Fe₃O₄@MoS₂ Core–Shell Composites: Preparation, Characterization, and Catalytic Application

Molybdenum disulfide (MoS₂) has received tremendous attention due to the earth-abundant composition and high catalytic activity. However, the catalytic activity of MoS₂ except electro- and photocatalytic has seldom been explored. Herein, Fe₃O₄@MoS₂ core–shell composites were prepared for the first time by <i>in situ</i> growth of MoS₂ nanosheets on the surfaces of Fe₃O₄ nanoparticles under different temperature, and the catalytic performance of the resulting composites was evaluated by using the catalytic reduction of 4-nitrophenol to 4-aminophenol. FE-SEM, TEM, XRD, and XPS analyses verified the core–shell structure with MoS₂ nanosheets of defect-rich and oxygen incorporation on the surfaces of Fe₃O₄ nanoparticles. Fe₃O₄@MoS₂ composites were found to exhibit a high catalytic activity for the reduction of 4-nitrophenol with the highest activity factor <i>k</i> = 3773 min^–1 g^–1. A plausible catalytic mechanism for the reduction of 4-nitrophenol was also proposed. This study presents an inexpensive, reusable, fast, and highly efficient catalyst for the reduction of 4-nitrophenol without noble metals

Visual Monitoring of Food Spoilage Based on Hydrolysis-Induced Silver Metallization of Au Nanorods

Colorimetric detection of biogenic amines, well-known indicators of food spoilage, plays an important role for monitoring of food safety. However, common colorimetric sensors for biogenic amines suffer from low color resolution or complicated design and intricate output for the end-users. Herein, we explored a simple but effective strategy for visual monitoring of biogenic amines with multiple color change based on hydrolysis-induced silver metallization reaction to tune the localized surface plasmon resonance (LSPR) adsorption of Au nanorods (NRs). The color change and blue shift of longitudinal LSPR peak of Au NRs were closely related to the concentration of biogenic amines. This strategy provided a simple, sensitive, robust, nondestructive, cost-effective, and user-friendly platform for in situ evaluating the freshness of foodstuffs

No Structure-Switching Required: A Generalizable Exonuclease-Mediated Aptamer-Based Assay for Small-Molecule Detection

The binding of small molecules to double-stranded DNA can modulate its susceptibility to digestion by exonucleases. Here, we show that the digestion of aptamers by exonuclease III can likewise be inhibited upon binding of small-molecule targets and exploit this finding for the first time to achieve sensitive, label-free small-molecule detection. This approach does not require any sequence engineering and employs prefolded aptamers which have higher target-binding affinities than structure-switching aptamers widely used in current small-molecule detecting assays. We first use a dehydroisoandrosterone-3-sulfate-binding aptamer to show that target binding halts exonuclease III digestion four bases prior to the binding site. This leaves behind a double-stranded product that retains strong target affinity, whereas digestion of nontarget-bound aptamer produces a single-stranded product incapable of target binding. Exonuclease I efficiently eliminates these single-stranded products but is unable to digest the target-bound double-stranded product. The remaining products can be fluorescently quantified with SYBR Gold to determine target concentrations. We demonstrate that this dual-exonuclease-mediated approach can be broadly applied to other aptamers with differing secondary structures to achieve sensitive detection of various targets, even in biological matrices. Importantly, each aptamer digestion product has a unique sequence, enabling the creation of multiplex assays, and we successfully demonstrate simultaneous detection of cocaine and ATP in a single microliter volume sample in 25 min via sequence-specific molecular beacons. Due to the generality and simplicity of this assay, we believe that different DNA signal-reporting or amplification strategies can be adopted into our assay for target detection in diverse analytical contexts