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Artificial enzyme mimics based on nanomaterials have attracted sustained attention owing to their multiple advantages compared with natural enzymes. However, there are a few enzyme self-cascade systems for the highly sensitive detection of analytical targets. Herein, we have described a self-cascade catalytic system based on single-component cupric oxide nanoparticles (CuO NPs) for an ultrasensitive fluorescent detection toward glutathione (GSH) and Ag + ions. The limit of detection is lower for nanomolar (nM) and picomolar (pM) levels for GSH and Ag + , respectively. To the best of our knowledge, for the first time, we find that CuO NPs possess the intrinsic GSH-oxidase and peroxidase-like activity as a dual-functional nanozyme, coupling with terephthalic acid (TA) and GSH to construct a self-cascade fluorescent system. The turn-on fluorescence signal of oxidation hydroxyterephthalate (TAOH) is generated in the presence of GSH. Then, the fluorescence of a reaction mixture is quenched after the addition of Ag + ions, operating as a turn-off switch. The turn-on−off switch allows the analysis of GSH and Ag + ions by a change of fluorescence status. The detection limits are 32 nM and 37 pM for GSH and Ag + , respectively. To the best of our knowledge, the approach presented in this work shows the highest sensitivity for Ag + detection among all reported fluorescent/colorimetric methods. Moreover, there is no obvious interference with the addition of other interferences without a masking agent. Our study opens a new avenue for the use of a single nanomaterial as an artificial enzyme self-cascade catalytic system for highly sensitive target analysis in biosensor, diagnosis, and environmental fields.

In this paper, we report a colorimetric sensor for the rapid, selective detection of ascorbic acid (AA) in aqueous solutions. Single-layered MnO nanosheets were established as an artificial oxidase; consequently colorless 3,3´,5,5´-tetramethylbenzidine (TMB) was oxidized to a blue product (oxTMB), with increase in absorbance at 650 nm. The absorbance of the reaction system decreased after introduction AA, which reduced MnO into Mn . Under optimum conditions, a detection limit of 62.81 nM for AA in aqueous solutions could be achieved. The linear response range for AA was 0.25-30 μM with a correlation coefficient of 0.996. Importantly, the MnO nanosheet-TMB chromogenic reaction exhibited great selectivity as there was no interference from other metal ions, amino acids and small biological molecules. The proposed colorimetric sensing of AA could be applied for fruit, juice and pharmaceutical samples. Moreover, the proposed sensor showed satisfying performance, including low cost, easy preparation, rapid detection, and good biocompatibility.

Current ambient ionization sources for mass spectrometry (MS) are typically connected to gas cylinders, high-voltage supply, injection pump, and other accessory equipment, which hinder the popularization of MS in the field of on-site detection. Here, we developed a wireless pocket-size "MasSpec Pointer" (weights 65 g) based on arc discharge powered by a 3.7 V polymer Li battery for ambient ionization MS. A high voltage of 5600 V and 20 kHz was generated from the boost coil to penetrate air and form a plasma. The relative standard deviation (RSD) of the high-voltage pulses is 3.8%, leading to a stable discharge and a good quantification performance. A mini diaphragm pump was used to cool the plasma from ∼600 to ∼40 °C and to blow the plasma into a jet, which facilitates sampling. MasSpec Pointer can work well at both positive-and negative-ion modes without any modification and can quickly test gaseous, liquid, or solid samples. The limit of detection of this device for atrazine (an agrochemical) is lower than 0.1 ng/mL. MasSpec Pointer has shown its ability to pinpoint the double-bond location of fatty acid isomers without derivatization reagents or light illumination. Agrochemicals from the surface of an apple and daily chemicals from the surface of a finger were detected successfully using MasSpec Pointer coupled with a miniature mass spectrometer. We believe the "point-and-shoot" device coupled with mini-MS brings the hope for an age of detecting chemicals on-site by nonprofessionals.

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A simple, visible and smartphone readable strategy has been proposed for the sensitive detection and discrimination of multiple proteins. By employing five different concentrations of NaCl salt, AuNP exhibited different aggregation behavior for different proteins because of differential ion strength, leading to diverse color changes. The sensing system could not only discriminate twelve proteins at the concentration of 50 nM in aqueous solution, but it could also discriminate these proteins at 200 nM in the presence of human urine with an accuracy of 100%. More importantly, based on the theory of chromatics, we could directly read out the color value using a smartphone to distinguish twelve proteins, pure Lys and HSA at various concentrations, and the mixture of these two proteins in the presence of human urine with no confusion after a hierarchical clustering analysis (HCA). The inexpensive and convenient colorimetric sensor array using the ubiquitous smartphone for signal readout has great potential for the point-of-care diagnosis without additional devices.

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