Zinc-Adeninate Metal−Organic Framework for Aqueous Encapsulation and Sensitization of Near-infrared and Visible Emitting Lanthanide Cations

Nanoscale Metal-Organic Frameworks for Therapeutic, Imaging, and Sensing Applications

Nanotechnology has played an important role in drug delivery and biomedical imaging over the past two decades. In particular, nanoscale metal-organic frameworks (nMOFs) are emerging as an important class of biomedically relevant nanomaterials due to their high porosity, multifunctionality, and biocompatibility. The high porosity of nMOFs allows for the encapsulation of exceptionally high payloads of therapeutic and/or imaging cargoes while the building blocks-both ligands and the secondary building units (SBUs)-can be utilized to load drugs and/or imaging agents via covalent attachment. The ligands and SBUs of nMOFs can also be functionalized for surface passivation or active targeting at overexpressed biomarkers. The metal ions or metal clusters on nMOFs also render them viable candidates as contrast agents for magnetic resonance imaging, computed tomography, or other imaging modalities. This review article summarizes recent progress on nMOF designs and their exploration in biomedical areas. First, the therapeutic applications of nMOFs, based on four distinct drug loading strategies, are discussed, followed by a summary of nMOF designs for imaging and biosensing. The review is concluded by exploring the fundamental challenges facing nMOF-based therapeutic, imaging, and biosensing agents. This review hopefully can stimulate interdisciplinary research at the intersection of MOFs and biomedicine.

Cd(ii)-nucleobase supramolecular metallo-hydrogels for in situ growth of color tunable CdS quantum dots

The integration of nanoscale materials into unconventional environments such as gels is a magnificent strategy towards development of engineered hybrid functional systems. Herein, the in situ formation of CdS quantum dots integrated into a metallogel formed through the coordination of Cd2+ with two pyrimidine nucleobases is reported. Thymine and uracil formed spontaneous hydrogels with nanofibrous morphology through coordinative interaction with Cd2+ ions at alkaline pH. Introduction of Na2S resulted in generation of CdS quantum dots within the hydrogels with tunable emission properties from blue to white to yellow. The quenching of emission of white light system was exploited for the sensing of Fe3+ and Cu2+ ions. Such a color tunable quantum dot incorporated metallogel system will find applications in energy harvesting and sensing.

Functionalization of Metal-Organic Frameworks for Photoactive Materials

Metal-organic frameworks (MOFs) are intriguing platforms with multiple functionalities. Additional functionalization of MOFs generates novel materials for various applications. Here, three main topics are examined regarding the functionalization of MOFs for use as photoactive materials. The first is chemical approaches for postsynthetic modification of the metal clusters and organic linkers in MOFs; that is, sites on pore surfaces and chemical trapping of photoactive moieties within the pores, which create materials with chemical functionalities for water splitting and CO₂ reduction by light. The second topic focuses on the functionalization of MOFs for photochemical response and the versatile applications of such materials. State-of-the-art research on functionalizing MOFs through photochemical reactions on the pore surface and within the pores as guests is also summarized. The third topic introduces the functionalization of MOFs for photofunctional materials, including photoluminescent tuning and integration, photoluminescent LED devices and barcodes, and photophysical applications for chemical sensing. Finally, conclusions and perspectives on the fields are given.

A reliable amplified fluorescence-enhanced chemosensor (Eu-MIL-61) for the directional detection of Ag+ in an aqueous solution

A robust gallium carboxylate Ga(OH)(btec)·0.5H₂O (MIL-61) was synthesized under mild hydrothermal conditions and selected as the parent MOF to prepare Eu-MIL-61 by post-synthetic modification due to the uncoordinated carbonyl group of the ligand (pyromellitic acid (H4btec)). The firm framework, permanent porosity, and non-coordinated sites make MIL-61 a good scaffold for immobilizing and sensitizing Eu³⁺. The products showed excellent luminescence and, more significantly, Eu-MIL-61 showed excellent selectivity with photoluminescence enhancement, fast response time, and low detection limit for Ag⁺ ions in the aqueous solutions. In addition, the frameworks were not susceptible to the acid-base property of solutions, which can be further applied to monitor the concentration of Ag⁺ in several daily water samples.

A Multifunctional Nanocage-based MOF with Tri- and Tetranuclear Zinc Cluster Secondary Building Units

A new Zn-cluster based MOF, [Zn21(BTC)11(μ3-OH)3(μ4-O)3(H2O)18]·21EtOH (1) (H3BTC = 1,3,5-benzenetricarboxylic acid), with two different types of cluster nodes has been successfully synthesized from Zn2+ and H3BTC under the solvothermal conditions. Single crystal X-ray diffraction studies reveal that 1 is a 3D trinodal (3,5,6)-c framework which features a large octahedral cage organized by nine Zn3O and nine Zn4O clusters SBUs and twenty-four triangular BTC3- linkers. The Eu3+/Tb3+-incorporated derivative of 1 with 0.251% Eu3+ and 0.269% Tb3+ exhibits tunable luminescence from yellow to white and then to blue-green by changing the excitation wavelength from 308 to 315 nm. Metal ion exchange with Cu2+ affords isomorphous Cu-based MOF with enhanced N2 and CO2 adsorption capacity. In addition, 1 can act as a selective luminescent sensor for Cu2+ and Al3+ ions.

Sensitized near infrared emission through supramolecular d → f energy transfer within an ionic Ru(ii)-Er(iii) pair

The newly synthesized ionic triple salt Ru-Er, {[Ru^II(bpy)₂(dbim)][Er^III(hfac)₄][CF₃COO]·H₂O} (bpy = 2,2'-bipyridine; hfac^- = hexafluoroacetylacetonate; dbim = 2,2'-dibenzimidazole) exhibits near-infrared (NIR) emission at 1535 nm by intermolecular Ru → Er (d → f) energy transfer across supramolecular interactions when pumped within the Ru(ii) ³MLCT band. It is the first such observation for a transition metal-lanthanide ionic pair.

Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements

In combination with porosity and tunability, light harvesting, energy transfer, and photocatalysis, are facets crucial for engineering of MOF-based sensors.

In vivo theranostics with near-infrared-emitting carbon dots-highly efficient photothermal therapy based on passive targeting after intravenous administration

Carbon dots that exhibit near-infrared fluorescence (NIR CDs) are considered emerging nanomaterials for advanced biomedical applications with low toxicity and superior photostability and targeting compared to currently used photoluminescence agents. Despite progress in the synthesis of NIR CDs, there remains a key obstacle to using them as an in vivo theranostic agent. This work demonstrates that the newly developed sulfur and nitrogen codoped NIR CDs are highly efficient in photothermal therapy (PTT) in mouse models (conversion efficiency of 59%) and can be readily visualized by photoluminescence and photoacoustic imaging. The real theranostic potential of NIR CDs is enhanced by their unique biodistribution and targeting. Contrary to all other nanomaterials that have been tested in biomedicine, they are excreted through the body's renal filtration system. Moreover, after intravenous injection, NIR CDs are accumulated in tumor tissue via passive targeting, without any active species such as antibodies. Due to their accumulation in tumor tissue without the need for intratumor injection, high photothermal conversion, excellent optical and photoacoustic imaging performance, and renal excretion, the developed CDs are suitable for transfer to clinical biomedical practice.

Zn(ii)–nucleobase metal–organic nanofibers and nanoflowers: synthesis and photocatalytic application

The interaction of Zn²⁺ ions with pure nucleobases guanine and cytosine under alkaline conditions leads to the formation of nanoscale metal–organic nanofibers and nanoflowers with excellent photocatalytic activity for the degradation of organic pollutant dyes.

A lanthanide functionalized MOF hybrid for ratiometric luminescence detection of an anthrax biomarker

A dual-emitting hybrid platform based on a lanthanide functionalized MOF was demonstrated as a fluorescent indicator for ratiometric detection of an anthrax biomarker—dipicolinic acid (DPA).

Hybrid materials based on transition metal–BTC–benzimidazole: solvent assisted crystallographic and structural switching

Five transition metal–BTC–BIm based coordination polymers with hierarchical structures were synthesized under hydro/solvothermal condition. Crystal structure analysis of the compounds showed that 1 forms a 1D chain, 2 and 3 have 2D layers, while 4 and 5 have three-dimensional architectures.

Syntheses, structures, luminescence and magnetic properties of seven isomorphous metal–organic frameworks based on 2,7-bis(4-benzoic acid)-N-(4-benzoic acid)carbazole

Seven isomorphous lanthanide metal–organic frameworks with special luminescence and magnetic properties are synthesized and characterized.

Lanthanide-Functionalized Metal-Organic Framework Hybrid Systems To Create Multiple Luminescent Centers for Chemical Sensing

Metal-organic frameworks (MOFs) possess an important advantage over other candidate classes for chemosensory materials because of their exceptional structural tunability and properties. Luminescent sensing using MOFs is a simple, intuitive, and convenient method to recognize species, but the method has limitations, such as insufficient chemical selectivity and signal loss. MOFs contain versatile building blocks (linkers or ligands) with special chemical reactivity, and postsynthetic modification (PSM) provides an opportunity to exploit and expand their unique properties. The linkers in most MOFs contain aromatic subunits that can readily display luminescence after ultraviolet or visible (typically blue) excitation, and this is the main luminescent nature of most MOFs. The introduction of photoactive lanthanide ions (Ln³⁺) into the MOF hosts may produce new luminescent signals at different positions from that of the MOF linker, but this depends on the intramolecular energy transfer (antenna effect) from the MOF (linkers) to the Ln³⁺ ions. Controlling the Ln³⁺ content in MOF hybrids may create multiple luminescent centers. The nature of the unique luminescent centers may cause different responses to sensing species (i.e., ratiometric sensing), which may provide a new opportunity for luminescence research with applications to chemical sensing. In this Account, recent research progress on using lanthanide-functionalized MOF hybrid materials to create multiple luminescent centers for chemical sensing is described. Here we propose a general strategy to functionalize MOF hosts with lanthanide ions, compounds, or other luminescent species (organic dyes or carbon dots) and to assemble types of photofunctional hybrid systems based on lanthanide-functionalized MOFs. Five main methods were used to functionalize the MOFs and assemble the hybrid materials: in situ composition, ionic doping, ionic exchange, covalent PSM, and coordinated PSM. Through the lanthanide functionalization, multiple (double or triple) luminescent centers were created with different luminescent bands in the visible region. Because of the different luminescent natures of the lanthanide ions, MOF linkers, and other species (organic dyes or carbon dots), they display different responses to sensing species. Currently, using these strategies, we have utilized a dual-response luminescent probe to realize chemical sensing of different types of cations (Fe³⁺/Fe²⁺, Hg²⁺, and Cd²⁺), anions (Cr₂O₇^2-/CrO₄^- and CO₃^2-), molecules (volatile organic compounds and O₂), special air pollutants (formaldehyde), and biomarkers of food spoilage as well as pH and temperature. Additionally, we have achieved triple-luminescence-response sensing of ions (Ag⁺, Hg²⁺, and S^2-) in complicated aqueous environments, which was developed using a logic operation.

Warm-White-Light-Emitting Diode Based on a Dye-Loaded Metal-Organic Framework for Fast White-Light Communication

A dye@metal-organic framework (MOF) hybrid was used as a fluorophore in a white-light-emitting diode (WLED) for fast visible-light communication (VLC). The white light was generated from a combination of blue emission of the 9,10-dibenzoate anthracene (DBA) linkers and yellow emission of the encapsulated Rhodamine B molecules. The MOF structure not only prevents dye molecules from aggregation-induced quenching but also efficiently transfers energy to the dye for dual emission. This light-emitting material shows emission lifetimes of 1.8 and 5.3 ns for the blue and yellow components, respectively, which are significantly shorter than the 200 ns lifetime of Y₃Al₅O₁₂:Ce³⁺ in commercial WLEDs. The MOF-WLED device exhibited a modulating frequency of 3.6 MHz for VLC, six times that of commercial WLEDs.

Dye@bio-MOF-1 Composite as a Dual-Emitting Platform for Enhanced Detection of a Wide Range of Explosive Molecules

By incorporating a cationic dye within a metal-organic framework (MOF) through an ion-exchange process, a responsive dye@bio-MOF-1 composite has been synthesized, serving as a dual-emitting platform for enhanced detection of different kinds of nitro-explosives, especially nitroalkanes, nitramines, and nitrate esters. The dye@bio-MOF-1 composite was constructed with free amines on their well-defined cavities, which is essential for the capture of explosives into their confined nanospace. It was observed that the encapsulation of explosives into the constructed dye@bio-MOF-1 composite could dramatically alter the luminescent properties of the dyes as well as the MOF skeletons owing to the size exclusivity and confinement-induced effect. For nitroaromatics, the dye@bio-MOF-1 composite exhibits turn-off responses via fluorescence quenching. Unexpectedly, the composite shows unique turn-on responses for aliphatic nitro-organics via confinement-induced enhancement, demonstrating enhanced ability to detecting different kinds of explosives selectively in aqueous solution. Furthermore, the dye@bio-MOF-1 film was facilely fabricated, making the chemical sensing more convenient and easier to realize the discrimination of the targeted explosives. The dual tunable responses indicate that dye@bio-MOF-1 composites are favorable materials for molecular sensing. On the basis of the host-guest properties of the constructed dye@bio-MOF-1 composite, our work can be further extended to sensing specific analytes with remarkable turn-on sensing properties, in particular those difficult to recognize with conventional methods.

A twofold interpenetrating three-dimensional heterometallic metal–organic framework with pcu topology based on 2-methylbiphenyl-4,4′-dicarboxylic acid

The 2-methylbiphenyl-4,4′-dicarboxylate (mbpdc2−) ligand has versatile coordination modes and can be used to construct multinuclear structures. Despite this, reports of the synthesis of coordination complexes involving this ligand are scarce. The title compound, poly[[triaquadi-μ3-hydroxido-hexakis(μ4-2-methylbiphenyl-4,4′-dicarboxylato)calcium(II)hexazinc(II)] monohydrate], {[CaZn6(C15H10O4)6(OH)2(H2O)3]·H2O} n , has been prepared by the hydrothermal assembly of Zn(NO3)2·6H2O, CaCl2 and 2-methylbiphenyl-4,4′-dicarboxylic acid. Two ZnII atoms adopt a four-coordinated distorted tetrahedral geometry by bonding to three O atoms from three different 2-methylbiphenyl-4,4′-dicarboxylate (mbpdc2−) dianionic ligands and one bridging hydroxide O atom. For the remaining ZnII atom, a five-coordinate environment is completed half the time by one carboxylate O atom, and then the same carboxylate O atom and an aqua O atom are present the other half of the time, giving a six-coordinate environment. The CaII atom is coordinated by six O atoms to give an octahedral coordination geometry. The supramolecular secondary building unit (SBU) is a hamburger-like heptanuclear unit (Zn6CaO30) and these units are interconnected through mbpdc2− carboxylate groups to generate a three-dimensional framework with the pcu topology. The single net leaves voids that are filled by mutual interpenetration of an independent equivalent framework in a twofold interpenetrating architecture. The title compound shows thermal stability up to 673 K. The excitation and luminescence data showed the emission of a bright-blue fluorescence.

Near-Infrared Photoluminescent Polymer-Carbon Nanodots with Two-Photon Fluorescence

Near-infrared-emissive polymer-carbon nanodots (PCNDs) are fabricated by a newly developed facile, high-output strategy. The PCNDs emit at a wavelength of 710 nm with a quantum yield of 26.28%, which is promising for deep biological imaging and luminescent devices. Moreover, the PCNDs possess two-photon fluorescence; in vivo bioimaging and red-light-emitting diodes based on these PCNDs are demonstrated.