Multifunctional Nanoscale Metal-Organic Layers for Ratiometric pH and Oxygen Sensing

Dopants Induce Persistent Room Temperature Phosphorescence in Triarylamine Boronate Esters

Purely organic materials exhibiting room temperature phosphorescence (RTP) are promising candidates for oxygen sensors and information encryption owing to their cost-effective and environmentally friendly nature. Herein, we report a bimolecular RTP system where DTBU acts as the guest and TBBU serves as the host. In contrast to previously reported results, we find that both pure DTBU and TBBU do not exhibit RTP in the solid state even under N2 atmosphere. A DTBU/TBBU system with a low doping ratio (0.1 mol %) exhibits persistent yellowish-green afterglow with a lifetime of 340 ms and is highly sensitive to oxygen. A DTBU/TBBU system with a higher doping ratio (10 mol %) maintains a phosphorescence lifetime of 179 ms under air. Applications of DTBU/TBBU at varied doping ratios in both oxygen sensing and information encryption are demonstrated. We propose that the T1 state of TBBU acts as an energy transfer intermediate between Tn and T1 of DTBU, ultimately leading to the generation of persistent RTP. Overall, this work demonstrates the critical importance of material purity in the design of RTP systems, and how an understanding of host-guest doping enables their photophysical properties to be precisely tuned.

Multisignals Sensing Platform for Highly Sensitive, Accurate, and Rapid Detection of p-Aminophenol Based on Adsorption and Oxidation Effects Induced by Defective NH2-Ag-nMOFs

Labile toxic pollutants detection remains a challenge due to the problem that a single method is prone to producing false-negative/-positive signals. The construction of a multisignal sensing platform with the advantages of different strategies is an effective way to solve this problem. Herein, a novel resonant light scattering (RLS), fluorescent and rapid visual multisignals sensing strategy for p-aminophenol (p-AP) detection was designed based on the adsorption and oxidation effects of defective amino-functionalized Ag-based nano metal-organic frameworks (NH2-Ag-nMOFs). In this reaction process, NH2-Ag-nMOFs with incomplete coordination oxidize H2O2 to produce singlet oxygen (1O2) which rapidly oxidizes p-AP, leading to the reduction of Ag+ to Ag0, thereby disrupting the structure of NH2-Ag-nMOFs and resulting in fluorescence quenching of NH2-Ag-nMOFs. Synchronously, owing to Ag0 aggregation and p-AP oxidation, the color of the system changed from colorless to purplish-red and pale brown within 20 s. The assay has realized the rapid naked-eye detection of 5 μM p-AP rapidly. Additionally, thanks to the intermolecular hydrogen bonding, NH2-Ag-nMOFs-p-AP aggregates formed, which enhanced the RLS signal. With the RLS signal, the designed multisignals sensing platform can analyze p-AP at a concentration as low as 11 nM and yield a wider dynamic response range than any single signal strategy reported before, which can quickly meet the measurement requirement of different actual samples. Overall, the proposed strategy without assembling various signal indicators presented an accurate, rapid, cost-effective, and sensitive multisignals sensing platform for p-AP analysis and has great prospects in labile toxic pollutants monitoring.

Ratiometric singlet oxygen self-detecting and oxygen self-supplying nanosensor for real-time photodynamic therapy feedback and therapeutic effect enhancement

Integration of singlet oxygen (¹O₂) detection that provides necessary therapeutic feedback into nanotheranostics for hypoxic tumor photodynamic therapy (PDT) is desirable but still challenging. Herein, we report a nanosensor (denominated PAPD) by combining dual-channel ratiometric sensing and oxygen-augmenting strategies, which synergistically realizes real-time ¹O₂ self-detection, O₂ self-supply and enhanced phototherapy. PAPD nanosensor is constructed by encapsulating anthracene-based ¹O₂ sensitive fluorophore (DPA) into porphyrin metal-organic frameworks (PCN-224), decorating gold nanoparticles (AuNPs) as nanoenzymes, and coating polyethylene glycol thiol (PEG-SH) by the Au-S bond. PCN-224 serves as ¹O₂ reference fluorescence (FL) agent and photosensitizer. Once PCN-224-induced ¹O₂ is synthesized, the dual-channel ratiometric FL signal of PAPD actualizes sensitive, accurate and dynamic ¹O₂ visualization and gives real-time therapeutic information correlated with the therapeutic progression. Additionally, the catalase-like activity of PAPD possesses in situ O₂ production via intracellular H₂O₂ decomposition and accelerates ¹O₂ yields for amplifying the tumor cell killing efficiency. Moreover, the ratiometric ¹O₂ self-detection affords the capacity to evaluate the O₂ self-supplying effect in tumor 4T1 cells. Consequently, the rational-designed nanosensor PAPD provides a paradigm for real-time therapeutic evaluation and precise hypoxic tumor treatment clinically.

Catalytic nanotechnology of X-ray photodynamics for cancer treatments

Photodynamic therapy (PDT) has been applied in cancer treatment because of its high selectivity, low toxicity, and non-invasiveness. However, the limited penetration depth of the light still hampers from reaching deep-seated tumors. Considering the penetrating ability of high-energy radiotherapy, X-ray-induced photodynamic therapy (X-PDT) has evolved as an alternative to overcome tissue blocks. As the basic principle of X-PDT, X-rays stimulate the nanoparticles to emit scintillating or persistent luminescence and further activate the photosensitizers to generate reactive oxygen species (ROS), which would cause a series of molecular and cellular damages, immune response, and eventually break down the tumor tissue. In recent years, catalytic nanosystems with unique structures and functions have emerged that can enhance X-PDT therapeutic effects via an immune response. The anti-cancer effect of X-PDT is closely related to the following factors: energy conversion efficiency of the material, the radiation dose of X-rays, quantum yield of the material, tumor resistance, and biocompatibility. Based on the latest research in this field and the classical theories of nanoscience, this paper systematically elucidates the current development of the X-PDT and related immunotherapy, and highlights its broad prospects in medical applications, discussing the connection between fundamental science and clinical translation.

Afterglow/Fluorescence Dual-Emissive Ratiometric Oxygen Probe for Tumor Hypoxia Imaging

Hypoxia is a common feature of many diseases such as solid tumors. The measurement and imaging of oxygen (O₂) are extremely important for disease diagnosis and therapy evaluation. In this work, the afterglow/fluorescence dual-emissive ratiometric O₂ probe based on a photochemical reaction-based afterglow system is reported. The afterglow is highly sensitive to O₂ because the O₂ content is directly related to the ¹O₂ yield and eventually affects the afterglow intensity. The O₂-insensitive fluorescence of an emitter can serve as an internal reference. As the O₂ concentration changes from 0.08 to 18.5 mg L^-1, the ratio value shows a remarkable 53-fold increase. Compared with the intensity of a single peak, the ratiometric signal can eliminate the interference of the probe concentration to achieve higher accuracy. This afterglow/fluorescence dual-emissive ratiometric O₂ probe is successfully applied to hypoxia imaging in tumor-bearing mice, which may further promote the development of O₂ sensing in the biomedical field.

Trityl Quinodimethane Derivatives as Unimolecular Triple-Function Extracellular EPR Probes for Redox, pH, and Oxygen

Electron paramagnetic resonance (EPR) spectroscopy and imaging coupled with the use of suitable probes is a promising tool for assessment of the tumor microenvironment (TME). Measurement of multiple TME parameters by EPR is very desirable but challenging. Herein, we designed and synthesized a class of negative-charged trityl quinodimethane MTPs as unimolecular triple-function extracellular probes for redox, pH, and oxygen (O2) levels. Using the deuterated analogue, dMTP5, which has an optimal pKa as well as high sensitivity to bioreduction and O2, we reasonably evaluated pH effects on efflux of reducing agents from HepG2 cells and cellular O2 consumption.

Effects of microsize on the biocompatibility of UiO67 from protein-adsorption behavior, hemocompatibility, and histological toxicity

The biocompatibility of metal-organic frameworks (MOFs) is necessary to humans but is far from being sufficiently addressed. This study focused on the effects of microsize on the biocompatibility of MOFs by selecting UiO67 with micron and submicron size as the MOFs models. Under the dose metric of surface area, the binding constant between UiO67 and human serum albumin (HSA) gradually increased with increased UiO67 size. Submicron UiO67 induced stronger conformational transformation and more greatly affected the protein surface hydrophobicity than micron UiO67. Micron UiO67 also inhibited the esterase-like activity of HSA through competitive inhibition mechanism, whereas submicron UiO67 inhibited it through noncompetitive inhibition mechanism. The size of UiO67 had little effect on hemocompatibility. A smaller size of UiO67, corresponded with a higher IC50 value for 293 T and LO2 cells, and the adsorption of HSA can effectively improve cytotoxicity. In vivo toxicity evaluations revealed that all UiO67 did not cause obvious distortion of organs, and they were metabolized primarily in the kidney. These results provided useful information about the toxicity of MOFs and experimental references for the development of MOFs-based engineering materials.

Porous matrix materials in optical sensing of gaseous oxygen

The review provides comparison of porous materials that act as a matrix for luminescent oxygen indicators. These include silica-gels, sol–gel materials based on silica and organically modified silica (Ormosils), aerogels, electrospun polymeric nanofibers, metal–organic frameworks, anodized alumina, and various other microstructured sensor matrices. The influence of material structure and composition on the efficiency of oxygen quenching and dynamic response times is compared and the advantages and disadvantages of the materials are summarized to give a guide for design and practical application of sensors with desired sensitivity and response time.

Polyoxometalate Modified by Zeolite Imidazole Framework for the pH-Responsive Electrodynamic/Chemodynamic Therapy

Electrodynamic therapy (EDT) and chemodynamic therapy (CDT) have the potential for future tumor treatment; however, their underlying applications are greatly hindered owing to their inherent drawbacks. The combination of EDT and CDT has been considered to be an effective way to maximize the superiorities of these two ROS-based methodologies. However, the development of novel nanomaterials with "one-for-all" functions still remains a big challenge. In this work, the polyoxometalate nanoparticles (NPs) were decorated using the zeolite imidazole framework (POM@ZIF-8) in order to integrate the EDT with CDT. The resulting POM@ZIF-8 NPs can effectively induce the generation of reactive oxygen species (ROS) via a catalytic reaction on the surface of POM NPs induced by an electric field (E). At the same time, POM@ZIF-8 NPs can catalyze the intracellular H₂O₂ into ROS via a Fenton-like reaction, thereby achieving the combination of EDT and CDT. Besides, since ZIF-8 is acid-responsive, it can protect normal tissues and avoid side effects. Of great note is that the cytotoxicity and the apoptosis rate of the POM@ZIF-8+E group (80%) were found to be significantly higher than that of the E group (55%). As a result, a high tumor inhibition phenomenon can be observed both in vitro and in vivo. The present study thus provides an alternative concept for combinational therapeutic modality with exceptional efficacy.

Luminescent Metal-Phenolic Networks for Multicolor Particle Labeling

The development of fluorescence labeling techniques has attracted widespread interest in various fields, including biomedical science as it can facilitate high-resolution imaging and the spatiotemporal understanding of various biological processes. We report a supramolecular fluorescence labeling strategy using luminescent metal-phenolic networks (MPNs) constructed from metal ions, phenolic ligands, and common and commercially available dyes. The rapid labeling process (<5 min) produces ultrathin coatings (≈10 nm) on diverse particles (e.g., organic, inorganic, and biological entities) with customized luminescence (e.g., red, blue, multichromatic, and white light) simply through the selection of fluorophores. The fluorescent coatings are stable at pH values from 1 to 8 and in complex biological media owing to the dominant π interactions between the dyes and MPNs. These coatings exhibit negligible cytotoxicity and their strong fluorescence is retained even when internalized into intracellular compartments. This strategy is expected to provide a versatile approach for fluorescence labeling with potential in diverse fields across the physical and life sciences.

Integration of Multiple Redox Centers into Porous Coordination Networks for Ratiometric Sensing of Dissolved Oxygen

The application of porphyrin metal-organic frameworks (MOFs) as a ratiometric electrochemical sensing platform is still unexplored. In this paper, we report a ratiometric electrochemical sensor by the integration of multiple redox centers into porphyrin MOFs for the detection of dissolved oxygen (DO). Specifically, the ferrocene (Fc) group was integrated into the nanosized PCN-222(Fe) (PCN = porous coordination networks) via acid-base reaction to synthesize the Fc@PCN-222(Fe) composite with two redox centers of the Fc group and Fe-porphyrin. The Fc group that is insensitive to DO serves as an internal reference, and the Fe-porphyrin in PCN-222(Fe) is a DO indicator. The ratios of the cathodic currents for the two redox centers exhibit a linear relationship with DO concentrations from 2.8 to 28.9 mg mL^-1 and a limit of detection of 0.3 mg mL^-1. In addition, the ratiometric electrochemical sensor has high selectivity and stability for DO sensing results from the Fc@PCN-222(Fe) composite. Because there are numerous redox centers, such as methylene blue and thionine, which can be integrated into MOFs, many MOF-based ratiometric electrochemical sensors can be simply developed for high-performance biosensing.

Mixed-Charge Nanocarriers Allow for Selective Targeting of Mitochondria by Otherwise Nonselective Dyes

Targeted delivery of molecular cargos to specific organelles is of paramount importance for developing precise and effective therapeutics and imaging probes. This work describes a disulfide-based delivery method in which mixed-charged nanoparticles traveling through the endolysosomal tract deliver noncovalently bound dye molecules selectively into mitochondria. This system comprises three elements: (1) The nanoparticles deliver their payloads by a kiss-and-go mechanism - that is, they drop off their dye cargos proximate to mitochondria but do not localize therein; (2) the dye molecules are by themselves nonspecific to any cellular structures but become so with the help of mixed-charge nanocarriers; and (3) the dye is engineered in such a way as to remain in mitochondria for a long time, up to days, allowing for observing dynamic remodeling of mitochondrial networks and long-term tracking of mitochondria even in dividing cells. The selectivity of delivery and long-lasting staining derive from the ability to engineer charge-imbalanced, mixed [+/-] on-particle monolayers and from the structural features of the cargo. Regarding the former, the balance of [+] and [-] ligands can be adjusted to limit cytotoxicity and control the number of dye molecules adsorbed onto the particles' surfaces. Regarding the latter, comparative studies with multiple dye derivatives we synthesized rationalize the importance of polar groups, long alkyl chains, and disulfide moieties in the assembly of fluorescent nanoconstructs and long-lasting staining of mitochondria. Overall, this strategy could be useful for delivering hydrophilic and/or anionic small-molecule drugs difficult to target to mitochondria by classical approaches.

Exploiting a New Approach to Destroy the Barrier of Tumor Microenvironment: Nano-Architecture Delivery Systems

Recent findings suggest that tumor microenvironment (TME) plays an important regulatory role in the occurrence, proliferation, and metastasis of tumors. Different from normal tissue, the condition around tumor significantly altered, including immune infiltration, compact extracellular matrix, new vasculatures, abundant enzyme, acidic pH value, and hypoxia. Increasingly, researchers focused on targeting TME to prevent tumor development and metastasis. With the development of nanotechnology and the deep research on the tumor environment, stimulation-responsive intelligent nanostructures designed based on TME have attracted much attention in the anti-tumor drug delivery system. TME-targeted nano therapeutics can regulate the distribution of drugs in the body, specifically increase the concentration of drugs in the tumor site, so as to enhance the efficacy and reduce adverse reactions, can utilize particular conditions of TME to improve the effect of tumor therapy. This paper summarizes the major components and characteristics of TME, discusses the principles and strategies of relevant nano-architectures targeting TME for the treatment and diagnosis systematically.

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.

Supramolecular metal-based nanoparticles for drug delivery and cancer therapy

Although conventional cancer therapies such as chemotherapy and radiotherapy prevail in clinic, they tend to have narrow therapeutic windows. Many chemotherapies have unfavorable pharmacokinetics while radiotherapy incurs radiotoxicity to normal tissues surrounding tumors. The chemical tunability of supramolecular metal-based nanoparticles (SMNPs) enables the incorporation of various therapeutics, including hydrophilic and hydrophobic chemotherapeutic drugs, photosensitizers, radiosensitizers, and biological therapeutics for more effective delivery to tumors. In this mini-review, we highlight recent advances in SMNPs, namely nanoscale coordination polymers and nanoscale metal-organic frameworks, for drug delivery and cancer therapy. We particularly focus on innovative uses of metal clusters, ligands, pores, and surface modifications to load various therapeutics into SMNPs and critical evaluations of the anticancer efficacies of SMNPs.

Nanoscale Metal-Organic Layer Isolates Phthalocyanines for Efficient Mitochondria-Targeted Photodynamic Therapy

Zinc-phthalocyanine (ZnPc) photosensitizers (PSs) have shown great potential in photodynamic therapy (PDT) owing to their strong absorption at long wavelengths (650-750 nm), high triplet quantum yields, and biocompatibility. However, the clinical utility of ZnPc PSs is limited by their poor solubility and tendency to aggregate in aqueous environments. Here we report the design of a new nanoscale metal-organic layer (nMOL) assembly, ZnOPPc@nMOL, with ZnOPPc [ZnOPPc = zinc(II)-2,3,9,10,16,17,23,24-octa(4-carboxyphenyl)phthalocyanine] PSs supported on the secondary building units (SBUs) of a Hf₁₂ nMOL for PDT. Upon irradiation, SBU-bound ZnOPPc PSs absorb 700 nm light and efficiently sensitize the formation of singlet oxygen by preventing aggregation-induced self-quenching of ZnOPPc excited states. With intrinsic mitochondria-targeting capability, ZnOPPc@nMOL showed exceptional PDT efficacy with >99% tumor growth inhibition and 40-60% cure rates on two mouse models of colon cancer.

Nanoscale Metal-Organic Layers Detect Mitochondrial Dysregulation and Chemoresistance via Ratiometric Sensing of Glutathione and pH

Mitochondrial dysregulation controls cell death and survival by changing endogenous molecule concentrations and ion flows across the membrane. Here, we report the design of a triply emissive nanoscale metal-organic layer (nMOL), NA@Zr-BTB/F/R, for sensing mitochondrial dysregulation. Zr-BTB nMOL containing Zr₆ secondary building units (SBUs) and 2,4,6-tris(4-carboxyphenyl)aniline (BTB-NH₂) ligands was postsynthetically functionalized to afford NA@Zr-BTB/F/R by exchanging formate capping groups on the SBUs with glutathione(GSH)-selective (2E)-1-(2'-naphthyl)-3-(4-carboxyphenyl)-2-propen-1-one (NA) and covalent conjugation of pH-sensitive fluorescein (F) and GSH/pH-independent rhodamine-B (R) to the BTB-NH₂ ligands. Cell imaging demonstrated NA@Zr-BTB/F/R as a ratiometric sensor for mitochondrial dysregulation and chemotherapy resistance via GSH and pH sensing.

Monolayer nanosheets formed by liquid exfoliation of charge-assisted hydrogen-bonded frameworks

Hydrogen-bonded organic frameworks (HOFs) are a diverse and tunable class of materials, but their potential as free-standing two-dimensional nanomaterials has yet to be explored. Here we report the self-assembly of two layered hydrogen-bonded frameworks based on strong, charge-assisted hydrogen-bonding between carboxylate and amidinium groups. Ultrasound-assisted liquid exfoliation of both materials readily produces monolayer hydrogen-bonded organic nanosheets (HONs) with micron-sized lateral dimensions. The HONs show remarkable stability and maintain their extended crystallinity and monolayer structures even after being suspended in water at 80 °C for three days. These systems also exhibit efficient fluorescence quenching of an organic dye in organic solvents, superior to the quenching ability of the bulk frameworks. We anticipate that this approach will provide a route towards a diverse new family of molecular two-dimensional materials.

Multifunctional metal-organic framework heterostructures for enhanced cancer therapy

Metal-organic frameworks (MOFs) are an emerging class of molecular crystalline materials built from metal ions or clusters bridged by organic linkers. By taking advantage of their synthetic tunability and structural regularity, MOFs can hierarchically integrate nanoparticles and/or biomolecules into a single framework to enable multifunctions. The MOF-protected heterostructures not only enhance the catalytic capacity of nanoparticle components but also retain the biological activity of biomolecules in an intracellular microenvironment. Therefore, the multifunctional MOF heterostructures have great advantages over single components in cancer therapy. In this review, we comprehensively summarize the general principle of the design and functional modulation of nanoscaled MOF heterostructures, and biomedical applications in enhanced therapy within the last five years. The functions of MOF heterostructures with a controlled size can be regulated by designing various functional ligands and in situ growth/postmodification of nanoparticles and/or biomolecules. The advances in the application of multifunctional MOF heterostructures are also explored for enhanced cancer therapies involving photodynamic therapy, photothermal therapy, chemotherapy, radiotherapy, immunotherapy, and theranostics. The remaining challenges and future opportunities in this field, in terms of precisely localized assembly, maximizing composite properties, and processing new techniques, are also presented. The introduction of multiple components into one crystalline MOF provides a promising approach to design all-in-one theranostics in clinical treatments.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

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

Due to its great relevance to environmental, biological, and chemical processes, the precise detection of pH or acidic/basic species is an ongoing and imperative need. In this context, pH-sensitive luminescent systems are highly desired. We reported a three-dimensional Zn(II) MOF synthesized from a bipyridyl-tetracarboxylic ligand and composed of 4-fold interpenetrated diamond frameworks. Because the steric hindrance in the ligand prevents metal coordination with the pyridyl group, the MOF features free basic N sites accessible to the small H⁺ ions, which renders pH responsivity. The aqueous dispersion exhibits an abrupt, high-contrast, and reversible on-off fluorescence transition in the narrow pH range of 5.4-6.2. The sensitive bistable system can be used for the precise monitoring of pH within the range and for use as a pH-triggered optical switch. The responsive mechanism through pyridyl protonation is collaboratively supported by data fitting, absorption spectra, and molecular orbital calculations. In particular, spectral and theoretical analyses reveal the destruction of n → π* transitions and the appearance of intramolecular charge-transfer transitions upon pyridyl protonation. Moreover, by virtue of the pH-responsive fluorescence, the MOF shows appealing sensing performance for the detection of 3-nitropropionic acid, a major mycotoxin in moldy sugar cane.

Metal hydrogen-bonded organic frameworks: structure and performance

Although great progress has been made in the design, synthesis, and performance expansion of porous materials, new porous materials with stable structures still need to be explored further. In recent years, porous molecular crystals formed by intermolecular interactions have attracted wide attention from chemists, especially metal hydrogen-bonded organic frameworks (M-HOFs) formed by connecting metal complexes through hydrogen bonds. Metal complexes with specific properties (e.g., magnetism, luminescence, sensing, and catalysis) can expand and develop the application of M-HOFs further. However, the huge volume, irregular shape, complex coordination modes, and interference of coordination bonds pose certain challenges in the synthesis and performance expansion of M-HOFs. In this frontier, we summarize the latest progress in the use of 3d, 4d, and 4f metal complexes for the synthesis of M-HOFs, and briefly introduce the performance expansion of these M-HOFs, which is expected to help expand new porous materials with stable structures and specific functions.

Ligand engineering to achieve enhanced ratiometric oxygen sensing in a silver cluster-based metal-organic framework

Ratiometric luminescent oxygen sensing based on dual fluorescence and phosphorescence emission in a single matrix is highly desirable, yet the designed synthesis remains challenging. Silver-chalcogenolate-cluster-based metal-organic frameworks that combine the advantages of silver clusters and metal-organic frameworks have displayed unique luminescent properties. Herein, we rationally introduce -NH₂ groups on the linkers of a silver-chalcogenolate-cluster-based metal-organic framework (Ag₁₂bpy-NH₂) to tune the intersystem crossing, achieving a dual fluorescence-phosphorescence emission from the same linker chromophore. The blue fluorescence component has a 100-nm gap in wavelength and 8,500,000-fold difference in lifetime relative to a yellow phosphorescence component. Ag₁₂bpy-NH₂ quantifies oxygen during hypoxia with the limit of detection of as low as 0.1 ppm and 0.3 s response time, which is visualized by the naked eye. Our work shows that metal cluster-based MOFs have great potential in luminescent sensing, and the longer-lived charge-separated states could find more photofunctional applications in solar energy transformation and photocatalysis.

Dual-Functional Proton-Conducting and pH-Sensing Polymer Membrane Benefiting from a Eu-MOF

Porous metal-organic frameworks (MOFs) have demonstrated a great potential in proton conduction and luminescence sensing due to functionalized nodes, ligands and channels, or pores. Herein, we prepared a hydrothermally stable Eu-MOF that also resisted acid and base using a bifunctional organic ligand containing carboxylic acid groups, which are easily coordinated to Eu ions, and Eu-phobic tetrazolyl groups as potential proton-hopping sites. The hydrogen bond network, which was constructed by the uncoordinated anionic tetrazolium and the coordinated and free water molecules, endowed this Eu-MOF with the highest proton conductivity of 4.45 × 10^-2 S/cm at 373 K and 93% relative humidity. The proton conductivity of the Nafion membrane containing this Eu-MOF increased 1.74 times. More interestingly, the hybrid membrane displayed luminescence pH sensing because the changeable protonation levels of uncoordinated tetrazolium groups along with the pH tuned the emission of embedded Eu-MOFs. Such a dual-functional MOF-based hybrid membrane including proton conduction and pH sensing is reported for the first time, which could open an avenue to the more practical application for functional MOFs.

Perimitochondrial Enzymatic Self-Assembly for Selective Targeting the Mitochondria of Cancer Cells

Emerging evidence indicates that mitochondria contribute to drug resistance in cancer, but how to selectively target the mitochondria of cancer cells remains less explored. Here, we show perimitochondrial enzymatic self-assembly for selectively targeting the mitochondria of liver cancer cells. Nanoparticles of a peptide-lipid conjugate, being a substrate of enterokinase (ENTK), encapsulate chloramphenicol (CLRP), a clinically used antibiotic that is deactivated by glucuronidases in cytosol but not in mitochondria. Perimitochondrial ENTK cleaves the Flag-tag on the conjugate to deliver CLRP selectively into the mitochondria of cancer cells, thus inhibiting the mitochondrial protein synthesis, inducing the release of cytochrome c into the cytosol and resulting in cancer cell death. This strategy selectively targets liver cancer cells over normal liver cells. Moreover, blocking the mitochondrial protein synthesis sensitizes the cancer cells, relying on glycolysis and/or OXPHOS, to cisplatin. This work illustrates a facile approach, selectively targeting mitochondria of cancer cells and repurposing clinically approved ribosome inhibitors, to interrupt the metabolism of cancer cells for cancer treatment.

Highly Stable Lanthanide Metal-Organic Framework as an Internal Calibrated Luminescent Sensor for Glutamic Acid, a Neuropathy Biomarker

Glutamic acid (Glu) is the most abundant excitatory neurotransmitter in the central nervous system, and an elevated level of Glu may indicate some neuropathological diseases. Herein, three isomorphic microporous lanthanide metal-organic frameworks (MOFs) [(CH₃)₂NH₂]₂[Ln₆(μ₃-OH)₈(BDC-OH)₆(H₂O)₆]·(solv)_x (ZJU-168; ZJU = Zhejiang University, H₂BDC-OH = 2-hydroxyterephthalic acid, Ln = Eu, Tb, Gd) were designed for the detection of Glu. ZJU-168(Eu) and ZJU-168(Tb) suspensions simultaneously produce the characteristic emission bands of both lanthanide ions and ligands. When ZJU-168(Eu) and ZJU-168(Tb) suspensions exposed to Glu, the fluorescence intensity of ligands increases while the emission of lanthanide ions is almost unchanged. By utilizing the emission of ligands as the detected signal and the emission of lanthanide ions as the internal reference, an internal calibrated fluorescence sensor for Glu was obtained. There is a good linear relationship between fluorescence intensity ratio and Glu concentration in a wide range with the detection limit of 3.6 μM for ZJU-168(Tb) and 4.3 μM for ZJU-168(Eu). Major compounds present in blood plasma have no interference for the detection of Glu. Furthermore, a convenient analytical device based on a one-to-two logic gate was constructed for monitoring Glu. These establish ZJU-168(Tb) as a potential turn-on, ratiometric, and colorimetric fluorescent sensor for practical detection of Glu.

Rational Design of "Three-in-One" Ratiometric Nanoprobes: Protein-Caged Dityrosine, CdS Quantum Dots, and Gold Nanoclusters

Recently, multiplexed ratiometric fluorescence sensors for detecting several analytes have received much interest because of their multifunctionality. Here, we fabricate a novel trinity fluorescent nanoprobe in which one small-molecule fluorophore, blue-emissive dityrosine (diTyr) residues, and two nanomaterial fluorophores, green-emissive CdS quantum dots (CdSQDs) and red-emissive gold nanoclusters (AuNCs), are cocaged in a bovine serum albumin (BSA) molecule. The large differences of Stokes shifts among diTyr residues, CdSQDs, and AuNCs ensure their emission at a single excitation wavelength. The nanoprobes can be facilely integrated using two-step synthetic reactions. DiTyr residues and AuNCs are formed and bound to the protein cage through the redox reaction between Au3+ and tyrosine residues of BSA, and the CdSQDs are followed to be conjugated to the modified BSA cage-templated CdS combination reaction. With established benign biocompatibility, the nanoprobes can ratiometrically detect intracellular glutathione by significantly enhancing the green emission of the conjugated CdSQDs. Likewise, the ratiometric sensing of solution alkalinity and tris(2-carboxyethyl)phosphine can be achieved using blue-emitted diTyr residues and red-emitted AuNCs as the responsive units, respectively, and the corresponding other two fluorophores as the reference signals. This study addresses a concept of trinity fluorescence ratiometric sensing system with multiple targets and optional references, which should be a promising pathway to meet the challenges from complexing biochemical environments and multivariate analysis.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.