Ingenious dual-emitting Ru@UiO-66-NH2 composite as ratiometric fluorescence sensor for detection of mercury in aqueous

Smartphone-based dual inverse signal MOFs fluorescence sensing for intelligent on-site visual detection of malachite green

The development of intelligent, sensitive, and visual methods for the rapid detection of veterinary drug residues is essential to ensure food quality and safety. Here, a smartphone-based dual inverse signal MOFs fluorescence sensing system was proposed for intelligent in-site visual detection of malachite green (MG). A UiO-66-NH2@RhB-dual-emission fluorescent probe was successfully synthesized in one step using a simple one-pot method. The inner filter effect (IFE) quenches the red fluorescence, while hydrogen bonding interaction enhances the blue fluorescence, enabling highly sensitive, accurate, and visual detection of MG dual inverse signals through fluorescence analysis. The probe showed great linearity over a wide range of 0.1-100 μmol/L, with a limit of detection (LOD) of 20 nmol/L. By integrating smartphone photography and RGB (red, green, and blue) analysis, accurate quantitative analysis of MG in water and actual fish samples can be achieved within 5 min. This developed platform holds great promise for the on-site detection of MG in practical applications, with the advantages of simplicity, cost-effectiveness, and rapidity. Consequently, it may open up a new pathway for on-site evaluation of food safety and environmental health.

Ultrasound-assisted synthesis of a Eu3+-functionalized ZnII coordination polymer as a fluorescent dual detection probe for highly sensitive recognition of HgII and L-Cys

A new ZnII coordination polymer (CP) based on 2,3-pyrazine dicarboxylic acid (H2pzdc) and 4,4'-bipyridine (bpy) (ZCP) was synthesized using a facile slow evaporation method. Single-crystal X-ray diffraction revealed that ZCP is a two-dimensional porous CP, [Zn2(pzdc)2(bpy)(H2O)2]n, with van der Waals forces as the dominant interaction within its layers forming a 63 network. Employing energetic ultrasound irradiation, nanoscale ZCP (nZCP) was successfully synthesized and Eu3+ ions were incorporated within its host lattice (Eu@nZCP). The resulting platform exhibits superior fluorescence characteristics and demonstrates notable optical durability. Therefore, it was used as a dual detection fluorescent sensing platform for the detection of mercury and L-cysteine (L-Cys) in aqueous media through a turn-off/on strategy. In the turn-off process, the fluorescence emission of Eu@nZCP progressively quenches by the addition of HgII via a photo-induced electron transfer (PET) mechanism. The fluorescence of Eu@nZCP is quenched to establish a low fluorescence background through the incorporation of HgII. This devised turn-on fluorescent system is suitable for the recognition of L-Cys (based on the strong affinity of L-Cys to the HgII ion) through a quencher detachment mechanism. This method attained a relatively wide linear range, spanning from 0.001 to 25 µM, with the low detection limit of 5 nM for the sensing of HgII. Also, the corresponding limit of detection (LOD) for L-Cys is 8 nM in a relatively wide linear range, spanning from 0.001 to 40 µM.

Metal-organic framework-based ratiometric point-of-care testing for quantitative visual detection of nitrite

Nitrite (NO2-) is categorized as a carcinogenic substance and is subjected to severe limitations in water and food. To safeguard the public's health, developing fast and convenient methods for determination of NO2- is of significance. Point-of-care testing (POCT) affords demotic measurement of NO2- and shows huge potential in future technology beyond those possible with traditional methods. Here, a novel ratiometric fluorescent nanoprobe (Ru@MOF-NH2) is developed by integrating UiO-66-NH2 with tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) through a one-pot approach. The special diazo-reaction between the amino group of UiO-66-NH2 and NO2- is responsible for the report signal (blue emission) with high selectivity and the red emission from [Ru(bpy)3]2+ offers the reference signal. The proposed probe shows obviously distinguishable color change from blue to red towards NO2- via naked-eye. Moreover, using a smartphone as the detection device to read color hue, ultra-sensitive quantitative detection of NO2- is achieved with a low limit of detection at 0.6 μΜ. The accuracy and repeatability determined in spiked samples through quantitative visualization is in the range of 105 to 117% with a coefficient of variation below 4.3%. This POCT sensing platform presents a promising strategy for detecting NO2- and expands the potential applications for on-site monitoring in food and environment safety assessment.

Real-time intelligent detection of ethephon based on a high-throughput ratiometric fluorescent probe

Ethephon (ETH) is a common pesticide, and its overuse has resulted in a variety of health problems for humans. However, the existing ETH detection methods are tedious and time-consuming, and real-time ETH identification remains a significant difficulty. To mitigate this concern, a dual-emission ratiometric fluorescent probe Ru@ZrMOF was rationally synthesized for the detection of ETH. In the presence of ETH, the emission peak at 435 nm gradually increased, while the peak at 600 nm remained constant, accompanied by the fluorescence color change from red, pink, blue-violet to blue. The fluorescence intensity ratio (F435/F600) demonstrated two linear relations with the ETH concentration ranges at 3 - 50 μM and 50 - 500 μM, with a lowest detection limit at 1 μM. This was attributed to the formation of Zr-O-P bonds which attenuated the ligand-metal charge transfer (LMCT) process, resulting in the recovery of blue fluorescence of the ligand 2-Aminoterephthalic acid (2-APDC). To validate the practical application of the developed platform, a YOLO v5x-based WeChat applet "96 Speckles" was developed, and a 96-well plate and smartphone-embedded 3D-printed portable toolbox was designed for the real-time intelligent detection of ETH. This smart platform allows for real-time and efficient ETH analysis in various real samples including apples, pears and tomatoes.

Intelligent quantitative recognition of sulfide using machine learning-based ratiometric fluorescence probe of metal-organic framework UiO-66-NH2/Ppix

Sulfides possess either high toxicity or play crucial physiological role such as gas transmitter dependent upon dosage, hence the significant for their rapid sensitive and selective concentration determination. Herein, a machine learning enhanced ratiometric fluorescence sensor was engineered for sulfide determination by incorporating the nanometal-organic framework (UiO-66-NH2) along with protoporphyrin IX (Ppix). The blue fluorescence at 431 nm originated from the moiety of UiO-66-NH2 by 365 nm excitation serves as an internal calibration reference signal, while the red fluorescence at 629 nm from the moiety of Ppix serves as the analytical signal, and the intensity is correlated to the amount of sulfides. The fluorescence color of the sensor gradually varies from blue to red upon sequential addition of copper and sulfide ions, resulting in RGB (Red, Green, Blue) feature values for corresponding sulfide concentrations, which facilities the advanced data processing techniques using machine learning algorithms. On the basis of fluorescence image fingerprint extraction and machine learning algorithms, an online data analysis model was developed to improve the precision and accuracy of sulfide determination. The established model employed Linear Discriminant Analysis (LDA) and was subjected to rigorous cross-validation to ensure its robustness. By analyzing the correlation between RGB feature values and sulfide concentrations, the study highlighted a significant positive relationship between the red feature values and sulfide concentrations. The application of machine learning techniques on the ratiometric fluorescence signal of the UiO-66-NH2/Ppix probe demonstrated its potential for intelligent quantitative determination of sulfides, offering a valuable and efficient tool for pollution detection and real-time rapid environmental monitoring.

Fabrication of Eu-MOFs rod-shaped nanospheres with dual emissions for ratiometric fluorescence detecting Hg2+ in water

The incorporation of novel nanostructure has been proven to significantly improve the performance of fluorescence-based sensors in terms of sensitivity, selectivity, and detection capability. Herein, a lanthanide metal-organic framework (BTC-Eu-BDC-NH2) with dual ligands of 2-aminobenzoic acid (BDC-NH2) and 1,3,5-benzene tricarboxylic acid (BTC) has been prepared for ratiometric fluorescent detection of Hg2+ through the rational one-step synthetic approach. Through adjusting the ratio of two ligands, this dual-ligands strategy not only provided two independent emissions at peaks of 435 nm and 615 nm to resist the influence of external conditions, but also introduced the visual detection with an obvious color change. Moreover, the specific rod-shaped nanospheres morphology substantially enlarged the surface area of BTC-Eu-BDC-NH2 to ensure good dispersion and rapid response during sensing. Upon the addition of Hg2+, the fluorescence at 435 nm of BTC-Eu-BDC-NH2 was obviously quenched because of the interaction between Hg2+ and -NH2 from the ligand, while the red fluorescence at 615 nm remains almost unchanged. As a result, the synthesized BTC-Eu-BDC-NH2 showed excellent performances for visual sensing detection of Hg2+ with a clear luminescent color conversion from blue to red, and the detecting range was 0-40 μM with a low detection limit of 67 nM. Finally, the developed sensor was applied to actual tap water, and a handy sensing kit was constructed by hydrogel with BTC-Eu-BDC-NH2, demonstrating its potential practical applications.

One-pot synthesis of nano Zr-based metal-organic frameworks for fluorescence determination of quercetin and Hg2

In this study, a nanoscale Zr-based metal-organic framework (nano-Zr-MOF) was prepared by one-pot method using meso-tetra(4-carboxyphenyl)porphyrin as organic ligand and Zr4+ as metal unit. The nanoscale structure endows it with excellent dispersion in water. The nano-Zr-MOF exhibited intense red fluorescence, which could be significantly quenched by the addition of quercetin, probably due to its electron-rich framework. The high selectivity for quercetin detection was verified with analogues and common ions as interfering agents. Moreover, the nano-Zr-MOF could be used as a highly selective and sensitive sensor for the detection of Hg2+. The detection limits for quercetin and Hg2+ were 0.026 μM and 0.039 μM, respectively. This fluorometric method was successfully applied to detect quercetin in red wine and food samples with satisfactory recoveries ranging from 83.7-112.3% and 81.8-115.9%, respectively. The recovery in detection of Hg2+ in lake water were ranging from 97.1-109.2%.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

Constructing an eco-friendly and ratiometric fluorescent sensor for highly efficient detection of mercury ion in environmental samples

Mercury ion (Hg2+) is a highly toxic and ubiquitous pollutant, whose effective detection has aroused widespread concern. A novel ratiometric fluorescent sensor has been designed to rapidly and efficiently detect Hg2+ based on blue/red carbon dots (CDs) with environmental friendliness. This sensor was well characterized via TEM, FTIR, XPS, UV-vis, and zeta potential analysis and displayed excellent fluorescence properties and stability. The fluorescence of blue CDs at 447 nm was significantly quenched with the addition of Hg2+ resulted from the static quenching, whereas that of red CDs at 650 nm remained invariable. A sensitive method for Hg2+ determination was constructed in the range of 0.05-7.0 nmol mL-1 with optimal conditions, and the detection limit was down to 0.028 nmol mL-1. Meanwhile, compared to other 17 metal ions, the ratiometric fluorescent sensor exhibited high selectivity for Hg2+. Furthermore, satisfied recoveries had also been obtained for measuring trace Hg2+ in practical environmental samples. This developed ratiometric fluorescent sensor provided a reliable, environmental-friendly, rapid, and efficient platform for the detection of Hg2+ in environmental applications.

Unlabelled LRET biosensor based on double-stranded DNA for the detection of anthraquinone anticancer drugs

Based on upconversion nanoparticles (UCNPs) as energy donor and herring sperm DNA (hsDNA) as molecular recognition element, an unlabelled upconversion luminescence (UCL) affinity biosensor was constructed for the detection of anthraquinone (AQ) anticancer drugs in biological fluids. AQ anticancer drugs can insert into the double helix structure of hsDNA on the surface of UCNPs, thereby shortening the distance from UCNPs. Therefore, the luminescence resonance energy transfer (LRET) phenomenon is effectively triggered between UCNPs and AQ anticancer drugs. Hence, AQ anticancer drugs can be quantitatively detected according to the UCL quenching rate. The biosensor showed good sensitivity and stability for the detection of daunorubicin (DNR) and doxorubicin (ADM). For the detection of DNR, the linear range is 1-100 μg·mL-1 with a limit of detection (LOD) of 0.60 μg·mL-1, and for ADM, the linear range is 0.5-100 μg·mL-1 with a LOD of 0.38 μg·mL-1. The proposed biosensor provides a convenient method for monitoring AQ anticancer drugs in clinical biological fluids in the future.

Efficient Degradation of Sulfamethoxazole by Diatomite-Supported Hydroxyl-Modified UIO-66 Photocatalyst after Calcination

Sulfamethoxazole (SMX) is a widely used antibiotic to treat bacterial infections prevalent among humans and animals. SMX undergoes several transformation pathways in living organisms and external environments. Therefore, the development of efficient remediation methods for treating SMX and its metabolites is needed. We fabricated a photo-Fenton catalyst using an UIO-66 (Zr) metal-organic framework (MOF) dispersed in diatomite by a single-step solvothermal method for hydroxylation (HO-UIO-66). The HO-UIO-66-0/DE-assisted Fenton-like process degraded SMX with 94.7% efficiency; however, HO-UIO-66 (Zr) is not stable. We improved the stability of the catalyst by introducing a calcination step. The calcination temperature is critical to improving the catalytic efficiency of the composite (for example, designated as HO-UIO-66/DE-300 to denote hydroxylated UIO-66 dispersed in diatomite calcined at 300 °C). The degradation of SMX by HO-UIO-66/DE-300 was 93.8% in 120 min with 4 mmol/L H2O2 at pH 3 under visible light radiation. The O1s XPS signatures signify the stability of the catalyst after repeated use for SMX degradation. The electron spin resonance spectral data suggest the role of h+, •OH, •O2-, and 1O2 in SMX degradation routes. The HO-UIO-66/DE-300-assisted Fenton-like process shows potential in degrading pharmaceutical products present in water and wastewater.

Formic Acid-Regulated Defect Engineering in Zr-Based Metal-Organic Frameworks toward Fluorescence Sensor for Sensitive Detection of Chlortetracycline

The elaborate defect-engineering of luminescent metal-organic frameworks (MOFs) allows them with enhanced sensing performance. A modulator-induced defect formation strategy is adopted in this paper, and the impact of the open-metal sites on sensing process is rationalized. It is demonstrated that the defect level can be tuned to a remarkable extent by controlling the amount of modulator. When a particular defect concentration is reached, the UiO-66-xFA can be acted as highly sensitive ratiometric fluorescence probes for chlortetracycline (CTE) determination with an ultralow detection limit of 9.9 nm. Furthermore, by virtue of the obvious variation in fluorescence chromaticity of probes from blue to yellow, a sensory hydrogels-based smartphone platform is proposed for visible quantitation of CTE by identifying the RGB values. A delicate device integrated with UV lamp and dark cavity has been developed for avoiding inconsistencies of ambient light and visual errors. Finally, the sensor obtains satisfactory results in the detection of actual seafood samples, with no significant differences from those of liquid chromatography-mass spectrometry. This approach anticipates a novel route to sensitize optical sensors through the design and synthesis of moderate defects in luminescent MOFs.

Target-Controlled Redox Reaction and Ru(II) Release of a Smart Metal-Organic Framework Nanomaterial for Highly Sensitive Ratiometric Homogeneous Electroanalysis of Cadmium(II)

In this work, a highly sensitive ratiometric homogeneous electroanalysis (HEA) strategy of cadmium(II) (Cd2+) was proposed via a Cd2+-controlled redox reaction and Ru(bpy)32+ (Ru(II)) release from a smart metal-organic framework (MOF) nanomaterial. For achieving this purpose, Ru(II) was entrapped ingeniously into the pores of an MOF material (UiO-66-NH2) and subsequently gated by the double-strand hybrids of a Cd2+-aptamer (Apt) and its complementary sequences (CP) to form a novel smart nanomaterial (denoted as Ru@UiO-66-NH2); meanwhile, Fe(III) was selected as an additional probe present in electrolyte to facilitate the Ru(II) redox reaction: Fe(III) + Ru(II) → Fe(II) + Ru(III). Owing to the strong binding effect of the Cd2+ target to the specific Apt, the Apt-CP hybridization at Ru@UiO-66-NH2 would be destroyed in the presence of Cd2+, and the related Apt was further induced away from the smart nanomaterial, leading to the opening of the gate and release of Ru(II). Meanwhile, the released Ru(II) was quickly oxidized chemically by Fe(III) to Ru(III). On the basis of the generated Ru(III) and consumed Fe(III), the ratio of the reduction currents between Ru(III) and Fe(III) exhibits an enhancement and it is dependent on the level of Cd2+; thus, a novel HEA strategy of Cd2+ was then designed. Under the optimal conditions, the HEA sensor shows a wide linearity ranging from 10.0 pM to 500.0 nM, and the achieved detection limit of Cd2+ is 3.3 pM. The as-designed ratiometric HEA strategy not only offers a unique idea to realize a simple and sensitive assay for Cd2+ but also possesses significant potential as an effective tool to be introduced for other target analysis just via altering the specific Apt.

Dual-ligand lanthanide metal-organic framework probe for ratiometric fluorescence detection of mercury ions in wastewater

By serving dipyridylic acid (DPA) and 2,5-dihydroxyterephthalic acid (DHTA) as the biligands, a novel lanthanide (Eu3+) metal-organic framework (MOF) namely Eu-DHTA/DPA was prepared for specific Hg2+ fluorescence determination. The dual-ligand approach can endows the resulting luminescent MOF with dual emission of ratiometric fluorescence and uniform size. Eu3+ produces intense red fluorescence when activated by the ligand DPA, while the other ligand DHTA produces yellow fluorescence. Under 273 nm excitation, the presence of Hg2+ in the monitoring environment causes an increase in the intensity of the DHTA fluorescence peak at 559 nm and a decrease in the intensity of the Eu3+ fluorescence peak at 616 nm. Hg2+ effectively quenches the fluorescence emission of the central metal Eu3+ in Eu-DHTA/DPA at 616 nm through a dynamic quenching effect. This recognition process occurs due to the coordination of Hg2+ with ligands such as benzene rings, carboxyl groups, and pyridine N in three-dimensional space. Hg2+ was detected by measuring the ratio between two fluorescence peaks (I559 nm/I616 nm) within the range 2-20 μM, achieving a remarkably low detection limit of 40 nM. The established ratiometric fluorescence method has been successfully applied to the determination of Hg2+ in industrial wastewater of complex composition. The method plays a crucial role in the rapid and sensitive monitoring of Hg2+ in real environmental samples. The recoveries ranged from 92.82% to 112.67% (n = 3) with relative standard deviations (RSD) below 4.8%. This study offers a convenient and effective method for constructing probes for Hg2+ monitoring, with practical applications in environmental monitoring.

A fluorescent probe based on aptamer gold nanoclusters for rapid detection of mercury ions

The mercuric ion (Hg2+) is a hazardous pollutant that is widely distributed in living organisms, foods, and environments with highly toxic and bio-accumulative properties. In the present study, a fluorescent probe based on aptamer gold nanoclusters (apt-AuNCs) was prepared for the ultrasensitive detection of Hg2+ in food. The principle underlying the prepared probe was the quenching of the fluorescence of apt-AuNCs in the presence of Hg2+ due to the strong metallophilic interactions between the 5d10 centers of Hg2+ and Au+. Under optimal conditions, the proposed fluorescent probe exhibited a linear relationship with Hg2+ concentration within the range of 2-200 nM (R2 = 0.9960). In addition, the limit of detection (LOD) was 0.0158 nM, which is below the Chinese standard value of 25 nM for Hg2+ in food. Furthermore, the apt-AuNCs were applied to detect Hg2+ in spinach and crawfish samples, with recovery percentages of 91.99%∼108.06%, meaning that apt-AuNCs could be used as a promising probe to detect Hg2+ in complex food samples.

Bimetallic metal-organic frameworks-based ratiometric fluorescence sensor for the quantitative detection of thiram in fruits samples

The identification of residual thiram (Tr) in foods is vital in view of its harmful effects on human health. Herein, a ratiometric fluorescence sensor (I₄₃₅/I₅₉₀) based on rhodamine B/NH₂-MIL-53(Al_0.75Fe_0.25) was constructed for the detection of Tr. Interestingly, the probe RhB/NH₂-MIL-53(Bim) assisted by Cu²⁺ could rapidly and sensitively recognize Tr with a low detection limit of 0.11 μg/mL in 10 min. The fluorescence sensing mechanism was investigated using fluorescence spectra, UV-Vis absorption spectra, the fluorescence lifetime and quantum yield. The results showed that the excellent sensing performance was attributed to fluorescence resonance energy transfer, electrostatic interaction, and photoinduced electron transfer. In addition, the practical application of this platform showed acceptable relative recoveries for Tr (84.03-107.81 %), and precisions were also achieved (relative standard deviation ≤ 8.69 %, n = 3). These results show that the presented herein can be applied to monitor the Tr content in real fruit samples.

Controllable synthesis of bifunctional magnetic carbon dots for rapid fluorescent detection and reversible removal of Hg2

Mercury is one of the most toxic heavy metals, whose identification and separation are crucial for environmental remediation. Till now, it remains a significant challenge upon simultaneous detection and removal of Hg²⁺. Herein, bifunctional probe magnetic carbon dots were synthesized and optimized via systematic structure manipulation of the carbon and iron precursors towards fluorescence, Hg²⁺ adsorption and magnetic separation. The probe exhibited blue emission at 440 nm with high quantum yield of 55 % and a high paramagnetism with the saturation magnetization value of 22.70 emu/g. Furthermore, the fluorescent detection of Hg²⁺ with limit of 5.40 nM and high selectivity were achieved through surface structure manipulation with moderate -NH₂, -SH and Fe contents. As a result, the magnetic removal of Hg²⁺ was consecutively effectuated with high removal efficiency of 98.30 %. The detection and recovery of Hg²⁺ in real samples were further verified and demonstrated the excellent environmental tolerance of probe. The reusability was viable with recycling at least three turns by external magnet. This work not only provides a promising approach for simultaneous detection and removal of heavy metal pollution, but also provides an excellent example as a versatile platform for multifunction integration via the structure manipulation for other applications.

Engineering of metal-organic frameworks (MOFs) for thermometry

Metal-organic frameworks (MOFs ) are excellent candidates for use in chemistry, material sciences and engineering thanks to their interesting qualitative features and potential applications. Quite interestingly, the luminescence of MOFs can be engineered by regulation of the ligand design, metal ion selection and encapsulation of guest molecules within the MOF cavity. Temperature is a very crucial physical parameter and the market share of temperature sensors is rapidly expanding with technology and medicinal advancement. Among the wide variety of available temperature sensors, recently MOFs have emerged as potential temperature sensors with the capacity to precisely measure the temperature. Lanthanide-based thermometry has advantages because of its ratiometric response ability, high quantum yield and photostability, and therefore lanthanide-based MOFs were initially focused on to construct MOF thermometers. As science and technology have gradually changed, it has been observed that with the inclusion of dye, quantum dots, etc. within the MOF cavity, it is possible to develop MOF-based thermometry. This review consolidates the recent advances of MOF-based ratiometric thermometers and their mechanism of energy transfer for determining the temperature (thermal sensitivity and temperature uncertainty). In addition, some fundamental points are also discussed, such as concepts for guiding the design of MOF ratiometric thermometers, thermometric performance and tuning the properties of MOF thermometers.

Bis-Schiff base cellulosic nanocrystals for Hg (II) removal from aqueous solution with high adsorptive capacity and sensitive fluorescent response

Mercury pollution in aqueous solutions is a severe problem in environmental protection and the contaminated water may cause serious risks to human health. Based on the constant development of adsorptive materials, adsorption technique is widely applied as an efficient and convenient approach to eliminate mercury species from waters. In this work, we report a one-pot procedure to prepare a bis-Schiff base cellulosic adsorbent to integrate the advantages of large adsorptive capacity and excellent fluorescent recognition towards mercury ions. The adsorption experiments demonstrate that sulfydryl-contained cellulosic nanocrystals exhibit specific affinity with mercury species and the adsorption capacity reaches as high as 624.8 mg/g at room temperature. Besides, the introduction of rhodamine moiety endows the material a 19 times enhancement of selective "off-on" fluorescent sensing while exposed to mercury. Additionally, the bifunctional adsorbent material shows high sensitivity towards mercury ions in aqueous solution with detection limits of as low as 8.29 × 10^-8 M for fluorescence and 5.9 × 10^-9 M for UV-vis spectrum, respectively. The fitting results of the adsorption models indicate a monolayer adsorption during the uptake of mercury ions and the removal process follows the pseudo-second order kinetics. Moreover, density functional theory studies are employed to further understand the adsorptive and responsive mechanisms.

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.

Detection of Heavy Metal Ions by Ratiometric Photoelectric Sensor

In recent years, heavy metal pollution has become increasingly serious. Heavy metals exist in an environment mainly in the form of ions (heavy metal ions, HMs). They can contaminate food, water, soil, and the atmosphere, leading to serious harm to plants and animals. With high bioavailability and nonbiodegradability, HMs can accumulate through biomagnification. Consequently, heavy metal pollution has become the cause of many fatal diseases threatening human health and ecological environment. Therefore, the accurate detection of HMs is vital and necessary. In this paper, the harm and limit standards of heavy metals were systematically summarized and the common analysis methods were overviewed and compared. Specifically, the latest research progress of ratiometric photoelectric sensor, including optical and electrical sensor, were mainly described. The research status and advantages and disadvantages of a photoelectric sensor were summarized. Furthermore, the future directions were proposed, which provided the reference for the further research and application of the ratiometric photoelectric sensor.

Thermal-controlled active sensor module using enzyme-regulated UiO-66-NH2/MnO2 fluorescence probe for total organophosphorus pesticide determination

An enzyme-regulated UiO-66-NH₂/MnO₂ fluorescence sensor, fully functionalized with spectrometric capacities, is developed for budget-friendly total organophosphorus pesticides (OPs) determination. The fluorescence probe, UiO-66-NH₂/MnO₂, is hydrothermally synthesized and morphologically examined. A specialized enzyme-catalyzed reaction, which can be gradually inhibited by OPs, is designed with participations of alkaline phosphatase (ALP) and sodium L-ascorbyl-2-phosphate (AAP). The reaction product of ascorbic acid (AA) decomposes MnO₂ and restores UiO-66-NH₂ fluorescence, establishing a relationship between OPs level and fluorescence intensity. Interactions among UiO-66-NH₂, MnO₂, OPs, and AA are clarified. Stepwise optimizations are performed to the UiO-66-NH₂/MnO₂ probe, ensuring considerable advantages as OPs affinity and fluorescence quenching behavior over rival nanomaterials. Analytical advances are magnified by fabricating an active sensor module, with self-acting thermal regulation for optimal enzyme activity. Under 4 and 20 °C environment, regulation period is less than 40 and 100 s. In total OPs determination for laboratorial and real-vegetable samples, this method exhibits uniform and log-linear responses to common species of OPs in a range as 1.0 × 10^-7~10 mg L^-1, and limit of detection is established as 8.9 × 10^-8 mg L^-1. Proposed readouts are validated with certified HPLC and recovery test. Relative errors and recovery rates are found as 2.7-6.4% and 95.8-102.6%, respectively.

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.

Colorimetric and ratiometric fluorescent dual-mode sensitive detection of Hg2+ based on UiO-66-NH2@Au composite

A colorimetric and ratiometric fluorescent dual-mode assay is constructed for sensitive and specific Hg²⁺ sensing based on UiO-66-NH₂ and Au composite (UiO-66-NH₂@Au). The addition of Hg²⁺ stimulates the peroxidase-like activity of UiO-66-NH₂@Au by the formation of Au-Hg amalgam, promoting the oxidizing of chromogenic substrate OPD to DAP with the aid of H₂O₂, which lead to the change of colorimetric and fluorescent signals. The absorbance of the sensing system at 450 nm is linear positive correlation with Hg²⁺ concentration of 30-1400 nM and the color of the solution under visible light shaded from light yellow to dark yellow. With the increase of Hg²⁺ concentration, the fluorescence signal at 570 nm (DAP) increased whereas that at 455 nm (intrinsic fluorescence of UiO-66-NH₂) decreased due to inner filter effect (IFE), the fluorescence intensity ratio (F₄₅₅/F₅₇₀) decreasing linearly with Log [Hg²⁺] over the range 60-1700 nM; the fluorescence emission of sensing system under UV excitation changed from blue to yellow, which can easily be discerned visually. This assay was successfully applied to the determination of Hg²⁺ in tap water and river water. The results indicate that the colorimetric and ratiometric fluorescent dual-mode assay based on UiO-66-NH₂@Au realized visual determination of Hg²⁺ rapidly and reliably, revealed application prospect in Hg²⁺ monitoring.

Detection and Sorption of Heavy Metal Ions in Aqueous Media by a Fluorescent Zr(IV) Metal-Organic Framework Functionalized with 2-Picolylamine Receptor Groups

Increasing global environmental pollution due to heavy metal ions raises the importance of research on new multifunctional materials for simultaneous detection and removal of these contaminants from water resources. In this study, we report a microporous 8-connected Zr⁴⁺ metal-organic framework (MOF) based on a terephthalate ligand decorated with a chelating 2-picolylamine side group (dMOR-2), which shows highly efficient fluorescence sensing and sorption of heavy metal cations. We demonstrate by detailed fluorescence studies the ability of a water-dispersible composite of dMOR-2 with polyvinylpyrrolidone for real-time detection of Cu²⁺, Pb²⁺, and Hg²⁺ in aqueous media. The limits of detection were found to be below 2 ppb for these species, while the system's performance is not affected by the presence of other potentially competitive ions. In addition, sorption studies showed that a composite of dMOR-2 with calcium alginate (dMOR-2@CaA) is an excellent sorbent for Pb²⁺ and Cu²⁺ ions with capacities of 376 ± 15 and 117 ± 4 mg per gram of dMOR-2@CaA, respectively, while displaying the capability for simultaneous removal of various heavy metal ions in low initial concentrations and in the presence of large excesses of other cationic species. Structural and spectroscopic studies with model ligands analogous to our material's receptor unit showed chelation to the 2-picolylamine moiety to be the main binding mode of metal ions to dMOR-2. Overall, dMOR-2 is shown to represent a rare example of a MOF, which combines sensitive fluorescence detection and high sorption capacity for heavy metal ions.

Dual-emitting metal-organic frameworks for ratiometric fluorescence detection of fluoride and Al3+ in sequence

Fluoride (F^-) and Al³⁺ are two common ions existing in drinking water and natural water bodies. Excessive intake of F^- can lead to serious health issues such as fluorosis and bone diseases while accumulated consumption of Al³⁺ may cause neurotoxicity-based diseases. Developing a fast, reliable, and sensitive sensor for visually detecting both F^- and Al³⁺ is of great significance. In the present work, a ratiometric fluorescence sensor was constructed by incorporating rhodamine B (RhB) in situ into a zirconium-based metal-organic framework, UiO-66-NH₂. The obtained nanocomposite UiO-66-NH₂@RhB exhibited similar octahedral structure to UiO-66-NH₂ with high BET surface area, and showed two emission peaks at 450 nm and 585 nm. The blue fluorescence from UiO-66-NH₂ was enhanced by the addition of F^- while subsequent Al³⁺ addition diminished the increased fluorescence intensity, and the red emission from RhB as the reference remained unchangeable to improve the detection precision. Under optimal conditions, detection of limits as low as 1.55 μM for F^- and 0.54 μM for Al³⁺ in aqueous solution were achieved with good selectivity. High recoveries in drinking water samples were also acquired, showing potential applications of this ratiometric fluorescence sensor for practical evaluation of F^- and Al³⁺.

Cost-Effective and Selective Fluorescent Chemosensor (Pyr-NH@SiO2 NPs) for Mercury Detection in Seawater

The release of mercury into the environment has adverse effects on humans and aquatic species, even at very low concentrations. Pyrene and its derivatives have interesting fluorescence properties that can be utilized for mercury (Hg²⁺) ion sensing. Herein, we reported the highly selective pyrene-functionalized silica nanoparticles (Pyr-NH@SiO₂ NPs) for chemosensing mercury (Hg²⁺) ions in a seawater sample. The Pyr-NH@SiO₂ NPs were synthesized via a two-step protocol. First, a modified Stöber method was adopted to generate amino-functionalized silica nanoparticles (NH₂@SiO₂ NPs). Second, 1-pyrenecarboxylic acid was coupled to NH₂@SiO₂ NPs using a peptide coupling reaction. As-synthesized NH₂@SiO₂ NPs and Pyr-NH@SiO₂ NPs were thoroughly investigated by ¹H-NMR, FTIR, XRD, FESEM, EDS, TGA, and BET surface area analysis. The fluorescent properties were examined in deionized water under UV-light illumination. Finally, the developed Pyr-NH@SiO₂ NPs were tested as a chemosensor for Hg²⁺ ions detection in a broad concentration range (0–50 ppm) via photoluminescence (PL) spectroscopy. The chemosensor can selectively detect Hg²⁺ ions in the presence of ubiquitous ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Ag⁺, and seawater samples). The quenching of fluorescence properties with Hg²⁺ ions (LOD: 10 ppb) indicates that Pyr-NH@SiO₂ NPs can be effectively utilized as a promising chemosensor for mercury ion detection in seawater environments.

A ratiometric sensor for selective detection of Hg2+ ions by combining second-order scattering and fluorescence signals of MIL-68(In)-NH2

Ratio fluorescence has attracted much attention because of its self-calibration properties. However, it is difficult to obtain suitable fluorescent materials with well-resolved signals simultaneously under one excitation. In this work, we report a different strategy, using MIL-68(In)-NH₂ as both the fluorescence element and the scattered light unit, and coupling the fluorescence and the scattered light to construct the fluorescence and scattered light ratio system. Based on the optical properties and the second-order scattering (SOS) of the material nanoparticles, the synthesized MIL-68(In)-NH₂ can be used to realize the ratio detection of Hg²⁺. Because the scattering intensity of small particle MIL-68(In)-NH₂ is weak, SOS is not obvious. When Hg²⁺ is introduced the coordination reaction between the amino nitrogen atoms of MIL-68(In)-NH₂ and Hg²⁺ make the particles larger, resulting in the decrease of fluorescence and the enhancement of SOS. As a result, a novel Hg²⁺ ratiometric detection method is developed by using the dual signal responses of the fluorescence and scattering. Under the optimal conditions (pH = 6, reaction time 5 min, room temperature, and the maximum excitation wavelength 365 nm), the linear range of the method is 0-100 μM, and the detection limit is 5.8 nM (Ksv = 9.89 × 10⁹ M^-1). In addition, the probe is successfully used to evaluate Hg²⁺ in actual water samples. Compared with the traditional method of recording only the fluorescence signal, the proposed fluorescence-scattering method provides a new strategy for the design of ratiometric sensors.

Silver nanoparticles combined with amino-functionalized UiO-66 for sensitive detection of glutathione

Herein, a fluorescent nanosensor has been constructed for detection of glutathione (GSH) based on NH₂-UiO-66 and AgNPs. The NH₂-UiO-66 was synthesized where 2-amino-terephthalic acid as the organic ligand and Zr⁴⁺ as the center metal ions. The AgNPs can enhance the fluorescence of NH₂-UiO-66 based on metal enhanced fluorescence (MEF) effect. Moreover, in the present of GSH, the fluorescence of NH₂-UiO-66@AgNPs was quenched via electrostatic interaction and Ag-S reaction. The present sensing strategy shows good linear relation with the concentration of GSH in the range of 0.2-1.0 μM and 1.0-30 μM, and the limit of detection is 79 nM. Furthermore, our fluorescent nanosensor was utilized to detect GSH in human serum with a recovery of 96.8-102.5%. The results indicated that NH₂-UiO-66@AgNPs is successfully applied for high sensitive and selective detection of GSH in human serum.

Facile synthesis of binary two-dimensional lanthanide metal-organic framework nanosheets for ratiometric fluorescence detection of mercury ions

Three boric acid-functionalized two-dimensional metal-organic frameworks (2D MOFs) nanosheets, namely, 2D Tb-bop, Eu-bop and Tb/Eu-bop nanosheets, were synthesized through simple mixing of a reaction mixture of Tb/Eu salts and 5-boronoisophthalic acid (5-bop) in the presence of triethylamine (TEA) at room temperature. The product had excellent mercury ions (Hg²⁺) sensing properties. The highly exposed boric acid sites on the surface of the 2D structure can undergo a transmetalation reaction with Hg²⁺ to promote the energy transfer between the ligand and the lanthanide ions, thus enhancing the emission of 2D Tb-bop and Eu-bop nanosheets and resulting in high sensitivity for Hg²⁺ sensing. Significantly, the Hg²⁺-induced transmetalation reaction in 2D binary Tb/Eu-bop nanosheets also enhanced the energy transfer between Tb³⁺ and Eu³⁺ nodes, showing a ratiometric fluorescence response toward Hg²⁺, and further improving the sensitivity. This novel 2D ratiometric fluorescence probe showed good linearity from 0.1 to 40 μM for Hg²⁺ with a detection limit of 4.83 nM. This work not only provided a simple and efficient strategy for synthesizing 2D MOFs but also yielded new insights for designing fluorescence probes with excellent sensing properties.

Simple Design Concept for Dual-Channel Detection of Ochratoxin A Based on Bifunctional Metal-Organic Framework

A simple fluorescence and electrochemical dual-channel biosensor based on bifunctional Zr(IV)-based metal-organic framework (Zr-MOF) was proposed to detect Ochratoxin A (OTA). The bifunctional Zr-MOF, with photoluminescence properties and enormous electroactive ligands, was exploited to load OTA-specific aptamers for designing signal probes, greatly simplifying the probe-fabrication process and improving sensing reliability. Upon specific recognition of aptamer toward OTA, the anchored probe was released from the sensing interface into the reaction solution. In this circumstance, the increased amount of the signal probe in reaction solution led to an enhanced fluorescence response, while the decreased amount of the signal probe on the sensing interface resulted in a diminished electrochemical response. According to the dual-channel signal change with increasing OTA concentration, the visual fluorescence strategy was established for intuitive OTA detection, and meanwhile, sensitive electrochemical assay with a detection limit of 0.024 pg/mL was also achieved with the help of one-step electrodeposition as a sensing platform. Moreover, the proposed dual-channel assay has been successfully applied to determine OTA levels in corn samples with rapid response, superior accuracy, and high anti-interference capability, providing a promising method for food safety monitoring.

Visual Hg(II) sensing in aqueous solution via a new 2,5-Bis(4-pyridyl)thiazolo[5,4-d]thiazole-based fluorescence coordination polymer

A new fluorescence coordination polymer [Zn(Py₂TTz)(5-OH-IPA)]_n (1) (Py₂TTz = 2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole, 5-OH-IPA = 5-hydroxyisophthalic acid dianion) was synthesized, which exhibited the characteristics of fluorescence quenching and bathochromic shift toward Hg(II) in aqueous solution at pH 7.00. Mechanism study showed that the interactions between Hg(II) ions and Py₂TTz ligands in 1 were responsible for the fluorescence emission change. Thanks to the specific interactions between 1 and Hg(II), excellent selectivity was achieved both in aqueous solution and in solid test paper. The detection limit of 1 for Hg(II) sensing was 125.76 nmol L^-1 and a linear rang was 1.00-10.00 μmol L^-1. More importantly, satisfactory recovery and accuracy of 1 for Hg(II) sensing were also obtained in buffer-free real water samples.

Ratiometric fluorescent sensor based on 2D MOF nanosheets modified by DNA for sensitive detection of Hg2

Mercury is highly toxic and can accumulate throughout the food cycle, leading to water contamination and foodstuffs pollution. Therefore, increasing attention has been paid to explore effective detection of Hg²⁺. Here, we report a sensitive Hg²⁺sensor based on single-stranded DNA (ssDNA) modified two-dimensional (2D) MOF nanosheets by a ratiometric fluorescent method. The chosen 2D MOF nanosheets possess intrinsic peroxidase-like catalytic ability, ssDNA adsorption and fluorescence quenching. We demonstrate that the adsorption of ssDNA can significantly improve the peroxidase mimetic activity of 2D MOF nanosheets, enhancing the fluorescence of substrate Amplex Red. Taking advantages of the favorable characteristics above, we fabricate an efficient Hg²⁺sensor. In the presence of Hg²⁺, the ssDNA is released from 2D MOF nanosheets, which results in a decreasing of peroxidase mimetic activity of 2D MOF nanosheets and a fluorescence enhancement of attached fluorophore. A linear relationship between ratiometric fluorescence of substrate and fluorophore and Hg²⁺concentrations is obtained. The detection limit is 5 nM, which is much lower than the maximal contamination level in drinking water (30 nM) by Word Health Organization. These findings show 2D MOF based ratiometric fluorescent sensor is a convenient and efficient strategy to detect Hg²⁺.

Ultrasensitive and turn-on homogeneous Hg2+ sensing based on a target-triggered isothermal cycling reaction and dsDNA-templated copper nanoparticles

In this work, an ultrasensitive and turn-on sensor for homogeneous Hg2+ detection has been constructed based on a target-triggered isothermal cycling reaction and rapid label-free signal output with dsDNA-templated copper nanoparticles (CuNPs). As the key component of the sensor, a hairpin DNA without any labels was designed to contain different functional sequence segments and to resist digestion by exonuclease due to the protruding 3'-terminus. In the presence of Hg2+, the formation of a T-Hg2+-T structure turned the protruding 3'-terminus of the hairpin DNA to a blunt end that could be efficiently digested by Exo III, accompanied by Hg2+ release, followed by another digestion cycle. Hence, the Hg2+-triggered isothermal cycling reaction accumulated numerous dsDNA templates that facilitated fluorescent CuNP generation and finally output an amplified signal used to identify the target. This protocol is capable of Hg2+ sensing in a concentration range of 5 orders of magnitude with a detection limit down to 3.9 pM. The as-constructed sensor also revealed high selectivity, as well as satisfactory results in recovery experiments of Hg2+ detection in real water samples.

Multifunctional bacteria-derived tags for advancing immunoassay analytical performance with dual-channel switching and antibodies bioactivity sustaining

Constructing multifunctional immunochromatographic assays (ICA) carriers with multiple signals and retaining bioactivity of monoclonal antibodies (mAbs) are conducive to the sensitive and accurate point-of-care testing (POCT). To fulfill this pressing need, a microorganism-based microsphere mediated dual-modal ICA (DICA) was developed for sensitive and reliable detection of zearalenone (ZEN). As the key to the biosensor, a superb biotag with an intact coccus morphology was designed based on Staphylococcus aureus biosynthesized quantum dots incorporating Ru(bpy)₃²⁺ (SAQDsRu), in which SA offered a specific recognition capacity for Fc region of mAbs, QDs endowed a naked-eye discernible colorimetric signal on the SA, and robust fluorescence signal that remedied for the insufficient brightness of QDs was derived from Ru(bpy)₃²⁺. The characterization of SAQDsRu-labeled mAb (SAQDsRu-mAb) probe demonstrated strong luminescence, excellent stability and high affinity with ZEN (affinity constant was approximately 1.723 × 10⁹ M^-1), which can significantly improve the detection sensitivity. Impressively, a portable sensing system was developed by the integration of SAQDsRu-DICA with a smartphone-based readout. After optimization, this DICA indicated a limit of detection reaching down to 0.008 ng/mL (colorimetric mode) and 0.0058 ng/mL (fluorescent mode), which were much lower than that of conventional gold nanoparticles-based ICA (0.1029 ng/mL), possessing favorable specificity and repeatability (relative standard deviation (RSD) < 10%). Moreover, the feasibility of the immunoassay was further assessed by measuring ZEN in real samples with satisfactory recoveries, and the results are good consistent with liquid chromatography tandem mass spectrometry (LC-MS/MS).