Dynamic Surface Modification of Metal-Organic Framework Nanoparticles via Alkoxyamine Functional Groups

External surface engineering of metal-organic framework nanoparticles (MOF NPs) is emerging as an important design strategy, leading to optimized chemical and colloidal stability. To date, most of the MOF surface modifications have been performed either by physical adsorption or chemical association of small molecules or (preformed) polymers. However, most of the currently employed approaches cannot precisely control the polymer density, and dynamic modifications at the surfaces on demand have been a challenging task. Here, we introduce a general approach based on covalent modification employing alkoxyamines as a versatile tool to modify the outer surface of MOF nanoparticles (NPs). The alkoxyamines serve as initiators to grow polymers from the MOF surface via nitroxide-mediated polymerization (NMP) and allow dynamic attachment of small molecules via a nitroxide exchange reaction (NER). The successful surface modification and successive surface polymerization are confirmed via time-of-flight secondary ion mass spectrometry (ToF-SIMS), size exclusion chromatography (SEC), and nuclear magnetic resonance (NMR) spectroscopy. The functionalized MOF NPs exhibit high suspension stability and good dispersibility while retaining their chemical integrity and crystalline structure. In addition, electron paramagnetic resonance spectroscopy (EPR) studies prove the dynamic exchange of two different nitroxide species via NER and further allow us to quantify the surface modification with high sensitivity. Our results demonstrate that alkoxyamines serve as a versatile tool to dynamically modify the surface of MOF NPs with high precision, allowing us to tailor their properties for a wide range of potential applications, such as drug delivery or mixed matrix membranes.

Tailoring the Gibbs Free Energy of the Nitroxide Exchange Reaction: Substituent and Solvent Effects

A quantum chemical study of the nitroxide exchange reaction is presented. Inspired by the recent use of this reaction in the synthesis of dynamic covalent polymer networks, we studied the influence of substituents and solvents on the Gibbs free energy, which plays a crucial role for both the reversibility of the reaction and the extent of product formation. We provide accurate benchmark values based on CCSD(T) and COSMO-RS theory for a series of structural modifications and make suggestions for improving the molecular building blocks used so far.

Dynamic porous organic polymers with tuneable crosslinking degree and porosity

Porous organic polymers (POPs) show enormous potential for applications in separation, organic electronics, and biomedicine due to the combination of high porosity, high stability, and ease of functionalisation. However, POPs are usually insoluble and amorphous materials making it very challenging to obtain structural information. Additionally, important parameters such as the exact molecular structure or the crosslinking degree are largely unknown, despite their importance for the final properties of the system. In this work, we introduced the reversible multi-fold nitroxide exchange reaction to the synthesis of POPs to tune and at the same time follow the crosslinking degree in porous polymer materials. We synthesised three different POPs based on the combination of linear, trigonal, and tetrahedral alkoxyamines with a tetrahedral nitroxide. We could show that modulating the equilibrium in the nitroxide exchange reaction, by adding or removing one nitroxide species, leads to changes in the crosslinking degree. Being able to modulate the crosslinking degree in POPs allowed us to investigate both the influence of the crosslinking degree and the structure of the molecular components on the porosity. The crosslinking degree of the frameworks was characterised using EPR spectroscopy and the porosity was determined using argon gas adsorption measurements. To guide the design of POPs for desired applications, our study reveals that multiple factors need to be considered such as the structure of the molecular building blocks, the synthetic conditions, and the crosslinking degree.

We synthesised three different POPs via a nitroxide exchange reaction and modulated their crosslinking degree. That allowed us to investigate the influence of the crosslinking degree and the structure of the molecular components on the porosity.

Dynamic covalent polymer networks via combined nitroxide exchange reaction and nitroxide mediated polymerization

This study explores the combination of nitroxide exchange reaction (NER) and nitroxide mediated polymerization (NMP) to prepare structurally tailored and engineered macromolecular networks with dynamically tunable strand lengths.

Profluorescent Fluoroquinolone-Nitroxides for Investigating Antibiotic⁻Bacterial Interactions

Fluorescent probes are widely used for imaging and measuring dynamic processes in living cells. Fluorescent antibiotics are valuable tools for examining antibiotic⁻bacterial interactions, antimicrobial resistance and elucidating antibiotic modes of action. Profluorescent nitroxides are 'switch on' fluorescent probes used to visualize and monitor intracellular free radical and redox processes in biological systems. Here, we have combined the inherent fluorescent and antimicrobial properties of the fluoroquinolone core structure with the fluorescence suppression capabilities of a nitroxide to produce the first example of a profluorescent fluoroquinolone-nitroxide probe. Fluoroquinolone-nitroxide (FN) 14 exhibited significant suppression of fluorescence (>36-fold), which could be restored via radical trapping (fluoroquinolone-methoxyamine 17) or reduction to the corresponding hydroxylamine 20. Importantly, FN 14 was able to enter both Gram-positive and Gram-negative bacterial cells, emitted a measurable fluorescence signal upon cell entry (switch on), and retained antibacterial activity. In conclusion, profluorescent nitroxide antibiotics offer a new powerful tool for visualizing antibiotic⁻bacterial interactions and researching intracellular chemical processes.

Recycling and self-healing of dynamic covalent polymer networks with a precisely tuneable crosslinking degree

Dynamic covalent polymer networks combine intrinsic reversibility with the robustness of covalent bonds, creating chemically stable materials that are responsive to external stimuli.

Impact of Light Intensity on Control in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization

Organic photoredox catalysts have been shown to operate organocatalyzed atom transfer radical polymerizations (O-ATRP) using visible light as the driving force. In this work, the effect of light intensity from white LEDs was evaluated as an influential factor in control over the polymerization and the production of well-defined polymers. We posit the irradiation conditions control the concentrations of various catalyst states necessary to mediate a controlled radical polymerization. Systematic dimming of white LEDs allowed for consideration of the role of light intensity on the polymerization performance. The general effects of decreased irradiation intensity in photoinduced O-ATRP were investigated through comparing two different organic photoredox catalysts: perylene and an 3,7-di(4-biphenyl) 1-naphthalene-10-phenoxazine. Previous computational efforts have investigated catalyst photophysical and electrochemical characteristics, but the broad and complex effects of varied irradiation intensity as an experimental variable on the mechanism of O-ATRP have not been explored. This work revealed that perylene requires more stringent irradiation conditions to achieve controlled polymer molecular weight growth and produce polymers with dispersities <1.50. In contrast, the 3,7-di(4-biphenyl) 1-naphthalene-10-phenoxazine is more robust, achieving linear polymer molecular weight growth under relative irradiation intensity as low as 25%, to produce polymers with dispersities <1.50. This finding is significant, as the discovery of highly robust catalysts is necessary to allow for the adoption of successful O-ATRP in a wide scope of conditions, including those which necessitate low light intensity irradiation.