Topics in Fluorescence Spectroscopy Advanced Concepts in Fluorescenc
Topics in Fluorescence Spectroscopy, Advanced Concepts in Fluorescence Sensing Part A: Small Molecule Sensing, is the ninth volume in the popular series Topics in Fluorescence Spectroscopy, edited by Drs. Chris D. Geddes and Joseph R. Lakowicz. This volum
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5.1. INTRODUCTION The miniaturized dimensions and high degree of control of molecular design offered by chemical synthesis render organic molecules attractive candidates for molecular electronics and photonics, especially for digital processing and communication. ' This includes molecular and supramolecular devices whose optical properties can be switched or modulated by external stimuh such as chemical,*"'" electrochemical,'^"'^ thermal, '""''' or optical"*"^'' inputs thereby producing a detectable signal, in some cases even on a single molecule level.^*"'" Such systems can be obtained by combining specific components suitably arranged and connected either via covalent bonds or intermolecular interactions with the interplay of the elementary acts performed by the respective modules determining the device's function.' "'' The key issue for the construction of favorable devices is the efficient transduction of an event into a measurable signal. This can be achieved upon combining a properly selected, externally addressable control module which can exist in two defined states of comparable and sufficient stability, that - dependent on the desired application - should or should not be reversibly convertible into each other, and a reporting module, efficiently and selectively communicating the switching event, i.e., external stimulus. The latter implies that, aside from the desired strong changes of the optical properties, the generated output should strongly differ irom any signals from an unspecific background, the sensor in a different (for instance oxidation or analyte binding) state or the switch in the ,^ERO" or „OFF' position. A particularly attractive signaling feature for sensing and switching applications is luminescence. Luminescence, that requires light absorption by the chromophore, is characterized by its intensity, spectral, and temporal distribution as well as (de)polarization, thus enabling multiparameter communication, and offers an Dr. Ute Resch-Genger, Federal Institute for Materials Research and Testing, Richard-Willstatter-Str. 11, D12489 Berlin, Germany. Dr. Gunther Hennrich, Dpto. de Quimica Organica, L 101, Facultad de Ciencias, Universidad Autonoma de Madrid, Cantoblanco 28049-Madrid, Spain. 189
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intrinsic selectivity typically via the two experimental parameters excitation and emission wavelength in combination with a high sensitivity down to the single molecule level/ Furthermore, signal accessing via remote control is possible. In the growing field of sensory devices and switches, fluorescent redox systems, whose emission properties can be controlled by electrochemically or chemically induced redox processes,'^""" have attracted increasing interest for applications in biology, (bio)chemistry, environmental analysis, and material sciences. This includes for instance fluorescent probes to map the electric potential across neurons, determine synaptic activity, and measure membrane potentials,'"'•''' the field of redox fluorometry, i.e., redox imaging of cells, tissu
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