A pH-Sensing Fluorescent Metal-Organic Framework: pH-Triggered Fluorescence Transition and Detection of Mycotoxin

An organic-organometallic CO-releasing material comprising 4,4'-bipyridine and molybdenum subcarbonyl building blocks

Over the past two decades, following the discovery of the important biological roles of carbon monoxide (CO), metal carbonyl complexes have been intensively studied as CO-releasing molecules (CORMs) for therapeutic applications. To improve the properties of "bare" low molecular weight CORMs, attention has been drawn to conjugating CORMs with macromolecular and inorganic scaffolds to produce CO-releasing materials (CORMAs) capable of storing and delivering large payloads of the gasotransmitter. A significant obstacle is to obtain CORMAs that retain the beneficial features of the parent CORMs. In the present work, a crystalline metal-organic framework (MOF) formulated as Mo(CO)3(4,4'-bipyridine)3/2 (Mobpy), with a structure based on Mo(CO)3 metallic nodes and bipyridine linkers, has been prepared in near quantitative yield by a straightforward reflux method, and found to exhibit CO-release properties that mimic those typically observed for molybdenum carbonyl CORMs. Mobpy was characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), FT-IR, FT-Raman and diffuse reflectance (DR) UV-vis spectroscopies, and 13C{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR. The release of CO from Mobpy was studied by the deoxy-myoglobin (deoxy-Mb)/carbonmonoxy-myoglobin (MbCO) UV-vis assay. Mobpy liberates CO upon contact with a physiological buffer in the dark, leading to a maximum released amount of 1.3-1.5 mmol g-1, after 1.5 h at 37 °C, with half-lives of 0.5-1.0 h (time to transfer 0.5 equiv. of CO to Mb). In the solid-state and under open air, Mobpy undergoes complete decarbonylation over a period of 42 days, corresponding to a theoretical CO-release of 7.25 mmol g-1.

Two isostructural Zn/Co-MOFs with penetrating structures: multifunctional properties of both luminescence sensing and conversion of CO2 into cyclic carbonates

Two new metal-organic frameworks (MOFs), namely, {[Zn(HL)(bpea)]·DMF}n (Zn-MOF-1) and {[Co(HL)(bpea)]·DMF}n (Co-MOF-2) (H3L = 3-(3,5-dicarboxybenzyloxy)benzoic acid, bpea = 1,2-di(pyridyl)ethane), were obtained by the reaction of H3L and N-containing ligand bpea with Zn(NO3)2·6H2O and Co(NO3)2·6H2O, respectively. The isomorphic Zn-MOF-1 and Co-MOF-2 featured a 3D penetrating framework with different stabilities, luminescence, and catalytic properties. Luminescence measurement indicated that Zn-MOF-1 could be used to detect Al3+ through a turn-on effect with a detection limit of 0.42 μM. The sensing mechanism experiments showed that the enhanced luminescence of Zn-MOF-1 toward Al3+ may be due to the weak interaction between Al3+ and Zn-MOF-1 and the absorbance-caused enhancement (ACE) mechanism. Meanwhile, both Zn-MOF-1 and Co-MOF-2 showed interesting CO2 adsorption properties and could catalyze the cycloaddition of CO2 to epoxides resulting in 96 and 92% ideal products within 12 hours, respectively. They can be cycled up to 5 times without significant loss of catalytic efficiency.

Site-Memory-Triggered Reversible Acronym Encryption in a Nitrogen-Rich Pore-Partitioned MOF for Ultrasensitive Monitoring of Roxarsone and Dichloran over Multiple Platform

Stimuli-responsive emission color modulation in fluorescent metal-organic frameworks (MOFs) promises luminescence-ink-based security application, while task-specific functionality-engineered pores can aid fast-responsive, discriminative, and ultralow detection of harmful organo-aromatics in the aqueous phase. Considering practical applicability, a self-calibrated fluoro-switch between encrypted and decrypted states is best suited for antiforgery measures, whereas image-based monitoring of organo-toxins by repetitive and handy methods over multiple platforms endorses in-field sensory potential. Herein, we constructed a mixed-ligand based chemically stable and bilayered-pillar MOF from -NH2-hooked pyridyl linker and tricarboxylate ligand that embraces negatively charged [Cd3(μ2-OH)(COO)6] node and shows pore-space-partitioning by nitrogen-rich flanked organic struts. Owing to the presence of a self-calibrating triazolylamine moiety-grafted auxiliary linker, this anionic MOF delineates reversible and multicyclic fluoro-swapping between protonated-encrypted and deprotonated-decrypted domains in the alternative presence of acid and base. Such pH-triggered, site-specific luminescence variation is utilized to construct highly regenerative anticounterfeiting labels for vivid acronym encryption. The intense fluorescence signature of the material is further harnessed in extremely selective and quick responsive sensing of harmful feed additive roxarsone (ROX) and dichloran (DCNA) pesticide in highly recyclable fashion with significant quenching and nanomolar limits of detection (ROX: 52 ppb; DCNA: 26.8 ppb). Notably, the ultrasensitive fluoro-detection of both these organo-toxins is successfully demonstrated via a handy paper-strip method as well as on the vegetable surface for real-time monitoring. Comprehensive density functional theory studies validate the electron transfer mechanism through redistribution of molecular orbital energy levels by each of the targeted analytes in this electron-rich framework besides evidencing MOF-analyte supramolecular interactions.

Two Stable Bifunctional Zinc Metal-Organic Frameworks with Luminescence Detection of Antibiotics and Proton Conduction

Functionalized crystalline solids based on metal-organic frameworks (MOFs) enable efficient luminescence detection and high proton conductivity, making them crucial in the realms of environmental monitoring and clean energy. Here, two structurally and functionally distinct zinc-based MOFs, [Zn(TTDPa)(bodca)]·H2O (1) and [Zn(TTDPb)(bodca)]·H2O (2), were successfully designed and synthesized using 3,6-di(pyridin-4-yl)thieno[3,2-b]thiophene (TTDPa) and 2,5-di(pyridin-4-yl)thieno[3,2-b]thiophene (TTDPb) as ligands, in the presence of bicyclo[2.2.2]octane-1,4-dicarboxylic acid (H2bodca). Both 1 and 2 display a three-dimensional (3D) structure with 5-fold interpenetration, and notably, 2 forms a larger one-dimensional pore measuring 17.16 × 10.81 Å2 in size. Fluorescence experiments demonstrate that 1 and 2 can function as luminescent sensors for nitrofurantoin (NFT) and nitrofurazone (NFZ) with low detection limits, remarkable selectivity, and good recyclability. A comprehensive analysis was conducted to investigate the differing sensing effects of compounds 1 and 2 and to explore potential sensing mechanisms. Additionally, at 328 K and 98% relative humidity, 1 and 2 exhibit proton conductivity values of 2.13 × 10-3 and 4.91 × 10-3 S cm-1, respectively, making them suitable proton-conducting materials. Hence, the integration of luminescent sensing and proton conductivity in monophasic 3D Zn-MOFs holds significant potential for application in intelligent multitasking devices.

Functional dynamics in framework materials

Dynamic crystalline materials have emerged as a unique category of condensed phase matter that combines crystalline lattice with components that display dynamic behavior in the solid state. This has involved a range of materials incorporating dynamic functional units in the form of stimuli-responsive molecular switches and machines, among others. In particular, it has been possible by relying on framework materials, such as porous molecular frameworks and other hybrid organic-inorganic systems that demonstrated potential for serving as scaffolds for dynamic molecular functions. As functional dynamics increase the level of complexity, the associated phenomena are often overlooked and need to be explored. In this perspective, we discuss a selection of recent developments of dynamic solid-state materials across material classes, outlining opportunities and fundamental and methodological challenges for their advancement toward innovative functionality and applications.

Highly pH-Responsive Sensor Based on a EuIII Metal-Organic Framework with Efficient Recognition of Arginine and Lysine in Living Cells

A lanthanide-based three-dimensional metal-organic framework with excellent water, acid/base, and solvent stability, namely {[(CH3)2NH2]0.7[Eu2(BTDBA)1.5(lac)0.7(H2O)2]·2H2O·2DMF·2CH3CN}n (JXUST-29, H4BTDBA = 4',4‴-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis([1,1'-biphenyl]-3,5-dicarboxylic acid), Hlac = lactic acid), has been synthesized and characterized. Since the N atoms of the thiadiazole group will not coordinate with lanthanide ions, JXUST-29 has a free basic N-site accessible to small H+ ions, which allows it to be used as a promising pH fluorescence sensor. Interestingly, the luminescence signal was significantly enhanced, with an approximately 54-fold enhancement in the emission intensity when the pH value was increased from 2 to 5, which is the typical behavior of pH probes. In addition, JXUST-29 can also be used as a luminescence sensor to detect l-arginine (Arg) and l-lysine (Lys) in an aqueous solution through fluorescence enhancement and the blue-shift effect. The detection limits were 0.023 and 0.077 μM, respectively. In addition, JXUST-29-based devices were designed and developed to facilitate detection. Importantly, JXUST-29 is also capable of detecting and sensing Arg and Lys in living cells.

Ratiometric fluorescent probe based on non-conjugated polymer dots for pH measurements in ordinary Portland cement-based materials

An organic fluorescent probe, citric acid-1,3-Propanediamine-Rhodamine B (CPR) was synthesized to determine pH of ordinary Portland cement requiring only a small sample size (less than 500 μL cement leachate). The SEM, XRD, and FTIR investigations demonstrate that citric acid-1,3-Propanediamine are polymer dots with a fusiform structure. Ratio pH probe is constructed by rhodamine B with polymer dots, which exhibits a linear response in high alkaline range. A 6-fold increase in fluorescence intensity (455 nm) is achieved at pH from 12.00 to 13.25. Combined with measurements of the isothermal calorimeter, mineral composition, and microscopic morphology, variation of pH is used to evaluate the changes of components during hydration. Furthermore, CPR can be applied to measure pH of high-dose pulverized fuel ash blending systems, the non-pure cement with slightly lower alkalinity.

Fluorescent "AND" logic gates for simultaneous detection of thiols and protons: photophysical properties, mechanism and bioimaging of living cells

Five fluorescent probes TP1-5 were demonstrated as two-input "AND" molecular logic gates for the detection of thiols and protons. The molecules were designed based on "thiol receptor-spacer₁-fluorophore-spacer₂-proton receptor" mode. The logic gates were constructed by employing maleimide, naphthalimide and morpholine (TP1-3)/N-methyl piperazine (TP4-5) as the thiol receptor, fluorophore and proton receptor, respectively. All probes show significant fluorescence enhancements upon addition of both protons and thiols. However, much weaker spectral responses were observed with the addition of only one single analyte. The fluorescence outputs, based on photoinduced electron transfer (PET) and (twisted) intramolecular charge transfer (TICT/ICT), were modulated by the proton receptor and linker. The length of spacer₁ affects the responses toward thiols, whereas spacer₂ influences the sensing performance toward protons. The difference between the pK_a values of morpholine (∼5.80) and N-methyl piperazine (∼7.10) enables us to detect thiols in divergent pH circumstances. TP1-3 exhibit an excellent "AND" logic function for simultaneous detection of protons and thiols as well as bioimaging thiols in weakly acidic living cells. However, TP4 and TP5 are not good candidates for executing "AND" logic operation possibly due to the stronger electron donating properties and steric effect of N-methyl piperazine.

Zinc(II) Carboxylate Coordination Polymers with Versatile Applications

This review considers the applications of Zn(II) carboxylate-based coordination polymers (Zn-CBCPs), such as sensors, catalysts, species with potential in infections and cancers treatment, as well as storage and drug-carrier materials. The nature of organic luminophores, especially both the rigid carboxylate and the ancillary N-donor bridging ligand, together with the alignment in Zn-CBCPs and their intermolecular interaction modulate the luminescence properties and allow the sensing of a variety of inorganic and organic pollutants. The ability of Zn(II) to act as a good Lewis acid allowed the involvement of Zn-CBCPs either in dye elimination from wastewater through photocatalysis or in pathogenic microorganism or tumor inhibition. In addition, the pores developed inside of the network provided the possibility for some species to store gaseous or liquid molecules, as well as to deliver some drugs for improved treatment.

Metal-organic frameworks for food contaminant adsorption and detection

Metal-organic framework materials (MOFs) have been widely used in food contamination adsorption and detection due to their large specific surface area, specific pore structure and flexible post-modification. MOFs with specific pore size can be targeted for selective adsorption of some contaminants and can be used as pretreatment and pre-concentration steps to purify samples and enrich target analytes for food contamination detection to improve the detection efficiency. In addition, MOFs, as a new functional material, play an important role in developing new rapid detection methods that are simple, portable, inexpensive and with high sensitivity and accuracy. The aim of this paper is to summarize the latest and insightful research results on MOFs for the adsorption and detection of food contaminants. By summarizing Zn-based, Cu-based and Zr-based MOFs with low cost, easily available raw materials and convenient synthesis conditions, we describe their principles and discuss their applications in chemical and biological contaminant adsorption and sensing detection in terms of stability, adsorption capacity and sensitivity. Finally, we present the limitations and challenges of MOFs in food detection, hoping to provide some ideas for future development.

Design and Detection of Cyanide Raman Tag pH-Responsive SERS Probes

As one of the most important parameters of biochemical analysis and detection, the pH value plays a very important role in cell function, food preservation and production, soil and water sources, and other applications. This makes it increasingly important to explore pH detection methods in depth. In this paper, a pH-responsive SERS probe based on the cyano Raman Tag was designed to realize pH sensing detection through the influence of the pH value of analytes on the displacement of the cyano Raman peak in the SERS probe. This cyano Raman tag exhibited not only excellent sensitivity in the liner range of pH 3.0-9.0 with a limit of detection (LOD) of pH 0.33, but also the anti-interference performance and stability (the relative standard deviation (RSD) was calculated to be 6.68%, n = 5). These results indicated that this pH SERS probe with the Raman cyano tag can provide new research ideas for future biological detection, bioimaging, and environmental detection.

MOF-Based Mycotoxin Nanosensors for Food Quality and Safety Assessment through Electrochemical and Optical Methods

Mycotoxins in food are hazardous for animal and human health, resulting in food waste and exacerbating the critical global food security situation. In addition, they affect commerce, particularly the incomes of rural farmers. The grave consequences of these contaminants require a comprehensive strategy for their elimination to preserve consumer safety and regulatory compliance. Therefore, developing a policy framework and control strategy for these contaminants is essential to improve food safety. In this context, sensing approaches based on metal-organic frameworks (MOF) offer a unique tool for the quick and effective detection of pathogenic microorganisms, heavy metals, prohibited food additives, persistent organic pollutants (POPs), toxins, veterinary medications, and pesticide residues. This review focuses on the rapid screening of MOF-based sensors to examine food safety by describing the main features and characteristics of MOF-based nanocomposites. In addition, the main prospects of MOF-based sensors are highlighted in this paper. MOF-based sensing approaches can be advantageous for assessing food safety owing to their mobility, affordability, dependability, sensitivity, and stability. We believe this report will assist readers in comprehending the impacts of food jeopardy exposure, the implications on health, and the usage of metal-organic frameworks for detecting and sensing nourishment risks.

A miniature chemiluminescence spectrometric system induced by atmosphere microplasma jet to avoid using hydrogen peroxide and catalyst

A miniature luminol chemiluminescence system based on atmosphere microplasma is proposed for detection without any catalysts. In our research, atmosphere microplasma jet is employed to oxidize luminol and produce chemiluminescence instead of H₂O₂. The transport of OH radicals to the plasma-liquid interface and induce the chemiluminescence. The weight of the system is only 3.6 kg (including a 1.2 kg laptop), and the power consumption of the microplasma is only 0.045 W. The mechanism of luminol chemiluminiscence induced by microplasma jet and generation of microplasma jet are investigated in this study. A 1 mL sample solution is sufficient for trace 3-NPA determination within an analysis time of 6 min. In the range of 0.03-10 mg L^-1, 3-NPA can be quantitatively analyzed along with a detection limit of 0.008 mg L^-1. In addition, the proposed system is employed for real-world samples detection, including water samples, brown sugar and tainted sugarcane, which demonstrates the reliability and practical feasibility of the detection method.

Expanding the Scope of Nanobiocatalysis and Nanosensing: Applications of Nanomaterial Constructs

The synergistic interaction between advanced biotechnology and nanotechnology has allowed the development of innovative nanomaterials. Those nanomaterials can conveniently act as supports for enzymes to be employed as nanobiocatalysts and nanosensing constructs. These systems generate a great capacity to improve the biocatalytic potential of enzymes by improving their stability, efficiency, and product yield, as well as facilitating their purification and reuse for various bioprocessing operating cycles. The different specific physicochemical characteristics and the supramolecular nature of the nanocarriers obtained from different economical and abundant sources have allowed the continuous development of functional nanostructures for different industries such as food and agriculture. The remarkable biotechnological potential of nanobiocatalysts and nanosensors has generated applied research and use in different areas such as biofuels, medical diagnosis, medical therapies, environmental bioremediation, and the food industry. The objective of this work is to present the different manufacturing strategies of nanomaterials with various advantages in biocatalysis and nanosensing of various compounds in the industry, providing great benefits to society and the environment.

pH-Stable Luminescent Metal-Organic Frameworks for the Selective Detection of Aqueous-Phase FeIII and CrVI Ions

The development of chemically stable metal-organic framework (MOF)-based luminescent platforms for toxic ion detection in an aqueous medium is highly challenging because most of the classical MOFs are prone to water degradation, and that is the reason why most of the MOF-based luminescent sensors use a nonaqueous medium for sensing. In this contribution, we report two new water-stable luminescent MOFs (Zn-MOF-1 and Zn-MOF-2), assembled from a mixed-ligand synthesis approach. Because of the presence of a hydrophobic trifluoromethyl group to the backbone and stronger metal-N coordination, these MOFs exhibit excellent stability not only in water but also in acidic/alkaline aqueous solutions (pH = 3-10). Here, we report a green sensing approach by exploiting the significant reduction in photoluminescence of these MOFs in the presence of toxic ions. Fe³⁺ and CrO₄^2-/Cr₂O₇^2- ions could be traced with a detection limit (LOD) in the micromolar range (0.045 and 0.745/0.33 μM for Zn-MOF-1; 125.2 and 114.2/83.5 μM for Zn-MOF-2). The mechanistic study reveals that competitive absorption of the excitation energy coupled with fluorescent resonance energy transfer are responsible for the turn-off quenching. The anti-interference ability and recyclability along with the pH stability gave these MOFs high potential to be used as practical sensors toward Fe^III and Cr^VI ions in water as a greenest medium.

A europium metal-organic framework for dual Fe3+ ion and pH sensing

Metal–organic frameworks (MOFs) with ratiometric sensing properties are desirable for many applications due to their intrinsic self-calibration. We report the re-assessment of the sensing properties of a MOF, originally reported as containing europium(III) and 2-hydroxyterephtalic acid, and having fluorescent ratiometric iron(III) sensing properties. Synchrotron single-crystal X-ray diffraction and proton nuclear magnetic resonance (¹H NMR) spectroscopy revealed that the MOF is composed of 2-methoxyterephthalate, not 2-hydroxyterephthalate as originally reported. We found that the MOF exhibits a sensor turn-off response towards Fe³⁺ ion concentrations in the range 0.5–3.7 ppm (band 425 nm), and a turn-on response towards a decrease of pH from 5.4 to 3.0 (band 375 nm), both resulting from the addition of acidic Fe³⁺ salt solution to a MOF suspension. Thus, the ratiometric sensing properties and the originally proposed mechanism no longer apply; our work reveals a dynamic quenching mechanism for the fluorescence turn-off response due to the presence of Fe³⁺ ions, and a ligand protonation mechanism for the turn-on response to a decrease in pH. Our work highlights the importance of a thorough investigation of the structure of any newly synthesized MOF, and, in the case of potential sensors, their selectivity and any environmental effects on their sensing behavior.

A microporous chromium-organic framework fabricated via solvent-assisted metal metathesis for C2H2/CO2 separation

Removal of CO₂ or C₂H₄ from C₂H₂ is still a challenging task due to their similar physical-chemical properties. Here, a bifunctional ligand decorated with amino and sulfoxide groups, 5',5''''-sulfonylbis (2'-amino-[1,1':3',1''-terphenyl]-4,4''-dicarboxylic acid) (H₄L), was employed to construct a new microporous iron-organic framework (Fe-MOF) with the formula [(Fe₃O)(L)_1.5(H₂O)₃]_n. This MOF can serve as a parent structure to obtain the isostructural Cr-MOF by solvent-assisted metal metathesis. Furthermore, the gas adsorption and separation performance of these two MOFs were systematically investigated. Compared to Fe-MOF, Cr-MOF shows a moderately higher CO₂, C₂H₂ and C₂H₄ uptake capacity. Additionally, Cr-MOF can selectively adsorb C₂H₂ over CO₂ and C₂H₄. The separation potential towards C₂H₂/C₂H₄ and C₂H₂/CO₂ was further established via IAST calculations of mixture adsorption equilibrium. IAST selectivity values of Cr-MOF are 3.4 for C₂H₂/C₂H₄ and 6.9 for C₂H₂/CO₂ at 298 K and initial pressure, indicating its potential C₂H₂ separation ability.

An AIE-Active probe for detection and bioimaging of pH values based on lactone hydrolysis reaction

Cellular pH homeostasis is essential for many physiological and pathological processes. pH monitoring is helpful for the diagnosis, treatment and prevention of disorders and diseases. Herein, we developed a ratiometric fluorescent pH probe (TCC) based on a coumarin derivative containing a highly active lactone ring. TCC exhibited a typical AIE effect and emitted blue fluorescence under weak acidic condition. When under weak basic condition, the active lactone moiety underwent a hydrolysis reaction to afford a water-soluble product, which gave red-shifted emission. The emission color change from blue through cyan and then to yellow within pH 6.5-9.0 which is approximate to the biological pH range. And the fluorescence color change along with pH value is reversible. Furthermore, TCC was successfully utilized in the detection of the intracellular pH change of live HeLa cells, which indicated that TCC had practical potential in biomedical research.

Ultrasensitive and highly selective "turn-on" fluorescent sensor for the detection and measurement of melatonin in juice samples

Melatonin (MLT), a hormone related to the regulation of brain functions, is directly related to sleep quality and is considered to be a possible adjuvant therapy for patients needing hospitalization for coronavirus disease 2019 pneumonia, and accurate measurement of MLT is crucial. Herein, a new, highly sensitive, and easy operation fluorescent probe was provided based on Zr metal-organic framework encapsulation into the molecularly imprinted polymer (MOF@MIP). By combining unique properties of MIP and fluorescent MOF, selectivity and operation of the applied method were significantly improved. Different characterization methods, such as XRD, FT-IR, and FE-SEM, were used to confirm the synthesis reliability. MOF@MIP was successfully used for the precise identification and ultrasensitive detection for trace amounts of MLT. The detection mechanism for the analytical system is based on the ''turn-on'' fluorescence (FL) signal in 404 nm. The findings proved that it is possible to detect trace amounts of MLT in real samples including grape, cherry, and sour cherry juice. The linear range and the limit of detection (LOD) for trace amounts of MLT were obtained as 1-100 ng/mL and 0.18 ng/mL, respectively.

Dysprosium(III) Metal-Organic Framework Demonstrating Ratiometric Luminescent Detection of pH, Magnetism, and Proton Conduction

A multifunctional metal-organic framework, (Hdmbpy)[Dy(H₂dobdc)₂(H₂O)]·3H₂O (Dy-MOF, H₄dobdc = 2,5-dihydroxyterephthalic acid, dmbpy = 4,4'-dimethyl-2,2'-bipyridine), was synthesized and structurally characterized. The metal center Dy^III is connected by four carboxyl groups to form the [Dy₂(CO₂)₄] binuclear nodes, which are further interconnected by eight separate H₂dobdc^2- ligands to form a three-dimensional (3D) framework including hydrophilic triangular channels and abundant hydrogen-bonding networks. Dy-MOF has good stability in aqueous solution as well as in harsh acidic or alkaline solutions (pH range: 2.0-12.0). Furthermore, the luminescence signal of Dy-MOF undergoes a visualized color change as the acidity of the solution alters, which is the typical behavior of pH ratiometric probe. At a 100% relative humidity, Dy-MOF exhibits a high proton conductivity σ (1.70 × 10^-4 S cm^-1 at 303 K; 1.20 × 10^-3 S cm^-1 at 343 K) based on the proton hopping mechanism, which can be classified as a superionic conductor with σ exceeding 10^-4 S cm^-1. Additionally, the ferromagnetic interaction and magnetic relaxation behavior are simultaneously achieved in Dy-MOF. Herein, the combination of luminescence sensing, magnetism, and proton conduction in a single-phase 3D MOF may offer great potential applications in smart multitasking devices.

A dual-emission polymer carbon nanoparticles for ratiometric and visual detection of pH value and bilirubin

Herein, we prepared a novel fluorescent polymer carbon nanoparticles by polymerizing dopamine (DA) and o-phenylenediamine (OPD) through oxidation of hydrogen peroxide. In a neutral environment, the synthesized fluorescent polymer carbon nanoparticles (PDA-OPD) exhibited two emission peaks at 460 nm and 540 nm with 400 nm excitation wavelength. In an acidic environment, the fluorescence emission peaks of PDA-OPD at 540 nm showed an obvious fluorescence quenching, and there existed a good linear relationship between the fluorescence ratio F₅₄₀/F₄₆₀ and environment pH value. In an alkaline environment, the fluorescence emission peak at 460 nm showed obvious fluorescence quenching after the addition of bilirubin, while a novel fluorescence emission peak at 560 nm emerged gradually. The PDA-OPD could be also used to detect bilirubin in the range of 0-400 μmol·L^-1.

Multifunctional Zr-MOF Based on Bisimidazole Tetracarboxylic Acid for pH Sensing and Photoreduction of Cr(VI)

Herein, a new luminescent zirconium MOF [Zr-BBI, BBI = 4,4',4″,4‴-(1,4-phenylenebis(1H-imidazole-2,4,5-triyl))tetrabenzoic acid] was successfully constructed by a rationally designed functionalized bisimidazole tetracarboxylic acid ligand. Zr-BBI consists of eight-connected Zr₆ clusters and four-connected BBI ligands. The high connection mode must be responsible for the high stabilities of Zr-BBI in both acidic and basic systems. Apart from the high stability, the inherent bisimidazole units endow Zr-BBI with the traits of intense fluorescent emission as well as the protonation/deprotonation behavior. Therefore, Zr-BBI could display a highly sensitive fluorescence response along with the varying pH values in the aqueous solutions and act as a pH sensing scaffold, especially in the pH value range from 4.6 to 7.12. Zr-BBI also shows good fluorescence detection performance toward Cr₂O₇^2- at a low concentration with a high K_SV value up to 6.49 × 10⁴ M^-1. Moreover, by the utilization of Zr-BBI as a catalyst, Cr(VI) could be effectively photoreduced to Cr(III) in aqueous solution under visible light irradiation, in which the introduction of a hole scavenger (benzyl alcohol) could further significantly enhance the photocatalytic efficiency. Compared to that of the recently representative MOFs, the k value of the photocatalytic reaction over Zr-BBI is as high as 0.073 min^-1. With the consideration of the presented results, Zr-BBI can serve as a multifunctional platform for efficiently sensing and photoreducing Cr₂O₇^2- in an aqueous system, which fully illustrates the feasibility that introducing specific functional groups in the framework of MOFs would enhance the related photocatalytic activity.

Stimuli-Responsive Zinc (II) Coordination Polymers: A Novel Platform for Supramolecular Chromic Smart Tools

The unique role of the zinc (II) cation prompted us to cut a cross-section of the large and complex topic of the stimuli-responsive coordination polymers (CPs). Due to its flexible coordination environment and geometries, easiness of coordination-decoordination equilibria, "optically innocent" ability to "clip" the ligands in emissive architectures, non-toxicity and sustainability, the zinc (II) cation is a good candidate for building supramolecular smart tools. The review summarizes the recent achievements of zinc-based CPs as stimuli-responsive materials able to provide a chromic response. An overview of the past five years has been organised, encompassing 1, 2 and 3D responsive zinc-based CPs; specifically zinc-based metallorganic frameworks and zinc-based nanosized polymeric probes. The most relevant examples were collected following a consequential and progressive approach, referring to the structure-responsiveness relationship, the sensing mechanisms, the analytes and/or parameters detected. Finally, applications of highly bioengineered Zn-CPs for advanced imaging technique have been discussed.

Visual pH Sensors: From a Chemical Perspective to New Bioengineered Materials

Many human activities and cellular functions depend upon precise pH values, and pH monitoring is considered a fundamental task. Colorimetric and fluorescence sensors for pH measurements are chemical and biochemical tools able to sense protons and produce a visible signal. These pH sensors are gaining widespread attention as non-destructive tools, visible to the human eye, that are capable of a real-time and in-situ response. Optical “visual” sensors are expanding researchers’ interests in many chemical contexts and are routinely used for biological, environmental, and medical applications. In this review we provide an overview of trending colorimetric, fluorescent, or dual-mode responsive visual pH sensors. These sensors include molecular synthetic organic sensors, metal organic frameworks (MOF), engineered sensing nanomaterials, and bioengineered sensors. We review different typological chemical entities of visual pH sensors, three-dimensional structures, and signaling mechanisms for pH sensing and applications; developed in the past five years. The progression of this review from simple organic molecules to biological macromolecules seeks to benefit beginners and scientists embarking on a project of pH sensing development, who needs background information and a quick update on advances in the field. Lessons learned from these tools will aid pH determination projects and provide new ways of thinking for cell bioimaging or other cutting-edge in vivo applications.

Highly Stable Acid-Induced Emission-Enhancing Cd-MOFs: Synthesis, Characterization, and Detection of Glutamic Acid in Water and Fe Ions in Acid

Two novel 3D fluorescent metal-organic frameworks (MOFs), [Cd(L)(bbibp)]_n (1) and [Cd(L)(bbibp)_0.5]_n (2), where H₂L = 4,4'-(4,4'-bipyridine-2,6-diyl)dibenzoic acid and bbibp = 4,4'-bis(benzoimidaz-1-yl)biphenyl, were acquired through a conventional method and characterized via IR spectra, single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, powder X-ray diffraction (PXRD), scanning electron microscopy, N₂ adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). The crystal framework of Cd-MOF 1 remained stable in the range of pH = 1.0-12.0. Interestingly, the emission peak of 1 showed a red shift and exhibited a fluorescence turn-on effect in an acidic environment. X-ray diffraction measurement revealed that the crystal structure of 1 remained unchanged after immersion in a pH = 1.0 solution. In addition, Cd-MOFs 1 and 2 displayed fluorescent quenching to l-glutamic acid with high sensitivity and selectivity. Meanwhile, 1 showed high selectivity in recognizing Fe³⁺ under acidic conditions, which made 1 capable of detecting Fe³⁺ in acidic industrial wastewater. Finally, the fluorescent sensing mechanism was carefully studied by PXRD, transient fluorescence lifetime, XPS, and UV spectroscopy.

Flexible and Semi-flexible Amide-Hydrazide Decorated Fluorescent Covalent Organic Frameworks as On-Off pH Responsive Proton Scavengers

One of the desired ways to improve robustness and crystallinity in the covalently linked reticular frameworks is through the incorporation of varied and key functionalities in these promising materials. In this work, we have strategically constructed a series of triazine-based amide-hydrazide linked COFs (CON, CONN, and CONNCO) by combining the flexible tri(4-formylphenoxy)cyanurate with semi-flexible 4-amino-N-(4-aminophenyl)benzamide, semi-flexible 4-aminobenzo-hydrazide, or flexible 4-amino-N-(4-aminobenzoyl)benzo-hydrazide linkages, respectively. The incorporation of an amide or a hydrazide functionality is the key to improving the crystallinity of the framework through strong intra- and interlayer H-bonding. The structural characterization of these COFs has been done with the help of numerous analytical methods. All three COFs exhibit good thermal and chemical stability in acid and base verified by PXRD and N₂ sorption studies. Their intra- and interlayer H-bonding control the rotation in eclipsed layers, which follows emissive nature. Their stability, linkage functionality, and wettability in water have been judiciously used in fluorescent on-off pH responsive proton scavenging. The protonation-deprotonation of the imine N and N-H bond of the amide or hydrazide linkage adorned in the pore walls of three COFs plays a vital role for such an outcome.

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.

Lighting up solid states using a rubber

It is crucial and desirable to develop green and high-efficient strategies to regulate solid-state structures and their related material properties. However, relative to solution, it is more difficult to break and generate chemical bonds in solid states. In this work, a rubbing-induced photoluminescence on the solid states of ortho-pyridinil phenol family was achieved. This rubbing response relied on an accurately designed topochemical tautomerism, where a negative charge, exactly provided by the triboelectric effect of a rubber, can induce a proton transfer in a double H-bonded dimeric structure. This process instantaneously led to a bright-form tautomer that can be stabilized in the solid-state settings, leading to an up to over 450-fold increase of the fluorescent quantum yield of the materials. The property can be repeatedly used due to the reversibility of the tautomerism, enabling encrypted applications. Moreover, a further modification to the structure can be accomplished to achieve different properties, opening up more possibilities for the design of new-generation smart materials.

Changes in molecular properties due to stimuli response are critically important for the development of new materials. However, most processes are slow or inefficient in the solid state. Here the authors demonstrate property switching in the solid state using a rubbing-induced tautomerism in multiple hydrogen-bonded donor-acceptor couples.

Various carboxylates induced eight Zn(ii)/Cd(ii) coordination polymers with fluorescence sensing activities for Fe(iii), Cr(vi) and oxytetracycline

Eight new Zn(ii)/Cd(ii) coordination polymers constructed from a naphthalene-amide-pyridyl ligand and various carboxylates were synthesized and characterized, which show multifunctional fluorescence responses for cations, anions and antibiotics.