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A one-pot method based on cyclic voltammetric scan was used to fabricate a glassy carbon electrode modified with nanocomposites consisting of poly(thionine) and amino-functionalized graphene quantum dots (afGQDs). Under near-neutral conditions, the dye polymer was effectively oxidized by hydroxyl radicals (·OH) that were derived from the copper-catalyzed Fenton-like reaction, and the cathodic peak current on the modified electrode greatly increased. The reaction of Cu2+ with thiourea (TU) and the generation of a complex, CuTU2+, led to the decrease of Cu2+/Cu+ species, which inhibited the Fenton-like reaction and reduced the electrochemical response change. Due to a displacement reaction, the addition of Hg2+ into the H2O2-Cu2+-TU system resulted in the release of cuprous ions that benefited the Fenton-like reaction. Under the following optimal conditions: 6 mg mL−1 afGQDs and the 25-cycle potential cycling for the fabrication of the modified electrode, pH 6.5, and the c Cu 2 + / c TU ratio of 1.0, the increasing extent of the cathodic peak current exhibited a good linear response to the logarithm of the Hg2+ concentration in the range of 1 pM–1 μM with a detection limit of 0.6 pM. Mercury ions in a water sample were determined with good recovery, ranging from 97 to 103%. The investigation on the uptake of Hg2+ into human vascular endothelial cells, HUVEC, shows that the cells incubated in the high-concentration glucose medium absorbed more mercury ions than HUVEC incubated in the normal medium. As a result, Hg2+ could lead to the greater damage to the former. [ABSTRACT FROM AUTHOR]

Copper is an important trace element involved in several physiological processes. The deficiency or excess of Cu in the human body may cause some serious diseases. EDTA has been widely employed in many industry fields owing to its excellent chelating ability. The poor biodegradability of EDTA makes itself a persistent substance in the natural environment. This work provided a fluorescence "on–off–on" strategy for the sequential determination of trace Cu2+ and EDTA. Amino-functionalized graphene quantum dots (afGQDs) were synthetized via the thermal pyrolysis of citric acid. Fluorescence resonance energy transfer (FRET) between afGQDs and 1-(2-pyridylazo)-2-naphthol (PAN) effectively quenched the fluorescence of this carbon-based nanomaterial. The generation of the Cu2+-PAN complex caused the increased FRET efficiency and the further fluorescence decline. The change of the fluorescence intensity sensitively responded to copper ions. The linear range and the limit of detection (LOD) were 1 nM-10 μM and 0.87 nM, respectively. EDTA could decompose the Cu2+-PAN complex and liberate PAN, which weakened the FRET efficiency and led to the fluorescence recovery. The increasing degree of the fluorescence intensity was closely related to EDTA within a concentration range from 10 nM to 10 μM with a LOD at 4 nM. Copper ions in the water and human serum samples and EDTA in the trypsin-EDTA sample were successfully detected based on the proposed fluorescence method. [ABSTRACT FROM AUTHOR]

A new method based on fluorescence spectroscopy for the sensitive determination of aminophylline (AP), an antiasthmatic drug, was developed in this work. Amino-functionalized graphene quantum dots (afGQDs) were synthesized based on a two-step method and they were characterized by transmission electron microscope, UV–vis absorption spectrum and infrared spectrum. The fluorescence of afGQDs was quenched by riboflavin (Rf) via both dynamic quenching and inner filter effect. Photoilluminated Rf-AP system in the presence of oxygen produced hydroxyl radicals (OH). The latter accepted electrons from afGQDs owing to a photo-induced electron transfer process and led to the further fluorescence decline. The changing extent of the fluorescence intensity was found to be proportional to the concentration of AP in the range of 0.10–10 μg mL−1 and the limit of detection arrived at 40 ng mL−1. The proposed method was successfully employed for the determination of AP in a pharmaceutical sample and the recovery rate varied in the range of 99%–106%. Unlabelled Image • The fluorescence of afGQDs was quenched by riboflavin due to dynamic quenching and IFE. • Photoilluminated riboflavin-aminophylline system produced hydroxyl radicals. • PET from afGQDs to OH led to the further decline of the fluorescence intensity. • A new method for the sensitive determination of aminophylline was developed. [ABSTRACT FROM AUTHOR]