Intracellular Imaging of Glutathione with MnO2 Nanosheet@Ru(bpy)32+-UiO-66 Nanocomposites

Aggregation-Resistant, Turn-On-Off Fluorometric Sensing of Glutathione and Nickel (II) Using Vancomycin-Conjugated Gold Nanoparticles

Glutathione (GSH) and nickel (II) cation have an indispensable role in various physiological processes, including preventing the oxidative damage of cells and acting as a cofactor for lipid metabolic enzymes. An imbalance in the physiological level of these species may cause serious health complications. Therefore, sensitive and selective fluorescent probes for the detection of GSH and nickel (II) are of great interest for clinical as well as environmental monitoring. Herein, vancomycin-conjugated gold nanoparticles (PEI-AuNP@Van) were prepared and employed for the detection of GSH and nickel (II) based on a turn-on-off mechanism. The as-synthesized PEI-AuNP@Van was ~7.5 nm in size; it exhibited a spherical shape with face-centered cubic lattice symmetry. As compared to vancomycin unconjugated gold nanoparticles, GSH led to the turn-on state of PEI-AuNP@Van, while Ni2+ acted as a fluorescence quencher (turn-off) without the aggregation of nanoparticles. These phenomena strongly justify the active role of vancomycin conjugation for the detection of GSH and Ni2+. The turn-on-off kinetics was linearly proportional over the concentration range between 0.05-0.8 µM and 0.05-6.4 μM. The detection limits were 205.9 and 90.5 nM for GSH and Ni2+, respectively; these results are excellent in comparison to previous reports. This study demonstrates the active role of vancomycin conjugation for sensing of GSH and Ni2+ along with PEI-AuNP@Van as a promising nanoprobe.

Enzyme-mediated Ru@UiO-66@MnO2 NSs/thiamine-based ratiometric fluorescence sensor for visual detection of organophosphorus pesticide residues

In view of the potential hazards of organophosphorus pesticides (OPs), this paper constructed a ratiometric fluorescent probe utilizing a functionalized metal-organic framework to detect OPs. Ru(bpy)3Cl2 was encapsulated inside UiO-66 as a reference signal, and MnO2 nanosheets (MnO2 NSs) were grown on the surface to obtain Ru@UiO-66@MnO2 NSs. Acetylcholinesterase catalyzed the decomposition of acetylcholine into reductive thiocholine, which consumed MnO2 NSs, thus restoring the Ru@UiO-66 fluorescence. Due to the enzymatic inhibition of OPs and the redox reaction between MnO2 NSs and thiamine, this probe emitted blue fluorescence in the presence of OPs. The probe achieved linear responses to dichlorvos and chlorpyrifos with LODs of 9.99 × 10-6 μg mL-1 and 9.99 × 10-5 μg mL-1. The probe exhibited a satisfactory recovery rate for OPs in green tea. Furthermore, a hydrogel detection platform was developed by embedding the probe into sodium alginate. Overall, this work provides a visual approach to detect OPs in agricultural products.

Fluorescence Resonance Energy Transfer (FRET)-Based Sensor for Detection of Foodborne Pathogenic Bacteria: A Review

Sensitive and rapid determination of foodborne pathogenic bacteria is of practical importance for the control and prevention of foodborne illnesses. Nowadays, with the prosperous development of fluorescence assays, fluorescence resonance energy transfer (FRET)-derived diagnostic strategies are extensively employed in quantitative analysis of different pathogenic bacteria in food-related matrices, which displays a rapid, simple, stable, reliable, cost-effective, selective, sensitive, and real-time way. Considering the extensive efforts that have been made in this field so far, we here discuss the up-to-date developments of FRET-based diagnostic approaches for the determination of key foodborne pathogens like Staphylococcus aureus, Escherichia coli, Vibrio parahaemolyticus, Salmonella spp., Campylobacter spp., and Bacillus cereus in complex food-related matrices. Moreover, the principle of this technology, the choosing standards of acceptor-donor pairs, and the fluorescence properties are also profiled. Finally, the current prospects and challenges in this field are also put forward.

Surface ligand-assisted synthesis and biomedical applications of metal-organic framework nanocomposites

Metal-organic framework (MOF) nanocomposites have recently gained intensive attention for biosensing and disease therapy applications owing to their outstanding physiochemical properties. However, the direct growth of MOF nanocomposites is usually hindered by the mismatched lattice in the interface between the MOF and other nanocomponents. Surface ligands, molecules with surfactant-like properties, are demonstrated to exhibit the robust capability to modify the interfacial properties of nanomaterials and can be utilized as a powerful strategy for the synthesis of MOF nanocomposites. Besides this, surface ligands also exhibit significant functions in the morphological control and functionalization of MOF nanocomposites, thus greatly enhancing their performance in biomedical applications. In this review, the surface ligand-assisted synthesis and biomedical applications of MOF nanocomposites are comprehensively reviewed. Firstly, the synthesis of MOF nanocomposites is discussed according to the diverse roles of surface ligands. Then, MOF nanocomposites with different properties are listed with their applications in biosensing and disease therapy. Finally, current challenges and further directions of MOF nanocomposites are presented to motivate the development of MOF nanocomposites with elaborate structures, enriched functions, and excellent application prospects.

Optical Immunosensor for the Detection of Listeria monocytogenes in Food Matrixes

In this paper, simple imine-based organic fluorophore 4-amino-3-(anthracene-9 yl methyleneamino) phenyl (phenyl) methanone (APM) has been synthesized via a greener approach and the same was used to construct a fluorescent immunoassay for the detection of Listeria monocytogenes (LM). A monoclonal antibody of LM was tagged with APM via the conjugation of the amine group in APM and the acid group of anti-LM through EDC/NHS coupling. The designed immunoassay was optimized for the specific detection of LM in the presence of other interfering pathogens based on the aggregation-induced emission mechanism and the formation of aggregates and their morphology was confirmed with the help of scanning electron microscopy. Density functional theory studies were done to further support the sensing mechanism-based changes in the energy level distribution. All photophysical parameters were measured by using fluorescence spectroscopy techniques. Specific and competitive recognition of LM was done in the presence of other relevant pathogens. The immunoassay shows a linear appreciable range from 1.6 × 10⁶-2.7024 × 10⁸ cfu/mL using the standard plate count method. The LOD has been calculated from the linear equation and the value is found as 3.2 cfu/mL, and this is the lowest LOD value reported for the detection of LM so far. The practical applications of the immunoassay were demonstrated in various food samples, and their accuracy obtained was highly comparable with the standard existing ELISA method.

Fluorescent Sensor Based on Magnetic Separation and Strand Displacement Amplification for the Sensitive Detection of Ochratoxin A

Ochratoxin A (OTA) is a common mycotoxin, and it is a significant threat to human health throughout the food chain. In this study, a sensitive and specific fluorescent sensor based on magnetic separation technology combined with chain displacement amplification was developed for fast and easy detection of OTA in food. The designed strand displacement amplification can improve the sensitivity for the detection, and the magnetic nanomaterials can provide a large surface area, thus enhancing the capture efficiency of the target from the sample. Based on those designs, the experimental results showed that the proposed method displayed excellent performance. The linearity range was 0.5-128.0 ng/mL. The detection limit was 0.125 ng/mL; the relative standard deviations were 3.92-7.71%. Additionally, the developed method was satisfactorily applied to determine OTA in wheat, corn, and red wine samples at three spiked levels (1.0, 8.0, and 64.0 ng/mL). The recoveries ranged from 85.45 to 107.8% for wheat flour, 101.34 to 108.35% for corn flour, and 91.15 to 93.80% for red wine, respectively. Compared with high-performance liquid chromatography, the proposed method showed a lower limit of detection and equal recovery. Hence, the designed method is a potential and good detecting tool for OTA residue analysis in complex matrix samples.

Silicon Nanoparticle-Based Ratiometric Fluorescence Probes for Highly Sensitive and Visual Detection of VB2

In this work, blue fluorescent silicon nanoparticles (SiNPs) were prepared by a simple one-step hydrothermal method using (3-aminopropyl) triethoxy silane (APTES) and eriochrome black T as raw materials. The SiNPs showed favorable water solubility, thermal stability, pH stability, salt tolerance, and photobleaching resistance. At an excitation wavelength of 376 nm, the SiNPs emitted bright blue fluorescence at 460 nm. In the presence of vitamin B₂ (VB₂), the fluorescence intensity (FL intensity) of the SiNPs at 460 nm decreased obviously, and a new peak appeared at 521 nm. Based on this, a novel ratiometric fluorescence method was established for VB₂ detection. There was a good linear relationship between the fluorescence intensity ratio (F ₅₂₁/F ₄₆₀) and VB₂ concentration from 0.5 to 60 μM with a detection limit of 135 nM. This method was successfully applied to detect VB₂ content in the samples of vitamin B₂ drugs and beverages. Additionally, a simple paper sensor based on the SiNPs was designed to visualize detection of VB₂. With the support of color recognition software on a smartphone, the visual quantitative analysis of VB₂ was realized, ranging from 40 to 800 μM.

Promoting sensitive colorimetric detection of hydroquinone and Hg2+ via ZIF-8 dispersion enhanced oxidase-mimicking activity of MnO2 nanozyme

Reducing the agglomeration and improving the dispersibility in water of two-dimensional (2D) nanozymes is one of the effective ways to improve their enzyme-like activity. In this work, we propose a method by constructing zeolitic imidazolate framework-8 (ZIF-8)-dispersed 2D manganese-based nanozymes to achieve the specific regulated improvement of oxidase-mimicking activity. By in-situ growth of manganese oxides nanosheets of MnO₂(1), MnO₂(2) and Mn₃O₄ on the surface of ZIF-8, the corresponding nanocomposites of ZIF-8 @MnO₂(1), ZIF-8 @MnO₂(2), and ZIF-8 @Mn₃O₄ were prepared at room temperature. The Michaelis-Menton constant measurements indicated that ZIF-8 @MnO₂(1) exhibits best substrate affinity and fastest reaction rate for 3,3',5,5'-tetramethylbenzidine (TMB). The ZIF-8 @MnO₂(1)-TMB system was exploited to detection of trace hydroquinone (HQ) based on the reducibility of phenolic hydroxyl groups. In addition, by employing the fact that the cysteine (Cys) with the excellent antioxidant capacity can bind the Hg²⁺ based on the formation of "S-Hg²⁺" bonds, the ZIF-8 @MnO₂(1)-TMB-Cys system was applied to detection of Hg²⁺ with high sensitivity and selectivity. Our findings not only provide a better understanding of the relationship between dispersion of nanozyme and enzyme-like activity, but also provide a general method for the detection of environmental pollutants using nanozymes.

Luminescent Guests Encapsulated in Metal-Organic Frameworks for Portable Fluorescence Sensor and Visual Detection Applications: A Review

Metal-organic frameworks (MOFs) have excellent applicability in several fields and have significant structural advantages, due to their open pore structure, high porosity, large specific surface area, and easily modifiable and functionalized porous surface. In addition, a variety of luminescent guest (LG) species can be encapsulated in the pores of MOFs, giving MOFs a broader luminescent capability. The applications of a variety of LG@MOF sensors, constructed by doping MOFs with LGs such as lanthanide ions, carbon quantum dots, luminescent complexes, organic dyes, and metal nanoclusters, for fluorescence detection of various target analyses such as ions, biomarkers, pesticides, and preservatives are systematically introduced in this review. The development of these sensors for portable visual fluorescence sensing applications is then covered. Finally, the challenges that these sectors currently face, as well as the potential for future growth, are briefly discussed.

Fluorescent Sensing of Glutathione and Related Bio-Applications

Glutathione (GSH), as the most abundant low-molecular-weight biological thiol, plays significant roles in vivo. Abnormal GSH levels have been demonstrated to be related to the dysfunction of specific physiological activities and certain kinds of diseases. Therefore, the sensing of GSH is emerging as a critical issue. Cancer, with typical high morbidity and mortality, remains one of the most serious diseases to threaten public health. As it is clear that much more concentrated GSH is present at tumor sites than at normal sites, the in vivo sensing of GSH offers an option for the early diagnosis of cancer. Moreover, by monitoring the amounts of GSH in specific microenvironments, effective diagnosis of ROS levels, neurological diseases, or even stroke has been developed as well. In this review, we focus on the fluorescent methodologies for GSH detection, since they can be conveniently applied in living systems. First, the fluorescent sensing methods are introduced. Then, the principles for fluorescent sensing of GSH are discussed. In addition, the GSH-sensing-related biological applications are reviewed. Finally, the future opportunities in in the areas of fluorescent GSH sensing-in particular, fluorescent GSH-sensing-prompted disease diagnosis-are addressed.

In-situ formation of MnO2 nanoparticles on Ru@SiO2 nanospheres as a fluorescent probe for sensitive and rapid detection of glutathione

Glutathione (GSH)-switched fluorescent assays have appealed much attention due to rapid signal changes of fluorescent probes. However, exposure to exterior environment of fluorescent probe causes photobleaching and premature leakage, leading to low sensitivity and poor photostability. Herein, luminescent SiO₂ nanoparticles encapsulated with Ru(bpy)₃²⁺ (Ru@SiO₂) were designed and synthesized as fluorescent probe to construct a GSH-switched fluorescent assay. The encapsulation of Ru(bpy)₃²⁺ in the SiO₂ nanoparticles could effectively prevent the leakage of Ru(bpy)₃²⁺ molecules, improving the photostability of probe. The fluorescence of Ru@SiO₂ nanoparticles was quenched by coating MnO₂ nanoparticles on Ru@SiO₂ surface (Ru@SiO₂@MnO₂ nanocomposites) through an in situ growth approach, which reduced background of the assay. The MnO₂ nanoparticles not only further inhibited the leakage of Ru(bpy)₃²⁺ molecules, but also could serve as a recognition unit of GSH. In the presence of GSH, the MnO₂ nanoparticles on the surface of Ru@SiO₂ nanoparticles were reduced to Mn²⁺, resulting the fluorescence recovery of Ru@SiO₂ nanoparticles. Thus, a signal-on fluorescent strategy was constructed for GSH detection. The assay displayed good analytical performance for GSH detection with a low detection limit of 16.2 nM due to excellent fluorescence quenching ability of MnO₂ nanoparticles and special role of Ru@SiO₂ nanoparticles to block probe leakage. The proposed assay was also applied to measure GSH levels in human serum samples. This work paves a new way to detect GSH with high sensitivity.

Dual-ligand lanthanide metal-organic framework for ratiometric fluorescence detection of the anthrax biomarker dipicolinic acid

Dipicolinic acid (DPA) is a unique biomarker of Bacillus anthracis. Development of a simple, fast, sensitive and timely DPA detection method is of great importance and interest for preventing mass disease outbreaks and treatment of anthrax. In this work, a novel lanthanide-doped fluorescence probe was constructed by coordination of Eu³⁺ with bifunctional UiO-66-(COOH)₂-NH₂ MOFs materials for efficient monitoring DPA. UiO-66-(COOH)₂-NH₂ MOFs were prepared using Zr⁴⁺ as a metal node, 1,2,4,5-benzenetetracarboxylic acid (H₄BTC) and 2-aminoterephthalic acid (NH₂-BDC) as bridging ligand through a simple one-pot synthesis method. By virtue their abundant carboxyl groups, UiO-66-(COOH)₂-NH₂ can readily grasp Eu³⁺ to form UiO-66-(COOH)₂-NH₂/Eu with coordinated water molecules at Eu sites. Upon interaction with DPA molecules, the coordinated H₂O molecules were replaced by DPA molecules which transfer energy to Eu³⁺ in UiO-66-(COOH)₂-NH₂/Eu and sensitize Eu³⁺ luminescence. Meanwhile, DPA has a characteristic absorption band at 270 nm, which overlapped with the excitation spectrum of NH₂-BDC, allowing the fluorescence of UiO-66-(COOH)₂-NH₂/Eu at 453 nm to be greatly quenched by DPA through inner filter effect (IFE). Therefore, the rationally designed UiO-66-(COOH)₂-NH₂/Eu complex not only exhibits strong hydrophilicity and high dispersion, but also serves as ratiometric fluorescence sensing platform for monitoring DPA concentration. This sensing platform showed a satisfactory linear relationship from 0.2 μM to 40 μM with a limit of detection of 25.0 nM and a noticeable fluorescence color change from blue to red, holding a great promise in practical applications.

Water Soluble Silicon Nanoparticles as a Fluorescent Probe for Highly Sensitive Detection of Rutin

In this work, water-soluble fluorescent silicon nanoparticles (SiNPs) were prepared by one-pot hydrothermal method using 3-(2-aminoethylamino)propyldimethoxymethylsilane (AEAPDMMS) as a silicon source and amidol as a reducing agent. The prepared SiNPs showed bright green fluorescence, excellent stability against photobleaching, salt tolerance, temperature stability, and good water solubility. Due to the internal filtration effect (IFE), rutin could selectively quench the fluorescence of the SiNPs. Based on such phenomena, a highly sensitive fluorescence method was established for rutin detection. The linear range and limit of detection (LOD) were 0.05-400 μM and 15.2 nM, respectively. This method was successfully applied to detect rutin in the samples of rutin tablets, Sophora japonica, fry Sophora japonica, and S. japonica carbon with satisfactory recovery.

An ingenious turn-on ratiometric fluorescence sensor for sensitive and visual detection of tetracyclines

To achieve facile and rapid detection of tetracyclines (TCs), herein, we fabricated an ingenious turn-on ratiometric fluorescence sensor (Ru@ZIF-8) based on embedding red-emitting Ru(bpy)₃²⁺ into zeolitic imidazolate framework-8 (ZIF-8). With the introduction of TCs, Ru@ZIF-8 system held the impervious red fluorescence, and generated green fluorescence which originated from the interaction between ZIF-8 and TCs, thereby achieving ratiometric fluorescence strategy through turn-on response signal and stable reference signal. Moreover, the ratiometric response accompanied discernible color change from red to green-yellow, which facilitated detection by naked eyes. The developed sensor exhibited prominent specificity and sensitivity, with detection limits of 2.4, 4.2, 1.6 and 7.2 nM for tetracycline, chlortetracycline, oxytetracycline and doxycycline, respectively. In addition, the satisfactory recoveries were obtained during detecting TCs in drink water, milk and beef, and the test paper-based sensor was successfully applied in real-time visual detection of TCs. All results indicated the feasibility and potential application of Ru@ZIF-8.

A sensitive fluorescence "turn on" nanosensor for glutathione detection based on Ce-MOF and gold nanoparticles

Glutathione (GSH) as an essential biothiol maintains redox homeostasis in human body, the aberrant level of it has been related to various diseases. In this work, we constructed a facile and environment-friendly strategy by using Ce based metal-organic frameworks and gold nanoparticles (AuNPs) for detection of GSH. The fluorescence intensity of the Ce-MOF was quenched by AuNPs, which is ascribed to the existence of fluorescence resonance energy transfer (FRET) and electrostatic interaction between Ce-MOFs and AuNPS. Because of the formation of Au-SH between AuNPs and GSH, the addition GSH induced the Ce-MOF/AuNPs and prevented the occurrence of FRET and electrostatic interaction between Ce-MOFs and AuNPS, which futher recovered the fluorescence of Ce-MOF. Under the optimized conditions, this "turn-on" sensing process revealed a high selectivity toward GSH and displayed good linearity in range of 0.2-32.5 μM with low detection limit of 58 nM. In addition, the practicability of the strategy was testified through analyzing GSH in real human serum samples.

Metal–organic frameworks (MOFs) as fluorescence sensors: principles, development and prospects

This review classifies the latest developments of MOF-based fluorescence sensors according to the analytes, and discusses the challenges faced by MOF-based fluorescence sensors and promotes some directions for future research.

Turning waste into treasure: chicken eggshell membrane derived fluorescent carbon nanodots for the rapid and sensitive detection of Hg2+ and glutathione

Herein, a green, economical, and waste-utilization approach is reported for the synthesis of water-soluble carbon nanodots (C-Dots) with a high fluorescence quantum yield of 19.5%. As a common protein-rich waste, eggshell membrane was selected as a cost-effective and ideal precursor to prepare C-Dots using the microwave method. The as-prepared C-Dots showed a maximum emission at 375 nm with an excitation wavelength at 235 nm. The fluorescent C-Dots were adopted as a sensitive probe for the rapid detection of Hg²⁺ and glutathione (GSH) based on the fluorescence off and on (turn-off-on) strategy. This was ascribed to the fact that Hg²⁺ could effectively quench the fluorescence of the C-Dots and GSH was able to prevent fluorescence quenching owing to the specific binding between Hg²⁺ and GSH. The designed method exhibited a high sensitivity and selectivity towards the detection of Hg²⁺ and GSH. Under the optimized conditions, the method showed a good linear relationship with Hg²⁺ concentration in the range from 100 nM to 50 μM with a detection limit of 32.0 nM. For GSH detection, it displayed a linear range from 50 nM to 10 μM with a detection limit of 9.8 nM. Moreover, this method was successfully applied to detect GSH in human serum samples. The eggshell derived fluorescent C-Dots pave the way for economical environmental and biological analyses.

Highly sensitive and selective detection and intracellular imaging of glutathione using MnO2 nanosheets assisted enhanced fluorescence of gold nanoclusters

Glutathione (GSH) plays a critical role in biological defense system and is associated with numerous human pathologies. However, it still remains a challenge for fluorescent detection of GSH over cysteine (Cys) and homocysteine (Hcy) because of their similar structures. In this work, MnO₂ nanosheets can efficiently quench the fluorescence of gold nanoclusters (Met-AuNCs) prepared by blending methionine and HAuCl₄ owing to their superior absorption capability. However, GSH can reduce MnO₂ nanosheets into Mn²⁺ which leads to the fluorescence recovery of Met-AuNCs. More intriguingly, GSH can dramatically and selectively enhance the fluorescence intensity of Met-AuNCs. Hence, a low background, ultrasensitive fluorescent detection of GSH was obtained with a detection limit of 68 nM. Moreover, the assay has been successfully used for GSH detection in human serum samples and cellular imaging with high selectivity over Cys and Hcy.

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.

MnO2 Nanospheres Assisted by Cysteine Combined with MnO2 Nanosheets as a Fluorescence Resonance Energy Transfer System for "Switch-on" Detection of Glutathione

Manganese dioxide nanosheets combined with cysteine-assisted emitting manganese dioxide nanospheres (Cys-MnO₂ nanospheres) is fabricated for the first time as an "off-on" fluorescence detection platform for glutathione (GSH). In this sensing system, Cys-MnO₂ nanospheres served as energy donors, while MnO₂ nanosheets were used as both energy acceptors and recognition units. MnO₂ nanosheets can effectively quench the fluorescence of Cys-MnO₂ nanospheres through the fluorescence resonance energy transfer (FRET). The addition of GSH could reduce MnO₂ nanosheets into Mn²⁺, disrupting the FRET process and restoring the fluorescence of Cys-MnO₂ nanospheres. Under the optimum conditions, the "switch-on" platform we established has a wide response to GSH with a range of 5-50 μM and 150-800 μM, as well as a superior specificity. Importantly, all components of the sensor are nontoxic, biocompatible, easily prepared, and have a high utilization of raw materials. Moreover, the sensing system achieved satisfactory results in human serum, showing a tremendous potential in the field of biomedicine.

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.

Heterogeneous electro-Fenton catalysis with self-supporting CFP@MnO2-Fe3O4/C cathode for shale gas fracturing flowback wastewater

Self-supporting electrodes have triggered great interests in improving electro-Fenton (EF) system for degradation of refractory organic pollutants. In this work, a novel self-supporting carbon fiber paper (CFP) electrode modified by transition metals, e.g. Fe and Mn, was fabricated and employed as a heterogeneous EF cathode. The prepared electrode exhibited excellent degradation for a number of typical organic pollutants along with superior stability. Remarkably, a high removal efficiency was achieved in the EF treatment of shale gas fracturing flowback wastewater. Results indicated that 65.2% TOC and 74.8% COD were eliminated after 4 h degradation. The residual COD value of the real wastewater was 80 mg L^-1, meeting the emission requirement of the integrated wastewater discharge standard (COD＜100 mg L^-1) with a low specific energy consumption of 6.9kWhkg^-1COD^-1. This work demonstrates a competing alternative for efficient decontamination of real wastewater using an electro-Fenton strategy with a low-cost electrode.

Encapsulation of Luminescent Guests to Construct Luminescent Metal-Organic Frameworks for Chemical Sensing

Metal-organic frameworks (MOFs), which are a class of coordination polymers constructed by metal ions or clusters with organic ligands, have emerged as exciting inorganic-organic hybrid materials with the superiorities of inherent crystallinity, adjustable pore size, clear structure, and high degree of functionalization. The MOFs have attracted much attention to develop good luminescent functional materials due to their inherent luminescent centers of both inorganic and organic photonic units. Furthermore, the pores within MOFs can also be used to encapsulate a large number of luminescent guest species, which provides a broader luminescent property for MOF materials. MOFs possess the incomparable multifunctional advantages of inorganic and organic luminescent materials. A large number of luminescent MOFs (LMOFs) have been synthesized for applications in sensing, white-light-emitting diodes (LED), photocatalysis, biomedicine, etc. This paper reviews the encapsulation of various luminescent guests such as lanthanide ions, dyes, quantum dots, and luminescent complexes in metal-organic frameworks to construct luminous sensors with single- or double-emission centers, as well as the research progress of these sensors in chemical sensing. Finally, the challenges in these fields were outlined and the prospects for future development were put forward.

Eu3+@metal-organic frameworks encapsulating carbon dots as ratiometric fluorescent probes for rapid recognition of anthrax spore biomarker

Anthrax spores pose a serious threat to human well-being and life, so it is highly desirable to develop a rapid, sensitive, and selective quantitative assay of calcium pyridine dicarboxylate (CaDPA) as a biomarker of anthrax spores. Herein, carbon dots (CDs) chelated Eu³⁺@metal-organic framework (Eu-MOFs) as dual-emissive ratiometric fluorescent (RF) probe was successfully fabricated by a simple one-pot in situ selective self-assembly synthetic strategy. The developed RF probe has an effective self-calibration function, which performs a highly sensitive and selective recognition of CaDPA in water and human serum sample. The blue-emitting CDs was employed as an effective fluorescence reference, while the Eu-MOFs exhibited enhanced red fluorescence signal through the coordination interaction with CaDPA chromophore. The sensing mechanism is attributed to that CaDPA can sensitize Eu³⁺ intrinsic luminescence due to the energy transfer from CaDPA to Eu³⁺. What's more interesting is that with the continuous drop of CaDPA, the emission color of CDs@Eu-MOF changes from blue to red. The results revealed that CDs@Eu-MOFs RF probe can detect CaDPA effectively in the range of 8-170 μg/L with good linear relationship, and exhibited a remarkable selectivity for CaDPA. More interestingly, a paper-based probe has also been devised for on-site detection of CaDPA. In addition, CaDPA is used as input signal to construct an IHIBITION logic gate device which performs the "off-on" mode. The constructed CDs@Eu-MOF probe can achieve exceptionally rapid, highly sensitive and selective detection of CaDPA, which can further expand the application prospects in environmental and biological analysis.

Design of a ratiometric fluorescence sensor based on metal organic frameworks and Ru(bpy)32+-doped silica composites for 17β-Estradiol detection

17β-Estradiol (E2), an important endocrine disrupting compound, could be quantitatively detected by fluorescence resonance energy transfer (FRET) aptasensor, designed in this paper. Metal organic frameworks have large specific surface area and easily modifiable groups, which are helpful for the construction of aptasensor. Specifically, streptavidin was immobilized on the synthesized MIL-53-NH₂ by covalent bonding, and further linked with the biotin modified E2 aptamer (apt) through specific bonding between avidin and biotin to obtain the FRET donor probe (MIL-53-apt). Meanwhile, complementary DNA (cDNA) modified Ru(bpy)₃²⁺-doped silica nanoparticles (RuSiO₂-cDNA) were prepared through covalent bonding. They acted as the FRET acceptor probe, since its absorption spectrum showed large overlap with the emission spectrum of MIL-53-apt. In the presence of E2, aptamer modified donor probes tended to bind with E2, owing to their higher selectivity and affinity. Therefore, the optimal distance between FRET pairs was broken, resulting in the fluorescence emission recovery of donor and the fluorescence emission of acceptor decreased. Under optimal conditions, this proposed aptasensor displayed sensitive detection of E2 ranging from 0.5 to 1000 nM with a detection limit of 0.2 nM. Furthermore, the sensor provides a promising method for rapid and sensitive detection of other small biological molecules.

Redox induced dual-signal optical sensor of carbon dots/MnO2 nanosheets based on fluorescence and second-order scattering for the detection of ascorbic acid

In order to detect ascorbic acid (AA) sensitively, a dual-signal optical sensor of a nanosystem with carbon dots (CDs)/MnO₂ nanosheets based on fluorescence and second-order scattering (SOS) has been constructed. Here, MnO₂ nanosheets, both as a fluorescence quencher and signal transducer of SOS, quench the blue fluorescence of CDs by an inner filter effect. Under the excitation of 300 nm, the nanosystem shows a fluorescence emission peak at 405 nm and a SOS peak at 610 nm, respectively. With the increase of AA , the lamellar structure of MnO₂ nanosheets is etched into a smaller nanostructure, causing a decrease of the fluorescence recovery of CDs (405 nm) and decrease of the SOS signal of the MnO₂ nanosheets (610 nm). According to the simultaneous changes of fluorescence and SOS signals, a dual-signal optical sensor toward AA is successfully constructed. Satisfactorily, the optical sensor for AA detection shows a detection limit of 88 and 105 nM for fluorescence and SOS, respectively. The practical application of the designed sensor is verified through the detection of AA content in vitamin C tablets, and satisfactory results are obtained Graphical Abstract A dual-signal sensor of fluorescence (FL) and second-order scattering (SOS) based on the carbon dot (CD) and MnO₂ nanosheet system for ascorbic acid (AA) detection is constructed, in which CDs are used for the FL mode and MnO₂ nanosheets are used for the SOS mode.

Manganese Oxide Nanomaterials: Synthesis, Properties, and Theranostic Applications

Despite the comprehensive applications in bioimaging, biosensing, drug/gene delivery, and tumor therapy of manganese oxide nanomaterials (MONs including MnO₂ , MnO, Mn₂ O₃ , Mn₃ O₄ , and MnO_x ) and their derivatives, a review article focusing on MON-based nanoplatforms has not been reported yet. Herein, the representative progresses of MONs on synthesis, heterogene, properties, surface modification, toxicity, imaging, biodetection, and therapy are mainly introduced. First, five kinds of primary synthetic methods of MONs are presented, including thermal decomposition method, exfoliation strategy, permanganates reduction method, adsorption-oxidation method, and hydro/solvothermal. Second, the preparations of hollow MONs and MON-based composite materials are summarized specially. Then, the chemical properties, surface modification, and toxicity of MONs are discussed. Next, the diagnostic applications including imaging and sensing are outlined. Finally, some representative rational designs of MONs in photodynamic therapy, photothermal therapy, chemodynamic therapy, sonodynamic therapy, radiotherapy, magnetic hyperthermia, chemotherapy, gene therapy, starvation therapy, ferroptosis, immunotherapy, and various combination therapy are highlighted.