Species fluorescence

Table 2. In acidic solution all isomers exhibit fluorescence. 4-Aminophenol shows two bands one at 300 nm common to all the isomers, and the second at 370 nm attributed to the existence of an additional aqueous ionic species. Fluorescence also exists in neutral solution, but is aboHshed at high pH values (3-13).

The fact that very few chemical species fluoresce works to both the advantage and disadvantage of fluorometry as a quantitative technique. Explain this. 

Other techniques, such as C.D. spectral change, have been used to demonstrate the presence of octa coordination for lanthanide ion in a system containing Eu(FOD)3 and alcohols or ketones (28). However, the anionic tetrakis complexes e.g. Eu(acac)i, Eu(benzac)i, Eu(DBM)i, Eu(BTFA)4, tend to dissociate into the tris-complex and L in alcoholic solution. The degree of dissociation depends on the complex as well as the polarity of the medium. In alcohol-DMF medium the dissociation is enhanced compared to the alcoholic solutions (29). The end product of these dissociation reaction may well be an octacoordinated species. Fluorescence emission from the coordinated europium ion was also helpful in estabhshing the nature of the species in solution 29). 

NHE-1 (sodium/hydrogen exchanger) Depends on cell type and species Fluorescent labeling, found at nuclear membrane Liver and cardiomyocytes in R. norvegicus No (Bkaily et al. 2004)... 

A widely-tunable, quasi-cw, ultraviolet laser source for exciting transient-species fluorescence in chemical kinetics experiments has been built and is described. The reaction between OH and C2H is shown to proceed through both OH addition and H-atom abstraction routes. 

One or two Al(III) cationic porphyrin(s) is (are) connected with a peripheral mono- or di-phenolic free base, and their photophysical properties are compared with previously described tin(IV), germanium(IV), and phosphorous(V) oxophilic porphyrins. Two processes can occur in these nonelectronically coupled species, the first being observed upon excitation of the aluminum porphyrin. When excited at 550 nm, the Al(II) species fluorescence is quenched by the free base component mostly by energy transfer, but concomitantly by electron transfer in polar media. The second process occurs upon excitation of the free base species, and consists in a poorly efficient (low <2h2%) electron transfer from the free base to the Al(ni) species. The same process is quite efficient in the case of P(V), Sn(IV), and Ge(IV) analogs with respective 0H2 values of 93, 82, and 69%. ... 

Important bimolecular processes in the excited state include complex formation and energy transfer processes. When these processes produce non-fluorescent species, fluorescence or phosphore>scence quenching aJone is observed. Quenching is the general word used to describe any... 

Figure 3.3c shows an example of time-domain fluorescence measurements. The emission decay measured at two different wavelengths, corresponding to and Fj., are plotted as a logarithmic function of intensity (counts) versus time. In the simplest case of irreversibly decaying fluorescent species, fluorescence decay is characterized by a single exponential function whose time constant is the excited-state lifetime. However, in the case of the exdted-state reaction such as tautomerization, it may be a sum of discrete exponentials, or an even more complicated function. In a heterogeneous environment, the system is often better characterized by a distribution of decay times. 

This is a rapidly developing field. Analytical procedures can be established by several methods specie fluorescence fluorescence quenching chemically induced fluorescence (e.g., chelation of non-fluorescent metal ions with fluorescent ligands) and enzymic reactions that produce fluorescent products.Sample concentrations and identities can be determined in solution, on powders, or on glasses. 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

Weston K D, Carson P J, DeAro J A and Buratto S K 1999 Single-molecule detection fluorescence of surface-bound species in vacuum Chem. Phys. Lett. 308 58-64... 

For low concentrations of the fluorescing species, where ebC is less than 0.01, this equation simplifies to... 

The intensity of fluorescence therefore, increases with an increase in quantum efficiency, incident power of the excitation source, and the molar absorptivity and concentration of the fluorescing species. 

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Selectivity The selectivity of molecular fluorescence and phosphorescence is superior to that of absorption spectrophotometry for two reasons first, not every compound that absorbs radiation is fluorescent or phosphorescent, and, second, selectivity between an analyte and an interferant is possible if there is a difference in either their excitation or emission spectra. In molecular luminescence the total emission intensity is a linear sum of that from each fluorescent or phosphorescent species. The analysis of a sample containing n components, therefore, can be accomplished by measuring the total emission intensity at n wavelengths. 

For electronically excited species, the emitted light can be used for spectroscopic purposes, as in fluorescence analysis. 

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

A high concentration of the fluorescent dye itself in a solvent or matrix causes concentration quenching. Rhodamine dyes exhibit appreciable concentration quenching above 1.0%. Yellow dyes, on the other hand, can be carried to 5 or even 10% in a suitable matrix before an excessive dulling effect, characteristic of this type of quenching, occurs. Dimerization of some dyes, particularly those with ionic charges on the molecules, can produce nonfluorescent species. 

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