Chemical Surface Modification of Self-Assembled Monolayers by Radical Nitroxide Exchange Reactions

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

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.

High-precision tabletop microplotter for flexible on-demand material deposition in printed electronics and device functionalization

Microstructuring, in particular, the additive functionalization of surfaces with, e.g., conductive or bioactive materials plays a crucial role in many applications in sensing or printed electronics. Mostly, the lithography steps are made prior to assembling functionalized surfaces into the desired places of use within a bigger device as a microfluidic channel or an electronic casing. However, when this is not possible, most lithography techniques struggle with access to recessed or inclined/vertical surfaces for geometrical reasons. In particular, for "on-the-fly" printing aiming to add microstructures to already existing devices on demand and maybe even for one-time trials, e.g., in prototyping, a flexible "micropencil" allowing for direct write under direct manual control and on arbitrarily positioned surfaces would be highly desirable. Here, we present a highly flexible, micromanipulator-based setup for capillary printing of conductive and biomaterial ink formulations that can address a wide range of geometries as exemplified on vertical, recessed surfaces and stacked 3D scaffolds as models for hard to access surfaces. A wide range of feature sizes from tens to hundreds of micrometer can be obtained by the choice of capillary sizes and the on-demand in situ writing capabilities are demonstrated with completion of a circuit structure by gold line interconnects deposited with the setup.

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.

KAT Ligation for Rapid and Facile Covalent Attachment of Biomolecules to Surfaces

The efficient and bioorthogonal chemical ligation reaction between potassium acyltrifluoroborates (KATs) and hydroxylamines (HAs) was used for the surface functionalization of a self-assembled monolayer (SAM) with biomolecules. An alkane thioether molecule with one terminal KAT group (S-KAT) was synthesized and adsorbed onto a gold surface, placing a KAT group on the top of the monolayer (KAT-SAM). As an initial test case, an aqueous solution of a hydroxylamine (HA) derivative of poly(ethylene glycol) (PEG) (HA-PEG) was added to this KAT-SAM at room temperature to perform the surface KAT ligation. Quartz crystal microbalance with dissipation (QCM-D) monitoring confirmed the rapid attachment of the PEG moiety onto the SAM. By surface characterization methods such as contact angle and ellipsometry, the attachment of PEG layer was confirmed, and covalent amide-bond formation was established by X-ray photoelectron spectroscopy (XPS). In a proof-of-concept study, the applicability of this surface KAT ligation for the attachment of biomolecules to surfaces was tested using a model protein, green fluorescent protein (GFP). A GFP was chemically modified with an HA linker to synthesize HA-GFP and added to the KAT-SAM under aqueous dilute conditions. A rapid attachment of the GFP on the surface was observed in real time by QCM-D. Despite the fact that such biomolecules have a variety of unprotected functional groups within their structures, the surface KAT ligation proceeded rapidly in a chemoselective manner. Our results demonstrate the versatility of the KAT ligation for the covalent attachment of a variety of water-soluble molecules onto SAM surfaces under dilute and biocompatible conditions to form stable, natural amide bonds.

Surface chemical reactions on self-assembled silane based monolayers

In this review, we aim to update our review "Chemical modification of self-assembled silane-based monolayers by surface reactions" which was published in 2010 and has developed into an important guiding tool for researchers working on the modification of solid substrate surface properties by chemical modification of silane-based self-assembled monolayers. Due to the rapid development of this field of research in the last decade, the utilization of chemical functionalities in self-assembled monolayers has been significantly improved and some new processes were introduced in chemical surface reactions for tailoring the properties of solid substrates. Thus, it is time to update the developments in the surface functionalization of silane-based molecules. Hence, after a short introduction on self-assembled monolayers, this review focuses on a series of chemical reactions, i.e., nucleophilic substitution, click chemistry, supramolecular modification, photochemical reaction, and other reactions, which have been applied for the modification of hydroxyl-terminated substrates, like silicon and glass, which have been reported during the last 10 years.

Adaptable and Reprogrammable Surfaces

Establishing control over chemical reactions on interfaces is a key challenge in contemporary surface and materials science, in particular when introducing well-defined functionalities in a reversible fashion. Reprogrammable, adaptable and functional interfaces require sophisticated chemistries to precisely equip them with specific functionalities having tailored properties. In the last decade, reversible chemistries-both covalent and noncovalent-have paved the way to precision functionalize 2 or 3D structures that provide both spatial and temporal control. A critical literature assessment reveals that methodologies for writing and erasing substrates exist, yet are still far from reaching their full potential. It is thus critical to assess the current status and to identify avenues to overcome the existing limitations. Herein, the current state-of-the-art in the field of reversible chemistry on surfaces is surveyed, while concomitantly identifying the challenges-not only synthetic but also in current surface characterization methods. The potential within reversible chemistry on surfaces to function as true writeable memories devices is identified, and the latest developments in readout technologies are discussed. Finally, we explore how spatial and temporal control over reversible, light-induced chemistries has the potential to drive the future of functional interface design, especially when combined with powerful laser lithographic applications.

Design, Modeling and Synthesis of 1,2,3-Triazole-Linked Nucleoside-Amino Acid Conjugates as Potential Antibacterial Agents

Copper-catalyzed azide-alkyne cycloadditions (CuAAC or click chemistry) are convenient methods to easily couple various pharmacophores or bioactive molecules. A new series of 1,2,3-triazole-linked nucleoside-amino acid conjugates have been designed and synthesized in 57–76% yields using CuAAC. The azido group was introduced on the 5′-position of uridine or the acyclic analogue using the tosyl-azide exchange method and alkylated serine or proparylglycine was the alkyne. Modeling studies of the conjugates in the active site of LpxC indicate they have promise as antibacterial agents.

Radical exchange reaction of multi-spin isoindoline nitroxides followed by EPR spectroscopy

The synthesis and exchange reaction of a rigid, isoindoline-functionalized tetraphenylmethane multi-spin system is described. The exchange reaction was followed using EPR spectroscopy.

A novel protecting group methodology for syntheses using nitroxides

The methoxyamine group represents an ideal protecting group for the nitroxide moiety. It can be easily and selectively introduced in high yield (typically >90%) to a range of functionalised nitroxides using FeSO4·7H2O and H2O2 in DMSO. Its removal is readily achieved under mild conditions in high yield (70-90%) using mCPBA in a Cope-type elimination process.

Chemical modification of polymer brushes via nitroxide photoclick trapping

The preparation of polymer brushes (PBs) bearing α-hydroxyalkylphenylketone (2-hydroxy-2-methyl-1-phenylpropan-1-one) moieties as photoreactive polymer backbone substituents is presented. Photoreactive polymer brushes with defined thicknesses (up to 60 nm) and high grafting densities are readily prepared by surface initiated nitroxide mediated radical polymerization (SINMP). The photoactive moieties can be transformed via Norrish-type I photoreaction to surface-bound acyl radicals. Photolysis in the presence of a persistent nitroxide leads to chemically modified PBs bearing acylalkoxyamine moieties as side chains resulting from trapping of the photogenerated acyl radicals with nitroxides. Application of functionalized nitroxides to the photochemical PB postmodification provides functionalized PBs bearing cyano, polyethylene glycol (PEG), perfluoroalkyl, and biotin moieties. As shown for one case, photochemical postfunctionalization of the PB through a mask using a biotin-conjugated nitroxide as the trapping reagent leads to the corresponding site-selective chemically modified PB, which is successfully used for site-specific streptavidin immobilization. Surface analysis of PBs was performed by contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR), fourier transform infrared (FTIR) spectroscopy, and fluorescence microscopy.