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This article reports on a novel fluorescence resonance energy transfer (FRET) system between harmine and silver nanoparticles (AgNPs), in which harmine acts as the donor and AgNPs act as the acceptor. As a result of FRET, harmine fluorescence is quenched efficiently with a corresponding Stern-Volmer constant of 3.61 × 10(11)  L/mol. It was found that upon addition of the anticancer drug, 6-thioguanine (6-TG), the fluorescence was recovered due to the competitive adsorption of this compound onto AgNPs. Based on this effect, a selective turn-on fluorescence sensor was developed for the determination of 6-TG. Under optimum conditions, the enhanced fluorescence intensity displays a linear relationship with the concentration of 6-TG in the range 1.5 × 10(-8) -7.5 × 10(-7)  M with a detection limit of 9.7 nM. The developed method was applied to the determination of this drug in a pharmaceutical preparation and human plasma samples.

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Abstract.A new analytical methodology based on the competitive aggregation in a dye-surfactant-drug system is developed for the determination of gemfibrozil. Eriochrome Blue Black R (EBBR) and Didodecyldimethylammonium bromide (DDABr) were the dye and surfactant used, respectively. In the proposed method, the anions of the dye bind to the cationic surfactant molecules to form dye-surfactant aggregates, which are monitored from changes in UV-Vis absorption features of the dye. In the ternary EBBR-DDABr-drug mixtures, the drug competes with the dye to interact with the surfactant, which results in a decrease in dye-surfactant aggregates formation. This, again, causes a change in absorption properties of the dye. The measurement parameter is the difference between the absorption of the dye in the presence and absence of the drug. In the appropriate experimental conditions the absorbance differences are directly proportional to the drug concentration. The influence of several experimental variables such as pH, concentrations of buffer, EBBR and DDABr on the measurement parameter were studied. Under the optimum conditions, the calibration graph was linear up to 6.0 µg ml −1 with the correlation coefficient of 0.998.The limit of detection and quantification were found to be 0.044 and 0.15 µg ml −1 , respectively. The method was validated and applied to the determination of gemfibrozil in pharmaceutical preparations.