Nanofabrication and applications of subwavelength optical probes: Chemical and biological sensors, light sources and exciton probes

The rapidly developing fiberoptic sensors and scanning probe microscopies call for more sophisticated probe fabrication. We have developed a new and controllable nanofabrication technique, photo-nanofabrication, based on near-field photo-chemical synthesis and nanometer optical sources. Photo-nanofabrication can produce subwavelength light and exciton probes with or without specific chemical or biological sensitivity. By applying near-field optics, we have successfully demonstrated a new concept of near-field photochemical synthesis, in which the dimension of a product is solely determined by the size of the light source. The most successful application to date is the development of the smallest fiberoptic chemical sensors. Specifically, a thousandfold miniaturization of an immobilized fiberoptic pH sensor has been achieved, leading to at least a millionfold decrease in necessary sample volume and to at least a hundredfold shorter response time. The sensors have high fluorescence intensity and excellent detection limit. New internal calibration methods have also been developed for accurate pH quantification. The newly developed optical sensors have been used in real time measurements of pH on individual, viable, intact rat conceptuses during the period of organogenesis. The sensors can discriminate pH changes of less than 0.1 pH unit in the physiologic pH range. In particular, static determinations of pH in rat conceptuses of varying gestational ages show decreasing pH with conceptal age. In these measurements only one single embryo is needed. In addition, chemical dynamic alterations in pH of intact rat conceptuses, in response to several variations in their environmental conditions, have been measured. This is the first time that such an experiment has been carried out on a single and live embryo. The ability of the sensors to measure pH changes, in real time, in the intact rat conceptus, demonstrates their potential application for dynamic analysis in small multicellular organisms and single cells. Furthermore, passive and active subwavelength light sources have been constructed with both micropipettes and fiberoptic tips. They have been used as exciton and light sources and in preliminary probe-to-sample distance regulated, Forster energy transfer studies as well as in studies of the probe-to-sample interfacial Kasha effect (external heavy atom effect). They were also used in supertip development for near-field scanning optical microscopy and for molecular exciton microscopy. A better understanding of active light sources has been achieved, and new preparation techniques have been developed.