Self-Assembly of Short Chain Poly- N-isopropylacrylamid Induced by Superchaotropic Keggin Polyoxometalates: From Globules to Sheets

Cluster-directed ionic framework supramolecular hydrogel with high-temperature tolerability and enhanced water evaporation

Supramolecular hydrogels have been constructed with flexible 2D ionic framework assemblies comprised of a stick-shaped di-cationic pseudo-rotaxane and polyanionic nanoclusters through electrostatic interactions. This type of small-molecule hydrogel exhibits excellent thermal stability at high temperature and shows an efficient reduction of water evaporation enthalpy.

Boron cluster leveraged polymeric building blocks

Boron cluster compounds (BCCs) are inorganic molecules characterized by their unique physical and chemical properties. Polymeric materials incorporating BCCs exhibit significant chemical and thermal stability, making them valuable for applications in biomedical fields, energy storage, ultrahigh stability materials, and π-conjugated luminochromic polymers. This review article aims to explore the primary methods for integrating these distinctive clusters into traditional carbon-based polymers. Both boron and carbon atoms possess catenation abilities, enabling the formation of extensive macromolecular structures. While carbon forms long linear chains, boron typically leads to three-dimensional polyhedral clusters. We first examine hybrid nanostructures, focusing on weak non-covalent interactions such as dihydrogen bonding, hydrophobic, and chaotropic effects between boron clusters and polymer chains. We then discuss classical chemical bonding approaches. Despite their inorganic nature, boron clusters can undergo exoskeletal substitution akin to organic counterparts, allowing their attachment as side groups to polymer repeating units. Additionally, polyhedral boron clusters can be incorporated into polymer backbones primarily through polycondensation reactions, resulting in hybrid macromolecules with exceptional physical and chemical attributes. Finally, we summarize the applications of BCC-containing polymeric materials, including their use in boron neutron capture therapy (BNCT), solid polymer electrolytes (SPEs) for metal ion batteries, and as electron acceptor groups in stimuli-responsive luminescent materials. In summary, BCC-containing polymeric materials are increasingly considered viable alternatives to traditional hydrocarbon-based polymers for biomedical applications, ion-conducting materials, luminescent materials, and temperature-resistant materials.

Unraveling the Role of Polyoxometalates-Based Superchaotropes on the Photophysics of Organic Molecules and Modulation of Water Dynamics in a Hydrophilic Block Copolymer Solution

Polyoxometalates (POMs) are composed of nanometric metal-oxide anions and have rich solution chemistry. In this class, Keggin POMs have been identified as the most influential inorganic additives for aqueous nonionic soft matter systems. POMs being at the borderline of classical ions and charged colloids possess fascinating solution properties; the present work aims to delve deeper into the interactions between nanoions and nonionic soft matters from a spectroscopic point of view. Our studies reveal that although of the same structural makeup, silicotungstic acid hydrate (SiW) and phosphotungstic acid hydrate (PW) affect the photophysics of Coumarin-480 (C-480) in poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) ((PEO)76-(PPO)30-(PEO)76, F-68) copolymeric media in a dissimilar manner. From time-resolved studies, we find a preference for SiW toward the intramolecular charge transfer state of C-480, whereas PW favors the locally emissive state of the probe. Further, from rotational relaxation studies, it appears that SiW renders a rigid environment around the probe molecule, while PW relaxes the copolymeric environment. Finally, the dynamic quenching mechanism of the added nanoions was unraveled, which showed a straightforward Förster mechanism for SiW but a short-range interaction was operative for PW. From Fourier transform infrared and 1H NMR, it can be concluded that both the nanoions interacted with the PPO moiety of the copolymer; yet, their contrasting effect on the photophysics has been rationalized as a consequence of charge density on the ions.

Controlling Protein Assembly with Superchaotropic Nano-Ions

In living systems, protein assemblies have essential functions, serving as structural supports, transport highways for molecular cargo, and containers of genetic material. The construction of protein assemblies, which involves control over space and time, remains a significant challenge in biotechnology. Here, we show that anionic boron clusters, 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] (COSAN-), and halogenated closo-dodecarboranes (B12X12 2-, X=H, Cl, or I), described as super-chaotropic nano-ions, induce the formation of 2D assemblies of model proteins, myoglobin, carbonic anhydrase, and trypsin inhibitor. We found that the nano-ion concentration reversibly controls the size of the protein assemblies. Furthermore, the secondary structures of the proteins are only slightly affected by assembly formation. For myoglobin, the formation of these assemblies even prevents temperature denaturation, highlighting a preservation effect of nano-ions. Our study reveals that inorganic boron-based nano-ions act as a reversible molecular glue for proteins, providing a potential starting point for the further development of controlled protein assemblies.

Cascading CRISPR/Cas and Nanozyme for Enhanced Organic Photoelectrochemical Transistor Detection with Triple Signal Amplification

Innovative signal amplification and transduction play pivotal roles in bioanalysis. Herein, cascading CRISPR/Cas and the nanozyme are integrated with electronic amplification in an organic photoelectrochemical transistor (OPECT) to enable triple signal amplification, which is exemplified by the miRNA-triggered CRISPR/Cas13a system and polyoxometalate nanozyme for OPECT detection of miRNA-21. The CRISPR/Cas13a-enabled release of glucose oxidase could synergize with peroxidase-like SiW12 to induce catalytic precipitation on the photogate, inhibiting the interfacial mass transfer and thus the significant suppression of the channel current. The as-developed OPECT sensor demonstrates good sensitivity and selectivity for miRNA-21 detection, with a linear range from 1 fM to 10 nM and an ultralow detection limit of 0.53 fM. This study features the integration of bio- and nanoenzyme cascade and electronic triple signal amplification for OPECT detection.

Polyoxometalate-Polymer Directed Macromolecular Architectonics of Silver Nanoparticles as Effective Antimicrobials

A novel inorganic-organic-inorganic ternary bioactive material formulated on antimicrobial peptide-based polymer has been reported. Supramolecular approach has been employed to incorporate molecularly crowded tyrosine-based polymer stabilized silver nanoparticles into membrane bound vesicles exploiting polyoxometalate-triggered surface templating strategy. Utilizing the covalent reversible addition fragmentation chain transfer (RAFT) polymerization and exploiting templated supramolecular architectonics at biopolymer interface, the bioactive ternary polymeric nanohybrids have been designed against Shigellosis leveraging the antibacterial activities of silver nanoparticle, cationic amphiphilic tyrosine polymer and inorganic polyoxometalate. The detail investigation against Shigella flexneri 2a cell line demonstrates that the collaborative mechanism of the ternary hybrid composite enhances the bactericidal activity in comparison to only polyoxometalate and polymer stabilized silver nanoparticle with an altered mechanism of action which is established via detailed biological analysis.

The effect of host size on binding in host-guest complexes of cyclodextrins and polyoxometalates

Harnessing flexible host cavities opens opportunities for the design of novel supramolecular architectures that accommodate nanosized guests. This research examines unprecedented gas-phase structures of Keggin-type polyoxometalate PW12O40 3- (WPOM) and cyclodextrins (X-CD, X = α, β, γ, δ, ε, ζ) including previously unexplored large, flexible CDs. Using ion mobility spectrometry coupled to mass spectrometry (IM-MS) in conjunction with molecular dynamics (MD) simulations, we provide first insights into the binding modes between WPOM and larger CD hosts as isolated structures. Notably, γ-CD forms two distinct structures with WPOM through binding to its primary and secondary faces. We also demonstrate that ε-CD forms a deep inclusion complex, which encapsulates WPOM within its annular inner cavity. In contrast, ζ-CD adopts a saddle-like conformation in its complex with WPOM, which resembles its free form in solution. More intriguingly, the gas-phase CD-WPOM structures are highly correlated with their counterparts in solution as characterized by nuclear magnetic resonance (NMR) spectroscopy. The strong correlation between the gas- and solution phase structures of CD-WPOM complexes highlight the power of gas-phase IM-MS for the structural characterization of supramolecular complexes with nanosized guests, which may be difficult to examine using conventional approaches.

Toward Distinguishing between the Superchaotropic and Hydrophobic Characters of Nanometric-Sized Ions in Interaction with PEGylated Surfaces

In this study, we explore the superchaotropic effect of various polyoxometalate or boron cluster nano-ions on hydrophilic neutral surfaces. Nano-ions, characterized by low charge densities, exhibit strong adsorption on non-ionic hydrophilic surfaces like PEGylated micelles. This adsorption phenomenon was attributed to the enthalpically favorable dehydration of nano-ions, the so-called superchaotropic effect. Here, we investigate the adsorption of three nano-ions, α-SiW12O404-, α-PW12O403-, and B12I122-, with decreasing charge density or increasing superchaotropicity (or hydrophobicity), on hydrophilic solid surfaces, PEGylated gold nanoparticles, and PEGylated gold-coated quartz crystal. Solid surfaces are devoid of hydrophobic regions, enabling the study of the subtle nuance between hydrophobic and superchaotropic effects. Unlike adsorption on PEGylated micelles, the adsorption constant decreases with a reduced charge density, aligning with the well-established principle that hydrophobic ions do not adsorb on hydrophilic surfaces. This research improves our understanding of the subtle difference between superchaotropic and hydrophobic effects in nano-ion adsorption phenomena.

Nonionic Microgels Adapt to Ionic Guest Molecules: Superchaotropic Nanoions

Microgels are commonly applied as solute carriers, where the size, density, and functionality of the microgels depend on solute binding. As representatives for ionic solutes with high affinity for the microgel, we study here the effect of superchaotropic Keggin polyoxometalates (POMs) PW12O403- (PW) and SiW12O404- (SiW) on the aqueous swelling and internal structure of nonionic poly(N-isopropylacrylamide) (pNiPAM) microgels by light scattering techniques and small-angle X-ray scattering. Due to their weak hydration, these POMs bind spontaneously to the microgels at millimolar concentrations. The microgels thus become charged and swell at low POM concentration, surprisingly without strongly increasing the volume phase transition temperature, and deswell at higher POM concentration. The swelling arises because of the osmotic pressure of dissociated counterions of the POMs, while the deswelling is due to POMs acting as physical cross-links in the microgels under screened electrostatics in NaCl or excess POM solution. This swelling/deswelling transition is sharper for PW than for SiW related to the lower charge density, weaker hydration, and stronger binding of PW. The POMs elicit qualitatively and quantitatively different swelling effects from ionic surfactants and classical salts. Moreover, the network softness and topology govern the swelling response upon POM binding. The softer the microgel, the stronger is the swelling response, while, inside the microgel, regions of high polymer density swell/contract more upon electric charging/cross-linking than regions with low polymer density. POM binding thus enables fine-tuning of microgel properties and highlights the role of network topology in microgel swelling. Because POMs decompose at an alkaline pH, these POM/microgel systems also exhibit pH-responsive swelling in addition to the typical temperature responsiveness of pNiPAM microgels.

Large anion binding in water

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

Hofmeister Effects Shine in Nanoscience

Hofmeister effects play a crucial role in nanoscience by affecting the physicochemical and biochemical processes. Thus far, numerous wonderful applications from various aspects of nanoscience have been developed based on the mechanism of Hofmeister effects, such as hydrogel/aerogel engineering, battery design, nanosynthesis, nanomotors, ion sensors, supramolecular chemistry, colloid and interface science, nanomedicine, and transport behaviors, etc. In this review, for the first time, the progress of applying Hofmeister effects is systematically introduced and summarized in nanoscience. It is aimed to provide a comprehensive guideline for future researchers to design more useful Hofmeister effects-based nanosystems.

When Ions Defy Electrostatics: The Case of Superchaotropic Nanoion Adsorption

Nanometer-sized anions, like polyoxometalates and borate clusters, bind to nonionic hydrated matter driven by the chaotropic effect, which arises from the favorable dehydration of the ions. Herein, we evaluate the adsorption and activity coefficient of the superchaotropic Keggin polyoxometalate SiW₁₂O₄₀^4- (SiW) on nonionic surfactant (C₈E₄) micelles by modeling small-angle X-ray and neutron-scattering spectra. Neither hard sphere nor electrostatic repulsion models reproduce the experimental activity coefficient of adsorbed SiW ions on the micelles. However, the activity and binding of SiW on the micelles is well-described by a Langmuir adsorption isotherm. These results imply that adsorbed SiW ions are non-interacting and "create" around themselves adsorption sites on the micelle. The temperature dependence of the adsorption constant showed that the SiW adsorption is enthalpically driven and entropically unfavorable, in line with the typical chaotropic thermochemical signature. The adsorption enthalpy can be split into an electrostatic term and a water-recovery term to evaluate and qualitatively predict the superchaotropicity of a nanoion.

Repulsive, but sticky - Insights into the non-ionic foam stabilization mechanism by superchaotropic nano-ions

HYPOTHESIS

The superchaotropic Keggin polyoxometalate α-SiW₁₂O₄₀^4- (SiW) was recently shown to stabilize non-ionic surfactant (C_18:1E₁₀) foams owing to electrostatic repulsion that arises from the adsorption of SiW-ions to the foam interfaces. The precise mechanism of foam stabilization by SiW however remained unsolved.

EXPERIMENTS

Imaging and conductimetry were used on macroscopic foams to monitor the foam collapse under free drainage and small angle neutron scattering (SANS) at a given foam height allowed for the tracking of the evolution of film thickness under quasi-stationary conditions. Thin film pressure balance (TFPB) measurements enabled to quantify the resistance of single foam films to external pressure and to identify intra-film forces.

FINDINGS

At low SiW/surfactant ratios, the adsorption of SiW induces electrostatic repulsion within foam films. Above a concentration threshold corresponding to an adsorption saturation, excess of SiW screens the electrostatic repulsion that leads to thinner foam films. Despite screened electrostatics, the foam and single foam films remain very stable caused by an additional steric stabilizing force consistent with the presence of trapped micelles inside the foam films that bridge between the interfaces. These trapped micelles can serve as a surfactant reservoir, which promotes self-healing of the interface leading to much more resilient foam films in comparison to bare surfactant foams/films.

The Counterion-Mediated Controllable Coacervation of Nano-Ions with Polyelectrolytes

Nano-ions can complex with polyelectrolytes for coacervates with hierarchical structures; however, the rational design of functional coacervations is still rare due to the poor understanding of their structure-property relationship from their complex interaction. Herein, 1 nm anionic metal oxide clusters, PW₁₂O₄₀^3-, with well-defined, mono-disperse structures are applied to complex with cationic polyelectrolyte and the system shows tunable coacervation via the alternation of counterions (H⁺ and Na⁺) of PW₁₂O₄₀^3-. Suggested from Fourier transform infrared spectroscopy (FT-IR) and isothermal titration studies, the interaction between PW₁₂O₄₀^3- and cationic polyelectrolytes can be modulated by the bridging effect of counterions via hydrogen bonding or ion-dipole interaction to carbonyl groups of polyelectrolytes. The condensed structures of the complexed coacervates are explored by small angle X-ray and neutron scattering techniques, respectively. The coacervate with H⁺ as counterions shows both crystallized and discrete PW₁₂O₄₀^3- clusters, with a loose polymer-cluster network in comparison to the system of Na⁺ which shows a dense packing structure with aggregated nano-ions filling the meshes of polyelectrolyte networks. The bridging effect of counterions helps understand the super-chaotropic effect observed in nano-ion system and provides avenues for the design of metal oxide cluster-based functional coacervates.

Superchaotropic Nano-ion Binding as a Gelation Motif in Cellulose Ether Solutions

Nanometer-sized anions (nano-ions) like polyoxometalates and boron clusters exhibit so-called superchaotropic behavior, which describes their strong binding to hydrated non-ionic matter in water. We show here that nano-ions, at millimolar concentrations, dramatically enhance the viscosity and induce gelation of aqueous solutions of non-ionic cellulose ethers (CEs), a class of widely utilized polymers known for their thickening and gel-forming ability. These phenomena arise from an interplay of attractive forces and repulsive electrostatic forces between CE-chains upon nano-ion binding. The attractive forces manifest themselves as aggregation of CE-chains into a physically crosslinked polymer network (gel). In turn, the electrostatic repulsions hamper the viscosity increase and gelation. Superchaotropic nano-ion binding emerges as a novel and general physical crosslinking motif for CE-solutions and exceeds by far the conventional thickening effects of classical salts and ionic surfactants.

Multifunctional Architectures of Cyclic Dipeptide Copolymers and Composites, and Modulation of Multifaceted Amyloid-β Toxicity

Alzheimer's disease (AD) is a major neurodegenerative disorder primarily characterized by the β-amyloid (Aβ42) misfolding and aggregation-associated multifaceted amyloid toxicity encompassing oxidative stress, neuronal death, and severe cognitive impairment. Modulation of Aβ42 aggregation via various structurally anisotropic macromolecular systems is considered effective in protecting neuronal cells. In this regard, we have developed a cyclic dipeptide (CDP)-based copolymer (CP) and explored its material and biomedical properties. Owing to the structural versatility, CDP-CP forms solvent-dependent anisotropic architectures ranging from dense fibers and mesosheets to vesicles, which are shown to interact with dyes and nanoparticles and mimic synthetic protocells, providing a conceptually new approach to achieve advanced functional materials with the hierarchical organization. CP upon interaction with gold nanoparticles (GNP) and polyoxometalate (POM) generated faceted architectures (CP-GNP) and the nanocomposite (CP-POM), respectively. CP-GNP and CP-POM have shown remarkable ability to inhibit Aβ42 aggregation, dissolve the preformed aggregates, and scavenge reactive oxygen species (ROS) to ameliorate multifaceted amyloid toxicity. In cellulo studies show that CP-GNP and CP-POM protect neuronal cells from Aβ42-induced toxicity and reduce lipopolysaccharide (LPS)-activated neuroinflammation at sub-micromolar concentration. To our knowledge, this is the first report on the hierarchical organization of CDP-CP into 1D-to-2D architectures and their organic-inorganic hybrid nanocomposites to combat the multifaceted amyloid toxicity.

A Fluorescein-Substituted Perbrominated Dodecaborate Cluster as an Anchor Dye for Large Macrocyclic Hosts and Its Application in Indicator Displacement Assays

Perhalogenated boron clusters derived from B₁₂Br₁₂^2-, a superchaotropic dianion with a globular icosahedral shape, serve as inorganic cavity binders for cyclodextrins (CDs), in particular for large CDs (γ-CD and δ-CD), with high binding affinity (K_a > 10⁶ M^-1) in aqueous solution. This opens the door for applications of this anchoring moiety by linking it to organic residues, prominently fluorescent dyes. We report here the synthesis of a novel fluorescein-substituted perbrominated dodecaborate cluster by a copper(I)-catalyzed azide-alkyne click reaction. The formation of host-guest inclusion complexes between the dodecaborate-modified fluorescein dye and CDs can be readily followed by optical titrations, which afforded a binding constant of ∼1 × 10⁴ M^-1 with γ-CD; that is, the cluster functionalization allows binding of an otherwise nonbinding dye to the macrocycle ("anchor dye"). The formation of the 1:1 host-guest inclusion complex between the dye and γ-CD occurs over a broad range of pH values, which allows its application as a sensitive reporter pair according to the indicator displacement method, e.g., for drug detection. In addition, the substituted dye shows outer-wall binding to cucurbiturils through the dodecaborate moiety, leading to the formation of aggregates and significant fluorescence quenching of the dye.

Polymeric Surfactant P84/Polyoxometalate α-PW12O403−—A Model System to Investigate the Interplay between Chaotropic and Hydrophobic Effects

Low charge density nanometric ions were recently shown to bind strongly to neutral hydrated matter in aqueous solution. This phenomenon, called the (super-)chaotropic effect, arises from the partial dehydration of both the nano-ion and the solute, leading to a significant gain in enthalpy. Here, we investigate the chaotropic effect of the polyoxometalate α-PW12O403− on the triblock copolymer P84: (EO)19(PO)43(EO)19 with (EO)19 the polyethoxylated and (PO)43 the polypropoxylated chains. The combination of phase diagrams, spectroscopic (nuclear magnetic resonance) and scattering (small angle neutron/X-ray scattering) techniques revealed that: (i) below the micellization temperature of P84, PW12O403− exclusively binds to the propylene oxide moiety of P84 unimers; and (ii) above the micellization temperature, PW12O403− mostly adsorbs on the ethylene oxide micellar corona. The preferential binding of the PW12O403− to the PPO chain over the PEO chains suggests that the binding is driven by the chaotropic effect and is reinforced by the hydrophobic effect. At higher temperatures, copolymer micellization leads to the displacement of PW12O403− from the PPO chain to the PEO chains. This study deepens our understanding of the subtle interplay between the chaotropic and hydrophobic effects in complex salt-organic matter solutions.

Chaotropic Effect as an Assembly Motif to Construct Supramolecular Cyclodextrin-Polyoxometalate-Based Frameworks

In aqueous solution, low-charged polyoxometalates (POMs) exhibit remarkable self-assembly properties with nonionic organic matter that have been recently used to develop groundbreaking advances in host-guest chemistry, as well as in soft matter science. Herein, we exploit the affinity between a chaotropic POM and native cyclodextrins (α-, β-, and γ-CD) to enhance the structural and functional diversity of cyclodextrin-based open frameworks. First, we reveal that the Anderson-Evans type polyoxometalate [AlMo₆O₁₈(OH)₆]^3- represents an efficient inorganic scaffold to design open hybrid frameworks built from infinite cyclodextrin channels connected through the disk-shaped POM. A single-crystal X-ray analysis demonstrates that the resulting supramolecular architectures contain large cavities (up to 2 nm) where the topologies are dictated by the rotational symmetry of the organic macrocycle, generating honeycomb (bnn net) and checkerboard-like (pcu net) networks for α-CD (C₆) and γ-CD (C₈), respectively. On the other hand, the use of β-CD, a macrocycle with C₇ ideal symmetry, led to a distorted-checkerboard-like network. The cyclodextrin-based frameworks built from an Anderson-Evans type POM are easily functionalizable using the molecular recognition properties of the macrocycle building units. As a proof of concept, we successfully isolated a series of compartmentalized functional frameworks by the entrapment of polyiodides or superchaotropic redox-active polyanions within the macrocyclic host matrix. This set of results paves the way for designing multifunctional supramolecular frameworks whose pore dimensions are controlled by the size of inorganic entities.

Improvement of the Hydrolytic Stability of the Keggin Molybdo- and Tungsto-Phosphate Anions by Cyclodextrins

Keggin-type molybdo- and tungsto-phosphate polyoxoanions are among the most popular polyoxometalates (POMs) but suffer from their limited stability at low pH in aqueous solution. Their superchaotropic properties generate strong supramolecular complexes with cyclodextrins (CDs), which significantly affect the hydrolytic stability of POM. This chaotropically driven stabilization effect was systematically monitored by ³¹P NMR spectroscopy covering a wide range of pH (from 0 to 8) and varying the nature of the CD (α-, β-, and γ-form). A shift of ca. two pH units of the stability domains of these POMs was found in the presence of two equivalents of γ-CD compared to pure water, leading to keep intact the PW₁₂O₄₀^3- anion without any decomposition up to pH 3.5 (versus 1.5 in pure water) and pH 2.5 for PMo₁₂O₄₀^3-, which begins to decompose even at pH 0 in pure water. The effect of the smaller CDs (α- and β-form) is much less pronounced (only 0.5 pH units shift of the stability domain) confirming the importance of host-guest size matching to form a sandwich-type inclusion complex and thus protect the POM structure against basic hydrolysis. Such complexation was further supported by ¹⁸³W and ¹H NMR spectroscopy. Finally, using quantitative ³¹P NMR analyses, the new speciation and formation constants of phospho-molybdates and phospho-tungstates in the presence of cyclodextrins are determined and compared to those previously reported in pure water or in the 50:50 water/1,4-dioxane mixture.

Cationic peptides template the assembly of polyoxometalates into ultrathin nanosheet with in-plane ordered arrangement

Ultrathin polyoxometalates nanosheets with in-plane alignment have been constructed in aqueous solution with the assistance of cationic peptides. Different POMs varying in topology, size, and charges could be templated into...

Wells-Dawson phosphotungstates as mushroom tyrosinase inhibitors: a speciation study

In order to elucidate the active polyoxotungstate (POT) species that inhibit fungal polyphenol oxidase (AbPPO4) in sodium citrate buffer at pH 6.8, four Wells-Dawson phosphotungstates [α/β-PV2WVI18O62]6- (intact form), [α2-PV2WVI17O61]10- (monolacunary), [PV2WVI15O56]12- (trilacunary) and [H2PV2WVI12O48]12- (hexalacunary) were investigated. The speciation of the POT solutions under the dopachrome assay (50 mM Na-citrate buffer, pH 6.8; L-3,4-dihydroxyphenylalanine as a substrate) conditions were determined by 183W-NMR, 31P-NMR spectroscopy and mass spectrometry. The intact Wells-Dawson POT [α/β-PV2WVI18O62]6- shows partial (~ 69%) disintegration into the monolacunary [α2-PV2WVI17O61]10- anion with moderate activity (Ki = 9.7 mM). The monolacunary [α2-PV2WVI17O61]10- retains its structural integrity and exhibits the strongest inhibition of AbPPO4 (Ki = 6.5 mM). The trilacunary POT [PV2WVI15O56]12- rearranges to the more stable monolacunary [α2-PV2WVI17O61]10- (~ 62%) accompanied by release of free phosphates and shows the weakest inhibition (Ki = 13.6 mM). The hexalacunary anion [H2PV2WVI12O48]12- undergoes time-dependent hydrolysis resulting in a mixture of [H2PV2WVI12O48]12-, [PV8WVI48O184]40-, [PV2WVI19O69(H2O)]14- and [α2-PV2WVI17O61]10- which together leads to comparable inhibitory activity (Ki = 7.5 mM) after 48 h. For the solutions of [α/β-PV2WVI18O62]6-, [α2-PV2WVI17O61]10- and [PV2WVI15O56]12- the inhibitory activity is correlated to the degree of their rearrangement to [α2-PV2WVI17O61]10-. The rearrangement of hexalacunary [H2PV2WVI12O48]12- into at least four POTs with a negligible amount of monolacunary anion interferes with the correlation of activity to the degree of their rearrangement to [α2-PV2WVI17O61]10-. The good inhibitory effect of the Wells-Dawson [α2-PV2WVI17O61]10- anion is explained by the low charge density of its protonated forms Hx[α2-PV2WVI17O61](10-x)- (x = 3 or 4) at pH 6.8.

Superchaotropic nano-ions as foam stabilizers

HYPOTHESIS

Weakly hydrated nanometric ions, called superchaotropes, were recently shown to adsorb strongly to non-ionic surfaces affecting drastically the surface's physical-chemical properties due to a charging effect. Superchaotropic ions could serve as stabilizing agents for non-ionic colloidal systems, such as non-ionic surfactant foams.

EXPERIMENTS

We study foams of the non-ionic surfactant BrijO10 (C_18:1E₁₀) without and in presence of the superchaotropic Keggin-ion SiW₁₂O₄₀^4- (SiW). The foams are investigated under free drainage conditions by image analysis and conductimetry to reveal the effect of SiW on the foam stability, liquid drainage, and bubble size. Additionally, small angle neutron scattering on the same foams, but in a dry quasi-stationary state, provides insight into effects of SiW on the foam films.

FINDINGS

SiW strongly stabilizes non-ionic surfactant foams at millimolar concentrations by inducing electrostatic repulsions between foam film interfaces resulting in thicker and monodisperse foam films. A similar effect is observed with the ionic surfactant sodium dodecylsulfate (SDS) but to a lesser extent and with a different mechanism. At the foam films' interface, SiW adsorbs to the polar non-ionic surfactant heads driven by the superchaotropic effect whereas DS^- anchors between non-ionic surfactant alkyl chains by the hydrophobic effect. The potential of superchaotropic ions as foam stabilizers is herein demonstrated.

Programmable two-dimensional nanocrystals assembled from POSS-containing peptoids as efficient artificial light-harvesting systems

Inspired by the formation of hierarchically structured natural biominerals (e.g., bone and tooth), various sequence-defined polymers have been synthesized and exploited for design and synthesis of functional hybrid materials. Here, we synthesized a series of organic-inorganic hybrid peptoids by using polyhedral oligomeric silsesquioxane (POSS) nanoclusters as side chains at a variety of backbone locations. We further demonstrated the use of these hybrid peptoids as sequence-defined building blocks to assemble a new class of programmable two-dimensional (2D) nanocrystals. They are highly stable and exhibit an enhanced mechanical property and electron scattering due to the incorporated POSS nanoclusters. By varying peptoid side-chain chemistry, we further demonstrated the precise displacement of a large variety of function groups within these 2D nanocrystals and developed a highly efficient aqueous light-harvesting system for live cell imaging. Because these 2D nanocrystals are biocompatible and highly programmable, we expect that they offer unique opportunities for applications.

Redox-Responsive Host-Guest Association between γ-Cyclodextrin and Mixed-Metal Keggin-Type Polyoxometalates

The complexation of Keggin-type polyoxometalates (POMs) with γ-cyclodextrin (γ-CD) leads to supramolecular inclusion assemblies in aqueous solution driven by a chaotropic effect. The strength of the interaction between γ-CD and oxidized or one-electron reduced POMs in a series of molybdenum and vanadium monosubstituted phospho- and silico-tungstates, [XW₁₁MO₄₀]^n- Keggin-type anions where X = P or Si and M = Mo^V/VI or V^IV/V, has been evaluated by isothermal titration calorimetry (ITC), NMR spectroscopy, and cyclic voltammetry. Such a study reveals that the host-guest binding constant K_1:1 increases strongly with the decrease of the global ionic charge of the POM unit. There is an almost one magnitude order of variation in K_1:1 per charge unit, where K_1:1 falls down from about 10⁵ M^-1 to values close to zero as ionic charge varies from 3- to 6-. Such POMs with molybdenum and vanadium addenda offer the possibility of tuning the host-guest association strength by the simple addition/removal of one electron to POMs, opening a new avenue for the design of smart materials through redox stimuli.

Aluminum-Substituted Keggin Germanotungstate [HAl(H2O)GeW11O39]4-: Synthesis, Characterization, and Antibacterial Activity

We report on the new monosubstituted aluminum Keggin-type germanotungstate (C₄H₁₂N)₄[HAlGeW₁₁O₃₉(H₂O)]·11H₂O ([Al(H₂O)GeW₁₁]^4–), which has been synthesized at room temperature via rearrangement of the dilacunary [γ-GeW₁₀O₃₆]^8– polyoxometalate precursor. [Al(H₂O)GeW₁₁]^4– has been characterized thoroughly both in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by cyclic voltammetry (CV) ¹⁸³W, ²⁷Al NMR and UV–vis spectroscopy. A study on the antibacterial properties of [Al(H₂O)GeW₁₁]^4– and the known aluminum(III)-centered Keggin polyoxotungstates (Al-POTs) α-Na₅[AlW₁₂O₄₀] (α-[AlW₁₂O₄₀]^5–) and Na₆[Al(AlOH₂)W₁₁O₃₉] ([Al(AlOH₂)W₁₁O₃₉]^6–) revealed enhanced activity for all three Al-POTs against the Gram-negative bacterium Moraxella catarrhalis (minimum inhibitory concentration (MIC) up to 4 μg mL^–1) and the Gram-positive Enterococcus faecalis (MIC up to 128 μg mL^–1) compared to the inactive Al(NO₃)₃ salt (MIC > 256 μg mL^–1). CV indicates the redox activity of the Al-POTs as a dominating factor for the observed antibacterial activity with increased tendency to reduction, resulting in increased antibacterial activity of the POT.

We report on the synthesis and thorough characterization of the new monosubstituted aluminum germanotungstate (C₄H₁₂N)₄[HAlGeW₁₁O₃₉(H₂O)]·11H₂O ([Al(H₂O)GeW₁₁]⁴⁻), which has been subjected to an antibacterial study including the previously reported α-Na₅[AlW₁₂O₄₀] and Na₆[Al(AlOH₂)W₁₁O₃₉]. All three aluminum-substituted polyoxotungstates (Al-POTs) revealed enhanced activity against Moraxella catarrhalis and Enterococcus faecalis compared to the inactive Al(NO₃)₃ salt. On the basis of cyclic voltammetry studies, the redox activity of the POTs is suggested to have an impact on their overall antibacterial activity.

Hofmeister effect in the Keggin-type polyoxotungstate series

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion.

Highlights on the Binding of Cobalta-Bis-(Dicarbollide) with Glucose Units

Metalla-bis-dicarbollides, such as the cobalta-bis-dicarbollide (COSAN) anion [Co(C₂ B₉ H₁₁ )₂ ]^- , have attracted much attention in biology but a deep understanding of their interactions with cell components is still missing. For this purpose, we studied the interactions of COSAN with the glucose moiety, which is ubiquitous at biological interfaces. Octyl-glucopyranoside surfactant (C8G1) was chosen as a model as it self-assembles in water and creates a hydrated glucose-covered interface. At low COSAN content and below the critical micellar concentration (CMC) of C8G1, COSAN binds to C8G1 monomers through the hydrophobic effect. Above the CMC of C8G1, COSAN adsorbs onto C8G1 micelles through the superchaotropic effect. At high COSAN concentrations, COSAN disrupts C8G1 micelles and the assemblies become similar to COSAN micelles but with a small amount of solubilized C8G1. Therefore, COSAN binds in a versatile way to C8G1 through either the hydrophobic or superchaotropic effect depending on their relative concentrations.

Rational Control of Self-Recognition of Macroionic γ-Cyclodextrin by Host-Guest Interaction with Super-Chaotropic Borate Cluster Ions

We report a feasible method to control self-recognition during the self-assembly of a hydrophilic macroion, phosphate-functionalized γ-cyclodextrin (γ-CD-P), though host-guest interactions. We confirmed that γ-CD-P can form a host-guest complex with a super-chaotropic anion, namely the B₁₂ F₁₂ ^2- borate cluster, by using NMR spectroscopy and isothermal titration calorimetry. The loaded γ-CD-P, which has a higher charge density, can be distinguished from the uncomplexed γ-CD-P, leading to self-sorting behavior during the self-assembly process, confirmed by the formation of two types of individual supramolecular structures (R_h of ca. 57 nm and 18 nm, determined by light scattering) instead of hybrid structures in mixed dilute solution. This self-recognition behavior is accounted for by the difference in intermolecular electrostatic interactions arising from the loading.

Co-ion Effects in the Self-Assembly of Macroions: From Co-ions to Co-macroions and to the Unique Feature of Self-Recognition

Macroions, as soluble ions with a size on the nanometer scale, show unique solution behavior different from those of simple ions and large colloidal suspensions. In macroionic solutions, the counterions are known to be important and well-explored. However, the role of co-ions (ions carrying the same type of charge as the macroions) is often ignored. Here, through experimental and simulation studies, we demonstrate the role of co-ions as a function of co-ion size on their interaction with the macroions (using {Mo₇₂Fe₃₀} and {SrPd₁₂} as models) and the related self-assembly into blackberry-type structures in dilute solutions. Several regimes of unique co-ion effects are clearly identified: small ions (halides, oxoacid ions), subnanometer-scaled bulky ions (lacunary Keggin and dodecaborate ions), and those with sizes comparable to the macroions. Small co-ions have no observable effect on the self-assembly of fully hydrophilic {Mo₇₂Fe₃₀}, while due to hydrophobic interaction and intermolecular hydrogen bonds, the small co-ions show influences on the self-assembly of hydrophobic {SrPd₁₂}. Subnanometer ions, a.k.a. "superchaotropic ions", are still too small to assemble into a blackberry by themselves, but they can coassemble with the macroions, showing a strong interaction with the macroionic system. When the co-ion size is comparable to that of the macroions, they assemble independently instead of assembling with the macroions, leading to the previously reported unique self-recognition phenomenon for macroions.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

How Nano-Ions Act Like Ionic Surfactants

Recently, nanometric ions were shown to adsorb to hydrated neutral surfaces and to bind to the cavities of macrocyclic molecules with an unexpectedly strong affinity arising from a solvent-mediated effect named superchaotropicity. We show here that nano-ions at low concentrations (μm range), similarly to anionic surfactants, induce the spontaneous transformation of a swollen lyotropic lamellar phase of non-ionic surfactant into a vesicle phase. This transition occurs when the neutral lamellae acquire charges, either by adsorption of the nano-ions onto, or by anchoring of the ionic surfactant into the lamellae. In contrast to ionic surfactants, nano-ions strongly dehydrate the neutral surfactant assemblies. As a conclusion, these purely inorganic nanometric ions act as alternatives to the widely used organic ionic surfactants.

Aqueous self-assembly of arginine and K8SiW11O39: fine-tuning the formation of a coacervate intended for sprayable anticorrosive coatings

Coacervates are commonly thought to be formed from the liquid-liquid phase separation of macromolecules, such as oppositely charged polyelectrolytes, proteins or peptides. Unlike conventional systems, we here show an entirely novel coacervate obtained from the self-assembly of arginine (Arg) and K₈[α-SiW₁₁O₃₉] (SiW₁₁) in water. The formation of the coacervate Arg/SiW₁₁ is confirmed by combined techniques, including turbidity, rheology, optical microscopy, and scanning and transmission electron microscopy. Assessment of the rheological response reveals that the complex coacervate exhibits shear thinning behaviour. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, elemental analysis and thermogravimetric analysis are used to characterize the coacervate. The thermodynamic parameters of the coacervation are monitored by isothermal titration calorimetry (ITC), which identifies that the formation of the coacervate by mixing of Arg and SiW₁₁ is driven by a combination of entropic and enthalpic effects. The resultant coacervate shows a typical upper critical solution temperature (UCST) phenomenon, which is strongly dependent on the concentration of the species. Furthermore, we demonstrate that the coacervation could be tuned by stoichiometry and pH. A phase diagram for the complexation of Arg and SiW₁₁ thus has been constructed using turbidity measurements. Such a phase diagram is a very useful tool for the preparation of coacervates from a specific combination of Arg and SiW₁₁. Finally, the acid induced gelation of the coacervate has been explored to fabricate an anticorrosive coating to protect a copper plate from exposure to acid vapour.

Twining Poly(polyoxometalate) Chains into Nanoropes

Organic polymers and inorganic clusters belong to two different disciplines and have completely different properties and structures. When a cluster is attached to the backbone of a polymer as a pendant, the resultant hybrid polymers (polyclusters) exhibit unique behaviours totally different from those of conventional polymers owing to the nanoscale size of the cluster and its particular interactions. Herein, the aggregation of a poly(polyoxometalate)-a polynorbornene backbone with inorganic polyoxometalate cluster pendants-upon addition of a non-solvent to its dilute solution is reported. A three-dimensional network of tangled and snake-like nanothreads was observed. Direct visualisation of individual nanoscale clusters enabled identification of single chains within the nanothreads. These observations suggest that during the process of aggregation, the hybrid polymer forms curved or extended chains as a consequence of an armouring effect in which the collapsed cluster pendants wrap around the backbone. The collapse occurs because they become less soluble in the solvent/non-solvent mixture. The extended chains then become entwined and form nanoropes consisting of multiple chains wound around each other. This study provides a deeper understanding of the nature of polyclusters and should also prove useful for their future development and application.

Chiral salen Cr(iii) complexes encapsulated in thermo-responsive polymer nanoreactors for asymmetric epoxidation of alkenes in water

Thermo-responsive polymer nanoreactors containing chiral salen Cr(iii) complexes exhibited unprecedented efficiency and facile reusability in asymmetric epoxidation of unfunctionalized olefins in water.