Radical detection fluorescence

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Li, B., Gutierrez, P. L., and Blough, N. V.. (1999) Trace determination ofhydroxyl radical using fluorescence detection, Methods in Enzymology 300 (Oxidants and Antioxidants, PartB), 202-216. 

We have constructed a monolayer assembly in which electron transfer occurs from excited cyanine dye J aggregates to a violo-gen electron acceptor. This process is manifested by efficient, rapid quenching of the dye fluorescence accompanied by slow, inefficient growth of relatively stable radicals, detected by absorption. If electron donors are added to either acceptor or dye layer, this radical yield can be enhanced. This enhancement or supersensitization can be as large as a factor of 3.5, depending on donor and conditions. The supersensitization is due to electron transfer from donor to either excited dye or the dye radical cation. The net effect is photocatalyzed electron transfer from donor to acceptor. Electron transport through the dye aggregate appears. to be necessary for supersensitization. 

Based on spectral characteristics and the similarity of the systems, radicals observed in milk powder and in freeze-dried cheese (Figure 3) may be similar to radicals detected in protein-containing lipid emulsions that were freeze-dried and stored in air for five weeks (24). The ESR signal increased up to about seven days of incubation and was considerably weaker after 21 days. The disappearance of radicals coincided with an increase in the fluorescence intensity of chloroform extracts of the freeze-dried emulsion, indicating formation of tertiary oxidation products (reactions between lipid oxidation products and protein/amino acids). 

With respect to using methyl viologen as electron relay, it might be of interest to note tlmt MV " can be oxidized by positive holes produced in illuminated colloidal semiconductors such as Ti02 Two oxidation products of MV are 1, 2 -di-hydro-l,r-dimethyl-2 -oxo-4,4 -bipyridylium chloride and 3,4-dihydro-l,r-dime-thyl-3-oxo-4,4 -bipyridylium chloride, which can readily be detected by their strong fluorescences at 516 nm and 528 nm, respectively. These products are also produced in the direct photolysis of MV " solutions and in the reaction of MV "" with OH radicals in homogeneous solution... 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Two-dimensional distributions of ground-state NO were detected by planar laser-induced fluorescence during the process of NO removal in a corona radical shower system in NO/dry air mixtures [57,58], The authors observed that the density of NO molecules decreased not only in the plasma region formed by the corona streamers and the downstream region of the reactor, but also in the upstream region of the reactor. They explained this behaviour by oxidation with ozone, which is transported upstream by electrohydrodynamic flow. 

St /G AG-0.14 V fluorescence quenching stilbene radical anion detected in transient absorption no Gs near injection site or in intervening sequence k = 1012-108 s 1 for 0-4 intervening A-T base-pairs (-3.4-17 A) exponential distance dependence of CT rate constant (3 0.6-0.7 A"1 small variations in k depending on whether G is in the A or T arm of the hairpin... 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

T. Ohyashiki, M. Nunomura, and T. Katoh, Detection of superoxide anion radical in phospholipid liposomal membrane by fluorescence quenching method using 1,3-diphenylisobenzofuran. Biochim. Biophys. Acta. 1421, 131-139 (1999). 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

Suzuki Y, Katagi T (2008) Novel fluorescence detection of free radicals generated in photolysis of fenvalerate. J Agric Food Chem 56 10811-10816... 

Larmor frequency, 39 353, 42 127 Laser desorption, Pt(lOO), wave types, 37 257 Laser-induced fluorescence, in detection of surface-generated gas-phase radicals, 35 150-160... 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

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