The self-assembled three-dimensional graphene nanohybrids with in situ-formed Fe3O4 and Pd nanoparticles on it (3DRGO_Fe3O4-Pd) are first synthesized by the one-pot solvothermal method, which have intrinsic peroxidase-like and oxidase-like activity. The catalytic mechanism is analyzed by the electron spin resonance (ESR), fluorescence, and electrochemical methods. The mimic enzyme catalytic activity of 3DRGO_Fe3O4-Pd is much higher than those of monometallic loaded nanohybrids and their physical mixture, probably caused by synergistic effect between Pd and Fe3O4 nanoparticles. The 3DRGO_Fe3O4-Pd nanohybrids was reproducible, stable, and reusable. After 10 cycles, the catalytic activity was still higher than 90%, and the morphology and structure were basically unchanged. Based on its high peroxidase-like activity, especially the enhanced affinity toward H2O2, a new colorimetric detection method for reduced glutathione (GSH) and glucose has been designed using H2O2 as an intermediary, which provides a simple, sensitive, and selective way to detect urine glucose of diabetes with a wide linear range and low detection limit.Keywords: colorimetry; dual-enzyme activity; glucose; one-pot; three-dimensional graphene nanohybrids

Fluorescence sensing of specific biological molecules by artificial chemosensors is a versatile technique. In the present work, a switch-on fluorescence sensor for rapid, sensitive, and selective sensing of glutathione (GSH) in food samples was developed. This method was based on the g-CNQDs–Hg2+ system, in which the initial fluorescence from g-CNQDs was quenched by Hg2+ with an electron transfer process. In the presence of GSH, the fluorescence sensor was switched to the “on” state, which was attributed to a competitive affinity of Hg2+ to GSH and the functional groups on the surface of g-CNQDs. Under the optimal conditions, the limit of detection (LOD) of 37 nM for GSH was achieved with a wide range of 0.16–16 μM. The repeatability was better than 5.3% for GSH in both standard and food samples (n = 3). Finally, this fluorescence sensor was successfully employed for the determination of GSH in various kinds of food samples with excellent recoveries. Furthermore, this application may pave a new way for fluorescence sensing of other substances in food samples.

•The CoxFe3−xO4 MNPs were firstly prepared by a simple coprecipitation method.•CoxFe3−xO4 MNPs could effectively catalyze the reaction between TMB and H2O2.•This colorimetric analytical method was convenient, economic and speedy.•The method had been applied to detection of DA in Shan Yao and human serum sample.In this study, cobalt doped magnetic composite nanoparticles (CoxFe3−xO4 MNPs) were firstly prepared through a simple and convenient coprecipitation approach. The characterization results from EDX, ICP-AES, TEM, XRD and XPS showed that the cobalt atoms might be located in the lattice position instead of the part of iron atoms. CoxFe3−xO4 MNPs possessed higher peroxidase-like activity comparing with Fe3O4 MNPs, although they were similar in crystal structure, size distribution and morphology. The as-prepared nanomaterials could effectively catalyze the reaction between 3,3,5,5-tetramethylbenzidine (TMB) and H2O2 under 40 °C within 15 min. Dopamine (DA) has some reducibility due to the existence of phenol hydroxyl group, which results in it can consume H2O2 and cause the blue shallowing of the reaction solution between H2O2 and TMB. A visual, sensitive and simple colorimetric method based on CoxFe3−xO4 MNPs as peroxidase mimetics was developed for detecting DA. Good linear relationship and recoveries for DA were obtained from 0.6 to 8.0 μM and 98.7 to 101.0%, respectively. The limit of detection (LOD) of the proposed method was calculated as 0.13 μM. And the relative standard deviation (RSD) was less than 4.0%. The visual method was successfully applied to DA detection in Common Yam Rhizome (Shan Yao) and human serum sample.CoxFe3−xO4 was proved to possess higher peroxidase-like activity comparing with Fe3O4 MNPs. It could effectively catalyze the reaction between 3,3,5,5-tetramethylbenzidine (TMB) and H2O2 under 40 °C within 15 min. So this proposed method was used for measuring dopamine. The color variation was very obvious on visual observation, which offered a convenient approach to detect DA by naked eye.

•It was the first time to apply Mn–ZnS QDs luminescence sensing system to detect Sudan dyes.•No complex process and expensive instruments except fluorescence spectrophotometer were needed.•The detect method was novel, simple and convenient.•The method showed high selectivity toward Sudan dyes detection.A novel luminescence sensing system has been designed for Sudan dyes detection based on the quenched fluorescence (FL) of oleic acid-functionalized Mn–ZnS quantum dots (QDs). The Sudan dyes could quench the fluorescence signal of Mn–ZnS QDs due to the so-called inner filter effect. Herein, it was demonstrated that this facile methodology could offer a rapid, reliable and selective detection of Sudan dyes, and the detection limits of Sudan I, II, III, and IV were 24.6, 32.7, 2.1, and 3.2 ng/mL, respectively. In this work, Sudan dyes were mainly detected in hot chili sauce, sausage, egg and tomato sauce, and Sudan I was chosen as spiked substance owning to its wide use as a coloring matter among Sudan dyes in many industrial and daily products. Furthermore, the recoveries of Sudan I from the standard fortified blank samples were in a range of 93.0–107.0%, and the relative standard deviations (RSDs) were lower than 2.8% (n = 3).Initially, the free Mn–ZnS QDs capped with oleic acid showed strong FL in solution. However, the FL of Mn–ZnS QDs was quenched significantly in the presence of Sudan dyes through the inner filter effect.

Study of the interaction between butyl p-hydroxybenzoate (butoben) and human serum albumin (HSA) has been performed by molecular modeling and multi-spectroscopic method. The interaction mechanism was predicted through molecular modeling first, then the binding parameters were confirmed using a series of spectroscopic methods, including fluorescence spectroscopy, UV–visible absorbance spectroscopy, circular dichroism (CD) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. The thermodynamic parameters of the reaction, standard enthalpy ΔH0 and entropy ΔS0, have been calculated to be −29.52 kJ mol−1 and −24.23 J mol−1 K−1, respectively, according to the Van’t Hoff equation, which suggests the van der Waals force and hydrogen bonds are the predominant intermolecular forces in stabilizing the butoben–HSA complex. Results obtained by spectroscopic methods are consistent with that of the molecular modeling study. In addition, alteration of secondary structure of HSA in the presence of butoben was evaluated using the data obtained from UV–visible absorbance, CD and FT-IR spectroscopies.Research highlights► The interaction between butyl p-hydroxybenzoate with HSA has been investigated for the first time. ► Molecular modeling study can provide theoretical direction for experimental design. ► Multi-spectroscopic method can provide the binding parameters and thermodynamic parameters. ► These results are important for food safety and human health when using parabens as a preservative.