Síntese, caracterização e aplicações analíticas de pontos quânticos (quantum dots) em amostras de interesse clínico e ambiental.

Abstract

This work deals with the application of quantum dots as fluorescent probes for the determination of organic and inorganic species in clinical and environmental samples. In the development of the work, two projects were developed, the first one was based on the application of carbon quantum dots (CDots) as fluorescent probes for determination of Cr(VI) in environmental samples (water and soil). CDots were prepared by pyrolysis of a mixture of molecular precursors citric acid and reduced glutathione. The synthesized CDots had a photoluminescent quantum yield of 69.4% and a particle size of about 2.5 nm. In the investigation of the interaction parameters between CDots and Cr(VI), it was observed the mechanism of static quenching with the formation of a supramolecular complex in the fundamental state. For the study of possible interferents to the method, Pb(II), Ba(II), Fe(II), Al(III) and Fe(III) were considered interfering to the method. After optimization of the main analytical parameters, the method presented a linear range of 0.10 – 12 mg mL -1 with limit of detection (LOD) of 0.03 mg mL-1 (pH 7), and recoveries of 90 – 105% and 99.5 - 107% for water and soil samples, respectively. Thus, the method proved to be sensitive, fast and versatile for applications in several environmental samples. The second project was based on the application of quantum dots of ZnS as fluorescent probes for the determination of thimerosal in vaccine samples. The ZnS QDs were synthesized from sodium sulfide solution (0.1 mol L-1) with zinc acetate (0.1 mol L-1) using N-acetyl-cysteine as the surface binding agent. The synthesized nanomaterial presented ex = 290 nm and em = 440 nm, with mean suspension diameter of 4.40 nm. In the optimization of the analytical parameters, pH and buffer system evaluation, it was observed that citric acid / phosphate buffer at pH 7 showed best response for the interaction of ZnS QDs with Thimerosal. In the evaluation of the quenching mechanism, it was observed that with increasing temperature, the values of Ksv and Kb decreased, indicating that the preferential mechanism of quenching is static. Based on the thermodynamic parameters, the interaction of ZnS QDs with thimerosal is a spontaneous, exothermic process and the negative entropy value may be a result of the accumulation of thimerosal on the surface of the QDs. The proposed method presented a linear range from 10 to 90 μg L-1 with LOD of 4.59 μg L-1, agreeing with other studies in the literature for the determination of mercury, since there are no similar systems with thimerosal. When performing the recovery test on samples of vaccines from market health centers in the city of Maceió, recovery ranged from 79,24 to 104,5%, indicating the accuracy of the method.