Fabrication of Hydrophilic Polymer Nanocontainers by Use of Supramolecular Templates

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and β-cyclodextrin-modified hyaluronan

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of β-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and β-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Biodegradable and Dual-Responsive Polypeptide-Shelled Cyclodextrin-Containers for Intracellular Delivery of Membrane-Impermeable Cargo

The transport of membrane impermeable compounds into cells is a prerequisite for the efficient cellular delivery of hydrophilic and amphiphilic compounds and drugs. Transport into the cell's cytosolic compartment should ideally be controllable and it should involve biologically compatible and degradable vehicles. Addressing these challenges, nanocontainers based on cyclodextrin amphiphiles that are stabilized by a biodegradable peptide shell are developed and their potential to deliver fluorescently labeled cargo into human cells is analyzed. Host-guest mediated self-assembly of a thiol-containing short peptide or a cystamine-cross-linked polypeptide shell on cyclodextrin vesicles produce short peptide-shelled (SPSVss ) or polypeptide-shelled vesicles (PPSVss ), respectively, with redox-responsive and biodegradable features. Whereas SPSVss are permeable and less stable, PPSVss effectively encapsulate cargo and show a strictly regulated release of membrane impermeable cargo triggered by either reducing conditions or peptidase treatment. Live cell experiments reveal that the novel PPSVSS are readily internalized by primary human endothelial cells (human umbilical vein endothelial cells) and cervical cancer cells and that the reductive microenvironment of the cells' endosomes trigger release of the hydrophilic cargo into the cytosol. Thus, PPSVSS represent a highly efficient, biodegradable, and tunable system for overcoming the plasma membrane as a natural barrier for membrane-impermeable cargo.

Self-Assembled Cationic Polypeptide Supramolecular Nanogels for Intracellular DNA Delivery

Supramolecular nanogels are an emerging class of polymer nanocarriers for intracellular delivery, due to their straightforward preparation, biocompatibility, and capability to spontaneously encapsulate biologically active components such as DNA. A completely biodegradable three-component cationic supramolecular nanogel was designed exploiting the multivalent host-guest interaction of cyclodextrin and adamantane attached to a polypeptide backbone. While cyclodextrin was conjugated to linear poly-L-lysine, adamantane was grafted to linear as well as star shaped poly-L-lysine. Size control of nanogels was obtained with the increase in the length of the host and guest polymer. Moreover, smaller nanogels were obtained using the star shaped polymers because of the compact nature of star polymers compared to linear polymers. Nanogels were loaded with anionic model cargoes, pyranine and carboxyfluorescein, and their enzyme responsive release was studied using protease trypsin. Confocal microscopy revealed successful transfection of mammalian HeLa cells and intracellular release of pyranine and plasmid DNA, as quantified using a luciferase assay, showing that supramolecular polypeptide nanogels have significant potential in gene therapy applications.

Cross-Linked Pillar[6]arene Nanosponges Fabricated by the Use of a Supra-Amphiphilic Template: Cargo Encapsulation and Overcoming Multidrug Resistance

We report a general fabrication method for water-soluble pillar[n]arene nanosponges (NS) by the use of a supra-amphiphilic template. For the first time, a supra-amphiphilic template is used to conveniently control the size of host molecule-based NS. The intrinsic cavity of water-soluble pillar[6]arene could stably encapsulate dyes (acridine orange and indocyanine green) and antitumor drugs (doxorubicin hydrochloride and mitoxantrone) by host-guest interaction. NS could deliver antitumor drugs to cancer cells. Multidrug resistance (MDR) of cancer cells (MCF-7/ADR) is overcome by the use of NS with a ninefold reduction in the IC₅₀ value compared to that of the free drug (3.4 μM vs 34.4 μM). Mechanistic studies show that stable encapsulation of the antitumor drug is the reason to overcome MDR.

Controlled Cellular Delivery of Amphiphilic Cargo by Redox-Responsive Nanocontainers

The specific transport of amphiphilic compounds such as fluorescently labeled phospholipids into cells is a prerequisite for the analysis of highly dynamic cellular processes involving these molecules, e.g., the intracellular distribution and metabolism of phospholipids. However, cellular delivery remains a challenge as it should not affect the physiological integrity and morphology of the cell membrane. To address this, polymer nanocontainers based on redox-responsive cyclodextrin (CD) amphiphiles are prepared, and their potential to deliver fluorescently labeled phospholipids to intracellular membrane compartments is analyzed. It is shown that mixtures of reductively degradable cyclodextrin amphiphiles and different phospholipids form liposome-like vesicles (CD-lipid vesicles, CSSLV) with a homogeneous distribution of each lipid. Host-guest-mediated self-assembly of a cystamine-crosslinked polymer shell on these CSSLV produces polymer-shelled liposomal vesicles (PSSCSSLV) with the unique feature of a redox-sensitive CSSLV core and reductively degradable polymer shell. PSSCSSLV show high stability and a redox-sensitive release of the amphiphilic cargo. Live cell experiments reveal that the novel PSSCSSLV are readily internalized by primary human endothelial cells and that the reductive microenvironment of the cells' endosomes triggers the release of the amphiphilic cargo into the cytosol. Thus, PSSCSSLV represent a highly efficient system to transport lipid-like amphiphilic cargo into the intracellular environment.

Deciphering and Controlling Structural and Functional Parameters of the Shells in Vesicle-Templated Polymer Nanocapsules

Vesicle-templated nanocapsules are prepared by polymerization of hydrophobic acrylic monomers and cross-linkers in the hydrophobic interior of self-assembled bilayers. Understanding the mechanism of capsule formation and the influence of synthetic parameters on the structural features and functional performance of nanocapsules is critical for the rational design of functional nanodevices, an emerging trend of application of the nanocapsule platform. This study investigated the relationship between basic parameters of the formulation and synthesis of nanocapsules and structural and functional characteristics of the resulting structures. Variations in the monomer/surfactant ratio, temperature of polymerization, and the molar fraction of the free-radical initiators were investigated with a multipronged approach, including shell thickness measurements using small-angle neutron scattering, evaluation of the structural integrity of nanocapsules with scanning electron microscopy, and determination of the retention of entrapped molecules using absorbance and fluorescence spectroscopy. Surprisingly, the thickness of the shells did not correlate with the monomer/surfactant ratio, supporting the hypothesis of substantial stabilization of the surfactant bilayer with loaded monomers. Decreasing the temperature of polymerization had no effect on the spherical structure of nanocapsules but resulted in progressively lower retention of entrapped molecules, suggesting that a spherical skeleton of nanocapsule forms rapidly, followed by filling the gaps to create the structure without pinholes. Lower content of initiators resulted in slower reactions, outlining the baseline conditions for practical synthetic protocols. Taken together, these findings provide insights into the formation of nanocapsules and offer methods for controlling the properties of nanocapsules in viable synthetic methods.

Photochemical preparation of gold nanoparticle decorated cyclodextrin vesicles with tailored plasmonic properties

We report a photochemical strategy for the preparation of plasmonic vesicles by the in situ formation of gold nanoparticles at the surface of cyclodextrin host vesicle templates decorated with photoactive guest polymers. Upon irradiation with UV light, these carefully designed polymer shells undergo a Norrish type I reaction to generate reducing radicals for the in situ reduction of gold salts and simultaneously provide a stabilizing matrix allowing for a dense decoration with discrete gold seeds. In a highly controlled growth procedure the gold particle size can be adjusted between 3 and 28 nm resulting in an increasing interparticle plasmonic coupling as revealed by a pronounced redshift of the surface plasmon resonance (SPR) band and an enhanced absorption at wavelengths above 600 nm. This unique combination of cyclodextrin vesicles capable of specifically recognizing guest molecules with a plasmonic particle shell displaying multiple interparticle gaps acting as electromagnetic hotspots shows great potential for surface-enhanced Raman scattering (SERS) applications.

Controlled and Tunable Loading and Release of Vesicles by Using Gigahertz Acoustics

Controllable exchange of molecules between the interior and the external environment of vesicles is critical in drug delivery and micro/nano‐reactors. While many approaches exist to trigger release from vesicles, controlled loading remains a challenge. Herein, we show that gigahertz acoustic streaming generated by a nanoelectromechanical resonator can control the loading and release of cargo into and from vesicles. Polymer‐shelled vesicles showed loading and release of molecules both in solution and on a solid substrate. We observed deformation of individual giant unilamellar vesicles and propose that the shear stress generated by gigahertz acoustic streaming induces the formation of transient nanopores, with diameters on the order of 100 nm, in the vesicle membranes. This provides a non‐invasive method to control material exchange across membranes of different types of vesicles, which could allow site‐specific release of therapeutics and controlled loading into cells, as well as tunable microreactors.

Formation of Polymeric Particles by Direct Polymerization on the Surface of a Supramolecular Template

Formation of polymeric materials on the surface of supramolecular assemblies is rather challenging because of the often weak noncovalent interactions between the self-assembled template and the monomers before polymerization. We herein show that the introduction of a supramolecular anion recognition motif, the guanidiniocarbonyl pyrrole cation (GCP), into a short Fmoc-dipeptide 1 leads to self-assembled spherical nanoparticles in aqueous solution. Negatively charged diacetylene monomers can be attached onto the surface of these nanoparticles, which, after UV polymerization, leads to the formation of a polymer shell around the self-assembled template. The hybrid supramolecular and polymeric nanoparticles demonstrate intriguing thermal hysteresis phenomena. The template nanoparticles could be disassembled upon treatment with organic base, which cleaved the Fmoc moiety on 1. This strategy thus showed that a supramolecular anion recognition motif allows the post-assembly formation of polymeric nanomaterials from anionic monomers around a cationic self-assembled template.

Controlled Self-Assembly of Multiple-Responsive Superamphiphilc Polymers Based on Host-Guest Inclusions of a Modified PEG with β-Cyclodextrin

Superamphiphilic polymers (SAPs) constructed by host-guest inclusion can self-assemble into various nanostructures in solution, which can find applications in many fields such as nanodevices, drug delivery, and template synthesis. Herein, we report the controlled self-assembly of multiple-responsive SAP based on a selective host-guest inclusion of β-cyclodextrin (β-CD) with a modified poly(ethylene glycol) (PEG) (FcC₁₁AzoPEG) consisting of a ferrocene (Fc) end group, a C₁₁ alkyl chain, an azobenzene (Azo) block, and a poly(ethylene glycol)methyl ether (PEG) chain. These SAPs can self-assemble into interesting nanostructures in water upon exposure to different stimuli because β-CD can be selectively included with different guests, such as Fc, Azo, and C₁₁ alkyl chain, under different stimuli. The inclusion complex of Fc with β-CD (Fc@β-CD SAP) can form nanowire micelles in aqueous solution. The nanowire micelles can be transformed into spindle micelles with the addition of oxidant because the majority of β-CDs dissociated from the complex Fc@β-CD SAP due to a conversion of Fc to Fc⁺ and will preferentially include with Azo group to form another dominant inclusion complex (Azo@β-CD SAP). After UV irradiation, the spindle micelles can be further transformed into spherical micelles because most of β-CDs are excluded from the complex Azo@β-CD SAP due to a trans- to cis-Azo conversion and then form a dominant inclusion complex with C₁₁ alkyl chains (C₁₁@β-CD SAP). This work not only demonstrates the selective host-guest inclusion of stimuli-responsive groups modified PEG with β-CD but also provides a useful approach for construction of diverse morphologies.

A mitochondria-targeting supramolecular photosensitizer based on pillar[5]arene for photodynamic therapy

A mitochondria-targeting supramolecular photosensitizer system TPP-QAS/WP5/DTAB was constructed based on a host-guest inclusion complex. The supramolecular system could efficiently release and activate TPP-QASs in an acidic environment, which have been demonstrated to preferentially accumulate in mitochondria. Singlet oxygen (¹O₂) could be in situ generated in mitochondria under light irradiation, further enhancing the PDT efficacy.

Hollow polymer nanocapsules: synthesis, properties, and applications

Hollow polymer nanocapsules (HPNs) have gained tremendous interest in recent years due to their numerous desirable properties compared to their solid counterparts.

Molecular Recognition and Immobilization of Ligand-Conjugated Redox-Responsive Polymer Nanocontainers

We present the preparation of ligand-conjugated redox-responsive polymer nanocontainers by the supramolecular decoration of cyclodextrin vesicles with a thin redox-cleavable polymer shell that displays molecular recognition units on its surface. Two widely different recognition motifs (mannose-Concanavalin A and biotin-streptavidin) are compared and the impact of ligand density on the nanocontainer surface as well as an additional functionalization with nonadhesive poly(ethylene glycol) is studied. Aggregation assays, dynamic light scattering, and a fluorometric quantification reveal that the molecular recognition of ligand-conjugated polymer nanocontainers by receptor proteins is strongly affected by the multivalency of interactions and the association strength of the recognition motif. Finally, microcontact printing is used to prepare streptavidin-patterned surfaces, and the specific immobilization of biotin-conjugated nanocontainers is demonstrated. As a prototype of a nanosensor, these tethered nanocontainers can sense a reductive environment and react by releasing a payload.

Surveying macrocyclic chemistry: from flexible crown ethers to rigid cyclophanes

This review features the progress made in the development of macrocycles since Pedersen's ground-breaking discovery of the crown ethers in 1967.

Supramolecular Double-Helix Formation by Diastereoisomeric Conformations of Configurationally Enantiomeric Macrocycles

Solid-state superstructures, resulting from assemblies programmed by homochirality, are attracting considerable attention. In addition, artificial double-helical architectures are being investigated, especially in relation to the ways in which homochiral small molecules can be induced to yield helical forms as a result of chiral induction. Herein, we report the highly specific self-assembly upon crystallization of a double-helical superstructure from an enantiopure macrocyclic dimer which adopts two diastereoisomeric conformations in a molar ratio of 1.5:1 in dimethyl sulfoxide. These two conformational diastereoisomers self-organize-and self-sort-in the crystalline phase in equimolar proportions to form two single-handed helices which are complementary to each other, giving rise to the assembly of a double helix that is stabilized by intermolecular [C-H···O] and π-π stacking interactions. The observed self-sorting phenomenon occurs on going from a mixed-solvent system containing two equilibrating conformational diastereoisomers, presumably present in unequal molar proportions, into the solid state. The diastereoisomeric conformations are captured upon crystallization in a 1:1 molar ratio in the double-helical superstructure, whose handedness is dictated by the choice of the enantiomeric macrocyclic dimer. The interconversion of the two conformational diastereoisomers derived from each configurationally enantiomeric macrocycle was investigated in CD₃SOCD₃ solution by variable-temperature ¹H NMR spectroscopy (VT NMR) and circular dichroism (VT CD). The merging of the resonances for the protons corresponding to the two diastereoisomers at a range of coalescence temperatures in the VT NMR spectra and occurrence of the isosbestic points in the VT CD spectra indicate that the two diastereoisomers are interconverting slowly in solution on the ¹H NMR time scale but rapidly on the laboratory time scale. To the best of our knowledge, the self-assembly of such solid-state superstructures from two conformational diastereoisomers of a homochiral macrocycle is a rare, if not unique, occurrence.

Silicon(IV) Phthalocyanine-Decorated Cyclodextrin Vesicles as a Self-Assembled Phototherapeutic Agent against MRSA

The host-guest complexation of a tailored Si(IV) phthalocyanine with supramolecular β-cyclodextrin vesicles (CDV) was studied, revealing a reduced aggregation of the photoactive center upon binding to the CDV, even in aqueous environments. For this purpose, a photosensitizing unit axially decorated with one adamantyl group and one pyridinium moiety on the other side was obtained by two successive click reactions on a bis-azido-functionalized derivative of Si(IV) phthalocyanine. To evaluate its potential as a photosensitizer against antibiotic-resistant bacteria, comparative studies of the photophysical properties including absorption and emission spectroscopy, lifetimes as well as fluorescence and singlet oxygen quantum yields were determined for the Si(IV) phthalocyanine alone and upon self-assembly on the CDV surface. In vitro phototoxicity against the methicillin-resistant Staphylococcus aureus (MRSA) USA300 was evaluated, showing an almost complete inactivation.

Polymer Cages as Universal Tools for the Precise Bottom-Up Synthesis of Metal Nanoparticles

A template synthesis allows the preparation of monodisperse nanoparticles with high reproducibility and independent from self-assembly requirements. Tailor-made polymer cages were used for the preparation of nanoparticles, which were made of cross-linked macromolecules with pendant thiol groups. Gold nanoparticles (AuNPs) were prepared in the polymer cages in situ, by using different amounts of cages versus gold. The polymer cages exhibited a certain capacity, below which the AuNPs could be grown with excellent control over the size and shape. Control experiments with a linear diblock copolymer showed a continuous increase in the AuNP size as the gold feed increased. This completely different behavior regarding the AuNP size evolution was attributed to the flexibility of the polymer chain depending on cross-linking. Moreover, the polymer cages were suitable for the encapsulation of AgNPs, PdNPs, and PtNPs by the in situ method.