Fluorescence field variation

A strong electric field (on the order of volts/angstrom) is applied to the tip of a sharp, single-crystal wire. The electrons tunnel into the vacuum and are accelerated along radial trajectories by Coulomb repulsion. When the electrons impinge on a fluorescent screen, variations of the electric field strength across the surface of the tip are displayed. 

Northrup, M.A., et al.. Direct measurement of interstitial velocity field variations in a porous medium using fluorescent-particle image velocimetry, Chem. Eng. Sci.. 48( I), 13-22 (1993). 

It is necessary to note that fluorescence characteristics demonstrate remarkable sensitivity to variations of physicochemical parameters of the environment. Therefore, such parameters as polarity, viscosity, temperature, electric potential, local electric field, pressure, pH, etc., can be registered successfully using the modem sensitive apparatus for fluorescence detection [1, 4—12]. As a consequence, fluorescent molecules are used successfully as molecular probes to study the local characteristics of physicochemical, biochemical and biological systems. 

As we have seen, an external plane wave can excite resonances of a particle, which leads to significant variation in fluorescence intensity. A fluorescent molecule located in or near a particle can also excite the resonances of the particle. This can be modeled by again considering the molecule as a classical point dipole and obtaining the fields due to the dipole from the solution to the boundary value problem. 

Because of the nature of electroporation, virtually any molecule can be introduced into cells. For transfer of DNA, the electroporation forces are important. An electrophoretic effect of the field causes the polyanion DNA to travel toward the positive electrode. Fluorescence studies have shown that DNA enters the cell through the pole facing the negative electrode, where the membrane is more destabilized and where the field will drive the DNA towards the center of the cell (245). Membrane resealing occurs after pore formation. Whereas pore formation happens in the microsecond time frame, membrane resealing happens over a range of minutes with variations depending on electrical parameters and temperature (246). 

Fluorescence burst analysis, a variation of FCS procedures that has an optimum configuration for simple presentation, uses a uniform nanoscopic flow channel with an optically perfect ceiling, uniform cross section, and periodic electrodes that can now be constructed by careful electron lithography techniques.55 By application of controlled electric fields, uniform plug flow of solution through... 

Rebscher and Pyell [62, 63] were the first to report the use of fluorescence detection. The baseline noise with ICFD was about twice that of OCFD. This could be attributed to the minute motions of the packed bed in the applied electric field and the diffuse scattering in the bed that increased the level of background fluorescence. R.S.D. s found for the retention times of polyaromatic hydrocarbons were less than 1.1 and < 0.4 % for ICFD and OCFD, respectively. Variations in the peak areas were about twice as high in ICFD (2.6-5.1%) compared to OCFD (1.4-2.3%). According to the authors, this could be due to variations in the temperature within the capillary. 

Figure 1. UV field illumination of a Plan Apo 100x lens (1.4 NA) derived with a fluorescent plastic slide and the intensity measurement of 10-micron Spherotech beads (obtained from Spherotech, Libertyville, IL, USA). This illustrates the problem of using a lens with improper field illumination to make comparative measurements on a sample. The field illumination pattern shows a bull s eye intensity pattern slightly off-center and the five beads located in different parts of the field to illustrate the variation in intensity occurring by using a lens that has improper field illumination. The intensity of beads was derived by a small Region of Interest (ROI) inside the bead. The five beads show a decrease in intensity relative to the bead in the center of the illumination. Although this figure was obtained with UV optics, it represents the type of field illumination that can also occur with visible light excitation. This pattern is also unacceptable, if a confocal laser scanning microscope optical system is used for a FISH study, as the maximum intensity should be in the center of the objective and not in the corner.

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