Fluorescent dyes and their supramolecular host/guest complexes with macrocycles in aqueous solution

Green Synthesis of Au-NPs on g-C3N4 Hybrid Nanomaterials Based on Supramolecular Pillar[6]arene and Its Applications for Catalysis

Gold nanoparticles (Au NPs) are installed in situ on the surfaces of graphitic carbon nitride (g-C₃N₄) based on supramolecular hydroxylatopillar[6]arene (P6). The Au NPs can be obtained via the redox reaction between HAuCl₄ and P6 without any NH₂-NH₂, NaBH₄, and other reductants, where AuCl₄ ^- is reduced to Au⁰ by the -OH groups in the presence of OH^-, and the -OH groups are oxidized into -COOH. First, P6 is loaded onto the surface of g-C₃N₄ via π-π interaction between P6 and g-C₃N₄, which offers a stabilized and reduced site for in situ anchoring of Au NPs. The hybrid nanomaterial Au-NPs@P6@g-C₃N₄ exhibits higher catalytic capability than the Pd/C catalyst in 4-nitrophenol (4-NP) reduction and methylene blue degradation, which opens a new avenue for designing more efficient hybrid nanomaterials for application in catalysis, sensing, and other fields.

A host-guest strategy for converting the photodynamic agents from a singlet oxygen generator to a superoxide radical generator

Type-I photosensitizers (PSs) generate cytotoxic oxygen radicals by electron transfer even in a hypoxic environment. Nevertheless, the preparation of type-I PSs remains a challenge due to the competition of triplet-triplet energy transfer with O2 (type-II process). In this work, we report an effective strategy for converting the conventional type-II PS to a type-I PS by host-guest complexation. Electron-rich pillar[5]arenes are used as an electron donor and macrocyclic host to produce a host-guest complex with the traditional electron-deficient type-II PS, an iodide BODIPY-based guest. The host-guest complexation promotes intermolecular electron transfer from the pillar[5]arene moiety to BODIPY and then to O2 by the type-I process upon light-irradiation, leading to efficient generation of the superoxide radical (O2 -˙). The results of anti-tumor studies indicate that this supramolecular PS demonstrates high photodynamic therapy efficacy even under hypoxic conditions. This work provides an efficient method to prepare type-I PSs from existing type-II PSs by using a supramolecular strategy.

Cucurbituril-Based Supramolecular Assemblies: Prospective on Drug Delivery, Sensing, Separation, and Catalytic Applications

Precise control over the stimuli-responsive noncovalent interactions operative in a complex molecular system has emerged as a convenient way to realize applications in the detection and sensing of trace analytes, metal ion separation, uptake-release, in situ nanoparticle synthesis, and catalytic activity. This feature article focuses on the attributes and advantages of noncovalent host-guest interactions involving cucurbituril homologues (CBs) with a wide range of organic and inorganic guests, starting from organic dyes to drugs, proteins, surfactants, metal ions, and polyoxometalates. The unique structural features of CBs provide interaction sites for cations at the portals, polyanions at the periphery, and hydrophobic groups in its cavity. The facile complexation and consequent compositional and geometrical arrangements of guests such as naphthalenediimides, coumarins, porphyrins, and triphenylpyrylium ions with the host CBs led to remarkable changes in many molecular properties, especially aggregation, the proton binding and release affinity, and novel emissive dimers, and each of such spectroscopic signatures have been appropriately channeled to drug delivery and activation to improve the antibacterial efficacy and shelf life of drugs by increasing their photostability. Several technological advantages have also been extracted from the interaction of CBs with inorganic guests as well. The interaction of CB7 with the heptamolybdate anion resulted in the precipitation of a hybrid complex material which enabled a convenient separation methodology for the use of clinically pure radioactive ^99mTc in diagnostic applications. Certain cucurbituril-based hybrid materials have been developed for enhanced SO₂ adsorption at low pressures, high-efficiency hydrogen production, and reversible catalytic systems. Thus, this feature article provides a glimpse of the vast potential of cucurbituril homologues with organic and inorganic guests and calls for a dedicated effort to explore supramolecular strategies for better sensors, therapeutics, smart drug delivery modules, and facile devices.

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

Transient Host-Guest Complexation To Control Catalytic Activity

Signal transduction mechanisms are key to living systems. Cells respond to signals by changing catalytic activity of enzymes. This signal responsive catalysis is crucial in the regulation of (bio)chemical reaction networks (CRNs). Inspired by these networks, we report an artificial signal responsive system that shows signal-induced temporary catalyst activation. We use an unstable signal to temporarily activate an out of equilibrium CRN, generating transient host–guest complexes to control catalytic activity. Esters with favorable binding toward the cucurbit[7]uril (CB[7]) supramolecular host are used as temporary signals to form a transient complex with CB[7], replacing a CB[7]-bound guest. The esters are hydrolytically unstable, generating acids and alcohols, which do not bind to CB[7], leading to guest reuptake. We demonstrate the feasibility of the concept using signal-controlled temporary dye release and reuptake. The same signal controlled system was then used to tune the reaction rate of aniline catalyzed hydrazone formation. Varying the ester structure and concentration gave access to different catalyst liberation times and free catalyst concentration, regulating the overall reaction rate. With temporary signal controlled transient complex formation we can tune the kinetics of a second chemical reaction, in which the signal does not participate. This system shows promise for building more complex nonbiological networks, to ultimately arrive at signal transduction in organic materials.

Supramolecular Detoxification of Macromolecular Biotoxin through the Complexation by a Large-Sized Macrocycle

Biotoxins are diverse, complex, and hypertoxic, ultimately serving as grave and lasting menaces to humanity. Here, it is aimed to introduce a new detoxification methodology for macromolecular biotoxin through complexation by a very large macrocycle. A 25-mer peptide isolated from Lycosa erythrognatha spider venom (LyeTxI) is selected as the model macromolecular biotoxin. Quaterphen[4]arene, with a side length of ≈1.6 nm, has a sufficient cavity to bind LyeTxI. Hence, the water-soluble derivative of Quaterphen[4]arene (H) is designed and synthesized. H exhibits an overall host-guest complexation toward LyeTxI, resulting in a considerably high association constant of (7.01 ± 0.18) × 10⁷  m^-1 . This encapsulation of peptide is interesting as traditional macrocycles can only engulf the amino acid residues of peptides due to their limited cavity size. In vitro assay verifies that complexation by H inhibits the interactions of LyeTxI with cell membranes, thereby reducing its cytotoxicity, suppressing hemolysis, and decreasing the release of lactate dehydrogenase. Notably, the intravenous administration of H has a significant therapeutic effect on LyeTxI-poisoned mice, alleviating inflammation and tissue damage, and markedly improving the survival rate from 10% to 80%. An efficient and potentially versatile approach is provided to detoxify macromolecular biotoxins, with giant macrocycle serving as an antidote.

Phosphorescent Host-Guest Complexes on the Basis of Polyhedral Oligomeric Silsesquioxane-Functionalized Metallotweezers

Phosphorescent host-guest systems have attracted considerable attention because of their intriguing properties and diverse applications. In this study, a polyhedral oligomeric silsesquioxane-functionalized gold(III) tweezer receptor has been designed and synthesized. It is capable of sandwiching platinum(II) terpyridine compounds into its cavity with a high noncovalent binding affinity (association constants: ∼10⁵ M^-1 in chloroform). The resulting heterometallic host-guest complexes exhibit enhanced phosphorescent emission compared with those of the individual species in chloroform, thanks to the prevention of vibration and rotation upon noncovalent complexation. They can further assemble into nanospheres in chloroform/diethyl ether (1:9, v/v) owing to phase segregation between the metallotweezer/guest motif and the peripheral polyhedral oligomeric silsesquioxane unit. When terpyridine platinum(II) chloride serves as the complementary guest, the resulting noncovalent system displays an intraligand emission at the individual host-guest complexed state yet excimeric emission at the supramolecular assembled state, yielding the phosphorescent solvatochromic behaviors. Overall, the polyhedral oligomeric silsesquioxane-functionalized metallotweezer combines guest encapsulation and supramolecular assembly capabilities, which provides new avenues for color-tunable phosphorescent materials.

Supramolecular Mitigation of the Cyanine Limit Problem

Currently, there is a substantial research effort to develop near-infrared fluorescent polymethine cyanine dyes for biological imaging and sensing. In water, cyanine dyes with extended conjugation are known to cross over the "cyanine limit" and undergo a symmetry breaking Peierls transition that favors an unsymmetric distribution of π-electron density and produces a broad absorption profile and low fluorescence brightness. This study shows how supramolecular encapsulation of a newly designed series of cationic, cyanine dyes by cucurbit[7]uril (CB7) can be used to alter the π-electron distribution within the cyanine chromophore. For two sets of dyes, supramolecular location of the surrounding CB7 over the center of the dye favors a nonpolar ground state, with a symmetric π-electron distribution that produces a sharpened absorption band with enhanced fluorescence brightness. The opposite supramolecular effect (i.e., broadened absorption and partially quenched fluorescence) is observed with a third set of dyes because the surrounding CB7 is located at one end of the encapsulated cyanine chromophore. From the perspective of enhanced near-infrared bioimaging and sensing in water, the results show how that the principles of host/guest chemistry can be employed to mitigate the "cyanine limit" problem.

Encapsulation of Trimethine Cyanine in Cucurbit[8]uril: Solution versus Solid‐State Inclusion Behavior

Inclusion of polymethine cyanine dyes in the cavity of macrocyclic receptors is an effective strategy to alter their absorption and emission behavior in aqueous solution. In this paper, the effect of the host‐guest interaction between cucurbit[8]uril (CB[8]) and a model trimethine indocyanine (Cy3) on dye spectral properties and aggregation in water is investigated. Solution studies, performed by a combination of spectroscopic and calorimetric techniques, indicate that the addition of CB[8] disrupts Cy3 aggregates, leading to the formation of a 1 : 1 host‐guest complex with an association constant of 1.5×10⁶ M⁻¹. At concentrations suitable for NMR experiments, the slow formation of a supramolecular polymer was observed, followed by precipitation. Single crystals X‐ray structure elucidation confirmed the formation of a polymer with 1 : 1 stoichiometry in the solid state.

Changes in Protonation Sites of 3-Styryl Derivatives of 7-(dialkylamino)-aza-coumarin Dyes Induced by Cucurbit[7]uril

The incorporation of a guest, with different basic sites, into an organized system (host), such as macrocycles, could stabilize, detect, or promote the formation of a certain protomer. In this context, this work aimed to study the influence of cucurbit[7]uril (CB7) on dyes such as 7-(dimethylamino)-aza-coumarins, which have more than one basic site along their molecular structure. For this, three 3-styryl derivatives of 7-(dialkylamino)-aza-coumarin dyes (SAC1-3) were synthesized and characterized by NMR, ESI-HRMS and IR. The spectral behaviour of the SACs in the absence and presence of CB7 was studied. The results showed large shifts in the UV-vis spectrum in acid medium: a hypsochromic shift of ≈5400 cm⁻¹ (SAC1-2) and ≈3500 cm⁻¹ (SAC3) in the absence of CB7 and a bathochromic shift of ≈4500 cm⁻¹ (SAC1-3) in the presence of CB7. The new absorptions at long and short wavelengths were assigned to the corresponding protomers by computational calculations at the density functional theory (DFT) level. Additionally, the binding mode was corroborated by molecular dynamics simulations. Findings revealed that in the presence of CB7 the heterocyclic nitrogen was preferably protonated instead of the dialkylamino group. Namely, CB7 induces a change in the protonation preference at the basic sites of the SACs, as consequence of the molecular recognition by the macrocycle.

Cucurbiturils mimicked by low polarizability solvents with pre-formed cavities: an empirical model to predict hydrocarbon selectivity

Relative binding affinities of a series of nine rigid hydrocarbons towards the cavity formed by a portion of the inner wall of cucurbit[8]uril (CB[8]) and a positive auxiliary guest were determined by competitive ¹⁹F NMR titrations in deuterium oxide. The corresponding free binding energies were corrected by the hydrocarbon computed solvation energies to obtain their free energies of transfer from the gas phase to the CB[8]/auxiliary guest cavity. These energies correlate linearly with the hydrocarbon static polarizabilities, thereby suggesting that the selectivity is driven, perhaps exclusively, by dispersive interactions between the hydrocarbons and the tailor-made cavity, regardless of the degree of unsaturation of the guests. The free energies of transfer also correlate linearly with the energy released upon introduction of the hydrocarbon into a pre-formed cavity extruded from a solvent (benzene) selected to mimic the polarity and polarizability of the CB[8]/auxiliary probe cavity – and this, with a unity slope. Among other features, this empirical model also accurately predicts the relative binding affinities of various rigid hydrocarbons to CB[6] and CB[7], as well as noble gases to CB[5], when the macrocycles are mimicked with pre-formed cavities in perfluorohexane or perfluorohexane/benzene mixtures, both being notoriously non-polar and non-polarizable environments.

Mimicking cucurbiturils with low polarizability solvents and pre-formed cavities allows the in silico prediction of their selectivities towards hydrocarbons and noble gases in aqueous solution.

Effects of Cations on HPTS Fluorescence and Quantification of Free Gadolinium Ions in Solution; Assessment of Intracellular Release of Gd3+ from Gd-Based MRI Contrast Agents

8-Hydroxypyrene-1,3,6-trisulfonate (HPTS) is a small, hydrophilic fluorescent molecule. Since the pKa of the hydroxyl group is close to neutrality and quickly responds to pH changes, it is widely used as a pH-reporter in cell biology for measurements of intracellular pH. HPTS fluorescence (both excitation and emission spectra) at variable pH was measured in pure water in the presence of NaCl solution or in the presence of different buffers (PBS or hepes in the presence or not of NaCl) and in a solution containing BSA. pKa values have been obtained from the sigmoidal curves. Herein, we investigated the effect of mono-, di-, and trivalent cations (Na+, Ca2+, La3+, Gd3+) on fluorescence changes and proposed its use for the quantification of trivalent cations (e.g., gadolinium ions) present in solution as acqua-ions. Starting from the linear regression, the LoD value of 6.32 µM for the Gd3+ detection was calculated. The effects on the emission were also analyzed in the presence of a combination of Gd3+ at two different concentrations and the previously indicated mono and di-valent ions. The study demonstrated the feasibility of a qualitative method to investigate the intracellular Gd3+ release upon the administration of Gd-based contrast agents in murine macrophages.

Colorful Luminescent Magnetic Supraparticles: Expanding the Applicability, Information Capacity, and Security of Micrometer-Scaled Identification Taggants by Dual-Spectral Encoding

(Sub)micrometer-scaled identification (ID) taggants enable direct identification of arbitrary goods, thereby opening up application fields based on the possibility of tracking, tracing, and anti-counterfeiting. Due to their small dimensions, these taggants can equip in principle even the smallest subcomponents or raw materials with information. To achieve the demanded applicability, the mostly used optically encoded ID taggants must be further improved. Here, micrometer-scaled supraparticles with spectrally encoded luminescent and magnetically encoded signal characteristics are reported. They are produced in a readily customizable bottom-up fabrication procedure that enables precise adjustment of luminescent and magnetic properties on multiple hierarchy levels. The incorporation of commonly used magnetic nanoparticles and fluorescent dyes, respectively, into polymer nanocomposite particles, establishes a convenient toolbox of magnetic and luminescent building blocks. The subsequent assembly of selected building blocks in the desired ratios into supraparticles grants for all the flexibility to freely adjust both signal characteristics. The obtained spectrally resolved visible luminescent and invisible magnetic ID signatures are complementary in nature, thus expanding applicability and information security compared to recently reported optical- or magnetic-encoded taggants. Additionally, the introduced ID taggant supraparticles can significantly enhance the coding capacity. Therefore, the introduced supraparticles are considered as next-generation ID taggants.

Enzyme assays with supramolecular chemosensors - the label-free approach

Enzyme activity measurements are essential for many research areas, e.g., for the identification of inhibitors in drug discovery, in bioengineering of enzyme mutants for biotechnological applications, or in bioanalytical chemistry as parts of biosensors. In particular in high-throughput screening (HTS), sensitive optical detection is most preferred and numerous absorption and fluorescence spectroscopy-based enzyme assays have been developed, which most frequently require time-consuming fluorescent labelling that may interfere with biological recognition. The use of supramolecular chemosensors, which can specifically signal analytes with fluorescence-based read-out methods, affords an attractive and label-free alternative to more established enzyme assays. We provide herein a comprehensive review that summarizes the current state-of-the-art of supramolecular enzyme assays ranging from early examples with covalent chemosensors to the most recent applications of supramolecular tandem enzyme assays, which utilize common and often commercially available combinations of macrocyclic host molecules (e.g. cyclodextrins, calixarenes, and cucurbiturils) and fluorescent dyes as self-assembled reporter pairs for assaying enzyme activity.

Supramolecular Fluorescent Probes for the Detection of Reactive Oxygen Species Discovered via High-Throughput Screening

Supramolecular fluorescent probes for the detection of reactive oxygen species (ROSs) are designed based on a pro-guest strategy. Nine commercially available fluorescent dyes, six host molecules, and a pro-guest are used to rapidly generate a library of 54 potential supramolecular probes. These potential supramolecular probes are screened in a high-throughput fashion using a plate reader to discover seven "hits" or workable probes. The mechanism is confirmed to be ROS-induced conversion from a low-binding-affinity pro-guest to a high-binding-affinity guest and the competitive displacement of the encapsulated fluorescent dye. The response to H₂O₂ of four supramolecular probes is found to be concentration-dependent and may be used for quantitative analysis of H₂O₂. The supramolecular probe is selectively responsive toward other oxidative agents, such as NaClO and Na₂SO₃. The cell study shows that supramolecular probes are capable of detecting H₂O₂ in human cancer cells (MCF-7 or HeLa).

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Monitoring Methionine Decarboxylase by Supramolecular Tandem Assay

Methionine is an essential amino acid involved in many physiological and pathological processes. Methionine starvation caused by methionine decarboxylase ( MetDC) degradation becomes a promising strategy for cancer treatment. Multistep colorimetric method, the present approach to monitor the MetDC activity, possesses drawbacks of the complicated process, low accuracy, and poor anti-interference due to indirect detecting. Herein, we report a facile and easy-to-use supramolecular tandem assay (STA) with cucurbit[7]uril and acridine orange reporter pair for the direct and real-time monitoring of MetDC activity. The applicability of this strategy for measuring enzyme-kinetic parameters and screening of inhibitors are also demonstrated. The STA for MetDC activity detection not only provides a feasible method for methionine-related disease diagnosing but also opens a perspective for cancer therapy.

Assembly and Applications of Macrocyclic-Confinement-Derived Supramolecular Organic Luminescent Emissions from Cucurbiturils

Cucurbit[n]urils (Q[n]s or CB[n]s), as a classical of artificial organic macrocyclic hosts, were found to have excellent advantages in the fabricating of tunable and smart organic luminescent materials in aqueous media and the solid state with high emitting efficiency under the rigid pumpkin-shaped structure-derived macrocyclic-confinement effect in recent years. This review aims to give a systematically up-to-date overview of the Q[n]-based supramolecular organic luminescent emissions from the confined spaces triggered host-guest complexes, including the assembly fashions and the mechanisms of the macrocycle-based luminescent complexes, as well as their applications. Finally, challenges and outlook are provided. Since this class of Q[n]-based supramolecular organic luminescent emissions, which have essentially derived from the cavity-dependent confinement effect and the resulting assembly fashions, emerged only a few years ago, we hope this review will provide valuable information for the further development of macrocycle-based light-emitting materials and other related research fields.

Complexation of specific residues by carboxylatopillar[6]arene for improving the zymolytic stability of arginine-containing peptides

A general strategy for improving the zymolytic stability against proteases is reported. Carboxylatopillar[6]arene (CP6A) could effectively bind arginine and arginine-containing peptides, thereby improving the stability of angiotensin peptides in the presence of trypsin by the complexation of the side chain of the arginine residue.

Synthesis of Fluorescent, DNA-Binding Benzo[b]indolonaphthyridinium Derivatives by a Misguided Westphal Condensation

A novel type of azoniahetarene, namely, benzo[b]indolonaphthyridinium, was unexpectedly formed by the reaction of N-alkylated β-carbolinium derivatives and the enolizable 1,2-cyclohexadione under typical conditions of a Westphal reaction. The products exhibit high fluorescence intensities in polar solvents (Φ_fl = 0.52-0.67) and bind to DNA by intercalation with high affinity (K_b = 1.5 × 10⁶ M^-1). Furthermore, under the same conditions, DNA-binding sempervirine derivatives were synthesized in a Westphal reaction from 1,2-diketones that have at least one non-enolizable α-carbon atom, which shows that the reaction pathway is determined by the substrate structure.

Cascade aryne insertion/vinylogous aldol reaction of vinyl-substituted β-keto/enol carbonyls

Cyclic and acyclic vinyl substituted β-keto/enol carbonyl substrates, on reaction with arynes, result in differentially substituted naphthyl carbocycles, hitherto difficult to synthesize with existing protocols. While the substitutions on the arynes have no role, the ring size of the cyclic β-keto/enol esters has a profound influence on the product formation.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Linked Rotaxane Structure Restricts Local Molecular Motions in Solution to Enhance Fluorescence Properties of Tetraphenylethylene

The restriction of local molecular motions is critical for improving the fluorescence quantum yields (FQYs) and the photostability of fluorescent dyes. Herein, we report a supramolecular approach to enhance the performance of fluorescent dyes by incorporating a linked rotaxane structure with permethylated α-cyclodextrins. Tetraphenylethylene (TPE) derivatives generally exhibit low FQYs in solution due to the molecular motions in the excited state. We show that TPE with linked rotaxane structures on two sides displays up to 15-fold higher FQYs. Detailed investigations with variable temperature ¹ H NMR, UV-Vis, and photoluminescence spectroscopy revealed that the linked rotaxane structure rigidifies the TPE moiety and thus suppresses the local molecular motions and non-radiative decay. Moreover, the linked rotaxane structure enhances the FQY of the dye in various solvents, including aqueous solutions, and improves the photostability through the inhibition of local molecular motions in the excited TPE.

A hypoxia-responsive supramolecular formulation for imaging-guided photothermal therapy

Photothermal agents (PTAs) based on organic small-molecule dyes emerge as promising theranostic strategy in imaging and photothermal therapy (PTT). However, hydrophobicity, photodegradation, and low signal-to-noise ratio impede their transformation from bench to bedside. In this study, a novel supramolecular PTT formulation by a stimuli-responsive macrocyclic host is prepared to overcome these obstacles of organic small-molecule PTAs. Methods: Sulfonated azocalix[4]arene (SAC4A) was synthesized as a hypoxia-responsive macrocyclic host. Taking IR780 as an example, the supramolecular nanoformulation IR780@SAC4A was constructed by grinding method, and its solubility, photostability, and photothermal conversion were evaluated. The hypoxia tumor-selective imaging and supramolecular PTT of IR780@SAC4A were further evaluated in vitro and in vivo. Results: IR780@SAC4A is capable of enhancing the solubility, photostability, and photothermal conversion of IR780 significantly, which achieve this supramolecular formulation with good imaging-guided PTT efficacy in vitro and in vivo. Conclusions: This study demonstrates that the supramolecular PTT strategy is a promising cancer theranostic method. Moreover, this supramolecular approach is applicative to construct kinds of supramolecular PTAs, opening a general avenue for extending smart PTT formulations.

Photomodulation of ultrastable host–guest complexes in water and their application in light-controlled steroid release

Host–guest complexation of dithienylethene photoswitches with cucurbit[8]uril leads to photoresponsive binding pairs with picomolar affinity in water.

Caged bulky organic dyes in a polyaromatic framework and their spectroscopic peculiarities

Here we report that the open large cavity of an M2L4 polyaromatic cage can efficiently bind sterically demanding organic dyes with coumarin, perylene bisimide, and porphyrin cores in aqueous solution. The spectroscopic properties of the caged dyes are largely modulated in the cavity.

Molecular recognition and spectral tuning of organic dyes in water by amide naphthotubes

Molecular recognition and spectral tuning of organic dyes were achieved in water by amide naphthotubes. The association affinity is up to 4.5 × 107 M−1, and great fluorescence enhancement was observed for styryl derivatives.

Effective taste masking of alkaloids by a water-soluble terphen[3]arene

The synthesis of first water-soluble 2,2’’,4,4’’-terphen[3]arene bearing sulphonatopropoxy moieties (STP3), and its effective alleviation of aversive response to alkaloids in vitro and in vivo by forming supramolecular complexes has been...

Rescuing the solid-state fluorescence of perylene diimide dyes by host–guest isolation

A host molecule with an open and flexible backbone was synthesized and is capable of recognizing various perylene diimide dyes. The host exhibits unique universality in improving the solid-state fluorescence of perylene diimide dyes.

Nonionic star polymers with upper critical solution temperature in aqueous solutions

Novel UCST star poly(2-ureido methacrylates) synthesized via the ARGET ATRP technique showed enhanced trapping abilities of model drug molecules.

Host–guest complexation of APP+ with cucurbit[7]uril. Theoretical and experimental studies on the supramolecular inhibition of its cytotoxicity on SERT

The supramolecular binding behavior of APP+ and CB[7] in aqueous solution was studied by different techniques. APP+ showed cytotoxicity towards HEK293 cells expressing hSERT. This cytotoxicity was inhibited by the treatment of the cells with CB[7].

Understanding Self-Assembled Pseudoisocyanine Dye Aggregates in DNA Nanostructures and Their Exciton Relay Transfer Capabilities

Progress has been made using B-form DNA duplex strands to template chromophores in ordered molecular aggregates known as J-aggregates. These aggregates can exhibit strong electronic coupling, extended coherent lifetimes, and long-range exciton delocalization under appropriate conditions. Certain cyanine dyes such as pseudoisocyanine (PIC) dye have shown a proclivity to form aggregates in specific DNA sequences. In particular, DX-tiles containing nonalternating poly(dA)-poly(dT) dinucleotide tracks (AT-tracks), which template noncovalent PIC dye aggregates, have been demonstrated to exhibit interesting emergent photonic properties. These DNA-based aggregates are referred to as J-bits for their similarity to J-aggregates. Here, we assemble multifluorophore DX-tile scaffolds which template J-bits into both contiguous and noncontiguous linear arrays. Our goal is to understand the relay capability of noncontiguous J-bit arrays and probe the effects that orientation and position have on the energy transfer between them. We find that linearly contiguous J-bits can relay excitons from an initial AlexaFluor 405 donor to a terminal AlexaFluor 647 acceptor across a distance of up to 16.3 nm. We observed a maximum increase in energy transfer of 41% in the shortest scaffold and an 11% increase in energy transfer across the maximum distance. However, in nonlinear arrays, exciton transfer is not detectable, even when off-axis J-bit-to-J-bit transfer distances were <2 nm. These results, in conjunction with the previous work on PIC-DNA systems, suggest that PIC-DNA-based systems may currently be limited to simple 1-D designs, which prevent isolating J-bits for enhanced energy-transfer characteristics until further understanding and improvements to the system can be made.

Supramolecular capture of highly polar amidosquaraine dye in water with nanomolar affinity and large turn-on fluorescence

A supramolecular dye-capture system comprising anionic amidosquaraine guest and macrocyclic tetralactam host exhibits nanomolar affinity and "turn on" visible fluorescence. Utility is demonstrated with a new fluorescent assay for liposome leakage induced by the biomedically important enzyme phospholipase A₂.

Enantioselective recognition of chiral acids by supramolecular interactions with chiral AIEgens

Novel chiral AIEgens bearing optically pure amino groups were synthesized and showed excellent discrimination for a series of chiral acidic compounds and amino acids. Interestingly, after supramolecular assembly with 4-sulfocalix[4]arene, the obtained complexes showed enhanced enantioselectivity for chiral acids.

Photoinduced Electron Transfer in Encapsulated Heterocycles by Cavitands

Host-guest complexation of small heterocyclic (guest) and macrocyclic cavitands (hosts) organic molecules is still to date a very popular, inexpensive approach that bypasses the burdens of conventional covalent synthesis. Understanding the selection criteria of these chemicals is crucial to the design and potential applications of their supramolecular assemblies. This review surveys examples within the last 15 years (2005-2020) of supramolecular complexes in which the interacting photoinduced electron transfer (PET)-based chromophore and quencher fragments are commonly used in the market with reported CAS numbers. It appears from this survey that the supramolecular effects can be directed to specifically disrupt PET when the nonemissive macrocycles separately encapsulate the fluorescent acceptor or donor molecules, among other specific factors, such as when inducing conformational changes or pK_a shift of the donor. On the contrary, synergetic encapsulation of both donor and acceptor molecules, formation of ternary self-assembly at the rim or encapsulation of one component while grafting the other onto the macrocycle, among other specific factors such as the modulation of the excited-state structure of donor, will lead to the enhancement of PET process. In the event the donor or acceptor molecules have multitopic structures, the PET process can repeatedly be switched on and off. It is generally concluded that understanding the criteria for the combination of these available products for the purpose of manipulating their PET efficiency should pave the way for the facile alternative generation of new noncovalently bonded host-guest supramolecular assemblies with a more specific design tailored for more advanced, diverse and economic applications such as chemical sensing, molecular gates, drug delivery and biolabeling.

An overview from simple host-guest systems to progressively complex supramolecular assemblies

Supramolecular chemistry involving macrocyclic hosts is a highly interdisciplinary and fast-growing research field in chemistry, biochemistry, and materials science. Host-guest based supramolecular assemblies, as constructed through non-covalent interactions, are highly dynamic in nature, and can be tuned easily using their responses to various external stimuli, providing a convenient approach to achieve excellent functional materials. Macrocyclic hosts, particularly cyclodextrins, cucurbit[n]urils, and calix[n]arenes, which have unique features like possessing hydrophobic cavities of different sizes, along with hydrophilic external surfaces, which are also amenable towards easy derivatizations, are versatile cavitands or host molecules to encapsulate diverse guest molecules to form stable host-guest complexes with many unique structures and properties. Interestingly, host-guest complexes possessing amphiphilic properties can easily lead to the formation of various advanced supramolecular assemblies, like pseudorotaxanes, rotaxanes, polyrotaxanes, supramolecular polymers, micelles, vesicles, supramolecular nanostructures, and so on. Moreover, these supramolecular assemblies, with varied morphologies and responsiveness towards external stimuli, have immense potential for applications in nanotechnology, materials science, biosensors, drug delivery, analytical chemistry and biomedical sciences. In this perspective, we present a stimulating overview, discussing simple host-guest systems to complex supramolecular assemblies in a systematic manner, aiming to encourage future researchers in this fascinating area of supramolecular chemistry to develop advanced supramolecular materials with superior functionalities, for their deployment in diverse applied areas.

Cyclodextrin-modified graphene quantum dots as a novel additive for the selective separation of bioactive compounds by capillary electrophoresis

Highly reliable separation and determination of various biologically active compounds were achieved using capillary electrophoresis (CE) based on β-cyclodextrin-functionalized graphene quantum dots (βcd-GQDs) as the background electrolyte additive. βcd-GQDs improve the separation efficiency between peaks of all analytes. No addition of surfactants or organic solvents was needed in the running buffer containing βcd-GQDs. Up to eight consecutive runs were acquired with high precision for the separation of resveratrol, pyridoxine, riboflavin, catechin, ascorbic acid, quercetin, curcumin, and even of several of their structural analogs. Baseline separation was achieved within just 13 min as a result of the effective mobility of the analytes along the capillary owing to the differential interaction with the additive. The proposed analytical method displayed a good resolution of peaks for all species selecting two absorption wavelengths in the diode array detector. Detection limits lower than 0.28 µg mL^-1 were found for all compounds and precision values were in the range of 2.1-4.0% in terms of the peak area of the analytes. The usefulness of the GQD-assisted selectivity-enhanced CE method was verified by the analysis of food and dietary supplements. The applicability to such complex matrices and the easy and low-cost GQD preparation open the door for routine analyses of food and natural products. The concept of using such a dual approach (macromolecules and nanotechnology) has been explored to tackle the separation of various bioactive compounds in nutritional supplements and food. Schematic illustration of the electrophoretic separation of the bioactive molecules in the capillary which is filled with the running solution without (top) and with βcd-GQDs (bottom). The fused silica capillary with negatively ionizable silanol groups at the wall. The voltage is applied at positive polarity at the outlet. R, riboflavin; r, resveratrol; P, pyridoxine; C, catechin; c, curcumin; A, ascorbic acid; Q, quercetin.

"Turn-Off" Supramolecular Fluorescence Array Sensor for Heavy Metal Ion Identification

A "turn-off" supramolecular fluorescence array sensor based on the host-guest complexes between fluorescence dyes and cucurbit[n]urils for sensing metal ions was developed. Three fluorescent probes (RhB@Q[7], H33342@2Q[7], and BRE@Q[7]) were used as the sensing units to construct a supramolecular fluorescence array sensor. The binding ability of the metal ions and cucurbituril-dye probes varied; therefore, the probes and metal ions produced different fluorescence responses. When combined with linear discriminant analysis (LDA), the qualitative and quantitative detection of seven metal ions was achieved. In analytical samples, the supramolecular fluorescence array sensor recognized and distinguish seven metal ions. These results provided new research ideas for the rapid analysis and real-time monitoring of different heavy metal ions.

Supramolecular Drug Delivery System from Macrocycle-Based Self-Assembled Amphiphiles for Effective Tumor Therapy

Intelligent drug delivery systems (DDSs) that can improve therapeutic outcomes of antitumor agents and decrease their side effects are urgently needed to satisfy special requirements of treatment of malignant tumors in clinics. Here, the fabrication of supramolecular self-assembled amphiphiles based on the host-guest recognition between a cationic water-soluble pillar[6]arene (WP6A) host and a sodium decanesulfonate guest (G) is reported. The chemotherapeutic agent doxorubicin hydrochloride (DOX) can be encapsulated into the formed vesicle (G/WP6A) to construct supramolecular DDS (DOX@G/WP6A). WP6A affords strong affinities to G to avoid undesirable off-target leakage during delivery. Nanoscaled DOX@G/WP6A is capable of preferentially accumulating in tumor tissue via enhanced permeability and retention (EPR) effect. After internalization by tumor cells, the abundant adenosine triphosphate (ATP) binds competitively with WP6A to trigger the disintegration of self-assembled vesicles with the ensuing release of DOX. In vitro and in vivo research confirmed that DOX@G/WP6A is not only able to promote antitumor efficacy but also reduce DOX-related systemic toxicity. The above favorable findings are ascribed to the formation of ternary self-assembly, which profits from the combination of the factors of the EPR effect and the ATP-triggered release.