Energy Transfer, Nanometer Crystals and Optical Namo-probes
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Energy Transfer, Nanometer Crystals and Opfical Nano-probes* Weihong Tan and Iaoug Kopelnan Department of Chemistry, The University of Michigan Ann Arbor, Michigan 48109
Abstract Nanometer light and exciton sources and probes have been prepared by adding various inorganic and organic crystals and molecularly doped polymers to micropipettes and nanofabricated optical fiber tips. Specifically, a new nanotechnology, near-field photonanofabrication, has been developed, leading to a thousandfold miniaturization of immobilized Fiber Optical Chemical Sensors and to a billionfold decrease in necessary sample volume. The response time has also been shortened by a factor of at least 100. Applications of these subwavelength probes include biological single cell analysis, supertip development, F3rster-energy transfer and Kasha quenching phenomena at the interface between the positionally controlled nanocrystal tip and its photoactive environment. Practically, this leads to enhanced sensitivity of optical probes, nano-optical chemical sensors and near-field exciton light sources.
UIntroduction Near-field Scanning Optical Microscopy (NSOM) has enabled researchers to optically examine a variety of specimens without being limited in resolution to one half the wavelength of light. The single most important task is the development of specialized subwavelength optical probes for NSOM and other scanning microscopy. Different types of subwavelength optical probes, both passive and active, with or without specific chemical sensitivity, for NSOM, Molecular Exciton Microscopy (MEM), Near-field Scanning Spectroscopy (NSS) and Fiber Optical Chemical Sensors (FOCS) have been fabricated by using micropipettes and nanofabricated optical fiber tips [1-3]. One of the most successful applications of those probes is the development of submicrometer FOCS. FOCS has played an increasingly important role in chemical and medical analysis [4]. They have several advantages compared to electro-chemical sensors. However, so far their physical size has been limited by that of commercial optical fibers. Conceptually it has often been assumed that the wavelength of light is the lowest limit for their ultimate size. Theoretically, sensors based on only one single molecule should be possible. The smaller the sensor, the better the detection limit, the sensitivity and the response time. Also, smaller sizes are preferable for nanomaterial and intracellular research-they enable non-invasive and spatially-resolved measurements. Furthermore, smaller sizes make FOCS and other light or exciton probes capable of fast (millisecond) monitoring of chemical and biological reactions, e.g. chemical kinetics inside restricted small dynamic domains, where non-classical rate laws apply [5). In this paper we first describe a new and controllable nanotechnology, photonanofabrication, based on nanofabricated optical fiber tips and near-field photopolymerization, to make nanometer optical fiber sensors with extremely short response times. The same technique can be used to prepare
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