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.

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.

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.

Dynamic Processes and Mechanisms Involved in Relaxations of Single-Chain Nano-Particle Melts

We present a combined study by quasielastic neutron scattering (QENS), dielectric and mechanical spectroscopy, calorimetry and wide-angle X-ray diffraction on single-chain nano-particles (SCNPs), using the corresponding linear precursor chains as reference, to elucidate the impact of internal bonds involving bulky cross-links on the properties of polymer melts. Internal cross-links do not appreciably alter local properties and fast dynamics. This is the case of the average inter-molecular distances, the β-relaxation and the extent of the atomic displacements at timescales faster than some picoseconds. Contrarily, the α-relaxation is slowed down with respect to the linear precursor, as detected by DSC, dielectric spectroscopy and QENS. QENS has also resolved broader response functions and stronger deviations from Gaussian behavior in the SCNPs melt, hinting at additional heterogeneities. The rheological properties are also clearly affected by internal cross-links. We discuss these results together with those previously reported on the deuterated counterpart samples and on SCNPs obtained through a different synthesis route to discern the effect of the nature of the cross-links on the modification of the diverse properties of the melts.

One-Shot Intrablock Cross-Linking of Linear Diblock Copolymer to Realize Janus-Shaped Single-Chain Nanoparticles

Developing an efficient and versatile process to transform a single linear polymer chain into a shape-defined nanoobject is a major challenge in the fields of chemistry and nanotechnology to replicate sophisticated biological functions of proteins and nucleic acids in a synthetic polymer system. In this study, we performed one-shot intrablock cross-linking of linear block copolymers (BCPs) to realize single-chain nanoparticles (SCNPs) with two chemically compartmentalized domains (Janus-shaped SCNPs). Detailed structural characterizations of the Janus-shaped SCNP composed of polystyrene-block-poly(glycolic acid) revealed its compactly folded conformation and compartmentalized block localization, similar to the self-folded tertiary structures of natural proteins. Versatility of the one-shot intrablock cross-linking was demonstrated using several different BCP precursors. In addition, the Janus-shaped SCNP produce miniscule microphase-separated structures.

Laser Photodissociation Action Spectroscopy for the Wavelength-Dependent Evaluation of Photoligation Reactions

The nitrile imine-mediated tetrazole-ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N-ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

Advances in the Multi-Orthogonal Folding of Single Polymer Chains into Single-Chain Nanoparticles

The folding of certain proteins (e.g., enzymes) into perfectly defined 3D conformations via multi-orthogonal interactions is critical to their function. Concerning synthetic polymers chains, the "folding" of individual polymer chains at high dilution via intra-chain interactions leads to so-called single-chain nanoparticles (SCNPs). This review article describes the advances carried out in recent years in the folding of single polymer chains into discrete SCNPs via multi-orthogonal interactions using different reactive chemical species where intra-chain bonding only occurs between groups of the same species. First, we summarize results from computer simulations of multi-orthogonally folded SCNPs. Next, we comprehensively review multi-orthogonally folded SCNPs synthesized via either non-covalent bonds or covalent interactions. Finally, we conclude by summarizing recent research about multi-orthogonally folded SCNPs prepared through both reversible (dynamic) and permanent bonds.

Remarkable untangled dynamics behavior of multicyclic branched polystyrenes

A typical multicyclic branched-topology polystyrene (c-BPS) with high molecular weight (30 K ≤ Mw MALLS ≤ 300 K g mol-1) and narrow dispersity (1.2 ≤ Đ ≤ 1.3) was efficiently synthesized by combining atom transfer radical polymerization (ATRP) and atom transfer radical coupling (ATRC) techniques. The topological constraints imposed by the presence of cyclic units and branch points had a marked influence on the entanglement behaviors of the polymer chains in solution. Therefore, c-BPS possesses the lowest loss modulus (G'') and viscosity (η), the highest diffusion coefficient (D0), the largest mesh size (ξ) and the fastest terminal relaxation (TR), compared with branched and linear precursors.

Stabilizing self-assembled nano-objects using light-driven tetrazole chemistry

Tetrazole-driven photochemistry is exploited for the stabilization of self-assembled morphologies via light-triggered nitrile imine carboxylic acid ligation (NICAL).

Collective Motions and Mechanical Response of a Bulk of Single-Chain Nano-Particles Synthesized by Click-Chemistry

We investigate the effect of intra-molecular cross-links on the properties of polymer bulks. To do this, we apply a combination of thermal, rheological, diffraction, and neutron spin echo experiments covering the inter-molecular as well as the intermediate length scales to melts of single-chain nano-particles (SCNPs) obtained through 'click' chemistry. The comparison with the results obtained in a bulk of the corresponding linear precursor chains (prior to intra-molecular reaction) and in a bulk of SCNPs obtained through azide photodecomposition process shows that internal cross-links do not influence the average inter-molecular distances in the melt, but have a profound impact at intermediate length scales. This manifests in the structure, through the emergence of heterogeneities at nanometric scale, and also in the dynamics, leading to a more complex relaxation behavior including processes that allow relaxation of the internal domains. The influence of the nature of the internal bonds is reflected in the structural relaxation that is slowed down if bulky cross-linking agents are used. We also found that any residual amount of cross-links is critical for the rheological behavior, which can vary from an almost entanglement-free polymer bulk to a gel. The presence of such inter-molecular cross-links additionally hinders the decay of density fluctuations at intermediate length scales.

Designing with Light: Advanced 2D, 3D, and 4D Materials

Recent achievements and future opportunities for the design of 2D, 3D, and 4D materials using photochemical reactions are summarized. Light is an attractive stimulus for material design due to its outstanding spatiotemporal control, and its ability to mediate rapid polymerization under moderate reaction temperatures. These features have been significantly enhanced by major advances in light generation/manipulation with light-emitting diodes and optical fiber technologies which now allows for a broad range of cost-effective fabrication protocols. This combination is driving the preparation of sophisticated 2D, 3D, and 4D materials at the nano-, micro-, and macrosize scales. Looking ahead, future challenges and opportunities that will significantly impact the field and help shape the future of light as a versatile and tunable design tool are highlighted.

Two colours of light drive PET–RAFT photoligation

By fusing the realms of photopolymerisation and photoligation, our contribution exploits two orthogonal wavelengths of visible light to readily synthesise and functionalise well defined polymers from a unique dual functionality RAFT agent.

Advances in the Phototriggered Synthesis of Single-Chain Polymer Nanoparticles

Clean use of photons from light to activate chemical reactions offers many possibilities in different fields, from chemistry and biology to materials science and medicine. This review article describes the advances carried out in last decades toward the phototriggered synthesis of single-chain polymer nanoparticles (SCNPs) as soft nanomaterials with promising applications in enzyme-mimicking catalysis and nanomedicine, among other different uses. First, we summarize some different strategies developed to synthesize SCNPs based on photoactivated intrachain homocoupling, phototriggered intrachain heterocoupling and photogenerated collapse induced by an external cross-linker. Next, we comprehensively review the emergent topic of photoactivated multifolding applied to SCNP construction. Finally, we conclude by summarizing recent strategies towards phototriggered disassembly of SCNPs.

Light-Healable Epoxy Polymer Networks via Anthracene Dimer Scission of Diamine Crosslinker

Two anthracene-based diamine crosslinkers were used to cure a range of commercially available monomers to produce four highly photoreversible crosslinked epoxy polymers. Through careful selection of the epoxy monomers used, the properties of the resultant polymer networks were varied to create a coating material that possessed room-temperature light-stimulated healing. Of the four coatings created, the best healing performance was exhibited by the two most flexible systems, both of these also exhibited the thermal and mechanical performance necessary for coatings. By using anthracene, the utilization of a wide range of wavelengths in the healing process is possible, which in applications such as industrial coatings would be of significant benefit.

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.

Photocuring of 4-arm coumarin-functionalised monomers to form highly photoreversible crosslinked epoxy coatings

Highly photoreversible photocurable crosslinked epoxy coatings that can heal substantial surface damage were formed by the synthesis of unique monomers.

Effect of Intra- versus Intermolecular Cross-Linking on the Supramolecular Folding of a Polymer Chain

Anfinsen's famous experiment showed that the restoration of catalytic activity of a completely unfolded ribonuclease A is only possible when the correct order of events is followed during the refolding process. Inspired by this work, the effect of structural constraints induced by covalent cross-links on the folding of a synthetic polymer chain via hydrogen-bonding interactions is investigated. Hereto, methacrylate-based monomers comprising either benzene-1,3,5-tricarboxamide (BTA)-based or coumarin-based pendants are copolymerized with n-butyl methacrylate in various ratios via reversible addition-fragmentation chain-transfer (RAFT) polymerization. To assess whether the folding and single-chain polymeric nanoparticle (SCPN) formation depend on the order of events, we compare two folding pathways. In the one case, we first covalently cross-link the coumarin pendants within the polymers in a solvent that prevents hydrogen bonding, after which hydrogen bonding is activated, inducing folding of the polymer. In the other case, we induce hydrogen-bonding interactions between tethered BTAs prior to covalent cross-linking of the coumarin pendants. A combination of circular dichroism (CD) spectroscopy, UV-vis spectroscopy, size-exclusion chromatography (SEC), and dynamic light scattering (DLS) is employed to understand the effect of the structural constraints on the folding behavior of these synthetic polymers. The results show that like in ribonuclease A, the order of events matters greatly and determines the outcome. Importantly, a hydrogen-bond-promoting solvent prevents the formation of SCPNs upon covalent cross-linking and results in multichain aggregates. In contrast, covalently cross-linking the polymer when no hydrogen bonds are present followed by inducing hydrogen bonding favors the formation of SCPNs above the UCST of the methacrylate-based polymer. To our surprise, the two systems show a fundamentally different response to changes in temperature, indicating that also in synthetic polymers differences in the folding pathway induce differences in the properties of the resultant nanostructures.

Intrinsically Fluorescent, Stealth Polypyrazoline Nanoparticles with Large Stokes Shift for In Vivo Imaging

Recent advances in super-resolution microscopy and fluorescence bioimaging allow exploring previously inaccessible biological processes. To this end, there is a need for novel fluorescent probes with specific features in size, photophysical properties, colloidal and optical stabilities, as well as biocompatibility and ability to evade the reticuloendothelial system. Herein, novel fluorescent nanoparticles are introduced based on an inherently fluorescent polypyrazoline (PPy) core and a polyethylene glycol (PEG) shell, which address all aforementioned challenges. Synthesis of the PPy-PEG amphiphilic block copolymer by phototriggered step-growth polymerization is investigated by NMR spectroscopy, size-exclusion chromatography, and mass spectrometry. The corresponding nanoparticles are characterized for their luminescent properties and hydrodynamic size in various aqueous environments (e.g., cell culture media). PPy nanoparticles particularly exhibit a large Stokes shift (Δλ = 160 nm or Δν > 7000 cm^-1 ) with visible light excitation and strong colloidal stability. While clearance by macrophages and endothelial cells is minimal, PPy displays good biocompatibility. Finally, PPy nanoparticles prove to be long circulating when injected in zebrafish embryos, as observed by in vivo time-lapse fluorescence microscopy. In summary, PPy nanoparticles are highly promising to be further developed as fluorescent nanodelivery systems with low toxicity and exquisite retention in the blood stream.

Making and Breaking Chemical Bonds by Chemiluminescence

Chemiluminescent (CL) reactions are powerful analytical tools and are present in commercially available everyday objects such as glow sticks. Herein, the photons generated by chemiluminescence are exploited to induce covalent bond breakage and formation, using a chemically generated photonic field at ambient temperature through space as energy transducer. Remarkably, the generated photons enable both the cleavage of species generating radicals as well as the execution of [2 + 2] cycloadditions, demonstrating that disparate types of reactions can be triggered. The herein-presented photochemical concept establishes the field of CL-induced photochemistry, which is poised to enable photochemical transformations in situations where physical light sources, such as lamps, LEDs, and lasers cannot be employed, including in biological environments.

Improvement and tuning of the performance of light-healable polymers by variation of the monomer content

High-performing crosslinked epoxy coatings that possess room temperature self-healing ability by the use of a newly synthesised dynamic diamine crosslinker.