Optochemically Responsive 2D Nanosheets of a 3D Metal–Organic Framework Material

Outstanding functional tunability underpinning metal–organic framework (MOF) confers a versatile platform to contrive next‐generation chemical sensors, optoelectronics, energy harvesters, and converters. A rare exemplar of a porous 2D nanosheet material constructed from an extended 3D MOF structure is reported. A rapid supramolecular self‐assembly methodology at ambient conditions to synthesize readily exfoliatable MOF nanosheets, functionalized in situ by adopting the guest@MOF (host) strategy, is developed. Nanoscale confinement of light‐emitting molecules (as functional guest) inside the MOF pores generates unusual combination of optical, electronic, and chemical properties, arising from the strong host–guest coupling effects. Highly promising photonics‐based chemical sensing opened up by the new guest@MOF composite systems is shown. By harnessing host–guest optochemical interactions of functionalized MOF nanosheets, detection of an extensive range of volatile organic compounds and small molecules important for many practical applications has been accomplished.     

Versatile and Switchable Responsive Properties of a Lanthanide‐Viologen Metal–Organic Framework

Metal–organic frameworks (MOFs) provide intriguing platforms for the design of responsive materials. It is challenging to mobilize as many components as possible of a MOF to collaboratively accomplish multiple responsive properties. Here, reversible photochromism, piezochromism, hydrochromism, ionochromism, and luminescence modulation of an ionic Eu(III) MOF is reported furnished by cationic electron‐deficient viologen units and exchangeable guest anions. Mechanistically, the extraordinarily versatile responsive properties are owed to electron transfer (ET), charge transfer (CT), and energy transfer, involving viologen as electron acceptor, anion as electron donor, luminescing Eu(III) as energy donor, and anion‐viologen CT complex or ET‐generated radical as energy acceptor (luminescence quencher). Moreover, guest anions and waters provide flexible handles to control the ET‐based responsive properties. Water release/reuptake or exchange with organic solvents can switch on/off the response to light, while reversible anion exchange can disenable or awaken the responses to pressure, light, and water release/reuptake. The impacts of water and anions on ET are justified by the high polarity and hydrogen‐bonding capability of water, the different electron donor strength of anions, and the strong I^?‐viologen CT interactions. The rich responsive behaviors have great implications for applications such as pressure sensors, iodide detection, and chemical logic gates.     

miRNA‐Specific Unlocking of Drug‐Loaded Metal–Organic Framework Nanoparticles: Targeted Cytotoxicity toward Cancer Cells

UiO‐68 metal–organic framework nanoparticles (NMOFs) are loaded with a doxorubicin drug (fluorescent dye analogs) and locked by means of structurally engineered duplex nucleic acid structures, where one strand is covalently linked to the NMOFs and the second strand is hybridized with the anchor strand. Besides the complementarity of the second strand to the anchor sequence, it includes the complementary sequence to the microRNAs (miRNA)‐21 or miRNA‐221 that is specific miRNA biomarker for MCF‐7 breast cancer cells or OVCAR‐3 ovarian cancer cells. In the presence of the respective miRNA biomarkers, the miRNA‐induced displacement of the strand associated with the anchor strand proceeds, resulting in the release of DNA/miRNA duplexes. The released duplexes are, however, engineered to be digested in the presence of exonuclease III, Exo III, a process that recycles the miRNAs and provides the autonomous amplified unlocking of the NMOFs and the release of the doxorubicin load (or the fluorescent dye analogs) even at low concentrations of miRNA. Preliminary cell experiments reveal that the respective NMOFs are unlocked by the miRNA‐21 or miRNA‐221, resulting in selective cytotoxicity toward MCF‐7 breast cancer cells or OVCAR‐3 ovarian cancer cells.     

Nanostructured Metal–Organic Framework (MOF)‐Derived Solid Electrolytes Realizing Fast Lithium Ion Transportation Kinetics in Solid‐State Batteries

Solid‐state batteries are hindered from practical applications, largely due to the retardant ionic transportation kinetics in solid electrolytes (SEs) and across electrode/electrolyte interfaces. Taking advantage of nanostructured UIO/Li‐IL SEs, fast lithium ion transportation is achieved in the bulk and across the electrode/electrolyte interfaces; in UIO/Li‐IL SEs, Li‐containing ionic liquid (Li‐IL) is absorbed in Uio‐66 metal–organic frameworks (MOFs). The ionic conductivity of the UIO/Li‐IL (15/16) SE reaches 3.2 × 10^?4 S cm^?1 at 25 °C. Owing to the high surface tension of nanostructured UIO/Li‐IL SEs, the contact between electrodes and the SE is excellent; consequently, the interfacial resistances of Li/SE and LiFePO₄/SE at 60 °C are about 44 and 206 Ω cm², respectively. Moreover, a stable solid conductive layer is formed at the Li/SE interface, making the Li plating/stripping stable. Solid‐state batteries from the UIO/Li‐IL SEs show high discharge capacities and excellent retentions (≈130 mA h g^?1 with a retention of 100% after 100 cycles at 0.2 C; 119 mA h g^?1 with a retention of 94% after 380 cycles at 1 C). This new type of nanostructured UIO/Li‐IL SEs is very promising for solid‐state batteries, and will open up an avenue toward safe and long lifespan energy storage systems.     

Enforcing Luminescence at Organic Nanointerfaces: Luminescence Spatial Confinement and Amplification in Molecular‐Based Core–Shell Nanoparticles

Fully organic core–shell nanoparticles that promote luminescence spatial confinement and enhancement at the core–shell nanointerface are designed and prepared. These molecular‐based bicomponent nanostructures give rise to very efficient directional excitation energy transfer from the shell to acceptor molecules in the core located at the core–shell nanointerface. A striking luminescence enhancement is observed with respect to the corresponding single‐component nanoparticles, which is ascribed to large local electric fields generated at the nanointerface between the polarizable molecular core and shell.     

Hybrid Nanomedicine Fabricated from Photosensitizer‐Terminated Metal–Organic Framework Nanoparticles for Photodynamic Therapy and Hypoxia‐Activated Cascade Chemotherapy

During photodynamic therapy (PDT), severe hypoxia often occurs as an undesirable limitation of PDT owing to the O₂‐consuming photodynamic process, compromising the effectiveness of PDT. To overcome this problem, several strategies aiming to improve tumor oxygenation are developed. Unlike these traditional approaches, an opposite method combining hypoxia‐activated prodrug and PDT may provide a promising strategy for cancer synergistic therapy. In light of this, azido‐/photosensitizer‐terminated UiO‐66 nanoscale metal–organic frameworks (UiO‐66‐H/N₃ NMOFs) which serve as nanocarriers for the bioreductive prodrug banoxantrone (AQ4N) are engineered. Owing to the effective shielding of the nanoparticles, the stability of AQ4N is well preserved, highlighting the vital function of the nanocarriers. By virtue of strain‐promoted azide–alkyne cycloaddition, the nanocarriers are further decorated with a dense PEG layer to enhance their dispersion in the physiological environment and improve their therapeutic performance. Both in vitro and in vivo studies reveal that the O₂‐depleting PDT process indeed aggravates intracellular/tumor hypoxia that activates the cytotoxicity of AQ4N through a cascade process, consequently achieving PDT‐induced and hypoxia‐activated synergistic therapy. Benefiting from the localized therapeutic effect of PDT and hypoxia‐activated cytotoxicity of AQ4N, this hybrid nanomedicine exhibits enhanced therapeutic efficacy with negligible systemic toxicity, making it a promising candidate for cancer therapy.     

Metal–Organic Framework Encapsulation for the Preservation and Photothermal Enhancement of Enzyme Activity

Interfacing biomolecules with functional materials is a key strategy toward achieving externally‐triggered biological function. The rational integration of functional proteins, such as enzymes, with plasmonic nanostructures that exhibit unique optical properties such as photothermal effect provides a means to externally control the enzyme activity. However, due to the labile nature of enzymes, the photothermal effect of plasmonic nanostructures is mostly utilized for the enhancement of the biocatalytic activity of thermophilic enzymes. In order to extend and utilize the photothermal effect to a broader class of enzymes, a means to stabilize the immobilized active protein is essential. Inspired by biomineralization for the encapsulation of soft tissue within protective exteriors in nature, metal–organic framework is utilized to stabilize the enzyme. This strategy provides an effective route to enhance and externally modulate the biocatalytic activity of enzymes bound to functional nanostructures over a broad range of operating environments that are otherwise hostile to the biomolecules.