Pseudo cage effect in double energy transfer

In some systems, the donor of a triplet—triplet energy transfer can be sensitized in its singlet state through a singlet—singlet energy transfer (Dexter mechanism), where the donor is the acceptor of the triplet transfer itself. As a consequence an extra acceptor molecule in the triplet energy transfer is present in the vicinity of the donor, thus enhancing the efficiency of the transfer process. Experiments show clearly this effect and a diffusional model gives semi-quantitative agreement with the experimental data.     

Fluorescent probes for detection of amphiphilic polymer hydrophobic microdomains: A comparative study between pyrene and molecular rotors

 Cationic amphiphilic polymers (e.g. polyvinylpyridinium bromides and polyvinylimidazo-lium bromides) adopt a compact coiled form in aqueous solutions. In the case of former polymers, the resulting hydrophobic microdomains are evidenced only by fluorescence spectroscopy if a molecular rotor (DMAC) is used as a fluorescent probe, while the behaviour of the latter can be studied in aqueous solution by using both types of fluorescent probes, i.e., pyrene and the molecular rotor. The purpose of the present investigation deals with a comparative study between the magnitude of the local viscosity and the polarity of the hydrophobic microdomains generated by poly(3-hexadecyl-1-vinylimidazolium bromide) in an aqueous medium using pyrene and DMAC as fluorescent probes. Moreover, the results are compared with the data obtained with micelles of conventional surfactants such as the homopolymer repetitive unit model and CTAB. Received: 3 June 1996 Accepted: 29 September 1996     

Industrial photochemistry XIV: Light distribution around an external source made of a fluorescent relay

The principle of the use of a fluorescent relay is described. Its aims is to transform undesirable wavelengths into active photons for a specific photoreaction and to improve the use of commercial mercury vapour lamps in photochemistry. The light profile in the photoreactor in the presence of a fluorescent relay has been established, using a sensor made of an optical fibre with a small integrating sphere; the experimental values are compared with the model (the lamp is assumed to be a quasi-linear source). The determination of a semi-empirical model for the light distribution, which considers the partial transmission of fluorescent light through the lamp (shadow effect), leads to the calculation of the extinction coefficient μ_O of the fluorescent solution corresponding to the best efficiency of the fluorescent relay.     

Three-Dimensional Fluorescence Microscopy of Endothelial Cells Labeled with Coumarins

Fluorescent coumarins were synthesized with the objective of introducing a glucosamine part in the chemical structure, with either hydroxyl or acetyl functions. The photophysical behavior was studied in organic solvents with different polarity and viscosity. The location of the fluorescent coumarins in endothelial cells was studied using fluorescence microscopy imaging, especially with a 3-D CELLscan instrument.     

Thermodynamic Investigation of the Micellization and the Adsorption of Short Perfluoroalkylated Diols via Fluorescence Spectroscopy and Tensiometry

The micellization and adsorption of two short chain perfluorodiols 3,3,4,4,5,5,6,6,6-nonafluorohexane-1,2-diol (nFHD) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctane-1,2-diol (tFOD) are examined from a thermodynamic point of view as a function of temperature and methanol content. The microenvironment of the fluorinated aggregates is evaluated by the fluorescence probe method using pyrene and a molecular rotor 1,1-dicyano-4-p-dimethylaminophenyl)-1,3-butadiene (DMAPhC). The formation of micellar aggregates being evidenced, the results are discussed in terms of the polarity and of the cohesion behavior of the micellar aggregates by taking into account the methanol (MeOH) effect. The critical micellization concentrations thus determined are compared with those given by surface tension measurements. Micellar and adsorption thermodynamic parameters such as Gibbs free energies, enthalpies, and entropies are determined together with the surface areas. The results are compared with literature data and discussed. A model for describing the adsorption process of the fluorinated compounds upon the influence of methanol is finally proposed. Copyright 2001 Academic Press.     

Investigation of acetone-butanol-ethanol (ABE) fermentation by fluorescence

Photophysical techniques have potential for the development of optical sensors in monitoring and controlling fermentors. In the particular case of acetone-butanol-ethanol (ABE) fermentation, carried out by bacteria of the speciesClostridium acetobutylicum, we have developed two studies based on fluorescence spectroscopy. First, we measured the intrinsic fluorescence of NADH related to bacteria metabolism, leading to a linear relationship between the NADH specific fluorescence and the specific rate of butyric acid production. At the same time, we have correlated enzymatic activities (acetate kinase, butyrate kinase, acetoacetate decarboxylase) with NADH specific fluorescence. Second, we studied the fluorescence polarization of extrinsic DPH (1,6-diphenyl-1,3,5-hexatriene) related to membrane fluidity. A simultaneous increase in both DPH anisotropy (order parameter increase) and butanol production is observed. Even though these results seem contradictory, because of the well-known fluidizing effect of butanol on lipids, they can be explained by a homeoviscous response ofC. acetobutylicum to the presence of butanol during fermentation. Thus the apparent changes in fluidity could be the result of the adaptative membrane alteration.     

Fiber-optic fluorescing sensors for nitrate and nitrite detection

Fiber-optic sensors allow remote analyses of chemical substances and they now find many applications in chemistry and biology [1,2]. The purpose of this short report is to give our first results in the development of optical-fiber chemical sensors. Among the numerous known spectrometric methods, we chose the fluorometric one, generally described as a suitable method for determining substances at the parts per million or parts per billion level, with the objective of analyzing nitrate and nitrite anions, using modifications of the fluorescence emission of suitable dyes. The detection of nitrates is based on the irreversible nitration of fluorescein, which leads to a subsequent inhibition of fluorescence emission [3]; determination of nitrites corresponds to their addition on 2,3-diaminonaphthalene, which on the contrary, improves the fluorescence emission [4]. To set up simple instrumentation, we are developing fiber-optic sensors. This consists of (i) realizing an extrinsic active optical fiber by chemical linkage of suitable fluorescent dyes on silica fiber involving silanization reaction (APTES) and chemical methods and (ii) designing an optical device which is appropriate for measurements with optical fibers. The threshold of detection, coating efficiency, and stability with time are presented.     

Protection and Labelling of Thymidine by a Fluorescent Photolabile Group

A fluorescent photolabile group including coumarin and MeNPOC moieties was synthesized to protect 5′‐OH terminal function of thymidine (T). Its photochemical and photophysical properties were studied, in particular the photocleavage (photodeprotection under a 365‐nm irradiation) is only lowered by a factor of two by addition of the fluorophore. Fluorescence properties of the coumarin probe are not changed upon irradiation, which is satisfactory for the application required, i.e., in situ synthesis of DNA microarrays.     

Use of fluorescent probes (pyrene and molecular rotor) for the detection of hydrophobic microdomains in amphiphilic polymers aqueous solutions

The purpose of the present investigation deals with a comparative study between the magnitudes of the local viscosity and/or the polarity of the hydrophobic microdomains generated by poly (3-hexadecyl-1-vinylimidazolium bromide) in an aqueous medium using a cinnamylidene molecular rotor and pyrene as fluorescent probes. Moreover, the results are compared with the data obtained with micelles of conventional surfactants such as the homopolymer repetitive unit model and CTAB.     

Fluorescent molecular rotors as probes for polymers

The use of a molecular rotor (1,1-dicyano-4-(4'-dimethylaminophenyl)-1,3-butadiene) as a fluorescent probe was proved to be of great interest for the study of polymers. First, the rotor can detect the critical time of the glass effect in the bulk polymerization of MMA into PMMA due to viscosity change, this will allow a better control of the process and is complementary to the information issued from the use of the fluorescent pyrene probe which is sensitive to the gel effect. Second, the cinnamylidene rotor was able to detect the formation of hydrophobic microdomains for cationic amphiphilic polymers in their aggregation modes when they were solubilized in water, both polarity and viscosity changes are playing a role. The possibility of incorporation of various molecular fluorescent rotors in polymers beads was also studied.