Hierarchical Assemblies of Dendrimers Embedded in Networks of Lanthanide-Based Supramolecular Polyelectrolytes

Research Status of Dendrimer Micelles in Tumor Therapy for Drug Delivery

Dendrimers are a family of polymers with highly branched structure, well-defined composition, and extensive functional groups, which have attracted great attention in biomedical applications. Micelles formed by dendrimers are ideal nanocarriers for delivering anticancer agents due to the explicit study of their characteristics of particle size, charge, and biological properties such as toxicity, blood circulation time, biodistribution, and cellular internalization. Here, the classification, preparation, and structure of dendrimer micelles are reviewed, and the specific functional groups modified on the surface of dendrimers for tumor active targeting, stimuli-responsive drug release, reduced toxicity, and prolonged blood circulation time are discussed. In addition, their applications are summarized as various platforms for biomedical applications related to cancer therapy including drug delivery, gene transfection, nano-contrast for imaging, and combined therapy. Other applications such as tissue engineering and biosensor are also involved. Finally, the possible challenges and perspectives of dendrimer micelles for their further applications are discussed.

Hierarchical self-assembly of metal-organic supramolecular fibers with lanthanide-derived functionalities

Achieving organized assembly structures with high complexity and adjustable functionalities is a central quest in supramolecular chemistry. In this report, we study what happens when a discotic benzene-1,3,5-tricarboxamide (BTA) ligand containing three dipicolinic acid (DPA) groups is allowed to coordinate with lanthanide (Ln) ions. A multi-BTA coordination cluster forms, which behaves as a type of "supramolecular monomer", stacking into fibers via hydrogen bonds enabled by multiple BTA cores. The fibrous morphology and size, as well as the packing unit and the process by which it grows, were investigated by light scattering measurements, luminescence spectra, TEM images and molecular simulation data. More notably, by selecting the kind of lanthanide or mixture of lanthanides that is incorporated, tunable luminescence and magnetic relaxation properties without compromising the fibrous structure can be realized. This case of hierarchical self-assembly is made possible by the special structure of our BTA-like building block, which makes non-covalent bond types that are different along the radial (coordination bonds) and axial (H-bonds) directions, respectively, each with just the right strength. Moreover, the use of lanthanide coordination leads to materials with metal-derived optical and magnetic properties. Therefore, the established approach demonstrates a novel strategy for designing and fabrication of multi-functional supramolecular materials.

Lanthanide and Ladder-Structured Polysilsesquioxane Composites for Transparent Color Conversion Layers

Ladder-type polysilsesquioxanes (LPSQs) containing phenyl as a high refractive index unit and cyclic epoxy as a curable unit were found to be excellent candidates for a transparent color conversion layer for displays due to being miscible with organic solvents and amenable to transparent film formation. Therefore, the LPSQs were combined with luminescent lanthanide metals, europium Eu(III), and terbium Tb(III), to fabricate transparent films with various emission colors, including red, orange, yellow, and green. The high luminescence and transmittance properties of the LPSQs-lanthanide composite films after thermal curing were attributed to chelating properties of hydroxyl and polyether side chains of LPSQs to lanthanide ions, as well as a light sensitizing effect of phenyl side chains of the LPSQs. Furthermore, Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy and nanoindentation tests indicated that the addition of the nanoparticles to the LPSQs moderately enhanced the epoxy conversion rate and substantially improved the wear resistance, including hardness, adhesion, and insusceptibility to atmospheric corrosion in a saline environment. Thus, the achieved LPGSG-lanthanide hybrid organic-inorganic material could effectively serve as a color conversion layer for displays.

Functional Nano-Objects by Electrostatic Self-Assembly: Structure, Switching, and Photocatalysis

The design of functional nano-objects by electrostatic self-assembly in solution signifies an emerging field with great potential. More specifically, the targeted combination of electrostatic interaction with other effects and interactions, such as the positioning of charges on stiff building blocks, the use of additional amphiphilic, π-π stacking building blocks, or polyelectrolytes with certain architectures, have recently promulgated electrostatic self-assembly to a principle for versatile defined structure formation. A large variety of architectures from spheres over rods and hollow spheres to networks in the size range of a few tenths to a few hundred nanometers can be formed. This review discusses the state-of-the-art of different approaches of nano-object formation by electrostatic self-assembly against the backdrop of corresponding solid materials and assemblies formed by other non-covalent interactions. In this regard, particularly promising is the facile formation of triggerable structures, i.e. size and shape switching through light, as well as the use of electrostatically assembled nano-objects for improved photocatalysis and the possible solar energy conversion in the future. Lately, this new field is eliciting an increasing amount of understanding; insights and limitations thereof are addressed in this article. Special emphasis is placed on the interconnection of molecular building block structures and the resulting nanoscale architecture via the key of thermodynamics.

Rational Polyelectrolyte Design Enables Multifunctional Polyion Complex Vesicles

Polyion complex (PIC) vesicles prepared by polyelectrolyte assembly have attracted extensive attention as distinctive carriers and nanoreactors, particularly for biological cargoes. However, the constrained regulation of their structure and functionality at this stage hinder the application of PIC vesicles. Herein, we design a new asymmetric assembly system, namely cationic-neutral-cationic triblock copolymer co-assembly with a supramolecular ionic coordination polymer. The former creates poly(ethylene oxide) (PEO) loops upon complexation, which are favorable for vesicle fabrication, while the coordination polyelectrolyte composed of metal ions and a dipicolinic acid (DPA)-based bis-ligand features well-defined functionalities depending on the incorporated metal ions. Thus, the rational combination allows controlled fabrication of PIC vesicles with a modulated structure and functionalities. Moreover, the encapsulation and release of hydrophilic dextran based on different PIC vesicles has been realized. Our design integrates the advantages of both triblock and coordination polymers, and therefore demonstrates a novel strategy for harmonious regulation of the structure and functionality of PIC vesicles. The revealed findings and achieved properties shall be inspirational for developing functional PIC vesicles and boosting their applications towards demand encapsulation and delivery.

Dendrimer-Based Polyion Complex Vesicles: Loops Make Loose

Associations of amphiphiles assume their various morphologies according to the so-called packing parameter under thermodynamic control. However, one may raise the question of whether polymers can always relax fast enough to obey thermodynamic control, and how this may be checked. Here, a case of polyion complex (PIC) assemblies where the morphology appears to be subject to kinetic control is discussed. Poly (ethylene oxide)-b-(styrene sulfonate) block copolymers are combined with cationic PAMAM dendrimers of various generations (2-7). The PEO-PSS diblocks, and the corresponding PSS-PEO-PSS triblocks should have nearly identical packing parameters, but surprisingly creat different assemblies, namely core-shell micelles and vesicles, respectively. Moreover, the micelles are very stable against added salt, whereas the vesicles are not only much more sensitive to added salt, but also appear to exchange matter on relevant time scales. The small and largely quenched early-stage precursor complexes are responsible for the morphological and dynamic differences, implying that kinetic control may also be a way to obtain particles with well-defined and useful properties. The exciting new finding that triblocks produce more "active" vesicles will hopefully trigger the exploration of more pathways, and so learn how to tune PICsomes toward specific applications.

Supramolecular virus-like particles by co-assembly of triblock polypolypeptide and PAMAM dendrimers

Virus-like particles are of special interest as functional delivery vehicles in a variety of fields ranging from nanomedicine to materials science. Controlled formation of virus-like particles relies on manipulating the assembly of the viral coat proteins. Herein, we report a new assembly system based on a triblock polypolypeptide C4-S10-BK12 and -COONa terminated PAMAM dendrimers. The polypolypeptide has a cationic BK12 block with 12 lysines; its binding with anionic PAMAM triggers the folding of the peptide's middle silk-like block and leads to formation of virus-like nanorods, stabilized against aggregation by the long hydrophilic "C" block of the polypeptide. Varying the dendrimer/polypeptide mixing ratio hardly influences the structure and size of the nanorod. However, increasing the dendrimer generation, that is, increasing the dendrimer size results in increased particle length and height, without affecting the width of the nanorod. The branched structure and well-defined size of the dendrimers allows delicate control of the particle size; it is impossible to achieve similar control over assembly of the polypeptide with linear polyelectrolyte as template. In conclusion, we report a novel protein assembling system with properties resembling a viral coat; the findings may therefore be helpful for designing functional virus-like particles like vaccines.

Efficient Synthesis of Stable Polyelectrolyte Complex Nanoparticles by Electrostatic Assembly Directed Polymerization

Polyelectrolyte complex nanoparticles with integrated advances of coacervate complexes and nanomaterials have attracted considerable attention as soft templates and functional nano-carriers. Herein, a facile and robust strategy, namely electrostatic assembly directed polymerization (EADP), for efficient and scalable preparation of stable coacervate nanoparticles is presented. With homo-polyelectrolyte PAA (polyacrylic acid) as template and out of charge stoichiometry, the cationic monomers are polymerized together with cross-linkers, which creates coacervate nanoparticles featuring high stability against salt through one-pot synthesis. The particle size can be tuned by varying the cross-linker amount and salt concentrations during the polymerization and the composition of nanoparticles, as well as the corresponding properties can be regulated by combining different charged blocks from both strong and weak ionic monomers. The strategy can tolerate both high monomer concentrations and increased volume of up to l L, which is favorable for scaled-up preparations. Moreover, the coacervate nanoparticles can be freeze-dried to produce a product in powder form, which can be redispersed without any effect on the particle size and size distribution. Finally, the obtained nanoparticles loaded with enzyme and Au nanoparticles exhibit enhanced catalytic performance, demonstrating a great potential for exploring various applications of coacervate particles as soft and functional nano-carriers.

Effects of pH on the Formation of PIC Micelles from PAMAM Dendrimers

Dendrimer-based PIC micelles are novel nanostructures from the assembly of dendrimers with polyion-neutral diblock copolymers. Because of the branched and three-dimensional structure of dendrimers, understanding the electrostatic assembly is challenging yet essential for manipulating the formation and property of the PIC micelles. Herein, we present the pH effects on the assembly of amine-terminated PAMAM dendrimers with PSS₉₂-b-PEO₁₁₃ diblock copolymers. The step-wise protonation of primary and tertiary amine groups of PAMAM allows us to manipulate the number of the positive charges by tuning pH. We find that the assembly based on the surface charges of PAMAM from G2 to G7 at pH 7 leads to well-defined micelles with high stability against salt. At pH 3, both the interior and surface charges contribute to the assembly, and the formed micelles are sensitive to ionic strength, namely, increasing salt concentration results in the formation of elongated (G2-G5) or bigger (G7) aggregates. Our study reveals the pH manipulation on the assembly of PAMAM dendrimers with linear polyelectrolytes and displays new findings that shall be helpful for understanding the assembly of asymmetric polyelectrolytes, as well as for designing new PIC micelles and functional soft nanocarriers.

Europium based coordination polyelectrolytes enable core-shell-corona micelles as luminescent probes

Core-shell-corona (CSC) micelles have multiple layers, which can serve as separate compartments. This property allows them to combine multiple functionalities in a single nanoparticle, with obvious application potential. Here, we propose a new type of CSC micelles with an apolar core and a polyelectrolyte complex shell incorporating coordination polymers. We obtain these particles by using a poly(styrene)-b-poly(vinyl pyridine)-b-poly(ethylene oxide) (PS-b-PVP-b-PEO) triblock copolymer with quaternized PVP blocks. This polymer leads to well-defined CSC micelles with a cationic shell, which allows us to entrap anionic coordination polymers without disturbing the micellar structure. Useful properties can be imported in this way, e.g., europium (Eu)-based coordination polymers endow the CSC micelles with strong luminescence. Moreover, copper ions (Cu2+) can quench the luminescence because they disturb the Eu-ligand coordination. Upon adding sulfide ions (S2-), copper ions precipitate as CuS and the Eu-ligand bond as well as the corresponding luminescence are restored. This effect is highly specific for Cu2+ and S2-: other cations or anions hardly interfere with this "on-off-on" luminescence response towards Cu2+ and S2-, demonstrating the selectivity of these CSC micelles as detectors of copper and sulfide ions.

Response of metal-coordination-based polyelectrolyte complex micelles to added ligands and metals

Polyelectrolyte complex based micelles have attracted significant attention due to their potential regarding bio-applications. Although the morphology and functions have been studied extensively, dynamic properties, particularly component exchange with other surrounding molecules, have remained elusive to date. Here, we show how micelles based on metal-ligand coordination complex coacervate-core micelles (M-C3Ms) respond to addition of extra ligand and metal ions. The micelles are prepared from a polycationic-neutral diblock copolymer and an anionic coordination polyelectrolyte, which is obtained by coordination between metal ions (lanthanides Ln³⁺ and Zn²⁺) and a bis-ligand (LEO) containing two dipicolinic acid (DPA) groups connected by a tetra-ethylene oxide spacer (4EO). Our findings show that the bis-ligand LEO is essential for the growth of coordination polymers and consequently the formation of micelles, leading to equilibrium structures with the same micellar composition and structure independent of the order of mixing. In other words, adding single DPA has no effect on the formed M-C3Ms. As for metal exchange, we find that added Zn²⁺ can replace some of the Ln³⁺ from Ln-C3Ms, leading to a hybrid coordination structure with both Ln³⁺ and Zn²⁺. We find that component exchange occurs in these coordination polyelectrolyte micelles, but it is more favorable in the direction of replacing the weak binding components with strong ones. Hence, the designed M-C3Ms based on the strong binding components, such as Ln-C3Ms, shall be relatively stable in biological surroundings, paving the way for the application of such particles as bio-imaging probes.

Hierarchical assembly of dual-responsive biomineralized polydopamine–calcium phosphate nanocomposites for enhancing chemo-photothermal therapy by autophagy inhibition

Hierarchically assembled biomineralized nanocomposites would be used to sensitize chemo-photothermal therapy by complementary autophagy inhibition.

Supramolecular crosslinks enable PIC micelles with tuneable salt stability and diverse properties

The stability of polyion complex (PIC) nanoparticles, like PIC micelles or PICsomes, in water is typically affected by added salt because salt screens the electrostatic driving force. This lack of salt stability seriously hampers numerous potential applications and a remedy is needed. Extending an earlier idea, we develop here a general strategy for preparing PIC micelles, with not only tuneable salt stability but also built-in functions. Using two different dipicolinic (DPA)-based ligands (a linear bis-ligand and a branched tris-ligand), as well as various metal ions we obtain anionic coordination polymers that subsequently co-assemble with a polycationic-neutral diblock copolymer to form PIC micelles. By a judicious choice of the metal ions and/or an appropriate mixture of the ligands we can create micellar cores with two types of reversible cross-links. In this way, we construct PIC micelles with not only tuneable and enhanced salt stability, but also tuned metal-derived properties, such as luminescence or magnetic relaxation. This non-covalent cross-link strategy, exclusively based on building block composition, is generally applicable with different metal ions and ligand combinations, and is therefore a robust approach for preparing stable and functional PIC micelles. Extension to other types of assemblies such as 'PICsomes' is possible, and therefore a range of applications becomes feasible.