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Quantum dots of Cd(Se,S) and fluorescent magnetic nanocomposites (Cd(Se,S)-magnetite) were used as photocatalytic agents in the photodegradation of methylene blue (MB) under UV irradiation at pH 6.5.Quantum dots and magnetic nanocomposites were characterized by X-ray diffraction (XRD), UV-Vis, photoluminescence and Fourier Transform Infrared (FT-IR) spectroscopy. The photo-induced degradation of MB was monitored using High Performance Liquid Chromatography (HPLC) at 660 nm and titanium dioxide (anatase and aeroxide P25 forms) was used as the photocatalyst standard. A degradation of 99.1% and 90.0% of MB was achieved in the presence of 160 mg L À1 -quantum dots and the magnetic nanocomposite, respectively, after 4.5 hours of UV-irradiation. Instead, 45.9% and 100% of MB degradation was achieved using 160 mg L À1 of TiO 2 anatase and aeroxide P25, respectively. The degradation products were studied by mass spectrometry (MS) and the results evidenced the formation of azure B, A, C and phenothiazine. The reuse of the magnetic nanocomposites (i.e., after one photodegradation cycle) allowed a maximum photo-degradation capacity of 65%. The results suggested that the nanocomposite has 10% less photodegradation capacity than the widely used catalytic agents such as TiO 2 aeroxide P25.

Size-exclusion chromatography (SEC) associated with High Performance Liquid Chromatography (HPLC) is a powerful tool to separate, purify and fractionalize materials based on size. In this paper, we present a SEC method that was developed for the separation of thiol-capped Cd(Se,S) quantum dots (QDs) synthesized in the aqueous phase. A HPLC system with a SEC column and a cascade of three detectors (UV/Vis, FLD and ELSD) was used to analyze the different size fractions of QDs. Nanocrystals-HPLC column interactions were suppressed using thioglycolic acid (TGA) as an ion pair agent. Five fractions, namely F1 to F5, of different sizes and tunable optical properties were isolated from the original QDs sample. The emission peaks for fractions F1 and F2 were red-shifted and the fractions F4 and F5 were blue-shifted compared with the original sample, which suggested the presence of nanocrystals having different sizes. Dynamic Light Scattering (DLS) confirmed that collected fractions exhibited different hydrodynamic diameters ranging from 98.2 nm (fraction F1) to 24.9 nm (fraction F5). Also, fractions F2 and F4 were functionalized with glutathione and analyzed by HPLC-SEC. Glutathione-capped Cd(Se,S) QDs showed an increase in their molecular weight, when compared to bare TGA-capped Cd(Se,S) QDs, without a remarkable change of their crystal size and optical properties. The developed SEC technique allows a fast and reproducible separation of water-stable Cd(Se,S) QDs. Collected fractions with tunable optical properties could have potential applications in the nanotechnology area such as bio-imaging and diagnostics, e.g. cell sorting.

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