Extending the Scope of "Living" Crystallization-Driven Self-Assembly: Well-Defined 1D Micelles and Block Comicelles from Crystallizable Polycarbonate Block Copolymers

Precision Stealth Nanofibers via PET-RAFT Polymerisation: Synthesis, Crystallization-driven Self-assembly and Cellular Uptake Studies

Stealth precision polymer nanofibers show great promise as therapeutic delivery systems. However, existing systems are largely limited to poly(ethylene glycol) (PEG) and suffer from challenging functionalization, hampering their translation. This work develops a modular, easily functionalizable platform for biocompatible stealth nanofibers based on a combination of ring-opening polymerisation (ROP), photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerisation, and crystallization-driven self-assembly (CDSA). Low length-dispersity poly(fluorenetrimethylenecarbonate)-b-poly(N-(2-hydroxypropyl) methacrylamide) (PFTMC-b-PHPMA) nanofibers may be produced in a single-step via CDSA, with a length that is dependent on the PHPMA DPn. Separately, living CDSA leads to nanofibers with length control between 30 nm and ca. 700 nm. Incorporation of fluorescein into the PET-RAFT polymerization results in fluorescent PFTMC-b-PHPMA block copolymers that can undergo CDSA, forming fluorescent nanoparticles for preliminary cell studies. PFTMC-b-PHPMA nanofibers exhibited minimal toxicity to cells as well as limited cellular association, in line with previous studies on neutral polymer nanofibers. In comparison, PFTMC-b-PHPMA nanospheres exhibited no cellular association. These results indicate that the unique shape and core-crystallinity of PFTMC-b-PHPMA nanofibers ideally positions them for use as therapeutic delivery systems. Overall, the results described herein provide the basis for a modular, easily functionalizable platform for precision stealth polymer nanofibers for a variety of prospective biomedical applications.

Liquid-Crystallization-Driven Self-Assembly toward Uniform Multi-Morphology Fried-Egg-Like Nanostructures

Liquid-crystallization-driven self-assembly (LCDSA) has recently emerged as an efficient strategy to create uniform one-dimensional (1-D), 2-D and 3-D nanostructures in a controlled manner. However, the examples of generation of uniform multi-morphology nanostructures from solution self-assembly of one single polymer sample are rare. Herein, we report the first example of preparation of multi-morphology fried-egg-like nanostructures consisting of an inner spherical/bowl-like core of uniform size and platelets protruded from the core by LCDSA of PAMAM-Azo6 (PAMAM=polyamidoamine, Azo=azobenzene) in methanol. It is disclosed that the different aggregation rates for PAMAM-Azo6 with varying contents of Azo units spontaneously separated nucleation and growth stages, which led to the formation of inner spherical/bowl-like cores ("seeds") firstly, followed by the formation of platelets protruded from the edges of inner core to give "imperfect" fried-egg-like nanostructures. Additional annealing of initially formed "imperfect" fried-egg-like micelles will promote the rearrangement of Azo units to give thermodynamically-favored "perfect" fried-egg-shaped micelles with a uniform dimension both in the core and whole structure. This work not only provides an efficient strategy to create uniform multi-morphology fried-egg-shaped nanostructures, but also reveals the essential impact of aggregation kinetics of liquid-crystalline-coil BCPs in the formation of multi-morphology nanostructures.

Uniform block copolymer nanofibers for the delivery of paclitaxel in 2D and 3D glioblastoma tumor models

Cancer treatment has transformed in recent years, with the introduction of immunotherapy providing substantial improvements in prognoses for certain cancers. However, traditional small molecule chemotherapeutics remain the major frontline of defence, and improving their delivery to solid tumors is of utmost importance for improving potency and reducing side effects. Here, length-controlled one-dimensional seed nanofibers (ca. 25 nm, ĐL = 1.05) were generated from poly(fluorenetrimethylenecarbonate)-block-poly(dimethylaminoethylmethacrylate) via living crystallization-driven self-assembly. Paclitaxel, with an encapsulation content ranging from 1 to 100 wt%, was loaded onto the preformed nanoparticles by solvent addition and evaporation. Drug loading was quantified by dynamic light scattering and transmission electron microscopy. Drug-loaded vectors were then incubated with U87 MG glioblastoma cells in a 2D cell assay for up to 72 h, and their anticancer properties were determined. It was observed that seed nanofibers loaded with 20 wt% paclitaxel were the most advantageous combination (IC50 = 0.48 μg mL-1), while pure seed nanofibers with no loaded drug displayed much lower cytotoxicity (IC50 = 11.52 μg mL-1). The IC50 of the loaded seed nanofibers rivaled that of the commercially approved Abraxane® (IC50 = 0.46 μg mL-1). 3D tumor spheroids were then cultured and subjected to the same stresses. Live/dead cell staining revealed that once more, seed nanofibers with 20 wt% paclitaxel, Abraxane®, and paclitaxel all exhibited similar levels of potency (55% viability), whereas control samples exhibited much higher cell viability (70%) after 3 days. These results demonstrate that nanofibers contain great potential as biocompatible drug delivery vehicles for cancer treatment as they exert a similar anticancer effect to the commercially available Abraxane®.

Two-dimensional (2D) quasi-living crystallization-driven self-assembly of polyethylene-b-hyperbranched polyglycidol diblock copolymers in solution

This paper presents a systematic investigation of the crystalline nucleation, micellization, two-dimensional (2D) growth of polyethylene-b-hyperbranched polyglycidol (PE-b-hbPG) copolymers in solutions during cooling and isothermal crystallization. As a result, lozenge-shaped monolayer or multilayer lamellar crystals were prepared by optimizing the "self-nucleation" conditions. The effect of crystallization temperatures (Tc), critical micelle temperature (CMT), selective solvents, and the topology of block copolymers (BCPs) on the growth of 2D lozenge-shaped crystals is extensively explored using TEM, AFM and in situ DLS techniques. The results demonstrate that the formation of a perfect lozenge-shaped monolayer crystal is contingent upon the relationship between CMT and Tc of the BCPs (CMT < Tc), as well as the isothermal crystallization temperature Tiso (CMT < Tiso < Tc). This significant finding provides a feasibility programme for the preparation of 2D lamellar crystals using the "self-nucleation" approach from an alternative viewpoint of the corona topology.

Independent Control of the Width and Length of Semiconducting 2D Nanorectangles via Accelerated Living Crystallization-Driven Self-Assembly

Self-assembly of conjugated polymers offers a powerful method to prepare semiconducting two-dimensional (2D) nanosheets for optoelectronic applications. However, due to the typical biaxial growth behavior of the polymer self-assembly, independent control of the width and length of 2D sheets has been challenging. Herein, we present a greatly accelerated crystallization-driven self-assembly (CDSA) system of polyacetylene-based conjugated polymer to produce 2D semiconducting nanorectangles with precisely controllable dimensions. In detail, rectangular 2D seeds with tunable widths of 0.2-1.3 μm were produced by changing the cosolvent% and grown in the length direction by uniaxial living CDSA up to 11.8 μm. The growth rate was effectively enhanced by tuning the cosolvent%, seed concentration, and temperature, achieving up to 27-fold increase. Additionally, systematic kinetic investigation yielded empirical rate equations, elucidating the relationship between growth rate constant, cosolvent%, seed concentration, and seed width. Finally, the living CDSA allowed us to prepare penta-block comicelles with tunable width, length, and height.

Ian Manners (1961-2023): Learning to Fly

Ian Manners, an internationally renowned main-group and materials scientist, passed away on December 3, 2023. He will be remembered as a brilliant researcher, an avid birdwatching enthusiast, Pink Floyd fan, and champion of his students who had a profoundly positive impact on those around him.

Mechanism of Action and Design of Potent Antibacterial Block Copolymer Nanoparticles

Self-assembled polymer nanoparticles are promising antibacterials, with nonspherical morphologies of particular interest as recent work has demonstrated enhanced antibacterial activity relative to their spherical counterparts. However, the reasons for this enhancement are currently unclear. We have performed a multifaceted analysis of the antibacterial mechanism of action of 1D nanofibers relative to nanospheres by the use of flow cytometry, high-resolution microscopy, and evaluations of the antibacterial activity of pristine and tetracycline-loaded nanoparticles. Low-length dispersity, fluorescent diblock copolymer nanofibers with a crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core (length = 104 and 472 nm, height = 7 nm, width = 10-13 nm) and a partially protonated poly(dimethylaminoethyl methacrylate) (PDMAEMA) corona (length = 12 nm) were prepared via seeded growth living crystallization-driven self-assembly. Their behavior was compared to that of analogous nanospheres containing an amorphous PFTMC core (diameter of 12 nm). While all nanoparticles were uptaken into Escherichia coli W3110, crystalline-core nanofibers were observed to cause significant bacterial damage. Drug loading studies indicated that while all nanoparticle antibacterial activity was enhanced in combination with tetracycline, the enhancement was especially prominent when small nanoparticles (ca. 15-25 nm) were employed. Therefore, the identified differences in the mechanism of action and the demonstrated consequences for nanoparticle size and morphology control may be exploited for the future design of potent antibacterial agents for overcoming antibacterial resistance. This study also reinforces the requirement of morphological control over polymer nanoparticles for biomedical applications, as differences in activity are observed depending on their size, shape, and core-crystallinity.

Length-Dependent Cellular Internalization of Nanobody-Functionalized Poly(2-oxazoline) Nanorods

The ability to target specific tissues and to be internalized by cells is critical for successful nanoparticle-based targeted drug delivery. Here, we combined "stealthy" rod-shaped poly(2-oxazoline) (POx) nanoparticles of different lengths with a cancer marker targeting nanobody and a fluorescent cell internalization sensor via a heat-induced living crystallization-driven self-assembly (CDSA) strategy. A significant increase in association and uptake driven by nanobody-receptor interactions was observed alongside nanorod-length-dependent kinetics. Importantly, the incorporation of the internalization sensor allowed for quantitative differentiation between cell surface association and internalization of the targeted nanorods, revealing unprecedented length-dependent cellular interactions of CDSA nanorods. This study highlights the modularity and versatility of the heat-induced CDSA process and further demonstrates the potential of POx nanorods as a modular nanomedicine platform.

Living Self-Assembly of Metastable and Stable Two-Dimensional Platelets from a Single Small Molecule

This study reports the design of a donor-acceptor (D-A) molecule with two fluorene units on each side of a benzothiadiazole moiety, which allows multiple intermolecular interactions to compete with one another so as to induce the evolution of the metastable 2D platelets to the stable 2D platelets during the self-assembly of the D-A molecule. Importantly, the living seeded self-assembly of metastable and stable 2D structures with precisely controlled sizes can be conveniently achieved using an appropriate supersaturated level of a solution of the D-A molecule as the seeded growth medium that can temporarily hold the almost-proceeding spontaneous nucleation from competing with the seeded growth. The stable 2D platelets with smaller area sizes exhibit higher sensitivity to gaseous dimethyl sulfide, illustrating that the novel living self-assembly method provides more available functional structures with controlled sizes for practical applications. The key finding of this study is that the new living methodology is separated into two independent processes: the elaborate molecular design for various crystalline structures as seeds and the application of a supersaturated solution with appropriate levels as the growth medium to grow the uniform structures with controlled sizes; this would make convenient and possible the living seeded self-assembly of rich 1D, 2D, and 3D architectures.

Understanding the Seeded Heteroepitaxial Growth of Crystallizable Polymers: The Role of Crystallization Thermodynamics

Seeded heteroepitaxial growth is a "living" crystallization-driven self-assembly (CDSA) method that has emerged as a promising route to create uniform segmented nanoparticles with diverse core chemistries by using chemically distinct core-forming polymers. Our previous results have demonstrated that crystallization kinetics is a key factor that determines the occurrence of heteroepitaxial growth, but an in-depth understanding of controlling heteroepitaxy from the perspective of crystallization thermodynamics is yet unknown. Herein, we select crystallizable aliphatic polycarbonates (PxCs) with a different number of methylene groups (xCH2, x = 4, 6, 7, 12) in their repeating units as model polymers to explore the effect of lattice match and core compatibility on the seeded growth behavior. Seeded growth of PxCs-containing homopolymer/block copolymer blend unimers from poly(ε-caprolactone) (PCL) core-forming seed platelet micelles exhibits distinct crystal growth behavior at subambient temperatures, which is governed by the lattice match and core compatibility. A case of seeded growth with better core compatibility and a smaller lattice mismatch follows epitaxial growth, where the newly created crystal domain has the same structural orientation as the original platelet substrate. In contrast, a case of seeded growth with better core compatibility but a larger lattice mismatch shows nonepitaxial growth with less-defined crystal orientations in the platelet plane. Additionally, a case of seeded growth with poor core compatibility and larger lattice mismatch results in polydisperse platelet micelles, whereby crystal formation is not nucleated from the crystalline substrate. These findings reveal important factors that govern the specific crystal growth during a seeded growth approach by using compositionally distinct cores, which would further guide researchers in designing 2D segmented materials via polymer crystallization approaches.

High-resolution cryo-electron microscopy structure of block copolymer nanofibres with a crystalline core

Seeded growth of crystallizable block copolymers and π-stacking molecular amphiphiles in solution using living crystallization-driven self-assembly is an emerging route to fabricate uniform one-dimensional and two-dimensional core-shell micellar nanoparticles of controlled size with a range of potential applications. Although experimental evidence indicates that the crystalline core of these nanomaterials is highly ordered, a direct observation of their crystal lattice has not been successful. Here we report the high-resolution cryo-transmission electron microscopy studies of vitrified solutions of nanofibres made from a crystalline core of poly(ferrocenyldimethylsilane) (PFS) and a corona of polysiloxane grafted with 4-vinylpyridine groups. These studies show that poly(ferrocenyldimethylsilane) chains pack in an 8-nm-diameter core lattice with two-dimensional pseudo-hexagonal symmetry that is coated by a 27 nm 4-vinylpyridine corona with a 3.5 nm distance between each 4-vinylpyridine strand. We combine this structural information with a molecular modelling analysis to propose a detailed molecular model for solvated poly(ferrocenyldimethylsilane)-b-4-vinylpyridine nanofibres.

Pathway Complexity in Supramolecular Copolymerization and Blocky Star Copolymers by a Hetero-Seeding Effect

This study unravels the intricate kinetic and thermodynamic pathways involved in the supramolecular copolymerization of the two chiral dipolar naphthalene monoimide (NMI) building blocks (O-NMI and S-NMI), differing merely by a single heteroatom (oxygen vs sulfur). O-NMI exhibits distinct supramolecular polymerization features as compared to S-NMI in terms of its pathway complexity, hierarchical organization, and chiroptical properties. Two distinct self-assembly pathways in O-NMI occur due to the interplay between the competing dipolar interactions among the NMI chromophores and amide-amide hydrogen (H)-bonding that engenders distinct nanotapes and helical fibers, from its antiparallel and parallel stacking modes, respectively. In contrast, the propensity of S-NMI to form only a stable spherical assembly is ascribed to its much stronger amide-amide H-bonding, which outperforms other competing interactions. Under the thermodynamic route, an equimolar mixture of the two monomers generates a temporally controlled chiral statistical supramolecular copolymer that autocatalytically evolves from an initially formed metastable spherical heterostructure. In contrast, the sequence-controlled addition of the two monomers leads to the kinetically driven hetero-seeded block copolymerization. The ability to trap O-NMI in a metastable state allows its secondary nucleation from the surface of the thermodynamically stable S-NMI spherical "seed", which leads to the core-multiarmed "star" copolymer with reversibly and temporally controllable length of the growing O-NMI "arms" from the S-NMI "core". Unlike the one-dimensional self-assembly of O-NMI and its random co-assembly with S-NMI, which are both chiral, unprecedentedly, the preferred helical bias of the nucleating O-NMI fibers is completely inhibited by the absence of stereoregularity of the S-NMI "seed" in the "star" topology.

Uniform Two-Dimensional Crystalline Platelets with Tailored Compositions for pH Stimulus-Responsive Drug Release

Two-dimensional, size-tunable, water-dispersible particle micelles with spatially defined chemistries can be obtained by using "living" crystallization-driven self-assembly (CDSA) approach. Nevertheless, a major obstacle of crystalline particles in drug delivery application is the difficulty in accessing to cargo within crystalline cores. In the present work, we design four different types of biocompatible two-dimensional platelets with a crystalline poly(ε-caprolactone) (PCL) core, a hydrophobic poly(4-vinylprydine) (P4VP) segment, and a water dispersible poly(N,N-dimethyl acrylamide) (PDMA) block in ethanol by seeded growth method. Transferring those uniform platelets with tailored compositions to an aqueous solution in the presence of a hydrophobic drug leads to efficient encapsulation of the cargo in the P4VP segments via hydrophobic interactions. These drug-loaded platelets exhibit pH-responsive release behavior in aqueous media due to the protonated-deprotonated process of P4VP blocks in acidic and neutral solutions. This work provides initial insight into biocompatible PCL platelets with low dispersity and precise chemistry control in stimulus-responsive drug delivery fields.

Multicompartment Nanoparticles by Crystallization-Driven Self-Assembly of Star Polymers: Combining High Stability and Loading Capacity

Herein novel multicompartment nanoparticles (MCNs) that combine high stability and cargo loading capacity are developed. The MCNs are fabricated by crystallization-driven self-assembly (CDSA) of a tailor-made 21 arm star polymer, poly(L-lactide)[poly(tert-butyl acrylate)-block-poly(ethylene glycol)]₂₀ [PLLA(PtBA-b-PEG)₂₀ ]. Platelet-like or spherical MCNs containing a crystalline PLLA core and hydrophobic PtBA subdomains are formed and stabilized by PEG. Hydrophobic cargos, such as Nile Red and chemotherapeutic drug doxorubicin, can be successfully encapsulated into the collapsed PtBA subdomains with loading capacity two orders of magnitude higher than traditional CDSA nanoparticles. Depolarized fluorescence measurements of the Nile Red loaded MCNs suggest that the free volume of the hydrophobic chains in the nanoparticles may be the key for regulating their drug loading capacity. In vitro study of the MCNs suggests excellent cytocompatibility of the blank nanoparticles as well as a dose-dependent cellular uptake and cytotoxicity of the drug-loaded MCNs.

Preparation and cellular uptake behaviors of uniform fiber-like micelles with length controllability and high colloidal stability in aqueous media

Fragmentation/disassembly of fiber-like micelles generated by living crystalline-driven self-assembly (CDSA) is usually encountered in aqueous media, which hinders the applications of micelles. Herein, we report the generation of uniform fiber-like micelles consisting of a π-conjugated oligo(p-phenylenevinylene) core and a cross-linking silica shell with grafted poly(ethylene glycol) (PEG) chains by the combination of living CDSA, silica chemistry and surface grafting-onto strategy. Owing to the presence of crosslinking silica shell and the outmost PEG chains, the resulting micelles exhibit excellent dispersity and colloidal stability in PBS buffer, BSA aqueous solution and upon heating at 80 °C for 2 h without micellar fragmentation/disassembly. The micelles also show negligible cytotoxicity toward both HeLa cervical cancer and HEK239T human embryonic kidney cell lines. Interestingly, micelles with L n of 156 nm show the "stealth" property with no significant uptake by HeLa cells, whereas some certain amounts of micelles with L n of 535 nm can penetrate into HeLa cells, showing length-dependent cellular uptake behaviors. These results provide a route to prepare uniform, colloidally stable fiber-like nanostructures with tunable length and functions derived for biomedical applications.

Length-Controlled Nanofiber Micelleplexes as Efficient Nucleic Acid Delivery Vehicles

Micelleplexes show great promise as effective polymeric delivery systems for nucleic acids. Although studies have shown that spherical micelleplexes can exhibit superior cellular transfection to polyplexes, to date there has been no report on the effects of micelleplex morphology on cellular transfection. In this work, we prepared precision, length-tunable poly(fluorenetrimethylenecarbonate)-b-poly(2-(dimethylamino)ethyl methacrylate) (PFTMC₁₆-b-PDMAEMA₁₃₁) nanofiber micelleplexes and compared their properties and transfection activity to those of the equivalent nanosphere micelleplexes and polyplexes. We studied the DNA complexation process in detail via a range of techniques including cryo-transmission electron microscopy, atomic force microscopy, dynamic light scattering, and ζ-potential measurements, thereby examining how nanofiber micelleplexes form, as well the key differences that exist compared to nanosphere micelleplexes and polyplexes in terms of DNA loading and colloidal stability. The effects of particle morphology and nanofiber length on the transfection and cell viability of U-87 MG glioblastoma cells with a luciferase plasmid were explored, revealing that short nanofiber micelleplexes (length < ca. 100 nm) were the most effective delivery vehicle examined, outperforming nanosphere micelleplexes, polyplexes, and longer nanofiber micelleplexes as well as the Lipofectamine 2000 control. This study highlights the potential importance of 1D micelleplex morphologies for achieving optimal transfection activity and provides a fundamental platform for the future development of more effective polymeric nucleic acid delivery vehicles.

AIE-Active, Stimuli-Responsive Fluorescent 2D Block Copolymer Nanoplatelets Based on Corona Chain Compression

Aggregation-induced emission (AIE) represents a powerful tool in nanoscience as a result of enhanced luminescence in the condensed state. Although AIEgenic materials have been utilized in a wide range of applications, well-defined self-assembled nanoparticles with tailorable and uniform dimensions and morphology remain challenging to access. Herein, we use the seeded growth, living crystallization-driven self-assembly (CDSA) method to prepare size-tunable and uniform AIE-active 2D nanoplatelets from amphiphilic block copolymer (BCP) precursors with a crystallizable core-forming block and a corona-forming block to which tetraphenylethene (TPE) groups were covalently grafted as AIE-active pendants. The nanoplatelets were formed as a result of a solvophobicity-induced 1D to 2D morphology preference change, which accompanied the seeded growth of a BCP with a quaternized corona-forming block bearing the TPE luminogen. The 2D nanoplatelets exhibited a solvent-responsive fluorescent emission, and examples with coronas containing homogeneously distributed AIE-active TPE groups and Hg(II)-capturing thymine units exhibited excellent performance as proof-of-concept "turn-on" sensors for Hg(II) detection with a rapid response, high selectivity, and a low detection limit (5-125 × 10^-9 M, i.e., 1-25 ppb). The fluorescence intensity was found to be nonlinear with respect to analyte concentration and to increase with the area of the nanoplatelet. This behavior is consistent with a cooperative mechanism based on changes in the steric compression of the corona chains, which gives rise to a restriction of the intramolecular motion (RIM) effect.

(Bio)degradable and Biocompatible Nano-Objects from Polymerization-Induced and Crystallization-Driven Self-Assembly

Polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) techniques have emerged as powerful approaches to produce a broad range of advanced synthetic nano-objects with high potential in biomedical applications. PISA produces nano-objects of different morphologies (e.g., spheres, vesicles and worms), with high solids content (∼10-50 wt %) and without additional surfactant. CDSA can finely control the self-assembly of block copolymers and readily forms nonspherical crystalline nano-objects and more complex, hierarchical assemblies, with spatial and dimensional control over particle length or surface area, which is typically difficult to achieve by PISA. Considering the importance of these two assembly techniques in the current scientific landscape of block copolymer self-assembly and the craze for their use in the biomedical field, this review will focus on the advances in PISA and CDSA to produce nano-objects suitable for biomedical applications in terms of (bio)degradability and biocompatibility. This review will therefore discuss these two aspects in order to guide the future design of block copolymer nanoparticles for future translation toward clinical applications.

Lateral growth of cylinders

The precise control of the shape, size and microstructure of nanomaterials is of high interest in chemistry and material sciences. However, living lateral growth of cylinders is still very challenging. Herein, we propose a crystallization-driven fusion-induced particle assembly (CD-FIPA) strategy to prepare cylinders with growing diameters by the controlled fusion of spherical micelles self-assembled from an amphiphilic homopolymer. The spherical micelles are heated upon glass transition temperature (T_g) to break the metastable state to induce the aggregation and fusion of the amorphous micelles to form crystalline cylinders. With the addition of extra spherical micelles, these micelles can attach onto and fuse with the cylinders, showing the living character of the lateral growth of cylinders. Computer simulations and mathematical calculations are preformed to reveal the total energy changes of the nanostructures during the self-assembly and CD-FIPA process. Overall, we demonstrated a CD-FIPA concept for preparing cylinders with growing diameters.

Bottom-up supramolecular assembly in two dimensions

The self-assembly of molecules in two dimensions (2D) is gathering attention from all disciplines across the chemical sciences. Attracted by the interesting properties of two-dimensional inorganic analogues, monomers of different chemical natures are being explored for the assembly of dynamic 2D systems. Although many important discoveries have been already achieved, great challenges are still to be addressed in this field. Hierarchical multicomponent assembly, directional non-covalent growth and internal structural control are a just a few of the examples that will be discussed in this perspective about the exciting present and the bright future of two-dimensional supramolecular assemblies.

The self-assembly of molecules in two dimensions (2D) is gathering attention from all disciplines across the chemical sciences. This perspective discusses the main strategies to direct the supramolecular self-assembly of organic monomers in 2D.

Exploring the "Living" Growth of Block Copolymer Nanofibers from Surface-Confined Seeds by In Situ Solution-Phase Atomic Force Microscopy

Living crystallization-driven self-assembly of polymeric and molecular amphiphiles is of growing interest as a seeded growth route to uniform 1D, 2D, and more complex micellar nanoparticles with controlled dimensions and a range of potential applications. Although most studies have been performed using colloidally stable seeds in bulk solution, growth of block copolymer (BCP) nanofibers from seeds confined to a surface is attracting increased attention. Herein, we have used atomic force microscopy (AFM) to undertake detailed studies of the growth of BCP nanofibers from immobilized seeds located on a Si surface. Through initial ex situ AFM studies and in situ AFM video analysis in solution, we determined that growth occurred in four stages, whereby an initial surface-bound growth regime transitions to surface-limited growth. As the nanofiber length increases, surface influence is diminished as the newly grown micelle segment is no longer bound to the Si substrate. Finally, a surface-independent regime occurs where nanofiber growth continues into bulk solution. In addition to the anticipated nanofiber elongation, our studies revealed occasional examples of AFM tip-induced core fragmentation. In these cases, the termini of the newly formed fragments were also active to further growth. Furthermore, unidirectional growth was detected in cases where the seed was oriented at a significant angle with respect to the surface, thereby restricting unimer access to one terminus.

Precision polymer nanofibers with a responsive polyelectrolyte corona designed as a modular, functionalizable nanomedicine platform

We describe the development of a modular, functionalizable platform for biocompatible core-shell block copolymer nanofibers of controlled length (22 nm – 1.3 μm) and low dispersity produced via living crystallization-driven...

Uniform, Length-Tunable Antibacterial 1D Diblock Copolymer Nanofibers

The rapid increase in antibiotic resistant strains of bacteria has led to an urgent need to develop new methods of treating bacterial infections. Antibacterial polymeric nanoparticles are of interest for...

Hydrophobic Cargo Loading at the Core-Corona Interface of Uniform, Length-Tunable Aqueous Diblock Copolymer Nanofibers with a Crystalline Polycarbonate Core

1D core-shell nanoparticles are considered to be among the most promising for biomedical applications such as drug delivery. The versatile living crystallization-driven self-assembly (CDSA) seeded growth method allows access to...

π-Conjugated-polymer-based nanofibers through living crystallization-driven self-assembly: preparation, properties and applications

π-Conjugated-polymer-based nanofibers (CPNFs) of controlled length, composition and morphology are promising for a broad range of emerging applications in optoelectronics, biomedicine and catalysis, owing to the morphological merits of fiber-like nanostructures and structural attributes of π-conjugated polymers. Living crystallization-driven self-assembly (CDSA) of π-conjugated-polymer-containing block copolymers (BCPs) has emerged as an efficient strategy to prepare CPNFs with precise dimensional and structural controllability by taking advantage of the crystallinity of π-conjugated polymers. In this review, recent advances in the generation of CPNFs have been highlighted. The influence of the structure of π-conjugated-polymer-containing BCPs and experimental conditions on the CDSA behaviors, especially seeded growth and self-seeding processes of living CDSA, has been discussed in detail, aiming to provide an in-depth overview of living CDSA of π-conjugated-polymer-containing BCPs. In addition, the properties of CPNFs as well as their potential applications have been illustrated. Finally, we put forward the current challenges and research directions in the field of CPNFs.

Adjusting the length of supramolecular polymer bottlebrushes by top-down approaches

Controlling the length of one-dimensional (1D) polymer nanostructures remains a key challenge on the way toward the applications of these structures. Here, we demonstrate that top-down processing facilitates a straightforward adjustment of the length of polyethylene oxide (PEO)-based supramolecular polymer bottlebrushes (SPBs) in aqueous solutions. These cylindrical structures self-assemble via directional hydrogen bonds formed by benzenetrisurea (BTU) or benzenetrispeptide (BTP) motifs located within the hydrophobic core of the fiber. A slow transition from different organic solvents to water leads first to the formation of µm-long fibers, which can subsequently be fragmented by ultrasonication or dual asymmetric centrifugation. The latter allows for a better adjustment of applied shear stresses, and thus enables access to differently sized fragments depending on time and rotation rate. Extended sonication and scission analysis further allowed an estimation of tensile strengths of around 16 MPa for both the BTU and BTP systems. In combination with the high kinetic stability of these SPBs, the applied top-down methods represent an easily implementable technique toward 1D polymer nanostructures with an adjustable length in the range of interest for perspective biomedical applications.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

Living supramolecular polymerization of fluorinated cyclohexanes

The development of powerful methods for living covalent polymerization has been a key driver of progress in organic materials science. While there have been remarkable reports on living supramolecular polymerization recently, the scope of monomers is still narrow and a simple solution to the problem is elusive. Here we report a minimalistic molecular platform for living supramolecular polymerization that is based on the unique structure of all-cis 1,2,3,4,5,6-hexafluorocyclohexane, the most polar aliphatic compound reported to date. We use this large dipole moment (6.2 Debye) not only to thermodynamically drive the self-assembly of supramolecular polymers, but also to generate kinetically trapped monomeric states. Upon addition of well-defined seeds, we observed that the dormant monomers engage in a kinetically controlled supramolecular polymerization. The obtained nanofibers have an unusual double helical structure and their length can be controlled by the ratio between seeds and monomers. The successful preparation of supramolecular block copolymers demonstrates the versatility of the approach.

Patchy Micelles with a Crystalline Core: Self-Assembly Concepts, Properties, and Applications

Crystallization-driven self-assembly (CDSA) of block copolymers bearing one crystallizable block has emerged to be a powerful and highly relevant method for the production of one- and two-dimensional micellar assemblies with controlled length, shape, and corona chemistries. This gives access to a multitude of potential applications, from hierarchical self-assembly to complex superstructures, catalysis, sensing, nanomedicine, nanoelectronics, and surface functionalization. Related to these applications, patchy crystalline-core micelles, with their unique, nanometer-sized, alternating corona segmentation, are highly interesting, as this feature provides striking advantages concerning interfacial activity, functionalization, and confinement effects. Hence, this review aims to provide an overview of the current state of the art with respect to self-assembly concepts, properties, and applications of patchy micelles with crystalline cores formed by CDSA. We have also included a more general discussion on the CDSA process and highlight block-type co-micelles as a special type of patchy micelle, due to similarities of the corona structure if the size of the blocks is well below 100 nm.

Morphologic design of sugar-based polymer nanoparticles for delivery of antidiabetic peptides

Zwitterionic polymer nanoparticles of diverse morphologies (spherical, cylindrical, and platelet-like) constructed from biocompatible sugar-based polymers are designed to extend the pharmacological activities of short- and long-acting insulin peptides, thereby providing potential for therapeutic systems capable of reducing the frequency of administration and improving patient compliance. Amphiphilic block copolymers composed of zwitterionic poly(d-glucose carbonate) and semicrystalline polylactide segments were synthesized, and the respective block length ratios were tuned to allow formation of nanoscopic assemblies having different morphologies. Insulin-loaded nanoparticles had similar sizes and morphologies to the unloaded nanoparticle counterparts. Laser scanning confocal microscopy imaging of three-dimensional spheroids of vascular smooth muscle cells and fibroblasts after treatment with LIVE/DEAD® stain and FITC-insulin-loaded nanoparticles demonstrated high biocompatibility for the nanoconstructs of the various morphologies and significant intracellular uptake of insulin in both cell lines, respectively. Binding of short-acting insulin and long-acting insulin glargine to nanoparticles resulted in extended hypoglycemic activities in rat models of diabetes. Following subcutaneous injection in diabetic rats, insulin- and insulin glargine-loaded nanoparticles of diverse morphologies had demonstrated up to 2.6-fold and 1.7-fold increase in pharmacological availability, in comparison to free insulin and insulin glargine, respectively. All together, the negligible cytotoxicity, immunotoxicity, and minimal cytokine adsorption onto nanoparticles (as have been demonstrated in our previous studies) provide exciting and promising evidence of biocompatible nanoconstructs that are poised for further development toward the management of diabetes.

Stealth nanorods via the aqueous living crystallisation-driven self-assembly of poly(2-oxazoline)s

The morphology of nanomaterials critically influences their biological interactions. However, there is currently a lack of robust methods for preparing non-spherical particles from biocompatible materials. Here, we combine 'living' crystallisation-driven self-assembly (CDSA), a seeded growth method that enables the preparation of rod-like polymer nanoparticles, with poly(2-oxazoline)s (POx), a polymer class that exhibits 'stealth' behaviour and excellent biocompatibility. For the first time, the 'living' CDSA process was carried out in pure water, resulting in POx nanorods with lengths ranging from ∼60 to 635 nm. In vitro and in vivo study revealed low immune cell association and encouraging blood circulation times, but little difference in the behaviour of POx nanorods of different length. The stealth behaviour observed highlights the promising potential of POx nanorods as a next generation stealth drug delivery platform.

Emerging applications for living crystallization-driven self-assembly

The use of crystallization as a tool to control the self-assembly of polymeric and molecular amphiphiles in solution is attracting growing attention for the creation of non-spherical nanoparticles and more complex, hierarchical assemblies. In particular, the seeded growth method termed living crystallization-driven self-assembly (CDSA) has been established as an ambient temperature and potentially scalable platform for the preparation of low dispersity samples of core-shell fiber-like or platelet micellar nanoparticles. Significantly, this method permits predictable control of size, and access to branched and segmented structures where functionality is spatially-defined. Living CDSA operates under kinetic control and shows many analogies with living chain-growth polymerizations of molecular organic monomers that afford well-defined covalent polymers of controlled length except that it covers a much longer length scale (ca. 20 nm to 10 μm). The method has been applied to a rapidly expanding range of crystallizable polymeric amphiphiles, which includes block copolymers and charge-capped homopolymers, to form assemblies with crystalline cores and solvated coronas. Living CDSA seeded growth methods have also been transposed to a wide variety of π-stacking and hydrogen-bonding molecular species that form supramolecular polymers in processes termed "living supramolecular polymerizations". In this article we outline the main features of the living CDSA method and then survey the promising emerging applications for the resulting nanoparticles in fields such as nanomedicine, colloid stabilization, catalysis, optoelectronics, information storage, and surface functionalization.

Towards scalable, low dispersity, and dimensionally tunable 2D platelets using living crystallization-driven self-assembly

Scalable low dispersity platelets were accessed through the self-assembly of crystallizable charge-terminated PFS homopolymers. The use of surfactant counteranions, as well as increasing the self-assembly temperature, improved structure fidelity.

Spherical Micelles with Nonspherical Cores: Effect of Chain Packing on the Micellar Shape

Self-assembly of amphiphilic polymers into micelles is an archetypical example of a "self-confined" system due to the formation of micellar cores with dimensions of a few nanometers. In this work, we investigate the chain packing and resulting shape of C n -PEOx micelles with semicrystalline cores using small/wide-angle X-ray scattering (SAXS/WAXS), contrast-variation small-angle neutron scattering (SANS), and nuclear magnetic resonance spectroscopy (NMR). Interestingly, the n-alkyl chains adopt a rotator-like conformation and pack into prolate ellipses (axial ratio ϵ ≈ 0.5) in the "crystalline" region and abruptly arrange into a more spheroidal shape (ϵ ≈ 0.7) above the melting point. We attribute the distorted spherical shape above the melting point to thermal fluctuations and intrinsic rigidity of the n-alkyl blocks. We also find evidence for a thin dehydrated PEO layer (≤1 nm) close to the micellar core. The results provide substantial insight into the interplay between crystallinity and molecular packing in confinement and the resulting overall micellar shape.

Evaluation of Self-Assembly Pathways to Control Crystallization-Driven Self-Assembly of a Semicrystalline P(VDF-co-HFP)-b-PEG-b-P(VDF-co-HFP) Triblock Copolymer

To date, amphiphilic block copolymers (BCPs) containing poly(vinylidene fluoride-co-hexafluoropropene) (P(VDF-co-HFP)) copolymers are rare. At moderate content of HFP, this fluorocopolymer remains semicrystalline and is able to crystallize. Amphiphilic BCPs, containing a P(VDF-co-HFP) segment could, thus be appealing for the preparation of self-assembled block copolymer morphologies through crystallization-driven self-assembly (CDSA) in selective solvents. Here the synthesis, characterization by 1H and 19F NMR spectroscopies, GPC, TGA, DSC, and XRD; and the self-assembly behavior of a P(VDF-co-HFP)-b-PEG-b-P(VDF-co-HFP) triblock copolymer were studied. The well-defined ABA amphiphilic fluorinated triblock copolymer was self-assembled into nano-objects by varying a series of key parameters such as the solvent and the non -solvent, the self-assembly protocols, and the temperature. A large range of morphologies such as spherical, square, rectangular, fiber-like, and platelet structures with sizes ranging from a few nanometers to micrometers was obtained depending on the self-assembly protocols and solvents systems used. The temperature-induced crystallization-driven self-assembly (TI-CDSA) protocol allowed some control over the shape and size of some of the morphologies.