Excitation, electronic fluorescence sensitization

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

Tunnelling electrons from a STM have also been used to excite photon emission from individual molecules, as has been demonstrated for Zn(II)-etioporphyrin I, adsorbed on an ultrathin alumina film (about 0.5 nm thick) grown on a NiAl(l 10) surface (Qiu et al, 2003). Such experiments have demonstrated the feasibility of fluorescence spectroscopy with submolecular precision, since hght emission is very sensitive to tip position inside the molecule. As mentioned before the oxide spacer serves to reduce the interaction between the molecule and the metal. The weakness of the molecule-substrate interaction is essential for the observation of STM-excited molecular fluorescence. 

Figure 21-1 also illustrates an atomic fluorescence experiment. Atoms in the flame are irradiated by a laser to promote them to an excited electronic state from which they can fluoresce to return to the ground state. Figure 21-4 shows atomic fluorescence from 2 ppb of lead in tap water. Atomic fluorescence is potentially a thousand times more sensitive than atomic absorption, but equipment for atomic fluorescence is not common. An important example of atomic fluorescence is in the analysis of mercury (Box 21-1). 

No. But that s OK because that kept on pushing us towards better and better experiments. I think that the last experiment that we published in Science this year may have made clear how exquisitely sensitive the electron transfer was to base-pair stacking. In this experiment we no longer appended metal complex intercalators but we simply looked at electron transfer from one modified base to another modified base. In this experiment we used two modified adenines as our fluorescent excited electron acceptor to oxidize guanines. The modified adenines were very similar in structure, very similar in redox characteristics, very similar in energetics. But when they were incorporated into DNA, one was well-stacked in the helix and one was... 

The fluorescence detector is a specific and concentration-sensitive detector. It is based on the emission of photons by electronically excited molecules. Fluorescence is especially observed for analytes with large conjugated ring systems, e.g., polynuclear aromatic hydrocarbons and their derivatives. In order to extend its applicability range, pre-column or post-column derivatization strategies have been developed [9]. 

Forster (fluorescence) resonance energy transfer (FRET) is the non-radiative energy transfer mechanism between donor chromophore in its excited electronic state and an acceptor chromophore. FRET is extremely sensitive to small distances as the energy transfer is inversely proportional to the distance between the donor and acceptor chromophore to the sixth power, making it highly suitable for imaging and sensing in biomedical. 

The different sensitive techniques of Doppler-limited laser spectroscopy discussed in the previous sections supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons, excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states the quantum efficiency rjk reaches 100% in many cases. For the detection of the laser-excited fluorescence, sensitive photomultipliers or intensified CCD cameras are available that allow, together with photon-counting electronics (Sect. 4.5), the detection of single fluorescence photons with an overall efficiency of 10 —10 including the collection efficiency 5 0.01—0.3 (Sect.6.3.1). 

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