Chloramphenicol (CAP) is a veterinary antibiotic that has been banned due to its severe side effects in humans. Through the application of manure, veterinary antibiotics can enter the soil, where they can be taken up by crops and vegetables and pose a potential health hazard to humans. Thus, it is highly desirable to develop a rapid and sensitive tool for on-site detection of CAP to ensure food safety and to control the abuse of antibiotics. To this end, nitrogen-doped graphene quantum dots (N-GQDs) were successfully prepared via microwave-assisted synthesis using citric acid and urea as carbon and nitrogen sources, respectively. Analytical results suggested that the interaction between N-GQDs and CAP could occurs via π-π stacking, which quenched N-GQD fluorescence. CAP spiked into chicken feed could be rapidly extracted with ethanol and quantified based on N-GQD fluorescence quenching without further separation. This method showed good recovery (97-102.6%), a low detection limit (1.8 ppm), and was not affected by interference from florfenicol, and thiamphenicol, legal substitute antibiotics. This method has excellent potential for determination of CAP in livestock feed and soil. K E Y W O R D Schloramphenicol, fluorescence quenching, graphene quantum dots, nitrogen doping

Tyrosinase inhibitors from natural products have applications in the pharmaceutical, food, and cosmetic industries because of the functions of tyrosinase in skin disorders and in the enzymatic browning of fruits. Current in vitro inhibitor screening assays are based on the inhibition of the oxidation of l‐3,4‐dihydroxyphenylalanine (l‐DOPA) mediated by a mushroom tyrosinase. However, in these assays, a tyrosinase inhibitor or an antioxidant could inhibit dopaquinone formation. In this study, we aimed to eliminate this ambiguity by using a microplate assay integrating tyrosinase‐immobilized magnetic nanoparticles (TYR‐MNPs) and a homemade magnetic microplate for high‐throughput screening. After incubating extracts of natural products with TYR‐MNPs, the magnetic nanoparticles are attracted to the bottoms of wells, the extracts are rinsed, and TYR‐MNPs react with l‐DOPA. This method can be used to screen compounds that interact with the active sites of the enzyme, or copper chelators that bind more strongly than tyrosinase to copper ions, distinguishing them from antioxidants or tyrosinase substrates. Integration with the homemade magnetic microplate enables high‐throughput inhibitor screening. Aloe vera flowers are crop by‐products, and litchi flowers fall after the blossom. Our work demonstrated that these flowers have tyrosinase inhibitory effects, thus increasing their value.

BackgroundThe enzyme-linked immunosorbent assay (ELISA) has been used for diagnosing medical and plant pathologies. In addition, it is used for quality-control evaluations in various industries. The ELISA is the simplest method for obtaining excellent results; however, it is time consuming because the immunoreagents interact only on the contact surfaces. Antibody-labeled magnetic particles can be dispersed in a solution to yield a pseudohomogeneous reaction with antigens which improved the efficiency of immunoreaction, and can be easily separated from the unreactive substances by applying a magnetic force. We used a homemade magnetic microplate, functional magnetic particles (MPs) and enzyme-labeled secondary antibody to perform the sandwich ELISA successfully.ResultsUsing antibody-labeled MPs enabled reducing the analysis time to one-third of that required in using a conventional ELISA. The secondary antibody conjugated with horseradish peroxidase (HRP) was affinity-bound to the analyte (IgG in this study). The calibration curve was established according to the measured absorbance of the 3, 3′, 5, 5′-tetramethybezidine–HRP reaction products versus the concentrations of standard IgG. The linear range of IgG detection was 114 ng/mL–3.5 ng/mL. The limit of detection (LOD) of IgG was 3.4 ng/mL. The recovery and coefficient of variation were 100% (±7%) and 116% (±4%) for the spiked concentrations of 56.8 ng/mL and 14.2 ng/mL, respectively.ConclusionPseudohomogeneous reactions can be performed using functional MPs and a magnetic microplate. Using antibody-labeled MPs, the analysis time can be reduced to one-third of that required in using a conventional ELISA. The substrate–enzyme reaction products can be easily transferred to another microplate, and their absorbance can be measured without interference by light scattering caused by magnetic microbeads. This method demonstrates great potential for detecting other biomarkers and in biochemical applications.Graphical AbstractA magnetic ELISA with convenient magnetic microplate.Electronic supplementary materialThe online version of this article (doi:10.1186/s13065-015-0088-1) contains supplementary material, which is available to authorized users.

A high-throughput and selective method for the determination of ʟ-dopa (levodopa) in complex formulations was developed. The method is based on the oxidation of ʟ-dopa to yield dopachrome using tyrosinase-labeled magnetic nanoparticles (TYR-MNPs) as the oxidation catalyst. TYR-MNP activity was retained at 75% after 20 reuse cycles, which is superior to previously reported systems that employ other substrates or cross-linkers for the immobilization of tyrosinase. In addition, the precision (< 3%), accuracy (recovery = 95-102%), and selectivity of the newly developed quantitative-analysis method for ʟ-dopa in complex polypill formulations meets the pharmaceutical industry's quality-control requirements; consequently this method can be applied to the routine analysis of complex formulations. The quality-control assay uses 96well microplates, which reduces the required volume of reagents, and the tyrosinase can easily be recycled and reused using an in-house-prepared magnetic microplate, thereby rendering the proposed method economical and less wasteful than existing methods.