies and therefore bring out important information for maximizing therapeutic efficacy and minimizing side effects.