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In this study, we have developed a sensitive approach to measure organophosphorus pesticides (OPs) using graphitic-phase C3N4 nanosheets (g-C3N4) combined with a nanomaterial-based quencher, MnO2 nanosheets (MnO2 NS). Since MnO2 NS can quench the fluorescence of g-C3N4 via the inner-filter effect (IFE), enzymatic hydrolysate (thiocholine, TCh) can efficiently trigger the decomposition of MnO2 nanosheets in the presence of acetylcholinesterase (AChE) and acetylthiocholine (ATCh), resulting in the fluorescence recovery of g-C3N4. OPs, as inhibitors to AChE activity, can prevent the generation of TCh and decomposition of MnO2 nanosheets while exhibiting fluorescence quenching. Therefore, the AChE-ATCh-MnO2-g-C3N4 system can be utilized to quantitatively detect OPs based on g-C3N4 fluorescence. Under optimal conditions, the linear ranges for the determination of parathion-methyl (PM) and 2,2-dichlorovinyl dimethyl phosphate (DDVP) were found to be 0.1–2.1 ng/mL and 0.5–16 ng/mL, respectively, with limits of detection of 0.069 ng/mL and 0.20 ng/mL, respectively. The advantages of this assay are user-friendliness, ease of use, and cost effectiveness compared to other more sophisticated analytical instruments. [ABSTRACT FROM AUTHOR]

Iron, cobalt and nitrogen co-doped carbon dots (FeCo/N-CDs) were fabricated firstly by a one pot hydrothermal method in this study. As well as exhibiting peroxidase-like activity, the prepared FeCo/N-CDs emitted fluorescence at 475 nm upon the excitation of 360 nm. A dual-signal biosensor relied on ratiometric fluorescence and colorimetric method was developed for detecting uric acid (UA) based on FeCo/N-CDs-catalyzed oxidation of OPD. Uricase catalyzed the production of H 2 O 2 from UA, and FeCo/N-CDs catalyzed H 2 O 2 into the •OH radicals, contributing to the oxidation of 2,3-diaminophenazine (DAP) from o -phenylenediamine (OPD). The color from colorless to yellow was observed and DAP can quench the fluorescence of FeCo/N-CDs via the inner filter effect (IFE), accompanied with a significant fluorescence enhancement of DAP at 580 nm. The fluorescence emission and UV–vis absorption intensities were thus affected by sequential additions of UA. Quantitative analysis of UA was based on the fluorescence intensity of DAP/FeCo/N-CDs (I 580 / I 475) and color records. The calibration curve was linear within the range of 0.2–150 μM and the limit of detection was low as 0.05 μM. The biosensor was successfully applied to the detection of UA in serum and urine samples. • Firstly, bifunctional FeCo/N-CDs were prepared by a one pot hydrothermal method with low cost and convenient. • A rapid and accurate ratiometric fluorescence and colorimetric detection for UA was established based on tFeCo/N-CDs. • By Comparison, the sensor performed an improvement with higher fluorescence quantum yield and highly catalytic activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • A enzyme-free sensing platform for glyphosate was constructed by dual read-out assay based on ultrathin g-C 3 N 4 nanosheets. • Ultrathin g-C 3 N 4 nanosheets exhibit enhanced fluorescence and more stability compared to bulk g-C 3 N 4 to detect glyphosate. • The assay can be carried out under common conditions and are free of influence by the deactivation of the enzyme variability. Herein, a sensing platform for the rapid quantitative detection of glyphosate with high sensitivity and selectivity was constructed by enzyme-free ratiometric fluorescence and colorimetric dual read-out assay based on ultrathin g-C 3 N 4 nanosheets (UCN NS). Fluorescent diaminophenylazine (DAP) with fluorescence was from the oxidation of o -phenylenediamine (OPD) in the presence of Cu2+. DAP could inspire a significant fluorescence quenching of nanosheets due to photo-induced electron transfer (PET), therefore exhibiting a ratiometric fluorescence response. In the presence of glyphosate, the strong chelation of Cu2+ by glyphosate inhibited the oxidation of OPD, which reduced the fluorescence intensity of DAP, while the fluorescence of the ultrathin nanosheets was restored. Moreover, the oxidation of OPD (colorless) to DAP (yellow) was a distinctly visible color change process, which provides well-suited conditions for colorimetric analysis. At the optimum experimental conditions, the detection concentration range of glyphosate was 0.001–0.48 μg/mL, and the limit of fluorometric detection was 0.49 ng/mL (S/N = 3). This assay was simple and easy to use for the detection of glyphosate in food safety and the environment with satisfactory results. [ABSTRACT FROM AUTHOR]