Dual catalytic activity of a cucurbit[7]uril-functionalized metal alloy nanocomposite for sustained hydrogen generation: hydrolysis of ammonia borane and electrocatalysts for the hydrogen evolution reaction.

H2 is one of the most attractive fuel alternatives to the existing fossil fuels that cause detrimental environmental issues. Thus, there has been an upsurge in the research on the production of green hydrogen. In this view, cucurbit[7]uril (CB7)-functionalized Co:Ni alloy nanocomposites with different compositions, reported here for the first time, were synthesized to synergise the catalytic activities of a nanoalloy and CB7 and screened for hydrogen generation via hydrolysis of ammonia borane (AB). The (Co85:Ni15)50:(CB7)50 nanocomposite exhibited enhanced catalytic performance for AB hydrolysis even at room temperature as compared to the nanoalloy without CB7. Efficient release of ammonia-free green H2 is ensured by the retention of NH3 by the surface functionalized CB7 macrocycles. For sustained release, a novel and cost-effective procedure was used to regenerate AB from the by-product, and the H2 release activity was verified to be on par with commercial AB. The used nanocomposite magnetically separated from the by-product solution was shown to be an efficient electrochemical catalyst for the hydrogen evolution reaction (HER). The cucurbit[7]uril-functionalized Co:Ni nanocomposite demonstrates remarkable dual catalytic performance to generate clean hydrogen from both the hydrolysis of AB at room temperature and the electrochemical HER, thus opening new avenues in supramolecular chemistry for developing noble metal-free catalysts with high activity and long-term stability.

Supramolecular assembly of coumarin 7 with sulfobutylether-ß-cyclodextrin for biomolecular applications.

Coumarins, in general, exhibit a wide range of photophysical characteristics and are highly sensitive to their microenvironment, and, therefore, their fluorescence characteristics have attracted immense attention as sensors in chemical and biological systems. In the present study, the supramolecular interaction of a bichromophoric coumarin dye, namely, Coumarin 7 (C7) with sulfobutylether-ß-cyclodextrin (SBE7ßCD) macrocyclic host at different pH conditions has been investigated by using optical spectroscopic techniques such as absorption, steady-state and time-resolved emissions, and circular dichroism measurements and compared with that of ßCD. Considerable enhancement in the fluorescence intensity and lifetime of C7 on complexation with SBE7ßCD proposes that non-radiative processes like TICT behavior are strictly hindered due to the confinement in the host cavity experienced by the C7 dye. The increase in the rotational correlation time evaluated from the fluorescence anisotropy decay kinetics further confirms the formation of tightly bound inclusion complexes. The binding constant values reveal that the monocationic form of dye at pH 3 shows ∼3 times stronger interaction with SBE7ßCD than the neutral form of dye at pH 7 due to strong electrostatic cation-anion interaction. SBE7ßCD:C7 exhibits an improved photostability and an upward pK a shift of 0.4 unit compared to the contrasting downward pK a shift of 0.5 with the ßCD. The enhanced fluorescence yield and increased photostability have been exploited for bioimaging applications, and better images were captured by staining the Drosophila fly gut with the SBE7ßCD:C7 complex. The enhancement in the binding interaction and the emission intensity were found to be responsive to external stimuli such as small competitive binders or metal ions and nearly quantitative dissociation of the complex was demonstrated to release the dye and would find stimuli-responsive applications.

Spectroscopic features of a perylenediimide probe for sensing amyloid fibrils: in vivo imaging of Aß-aggregates in a Drosophila model organism.

Customised perylenediimide (PDI) chromophores find diverse applications not only as chemosensors, inorganic-organic semiconductors, photovoltaics, photocatalysts, etc., but also in protein surface engineering, bio-sensors and drug delivery systems. This study focuses on the interaction of a custom synthesized phenylalanine derivatized perylenediimide (L-Phe-PDI) dye with a model protein, insulin, and its structurally distinct fibrils to develop fluorescence sensors for fibrillar aggregates and in vivo imaging applications. Detailed photophysical studies revealed that L-Phe-PDI gets aggregated in the presence of insulin and causes emission quenching at pH 7.4, which in the absence of insulin occurs only at pH ∼2. During in vitro incubation of insulin to its fibrils, the fluorescence intensity of the L-Phe-PDI probe is enhanced to ∼150 fold in a two-stage manner, manifesting the pathways of structural transformation to ß-sheet rich mature fibrils. The in vivo sensing has further been validated in living models of the Aß-mutant Drosophila fly, which is known to develop progressive neurodegeneration comparable to that of human brains with Alzheimer's disease (AD). Bioimaging of the L-Phe-PDI treated Aß-mutant Drosophila documented the blood-brain/blood-retina-barrier cross-over ability of L-Phe-PDI with no toxic effects. Comparison of the fibrillar images from the brain and eye region with the reference thioflavin T (ThT) probe established the uptake of L-Phe-PDI by the aggregate/fibrillar moieties. The samples from L-Phe-PDI-treated flies apparently displayed reduced fibrillar spots, a possible case of L-Phe-PDI-induced disintegration of fibrillar aggregates at large, an observation substantiated by the improved phenotype activities as compared to the untreated flies. The findings reported both in vitro and in vivo with the L-Phe-PDI material for the first time open up avenues to explore the therapeutic potential of custom-designed PDI derivatives for amyloid fibril sensors and bioimaging.

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.

Fibril-induced neurodegenerative disorders in an Aß-mutant Drosophila model: therapeutic targeting using ammonium molybdate.

The ability of polyanionic molybdate to inhibit and degrade protein fibrils both in vitro (insulin protein) and in vivo (Drosophila fly model) has been demonstrated. We establish the disappearance of fibrillar structures and recovery from neurodegenerative disorders in molybdate-treated Aß42-mutant Drosophila flies as compared to the untreated ones, corroborating the therapeutic ability of ammonium molybdate towards the treatment of Alzheimer's disease.

Metal-Free Supramolecular Catalytic Hydrolysis of Ammonia Borane through Cucurbituril Nanocavitands.

Ammonia borane (AB) is considered a potential "on-board" hydrogen storage material. However, its implementation as a hydrogen reservoir in fuel cells is lacking due to the extremely slow release of hydrogen at room-temperature hydrolysis. In this study, a metal-free supramolecular strategy is demonstrated at room temperature to increase the hydrolysis rate and yield of hydrogen along with significant reduction in ammonia release by using cucurbit[5/8]uril (CB5/CB8) nanocavitands as catalysts. The complex of AB with CB stabilizes the ammonium ion at the host portals, which reduces ammonia release and enhances hydrogen yield. The complexation brings down the activation energy of hydrolysis from 103.8 to ∼27.5 kJ mol-1 (for CB5), a value close to the Pt/Pd nanoparticle-based catalysts reported so far. The high catalytic performance and reusability of CB catalysts at very low concentration make AB a promising supramolecular alternative for a sustainable "on-board" energy source.

Supramolecular interaction of sanguinarine dye with sulfobutylether-ß-cyclodextrin: modulation of the photophysical properties and antibacterial activity.

The noncovalent host-guest interaction of sanguinarine (SGR), a benzophenanthridine alkaloid, with a nontoxic, water soluble sulfobutylether-ß-cyclodextrin (SBE7ßCD, commercially available as Captisol) macrocyclic host has been investigated using ground-state optical absorption, and steady-state and time-resolved fluorescence measurements. The pH-dependent changes in the absorbance of the dye at 327 nm showed a pK a value of 7.5, which has been shifted to 8.1 in the presence of SBE7ßCD. The changes in the pK a values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE7ßCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB7ßCD (K = 1.2 × 104 M-1) as compared to the neutral form (K = 3.9 × 103 M-1) which leads to a moderate upward pK a shift (pK a values of SGR shifted by more than 0.6 units). The subsequent fluorescence "turn off" was demonstrated to be responsive to chemical stimuli, such as metal ions (Ca2+ ions). Upon addition of Ca2+ ions, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence "turn on". Apart from improving the stability under ambient light conditions, the upward pK a shift of SGR in the presence of SBE7ßCD results in increasing the antibacterial activity of the SBE7ßCD:SGR complex compared to that of the free dye towards four pathogenic micro-organisms at the physiological pH range. This work further compares SGR interaction with parent ß-cyclodextrin.

Effect of Bi3+ addition on the upconversion luminescence properties of NaYbF4 :Er.

In this study, Bi3+ incorporation in NaYbF4 :Er lattice and its influence on upconversion luminescence properties have been investigated in detail using techniques such as temperature-dependent luminescence, Fourier transform infrared spectroscopy and X-ray diffraction (XRD). The study was carried out to develop phosphors with improved upconversion luminescence. From photoluminescence and lifetime measurements it is inferred that luminescence intensity from NaYbF4 :Er increases with Bi3+ addition. The sample containing 50 at.% Bi3+ ions exhibited optimum upconversion luminescence. Increased distance between Yb3+ -Yb3+ and Er3+ -Er3+ due to Bi3+ incorporation into the lattice and associated decrease in the extent of dipolar interaction/self-quenching are responsible for increase in lifetime values and luminescence intensities from Er3+ ions. Incorporation of Bi3+ into NaYbF4 :Er lattice reduced self-quenching among Yb3+ -Yb3+ ions and this facilitated energy transfer from Yb3+ to Er3+ . This situation also explains decrease in the extent of temperature-assisted quenching of emission from thermally coupled 2 H11/2 and 4 S3/2 levels of Er3+ . Based on Rietveld refinement of XRD patterns it was confirmed that a maximum of 10 at.% of Bi3+ added was incorporated into the NaYbF4 :Er lattice and the remaining complex co-exists as a BiOF phase. These results are of significant interest in the area of development of phosphors based on Yb3+ -Er3+ upconversion luminescence.

Cooperative enhancement of antibacterial activity of sanguinarine drug through p-sulfonatocalix[6]arene functionalized silver nanoparticles.

The amelioration of antibacterial efficacy along with the reduced minimum inhibitory concentration (MIC) of sanguinarine (SGR) drug have been demonstrated through the uptake of SGR by p-sulfonatocalix[6]arene functionalized silver nanoparticles. The large upward pKa shift and enhanced stability of SGR resulting from the favorable supra-nanomolecular strategy are deciphered into an improved antibacterial drug against different pathogenic micro-organisms including multi drug resistant bacteria.

Redox-mediated Negative Differential Resistance (NDR) Behavior in Perylenediimide Derivative: A Supramolecular Approach.

Deaggregated perylenediimide (PDI) derivatives exhibit exceptionally high quantum yields, photostability and appropriate molecular features for organic electronics. This work demonstrates a metal-dye-metal framework with a large and stable negative differential resistance (NDR) at ambient conditions, built using a supramolecular strategy. The deaggregation achieved through the encapsulation of the bay-substituted phenyl groups of aggregated (l/d)-Phe-PDI dyes by the ß-CD macrocyclic host is validated through detailed spectroscopic and imaging techniques. The host-guest interaction resulted in a dramatic enhancement in the emission yield from 0.28 to 0.90. In the thin film deposits, the ß-CD/(l/d)-Phe-PDI complex displayed well-connected sheet-like morphology, whereas the uncomplexed (l/d)-Phe-PDI dye remained as scattered lumps. The large and reversible I-V characteristics displaying strong NDR behavior is attributed to the oxidation/reduction processes involving the rigid π-rich PDI core and is stable at least for about six months at ambient conditions, a promising system for organic electronics applications.

Supramolecular Assembly Induced Emission of Thiazole Orange with Sulfobutylether ß-cyclodextrin: A Stimuli-Responsive Fluorescence Sensor for Tyramine.

Modulation and control of stimuli responsive features of molecular assemblies in organized assemblies/cavitand macrocycles have received immense attention in many areas. In this study, we have established the formation of a discrete molecular assembly of thiazole orange (TO) dyes at the portals of the sulfobutylether ß-cyclodextrin (SBE7 ßCD) macrocycle leading to the evolution of a strong and distinct emission band from aggregated TO. The supramolecular assembly promoted portal aggregation of TO in its 1 : 4 (SBE7 ßCD : TO) composition, characterized by absorption, fluorescence, circular dichroism, ITC and 1 H NMR measurements, was probed to be selectively responsive to tyramine among other biogenic amines/neurotransmitters. For the first time, the different extent of emission quenching of SBE7 ßCD : TO assembly in the presence of biogenic amines/neurotransmitters is translated to achieve a selective on-off fluorescence sensor for the detection of tyramine against other neurotransmitters with a limit-of-detection (LOD) as low as ∼575 nM (79 ppb). The emission features of the assembly with changes in temperature is found to be highly reproducible even after several temperature cycles and is promising to design an optical supramolecular thermometer in the ambient temperature range.

Supramolecular Nanorods of (N-Methylpyridyl) Porphyrin With Captisol: Effective Photosensitizer for Anti-bacterial and Anti-tumor Activities.

Porphyrins, especially the 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin (TMPyP), are well-accepted as photosensitizers due to strong absorption from visible to near-infrared region, good singlet oxygen quantum yields as well as chemical versatility, all of which can be further modulated through planned supramolecular strategies. In this study, we report the construction of supramolecular nanorods of TMPyP dye/drug with captisol [sulfobutylether-ß-cyclodextrin (SBE7ßCD)] macrocycle through host-guest interaction. The availability of four cationic N-methylpyridyl groups favors multiple binding interaction with the captisol host, building an extended supramolecular assembly of captisol and TMPyP. In addition to the spectroscopic characterizations for the assembly formation, the same has been pictured in SEM and FM images as nanorods of ~10 µm in length or more. Complexation of TMPyP has brought out beneficial features over the uncomplexed TMPyP dye; enhanced singlet oxygen yield, improved photostability, and better photosensitizing effect, all supportive of efficient photodynamic therapy activity. The Captisol:TMPyP complex displayed enhanced antibacterial activity toward E. coli under white light irradiation as compared to TMPyP alone. Cell viability studies performed in lung carcinoma A549 cells with light irradiation documented increased cytotoxicity of the complex toward the cancer cells whereas reduced dark toxicity is observed toward normal CHO cells. All these synergistic effects of supramolecular nanorods of Captisol-TMPyP complex make the system an effective photosensitizer and a superior antibacterial and antitumor agent.

A Supramolecular Approach for Enhanced Antibacterial Activity and Extended Shelf-life of Fluoroquinolone Drugs with Cucurbit[7]uril.

The host-guest interactions of a third-generation fluoroquinone, danofloxacin (DOFL), with the macrocyclic host cucurbit[7]uril (CB7) have been investigated at different pH values (~3.5, 7.5, and 10). The photophysical properties have been positively affected, that is, the fluorescence yield and lifetime increased, as well as the photostability of DOFL improved in the presence of CB7. The antibacterial activity of DOFL is enhanced in the presence of CB7, as tested against four pathogenic bacteria; highest activity has been found towards B. cereus and E. coli, and lower activity towards S. aureus and S. typhi. The antibacterial activity of two additional second-generation fluoroquinones, i.e., norfloxacin and ofloxacin, has also been investigated in the absence as well as the presence of CB7 and compared with that of DOFL. In case of all drugs, the minimum inhibitory concentration (MIC) was reduced 3-5 fold in the presence of CB7. The extended shelf-life (antibacterial activity over time) of the fluoroquinone drugs in the presence of CB7, irrespective of four types of bacteria, can be attributed to the enhanced photostability of their CB7 complexes, which can act as better antibiotics with a longer expiry date than uncomplexed DOFL.

Ultra-Bright Rhodamines with Sulfobutylether-ß-Cyclodextrin: A Viable Supramolecular Dye Laser in Aqueous Medium.

Although aqueous dye lasers are much sought after, they have been of no practical use, as laser dyes show a strong tendency for aggregation in water, thus diminishing their optical output. Contributing towards this shortcoming, we studied the noncovalent interactions of two prominent laser dyes, namely, rhodamine 6G and rhodamine B, with a water soluble macrocyclic host, sulfobutylether-ß-cyclodextrin (SBE7 ßCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE7 ßCD host indicated adequate complex formation between dye and host (K∼104  M-1 ). A combination of various photophysical parameters evaluated from measurements such as Job plot, changes in the fluorescence lifetime/anisotropy values, and favorable thermodynamic parameters from isothermal titration calorimetric measurements adjudicated a 1 : 1 stoichiometric complex formation between dye and SBE7 ßCD host. Consequently, SBE7 ßCD prevents dye aggregation/adsorption and present rhodamine dyes in their monomeric forms with enhanced fluorescence yield and brightness. These vital parameters were utilized to optimize and demonstrate cost-effective supramolecular broad-band and narrow-band aqueous dye laser systems with improved lasing efficiencies (∼25 % higher for the SBE7 ßCD : RhB system and ∼10 % higher for SBE7 ßCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.

pH-responsive molecular assemblies of pyridylbutadiene derivative with cucurbit[7]uril.

The interaction between red-emitting pyridinium derivative [4-((1E,3E)-4-(4-(dimethylamino)phenyl)buta-1,3-dien-1-yl)-1-methylpyridin-1-ium] (DABP) and cucurbit[7]uril was investigated using optical absorption, steady-state and time-resolved fluorescence and anisotropy measurements. The changes in the UV/Vis absorption, fluorescence spectral characteristics and lifetime, with a change in pH, reveal efficient binding of CB7 to the dye molecule. The cucurbit[7]uril encapsulated DABP brings ∼1.5 units upward pK a shift. The formation of supramolecular assemblies with 1 : 1 and 2 : 1 host-guest stoichiometries with CB7 at different pH conditions have been verified from 1H NMR, isothermal titration calorimetric studies and geometry optimization calculations. CB7-dye complexation and the ensuing morphological changes were revealed by SEM, AFM and optical microscopy images. This pH-responsive supramolecular assemblies of red-emitting DABP dye can find potential applications in biological imaging, optical pH-sensor and the construction of building blocks for the supramolecular architectures.

Detection, inhibition and disintegration of amyloid fibrils: the role of optical probes and macrocyclic receptors.

Amyloid fibrils are formed by the aberrant aggregation of proteins into highly ordered ß-sheet structures and are believed to be the root cause of several neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Prion diseases, etc. and have been the subject of extensive biochemical, biophysical and clinical studies. Developing methods for the early detection of fibril formation using optical spectroscopic techniques and inhibition/disintegration of amyloid fibrils/plaques by introducing small molecules have been a major challenge to establish a clinically facile therapeutic intervention to combat these neurodegenerative diseases. This feature article provides an account of the recent reports from different research groups, including ours, on the optical detection, and inhibition/disintegration of mature fibrils using fluorescent probes and macrocyclic hosts such as cucurbiturils, calixarenes and cyclodextrins. Site specific or spectrally distinct fluorescence emission from a large number of fluorophores in a broad spectral region has been used to detect the fibrillation of different proteins/peptides, mainly insulin, α-synuclein, transthyretin, barstar, lysozyme, Aß40 peptide, etc. On the one hand, while macrocyclic receptors modify the inter-protein interactions through molecular recognition of amino acid residues leading to the inhibition of amyloid fibrillation, on the other hand, one of the cavitands, p-sulfonatocalixarenes, has been demonstrated to cause the disintegration of mature fibrils, effectively through surface charge interactions, which destabilize the extended fibrillar structure into soluble or fine particles. Beneficially, the presence of extrinsic p-sulfonatocalix[4/6]arenes did not introduce any additional toxicity to the cell viability, which advocates its potential utility as a therapeutic for amyloidosis.

Stimuli-Responsive Cucurbit[7]uril-Mediated BSA Nanoassembly for Uptake and Release of Doxorubicin.

We report the construction of a non-toxic nanoassembly of bovine serum albumin (BSA) protein and the cucurbit[7]uril macrocycle as well as its stimuli-responsive breakage with adamantylamine or pH, which restores the protein structure and recognition properties. The assembly showed efficient loading and controlled release of a standard drug, doxorubicin (DOX), and the same was validated in live cells. The cell viability studies documented that the DOX-loaded assembly mask the cytotoxicity of DOX and the toxicity can be revived at the target on demand, triggering its therapeutic activation. This is found to be more effective in the cancer cells. In addition, such host-assisted protein assemblies are also highly promising for stabilizing/protecting the native protein structure, a viable approach to prevent/inhibit protein misfolding and aggregation.

A Viologen-Perylenediimide Conjugate as an Efficient Base Sensor with Solvatochromic Property.

A viologen-perylenediimide conjugate, denoted PDEV, is prepared for efficient base sensing. The conjugate shows solvatochromic behavior as well. The base sensitivity of viologen is purposefully coupled with the emission property of perylenediimide (PDI) to lower the detection limit. PDEV shows base-sensing ability at the ppb level, which is at least three orders of magnitude lower than those of previously reported sensors. The probe is sensitive toward solvent polarity and generates different shades of colors according to the polarity of the medium (solvent). The photophysical properties show a linear correlation with the solvent polarity, and this makes it an efficient solvatochromic agent. On the other hand, the generation of viologen radical cations by bases affects the aggregation and consequently the absorption and emission behavior of the PDI core. The effect of bases can also be visualized, because the probe generates different colors in the presence of bases, both under normal and under UV light. Organic amines can be detected even in the crystalline state, since the dark red color of the PDEV crystals changes to purple in a reversible fashion on exposure to amine vapors. An easy and practical paper-based tool created by using the probe can efficiently be used to detect solvent polarity and presence of bases optically.