Linker-Eliminated Nano Metal-Organic Framework Fluorescent Probe for Highly Selective and Sensitive Phosphate Ratiometric Detection in Water and Body Fluids

Principal component analysis-assisted zirconium-based metal-organic frameworks/DNA biosensor for the analysis of various phosphates

Considering the diversity of phosphates and their pivotal roles in physiological processes, detection of various phosphates related to their metabolism is urgent but challenging. Herein, we design a biosensor with zirconium-based MOFs (Zr-MOFs) and fluorophore-modified single-stranded DNA (F-ssDNA) for the analysis of phosphates. Relying on the interaction between Zr clusters and phosphate backbone, F-ssDNA is anchored on the surface of Zr-MOFs, inducing fluorescence resonance energy transfer (FRET) and subsequently quenching the fluorescence of F-ssDNA. Meanwhile, phosphates with different numbers of phosphate groups, molecular structures and coordination environments are able to adjust the FRET between Zr-MOFs and F-ssDNA via a site-occupying effect, recovering the fluorescence of F-ssDNA in distinct cases, which may result in diverse fluorescence signals. Consequently, seventeen phosphates and four phosphate mixtures are discriminated with the assistance of principal component analysis. These results provide new insight into the application of Zr-MOFs and broaden the path for the development of analytical methods for phosphates.

A fluorescence based dual sensor for Zn2+ and PO43- and the application of soft computing methods to predict machine learning outcomes

A phenolphthalein-based Schiff base, 3,3-bis-{4-hydroxy-3-[(pyridine-2-ylmethylimino)-methyl]-phenyl}-3H-isobenzofuran-1-one (PAP), has been synthesized and used for selective fluorescence 'turn on' and 'turn off' sensing of Zn2+ and PO43- respectively. The limit of detection using the 3σ method for Zn2+ is found to be 19.3 nM and that for PO43- is 8.3 μM. The sensing mechanism of PAP for Zn2+ ions has been explained by 1H NMR, 13C NMR, TRPL, ESI-MS, FT-IR, and DFT based calculations. Taking advantage of this fluorescence 'on-off' behavior of PAP in the sequential presence of Zn2+ and PO43- a two input fuzzy logic (FL) operation has been constructed. The chemosensor PAP can thus act as a metal ion and anion responsive molecular switch, and its corresponding emission intensity is used to mimic numerous FL functions. To replace various expensive, time-consuming experimental procedures, we implemented machine learning soft computing tools, such as fuzzy-logic, artificial neural networks (ANNs), and adaptive neuro-fuzzy inference systems (ANFIS), to correlate as well as predict the fluorescence intensity in the presence of any equivalent ratio of Zn2+ and PO43-. The statistical performance measures (MSE and RMSE, for example) show that the projected values of the cation and anion sensing data by the ANFIS network are the best and closer to the experimental values.

Tuning the Fluorometric Sensing of Phosphate on UiO-66-NH2(Zr, Ce, Hf) Metal Nodes

Metal-organic frameworks (MOFs) with intrinsic luminescent properties, modular structure, and tunable electronic properties, provide unique opportunities for designing target-specific molecular sensors by systematically choosing their constituent building blocks. We report a simple one-step MOF-based sensing platform for phosphate (P) detection that combines the luminescent properties of 2-aminoterephthalic acid (ATA) with the affinity of rationally selected nodes in UiO-66-NH2 to bind with P. This MOF possesses an electron-donating amine group that controls the light-harvesting characteristics of the linkers. Substituting Zr6 node with Ce6 or Hf6 results in a series of isostructural MOFs with distinct optical properties that are nonexistent in the unsubstituted MOF. We have utilized these MOFs to quantitatively measure P, using its ability to bind strongly to metal nodes inhibiting the LMCT process and altering the linker's photon emission. Using this system, detection limits of 4.5, 7.2 and 10.5 μM were obtained for the UiO-66-NH2(Ce), UiO-66-NH2, and UiO-66-NH2(Hf) respectively, adopting a straightforward single step procedure. These results demonstrate that the selection of metal nodes in a series of isostructural MOFs can be used to modulate their electronic properties and create sensing probes possessing the desired characteristics needed for the detection of environmental contaminants.

Synthesis of rare-earth metal-organic frameworks to construct high-resolution sensing array for multiplex anions detection, cell imaging and blood phosphorus monitoring

Accurate detection and differentiation of multiple anions is still a difficult problem due to their wide variety, structural similarity, and mutual interference. Hence, four rare-earth metal-organic frameworks (RE-MOFs) including Dy-MOFs, Er-MOFs, Tb-MOFs and Y-MOFs are successfully prepared by using TCPP as the ligand and rare-earth ions as the metal center via coordination chelation. It is found that 7 anions can light up their fluorescence. Thus, a high-resolution sensing array based on RE-MOFs nanoprobes is employed to differentiate these anions from intricate analytes in real-time scenarios. The distinctive host-guest response promotes the RE-MOFs nanoprobes to selectively extract the target anions from the complex samples. By taking advantage of the cross-response between RE-MOFs nanoprobes and anions, it allows to create an array for detecting target analytes using pattern recognition. Additionally, RE-MOFs nanoprobes also facilitate the quantitative analysis of these anions (PO43-, H2PO4-, HPO42-, F-, S2-, CO32- and C2O42-). More importantly, the exceptional effectiveness of this method has been demonstrated through various successful applications, including quality monitoring of 8 toothpaste brands, intracellular phosphate imaging, and blood phosphorus detection in mice with vascular calcification. These findings provide robust evidence for the efficacy and reliability of the RE-MOFs nanoprobes array for anion recognition.

An intelligent "chemical tongue" for high-order monitoring ATP-related physiological phosphates and ATP hydrolysis through diverse transduction principles

For discriminating diverse analytes and monitoring a specific chemical reaction, the emerging multi-channel "chemical nose/tongue" is challenging multi-material "chemical nose/tongue". The former contributes greatly to integrating different transduction principles from a single sensing material, avoiding the need for complex design, high cost, and tedious operation involved with the latter. Therefore, this high-order sensing puts a particular emphasis on the effects of encapsulation. Herein, the plasmonic gold nanoparticles (Au NPs) are encapsulated as a core into the fluorescent guanine monophosphate-Tb3+ infinite coordination polymer nanoparticles (GMP-Tb ICPs) to obtain a core-shell nanocomposite named Au NPs@GMP-Tb ICPs. Hence, a dual-channel "chemical tongue" based on Au NPs@GMP-Tb ICPs is present to realize high-order sensing of adenosine triphosphate (ATP)-related physiological phosphates and the monitoring of ATP hydrolysis. Considering the affinity of Tb3+ towards P-O bonds, four inorganic phosphates and three nucleotide phosphates with different phosphate group numbers and steric hindrance effect directly regulate two stimulus responses (fluorescence intensity and UV-vis absorbance) of Au NPs@GMP-Tb ICPs. Robust statistical methods, such as linear discriminant analysis and hierarchical cluster analysis, are used to recognize each phosphate by the developed sensor array either in the aqueous solution or in complex media such as serum, together with efficiently monitored ATP hydrolysis at different intervals. These findings and blind test clarify that the designed "chemical tongue" guarantees interference resistance and strengthens analytical capacity, together with offering valuable insight into "lab-on-a-nanoparticle" development for monitoring specific chemical reactions.

Upconversion luminescence nanosensor for detection of Fe3+ and phosphate ion based on the inner-filter effect

In this work, an upconversion luminescence (UCL) nanosensor for fast detection of ferric ion (Fe3+) and phosphate ion (Pi) is developed based on the inner-filter effect (IFE) between NaYF4:Yb/Er upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex. Fe3+-HB complex has strong absorption band (450-650 nm), which overlaps with the green emission peak of UCNPs at 545 nm. By adding Fe3+ and Pi, the UCNPs-HB system produces the red-shift change of absorption spectrum, which leads to the "on-off-on" process of IFE. So, with the specific recognition ability of HB for Fe3+ and the competitive complexation of Pi for Fe3+, the proposed nanosensor utilizes the UCL change to achieve the detection of the targets. For the detections of Fe3+, the linear range is 10-600 μM with a limit of detection (LOD) of 2.62 μM, and for Pi, the linear range is 5-100 μM with a LOD of 1.25 μM. The results for selectivity, precision, and recovery test are also satisfactory. Furthermore, the real sample detection shows that the proposed nanaosensor has a great potential in environmental and biological systems. An upconversion luminescence (UCL) nanosensor based on the inner-filter effect (IFE) between upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex for the detection of Fe3+ and phosphate ion has been proposed, which is promising to be a convenient and sensitive assay for monitoring Fe3+ and phosphate ion in different environments and biological systems.

Reversible and Turn-On Fluorescence Detection of Phosphate in Aqueous Solution and Living Cell Imaging by Supramolecular Metallacycles with AIE-Active Ligands

Luminescent supramolecular metallacycles have attracted great interest as a new promising class of sensing substrates. In this work, two tetraphenylethene (TPE)-based diimidazole and dipyrazole ligands with the aggregation-induced emission (AIE) feature were designed for the construction of supramolecular tetragonal metallacycles 1-4 with two 90° mononuclear [(bpy)M]2+ or dinuclear [(bpy)2M2]4+ acceptors (bpy = 2,2'-dipyridine; M = Pd, Pt), in which the fluorescence can be quenched to an "off" state due to the ligand-to-metal charge transfer (LMCT). Metallacycle 1 was utilized as a fluorescence sensor for phosphate (PO43-) detection in aqueous solution by means of disassembly, leading to the release of the ligand. Additionally, the metallacycle can be regenerated through self-assembly via the introduction of Pd(II) acceptors. PO43- was detected using TPE-based metallacycles over a wide concentration range, with a detection limit as low as 2.1 × 10-8 M. Furthermore, sensor 1 also presented the semiquantitative visual detection ability for PO43- in the test paper mode via fluorescence changes. The aforementioned studies not only enhance the current research on fluorescent materials but also offer a strategy for the creation of stimuli-responsive supramolecular coordination complexes.

Enhancing the Electrochemiluminescence of Porphyrin via Crystalline Networks of Metal-Organic Frameworks for Sensitive Detection of Cardiac Troponin I

Porphyrins easily aggregate due to unfavorable π-π accumulation, causing luminescent quenching in the aqueous phase and subsequently reducing luminescent efficiency. It is a feasible way to immobilize porphyrin molecules through metal-organic framework materials (MOFs). In this study, 5,10,15,20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) was introduced into the metal-organic skeleton (PCN-224) as a ligand. The result showed that the electrochemiluminescence (ECL) and photoluminescence (PL) efficiency of the MOF skeleton was 8.2 and 6.5 times higher than TCPP, respectively. Impressively, the periodic distribution of porphyrin molecules in the MOF framework can overcome the bottleneck of porphyrin aggregation, resulting in the organic ligand TCPP participating in the electron transfer reaction. Herein, based on the PCN-224, a sandwich-type ECL immunosensor was constructed for the determination of cardiac troponin I (cTnI). It provided sensitive detection of cTnI in the range of 1 fg/mL to 10 ng/mL with a detection limit of 0.34 fg/mL. This work not only innovatively exploited a disaggregation ECL (DIECL) strategy via the crystalline framework of MOF to enhance the PL and ECL efficiency of porphyrin but also provided a promising ECL platform for the ultrasensitive monitoring of cTnI.

Kill two birds with one stone: Ratiometric sensing of phosphate via a single-component probe with fluorescence-scattering dual-signal response behavior

Ratiometric fluorescence sensors gain stronger anti-interference ability via self-calibration. Nevertheless, ratiometric analysis of phosphate (Pi) still faces problems such as complicated construction process of dual emission probes and possible interferences from outputting mono-category fluorescent signal. Herein, we propose a "kill two birds with one stone" strategy to address these challenges, by simply introducing a single-component probe, porphyrin paddlewheel framework-3 (PPF-3) nanosheets without modification, encapsulation or complex, to integrate fluorescence (FL)-second-order scattering (SOS) dual-signal for ratiometric detection of Pi. PPF-3 nanosheets are constructed by coordination of Co²⁺ with 5,10,15,20-tetrakis(4-carboxyl-phenyl)-porphyrin (TCPP) ligands, displaying weak FL and strong SOS, two different and independent signals. In the response system to Pi, Co²⁺ and TCPP serve as the recognition element and signal unit, respectively. After interacting with Pi, the high affinity for Co²⁺ makes Pi snatch Co²⁺ from the PPF-3 nanosheets, causing their structure disassembly (SOS decrease) and TCPP release (FL increase). Finally, the FL-SOS ratiometric platform is successfully employed to access Pi in real water samples. Synchronous collection of FL and SOS from the single-component probe provides a simpler and more efficient way on ratiometric sensor design as well as a new useful technique for monitoring target-induced aggregation and disaggregation behavior.

Ratiometric fluorescent sensing of phosphate ion in environmental water samples using flavin mononucleotide-functionalized Fe3O4 particles

Phosphate ion (PO₄^3-) serves as an important nutrient carrier to support the growth of aquatic animals and plants in aquatic systems. However, excess concentrations of PO₄^3- are the key factor responsible for eutrophication, resulting in rapid deterioration of water quality. Therefore, accurate determination of PO₄^3- is of great significance in water quality and security. In this study, flavin mononucleotide (FMN), an intracellular form of vitamin B₂, was used as fluorophore. A novel "off-on" fluorescent sensing platform (FMN@Fe₃O₄) was fabricated for selective and sensitive detection of PO₄^3-, and showed excellent fluorescence response and good selectivity for PO₄^3- detection. With the addition of PO₄^3-, the fluorescence intensity restored is proportional to PO₄^3- concentration in the quantification range of 50 nM-0.75 μM with a limit of detection as low as 20 nM (0.62 μg.L^-1, calculated by P element). An adsorption/desorption sensing mechanism via an in-depth analysis of the interfacial interaction between PO₄^3- and FMN@Fe₃O₄ is proposed. FMN is first adsorbed by its terminal phosphate group on Fe₃O₄ particles to quench fluorescence. Free PO₄^3- replaces the adsorbed FMN and restores the quenched fluorescence to achieve the aim of PO₄^3- detection. In addition, this sensing system has been successfully validated in real water sample analysis and all reagents involved are nontoxic, environmentally benign, and easily-available. Therefore, this assay has great applicability in water quality monitoring.

Assessing the Progression of Early Atherosclerosis Mice Using a Fluorescence Nanosensor for the Simultaneous Detection and Imaging of pH and Phosphorylation

The inflammatory microenvironment involves changes in pH and protein phosphorylation state and is closely related to the occurrence and development of atherosclerosis (AS). Herein, we constructed a dual-detection fluorescence nanosensor PCN-NP-HPZ based on post modification of MOFs, which realized the simultaneous detection and imaging of pH and phosphorylation through the pH-sensitive group piperazine and the Zr^IV node of the MOFs. The sensors were used to monitor changes in blood pH and phosphate levels at different time stages during atherosclerotic plaque formation. Two-photon fluorescence imaging was also performed in the vascular endothelium. Blood tests combined with two-photon fluorescence images indicated that in the early stage of AS, blood and tissue pH levels were lower than that of the normal mice, while phosphate and tissue phosphorylation levels were higher than that of the normal mice. The present study provides a new analysis method for the assessment of early atherosclerotic disease.

New insight into the application of fluorescence platforms in tumor diagnosis: From chemical basis to clinical application

Early and rapid diagnosis of tumors is essential for clinical treatment or management. In contrast to conventional means, bioimaging has the potential to accurately locate and diagnose tumors at an early stage. Fluorescent probe has been developed as an ideal tool to visualize tumor sites and to detect biological molecules which provides a requirement for noninvasive, real-time, precise, and specific visualization of structures and complex biochemical processes in vivo. Rencently, the development of synthetic organic chemistry and new materials have facilitated the development of near-infrared small molecular sensing platforms and nanoimaging platforms. This provides a competitive tool for various fields of bioimaging such as biological structure and function imaging, disease diagnosis, in situ at the in vivo level, and real-time dynamic imaging. This review systematically focused on the recent progress of small molecular near-infrared fluorescent probes and nano-fluorescent probes as new biomedical imaging tools in the past 3-5 years, and it covers the application of tumor biomarker sensing, tumor microenvironment imaging, and tumor vascular imaging, intraoperative guidance and as an integrated platform for diagnosis, aiming to provide guidance for researchers to design and develop future biomedical diagnostic tools.

Size- and shape-dependent cytotoxicity of nano-sized Zr-based porphyrinic metal-organic frameworks to macrophages

The wide utilization of nano-sized metal-organic frameworks (NMOFs) leads to inevitable health risks to humans. Previous studies on health risks of NMOFs mainly focus on the cytotoxic tests of typical NMOFs，but lack sufficient studies on the effects of physiochemical characteristics of NMOFs on the cytotoxicity and the related mechanisms. Here, four kinds of Zr-based porphyrinic NMOFs (PCNs), including spherical 30, 90, and 180 nm PCN-224 and rod-like 90 nm PCN-222, were taken as a proof of the concept to investigate the effects of the size and shape of NMOFs on the cytotoxicity and related mechanisms to macrophages. The 30 nm spherical PCN-224 induced significant rupture of cell membrane and dissolved in lysosome, leading to the most significant cell necrosis among the studied other nano-sized PCNs. However, other studied PCNs showed insignificant membrane rupture and their dissolution in lysosome. Furthermore, the 90 nm-sized PCN-224 led to much more significant cell necrosis by inducing lysosome damage and inhibiting of autophagy flux than the rod-like 90 nm PCN-222. These findings reveal the size- and shape-dependent cytotoxicity of PCNs and the related mechanisms and are helpful to the assessment of the potential health risks of NMOFs and the safe application of NMOFs.

Cobalt-Based Metal-Organic Framework Nanoparticles with Peroxidase-like Catalytic Activity for Sensitive Colorimetric Detection of Phosphate

Appropriate addition of phosphate salt in food can improve the food quality and taste. However, extensive intake of phosphate salt may lead to some human diseases such as hyperphosphatemia and renal insufficiency. Thus, it is essential to establish a cost-effective, convenient, sensitive, and selective method for monitoring phosphate ion (Pi) to ensure food quality control. In this work, a Co-based metal-organic frameworks (Co-MOF) nanomaterial with dual functions (peroxidase-like activity and specific recognition) was designed for acting as a catalytic chromogenic platform for sensitive detection of Pi. The Co2+ nodes not only provide high enzyme-like activity to catalyze the 3,3′,5,5′--tetramethylbenzidine (TMB) substrate to blue oxTMB (652 nm) but also act as selective sites for Pi recognition. The use of cationic organic ligands (2-methylimidazole) and cationic metal ions (Co2+) endows the Co-MOF with a strong positive surface charge, which is beneficial to the capture of negative-charged Pi and the dramatically suppressed TMB oxidation. When Pi exists, it specifically adsorbs onto the Co-MOF through the Co-O-P bond and the strong electrostatic interaction, leading to the change of surface charge on Co-MOF. The peroxidase-like catalytic activity of Co-MOF is thus restrained, causing a different catalytic effect on TMB oxidation from that without Pi. Based on this principle, a colorimetric assay was established for rapid and sensitive detection of Pi. A good linear relationship was obtained between Pi concentration and the absorbance at 652 nm, with a linear range of 0.009–0.144 mg/L and a detection limit of 5.4 μg/L. The proposed assay was applied to the determination of Pi in actual food samples with recoveries of 92.2–108% and relative standard deviations (RSDs) of 2.7–7.3%, illustrating the promising practicality for actual samples analysis.

Enhanced Ultrasensitive Photoelectrochemical Probe for Phosphate Detection in Water Based on a Zirconium-Porphyrin Framework

Excessive phosphate poses a serious ecological and human health risk, and thereby, monitoring its trace concentration is of great significance to environmental protection and human health. In this work, a zirconium-porphyrin framework (PCN-222) with excellent stability and unique luminescence properties was designed to modify the surface of the indium tin oxide electrode, which was first used as a photoelectrochemical (PEC) probe for phosphate detection. The PCN-222-modified PEC probe demonstrated an excellent selectivity and stability and indicated a linear response to phosphate in the range of 0-10⁶ nM with a limit of detection (LOD) as low as 1.964 nM. To the best of our knowledge, this is the phosphate probe with the lowest LOD, and this is also the first signal-on PEC probe toward phosphate based on PCN-222. More importantly, the PEC probe can be validated for the good applicability of trace phosphate detection in real water samples, indicating a good application prospect. Finally, a series of electrochemical and spectroscopic studies have proved that phosphate can bind to the indium tin oxide (ITO)/PCN-222 electrode, which shortens the distance of the space charge region while reducing the bandwidth and thus facilitates the transfer of photogenerated electrons across the energy band barrier to reduce O₂ in the electrolyte, producing an enhanced cathodic photocurrent signal. The proposed strategy of the highly sensitive PEC probe provides a promising platform for more effective label-free phosphate monitoring in the environment and organisms.

Imine-Linked Covalent Organic Framework with a Naphthalene Moiety as a Sensitive Phosphate Ion Sensing

Due to the excellent ion-sensing potential of covalent organic frameworks (COFs), the new imine-linked conjugated COF (IC-COF) is synthesized through a water-based synthesis reaction between 1,5-diaminonaphthalene and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine to create a luminescence sensor. It is noteworthy that the green synthesized IC-COF shows excellent selectivity to phosphate ions (PO₄^3-) with a detection limit of 0.61 μM. The recyclability performance of IC-COF is high, indicating that it can be reused without a significant reduction in performance (5.2% decline after 5 cycles). Theoretical calculations using the density functional theory are performed on the IC-COF-PO₄^3- and IC-COF-Cu⁺ complexes to explore the sensing mechanism. The fluorescence quenching in the presence of PO₄^3- ions is attributed to the difference between PO₄^3- binding sites to the IC-COF compared to Cu⁺, which leads to the considerable change in the IC-COF absorption spectrum from 400 to 600 nm.

SERS Determination of Trace Phosphate in Aquaculture Water Based on a Rhodamine 6G Molecular Probe Association Reaction

Although phosphate (Pi) is a necessary nutrient for the growth of aquatic organisms, the presence of excess Pi leads to water eutrophication; thus, it is necessary to accurately determine the content of Pi in water. A method for the determination of trace Pi in aquaculture water was developed based on surface-enhanced Raman spectroscopy (SERS) combined with rhodamine 6G (R6G)-modified silver nanoparticles (AgNPs) as the active substrate. The adsorption of R6G on the AgNP surfaces led to a strong SERS signal. However, in the presence of Pi and ammonium molybdate, phosphomolybdic acid formed, which further associated with R6G to form a stable R6G-PMo₁₂O₄₀³⁻ association complex, thereby hindering the adsorption of R6G on the AgNPs, and reducing the SERS intensity; this sequence formed the basis of Pi detection. The decrease in the SERS intensity was linear with respect to the Pi concentration (0.2–20 μM), and the limit of detection was 29.3 nM. Upon the application of this method to the determination of Pi in aquaculture water, a recovery of 94.4–107.2% was obtained (RSD 1.77–6.18%). This study provides an accurate, rapid, and sensitive method for the trace determination of Pi in aquaculture water, which is suitable for on-site detection.

A Comprehensive Review on Water Quality Monitoring Devices: Materials Advances, Current Status, and Future Perspective

Water quality monitoring has become more critical in recent years to ensure the availability of clean and safe water from natural aquifers and to understand the evolution of water contaminants across time and space. The conventional water monitoring techniques comprise of sample collection, preservation, preparation, tailed by laboratory testing and analysis with cumbersome wet chemical routes and expensive instrumentation. Despite the high accuracy of these methods, the high testing costs, laborious procedures, and maintenance associated with them don't make them lucrative for end end-users and field testing. As the participation of ultimate stakeholders, that is, common man for water quality and quantity can play a pivotal role in ensuring the sustainability of our aquifers, thus it is essential to develop and deploy portable and user-friendly technical systems for monitoring water sources in real-time or on-site. The present review emphasizes here on possible approaches including optical (absorbance, fluorescence, colorimetric, X-ray fluorescence, chemiluminescence), electrochemical (ASV, CSV, CV, EIS, and chronoamperometry), electrical, biological, and surface-sensing (SPR and SERS), as candidates for developing such platforms. The existing developments, their success, and bottlenecks are discussed in terms of various attributes of water to escalate the essentiality of water quality devices development meeting ASSURED criterion for societal usage. These platforms are also analyzed in terms of their market potential, advancements required from material science aspects, and possible integration with IoT solutions in alignment with Industry 4.0 for environmental application.

Research Update of Emergent Sulfur Quantum Dots in Synthesis and Sensing/Bioimaging Applications

Due to their unique optical property, low toxicity, high hydrophilicity, and low cost, sulfur quantum dots (SQDs), an emerging luminescent nanomaterial, have shown great potential in various application fields, such as sensing, bioimaging, light emitting diode, catalysis, and anti-bacteria. This minireview updates the synthetic methods and sensing/bioimaging applications of SQDs in the last few years, followed by discussion of the potential challenges and prospects in their synthesis and sensing/bioimaging applications, with the purpose to provide some useful information for researchers in this field.

Two robust Zn(ii)-organic frameworks as dual-functional fluorescent probes for efficient sensing of enrofloxacin and MnO4− anions

Two robust Zn-MOFs were employed as visual and ultra-sensitive indicators toward enrofloxacin (ENR) and MnO4− anions in aqueous phase.

A Two-Fold Interpenetrated Dual-Emitting Luminescent Metal-Organic Framework as a Ratiometric Sensor for Chromium(III)

A novel two-fold interpenetrated metal-organic framework, namely Co-EDDA, was synthesized by hydrothermal reaction of 5,5'-[ethane-1,2-diylbis(oxy)]diisophthalic acid (H₄EDDA), Co(NO₃)₂·6H₂O, and 1,4-di(1H-imidazol-1-yl)benzene in water in an alkaline environment and structurally characterized. Co-EDDA could display clear dual-emission signals at 350 and 430 nm, representing the charge transfer emission between metal ions and the ligand and the ligand-based emission, respectively, which represents the ratiometric luminescence response to chromium(III) with high selectivity and sensitivity (limit of detection of 0.54 μM). Comprehensive studies indicate that the detection can be attributed to the interaction between the Cr³⁺ ions and the O atoms on the ether bond in Co-EDDA.

Rapid and accurate detection of phosphate in complex biological fluids based on highly improved antenna sensitization of lanthanide luminescence

Rapid and accurate detection of phosphate (Pi) in complex biological fluid is of critical importance for timely warning of Pi accumulation and monitoring Pi related pathological process. Up to date, various luminescent probes have been developed for Pi determination in aqueous media. However, the huge obstacles of the current probes suffer from the inherent issues such as time-consuming, tedious preparation and unavoidable background interference during Pi detection. To circumvent this limitation, we proposed a universal and facile strategy to fabricate a novel sensitizer-Ln³⁺@surfactant micelle probe with time-resolved luminescent (TRL) superiority through the self-assembly of sensitizer, Ln³⁺ and surfactant. Through this design, the sensitizer-Ln³⁺ chelate can be encapsulated into the surfactant constructed micelle and Ln³⁺ luminescence can be substantially lighted up through the effective energy transfer from the coordinated sensitizer and the assistance of Triton X-100. Such high TRL signal can be sensitively and specifically quenched by Pi, which was attributed to the specific coordination competition between sensitizer and Pi towards Ln³⁺. Benefitting from the background-free interference and highly sensitive TRL response of the sensitizer-Ln³⁺@surfactant probe, we achieved the rapid, selective and sensitive detection of Pi in the range of 0.5-120 μM with a limit of detection (LOD) of 0.19 μM. Furthermore, the accuracy of the proposed method based on the Ln³⁺ involved micelle probes was further verified through the quantitation of Pi in real biological samples.

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

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

"Switch-on" fluorescence sensing platform based on porphyrin metal-organic frameworks for rapid and specific detection of zinc ion

Zinc ion (Zn²⁺) is a necessary transition metal ion in the human body and plays a vital role in biological systems. Therefore, it is meaningful for rapid, sensitive, and specific detection of Zn²⁺. Herein, a "switch-on" fluorescence sensing platform for the detection of Zn²⁺ was successfully established. In this work, we report an optical nanoprobe with amino functional groups based on porphyrin metal-organic frameworks (PCN-NH₂) through a simple treatment method. The modification of the salicylaldehyde (SA) through the C=N acts as a quencher of PCN-NH₂ and the ligand of Zn²⁺ can effectively inhibit the fluorescence emission of PCN-NH₂. The complex of porphyrin-based metal-organic framework and salicylaldehyde (PCN/SA) acts as the fluorescence sensing probe. The presence of Zn²⁺ results in the new emission peak at 448 nm due to the formation of a complex between Zn²⁺ and SA. The corresponding fluorescence signal changes were used to accurately detect the concentration of Zn²⁺. The fluorescence sensing platform shows advantages, including rapid response, high sensitivity, and excellent specificity with a linear detection range of Zn²⁺ between 0.1 and 250 μM, and a low detection limit (LOD) of 0.07 μM. And the feasibility of the method was verified for the quantification of Zn²⁺ in cell lysate, environmental water, and plasma.

One-component nano-metal-organic frameworks with superior multienzyme-mimic activities for 1,4-dihydropyridine metabolism

Although a number of nanozymes have been developed, it is still difficult to develop single-component nanozyme with overall high multienzyme-like activities. In this study, the nanosized metal-organic frameworks (nano-MOFs) FePCN (PCN stands for porous coordination network) was synthesized by integrating zirconium and iron ions with different catalytic property on single-component MOFs and exhibited superior intrinsic multienzyme-like activities, namely oxidase-, peroxidase- and phosphatase-mimicking activity. The catalytic active sites of oxidase- and peroxidase-, and phosphatase-like activity of FePCN were Fe-centers and Zr-O clusters, respectively. Based on the intrinsic oxidase-like activity and the similarity of molecular structures between cytochrome P450 oxidase (CYP) cofactors and the organic linker in FePCN, FePCN exhibited high CYP-like activity to catalyze the oxidation of hypotensive drug 1,4-dihydropyridine (1,4-DHP) into diethyl 2,6-dimethylpyridine-3,5-dicarboxylate (DDPD) and the yield of DDPD reached over 80%. Moreover, as peroxidase- and phosphatase-mimics, FePCN was successfully applied to detecting H₂O₂ under neutral condition and catalyzing the dephosphorylation of adenosine triphosphate (ATP), respectively. This study provides a feasible way for rational design one-component nanomaterials as multienzyme-mimics.

Understanding disorder and linker deficiency in porphyrinic zirconium-based metal-organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221

Porphyrin-based metal–organic frameworks (MOFs), exemplified by MOF-525, PCN-221, and PCN-224, are promising systems for catalysis, optoelectronics, and solar energy conversion. However, subtle differences between synthetic protocols for these three MOFs give rise to vast discrepancies in purported product outcomes and description of framework topologies. Here, based on a comprehensive synthetic and structural analysis spanning local and long-range length scales, we show that PCN-221 consists of Zr₆O₄(OH)₄ clusters in four distinct orientations within the unit cell, rather than Zr₈O₆ clusters as originally published, and linker vacancies at levels of around 50%, which may form in a locally correlated manner. We propose disordered PCN-224 (dPCN-224) as a unified model to understand PCN-221, MOF-525, and PCN-224 by varying the degree of orientational cluster disorder, linker conformation and vacancies, and cluster–linker binding. Our work thus introduces a new perspective on network topology and disorder in Zr-MOFs and pinpoints the structural variables that direct their functional properties.

Zirconium-based metal–organic frameworks have defective structures that are useful in catalysis and gas storage. Here, the authors study the interplay between cluster disorder and linker vacancies in PCN-221 and propose a new structure model with tilted Zr₆O₄(OH)₄ clusters rather than Zr₈O₆ clusters.

A Ratiometric and Visual Sensing of Phosphate by White Light Emitting Quantum Dot Complex

Ratiometric and visual sensing of phosphate by using a white light emitting quantum dot complex (WLE QDC) is reported herein. The WLE QDC comprised of Mn²⁺-doped ZnS quantum dot (with λ_em = 585 nm) and surface zinc quinolate (ZnQS₂) complex (with λ_em= 480 nm). The limit of detection was estimated to be of 5.9 nM in the linear range of 16.6-82.6 nM. This was accomplished by monitoring the variations in the photoluminescence color, intensity ratio (I₄₈₀/I₅₈₅), chromaticity and hue of the WLE QDC in the presence of phosphate. The high selectivity and sensitivity of WLE QDC toward phosphate was observed. The chemical interaction of ZnQS₂ (present in WLE QDC) with phosphate might have led to the observed specificity in photoluminescence changes. The presented WLE QDC was successfully employed for the quantification of phosphate in samples prepared using environmental water and commercial fertilizer.

Enzymatic Behavior Regulation-Based Colorimetric and Electrochemiluminescence Sensing of Phosphate Using the Cobalt Oxyhydroxide Nanosheet

In this work, a convenient and flexible assay for colorimetric and electrochemiluminescence (ECL) sensing of phosphate was proposed based on the enzymatic behavior regulation of the cobalt oxyhydroxide (CoOOH) nanosheet. CoOOH as a novel nanoenzyme exhibited a peroxidase-like activity, which could catalyze different substrates such as 2, 2'-azinobis-3-ethylbenzthiazoline-6-sulfonate (ABTS) and 4-chloro-1-naphthol (4-CN) with hydrogen peroxide (H₂O₂) as the electron acceptor. Phosphate could specifically regulate the enzymatic behavior of the CoOOH nanosheet via the deactivating effect. A high level of phosphate enabled a weak color change of ABTS, which offered a "turn-off" model of the colorimetric assay with a limit of detection of 0.673 μM. Based on the similar enzymatic behavior, this strategy could then be applied in the ECL assay utilizing l-arginine-6-aza-2-thiothymine-protected gold nanoclusters (Arg-ATT-AuNCs) as ECL signal indicators. Specifically, 4-CN was catalyzed to generate the precipitate and lead to the quenching on ECL emission. Different from colorimetric behavior, phosphate with a high concentration could induce strong ECL performance, which enabled the "turn-on" model of the ECL assay with a more sensitive determination down to 0.434 nM. This flexible enzymatic behavior regulation could then allow the phosphate measurement in environmental samples including tap water and river water with satisfactory accuracy, which holds the potential in the field of environmental protection.

Ratiometric fluorescence for sensitive detection of phosphate species based on mixed lanthanide metal organic framework

Phosphate (PO₄^3-) plays a major role in aquatic ecosystems and biosystems. Developing a highly sensitive and selective ratiometric fluorescence probe for detection of PO₄^3- is of great significance to the ecological environment and human health. In this work, a novel dual lanthanide metal organic framework was synthesized via hydrothermal reaction based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand (designated as Tb-Ce-MOFs). The fluorescence of Tb-Ce-MOFs shows emission at 375 nm. In the presence of PO₄^3-, with increased concentration of PO₄^3-, the fluorescence intensity of Tb-Ce-MOFs at 500 nm and 550 nm increased, while the intensity at 375 nm was reduced. Hence, ratiometric fluorescence detecting of PO₄^3- can be achieved by measuring the ratio of fluorescence at 550 nm (FL₅₅₀) to 375 nm (FL₃₇₅) in the fluorescent spectra of the Tb-Ce-MOFs. In this sensing approach, the Tb-Ce-MOFs probe exhibits highly sensitive and selective for detection of PO₄^3-. The limit of detection is calculated to be 28 nM and the detection range is 0.1 to 10 μM. In addition, the Tb-Ce-MOFs were used in the detection of PO₄^3- in real samples. We design and synthesize a mixed lanthanide metal organic framework fluorescence probe (Tb-Ce-MOFs) for ratiometric fluorescence for the detection of PO₄^3- based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand.

Nanoscale Metal-Organic Frameworks as Fluorescence Sensors for Food Safety

Food safety has attracted attention worldwide, and how to detect various kinds of hazardous substances in an efficient way has always been a focus. Metal-Organic Frameworks (MOFs) are a class of hybrid porous materials formed by organic ligand and metal ions. Nanoscale MOFs (NMOFs) exhibit great potential in serving as fluorescence sensors for food safety due to their superior properties including high accuracy, great stability, fast response, etc. In this review, we focus on the recent development of NMOFs sensing for food safety. Several typical methods of NMOFs synthesis are presented. NMOFs-based fluorescence sensors for contaminants and adulterants, such as antibiotics, food additives, ions and mycotoxin etc. are summarized, and the sensing mechanisms are also presented. We explore these challenges in detail and provide suggestions about how they may be surmounted. This review could help the exploration of NMOFs sensors in food related work.

Recent advances in the applications of nano-agrochemicals for sustainable agricultural development

Modern agricultural practices have triggered the process of agricultural pollution. This process can cause the degradation of eco-systems, land, and environment owing to the modern-day by-products of agriculture. The substantial use of chemical fertilizers, pesticides, and, contaminated water for irrigation cause further damage to agriculture. The current scenario of the agriculture and food sector has therefore become unsustainable. Nanotechnology has provided innovative and resourceful frontiers to the agriculture sector by contributing practical applications in conventional agricultural ways and practices. There is a large possibility that agri-nanotechnology can have a significant impact on the sustainable agriculture and crop growth. Recent research has shown the potential of nanotechnology in improving the agriculture sector by enhancing the efficiency of agricultural inputs and providing solutions to agricultural problems for improving food productivity and security. The prospective use of nanoscale agrochemicals such as nanofertilizers, nanopesticides, nanosensors, and nanoformulations in agriculture has transformed traditional agro-practices, making them more sustainable and efficient. However, the application of these nano-products in real field situations raises concern about nanomaterial safety, exposure levels, and toxicological repercussions to the environment and human health. The present review gives an insight into recent advancements in nanotechnology-based agrochemicals that have revolutionized the agriculture sector. Further, the implementation barriers related to the nanomaterial use in agriculture, their commercialization potential, and the need for policy regulations to assess possible nano-agricultural risks are also discussed.

Four dual-functional luminescent Zn(ii)-MOFs based on 1,2,4,5-benzenetetracarboxylic acid with pyridylbenzimidazole ligands for detection of iron(iii) ions and acetylacetone

Four Zn-MOFs with different topological types were synthesized and characterized, MOFs 1–4 have excellent sensitivity, selectivity, recyclability and structural stabilities for detecting acac/Fe³⁺ in the naked eye range.

Porphyrinic Metal-Organic Framework Nanorod-Based Dual-Modal Nanoprobe for Sensing and Bioimaging of Phosphate

Herein, a dual-modal fluorescent/colorimetric "Signal-On" nanoprobe based on PCN-222 nanorods (NRs) toward phosphate was proposed for the first time. Due to the high affinity of the zirconium node in PCN-222 NRs for phosphate, the structure collapse of PCN-222 NRs was triggered by phosphate, resulting in the release of the tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligand from PCN-222 NRs as well as the enhancement of fluorescence and absorbance signals. The PCN-222 NR-based nanoprobe could be employed for phosphate detection over a wide concentration range with a detection limit down to 23 nM. The practical application of the PCN-222 NR-based nanoprobe in real samples was evaluated. Moreover, benefitting from the good biocompatibility and water dispersibility of PCN-222 NRs, this nanoprobe was successfully employed in the intracellular imaging of phosphate, revealing its promising application in the biological science. The present work would greatly extend the potential of nanostructured MOFs in the sensing and biological fields.

MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr⁴⁺ ion and 2-aminoterephthalic acid (BDC-NH₂). The high affinity of the Zr⁴⁺ ion to phosphate species improves sensing selectivity. The surface -NH₂ reacts with 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. The hydrogen bond formed between the hydroxyl group in Tp and imine nitrogen realizes excited-state intramolecular proton transfer; therefore, multiemission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470, and 613 nm for UiO@COF1 and at 370, 470, and 572 nm for UiO@COF2. When phosphate ion is added in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from the keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multiemission, the affinity of Zr⁴⁺ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multiemission, affinity, and structural selectivity to improve the sensing performance for differentiation and ratiometric detection.

Colorimetric assay for the sensitive detection of phosphate in water based on metal–organic framework nanospheres possessing catalytic activity

For the first time, the inhibition of the catalytic activity of Cu-MOF caused by phosphate was used for phosphate detection.