Confinement inside MOFs Enables Guest-Modulated Spin Crossover of Otherwise Low-Spin Coordination Cages

Confinement of discrete coordination cages within nanoporous lattices is an intriguing strategy to gain unusual properties and functions. We demonstrate here that the confinement of coordination cages within metal-organic frameworks (MOFs) allows the spin state of the cages to be regulated through multilevel host-guest interactions. In particular, the confined in situ self-assembly of an anionic FeII4L6 nanocage within the mesoporous cationic framework of MIL-101 leads to the ionic MOF with an unusual hierarchical host-guest structure. While the nanocage in solution and in the solid state has been known to be invariantly diamagnetic with low-spin FeII, FeII4L6@MIL-101 exhibits spin-crossover (SCO) behavior in response to temperature and release/uptake of water guest within the MOF. The distinct color change concomitant with water-induced SCO enables the use of the material for highly selective colorimetric sensing of humidity. Moreover, the spin state and the SCO behavior can be modulated also by inclusion of a guest into the hydrophobic cavity of the confined cage. This is an essential demonstration of the phenomenon that the confinement within porous solids enables an SCO-inactive cage to show modulable SCO behaviors, opening perspectives for developing functional supramolecular materials through hierarchical host-guest structures.

Supramolecular Host-Guest Strategy for the Accelerating Detection of Nitroreductase

Identifying nitroreductase (NTR) with fluorescent techniques has become a research hotspot, due to its good sensitivity and selectivity toward the early-stage cancer diagnosis and monitoring. Herein, a host-guest reporter (NAQA⊂Zn-MPPB) is successfully achieved by encapsulating the NTR probe NAQA into a new NADH-functioned metal-organic cage Zn-MPPB, which makes the reporter for ultrafast detection of NTR within dozens of seconds in solution. The host-guest strategy fuses the Zn-MPPB and NAQA to form a pseudomolecule material, which changes the reaction process of NTR and NAQA from a double substrates mechanism to a single substrate one, and accelerates the reduction efficiency of NAQA. This advantage make the new host-guest reporter exhibit a linear relationship between emission changes and NTR concentration, and it shows better sensitively toward NTR than that of NAQA. Additionally, the positively charged water-soluble metal-organic cage can encapsulate NAQA in the cavity, promote it to dissolve in an aqueous environment, and facilitate their accumulation into tumor cells. As expected, such host-guest reporter displays a fast and high efficiently imaging capability toward NTR in tumor cells and tumor-bearing mice, and flow cytometry assay is conducted to corroborate the capability as well, implying the considerably potential of host-guest strategy for early tumor diagnosis and treatment.

Block Co-PolyMOC Micelles and Structural Synergy as Composite Nanocarriers

Conventional micelles of amphiphilic block copolymers (BCPs) disassemble into individual polymer chains upon dilution to a critical concentration, which causes the premature release of the encapsulated drugs and reduces the drug's bioavailability. Here, by integrating the emerging metal-organic cage (MOC) materials with BCPs, we introduce a new type of composite micellar nanoparticles, block co-polyMOC micelles (or BCPMMs), that are self-assembled in essence yet remarkably stable against dilution. BCPMMs are fabricated via a stepwise assembly strategy that combines MOCs and BCPs in a well-defined, unimolecular core-shell structure. The synergistical interplay between the two components accounts for the particle stability: the MOC core holds BCPs firmly in place and the BCPs increase the MOC's bioavailability. When used as nanocarriers for anticancer drugs, BCPMMs showed an extended blood circulation, a favorable biodistribution, and eventually an improved treatment efficacy in vivo. Given the versatility in designing MOCs and BCPs, we envision that BCPMMs can serve as a modular platform for robust, multifunctional, and tunable nanomedicine.

Robust Heterogeneous Photocatalyst for Visible-Light-Driven Hydrogen Evolution Promotion: Immobilization of a Fluorescein Dye-Encapsulated Metal-Organic Cage on TiO2

The design of artificial photocatalytic devices that simulates the ingenious and efficient photosynthetic systems in nature is promising. Herein, a metal-organic cage [Pd₆(NPyCzPF)₁₂]¹²⁺ (MOC-PC6) integrating 12 organic ligands NPyCzBP and 6 Pd²⁺ catalytic centers is designed, which is well defined to include organic dye fluorescein (FL) for constructing a supramolecular photochemical molecular device (SPMD) FL@MOC-PC6. Photoinduced electron transfer (PET) between MOC-PC6 and the encapsulated FL has been observed by steady-state and time-resolved emission spectroscopy. FL@MOC-PC6 is successfully heterogenized with TiO₂ by a facile sol-gel method to achieve a robust heterogeneous FL@MOC-PC6-TiO₂. The close proximity between the Pd²⁺ catalytic site and FL included in the cage enables PET from the photoexcited FL to Pd²⁺ sites through a powerful intramolecular pathway. The photocatalytic hydrogen production assessments of the optimized 4 wt % FL@MOC-PC6-TiO₂ demonstrate an initial H₂ production rate of 2402 μmol g^-1 h^-1 and a turnover number of 4356 within 40 h, enhanced by 15-fold over that of a homogeneous FL@MOC-PC6. The effect of the MOC content on photocatalytic H₂ evolution (PHE) is investigated and the inefficient comparison systems, such as MOC-PC6, MOC-PC6-TiO₂, FL-sensitized MOC-PC6/FL-TiO₂, and analogue FL/MOC-PC6-TiO₂ with free FL, are evaluated. This study provides a creative and distinctive approach for the design and preparation of novel heterogeneous SPMD catalysts based on MOCs.

Constructing Heterogeneous Direct Z-Scheme Photocatalysts Based on Metal-Organic Cages and Graphitic-C3N4 for High-Efficiency Photocatalytic Water Splitting

The development of artificial devices that mimic the highly efficient and ingenious photosystems in nature is worthy of in-depth study. A metal-organic cage (MOC) Pd₂(M-4)₄(BF₄)₄, denoted as MOC-Q1, integrating four organic photosensitized ligands M-4 and two Pd²⁺ catalytic centers is designed for a photochemical molecular device (PMD). MOC-Q1 is successfully immobilized on graphitic carbon nitride (g-C₃N₄) by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C₃N₄/MOC-Q1 photocatalyst for H₂ generation under visible light. The optimized g-C₃N₄/MOC-Q1 (2 wt %) system shows high hydrogen evolution activity (4495 μmol g^-1 h^-1 based on the catalyst mass) and exhibits stable performances for 25 h (a turnover number of 19,268 based on MOC-Q1), significantly outperforming pure MOC-Q1, g-C₃N₄, and comparsion materials Pd/g-C₃N₄/M-4, which is the highest one of all reported heterogeneous MOC-based photocatalysts under visible irradiation. This enhancement can be ascribed to the synergistic effects of high-efficient electron transfer, extended visible-light response region, and good protective environment for MOC-Q1 arising from an efficient direct Z-scheme heterostructure of g-C₃N₄/MOC-Q1. This rationally designed and synthesized MOC/g-C₃N₄-based heterogeneous PMD is expected to have great potential in photocatalytic water splitting.

Kinetics of Toehold-Mediated DNA Strand Displacement Depend on FeII4L4 Tetrahedron Concentration

The toehold-mediated strand displacement reaction (SDR) is a powerful enzyme-free tool for molecular manipulation, DNA computing, signal amplification, etc. However, precise modulation of SDR kinetics without changing the original design remains a significant challenge. We introduce a new means of modulating SDR kinetics using an external stimulus: a water-soluble Fe^II₄L₄ tetrahedral cage. Our results show that the presence of a flexible phosphate group and a minimum toehold segment length are essential for Fe^II₄L₄ binding to DNA. SDRs mediated by toehold ends in different lengths (3-5) were investigated as a function of cage concentration. Their reaction rates all first increased and then decreased as cage concentration increased. We infer that cage binding on the toehold end slows SDR, whereas the stabilization of intermediates that contain two overhangs accelerates SDR. The tetrahedral cage thus serves as a versatile tool for modulation of SDR kinetics.

Nanosheets and Hydrogels Formed by 2 nm Metal-Organic Cages with Electrostatic Interaction

We report the mechanism of hydrogel formation in dilute aqueous solutions (>15 mg/mL) by 2 nm metal-organic cages (MOCs). Experiments and all-atom simulations confirm that with the addition of small electrolytes, the MOCs self-assemble into 2D nanosheets via counterion-mediated attraction because of their unique molecular structure and charge distribution as well as σ-π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroion systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (∼1.5 wt %) MOC concentrations, with additional help from hydrophobic and partial π-π interactions similar to the gelation of graphene oxides.

One-/Two-Photon Excited Cell Membrane Imaging and Tracking by a Photoactive Nanocage

Cell membrane imaging by predesigned molecular and supramolecular photoluminescence probes is of great importance in understanding the nano-biointeractions for potential applications in cellular tracking, drug delivery, cancer diagnosis, and treatment. Herein, we report an effective strategy for cell membrane imaging in both living cell and tissue levels on the basis of a multifunctional nanocage (MOC-16) integrating one-/two-photon active phosphorescence, high charges, balanced hydrophobicity/lipophilicity, and proton sensitivity attributes. The intrinsic optical characters, including strong one-/two-photon excitation and pH-dependent red emission, make MOC-16 powerful optical probes for membrane imaging in living cell and tissue levels under both visible and near-infrared irradiations. Meanwhile, the highly positive charges of +28 endow MOC-16 with adequate water solubility and deprotonation ability while still maintaining its hydrophobicity, thus enabling balanced hydrophobic-lipophilic interactions at the nano-biointerface to facilitate a pH-dependent membrane absorption within the biological pH range of 5.3-7.4. However, the low-charged RuL₃ metalloligand or polyethylene glycol (PEG)-modified MOC-16^PEG with less hydrophobicity cannot offer enough nano-biointeractions for cell membrane tracking. These findings advance the fundamental understanding of nano-biointerface interactions of MOCs with cell membranes and provide further guidance in their biological applications.

Conversion of Metal-Organic Cage to Ligand-Free Ultrasmall Noble Metal Nanocluster Catalysts Confined within Mesoporous Silica Nanoparticle Supports

Supported ultrasmall noble metal nanocluster-based (UNMN-based) catalysts are one of the most important classes of solid materials for heterogeneous catalysis. In this work, we present a novel strategy for the controlled synthesis of ligand-free UNMN nanocatalysts based on in situ reduction of a palladium-based (Pd-based) metal-organic cage (MOC) confined within monosized, thiol-modified mesoporous silica nanoparticle (MSN) supports. By taking advantage of the high mutual solubility of MOCs and MSNs in DMSO and the strong interactions between the thiol-modified MSN pore wall and MOC surface, a good dispersion of MOC molecules was achieved throughout the MSN support. The close correspondence of the MSN pore diameter (ca. 5.0 nm) with the diameter of the MOC (ca. 4.0 nm) confines MOC packing to approximately a monolayer. Based on this spatial constraint and electrostatic binding of the MOC to the thiol-modified MSN pore surface, in situ MOC reduction followed by metal atom diffusion, coalescence, and anchoring on the active sites resulted in ligand-free Pd-based UNMNs of approximately 0.9 ± 0.2 nm in diameter decorating the MSN pore surfaces. Control experiments of the reduction of a conventional palladium source or the reduction of free, unconstrained cages in solution under the same conditions only produced large metal nanocrystals (NP, >2 nm), confirming the importance of confined reduction to achieve a highly catalytically active surface. In light of this strategy, two catalytic experiments including the reaction of 4-nitrophenol to 4-aminophenol and the Suzuki C-C coupling reaction show superior catalytic activity of the engineered MSN-supported UNMN nanocatalysts compared to their free form and state of the art commercial catalysts. We believe that our new strategy will open new avenues for artificially designed UNMN-inspired nanoarchitectures for wide applications.