MnO2 Nanosheet-Carbon Dots Sensing Platform for Sensitive Detection of Organophosphorus Pesticides

A Review on Carbon Dots: Synthesis, Characterization and Its Application in Optical Sensor for Environmental Monitoring

The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.

Sensitive CTC analysis and dual-mode MRI/FL diagnosis based on a magnetic core-shell aptasensor

Synergizing the sensitive circulating tumor cell (CTC) capture, detection, release and the specific magnetic resonance/fluorescence (MR/FL) imaging for accurate cancer diagnosis is of great importance for cancer treatment. Herein, EcoR1-responsive complementary pairing of two ssDNA with a fluorescent P₀ aptamer, which can specifically bind with the overexpressed MUC1 protein on cancer cells, was covalently modified to SiO₂@C-coated magnetic nanoparticles for preparing a special nanoparticle-mediated FL turn-on aptasensor (FSC-D-P₀). This aptasensor can selectively capture/enrich CTC and thus achieve sensitive CTC detection/imaging in even the blood due to its stable targeting, unique magnetic properties and the regulated interactions between the quencher and the fluorescent groups. Meanwhile, FSC-D-P₀ can release the captured CTC for further downstream analysis upon the EcoR1 enzyme-triggered cleavage of the double-stranded DNA (dsDNA). Most importantly, this aptasensor can distinctly avoid false positivity of MRI via multiple targeting mechanisms. Thus, the sensitive CTC capture, detection, release and accurate MR/FL imaging were synergistically combined into a single platform with good biocompatibility, promising a robust pattern for clinical tumor diagnosis in vitro and in vivo.

Ag-EDTA-Zr and Au-EDTA-Zr Nanocomposites for the Quantitative Determination of Some Organophosphate Pesticides in Water and Tomatoes

Background

Organophosphate pesticides (OP) pose risks to health and the environment, and monitoring them is an urgent task.

Objective

Development of a method for their determination in water and food.

Method

A simple photometric method for the determination of some OPs was demonstrated. Ag-EDTA-Zr and Au-EDTA-Zr nanoparticles were synthesized and were applied for the determination of some OPs (dimethoate, chlorpyrifos, and diazinon).

Results

It was demonstrated that silver nanoparticles provide more sensitivity in comparison to gold nanoparticles. As a result, Ag-EDTA-Zr nanoparticles were further applied for the determination of these pesticides in tomatoes.

Conclusions

The lowest detectable concentrations for dimethoate, diazinon, and chlorpyrifos in water reached 0.1 (SD 0.082), 0.4 (SD 0.075), and 0.4 mg/L (SD 0.088), respectively, whereas in tomato the lowest detectable concentrations for these pesticides were 0.5 (SD 0.789), 2 (SD 0.085), and 0.5 mg/L (SD 0.088). The sensitivity of detection in tomatoes is lower than in water; however, these concentrations are comparable with allowable concentrations of OP pesticides.

Highlights

A novel approach for quantitative detection of OPs was developed. Ag-EDTA-Zr and Au-EDTA-Zr nanocomposites interact with these pesticides, and, as a result, optical changes of these nanoparticles take place. These changes correlate with the concentration of pesticides.

Octopamine modulates insect mating and Oviposition

The neuro-mechanisms that regulate insect reproduction are not fully understood. Biogenic amines, including octopamine, are neuromodulators that have been shown to modulate insect reproduction in various ways, e.g., promote or inhibit insect mating or oviposition. In this study, we examined the role of octopamine in regulating the reproduction behaviors of a devastating underground insect pest, the dark black chafer (Holotrichia parallela). We first measured the abundance of octopamine in different neural tissues of the adult chafer pre- and post-mating, demonstrating that octopamine decreased in the abdominal ganglia of females but increased in males post-mating. We then fed the adult H. parallela with a concentration gradient of octopamine to test the effects on insect reproductive behaviors. Compared with its antagonist mianserin, octopamine at the concentration of 2 µg/mL resulted in the highest increase in males' preference for sex pheromone and females' oviposition, whereas the mianserin-treatment increased the survival rate and prolonged the lifespan of H. parallela. In addition, we did not observe significant differences in egg hatchability between octopamine and mianserin-treated H. parallela. Our results demonstrated that octopamine promotes H. parallela mating and oviposition with a clear low dosage effect, illustrated how neural substrates modulate insect behaviors, and provided insights for applying octopamine in pest management.

Glyphosate sensing in aqueous solutions by fluorescent zinc(II) complexes of [9]aneN3-based receptors

Herein we describe the binding abilities of Zn(II) complexes of [12]aneN4- (L1) and [9]aneN3-based receptors (L2, L3) towards the herbicides N-(phosphonomethyl)glycine (glyphosate, H3PMG) and 2-amino-4-[hydroxy(methyl)phosphoryl]butanoic acid (glufosinate, H2GLU), and also aminomethylphosphonic acid (H2AMPA), the main metabolite of H3PMG, and phosphate. All ligands form stable Zn(II) complexes, whose coordination geometries allow a possible interaction of the metal center with exogenous anionic substrates. Potentiometric studies evidenced the marked coordination ability of the L2/Zn(II) system for the analytes considered, with a preferential binding affinity for H3PMG over the other substrates, in a wide range of pH values. 1H and 31P NMR experiments supported the effective coordination of such substrates by the Zn(II) complex of L2, while fluorescence titrations and a test strip experiment were performed to evaluate whether the H3PMG recognition processes could be detected by fluorescence signaling.

Mechanisms for carbon dots-based chemosensing, biosensing, and bioimaging: A review

Due to the favorable biocompatibility, photostability and fluorescence emissions, carbon dots (CDs) are being widely investigated as fluorescent probes. Current CD-based fluorescent probe designs depend largely on conventional fluorescence sensing mechanisms, for e.g. the inner filter effect, photoinduced electron transfer, and Förster resonance energy transfer. Although these mechanisms have been successful, it is still desirable to introduce new sensing mechanisms. In recent years, emerging mechanisms such as aggregation-induced emission, hydrogen-bond induced emission, and intramolecular charge transfer have been developed for CD-based probes. This review summarizes both conventional and emerging mechanisms, and discuss CDs in the context of chemosensing, biosensing, and bioimaging. We provide an outlook for several other mechanisms such as CN isomerization, the short-wavelength inner filter technique, excited-state intramolecular proton transfer, and twisted intramolecular charge transfer, which have been applied to organic fluorescent probes design but not as much in CD-based sensing systems. We envision that this review will provide insights that inspire further development of CD-based fluorescent probes as for biological applications.

A Yellow Fluorescence Probe for the Detection of Oxidized Glutathione and Biological Imaging

It is well-known that the ratio of reduced l-glutathione (GSH) to oxidized l-glutathione (GSSG) is a vital biomarker for monitoring overall cellular health, thus detecting the intracellular concentration of glutathione is of great significance. Recently, an increasing number of reports have published various methods for GSH detection, but studies on the detection of GSSG are still rare. Here, we report a kind of new yellow fluorescent carbon dots (CDs) for the detection of GSSG through a fluorescence "off-on" process. Because the surface is rich in amino groups, the CDs show a positive potential. When the concentration of GSSG was continuously increased, the CDs' fluorescence dropped sharply, while the fluorescence gradually recovered after the addition of sodium sulfide. The phenomenon of fluorescence quenching is linear with the concentration of the quencher (GSSG)(0-200 μM), and 0.18 μM is calculated as the detection limit. More interestingly, as a fluorescent probe, the CDs can be further used for fluorescence imaging in living cells and zebrafish.

Graphitic-phase C3N4 nanosheets combined with MnO2 nanosheets for sensitive fluorescence quenching detection of organophosphorus pesticides

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.

Construction of a copper nanocluster/MnO2 nanosheet-based fluorescent platform for butyrylcholinesterase activity detection and anti-Alzheimer's drug screening

An abnormal level of butyrylcholinesterase (BChE) activity is highly connected with hepatic damage and Alzheimer's disease. Herein, a facile and efficient method was proposed for BChE detection by incorporating polyethyleneimine-capped copper nanoclusters (PEI-CuNCs) with manganese dioxide (MnO₂) nanosheets. The emission of PEI-CuNCs can be significantly quenched by MnO₂ nanosheets via the inner filter effect. With the addition of BChE, the hydrolysis of butyrylthiocholine iodide produces thiocholine which can reduce MnO₂ nanosheets to Mn²⁺, thus resulting in the fluorescence recovery of PEI-CuNCs. Based on that, a fluorescence "turn-on" sensing platform for BChE activity determination was constructed with a detection limit of 2.26 U L^-1. This sensing method is able to detect BChE in human serum samples and identify the serums of normal persons and cirrhotic patients effectively, indicating its great potential in the clinical diagnosis of liver diseases. Furthermore, the approach can also be used to screen BChE inhibitors, which are promising medications to alleviate the symptoms of Alzheimer's disease.

Graphene Quantum Dot-Based Optical Sensing Platform for Aflatoxin B1 Detection via the Resonance Energy Transfer Phenomenon

An optical sensing platform for the detection of an important mycotoxin, aflatoxin B1 (AFB1), in the absence of a bioactive environment is explored. In this work, a fluorescence-based sensing technique was designed by combining graphene quantum dots (GQDs) and AFB1 via fluorescence quenching, where AFB1 acts as the quencher of GQD fluorescence. GQDs were synthesized through a single-step hydrothermal reaction from the leaves of "curry tree" (Murraya Koenigii) at 200 °C. The fluorescent GQDs were quenched by AFB1 (quencher), which itself is detecting the analyte. Hence, this study reports the direct sensing of the mycotoxin AFB1 without the involvement of inhibitors or biological entities. The possible mode of quenching is the nonradiative resonance energy transfer between the GQDs and the AFB1 molecules. This innovative sensor could detect AFB1 in the range from 5 to 800 ng mL^-1 with a detection limit of 0.158 ng mL^-1. The interferent study was also carried out in the presence of different mycotoxins and carbohydrates (d-fructose, cellulose, and starch), which demonstrated the high selectivity and robustness of the sensor in the complex sample matrix. The recovery percentage of the spiked samples was also calculated to be up to 106.8%. Thus, this study reports the first GQD based optical sensor for AFB1.

Phosphate-triggered ratiometric fluoroimmunoassay based on nanobody-alkaline phosphatase fusion for sensitive detection of 1-naphthol for the exposure assessment of pesticide carbaryl

The excessive use of carbaryl has resulted in the risk of its exposure. In this study, we isolated six nanobodies (Nbs) from a camelid phage display library against the biomarker of carbaryl, 1-naphthol (1-NAP). Owing to its characteristics of easy genetic modifications, we produced a nanobody-alkaline phosphatase (Nb-CC4-ALP) fusion protein with good stability. A dual-emission system based ratiometric fluoroimmunoassay (RFIA) for quick and highly sensitive determination of 1-NAP was developed. Silicon nanoparticles (SiNPs) was used as an internal reference and for aggregation-induced emission enhancement (AIEE) of gold nanoclusters (AuNCs), while AuNCs could be quenched by MnO2 via oxidation. In the presence of ALP, ascorbic acid phosphate (AAP) can be transformed into ascorbic acid (AA), the later can etch MnO2 to recover the fluorescence of the AuNCs. Based on optimal conditions, the proposed assay showed 220-fold sensitivity improvement in comparison with conventional monoclonal antibody-based ELISA. The recovery test of urine samples and the validation by standard HPLC-FLD demonstrated the proposed assay was an ideal tool for screening 1-NAP and provided technical support for the monitoring of carbaryl exposure.

High-Throughput Aptamer Microarrays for Fluorescent Detection of Multiple Organophosphorus Pesticides in Food

A novel high-throughput aptamer microarray fluorescent method based on thioflavin T (ThT) was established for the sensitive detection of phoxim, parathion, fensulfothion, and isocarbophos. In this work, the aptamers in binding buffer tended to have the antiparallel G-quadruplex structure, which can bind ThT and release its potential fluorescence signal. However, when the organophosphorus pesticides (OPs) were present, partial aptamers preferred to bind them, forcing the displacement of ThT from the G-quadruplex and resulting in the significant decrease in fluorescence signal. Under optimal experimental conditions (12T spacer, 300 nM aptamer, and 80 μM ThT), the OP aptamer microarray has low limits of detection of 25.4 ng/mL for phoxim, 12.0 ng/mL for parathion, 7.7 ng/mL for fensulfothion, and 9.9 ng/mL for isocarbophos. The accuracy and reliability of the method is further verified by testing the recovery rate of OPs spiked in two different complicated sample matrices (pears and radishes). It is worth mentioning that not only the developed aptamer microarray technology has low sensitivity and a broad spectrum, but it also allows for high-throughput and rapid analysis of a variety OPs, which overcomes some of the shortcomings of other OP detection methods.

Rational construction of a robust metal-organic framework nanozyme with dual-metal active sites for colorimetric detection of organophosphorus pesticides

While nanomaterials with enzyme-mimicking activities are emerging as promising candidates in the colorimetric detection of organophosphorus pesticides (OPs), the catalytic activities and recognition ability to analyte of most nanozymes are inherently deficient. In this work, we introduced manganese ions into a typical iron based MOF (Fe-MIL(53)) via a one-pot hydrothermal reaction strategy, which brought out a catalytically favorable bimetallic Mn/Fe-MIL(53) MOF nanozyme. The catalytic performance of Mn/Fe-MIL(53) is superior to that of pure Fe-MIL (53) and the mechanism for superior catalytic activity of material is revealed by active species scavenging experiments and X-ray photoelectron spectroscopy (XPS). Besides, the introduction of manganese endows the material with the characteristic of being specially destroyed by choline, which motivates the establishment of a simple, selective and sensitive colorimetric strategy for OPs detection. The proposed colorimetric strategy could quantify the methyl parathion and chlorpyrifos in the concentration range of 10-120 nM and 5-50 nM, respectively. The low detection limit of 2.8 nM for methyl parathion and 0.95 nM (3 S/N) for chlorpyrifos were achieved. Good recoveries were obtained when applied in the real sample detection. Our work paves the way to boost catalytic performance of MOF nanozymes, which will be useful in biosensing.

Dual-mode fluorescence and colorimetric detection of pesticides realized by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles

The great threat of pesticide residues to the environment and human health has drawn widespread interest to explore approaches for pesticide monitoring. Compared to commonly developed single-signal pesticide assays, multi-mode detection with inherent self-validation and self-correction is expected to offer more reliable and anti-interference results. However, how to realize multi-mode analysis of pesticides still remains challenging. Herein, we propose a dual-mode fluorescence and colorimetric method for pesticide determination by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles (CPNs(Ⅳ)). The CPNs(Ⅳ) exhibit good oxidase-like activity of catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to its blue oxide, offering a visible color signal; by employing acid phosphatase (ACP) to hydrolyze ascorbic acid 2-phosphate (AAP), the generated ascorbic acid (AA) can chemically reduce the CPNs(Ⅳ) to CPNs(Ⅲ), which exhibit a remarkable fluorescence signal but lose the oxidase-mimicking ability to trigger the TMB chromogenic reaction; when pesticides exist, the enzymatic activity of ACP is restrained and the hydrolysis of AAP to AA is blocked, leading to the recovery of the catalytic TMB chromogenic reaction but the suppression of the fluorescence signal of CPNs(Ⅲ). According to this principle, by taking malathion as a pesticide model, dual-mode 'off-on-off' fluorescence and 'on-off-on' colorimetric detection of the pesticide with good sensitivity was realized. Excellent interference-tolerance and reliability were verified by applying it to analyze the target in real sample matrices. With good performance and practicability, the proposed dual-mode approach shows great potential in the facile and reliable monitoring of pesticide residues.

Nanoflowers: A New Approach of Enzyme Immobilization

Enzymes are biocatalysts known for versatility, selectivity, and brand operating conditions compared to chemical catalysts. However, there are limitations to their large-scale application, such as the high costs of enzymes and their low stability under extreme reaction conditions. Immobilization techniques can efficiently solve these problems; nevertheless, most current methods lead to a significant loss of enzymatic activity and require several steps of activation and functionalization of the supports. In this context, a new form of immobilization has been studied: forming organic-inorganic hybrids between metal phosphates as inorganic parts and enzymes as organic parts. Compared to traditional immobilization methods, the advantages of these nanomaterials are high surface area, simplicity of synthesis, high stability, and catalytic activity. The current study presents an overview of organic-inorganic hybrid nanoflowers and their applications in enzymatic catalysis.

Recent advances in synthesis and modification of carbon dots for optical sensing of pesticides

Serious threat from pesticide residues to the ecosystem and human health has become a global concern. Developing reliable methods for monitoring pesticides is a world-wide research hotspot. Carbon dots (CDs) with excellent photostability, low toxicity, and good biocompatibility have been regarded as the potential substitutes in fabricating various optical sensors for pesticide detection. Based on the relevant high-quality publications, this paper first summarizes the current state-of-the-art of the synthetic and modification approaches of CDs. Then, a comprehensive overview is given on the recent advances of CDs-based optical sensors for pesticides over the past five years, with a particular focus on photoluminescent, electrochemiluminescent and colorimetric sensors regarding the sensing mechanisms and design principles by integrating with various recognition elements including antibodies, aptamers, enzymes, molecularly imprinted polymers, and some nanoparticles. Novel functions and extended applications of CDs as signal indicators, catalyst, co-reactants, and electrode surface modifiers, in constructing optical sensors are specially highlighted. Beyond an assessment of the performances of the real-world application of these proposed optical sensors, the existing inadequacies and current challenges, as well as future perspectives for pesticide monitoring are discussed in detail. It is hoped to provide powerful insights for the development of novel CDs-based sensing strategies with their wide application in different fields for pesticide supervision.

A FRET Approach to Detect Paraoxon among Organophosphate Pesticides Using a Fluorescent Biosensor

The development of faster, sensitive and real-time methods for detecting organophosphate (OP) pesticides is of utmost priority in the in situ monitoring of these widespread compounds. Research on enzyme-based biosensors is increasing, and a promising candidate as a bioreceptor is the thermostable enzyme esterase-2 from Alicyclobacillus acidocaldarius (EST2), with a lipase-like Ser-His-Asp catalytic triad with a high affinity for OPs. This study aimed to evaluate the applicability of Förster resonance energy transfer (FRET) as a sensitive and reliable method to quantify OPs at environmentally relevant concentrations. For this purpose, the previously developed IAEDANS-labelled EST2-S35C mutant was used, in which tryptophan and IAEDANS fluorophores are the donor and the acceptor, respectively. Fluorometric measurements showed linearity with increased EST2-S35C concentrations. No significant interference was observed in the FRET measurements due to changes in the pH of the medium or the addition of other organic components (glucose, ascorbic acid or yeast extract). Fluorescence quenching due to the presence of paraoxon was observed at concentrations as low as 2 nM, which are considered harmful for the ecosystem. These results pave the way for further experiments encompassing more complex matrices.

MnO2/DNAzyme-mediated ratiometric fluorescence assay of acetylcholinesterase

A ratiometric fluorescent probe (MnO2/DNAzyme) is constructed. In the presence of AChE, the product thiocholine reduces MnO2 to Mn2+. The released H1 strands hybridizes with H2 strands to activate DNAzyme and cause cleavage of DNA-F signal probe.

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

A ratiometric fluorescence platform composed of MnO2 nanosheets and nitrogen, chlorine co-doped carbon dots and its logic gate performance for glutathione determination

Illustration of the principle of a dual-emission ratiometric fluorescence strategy for the selective detection of GSH based on an N, Cl-CD-assisted MnO2 nanosheet–OPD system.

Integrating smartphone-assisted ratiometric fluorescent sensors with in situ hydrogel extraction for visual detection of organophosphorus pesticides

Rapid, reliable and on-site detection of organophosphorus pesticides (OPs) on fruit or vegetable surfaces is necessary in real life.

Recent advances and perspectives of enzyme-based optical biosensing for organophosphorus pesticides detection

The overuse or abuse of organophosphorus pesticides (OPs) can bring about severe contamination problems in foodstuff and the environment, which will seriously threaten human health and the ecosystem's cycle. Hence, it is in high demand to establish sensitive, portable, specific, and cost-effective methods for monitoring OPs to control food safety, protect the ecosystem, and prevent disease. The optical biosensor with enzyme as bio-recognition elements has been an effective alternative for OPs detection. Herein, we firstly introduce various enzymes, sensing mechanisms, advantages and disadvantages used as bio-recognition elements in optical sensing for OPs detection. Then, we review various optical biosensing strategies based on enzymes as recognition elements that were ingeniously designed and successfully utilized for OPs detection, with a particular emphasis on photoluminescence (PL), chemiluminescence (CL), electrochemiluminescence (ECL), and colorimetric (CM) biosensing strategies. We not only highlight the state-of-art developments and the construction strategies of the enzyme-based optical biosensing method but also summarize the existing deficiencies, current challenges, and the future perspectives of OPs detection.

Electrochemical detection of methyl-paraoxon based on bifunctional cerium oxide nanozyme with catalytic activity and signal amplification effect

A new electrochemical sensor for organophosphate pesticide (methyl-paraoxon) detection based on bifunctional cerium oxide (CeO₂) nanozyme is here reported for the first time. Methyl-paraoxon was degraded into p-nitrophenol by using CeO₂ with phosphatase mimicking activity. The CeO₂ nanozyme-modified electrode was then synthesized to detect p-nitrophenol. Cyclic voltammetry was applied to investigate the electrochemical behavior of the modified electrode, which indicates that the signal enhancement effect may attribute to the coating of CeO₂ nanozyme. The current research also studied and discussed the main parameters affecting the analytical signal, including accumulation potential, accumulation time, and pH. Under the optimum conditions, the present method provided a wider linear range from 0.1 to 100 μmol/L for methyl-paraoxon with a detection limit of 0.06 μmol/L. To validate the proof of concept, the electrochemical sensor was then successfully applied for the determination of methyl-paraoxon in three herb samples, i.e., Coix lacryma-jobi, Adenophora stricta and Semen nelumbinis. Our findings may provide new insights into the application of bifunctional nanozyme in electrochemical detection of organophosphorus pesticide.

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A new electrochemical method for methyl-paraoxon detection by using bifunctional nanozyme was presented.

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The cerium oxide nanozyme modified glassy carbon electrode was prepared to improve the sensitivity.

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The developed method has been successfully applied in three herbal plant samples.

Sensitivity Dependence on the Crystal Forms of a Fluorescence Quencher for Silicon Quantum Dots and Its Use in Acetylcholinesterase Assay

Acetylcholinesterase (AChE) plays crucial roles in the nervous system, and thus the reliable assay of its activity is of great significance for the diagnosis of nervous diseases. In this work, we report a fluorescent sensing platform with silicon quantum dots (Si-QDs) as a fluorescence oscillator and nano iron oxyhydroxide (α-, β-, and γ-FeOOH) as a quencher for the assay of AChE. FeOOH with α-, β-, and γ-crystal forms quenches the fluorescence of Si-QDs at λ_ex/λ_em = 350/438 nm, which is retrieved in the presence of AChE and its substrate acetylthiocholine (ATCh) to provide an off-on strategy with a high signal/noise ratio. It is interesting that the sensitivity of AChE sensing is closely related to the crystal forms of FeOOH, with the highest sensitivity by adopting α-FeOOH as the quencher. A linear calibration is achieved within 0.02-1.4 U/L along with a limit of detection of 0.016 U/L. The sensing strategy was demonstrated by the AChE assay in human blood, plasma, and hemocytes.

New Fluorescent Probes for the Sensitive Determination of Glyphosate in Food and Environmental Samples

In this paper, a dual-functional probe, 2-(benzothiazol)-4-(3-hydroxy-4-methylphenyl) imino phenol (BHMH), was synthesized and characterized for the simultaneous detection of Cu2+ and Fe3+ in dimethyl sulfoxide/4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (DMSO/HEPES) (1:4, v/v, pH = 6.0). The limits of detections (LODs) for Cu2+ and Fe3+ were 9.05 and 48 nM, respectively. Based on the competitive coordination, the complex BHMH-Cu2+/Fe3+ exhibited good sensitivity and selectivity for glyphosate. The LODs of BHMH-Cu2+ and BHMH-Fe3+ for glyphosate were 0.41 and 0.63 μM, respectively. The probe quantitatively detected glyphosate in tap water, Songhua River water, local water and soil, and food samples. The colorimetric on-site glyphosate sensing through the probe BHMH-Cu2+ was also studied based on smartphones. BHMH and BHMH-Cu2+/Fe3+ exhibited outstanding imaging capabilities for Cu2+, Fe3+, and glyphosate in living cells with low cytotoxicity, especially the first time for glyphosate.

Point-of-care testing of butyrylcholinesterase activity through modulating the photothermal effect of cuprous oxide nanoparticles

Butyrylcholinesterase (BChE) is an important indicator for clinical diagnosis of liver dysfunction, organophosphate toxicity, and poststroke dementia. Point-of-care testing (POCT) of BChE activity is still a challenge, which is a critical requirement for the modern clinical diagnose. A portable photothermal BChE assay is proposed through modulating the photothermal effects of Cu₂O nanoparticles. BChE can catalyze the decomposition of butyrylcholine, producing thiocholine, which further reduce and coordinate with CuO on surface of Cu₂O nanoparticle. This leads to higher efficiency of formation of Cu₉S₈ nanoparticles, through the reaction between Cu₂O nanoparticle and NaHS, together with the promotion of photothermal conversion efficiency from 3.1 to 59.0%, under the excitation of 1064 nm laser radiation. An excellent linear relationship between the temperature change and the logarithm of BChE concentration is obtained in the range 1.0 to 7.5 U/mL, with a limit of detection of 0.076 U/mL. In addition, the portable photothermal assay shows strong detection robustness, which endows the accurate detection of BChE in human serum, together with the screening and quantification of organophosphorus pesticides. Such a simple, sensitive, and robust assay shows great potential for the applications to clinical BChE detection and brings a new horizon for the development of temperature based POCT.

Ratiometric sensing of butyrylcholinesterase activity based on the MnO2 nanosheet-modulated fluorescence of sulfur quantum dots and o-phenylenediamine

Butyrylcholinesterase (BChE) can modulate the expression level of cholinesterase, which emerges as an important clinical diagnose index. However, the currently reported assays for BChE are suffering from the problem of interferences. A ratiometric fluorescence assay was developed based on the MnO₂ nanosheet (NS)-modulated fluorescence of sulfur quantum dots (S-dots) and o-phenylenediamine (OPD). MnO₂ NS can not only quench the fluorescence of blue emissive S-dots, but also enhance the yellow emissive OPD by catalyzing its oxidation reactions. Upon introducing BChE and substrate into the system, their hydrolysate can reduce MnO₂ into Mn²⁺, leading to the fluorescence recovery of S-dots and failure of OPD oxidation. BChE activity can be quantitatively detected by recording the change of fluorescence signals in the blue and yellow regions. A linear relationship is observed between the ratio of F₄₃₅/F₅₆₀ and the concentration of BChE in the range 30 to 500 U/L, and a limit of detection of 17.8 U/L has been calculated. The ratiometric fluorescence assay shows an excellent selectivity to acetylcholinesterase and tolerance to various other species. The method developed  provides good detection performances in human serum medium and for screening of  inhibitors.

Sensitive fluorescence and visual detection of organophosphorus pesticides with a Ru(bpy)32+-ZIF-90-MnO2 sensing platform

Fluorescence sensing organophosphorus pesticides (OPs) is of great importance for both food safety and global environment; however, the reported fluorescent probes are usually directly exposed to the external environment, resulting in premature leakage or photobleaching and thus limiting their photostability and assay sensitivity. In this work, a fluorescent sensing platform consisting of a novel luminescent metal-organic framework (Ru(bpy)₃²⁺-ZIF-90) and manganese dioxide nanosheets (MnO₂ NSs) was prepared for sensing OPs. Due to the protection and improvement in the fluorescence of Ru(bpy)₃²⁺ by ZIF-90, the Ru(bpy)₃²⁺-ZIF-90 probe displayed remarkable photostability and high stability in water. By virtue of the high stability of Ru(bpy)₃²⁺-ZIF-90, as well as the outstanding fluorescence quenching and notable recognition ability of the MnO₂ NSs, this sensing platform provided excellent detection capability for parathion-methyl, with a wide concentration range of 0.050-60 ng mL^-1 and a low detection limit of 0.037 ng mL^-1. Additionally, the system exhibited a visual color change with the concentration of the OPs under sunlight. Moreover, satisfactory recoveries ranging from 93.3% to 103.6% were obtained for the real samples. The results indicated that the Ru(bpy)₃²⁺-ZIF-90-MnO₂-based OP sensing platform is promising for applications in food safety and environmental monitoring.

Synthesis and modification of carbon dots for advanced biosensing application

There has been an explosion of interest in the use of nanomaterials for biosensing applications, and carbonaceous nanomaterials in particular are at the forefront of this explosion. Carbon dots (CDs), a new type of carbon material, have attracted extensive attention due to their fascinating properties, such as small particle size, tunable optical properties, good conductivity, low cytotoxicity, and good biocompatibility. These properties have enabled them to be highly promising candidates for the fabrication of various high-performance biosensors. In this review, we summarize the top-down and bottom-up synthesis routes of CDs, highlight their modification strategies, and discuss their applications in the fields of photoluminescence biosensors, electrochemiluminescence biosensors, chemiluminescence biosensors, electrochemical biosensors and fluorescence biosensors. In addition, the challenges and future prospects of the application of CDs for biosensors are also proposed.

Aggregation-induced emission luminogen@manganese dioxide core-shell nanomaterial-based paper analytical device for equipment-free and visual detection of organophosphorus pesticide

Organophosphorus pesticide (OP) residues have gathered considerable attention because of their significant threat to society development and healthy life. Developing a sensitive and practical OPs sensor is highly urgent, whereas remains a huge challenge. To this end, we fabricated a high-performance fluorescence paper analytical device (PAD) for apparatus-free and visual sensing of OPs based on aggregation-induced emission (AIE) luminogen's bright emission in aggregated state, unique response of MnO₂ to thiol compounds, and difference of MnO₂ and Mn²⁺ in quenching fluorescence. AIE nanoparticles PTDNPs-0.10 and MnO₂ respectively acted as core and shell to prepare PTDNPs@MnO₂, which possessed high stability and were dripped on cellulose paper's surface to fabricate AIE-PAD. The sensing mechanism is that OPs-treated acetylcholinesterase (AChE) prevents the formation of thiocholine, thereby minimizing the reduction of MnO₂ into Mn²⁺ and changing the output signal. As a result, equipment-free and visual sensing of OPs was acquired with limit of detection of 1.60 ng/mL. This work justifies the feasibility of applying core-shell material to develop high-performance sensor and substituting complex/expensive solution-phase sensor with PAD, providing a new avenue to bring OPs analysis out of the lab and into the world.

CDs-MnO2-TPPS Ternary System for Ratiometric Fluorescence Detection of Ascorbic Acid and Alkaline Phosphatase

Manganese dioxide (MnO₂) nanosheet-based fluorescence sensors often use oxidase-like activity or wide absorption spectrum for detection of antioxidants. In those strategies, MnO₂ nanosheets were reduced to Mn²⁺ by antioxidants. However, few strategies emphasize the role of Mn²⁺ obtained from MnO₂ reduction in the design of the fluorescence sensor. Herein, we expanded the application of a MnO₂ nanosheet-based fluorescence sensor by involving Mn²⁺ in the detection process using ascorbic acid (AA) as a model target. In this strategy, carbon dots (CDs), MnO₂ nanosheets, and tetraphenylporphyrin tetrasulfonic acid (TPPS) comprise a ternary system for ratiometric fluorescence detection of AA. Initially, CDs were quenched by MnO₂ nanosheets based on the inner filter effect, while TPPS maintained its fluorescence intensity. After the addition of AA, MnO₂ nanosheets were reduced to Mn²⁺ so that the fluorescence intensity of CDs was recovered and TTPS was quenched by coordination with Mn²⁺. Overall, AA triggered an emission intensity increase at 440 nm for CDs and a decrease at 640 nm for TPPS. The ratio intensity of CDs to TPPS (F ₄₄₀/F ₆₄₀) showed a good linear relationship from 0.5 to 40 μM, with a low detection limit of 0.13 μM for AA detection. By means of the alkaline phosphatase (ALP)-triggered generation of AA, this strategy can be applied for the detection of ALP in the range of 0.1-100 mU/mL, with a detection limit of 0.04 mU/mL. Furthermore, this sensor was applied to detect AA and ALP in real, complex samples with ideal recovery. This novel platform extended the application of MnO₂ nanosheet-based fluorescence sensors.

A silver@gold nanoparticle tetrahedron biosensor for multiple pesticides detection based on surface-enhanced Raman scattering

The detection of multiple pesticides in food and environment is of great importance for human health and safety. In this study, the DNA backbone structure and Ag@Au nanoparticles (NPs) to construct a nano-tetrahedron with the help of the surface-enhanced Raman scattering (SERS) effect by controlling the formation of SERS hotspots and subsequently realized the simultaneous detection of multiple pesticides. The DNA aptamers corresponding to the three pesticides of profenofos, acetamiprid and carbendazim were embedded into the three edges of the DNA tetrahedral skeleton, and the tetrahedral corners were connected to modify the Ag@Au NPs with different Raman signaling molecules. When aptamers recognize the related pesticides, the DNA backbone is deformed. Then Ag@Au NPs approach to each other with SERS hotspots formed and the intensity of the Raman signal increased, realizing the detection of the pesticide content. The biosensor constructed from the SERS substrate with higher sensitivity and lower detection limit (profenofos: 0.0021 ng mL^-1; acetamiprid: 0.0046 ng mL^-1; carbendazim: 0.0061 ng mL^-1). The practicability of this proposed method was verified by adding the recovery rate detection and the accuracy of the method was examined by the analysis of the HPLC-MS method. The proposed SERS biosensor could distinguish and detect three pesticides in food and environmental samples with high sensitivity and low detection limit that can be used in practical applications.

Biomass-derived carbon nanospheres decorated by manganese oxide nanosheets, intercalated into polypyrrole, as an inside-needle capillary adsorption trap sorbent for the analysis of linear alkylbenzenes

Carbon nanospheres (CNSs) were derived hydrothermally from biomass (orange peels) and decorated by manganese dioxide (MnO₂) nanosheets. The MnO₂/CNSs nanocomposite was intercalated into polypyrrole (PPy) during flow-through in-situ electropolymerization of pyrrole on the surface of the inner wall of a stainless-steel needle to prepare an inside-needle capillary adsorption trap (INCAT) device. The surface morphology, thermogravimetric behavior, sorption characteristics, and structure of the MnO₂/CNSs@PPy nanocomposite were characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), energy dispersive X-ray analysis (EDX), nitrogen physisorption by the Brunauer-Emmett-Teller (BET) method, dynamic light scattering (DLS) size distribution, and Fourier-transform infrared spectrometry (FT-IR). The INCAT device was coupled with GC-FID and applied for dynamic headspace analysis of linear alkyl benzenes (LABs) in wastewater samples. The effective experimental variables on the extraction efficiency was optimized using a central composite design (CCD) based on response surface methodology (RSM). Under the optimal conditions, the limits of detection (LODs) were in the range of 0.5-1.0 ng mL^-1. The calibration plots were linear over the range of 0.01-10 μg mL^-1. The relative standard deviations (RSDs%) for intra-day, inter-day, and inter-INCAT precision were calculated 5.3-8.3%, 9.4-13.5%, and 13.6-16.9%, respectively. The developed technique was employed successfully for the analysis of LABs in water and wastewater samples with average recovery values ranging from 92 to 109%. A single INCAT device was used more than 90 times without significant change in its extraction capability.

Inorganic Recognizer-Assisted Homogeneous Electrochemiluminescence Determination of Organophosphorus Pesticides via Target-Controlled Emitter Release

Given the relevance of organophosphorus pesticides (OPs) with food safety, it is highly urgent to develop sensitive and reliable sensors for OPs. However, most of the OP sensors are developed based on colorimetric and fluorescent techniques, which are limited to severe interference of color and fluorescence from pigments and organic acids in agricultural crops. Herein, we develop an inorganic recognizer-based homogeneous electrochemiluminescence (ECL) sensor for the highly sensitive and credible determination of OPs based on manganese dioxide and tris(2,2'-bipyridine)ruthenium [Ru(bpy)₃]²⁺. Through electrostatic interaction, manganese dioxide nanoflakes-[Ru(bpy)₃]²⁺ nanocomposites (MnNFs-Ru) are formed and exhibit a weak ECL signal due to the confinement of [Ru(bpy)₃]²⁺ in MnNFs-Ru. Interestingly, MnNFs-Ru are capable of recognizing thiols due to the analyte-initiated reduction of MnNFs into Mn²⁺ and release of [Ru(bpy)₃]²⁺ from MnNFs-Ru into solution. Further, MnNFs-Ru are employed for the homogeneous ECL determination of OPs, where acetylcholinesterase (AChE) catalyzes the hydrolysis of acetylthiocholine (ATCh) into thiocholine, which in turn decomposes MnNFs of MnNFs-Ru into Mn²⁺, and OPs inhibit AChE activity. This study widens the application of inorganic recognizers from colorimetry/fluorescence to homogeneous ECL and effectively avoids the interference of color and fluorescence, opening up a new path to the development of high-performance OP sensors and supplying a promising tool for guaranteed OP-related food safety.

MOF@MnO2nanocomposites prepared usingin situmethod and recyclable cholesterol oxidase-inorganic hybrid nanoflowers for cholesterol determination

In this work, through thein situgrowth of MnO₂nanosheets on the surface of terbium metal-organic frameworks (Tb-MOFs), MOF@MnO₂nanocomposites are prepared and the fluorescence of Tb-MOFs is quenched significantly by MnO₂. Additionally, the hybrid nanoflowers are self-assembled by cholesterol oxidase (ChOx) and copper phosphate (Cu₃(PO₄)₂·3H₂O). Then a new strategy for cholesterol determination is developed based on MOF@MnO₂nanocomposites and hybrid nanoflowers. Cholesterol is oxidized under the catalysis of hybrid nanoflowers to yield H₂O₂, which further reduces MnO₂nanosheets into Mn²⁺. Hence, the fluorescence recovery of Tb-MOFs is positively correlated to the concentration of cholesterol in the range of 10 to 360μM. The limit of detection (LOD) of cholesterol is 1.57μM. On the other hand, the hierarchical and confined structure of ChOx-inorganic hybrid nanoflowers greatly improve the stability of the enzyme. The activity of hybrid nanoflowers remains at a high level for one week when stored at room temperature. Moreover, the hybrid nanoflowers can be collected by centrifugation and reused. The activity of hybrid nanoflowers can continue at a high level for five cycles of determination. Therefore, it can be concluded that the hybrid nanoflowers are more stable and more economic than free enzymes, and they show a similar sensitivity and specificity to cholesterol compared with free ChOx. Finally, this strategy has been further validated for the determination of cholesterol in serum samples with satisfactory recoveries.

Fluorescence-Based Sensing of Pesticides Using Supramolecular Chemistry

The detection of pesticides in real-world environments is a high priority for a broad range of applications, including in areas of public health, environmental remediation, and agricultural sustainability. While many methods for pesticide detection currently exist, the use of supramolecular fluorescence-based methods has significant practical advantages. Herein, we will review the use of fluorescence-based pesticide detection methods, with a particular focus on supramolecular chemistry-based methods. Illustrative examples that show how such methods have achieved success in real-world environments are also included, as are areas highlighted for future research and development.

Advances in single-molecule fluorescent nanosensors

Single-molecule detection represents the ultimate sensitivity in measurement science with the characteristics of simplicity, rapidity, low sample consumption, and high signal-to-noise ratio and has attracted considerable attentions in biosensor development. In recent years, a variety of functional nanomaterials with unique chemical, optical, mechanical, and electronic features have been synthesized. The integration of single-molecule detection with functional nanomaterials enables the construction of novel single-molecule fluorescent nanosensors with excellent performance. Herein, we review the advance in single-molecule fluorescent nanosensors constructed by novel nanomaterials including quantum dots, gold nanoparticles, upconversion nanoparticles, fluorescent conjugated polymer nanoparticles, nanosheets, and magnetic nanoparticles in the past decade (2011-2020), and discuss the strategies, features, and applications of single-molecule fluorescent nanosensors in the detection of microRNAs, DNAs, enzymes, proteins, viruses, and live cells. Moreover, we highlight the future direction and challenges in this area. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > In Vitro Nanoparticle-Based Sensing Diagnostic Tools > Diagnostic Nanodevices.

Fluorescent Determination of Butyrylcholinesterase Activity and Its Application in Biological Imaging and Pesticide Residue Detection

Butyrylcholinesterase (BChE) is an essential human cholinesterase relevant to liver conditions and neurodegenerative diseases, which makes it a pivotal biomarker of health. It therefore remains challenging and highly desired to elaborate efficient chemical tools for BChE with simple operations and satisfactory working performance. In this work, a background-free detection strategy was built by virtue of the judicious coupling of a specific BChE-enzymatic reaction and in situ cyclization. High sensitivity with a low limit of detection (LOD) of 0.075 μg/mL could be readily achieved from the blank background and the as-produced emissive indicators, and the specific reaction site contributed to the high selectivity over other bio-species even acetylcholinesterase (AChE). In addition to the multifaceted spectral experiments to verify the sensing mechanism, this work assumed comprehensive studies on the application. The bio-investigation ranged from cells to an organism, declaring a noteworthy prospect in disease diagnosis, especially for Alzheimer's disease (AD), a common neurodegenerative disease with over-expressed BChE. Moreover, its excellent work for inhibition efficacy elucidation was also proved with the accuracy IC₅₀ of tacrine for BChE (8.6 nM), giving rise to an expanded application for trace pesticide determination.

Construction of an MnO2 nanosheet array 3D integrated electrode for sensitive enzyme-free glucose sensing

MnO₂ based electrochemical enzyme-free glucose sensors remain significantly limited by their low electronic conductivity and associated complex preparation. In this paper, an MnO₂ nanosheet array supported on nickel foam (MnO₂ NS/NF) was prepared using a simple hydrothermal synthesis and employed as a 3D integrated electrode for enzyme-free glucose detection. It was found that MnO₂ NS/NF shows high performance with a wide linear range from 1 μM to 1.13 mM, a high sensitivity of 6.45 mA mM^-1 cm^-2, and a low detection limit of 0.5 μM (S/N = 3). Besides, MnO₂ NS/NF shows high selectivity against common interferences and good reliability for glucose detection in human serum. This work demonstrates the promising role of MnO₂ NS/NF as an efficient integrated electrode in enzyme-free glucose detection with high performance.