Cucurbit[10]uril-based chemistry

Supramolecular assemblies with aggregation-induced emission for in situ active imaging-guided photodynamic therapy of cancer cells

Photodynamic therapy (PDT) has attracted widespread attention as a novel non-invasive anticancer approach. However, the diminished photosensitivity and limited oxygen exposure caused by the aggregation of traditional photosensitizers greatly impair its overall therapeutic efficacy. Herein, a series of water-soluble aggregation-induced emission luminogens (AIEgens) with triphenylamine as skeleton were synthesized and exhibited bright Near-infrared (NIR) emission and strong reactive oxygen species (ROS) generation. Through host-guest complexation between the multicharged triphenylamine units on AIEgens and cucurbit[10]uril (CB[10]) host molecule, supramolecular nanoassemblies were constructed and exhibited negligible phototoxicity to normal cells due to their limited oxygen contact. In contrast, the efficient release of AIEgens from nanoassemblies through competitive binding of overexpressed peptides in cancer cells with CB[10], enabled the full exploitation of the photosensitivity of AIEgens to produce highly efficient ROS, achieving selective ablation of cancer cells. Moreover, due to the restriction of intramolecular motion (RIM) upon anchored on organelle membranes through electrostatic interactions, the cationic AIEgens with weak fluorescence in physiological environment exhibited intense fluorescence emission, thus realizing imaging-guided PDT. This work may open up an avenue for the development of simple and feasible smart responsive nanomaterials for cancer treatment using supramolecular host-guest complexation strategy.

Supramolecular DNA nanogels through host-guest interaction for targeted drug delivery

DNA hydrogels have been demonstrated with the advantages of good stability, easy modification, and extraordinary biocompatibility, which enables their great application prospects in biosensing, tissue engineering, and biomedicine. Based on the host-guest recognition properties of cucurbit[8]uril (CB[8]), we proposed a general method for constructing functional supramolecular DNA nanogels. Guest molecules have been conjugated into the DNA building units, which could be further crosslinked with CB[8] to construct supramolecular DNA nanogels. At the same time, the aptamer has also been modified into the hydrogel network to achieve cell targeting. These supramolecular DNA nanogels have been demonstrated with a controllable size and multiple stimuli responses, in addition to the excellent biocompatibility, stability and good targeting drug transport ability. Such a host-guest based strategy will provide a molecular library as a "toolbox" for the functionalization of DNA nanogels.

Emerging sensing platforms based on Cucurbit[n]uril functionalized gold nanoparticles and electrodes

Cucurbit[n]urils (CB[n]s, n = 5-8, 10, and 14), synthetic macrocycles with unique host-guest properties, have triggered increasing research interest in recent years. Gold nanoparticles (Au NPs) and electrodes stand out as exceptional substrates for sensing due to their remarkable physicochemical characteristics. Coupling the CB[n]s with Au NPs and electrodes has enabled the development of emerging sensing platforms for various promising applications. However, monitoring the behavior of analytes at the single-molecule level is currently one of the most challenging topics in the field of CB[n]-based sensing. Constructing supramolecular junctions in a sensing platform provides an ideal structure for single-molecule analysis, which can provide insights for a fundamental understanding of supramolecular interactions and chemical reactions and guide the design of sensing applications. This feature article outlines the progress in the preparation of the CB[n] functionalized Au NPs and Au electrodes, as well as the construction and application of supramolecular junctions in sensing platforms, based on the methods of recognition tunneling (RT), surface-enhanced Raman spectroscopy (SERS), single-molecule force spectroscopy (SMFS), and electrochemical sensing (ECS). A brief perspective on the future development of and challenges in CB[n] mediated sensing platforms is also covered.

Synthesis of Naphthalimide Derivatives and Their Luminescence upon Complexation with Cucurbit[n]uril Hosts

A series of naphthalimide derivatives are synthesized and their binding behavior upon complexation with cucurbit[n]urils (CB[n]s) has been investigated. With a heavy atom (bromine) on the naphthalimide core, 4-bromo-1,8-naphthalimide derivatives 1-4 show short room-temperature phosphorescence (RTP) lifetimes with low quantum yields. Their RTP properties are significantly enhanced in the presence of CB[8] or CB[10] both in aqueous solution and solid state owing to the efficient suppression of nonradiative decay and isolation of quenching factors by the rigid cavity of CB[n]. Without the bromine atom, 1,8-naphthalimide derivatives 5 and 6 show strong excimer emission upon complexation with CB[10] accompanied by fluorescence transition from blue to cyan. The fluorescence colors of 4-(dimethylamino)-1,8-naphthalimide derivatives 7 and 8 change from blue to white to yellow with the addition of CB[n]. This host-guest complexation strategy to modulate the luminescence of the luminophore would further broaden the application of luminescent materials.

The host-guest inclusion driven by host-stabilized charge transfer for construction of sequentially red-shifted mechanochromic system

Developing more extensive methods to understand the underlying structure-property relationship of mechanochromic luminescent molecules is demanding but remains challenging. Herein, the effect of host-guest interaction on the mechanochromic properties of organic molecules is illustrated. A series of pyridinium-functionalized triphenylamine derivatives show bathochromic-shifted emission upon mechanical stimulation. These derivatives bind to cucurbit[8]uril to form homoternary host-guest inclusion complexes through host-stabilized intermolecular charge transfer interactions. Remarkably, the homoternary complexes exhibit longer emission than that of free guests in the solid state (even longer than ground guests), and a further bathochromic-shifted emission is observed upon grinding. Additionally, a heteroternary complex constructed through the encapsulation of pyrene (donor) and pyridinium (acceptor) guest pair in cucurbit[8]uril also displays the mechanochromic luminescent property. This work not only discloses the effect of host-guest inclusion on the mechanochromic property of organic molecules, but also provides a principle and a facile way to design the sequentially red-shifted mechanochromic materials.

Supramolecular Chemotherapy with Cucurbit[n]urils as Encapsulating Hosts

The cucurbit[n]urils (CB[n]) belong to the field of relatively young supramolecules which act as containers for a large variety of guests and are being explored extensively for their numerous biomedical applications. This includes drug formulation and delivery, controlled drug release, photodynamic therapy, sensing for bioanalytical purposes, etc. These supramolecular host-guest systems have distinctive recognition properties and have successfully been shown to enhance the in vitro and in vivo utility of various chemotherapeutic agents. The CB[n]s are tailored to optimize their application in payload delivery and diagnostics and in lowering the toxicity of existing drugs. This review has listed the recent studies on working mechanisms and host-guest complexation of the biologically vital molecules with CB[n] and highlighted their implementation in anticancer therapeutics. Various modifications in CB-drug inclusion compounds like CB supramolecular nanoarchitectures along with application in photodynamic therapy, which has shown potential as targeted drug delivery vehicles in cancer chemotherapy, have also been discussed.

A High-Affinity and Removable Iminium Dication Guest for Recycling of Cucurbit[7]uril Materials

An iminium dication 1 was designed as a high-affinity and easily removable guest for cucurbit[7]uril (CB[7]). X-ray crystallographic analysis shows that maximized N⁺···O═C ion-dipole interactions and perfect packing coefficient are responsible for the high affinity of 1 for CB[7] (K_a > 10¹¹ M^-1). Under alkaline conditions, included 1 in CB[7]·1 is hydrolyzed into 2,6-adamantanedione and pyrrolidine that can be fully removed by further extraction, enabling the recycling of CB[7] materials.

Sequential Formation of Heteroternary Cucurbit[10]uril (CB[10]) Complexes

The globular and monocationic guest molecule trimethyl-azaphosphatrane (AZAP, a protonated Verkade superbase) was shown to form a host:guest 1 : 1 complex with the cucurbit[10]uril (CB[10]) macrocycle in water. Molecular dynamics calculations showed that CB[10] adopts an 8-shape with AZAP occupying the majority of the internal space, CB[10] contracting around AZAP and leaving a significant part of the cavity unoccupied. This residual space was used to co-include planar and monocationic co-guest (CG) molecules, affording heteroternary CB[10]⋅AZAP⋅CG complexes potentially opening new perspectives in supramolecular chemistry.

Cucurbit[10]uril-mediated Supramolecular Assembly for Optically Tunable Dimers and Near White-light Emissive Materials

Cucurbit[10]uril (Q[10]), the cucurbit[ n ]uril with the greatest cavity, exhibits several new features in the development of the host-guest complex. Thus, based on Q[10] and π-conjugated molecule, oligo(p-phenylenevinylene) derivative (OPVCOOH), the host-guest complexes with three different interaction ratios of 1:2, 2:2, and 3:2 assemblies (Q[10]: guest) were fabricated. Depending on the host/guest ratio, the emission color of these complexes ranged from blue to yellow-green. The extra Fe 2+ coordinated with a bare carboxyl group of the Q[10]-OPVCOOH (3:2) assembly, obstructing its rotaxane structure and forming Q[10]-OPVCOOH-Fe 2+ assembly, which may be used as a coating for near-white LED bulbs.

CB[10]-driven self-assembly of a homotrimer from a symmetric organic dye: tunable multicolor fluorescence and higher solid-state stability than that of a CB[8]-included homodimer

A symmetric organic dye can form a highly stable homotrimer in the cavity of CB[10], which exhibits unique multicolour fluorescence different from that of the single molecule or its dimer.

Study on the interactions between melamine-cored Schiff bases with cucurbit[n]urils of different sizes and its application in detecting silver ions

Three different complexes, TMeQ[6]-TBT, Q[7]-TBT, and Q[8]-TBT are constructed by three different cucurbiturils and synthesized by guest melamine-cored Schiff bases (TBT) through outer-surface interaction and host-guest interactions. TBT forms a TMeQ[6]-TBT complex with TMeQ[6] through outer-surface interaction, while Q[7]-TBT and Q[8]-TBT form complexes with Q[7,8] through host-guest interactions. Among them, Q[7]-TBT is selected as a UV detector for the detection of silver ions (Ag+). This work makes full use of the characteristics of each cucurbituril and melamine-cored Schiff base to construct a series of complexes and these are applied to metal detection.

Achieving Enhanced Photochromic Properties of Diarylethene through Host-Guest Interaction in Aqueous Solution

Diarylethene (DTE) has been widely used in fluorescence probes, molecular logic gates, optical data-storage devices owing to the excellent photochromic property, while constructing high-performance photochromic DTE in aqueous media remains a big challenge. Herein we present several host-guest systems formed between cucurbit[n]uril (CB[n], n=7, 8, 10) and two water-soluble DTE derivatives 1 and 2. It was found that host-guest interactions not only affect the photophysical properties of photochromic guests, but also make great differences on the photoreaction process. Different host-guest binding behaviors also lead to different effects on the photochromic properties of guests. In the presence of CB[n], both 1 and 2 showed enhanced emission and higher fluorescence quenching ratio at photostationary state. Besides, CB[10]⋅1 exhibited faster response rate in cyclization reaction and better photofatigue resistance than free 1 in aqueous solution, while the supramolecular assembly of (CB[8])_n ⋅(2)_n showed slower response rate in both directions of the reversible photoreaction. Besides, the photofatigue resistance of 2 can be greatly improved through binding with CB[7]. Our results suggest that host-guest interactions could be an efficient way to improve photochromic properties of DTE in aqueous solution.

Tunable White-Light Emissions of Azapyrene Derivatives with Cucurbit[n]uril Hosts in Aqueous Solution

Cucurbit[n]uril (CB[n])-mediated assembly of π-conjugated azapyrene derivatives with rigid aromatic rings as bridging units into optically tunable complexes is reported. Due to the hindrance of rotation of diazapyrene moieties and the enhancement of intramolecular charge transfer of chromophore guests within the cavity of the CB[8] host, color tuning including white-light emission was easily achieved by introducing CB[8] into the guest aqueous solution, therefore suggesting a feasible strategy for the creation of tunable white-light emission materials through CB[n]-based host-guest interactions.

Triple Stack of a Viologen Derivative in a CB[10] Pair

A viologen-phenylene-imidazole (VPI) conjugate, previously shown to be singly complexed by CB[7] and doubly bound by CB[8], is herein shown to form antiparallel triple stacks in water with cucurbit[10]uril (CB[10]), pairwise complexing the guest trimer. The quinary host:guest 2:3 complex showed features assignable to charge-transfer interactions. Under reductive conditions, CB[10] could solubilize a VPI radical, even though CB[10] and reduced VPI are almost insoluble, thereby illustrating a possible new application for CB[10].

Cucurbituril-Oriented Nanoplatforms in Biomedical Applications

Cucucrbituril (CB) belongs to a family of macrocycles that are easily accessible. Their structural specificity provides excellent molecular recognition capabilities, with the ability to be readily chemically modified. Because of these properties, researchers have found CB to be a useful molecular carrier for delivering drug molecules and therapeutic biomolecules. Their significance lies in the fact that CB not only increases the solubility and stability of an encapsulated guest but also provides the possibility to achieve targeted delivery of the guest molecule. Therefore, the emergence of CB undoubtedly provides opportunities for the development of targeted drug delivery in an era where intelligent drugs have attracted considerable attention. It has also been found that CB can enhance fluorescent dyes, allowing the preparation of biosensors with enhanced sensitivity for use in clinical settings. In the present review, the acquisition, properties, and structural modifications of CB are first comprehensively described, and then the value of this macrocycle in applications within the medical field is discussed. In addition, we have also summarized patent applications of CB in this field over recent years, aiming to illustrate the current status of developments of this molecule. Finally, we discuss the challenges faced by CB in the medical field and future trends in its development.

Expected and unexpected photoreactions of 9-(10-)substituted anthracene derivatives in cucurbit[n]uril hosts

By arranging substrates in a "reaction ready" state through noncovalent interactions, supramolecular nanoreactors/catalysts show high selectivity and/or rate acceleration features. Herein, we report the host-guest complexation of 9-(10-)substituted anthracene derivatives (G1-G3) with cucurbit[n]uril (CB[n], n = 8, 10), and the photoreactions of these derivatives in the presence of CB[n] hosts. Both CB[10] and CB[8] showed no obvious effects on the photoreaction of 9,10-disubstituted derivative G1. For G2 and G3, CB[10] operated as either a nanoreactor or catalyst (10%) for the photodimerization of two compounds with high selectivity and high yield. However, although CB[8] formed a 1 : 2 complex with G2, as also observed with CB[10], the photosolvolysis product (9-anthracenemethanol) was obtained quantitatively after photoirradiation of the CB[8]·2G2 complex. This unexpected photosolvolysis was rationalized by a plausible catalytic cycle in which anthracene acts as a photoremovable protecting group (PPG) and the carbonium ion intermediate is stabilized by CB[8].

Host-guest interaction-mediated fabrication of a hybrid microsphere-structured supramolecular hydrogel showing high mechanical strength

The introduction of structured microsphere composites into hydrogels is found to improve their mechanical strength capability. Herein, chitosan microspheres were functionalized with poly(acrylamide-co-1-benzyl-3-vinylimidazolium bromide) (CS-P(AM-G)), which was synthesized through an in situ copolymerization of acrylamide and a guest functional monomer. Supramolecular hydrogels were fabricated by dynamic host-guest interactions between guest units and the host molecule cucurbit[8]uril (CB[8]). Investigations on the mechanical properties of the hydrogels show that the tensile stress and the compress stress of the hydrogels are five times higher than those of CB[8] hydrogels without CS, and the healing efficiency of the hydrogels at room temperature is 88% after 24 h. The results show that CS microspheres serve as both polyfunctional initiating and cross-linking centers, whereas the dynamic host-guest interactions endow the hydrogels with a higher self-healing property. The process provides a novel method for the production of tough and self-healing supramolecular hydrogels with various potential applications.

Water-Soluble 3D Covalent Organic Framework that Displays an Enhanced Enrichment Effect of Photosensitizers and Catalysts for the Reduction of Protons to H2

Covalent organic frameworks (COFs) are emerging porous polymers that have 2D or 3D long-range ordering. Currently available COFs are typically insoluble or decompose upon dissolution, which remarkably restricts their practical implementations. For 3D COFs, the achievement of noninterpenetration, which maximizes their porosity-derived applications, also remains a challenge synthetically. Here, we report the synthesis of the first highly water-soluble 3D COF (sCOF-101) from irreversible polymerization of a preorganized supramolecular organic framework through cucurbit[8]uril (CB[8])-controlled [2 + 2] photodimerization. Synchrotron X-ray scattering and diffraction analyses confirm that sCOF-101 exhibits porosity periodicity, with a channel diameter of 2.3 nm, in both water and the solid state and retains the periodicity under both strongly acidic and basic conditions. As an ordered 3D polymer, sCOF-101 can enrich [Ru(bpy)₃]²⁺ photosensitizers and redox-active polyoxometalates in water, which leads to remarkable increase of their photocatalytic activity for proton reduction to produce H₂.

Emission enhancement of cationic tetraphenylethylene derivatives by encapsulation in a cucurbit[10]uril host in water

Encapsulation of cationic tetraphenylethylene guests in a cucurbit[10]uril host in water results in strong emission enhancement of these guests since the host–guest interaction caused restriction of intramolecular rotation.

Cucurbit[7]uril-anchored polymer vesicles enhance photosensitization in the nucleus

Effective photosensitizers (PSs) are highly desirable in many applications, such as photodynamic therapy and catalytic chemistry. Here, we demonstrated that vesicles of cucurbit[7]uril (Q[7] or CB[7])-anchored polymers enhanced photosensitization in the nucleus. The polyacrylic acid chain spacer triggered Q[7] polymers on the surfaces of the vesicles at a regular distance, thus not only leading to efficient inhibition of the aggregation induced self-quenching of the porphyrin based cationic PS in aqueous solution but also maintaining the PS at high concentration on the nanoscale via stable host-guest interactions. Further experiments indicated that Q[7] polymer based vesicles as a PS loading vehicle had a high penetration depth, entering the nuclei of cancer cells. Therefore, highly enhanced photosensitization and efficient anticancer effects were achieved.

Host-guest interactions in nor-seco-cucurbit[10]uril: novel guest-dependent molecular recognition and stereoisomerism

The unique monomer and excimer fluorescence emissions of pyrene were first exploited as distinctly photophysical signals to identify the possible diastereomers of guests within nor-seco-cucurbit[10]uril (NS-CB[10]) cavities. Further experiments revealed that balancing the hydrophilic and hydrophobic effects of the guest in aqueous solution can improve the molecular recognition and binding ability of NS-CB[10].

Fabrication, characterization and adsorption properties of cucurbit[7]uril-functionalized polycaprolactone electrospun nanofibrous membranes

The fabrication of electrospun nanofibers comprising cucurbit[7]uril (CB[7]) and poly(ε-caprolactone) (PCL) is reported. Various techniques such as SEM, FTIR, XRD, DSC and TG were utilized to characterize the morphology, composition and properties of the nanofibers. Uniform bead-free electrospun nanofibers were obtained from PCL/CB[7] mixed solutions and the average fiber diameter of the nanofibers increases with the increase of CB[7] content. The nanofibers are composed of a physical mixture of PCL and CB[7], and CB[7] itself is present in the PCL fiber matrix in an uncomplexed state. The static adsorption behavior of the PCL/CB[7] nanofibers towards methylene blue (MB) was also preliminary investigated. The results indicate that the adsorption of MB onto the nanofibrous membranes fits the second-order kinetic model and Langmuir isotherm model.

Reversible morphological tuning of DNA–perylenebisdiimide assemblies through host–guest interaction

A new supramolecular host–guest strategy is developed to reversibly control the morphology of DNA–perylenebisdiimide assemblies in aqueous solution.

Controllable Synthesis and Catalytic Performance of Gold Nanoparticles with Cucurbit[n]urils (n = 5⁻8)

A series of gold nanoparticles (AuNPs) was prepared in situ with different cucurbit[n]urils (CB[n]s) in an alkaline aqueous solution. The nanoparticle sizes can be well controlled by CB[n]s (n = 5, 6, 7, 8) with different ring sizes. The packing densities of CB[5⁻8] and free surface area on AuNPs were determined. A direct relationship was found between the ring size and packing density of CB[n]s with respect to the AuNP-catalyzed reduction of 4-nitrophenol in the presence of NaBH₄. The larger particle size and higher surface coverage of bigger CB[n]-capped AuNPs significantly decreased the catalytic activity. Furthermore, this work could lead to new applications that utilize AuNPs under an overlayer of CB[n]s for catalysis, sensing, and drug delivery.

A Highly Selective and Strong Anti-Interference Host-Guest Complex as Fluorescent Probe for Detection of Amantadine by Indicator Displacement Assay

Amantadine (AMA) and its derivatives are illicit veterinary drugs that are hard to detect at very low concentrations. Developing a fast, simple and highly sensitive method for the detection of AMA is highly in demand. Here, we designed an anthracyclic compound (ABAM) that binds to a cucurbit[7]uril (CB[7]) host with a high association constant of up to 8.7 × 10⁸ M⁻¹. The host-guest complex was then used as a fluorescent probe for the detection of AMA. Competition by AMA for occupying the cavity of CB[7] allows ABAM to release from the CB[7]-ABAM complex, causing significant fluorescence quenching of ABAM (indicator displacement assay, IDA). The linear range of the method is from 0.000188 to 0.375 μg/mL, and the detection limit can be as low as 6.5 × 10⁻⁵ μg/mL (0.35 nM). Most importantly, due to the high binding affinity between CB[7] and ABAM, this fluorescence host-guest system shows great anti-interference capacity. Thus, we are able to accurately determine the concentration of AMA in various samples, including pharmaceutical formulations.

Enhancement of metal–metal interactions inside a large-cavity synthetic host in water

A new host–guest strategy is developed to enhance metal–metal interactions between metal–terpyridyl complexes in aqueous solution.

Effects of cucurbit[n]uril (n = 7, 8, 10) hosts on the formation and stabilization of a naphthalenediimide (NDI) radical anion

The binding interactions of a NDI diammonium dichloride salt with cucurbit[n]uril (CB[n], n = 7,8, 10) and the formation process of a NDI radical anion upon photoexcitation of the NDI derivative in the presence of a CB[n] host have been investigated.