Study of graphene dispersions in sodium dodecylsulfate by steady-state fluorescence of pyrene.

HYPOTHESIS: Aqueous solutions of ionic surfactants allow the exfoliation of graphene, that can be explained considering the adsorption model of ionic surfactants to hydrophobic surfaces. For many years, pyrene has been used as a fluorescent probe because its sensitivity to the micro-environment. The study of pyrene fluorescence in the presence of different graphene dispersions in an ionic surfactant, would improve the knowledge of the graphene-surfactant interactions. EXPERIMENTS: Different dispersions of graphene in sodium dodecylsulfate were prepared at different weight ratios 0.5, 1 and 2%. The dispersions have been studied by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The influence of the dispersions on the pyrene fluorescence has been investigated. FINDINGS: The graphene sheets modified by the surfactant quench the fluorescence of pyrene, which depends on the amount of graphene, the concentration of surfactant and the weight ratio. For surfactant concentrations below the critical micelle concentration, the quenching effect is higher as the weight ratio increases. Once this concentration is reached, the fluorescence increases slightly and then levels off. This behavior has been explained by the adsorption model. For a constant surfactant concentration, two straight lines can be observed in the Stern-Volmer plots whose cut-off point is approximately 20 mg L-1 of graphene.

Retinol fluorescence in lecithin/n-butanol/water aggregates: a new improvement for its analysis in cosmetics without pretreatment.

The possibilities of different media formed by lecithin/n-butanol (n-BuOH)/water ternary mixtures for the analysis of all-trans-retinol by fluorescence have been studied. Fluorescence intensity of retinol increases in the presence of different types of aggregates formed in these media. Analytical features are good, the detection limit and quantification limit have micrograms per liter levels, and the linear range and sensitivity are appropriate to determine retinol in cosmetic samples. The analysis of retinol in anti-wrinkle creams can be achieved directly without any pretreatment of the sample. The vesicles built up from a biocompatible surfactant (lecithin) in aqueous solution with a low amount of n-BuOH permit an appropriated media for a simple, rapid, and sensitive analytical method. This method has a linear range between 64.1 and 800 µg L(-1), a sensitivity of 202.3 L mg(-1), and a low detection and quantification limit at 19.2 and 64.1 µg L(-1), respectively.

Improvement in retinol analysis by fluorescence and solid phase extraction (SPE) in micellar medium.

In this paper, the determination of retinol, the structure with the most activity as vitamin A, was carried out in an aqueous micellar medium with a low quantity of a short-chain alcohol. The analytical technique used in this work was fluorescence, which gave us very much information qualitative and quantitative. The sensitivity of the method is higher than that obtained in other media; the detection limit is 0.03 mg L(-1) and retinol was stable in solution for at least 5 days. The use of solid phase extraction (SPE) for organic samples, allowed us to change the organic matrix by a mixture CTAB 5%/n-butanol 10%/water 85% w/w/w with recoveries in retinol spiked samples close to 100%. In addition, the combination of SPE and fluorescence is a good preconcentration technique, sensitive and fast for the identification and determination of retinol, simultaneously.

Validation of a screening method for the simultaneous identification of fat-soluble and water-soluble vitamins (A, E, B1, B2 and B6) in an aqueous micellar medium of hexadecyltrimethylammonium chloride.

Simultaneous determination of the fat-soluble vitamins A and E and the water-soluble vitamins B1, B2 and B6 has been carried using a screening method from fluorescence contour graphs. These graphs show different colour zones in relation to the fluorescence intensity measured for the pair of excitation/emission wavelengths. The identification of the corresponding excitation/emission wavelength zones allows the detection of different vitamins in an aqueous medium regardless of the fat or water solubility of each vitamin, owing to the presence of a surfactant which forms micelles in water at the used concentration (over the critical micelle concentration). The micelles dissolve very water insoluble compounds, such as fat-soluble vitamins, inside the aggregates. This approach avoids the use of organic solvents in determining these vitamins and offers the possibility of analysing fat- and water-soluble vitamins simultaneously. The method has been validated in terms of detection limit, cut-off limit, sensitivity, number of false positives, number of false negatives and uncertainty range. The detection limit is about microg L(-1). The screening method was applied to different samples such as pharmaceuticals, juices and isotonic drinks.

Determination of vitamins A and E in milk samples by fluorescence in micellar media.

The fat-soluble vitamins A and E in milk samples were determined by fluorescence at room temperature in an aqueous media of micellar solutions. Different types of surfactants were studied; the cationic hexadecyltrimethylammonium bromide (CTAB), the anionic sodium dodecylsulfate (SDS) and the non-ionic polyoxyethylene(23)laurylether (Brij 35). The detection limits ranged between 50 and 90 ng.L-1 for both vitamins in CTAB and Brij 35. The method has been applied to the determination of vitamins A and E in milk samples.

Determination of antihistamines based on the formation of mixed aggregates with surfactants.

The determination of antihistamines based on the measurement of the critical micelle concentration (c.m.c.) of mixed sodium dodecyl sulfate (SDS)-antihistamine aggregates is proposed. The dye Coomassie Brilliant Blue G (CBBG) was used as a photometric probe for the rapid determination of c.m.c.s. The micellar properties of these drugs permitted the determination of diphenhydramine, antazoline, tripelennamine, diphenylpyraline and clemizole at the micron level with detection limits ranging between 0.1 and 0.7 microns. Hence the proposed method surpassed existing alternative photometric methods used routinely in the quality control of these drugs in sensitivity and featured similar detection limits to fluorimetric methods. The relative standard deviation for 6 micron diphenhydramine was 3.7%. The method was applied to the determination of these drugs in pharmaceutical preparations (solutions, capsules, creams and pills). which were analyzed directly after dissolution of the samples in distilled water.

Quantitation of tricyclic antidepressant drugs based on the formation of mixed aggregates with surfactants.

A new approach for the quantitation of drugs, based on the measurement of the critical micelle concentration of mixed surfactant-drug aggregates, is proposed. This methodology involves the photometric titration of drugs in an aqueous medium using a surfactant as titrant. The Coomassie Brilliant Blue G (CBBG) dye, negatively charged, is used as a photometric probe. The analytical applicability of this approach is demonstrated by quantifying tricyclic antidepressants (TCAs) such as imipramine, desimipramine, amitriptylin, and nortriptylin. Aggregates studied included TCA-sodium dodecyl sulfate (SDS) and TCA-Triton X-100 mixtures. Because of the opposite charge of TCAs and SDS, which causes strong synergistic effects in the mixture relative to the properties of their individual components, this anionic surfactant was selected for the quantitation of TCAs. Pharmaceutical preparations can be analyzed directly after dissolution of the samples in water or ethanol. The detection limit achieved for the studied drugs is about 0.12 mg L-1, so the proposed method surpasses existing alternative photometric methods in sensitivity and features a detection limit similar to fluorimetric methods. The relative standard deviation for 0.8 mg L-1 of TCA is 2.6%.

Spectrophotometric determination of copper(II), nickel(II) and cobalt(II) as complexes with sodium diethyldithiocarbamate in cationic micellar medium of hexadecyltrimethylammonium salts.

The determination of copper(II), nickel(II) and cobalt(II) was carried out as diethyldithiocarbamate (DDTC) complexes in presence of aqueous solutions of cationic surfactants of hexadecyltrimethylammonium bromide, chloride and hydroxide (CTAB, CTAC, CTAOH). The presence of micellar systems avoids the previous step of solvent extraction necessary to the formation of the DDTC complexes in absence of micelles. The influence of the different micellar counterions on the analytical characteristics (sensitivity and detection limits) of the proposed method for spectrophotometric determination of Cu(II), Ni(II) and Co(II) was studied.