An "on-off" ratio photoluminescence sensor based on catalytically induced PET effect by Fe3O4 NPs for the determination of coumarin

A molecularly imprinted ratiometric fluorescence sensor based on blue/red carbon quantum dots for the visual determination of thiamethoxam

Neonicotinoids such as thiamethoxam (TMX) were widely used in agricultural production and tended to accumulate in the environment, potentially harming human and ecosystem health. To enable widespread monitoring of TMX residues, it was essential to design a reliable and sensitive detection method. Here, we developed a novel smartphone-enablled molecularly imprinted ratiometric fluorescence sensing system for selective on-site detection of TMX. It was based on blue-emission carbon dots (CDs) wrapped with a molecularly imprinted layer (B-CDs@MIPs), which provided the response signal, while red-emission CDs (R-CDs) served as an internal reference. The fluorescence signal ratio of the sensor increased with the TMX concentration, resulting in an obvious fluorescence color change from red to blue. The sensor exhibited a satisfactory limit of detection (LOD) of 13.5 nM in fluorescence analysis while LOD of 70.1 nM in visual determination. In addition, the sensing system was validated using food and environment samples, exhibiting recoveries from 91.40% to 105.7%, indicating excellent reliability for TMX detection in actual samples. Thus, the sensing system developed in this study offered promising prospects for visual detection of pesticide residues in complex environmental samples.

Ratio fluorescence test strip visualized by amino-functionalized metal-organic framework for rapid sensing of glyphosate

As glyphosate is a broad-spectrum herbicide extensively used in agriculture worldwide, rapid glyphosate detection is essential for food safety and human health. Herein, a ratio fluorescence test strip was prepared and coupled with an amino-functionalized bismuth-based metal-organic framework (NH₂-Bi-MOF) that bonded with copper ion for rapid visualization and determination of glyphosate. NH₂-Bi-MOF had excellent fluorescence performance, and the copper ion, a Lewis acid, was selected as the quencher. The strong chelation of glyphosate with copper ion and its quick interaction with NH₂-Bi-MOF would turn on the fluorescence signal, thus enabling the quantitative sensing of glyphosate, with a linear range of 0.10-200 µmol L^-1, and recoveries between 94.8% and 113.5%. The system was then expanded to a ratio fluorescence test strip, in which the fluorescent ring sticker was set as a binding-in self-calibration to reduce errors from the angle and light dependency. The method realized the visual semi-quantitation referring to a standard card, as well as the ratio quantitation using the gray value output with LOD of 0.82 µmol L^-1. And the as-developed test strip was accessible, portable, and reliable, thus offering a platform for the rapid on-site detection of glyphosate and other residual pesticides.

A biosensor based on Fe3O4@MXene-Au nanocomposites with high peroxidase-like activity for colorimetric and smartphone-based detection of glucose

A novel magnetic nanozyme Fe₃O₄@MXene-Au nanocomposite, which possessed higher peroxidase-like activity than that of Fe₃O₄ nanoparticles and Fe₃O₄@MXene nanocomposites, was developed. The outstanding magnetic properties of the nanozyme endowed it with the ability of simple and rapid separation, achieving great recyclability. Based on Fe₃O₄@MXene-Au nanocomposites and glucose oxidase (Glu Ox), a highly selective colorimetric biosensor for glucose detection was developed. Fe₃O₄@MXene-Au nanocomposites can catalyze H₂O₂ produced from glucose catalyzed by glucose oxidase to ·OH and oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB) with a significant absorbance at 652 nm. The linear range of glucose was 0-1.4 mM under optimal conditions, with a limit of detection (LOD) of 0.11 mM. Glucose in human whole blood was successfully detected with satisfactory recoveries. Furthermore, a facile agarose hydrogel detection platform was designed. With smartphone software, glucose detection can be realized by the agarose hydrogel platform, demonstrating the potential in on-site and visual detection of glucose.

Highly selective detection of epinephrine by a "turn-off" fluorescent sensor based on N-doped carbon quantum dots

Epinephrine (EP) is a catecholamine hormone with a variety of physiological activities. Monitoring the concentration of EP in drugs, food, biological samples and cosmetics is of great significance for their quality control. Herein, a novel fluorescence sensing method was designed for the high-specificity detection of EP based on N-doped carbon quantum dots (N-CDs). The EP could interact with the fluorescent senor of N-CDs which emits blue fluorescence to produce concentration- dependent fluorescence quenching through the photo-induced electron transfer (PET). The established sensing method has good linearity in the range of 0.5-10 μM with the LOD of 0.15 μM. More importantly, it is highly selective because similar components with phenolic hydroxyl groups or primary amino groups, even norepinephrine (NEP), could not interfere with the detection. This method can provide a low-cost, rapid and simple new way for the detection of EP, and has a good application prospect in point-of-care assay and in situ test.

Facile preparation of Fe3O4@Pt nanoparticles as peroxidase mimics for sensitive glucose detection by a paper-based colorimetric assay

A simple strategy to rapidly detect glucose was developed by utilizing core (Fe₃O₄)-shell (Pt) magnetic nanoparticles (Fe₃O₄@Pt NPs) as a nanoenzyme and a paper-based colorimetric sensor. In the presence of H₂O₂, Fe₃O₄@Pt NPs catalyze the redox reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and generate a colour change from colourless to blue. On this basis, a colorimetric glucose sensing method assisted by glucose oxidase (GOx) was developed. Under the optimal conditions, the detection limits of the proposed assay for H₂O₂ and glucose were 0.36 µM and 1.27 µM, respectively. Furthermore, the fabricated colorimetric method was successfully applied to analyze glucose concentrations by using a paper device as a measuring platform without a spectrometer. In addition, this method exhibited satisfactory recovery for glucose detection in human serum samples and urine samples, which satisfied the requirements for normal detection of real samples. This study provides a good candidate for health monitoring of glucose and also expands the applications of nanoenzymes and paper-based colorimetric assays in point-of-care testing.

Ratiometric fluorescence sensing with logical operation: Theory, design and applications

The construction of ratiometric fluorescence sensing logic systems has gradually become a hot topic in fluorescence analysis, due to the multi-target analysis potential of logic operations and the high specificity and selectivity of ratiometric fluorescence sensing. In this paper, the basic principles of various logic functions implemented in ratiometric fluorescence detection are discussed in the context of sensing mechanisms, and the strategies for constructing logic systems in different ratiometric fluorescence sensing application areas are summarized. Although there are limitations such as cumbersome operations and complicated experiments, ratiometric fluorescence sensing logic circuits that combine the visualization of logic operations and the accuracy of ratiometric fluorescence are still worthy of in-depth study. This review may be useful for researchers interested in the construction of logic operations based on ratiometric fluorescence sensing applications.