A Selective Fluorescent Nanoprobe Based on Graphene Quantum Dots and Hg2+ for the Determination of Tetracycline in Biological Samples.

A simple, sensitive, and selective fluorometric method based on graphene quantum dots and Hg2+ is presented for the determination of tetracycline. The fluorescence emission of graphene quantum dots at 463 nm decreased in the presence of Hg2+ ions due to its electrostatic interaction with the negatively charged surface of quantum dots at pH = 8.0. The addition of tetracycline to this system resulted in the retrieval of the fluorescence emission of the graphene quantum dots proportional to the tetracycline concentration. This is because of the interaction between tetracycline and Hg2+ that results in the release of the quantum dots' surface. Under the optimized conditions, the calibration curve indicated good linearity in the range of 2.0-44.0 nmol L-1 with a detection limit of 0.52 nmol L-1 for tetracycline. The designed nanoprobe was capable of the determination of tetracycline in serum and urine samples.

Selective and Sensitive Fluorometric Determination of Piroxicam Based on Nitrogen-doped Graphene Quantum Dots and Gold Nanoparticles Coated with Phenylalanine.

In this study, a sensitive fluorimetric method is proposed for the determination of piroxicam using nitrogen graphene quantum dots (N-GQDs) and gold nanoparticles coated with phenylalanine. The fluorescence emission of N-GQDs at 440 nm decreases with the increase of gold nanoparticles coated with phenylalanine. However, the addition of piroxicam causes the release of gold nanoparticles from the surface of quantum dots followed by the retrieval of the fluorescence emission of N-GQDs. Under the optimum conditions, the calibration graph was linear in the concentration range of 2.0-35.0 nmol L-1 for piroxicam with a limit of detection of 0.11 nmol L-1. The developed method was successfully applied for the determination of piroxicam in urine and serum samples.

Chemiluminescence determination of dopamine using N, P-graphene quantum dots after preconcentration on magnetic oxidized nanocellulose modified with graphene quantum dots.

A novel sorbent consisting of magnetic oxidized nanocelluloses modified with graphene quantum dots was prepared and used for the separation and preconcentration of dopamine. The eluted dopamine from the sorbent was determined by a designed chemiluminescence system containing luminol, H2O2, Fe3+, and graphene quantum dots doped with nitrogen and phosphorus. Graphene quantum dots cause an increase in the intensity of the chemiluminescence system of luminol-H2O2, but the presence of Fe3+ ions in this system decreases its intensity because of the sorption of the Fe3+ ions on the surface of P, N-graphene quantum dots. However, the addition of dopamine resulted in the retrieval of P, N-graphene quantum dots, as well as the chemiluminescence intensity, due to the formation of its complex with Fe3+. The sorbent made of magnetic oxidized nanocelluloses modified with graphene quantum dots was characterized by various analytical techniques, and the effective parameters on the extraction of dopamine were investigated and optimized. Under the optimized conditions, the method displayed good linearity in the concentration range 0.25-17.5 µg L-1 for dopamine (R2 = 0.9918) with a limit of detection of 0.054 µg L-1. The intra- and inter-day relative standard deviations at a 10.0 µg L-1 concentration level of dopamine (n = 6) were 2.6 and 4.1%, respectively. This method was successfully applied to the extraction and determination of dopamine in human serum and urine samples.

Determination of lamotrigine by fluorescence quenching of N-doped graphene quantum dots after its solid-phase extraction using magnetic graphene oxide.

A sensitive fluorescent nanoprobe is reported for the determination of lamotrigine after its preconcentration by magnetic graphene oxide nanocomposite. The fluorescent nanoprobe is based on the quenching effect of lamotrigine on the nitrogen graphene quantum dots fluorescence at 440 nm, through strong hydrogen bonding. Under optimum conditions, the quenching fluorescent intensity of nitrogen graphene quantum dots shows linearity with the lamotrigine concentration in the range of 2.0-45.0 µg L-1, limits of detection (LOD), and quantification of 0.39 and 1.28 µg L-1 respectively. The parameters affecting the extraction and determination of lamotrigine were optimized via the central composite design (CCD) and one at the time method, respectively. The developed method was successfully employed for the extraction and quantification of lamotrigine in biological samples.

Fabrication of a sensitive colorimetric nanosensor for determination of cysteine in human serum and urine samples based on magnetic-sulfur, nitrogen graphene quantum dots as a selective platform and Au nanoparticles.

A novel colorimetric nanosensor is reported for the selective and sensitive determination of cysteine using magnetic-sulfur, nitrogen graphene quantum dots (Fe3O4/S, N-GQDs), and gold nanoparticles (Au NPs). Thus, S, N-GQDs was firstly immobilized on Fe3O4 nanoparticles through its magnetization in the presence of Fe3+ in the alkali solution. The prepared Fe3O4/S, N-GQDs were dispersed in cysteine solution resulting in its quick adsorption on the surface of the Fe3O4/S, N-GQDs through hydrogen bonding interaction. Then, Au NPs solution was added to this mixture that after a short time, the color of Au NPs changed from red to blue, the intensity of surface plasmon resonance peak of Au NPs at 530 nm decreased, and a new peak at a higher wavelength of 680 nm appeared. The effective parameters on cysteine quantification were optimized via response surface methodology using the central composite design. Under optimum conditions, the absorbance ratio (A680/A530) has a linear proportionality with cysteine concentration in the range of 0.04-1.20 µmol L-1 with a limit of detection of 0.009 µmol L-1. The fabrication of the reported nanosensor is simple, fast, and is capable of efficient quantification of ultra traces of cysteine in human serum and urine real samples.

Design of a pseudo stir bar sorptive extraction using graphenized pencil lead as the base of the molecularly imprinted polymer for extraction of nabumetone.

Molecularly imprinted polymer (MIP) was synthesized through the coprecipitation method on the graphene oxide anchored pencil lead as a substrate for the first time and applied as an efficient sorbent for pseudo stir bar sorptive extraction of nabumetone. The extracted analyte was determined by a novel spectrophotometric method based on the aggregation of silicate sol-gel stabilized silver nanoparticles in the presence of the analyte. The synthesized polymer was characterized using Fourier transform infrared spectroscopy and field emission scanning electron microscopy. Optimization of important parameters affecting the extraction efficiency was done using central composite design whereas the spectrophotometric method was optimized via one at a time variable. Under the optimal conditions, the calibration curve exhibited linearity in the concentration range of 1.5-20.0 µg L-1. A limit of detection of 0.20 µg L-1, an enhancement factor of 393 and relative standard deviations (at 10 µg L-1, n = 6) of 4.6% and 8.1% for intra- and inter-day analysis were obtained. The developed procedure was successfully utilized for the quantification of traces of nabumetone in tap water and biological samples with the complex matrix including human urine and serum.

Selective fluorometric determination of sulfadiazine based on the growth of silver nanoparticles on graphene quantum dots.

A sensitive fluorometric assay is described for the direct determination of the antibiotic sulfadiazine. Silver nanoparticles placed on graphene quantum dots (Ag NP-GQDs) were synthesized by reduction of AgNO3 with sodium borohydride in the presence of GQDs. The growth of Ag NPs on the surface of the GQDs causes quenching of the blue fluorescence of the GQDs with an emission maximum at 470 nm by surface-enhanced energy transfer. If sulfadiazine is added, it interacts with Ag NPs and fluorescence is restored. Under optimal conditions, the fluorescence increases linearly in the sulfadiazine concentration range of 0.04-22.0 µM. The detection limit is 10 nM with relative standard deviations of 2.3 and 4.2 (at 10 µM of sulfadiazine; for n = 6) for intra- and inter-day assays. Graphical abstractSchematic representation of sulfadiazine determination using Ag NP-GQDs as a fluorescent nanoprobe.