Cleavable azobenzene linkers for the design of stimuli-responsive materials

The azo linkage (NN) is one of the very few functional groups in organic chemistry that exhibits sensitivity towards thermal, chemical, photochemical, and biological stimuli. Consequently, this property has given rise to a distinct class of responsive materials. For example, thermal sensitivity has led to generation of free radical initiators useful in curing and polymerization applications. Chemically-induced cleavage has aided the development of self-immolative polymers and reactive scaffolds for proteomics applications. Photo-isomerization capability has given rise to photo-responsive systems. Azobenzene cleavage in biologically reducing environments, such as that of the colon, and under tumor hypoxia conditions has led to diagnostic, therapeutic, and delivery materials. Such conditions have also allowed for control over formation (assembly) and disruption (disassembly) of micellar nanoparticles. The aim of this review article is to look beyond the prevalent photosensitivity aspect of the aromatic azo compounds and draw attention to the azo scission reaction as a trigger of the change in the structure and properties of organic materials. Thus, the main discussion begins with the mechanism of the reductive cleavage. Then, its application in the design of molecules that can be activated as drugs and fluorescent sensors, (nano)materials with potential to release active substances, and polymers with side-chain and main-chain self-immolative capacity is discussed. Finally, the status and future challenges in this field are discussed.

Single Chain Nanoparticles in Catalysis

Over the last six decades folded polymer chains-so-called Single Chain Nanoparticles (SCNPs)-have evolved from the mere concept of intramolecularly crosslinked polymer chains to tailored nanoreactors, underpinned by a plethora of techniques and chemistries to tailor and analyze their morphology and function. These monomolecular polymer entities hold critical promise in a wide range of applications. Herein, we highlight the exciting progress that has been made in the field of catalytically active SCNPs in recent years.

Thermoresponsive swelling of photoacoustic single-chain nanoparticles

NIR-fluorescent LCST-type single-chain nanoparticles (SCNPs) change their photophysical behaviour upon heating, caused by depletion of water from the swollen SCNP interiors. This thermoresponsive effect leads to a fluctuating photoacoustic (PA) signal which can be used as a contrast mechanism for PA imaging.

Simultaneously controlling conformational and operational stability of single-chain polymeric nanoparticles in complex media

Single-chain polymeric nanoparticles (SCPNs) comprising a solvatochromic pyrazoline adduct show conformational and operational stability in complex media and in cellular compartments; the connectivity of the adduct is crucial in modulating interactions with the surrounding media.

Single-chain polymer nanoparticles in biomedical applications

Single-chain polymer nanoparticles (SCNPs) are a well-defined and uniquely sized class of polymer nanoparticles. The advances in polymer science over the past decades have enabled the development of a variety of intramolecular crosslinking systems, leading to particles in the 5-20 nm size regime. Which is aligned with the size regime of proteins and therefore making SCNPs an interesting class of NPs for biomedical applications. The high modularity of SCNP design and the ease of their functionalization have led to growing research interest. In this review, we describe different crosslinking systems, as well as the preparation of functional SCNPs and the variety of biomedical applications that have been explored.

Tuning the Internal Compartmentation of Single-Chain Nanoparticles as Fluorescent Contrast Agents

Controlling the internal structures of single-chain nanoparticles (SCNPs) is an important factor for their targeted chemical design and synthesis, especially in view of nanosized compartments presenting different local environments as a main feature to control functionality. We here design SCNPs bearing near-infrared fluorescent dyes embedded in hydrophobic compartments for use as contrast agents in pump-probe photoacoustic (PA) imaging, displaying improved properties by the location of the dye in the hydrophobic particle core. Compartment formation is controlled via single-chain collapse and subsequent crosslinking of an amphiphilic polymer using external crosslinkers in reaction media of adjustable polarity. Different SCNPs with hydrodynamic diameters of 6-12 nm bearing adjustable label densities are synthesized. It is found that the specific conditions for single-chain collapse have a major impact on the formation of the desired core-shell structure, in turn adjusting the internal nanocompartments together with the formation of excitonic dye couples, which in turn increase their fluorescence lifetime and PA signal generation. SCNPs with the dye molecules accumulate at the core also show a nonlinear PA response as a function of pulse energy-a property that can be exploited as a contrast mechanism in molecular PA tomography.

Fluorescence turn-on by photoligation - bright opportunities for soft matter materials

Photochemical ligation has become an indispensable tool for applications that require spatially addressable functionalisation, both in biology and materials science. Interestingly, a number of photochemical ligations result in fluorescent products, enabling a self-reporting function that provides almost instantaneous visual feedback of the reaction's progress and efficiency. Perhaps no other chemical reaction system allows control in space and time to the same extent, while concomitantly providing inherent feedback with regard to reaction success and location. While photoactivable fluorescent properties have been widely used in biology for imaging purposes, the expansion of the array of photochemical reactions has further enabled its utility in soft matter materials. Herein, we concisely summarise the key developments of fluorogenic-forming photoligation systems and their emerging applications in both biology and materials science. We further summarise the current challenges and future opportunities of exploiting fluorescent self-reporting reactions in a wide array of chemical disciplines.

En Route to Stabilized Compact Conformations of Single-Chain Polymeric Nanoparticles in Complex Media

Precise control over the folding pathways of polypeptides using a combination of noncovalent and covalent interactions has evolved into a wide range of functional proteins with a perfectly defined 3D conformation. Inspired hereby, we develop a series of amphiphilic copolymers designed to form compact, stable, and structured single-chain polymeric nanoparticles (SCPNs) of defined size, even in competitive conditions. The SCPNs are formed through a combination of noncovalent interactions (hydrophobic and hydrogen-bonding interactions) and covalent intramolecular cross-linking using a light-induced [2 + 2] cycloaddition. By comparing different self-assembly pathways of the nanoparticles, we show that, like for proteins in nature, the order of events matters. When covalent cross-links are formed prior to the folding via hydrophobic and supramolecular interactions, larger particles with less structured interiors are formed. In contrast, when the copolymers first fold via hydrophobic and hydrogen-bonding interactions into compact conformations, followed by covalent cross-links, good control over the size of the SCPNs and microstructure of the hydrophobic interior is achieved. Such a structured SCPN can stabilize the solvatochromic dye benzene-1,3,5-tricarboxamide-Nile Red via molecular recognition for short periods of time in complex media, while showing slow exchange dynamics with the surrounding complex media at longer time scales. The SCPNs show good biocompatibility with cells and can carry cargo into the lysosomal compartments of the cells. Our study highlights the importance of control over the folding pathway in the design of stable SCPNs, which is an important step forward in their application as noncovalent drug or catalyst carriers in biological settings.

Fluorescence monitoring of the degradation evolution of aliphatic polyesters

To provide lifecycle monitoring for degradable polymers, we have proposed a three-dimensional fluorescence monitoring and quantification method to simultaneously study the thermal and photothermal degradation by combining the intrinsic conjugation and probe-labelled carboxyl of poly(butylene adipate-co-terephthalate) (PBAT).

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Passerini Multicomponent Reactions Enabling Self-Reporting Photosensitive Tetrazole Polymers

We introduce the synthesis of photosensitive tetrazole monomers via Passerini multicomponent reactions (MCRs). We exploit the MCR's tolerance toward various functional groups under mild, catalyst-free conditions in a one-pot reaction setup to generate tetrazole-containing monomers featuring a methacrylic moiety, which enables their subsequent reversible addition-fragmentation chain transfer (RAFT) polymerization. By employing tetrazoles with either a 4-methoxy phenyl or a pyrene substituent, further modifications of the polymers in a wavelength-orthogonal, self-reporting fashion upon irradiation with either UV or visible light become possible.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Elemental Sulfur Mediated Novel Multicomponent Redox Polycondensation for the Synthesis of Alternating Copolymers Based on 2,4-Thiophene/Arene Repeating Units

A sulfur-based self-condensation method is investigated as an efficient tool for the synthesis of polythiophene derivatives. The reaction proceeds through multicomponent redox polycondensation between readily available diketone compounds and elemental sulfur in the presence of a Brønsted acid/base pair. Six different diketone derivatives have been screened and the polymerization is generalized by the synthesis of so-far-unprecedented alternating copolymers based on 2,4-thiophene/arene repeating units. By exploiting microwave heating the synthetic procedure is optimized, particularly for alternating copolymers containing aryl and thiophene units, such that a copolymer can be synthesized in only 24 h compared to the conventional process taking 6 d, yielding polymers within the same apparent weight average molar mass (M_w ). All obtained copolymers are analyzed in detail using size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), attenuated total reflectance infrared spectroscopy (ATR-IR), thermal gravimetric analysis and differential scanning calorimetry (DSC).

Using nickel to fold discrete synthetic macromolecules into single-chain nanoparticles

Sequence-defined macromolecules were prepared with a thiolactone-based platform whereby ligand functionalities were introduced along the backbone enabling a nickel induced formation of single-chain nanoparticles.

A general strategy for degradable single-chain nanoparticles via cross-linker mediated chain collapse of radical copolymers

Radical ring-opening copolymerization (rROP) between 2-methylene-1,3-dioxepane (MDO) and methacrylic acid N-hydroxysuccinimide ester (NHSMA) furnishes a reactive polyester-based linear copolymer precursor. Subsequent cross-linker mediated chain collapse affords degradable single-chain nanoparticles (DSCNPs). This methodology is an experimentally robust and straightforward route to main-chain degradable polymeric nanoparticles in the sub-30 nm size range.

Hypersensitive azobenzenes: facile synthesis of clickable and cleavable azo linkers with tunable and high reducibility

The aim of this work is to show that by increasing the number of donor substituents in a donor/acceptor system, the sensitivity of the azobenzene linkage towards a reductive cleavage reaction can be enhanced to unprecedented high levels. For instance, in a triple-donor system, less than a second constitutes the half-life of the azo (N[double bond, length as m-dash]N) bond. Synthetic access to such redox active scaffolds is highly practical and requires only 1-2 synthetic steps. The fundamental molecular design is also adaptable. This is demonstrated through scaffold functionalization by azide, tetraethylene glycol, and biotin groups. The availability of the azide group is shown in a copper-free 'click' reaction suitable in context with protein conjugation and proteomics application. Finally, the clean nature of the scission process is demonstrated with the help of liquid chromatography coupled with mass analysis. This work, therefore, describes development of cleavable azobenzene linkers that can be accessed with synthetic ease, can be multiply functionalized, and show a clean and rapid response to mild reducing conditions.

Hybrid polymers bearing oligo-l-lysine(carboxybenzyl)s: synthesis and investigations of secondary structure

Hybrid polymers of peptides resembling (partially) folded protein structures are promising materials in biomedicine, especially in view of folding-interactions between different segments. In this study polymers bearing repetitive peptidic folding elements, composed of N-terminus functionalized bis-ω-ene-functional oligo-l-lysine(carboxybenzyl(Z))s (Lys_n) with repeating units (n) of 3, 6, 12, 24 and 30 were successfully synthesized to study their secondary structure introduced by conformational interactions between their chains. The pre-polymers of ADMET, narrowly dispersed Lys_ns, were obtained by ring opening polymerization (ROP) of N-carboxyanhydride (NCA) initiated with 11-amino-undecene, following N-terminus functionalization with 10-undecenoyl chloride. The resulting Lys_ns were subsequently polymerized via ADMET polymerization by using Grubbs’ first generation (G1) catalyst in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) generating the ADMET polymers (A-[Lys_n]_m) (m = 2–12) with molecular weights ranging from 3 to 28 kDa, displaying polydispersity (Đ) values in the range of 1.5–3.2. After chemical analyses of Lys_ns and A-[Lys_n]_ms by ¹H-NMR, GPC and MALDI-ToF MS, secondary structural investigations were probed by CD spectroscopy and IR spectroscopy in 2,2,2-trifluoroethanol (TFE). In order to study A-[Lys_n]_ms with defined molecular weights and low polydispersity values (Đ = 1.03–1.48), the ADMET polymers A-[Lys_n=3]_m=3 and A-[Lys_n=24]_m=4 were fractionated by preparative GPC, and subsequently analysed by ¹H-NMR, analytical GPC, MALDI-ToF MS and CD spectroscopy. We can demonstrate the influence of chain length of the generated polymers on the formation of secondary structures by comparing Lys_ns with varying n values to the ADMET-polymers with the help of spectroscopic techniques such as CD and FTIR-spectroscopy in a helicogenic solvent.

We demonstrate the influence of chain length of segmented polymers bearing dynamic folding elements onto the formation of secondary structures with the help of spectroscopic techniques such as CD and FTIR-spectroscopy in a helicogenic solvent.

Bioactivatable reductive cleavage of azobenzene for controlling functional dumbbell oligodeoxynucleotides

Application of stimuli-responsive bioactive molecules is an attractive strategy due to use for target special tissues and cells. Here, we reported synthesis of an azo-linker, 2,2'-dimethoxyl-4,4'-dihydroxymethylazobenzene (mAzo), which was more effectively recognized and cleaved by reducing glutathione (GSH) via comparing with 4,4'-dihydroxymethylazobenzene (Azo). In addition, mAzo is further exploited to engineer dumbbell asODNs, which could result in the release of asODNs and thus modulate their hybridization to target nucleic acids. The present study is the first example to disclose efficient reductive cleavage of azobenzene by GSH to generate aromatic amine. This would provide a valuable strategy for tunable cell-specific release of ODNs and modulation of known disease-causing gene expression in cancer cells.

Soft Matter Technology at KIT: Chemical Perspective from Nanoarchitectures to Microstructures

Bioinspiration has emerged as an important design principle in the rapidly growing field of materials science and especially its subarea, soft matter science. For example, biological cells form hierarchically organized tissues that not only are optimized and designed for durability, but also have to adapt to their external environment, undergo self-repair, and perform many highly complex functions. Being able to create artificial soft materials that mimic those highly complex functions will enable future materials applications. Herein, soft matter technologies that are used to realize bioinspired material structures are described, and potential pathways to integrate these into a comprehensive soft matter research environment are addressed. Solutions become available because soft matter technologies are benefitting from the synergies between organic synthesis, polymer chemistry, and materials science.

Supramolecular Single-Chain Polymeric Nanoparticles

Nature has unparalleled control over the conformation and dynamics of its folded macromolecular structures. Nature’s ability to arrange amino acids into a precise spatial organization by way of folding allows proteins to fulfill specific functions in an extremely efficient manner. Chemists and materials scientists have used the delicate structure–function relationships observed in proteins to elucidate nature’s design principles. These insights have led to the development of various revolutionary macromolecular architectures, mimicking the structural features of proteins. In this review, we focus on the folding of single polymer chains into well-defined nanoparticles using supramolecular interactions and their possible use as enzyme mimics.

Self-reporting visible light-induced polymer chain collapse

We introduce a facile photoinduced self-reporting crosslinking methodology for the compaction of polymer chains in highly diluted solution.

Folding polymer chains with visible light

A simple and versatile tool for generating fluorescent single chain polymer nanoparticles with visible light.

Advances in Fluorescent Single-Chain Nanoparticles

Fluorophore molecules can be monitored by fluorescence spectroscopy and microscopy, which are highly useful and widely used techniques in cell biology, biochemistry, and medicine (e.g., biomarker analysis, immunoassays, cancer diagnosis). Several fluorescent micro- and nanoparticle systems based on block copolymer micelles and cross-linked polymer networks, quantum dots, π-conjugated polymers, and dendrimers have been evaluated as optical imaging systems. In this review, we highlight recent advances in the construction of fluorescent single-chain nanoparticles (SCNPs), which are valuable artificial soft nano-objects with a small tunable size (as small as 3 nm). In particular, the main methods currently available to endow SCNPs with fluorescent properties are discussed in detail, showing illustrative examples.

Fluorescent Glyco Single-Chain Nanoparticle-Decorated Nanodiamonds

We introduce the light-induced collapse of single glycopolymer chains in water generating fluorescent glyco single-chain nanoparticles (SCNPs) and their subsequent functionalization onto nanodiamonds. The glycopolymer precursors are prepared by polymerizing an acetylated mannose-based methacrylate monomer followed by a deprotection and postpolymerization functionalization step, introducing profluorescent photoactive tetrazole groups and furan-protected maleimide moieties. Subsequent UV irradiation in highly diluted aqueous solution triggers intramolecular tetrazole-mediated cycloadditions, yielding glyco SCNPs featuring fluorescence as well as lectin binding properties. The obtained SCNPs are coated onto nanodiamonds by adsorption, and the obtained hybrid nanoparticles are in depth characterized in terms of size, functionality, and bioactivity. Different coating densities are achieved by altering the SCNP concentration. The prepared nanoparticles are nontoxic in mouse RAW 264.7 macrophages. Furthermore, the fluorescence of the SCNPs can be exploited to image the SCNP-coated nanodiamonds in macrophage cells via confocal fluorescence microscopy.

Exploring structural effects in single-chain “folding” mediated by intramolecular thermal Diels–Alder chemistry

We describe a method to fold single polymer chains into nanoparticles using simple thermal Diels–Alder (DA) chemistry.

De-assembly of assembled Pt1Ag12 units: tailoring the photoluminescence of atomically precise nanoclusters

De-assembly of assembled Pt₁Ag₁₂-units renders a blue-shift of the photoluminescent emission as well as an enhancement of the quantum yield.

Characterizing single chain nanoparticles (SCNPs): a critical survey

We provide the results of a critical literature survey on the reported sizes of single chain polymer nanoparticles (SCNPs) employing different techniques.