A novel fluorescence "turn off-on" nanosensor for sensitivity detection acid phosphatase and inhibitor based on glutathione-functionalized graphene quantum dots

An off-on fluorescence method for acid phosphatase assay based on the inner filter effect of MnO2 nanosheets on vitamin B2

Fluorescence analysis has attracted much attention due to its rapidity and sensitivity. The present work describes a novel fluorescence detection method for acid phosphatase (ACP) on the basis of inner-filter effect (IFE), where MnO2 nanosheets (MnO2 NSs) and vitamin B2 (VB2) are served as absorbers and fluorophores, respectively. In the absence of ACP, the absorption band of MnO2 NSs overlaps well with the excitation band of VB2, resulting in effective IFE and inhibition of VB2 fluorescence. In the presence of ACP, 2-phospho-L-ascorbic acid trisodium salt (AAP) is hydrolyzed to generate ascorbic acid (AA), which efficiently trigger the reduction of MnO2 NSs into Mn2+ ions, causing the weakening of the MnO2 NSs absorption band and the recovery of VB2 fluorescence. Further investigation indicates that the fluorescence recovery degree of VB2 increases with the increase of ACP concentration. Under selected experimental conditions, the proposed method can achieve sensitive detection of ACP in the ranges of 0.5-4.0 mU/mL and 4.0-15 mU/mL along with a limit of detection (LOD) as low as 0.14 mU/mL. Finally, this method was successfully applied for the detection of ACP in human serum samples with satisfactory recoveries in the range of 95.0 %-108 %.

An Updated Review on Functionalized Graphene as Sensitive Materials in Sensing of Pesticides

Numerous chemical pesticides were employed for a long time to manage pests, but their uncontrolled application harmed the health and the environment. Accurately quantifying pesticide residues is essential for risk evaluation and regulatory purposes. Numerous analytical methods have been developed and utilized to achieve sensitive and specific detection of pesticides in intricate sampl es like water, soil, food, and air. Electrochemical sensors based on amperometry, potentiometry, or impedance spectroscopy offer portable, rapid, and sensitive detection suitable for on-site analysis. This study examines the potential of electrochemical sensors for the accurate evaluation of various effects of pesticides. Emphasizing the use of Graphene (GR), Graphene Oxide (GO), Reduced Graphene Oxide (rGO), and Graphdiyne composites, the study highlights their enhanced performance in pesticide sensing by stating the account of many actual sensors that have been made for specific pesticides. Computational studies provide valuable insights into the adsorption kinetics, binding energies, and electronic properties of pesticide-graphene complexes, guiding the design and optimization of graphene-based sensors with improved performance. Furthermore, the discussion extends to the emerging field of biopesticides. While the GR/GO/rGO based sensors hold immense future prospects, and their existing limitations have also been discussed, which need to be solved with future research.

Physico-Chemical Properties of CdTe/Glutathione Quantum Dots Obtained by Microwave Irradiation for Use in Monoclonal Antibody and Biomarker Testing

In this report, we present the results on the physicochemical characterization of cadmium telluride quantum dots (QDs) stabilized with glutathione and prepared by optimizing the synthesis conditions. An excellent control of emissions and the composition of the nanocrystal surface for its potential application in monoclonal antibody and biomarker testing was achieved. Two samples (QDYellow, QDOrange, corresponding to their emission colors) were analyzed by dynamic light scattering (DLS), and their hydrodynamic sizes were 6.7 nm and 19.4 nm, respectively. Optical characterization by UV-vis absorbance spectroscopy showed excitonic peaks at 517 nm and 554 nm. Photoluminescence spectroscopy indicated that the samples have a maximum intensity emission at 570 and 606 nm, respectively, within the visible range from yellow to orange. Infrared spectroscopy showed vibrational modes corresponding to the functional groups OH-C-H, C-N, C=C, C-O, C-OH, and COOH, which allows for the formation of functionalized QDs for the manufacture of biomarkers. In addition, the hydrodynamic radius, zeta potential, and approximate molecular weight were determined by dynamic light scattering (DLS), electrophoretic light scattering (ELS), and static light scattering (SLS) techniques. Size dispersion and the structure of nanoparticles was obtained by Transmission Electron Microscopy (TEM) and by X-ray diffraction. In the same way, we calculated the concentration of Cd2+ ions expressed in mg/L by using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES). In addition to the characterization of the nanoparticles, the labeling of murine myeloid cells was carried out with both samples of quantum dots, where it was demonstrated that quantum dots can diffuse into these cells and connect mostly with the cell nucleus.

Development of a fluorometric and colorimetric dual-mode sensing platform for acid phosphatase assay based on Fe3+ functionalized CuInS2/ZnS quantum dots

BACKGROUND

The spectral dual-mode response towards analyte has been attracted much attention, benefiting from the higher detection accuracy of such strategy in comparison to single signal readout. However, the currently reported dual-mode sensors for acid phosphatase (ACP) activity are still limited, and most of them more or less exist some deficiencies, such as complicated construction procedure, high-cost, poor biocompatibility, aggregation-caused quenching and limited emission capacity.

RESULTS

Herein, we employed Fe3+ functionalized CuInS2/ZnS quantum dots (CIS/ZnS QDs) as nanosensor to develop a novel fluorometric and colorimetric dual-mode assay for ACP activity, combing with ACP-triggered hydrolysis of ascorbic acid 2-phosphate (AAP) into ascorbic acid (AA). The Fe3+ binding to CIS/ZnS QDs can be reduced into Fe2+ during the determination, resulting in the dramatically weakened photoinduced electron transfer (PET) effect and the disappearance of competition absorption. Thus, a highly sensitive ACP assay in the range of 0.22-12.5 U L-1 through fluorescence "turn-on" mode has been achieved with a detection of limit (LOD) of 0.064 U L-1. Meanwhile, the ACP activity can also be quantified by spectrophotometry based on the chromogenic reaction of the formed Fe2+ with 1,10-phenanthroline (Phen). Moreover, the designed nanosensor with good biocompatibility was successfully applied to image and monitor the ACP levels in living cells.

SIGNIFICANCE

We believe that the proposed method has remarkable advantages and potential application for ACP assay in terms of the high accuracy, simplicity, low cost, as well as its adequate sensitivity.

Phosphorescence, fluorescence, and colorimetric triple-mode sensor for the detection of acid phosphatase and corresponding inhibitor

Acid phosphatase (ACP) as a clinical diagnostic biomarker for several pathophysiological diseases has aroused widespread interest. Compared to commonly developed single-mode ACP detection technology, the multi-mode detection method with self-validation can provide more reliable results. Herein, we proposed a triple-mode phosphorescence, fluorescence, and colorimetric method for ACP detection in combination with CDs@SiO₂. HAuCl₄ with oxidase-like activity can catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxide TMB (TMBox), offering absorption signals and quenching the phosphorescence and fluorescence of CDs@SiO₂ based on the internal filtration effect (IFE). ACP can hydrolyze ascorbic acid 2-phosphate (AAP) to yield ascorbic acid (AA), thereby reducing TMBox to TMB, triggering solution fading and restoring phosphorescence and fluorescence signals. When the ACP inhibitor malathion is present, the reduction of TMBox is hindered, which successively led to the suppression of CDs@SiO₂ phosphorescence and fluorescence signal recovery. According to these principles, triple-mode ACP (LOD = 0.0026 mU mL^-1) and malathion detections (LOD = 0.039 μg mL^-1) with favorable accuracy and sensitivity are realized. With simplicity, robustness, and versatility, the triple-mode sensor can be extended to the detection of the AAP hydrolase family and the screening of corresponding inhibitors.

Sensitive acid phosphatase assay based on light-activated specific oxidase mimic activity

Acid phosphatase (ACP) is a key marker in the diagnosis of many diseases. However, exploiting a simple and sensitive sensor for the real-time quantitative analysis of ACP is still challenging. Herein, we attempted to develop a sensitive colorimetric sensing strategy for the detection of ACP based on light-activated oxidase mimic property of carbon dots (CDs). The synthesized CDs were proved to be capable of intrinsic light-activated oxidase mimic activity, which could generate reactive oxygen species to oxidize chromogenic substrate under ultraviolet light stimulation. Interestingly, this light-activated oxidase mimic behavior would be effectively suppressed by the antioxidant ascorbic acid (AA), a product from the hydrolysis of 2-phospho-L-ascorbic acid trisodium (AAP) mediated by ACP. Based on the above property, a facile and sensitive colorimetric sensing method for ACP was developed. Under the optimal conditions, the linear range for ACP 0.1-5.5 U/L, and the detection limit was 0.056 U/L. Compared with conventional nanozyme based ACP assay systems, the catalytic activity of light-activated nanozyme could be conveniently regulated by switching the light on and off, which made it easier to precisely control the extent of the reaction and ensured the accuracy of the assay. In addition, the proposed sensing system would be readout directly by the naked eye or smartphone-based RGB analysis system, and have been successfully applied to analyze diluted in diluted fetal bovine serum and urine samples spiked with ACP. All these results indicated that this approach holds good promise for future applications in clinical analysis and point-of-care (POC) biosensor platforms.

Quantifying H2O2 by ratiometric fluorescence sensor platform of N-GQDs/rhodamine B in the presence of thioglycolic acid under the catalysis of Fe3

In the presence of thioglycolic acid (TGA) and under the catalysis of Fe³⁺, a simple, rapid, sensitive, selective and effective ratiometric fluorescence sensor platform based on the mixed physically blue nitrogen-doped graphene quantum dots (N-GQDs) as probe signals and orange rhodamine B as internal standard signals has been constructed for analysis of H₂O₂ in human serum. TGA is the key factor for fluorescence response toward H₂O₂ by N-GQDs and the mechanism is H₂O₂ reacts speedily with TGA under the catalysis of Fe³⁺, and produces intermediate of superoxide anions (O₂^-), which accepts electrons from N-GQDs, and generates graphene oxide, causing the fluorescence quench of N-GQDs. Compared with N-GQDs probe, the sensitivity of the ratiometric fluorescence sensor platform of N-GQDs/rhodamine B for analysis of H₂O₂ has been improved by nearly 5-folds. Under the optimum conditions, F_λ=580nm/F_λ=440nm has a good linear relationship with the concentration of H₂O₂ and the detection limit of H₂O₂ is 0.46 μmol/L with 3.5% RSD. The established sensor platform has been successfully used for probing H₂O₂ in human serum with satisfactory results. The superior performance of the probe lies in its high selectivity and can be directly employed in detecting H₂O₂ in serum samples without any sample pretreatment procedures.

A fluorescence aptasensor based on GSH@GQDs and RGO for the detection of Glypican-3

Glypican-3 (GPC3), a heparin sulfate proteoglycan, is a potential diagnostic and therapeutic target for hepatocellular carcinoma. In this paper, a novel fluorescent aptasensor for GPC3 detection is constructed via glutathione@graphene quantum dots-labeled GPC3 aptamer (GSH@GQDs-GPC3_Apt) as a fluorescence probe. First, GSH@GQDs is screened out with higher fluorescence intensity, which emits bright blue fluorescence under ultraviolet light. Then, the fluorescence-labeled GSH@GQDs-GPC3_Apt probe is formed by the combination of amination GPC3_Apt and GSH@GQDs using EDC/NHS coupled reaction. Under hydrogen bond and π-π interaction/stacking, the fluorescence of GSH@GQDs-GPC3_Apt could be quenched by reductive graphene oxide (RGO) with the help of the photoinduced electron transfer and the fluorescence resonance energy transfer mechanism. In the presence of GPC3, the GSH@GQDs-GPC3_Apt specifically recognizes and binds to GPC3, giving rise to the change of secondary structure of GPC3_Apt to form the GPC3/GPC3_Apt-GSH@GQDs complex, which would lead to the disintegration of the GSH@GQDs-GPC3_Apt-RGO compound. Therefore, the energy transfer process is blocked and the fluorescence intensity is restored, enabling a highly sensitive response to GPC3. When the concentration of GPC3 is from 5.0 ng/mL to 150.0 ng/mL, the fluorescence recovery rate is well linearly related to GPC3 concentration with the limit of detection of 2.395 ng/mL (S/N = 3). This strategy shows recoveries from 98.31% to 101.89% in human serum samples and provides simple, fast and cheap analysis of GPC3, which suggests that it has great potential applications in clinical diagnosis for hepatocellular carcinoma.

Green- and Red-Emitting Fluorescent Silicon Nanoparticles: Synthesis, Mechanism, and Acid Phosphatase Sensing

Until now, the green and facile synthesis of multicolor fluorescent silicon nanoparticles (SiNPs) with favorable biocompatibility for cellular imaging and biosensors is still a challenge. Herein, a facile one-step room temperature method for preparing fluorescent SiNPs displayed different emission wavelengths was reported. Green and red fluorescent SiNPs (G-SiNPs and R-SiNPs) were synthesized by adjusting the concentration of the reducing agent 2,4-diaminophenol hydrochloride when the amount of N-[3-(trimethoxysilyl)-propyl]-ethylenediamine was consistent. Characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, the results revealed that the G-SiNPs and R-SiNPs were assembled by polymerization of different building blocks, and the emission characteristics of these SiNPs were attributed to the difference in their structural composition and particle size. Interestingly, these fluorescent SiNPs exhibited excellent water solubility, salt tolerance, pH stability, photobleaching resistance, and low cytotoxicity, which facilitated multicolor cell imaging, and further led to these SiNPs were highly attractive in a variety of applications, such as multi-channel sensing and biological imaging. Furthermore, the R-SiNPs have shown the potential to detect acid phosphatase, which is a biomarker of prostate cancer.

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.

Recent progress on graphene quantum dots-based fluorescence sensors for food safety and quality assessment applications

The versatile photophysicalproperties, high surface-to-volume ratio, superior photostability, higher biocompatibility, and availability of active sites make graphene quantum dots (GQDs) an ideal candidate for applications in sensing, bioimaging, photocatalysis, energy storage, and flexible electronics. GQDs-based sensors involve luminescence sensors, electrochemical sensors, optical biosensors, electrochemical biosensors, and photoelectrochemical biosensors. Although plenty of sensing strategies have been developed using GQDs for biosensing and environmental applications, the use of GQDs-based fluorescence techniques remains unexplored or underutilized in the field of food science and technology. To the best of our knowledge, comprehensive review of the GQDs-based fluorescence sensing applications concerning food quality analysis has not yet been done. This review article focuses on the recent progress on the synthesis strategies, electronic properties, and fluorescence mechanisms of GQDs. The various GQDs-based fluorescence detection strategies involving Förster resonance energy transfer- or inner filter effect-driven fluorescence turn-on and turn-off response mechanisms toward trace-level detection of toxic metal ions, toxic adulterants, and banned chemical substances in foodstuffs are summarized. The challenges associated with the pretreatment steps of complex food matrices and prospects and challenges associated with the GQDs-based fluorescent probes are discussed. This review could serve as a precedent for further advancement in interdisciplinary research involving the development of versatile GQDs-based fluorescent probes toward food science and technology applications.

Graphene oxide-based fluorescent biosensors and their biomedical applications in diagnosis and drug discovery

Graphene oxide (GO), an oxidized derivative of graphene, has received much attention for developing novel fluorescent bioanalytic platforms due to its remarkable optical properties and biocompatibility. The reliable performance and robustness of GO-based biosensors have enabled various applications in the biomedical field including diagnosis and drug discovery. Here, recent advances in the development of GO-based fluorescent biosensors are overviewed, particularly nucleic acid detection and enzyme activity assay. In addition, practical applications in biomarker detection and high-throughput screening are also examined. Lastly, basic design principles and remaining challenges of these types of biosensors are discussed for further progress.

A feasible self-assembled near-infrared fluorescence sensor for acid phosphatase detection and cell imaging

The single signal amplification strategy is significant for detecting various disease biomarkers but is restricted by its limited accuracy. The multi-signal and multi-mode methods have overcome this deficiency. Acid phosphatase (ACP) is an important intracellular enzyme but one-step cell imaging material-based probes are scarce for ACP. Herein, we designed a one-step self-assembled polymer probe using neutral red (NR), modified-(pyridoxal-5'-phosphate (PLP)) and Eu³⁺. The polymer exhibited non-emission and excellent stability. Upon the catalytic hydrolysis reaction of ACP, the polymer exhibited two strong fluorescence signals at 373 nm and 613 nm and an appreciable decline of absorbance at 395 nm. The probe has excellent selectivity and higher sensitivity with a limit of detection as low as 0.02 mU mL^-1. It possesses favorable biocompatibility and has been successfully used to detect and image intracellular ACP in several living cells.

Construction of a Carbon Dots/Cobalt Oxyhydroxide Nanoflakes Biosensing Platform for Detection of Acid Phosphatase

Because abnormal acid phosphatase (ACP) can disrupt the normal physiological processes, determination of ACP level is extremely important for early diagnosis, treatment, and prognostic evaluation of diseases. Herein, a fluorescence platform for monitoring ACP level was established based on the assembly of red-emitting carbon dots (RCDs) on cobalt oxyhydroxide (CoOOH) nanoflakes. RCDs displayed excellent water solubility, pH stability, salt resistance, and photobleaching resistance. Interestingly, the fluorescence of the RCDs assembled on the surface of the CoOOH nanoflakes could be quenched due to the energy transfer caused by the nanoflakes. However, the ascorbic acid (AA) produced by the hydrolysis of ascorbic acid-2-phosphate trisodium salt (AAP) catalyzed by ACP could quickly and effectively reduce CoOOH nanoflakes, leading to the fluorescence recovery of the RCDs. Therefore, an "off-on" biosensor platform for rapid, sensitive, and selective detection of ACP was constructed with a limit of detection of 0.25 mU/L. With the assistance of the biosensor, the level of ACP in human serum samples was evaluated, and the spike recovery values ranged from 94.0% to 104.5%.

Fabrication of a sensitive colorimetric nanosensor for determination of cysteine in human serum and urine samples based on magnetic-sulfur, nitrogen graphene quantum dots as a selective platform and Au nanoparticles

A novel colorimetric nanosensor is reported for the selective and sensitive determination of cysteine using magnetic-sulfur, nitrogen graphene quantum dots (Fe₃O₄/S, N-GQDs), and gold nanoparticles (Au NPs). Thus, S, N-GQDs was firstly immobilized on Fe₃O₄ nanoparticles through its magnetization in the presence of Fe³⁺ in the alkali solution. The prepared Fe₃O₄/S, N-GQDs were dispersed in cysteine solution resulting in its quick adsorption on the surface of the Fe₃O₄/S, N-GQDs through hydrogen bonding interaction. Then, Au NPs solution was added to this mixture that after a short time, the color of Au NPs changed from red to blue, the intensity of surface plasmon resonance peak of Au NPs at 530 nm decreased, and a new peak at a higher wavelength of 680 nm appeared. The effective parameters on cysteine quantification were optimized via response surface methodology using the central composite design. Under optimum conditions, the absorbance ratio (A₆₈₀/A₅₃₀) has a linear proportionality with cysteine concentration in the range of 0.04-1.20 μmol L^-1 with a limit of detection of 0.009 μmol L^-1. The fabrication of the reported nanosensor is simple, fast, and is capable of efficient quantification of ultra traces of cysteine in human serum and urine real samples.

A dual-signal fluorescent probe for detection of acid phosphatase

Acid phosphatase has become a significant indicator of prognostic and medical diagnosis, and its dysfunction may lead to a series of diseases. A novel dual-signal fluorescence method for acid phosphatase detection based on europium polymer (europium-pyridine dicarboxylicacid-adenine) and pyridoxal phosphate (PLP) was proposed. PLP coordinated with europium polymer via Eu³⁺ and P-O bonds, and the fluorescence of europium polymer was quenched due to the photoinduced electron transfer (PET) effect between aldehyde and europium polymer. Upon addition of acid phosphatase, the PLP was transformed to phosphate (Pi) and pyridoxal (PL). The PL was released from the surface of europium polymer, and the blue emission was enhanced due to the formation of internal hemiacetal, while the fluorescence of europium polymer recovered. The blue (PL) and red emission (Eu³⁺) were positively correlated with acid phosphatase activity; thus the sensitive assay of acid phosphatase was effectively achieved. The two signals were applied to determine the acid phosphatase with limits of detection (LOD) of 0.04 mU/mL and 0.38 mU/mL, and the linear ranges were 0.13-5.00 mU/mL and 1.25-20.00 mU/mL, respectively. The probe can be used to trace the acid phosphatase in biological systems and holds promise for use in clinical diagnosis and early prevention.

Smartphone based highly sensitive visualized detection of acid phosphatase enzyme activity

With the rapid development of point-of-care (POC) technologies, development of sensitive method featured with fast analysis and affordable devices has become an emerging requirement for practical applications. In this study, we introduced a smartphone-based RGB analysis system for the sensitive detection of acid phosphatase (ACP) enzyme activity. In the presence of ACP, l-ascorbic acid 2-phosphate (AAP) can be converted into ascorbic acid (AA), which can reduce Ag⁺ to Ag⁰ and format the Au@Ag core-shell nanostructure. This morphology change of the Au@Ag core-shell would trigger a significant color variation (pink to yellow). A good linear relationship between the RGB model parameter and the concentration of ACP could be obtained with a detection limit of 0.1 U L^-1. Moreover, this sensing strategy is suitable for the detection of ACP in practical serum samples. Thus, this simple but powerful protocol has great potential application for on-site detection of ACP in future complex biological samples.

Computational studies and biosensory applications of graphene-based nanomaterials: a state-of-the-art review

Graphene, graphene oxide (GO) and graphene quantum dots (GQDs) are expected to play a vital role in the diagnosis of severe ailments. Computer-based simulation approaches are helpful for understanding theoretical tools prior to experimental investigation. These theoretical tools still have a high computational requirement. Thus, more efficient algorithms are required to perform studies on even larger systems. The present review highlights the recent advancement in structural confinement using computer simulation approaches along with biosensory applications of graphene-based materials. The computer simulation approaches help to identify the interaction between interacting molecules and sensing elements like graphene sheets. The simulation approach reduces the wet-lab experiment time and helps to predict the interaction and interacting environment. The experimental investigation can be tuned at a molecular level easily to predict small changes in structural configuration. Here, the molecular simulation study could be useful as an alternative to actual wet experimental approaches. The sensing ability of graphene-based materials is a result of interactions like hydrogen bonding, base-base interaction, and base-to-pi interaction to name a few. These interactions help in designing and engineering a substrate for sensing of various biomolecules.

Smartphone colorimetric assay of acid phosphatase based on a controlled iodine-mediated etching of gold nanorods

A simple but efficient colorimetric assay was developed for the detection and quantification of acid phosphatase (ACP) using a smartphone. This strategy is based on target-controlled iodine-mediated etching of gold nanorods (AuNRs). Due to effective hydrolysis of the substrate pyrophosphate (PPi) by ACP, chelated Cu²⁺ with PPi was released, which promoted the redox reaction with an iodide ion (I^-), leading to the formation of I₃^-. As the etching agent of AuNRs, I₃^- caused a blueshift of the localized surface plasmon resonance peak and, more importantly, an observable color change. The vivid colors were recorded with a smartphone camera and directly analyzed using an image-processing app. On the basis of the direct correlation between ACP concentration and the etching degree of AuNRs as well as color change, this smartphone nanocolorimetry technique showed a good linear response toward ACP over the range of 0-15.0 U/L, with a detection limit of 0.97 U/L. Using the standard addition method, the practical applicability of the proposed smartphone-based assay was successfully demonstrated by determining ACP in human serum samples, with results consistent with those obtained by UV-Vis spectrophotometry.

A dual-mode nanoprobe for the determination of parathion methyl based on graphene quantum dots modified silver nanoparticles

We developed a highly sensitive and selective method for double-signal analysis (fluorescence and ultraviolet-visible spectrophotometry) of organophosphorus pesticides (OPs), based on reversible quenching of graphene quantum dots (GQDs; fluorophores) with silver nanoparticles (AgNPs; absorbers). We used acetylcholinesterase to catalytically convert acetylthiocholine into thiocholine. In turn, by competitive binding to the AgNPs, the produced thiocholine displaces AgNPs from the GQDs and thus induces fluorescence recovery. However, OP analytes inhibit the activity of acetylcholinesterase and, in so doing, retain the silver-graphene nanoparticle complex and fluorescence quenching. The degree of quenching is proportional to the concentration of OPs; the detection limit is as low as 0.017 μg/L. The ultraviolet-visible absorption of GQDs/AgNPs at 390 nm decreases-because of AgNP aggregation that occurs after desorption from the GQDs-and the absorbance is linearly proportional to the OP concentration. Our system has good selectivity to substances that are commonly present in water and vegetables. We successfully applied our method to OP analysis in water, apple, and carrot samples.

Fluorometric determination of the activity of alkaline phosphatase based on a system composed of WS2 quantum dots and MnO2 nanosheets

A fluorometric method is described for the detection of alkaline phosphatase (ALP) activity. It is based on the use of the product of hydrolysis of the drug amifostine (a thiophosphoester) by ALP. It is known that MnO₂ nanosheets quench the blue fluorescence of tungsten disulfide quantum dots (WS₂ QDs) which have excitation/emission wavelengths of 320/448 nm. However, in the presence of ALP and amifostine, the product of hydrolysis [2-(3-aminopropylamino)ethanethiol] triggers the decomposition of the MnO₂ nanosheets. This results in the recovery of fluorescence. Based on this finding, an assay for ALP activity was developed that works in the 0.09-1.6 U L^-1 range, with a 40 mU L^-1 detection limit. The relative standard deviation is 1.87% for five repeated measurements of 0.8 U L^-1 ALP. The method was applied to the analysis of ALP in real samples and gave satifactory results. Graphical abstractSchematic representation of a fluorometric method for determination of the activity of alkaline phosphatase (ALP). The fluorescence of a system composed of WS₂ quantum dots and MnO₂ nanosheets is quenched. Hydrolysis of the cytoprotective adjuvant amifostine (a phosphothioester) by ALP leads to a thiol that causes the decomposition of the MnO2 nanosheets. As a result, the blue fluorescence of the system becomes increasingly restored.

Carbon dots as an "on-off-on" fluorescent probe for detection of Cu(II) ion, ascorbic acid, and acid phosphatase

Carbon dot (CD)-based fluorescent probes have been widely exploited; however, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, we develop a convenient and simple CD-based "on-off-on" fluorescent probe for detection of copper(II) ion (Cu²⁺), ascorbic acid (AA), and acid phosphatase (ACP). Cu²⁺ leads to the fluorescence quenching of CDs. The limit of detection (LOD) for Cu²⁺ is 2.4 μM. When AA is added into the CDs + Cu²⁺ solution, Cu²⁺ is reduced by AA to Cu⁺, causing the fluorescence recovery of CDs. The fluorescent intensity linearly correlates with the concentration of AA in the range of 100-2800 μM with LOD of 60 μM. Besides, the probe has potential application for detection of AA in real samples such as VC tablets, orange juice, and fresh orange. The probe can also indirectly detect ACP that enzymatically hydrolyzes ascorbic acid-phosphate (AAP) to produce AA. This work expands the application of CDs in the multi-component detection and provides a facile fluorescent probe for detection of AA in real samples. Graphical abstract.