Joseph Georges

Fluorescence quantum yield of rhodamine 6G in ethanol as a function of concentration using thermal lens spectrometry

Abstract Thermal lens spectrometry has been used to measure the fluorescence quantum yield of rhodamine 6G in ethanol. The absolute Φf (0.94), obtained at low concentrations, is found to be independent of concentration up to 2 × 10−4 M. Relative fluorescence experiments corrected for inner-filter effects corroborate the photothermal data.

Comparative study of the luminescent properties of europium and terbium coordinated with thenoyltrifluoroacetone or pyridine-2,6-dicarboxylic acid in aqueous solutions

The chelates formed with europium(III) and terbium(III) coordinated with either thenoyltrifluoroacetone or pyridine-2,6-dicarboxylic acid were studied. The formation and the composition of the chelates were compared with respect to the number of donor groups of the ligand and the coordination number of the lanthanide ion. The luminescent properties of the chelates were investigated and compared in terms of the relative luminescence intensity, the linear dynamic range and the detection limit. The results are discussed with respect to the stoichiometry of the chelate, the lifetime of the emitting level and the energy gap between the donor level of the ligand and the emitting level of the ion.

Electrochemistry of ferrocene in anionic, cationic and nonionic micellar solutions. Effet of the micelle solubilization of the half-wave potentials

Abstract The redox properties of the ferrocene/ferricinium Fc/Fc + couple were studied in aqueous 0.1 M NaCl solution of sodium dodecylsulphate (SDS), N -cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-23 lauryl ether (Brij 35). Solubilities of ferrocene were measured by spectrophotometry and by limiting current measurements in direct current voltammetry. The reversible half-wave potentials were determined by cyclic and differential pulse voltammetry. The results are discussed on the basis of the micelle solubilization equilibrium of both the reductant Fc and the oxidant Fc + . Indeed, diffusion coefficient ratios of the oxidant to the reductant, measured by coulometry, show that the micelle solubilization of the electrogenerated ferricinium cation Fc + must also be considered in the nonionic and the anionic micellar solutions. Thus in every solution studied, a standard potential of Fc/Fc + in water may be obtained from the experimental values of the half-wave potentials, the partition coefficients of Fc and Fc + and the ratio of their diffusion coefficients. The behaviour of ferrocene in the SDS solutions and the effect produced by the addition of pentanol seem to confirm that SDS forms micelles with a less ordered structure than the other surfactants studied. The invariance of the half-wave potentials of ferrocene in pure SDS solutions whatever the surfactant and ferrocene concentrations suggests the use of this solute-solvent couple as a reference potential system.

Sensitive detection of tetracyclines using europium-sensitized fluorescence with EDTA as co-ligand and cetyltrimethylammonium chloride as surfactant

The determination of tetracyclines (TC) in aqueous solutions, based on europium-sensitized fluorescence, has been improved using EDTA as co-ligand and cetyltrimethylammonium chloride as surfactant. The method involves working in slightly alkaline solutions with the formation of a new chelate where the lanthanide ion is bound to the beta-diketone group. The method is about 6 times more sensitive than that with the Eu-TC-Triton system and LODs are 2.5 x 10(-10), 5 x 10(-10), 1.5 x 10(-9) and 2 x 10(-9) mol l-1 for TC, oxytetracycline, chlortetracycline and doxycycline, respectively. The method has been applied to the determination of TC in calf serum without sample pretreatment. The mean recovery was close to 102% and the lowest concentration attainable in serum samples was better than 0.1 microgram ml-1.

Study of europium-sensitized fluorescence of tetracycline in a micellar solution of Triton X-100 by fluorescence and thermal lens spectrometry

Abstract Europium fluorescence sensitized by tetracycline, via the formation of an organic chelate, was used for the sensitive detection of tetracycline. The efficiency of the intramolecular energy transfer and of the resulting ion luminescence was studied by fluorescence and thermal lens spectrometry. The sensitivity of the method depends on the pH and the concentration of the emitting ion and it is increased by the addition of Triton X-100 to the aqueous solution. The fluorescence enhancement obtained in the micellar medium is correlated with an increase in the fluorescence lifetime of europium emission. Detection limits for tetracycline are 1.0 X 10 −8 and 3 X 10 −9 M in the absence and presence of Triton X-100, respectively.

Limitations Arising from Optical Saturation in Fluorescence and Thermal Lens Spectrometries Using Pulsed Laser Excitation: Application to the Determination of the Fluorescence Quantum Yield of Rhodamine 6G

The high incident irradiances available with pulsed lasers can lead to a significant depletion of the ground-state population of the chromophore and to optical saturation effects. As a result, the optical absorption coefficient decreases as a function of the excitation energy and, because the amount of energy released by radiative and nonradiative relaxation processes depends on the amount of energy absorbed, nonlinear energy-dependent signals are obtained. Therefore, large errors can be introduced when fluorescence and photothermal data are used to determine fluorescence quantum yields. This work provides experimental results describing the effects of optical saturation on fluorescence and thermal lens measurements for rhodamine 6G in various media and over a wide energy range. It is shown that, when optical saturation is avoided, the photothermal method gives accurate absolute values of Φf ranging from 0.93 to 0.95, depending on the solvent. On the contrary, fluorescence measurements seem to be sensitive to additional experimental artifacts that are more difficult to characterize and to eliminate. Index Headings: Optical saturation; Thermal lens spectrometry; Fluorescence; Rhodamine 6G; Fluorescence quantum yield.

Improved detection of salicylic acids using terbium-sensitized luminescence in aqueous micellar solutions of cetyltrimethylammonium chloride

The determination of salicylic, p-aminosalicylic and 5-fluorosalicylic acids was investigated using terbium-sensitized luminescence in aqueous solutions. Formation of a ternary chelate between terbium, EDTA and the salicylic acid requires dissociation of the phenol group which is adjacent to the dissociated carboxylic group. The reaction is obtained in alkaline solutions and is enhanced in the presence of cetyltrimethylammonium chloride. As evidenced by absorbance and fluorescence measurements, the cationic surfactant plays an important role in the formation of the ternary chelate and then terbium luminescence depends mainly on the extent of chelate formation. Linearity is found over more than four orders of magnitude and detection limits are in the range (2-4) x 10(-10) mol l-1 for the three acids.

Prediction of the calibration curves for the analysis of high absorbances using mode-mismatched dual-beam thermal lens spectrometry with chopped continuous wave laser excitation

Thermal lens spectrometry is a sensitive method especially designed for the analysis of weak absorbances. Recently, a convenient mathematical expression has been derived for steady-state and time-resolved measurements, using a mode-mismatched dual-beam optical configuration. This work discusses the validity of the theoretical predictions, over a wide range of concentrations including high absorbance solutions, when a chopped continuous wave laser is used to irradiate the sample. Depending on the chopping frequency and on the thermo-optical properties of the solvent, experimental calibration curves can differ significantly from the theoretical ones because of a cumulative effect induced by the chopped excitation beam on the sample. However, a good agreement is achieved when the chopping disc is arranged in such a way that the dark period following each irradiation period is long enough to allow the sample to relax to its initial thermal state before being excited again.

Investigation of chelate formation, intramolecular energy transfer and luminescence efficiency and lifetimes in the Eu–thenoyltrifluoroacetone–trioctylphosphine oxide–Triton X-100 system using absorbance, fluorescence and photothermal measurements

The formation and the photophysical properties of the europium-thenoyltrifluoroacetone (TTA) trioctylphosphine oxide (TOPO)-Triton X-100 chelate were investigated. When the medium is buffered with acetate, there is a strong competition between acetate and TTA for coordination with europium ions. When TOPO is added into the solution, the Eu-TTA-TOPO ternary chelate forms more easily, probably because the coligand acts as a synergic agent and would favour the formation of the enol form of TTA. Although the stoichiometric composition of the chelate is expected to be Eu(TTA)3(TOPO)2, the Eu-TTA and the Eu-TOPO mole ratios may be within 2-3 and 1-2, respectively, depending on the composition of the solution. However, the fluorescent properties of the chelate seem to be mainly dominated by its actual concentration into the solution rather than by its composition. Time resolution of europium emission spectra in the microsecond range has shown that energy transfer occurs from the TTA ligand to the 5D1 level of europium. Then, the emitting 5D0 level is populated through non-radiative deactivation of 5D1. The observed lifetimes of the 5D1 and 5D0 states are 1.25 and 860 micros, respectively. The overall fluorescence quantum yield of the chelate, measured by the photothermal method, is found to be 0.22. On the basis of the time-resolved photothermal experiments, the fluorescence quantum yield of the 5D0 state is expected to be > 0.8 and the energy transfer efficiency < 0.28.

USE OF THERMAL LENS SPECTROMETRY FOR THE INVESTIGATION OF DIMERIZATION EQUILIBRIA OF RHODAMINE 6G IN WATER AND AQUEOUS MICELLAR SOLUTIONS

Abstract The aggregation and the fluorescent properties of rhodamine 6G in aqueous solutions have been investigated using thermal lens spectrometry in addition to conventional absorption and fluorescence experiments. The results show that, when the compound is highly fluorescent, the photothermal method is very sensitive to detect the formation of dimers. It allowed the determination of the absolute fluorescence quantum yield of the monomer and of the dimerization constant. The experiments, carried out over a wide range of concentrations, show that while the non-active absorption by dimers is the main quenching process at low concentrations, the decrease in the fluorescence quantum yield of the dye at higher concentrations could also result from migrational quenching involving the non-radiative energy transfer from excited monomers to non-fluorescent dimers. Solubilization of rhodamine 6G into micellar solutions has a positive effect on the deaggregation of dimers for dye concentrations up to at least 4×10 −4 M. In solutions containing 2% and 10% of anionic SDS or nonionic Brij 35, respectively, dimers are absent and the fluorescence efficiency of the rhodamine is completely restored. On the contrary, in solutions containing 4% of cationic CTABr, the absolute fluorescence quantum yield of monomers is significantly decreased and dimerization still occurs to a small extent.