A study of 12-tungstosilicic and 12-molybdophosphoric acids in solution

Development of chemiluminescent systems and devices for analytical applications

Chemiluminescence (CL) refers to the light-emitting phenomenon resulting from chemical reactions. Due to its simplicity in terms of instrumentation and high sensitivity, CL plays a critical role in analytical chemistry and has developed rapidly in recent years. In this review, we discuss the efforts made by our group in the field of CL. This includes exploring new luminophores that function under neutral pH conditions, developing oxidant- and reactive oxygen species-based coreactants (e.g. artemisinin and thiourea dioxide) for luminol and lucigenin CL, utilizing nanomaterial-based CL signal amplification and employing innovative ultrasound devices for CL and their analytical applications. We discussed the CL amplification mechanisms of these systems in detail. Finally, we summarize the challenges and prospects for the future development of CL.

Detailed Study on the Interfacial Interaction between Different Polyoxometalates and Tetronic Block Copolymers Exploring the Langmuir-Blodgett Technique

Polyoxometalates (POMs) interact with various biologically relevant entities. A basic understanding of this interaction is very important for various applications in the biological field. In this work, the focus is on the study of the interaction between tetronics and Keggin POMs. T701 and T90R4 are the two tetronics considered here; they have different solubilities in water due to different PPO/PEO ratios. The arrangement of PPO and PEO is also different with respect to the central ethylenediamine groups. Three different Keggin-type POMs, phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), with different charge densities are chosen for an elaborate investigation using Langmuir-Blodgett technique. The observation is analyzed thoroughly, which shows both electrostatic interaction and adsorption of POMs on the PPO blocks of the tetronics due to the chaotropic effect, which is responsible for the binding of POMs (in subphase) with the tetronic monolayer. This interaction results in an expanded yet rigid monolayer for POM-tetronic association on the surface. Surface pressure vs mean molecular area isotherm is the key characterization to reach the conclusion. UV-vis spectroscopy, NMR, ITC, ellipsometric studies, FTIR, and SEM also serve as supportive characterization techniques.

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.

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.

A new acetylcholinesterase allosteric site responsible for binding voluminous negatively charged molecules - the role in the mechanism of AChE inhibition

Acetylcholinesterase (AChE) inhibitors are important in the treatment of neurodegenerative diseases. Two inhibitors, 12-tungstosilicic acid (WSiA) and 12-tungstophosphoric acid (WPA), which have polyoxometalate (POM) type structure, have been shown to inhibit AChE activity in nM concentration. Circular dichroism and tryptophan fluorescence spectroscopy demonstrated that the AChE inhibition was not accompanied by significant changes in the secondary structure of the enzyme. The molecular docking approach has revealed a new allosteric binding site, termed β-allosteric site (β-AS), which is considered responsible for the inhibition of AChE by POMs. To the best of our knowledge, this is the first study reporting a new allosteric site that is considered responsible for AChE inhibition by voluminous and negatively charged molecules such as POMs. The selected POMs were further subjected to genotoxicity testing using human peripheral blood cells as a model system. It was shown that WSiA and WPA induced a mild cytostatic but not genotoxic effects in human lymphocytes, which indicates their potential to be used as medicinal drugs. The identification of non-toxic compounds capable of binding to an allosteric site that so far has not been considered responsible for enzyme inhibition could be fundamental for the development of new drug design strategies and the discovery of more efficient AChE modulators.

Silicotungstic acid as a highly efficient coreactant for luminol chemiluminescence for sensitive detection of uric acid

Herein, we report luminol-silicotungstic acid (STA) chemiluminescence (CL) for the first time. The luminol-STA system resulted in remarkable CL enhancement (65 times) compared with the known classical luminol-H₂O₂ system because of the generation of the strong oxidizing agent tungsten trioxide from STA. Based on the quenching effect of uric acid, the new CL system is applied for the sensitive and selective assay of uric acid in its pure state (LOD 0.75 nM) and in real human urine with excellent recoveries in the range of 99.6-102.3%. Furthermore, this system permits the efficient detection of STA (LOD, 0.24 μM).

One-pot synthesis at room temperature of epoxides and linalool derivative pyrans in monolacunary Na7PW11O39-catalyzed oxidation reactions by hydrogen peroxide

In this work, we describe a new one-pot synthesis route of valuable linalool oxidation derivatives (i.e., 2-(5-methyl-5-vinyltetrahydrofuran-2-yl propan-2-ol) (1a)), 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol (1b) and diepoxide (1c), using a green oxidant (i.e., hydrogen peroxide) under mild conditions (i.e., room temperature). Lacunar Keggin heteropolyacid salts were the catalysts investigated in this reaction. Among them, Na₇PW₁₁O₃₉ was the most active and selective toward oxidation products. All the catalysts were characterized by FT-IR, TG/DSC, BET, XRD analyses and potentiometric titration. The main reaction parameters were assessed. Special attention was dedicated to correlating the composition and properties of the catalysts and their activity.

In this work, the catalyst Na₇PW₁₁O₃₉ was used in a new one-pot synthesis route to oxidize linalool to valuable derivatives (i.e., vinyltetrahydrofuran, vinyltetrahydropyran, and diepoxide), using a green oxidant (H₂O₂) at room temperature.

Unraveling the role of the lacunar Na7PW11O39 catalyst in the oxidation of terpene alcohols with hydrogen peroxide at room temperature

The Na₇PW₁₁O₃₉ lacunar salt was the most active among Keggin lacunar salts (Na₈SiW₁₁O₃₉ and Na₇PMo₁₁O₃₉), a saturated salt (Na₇PW₁₂O₄₀) and heteropolyacid catalysts.

Polyoxometalates in solution: speciation under spotlight

The review covers stability and transformations of classical polyoxometalates in aqueous solutions and provides their ion-distribution diagrams over a wide pH range.

Better understanding and applications of ammonium 12-molybdophosphate-based diffusive gradient in thin film techniques for measuring Cs in waters

This study deals with further and systematic laboratory evaluation of the already known ammonium 12-molybdophosphate (AMP)-diffusive gradient in thin film (DGT) method, which is used for measuring total Cs concentration in environmental waters. This study confirms that the AMP-binding gel is not stable for pH > 6. In order to reveal a potential impact of AMP degradation on DGT application, time-series experiments were performed by deploying AMP-DGT samplers in Cs-doped moderately basic soft and hard water up to total AMP-binding gel degradation (60 and 175 h of deployment time, respectively). Linear accumulation of Cs by AMP-DGT samplers was observed up to 48 and 58 h in hard and soft waters, respectively. For this deployment time range, AMP-DGT measured over 77 ± 10 and 94 ± 16% of total Cs concentration in hard and soft water, respectively. The difference in DGT response was attributed to Ca²⁺ and Mg²⁺ competition reducing the uptake of AMP-DGT samplers in hard water. Shrinkage of agarose-polyacrylamide diffusive gel was experimentally observed only in hard water due to more intensive AMP-binding gel degradation in hard water. Even if the AMP-DGT response was not impacted in this study, it is recommended to use agarose hydrogel as standard diffusive gel. Based on the experience obtained from this detailed validation process, the authors propose a number of key requirements that need to be considered when developing DGT devices, with testing the performance over longer deployment times being critical. Graphical abstract ᅟ.

An unexpected behavior of H3PMo12O40 heteropolyacid catalyst on the biphasic hydrolysis of vegetable oils

Fatty acids (FA) are key ingredients in formulating numerous high-value chemicals.

Study of the decomposition pathway of 12-molybdophosphoric acid in aqueous solutions by micro Raman spectroscopy

Micro Raman spectroscopy was applied to investigate the speciation of heteropoly and isopoly molybdates in 0.05 and 0.005 M aqueous solutions of 12-molybdophosphoric acid at pH values between 1 and 6. For comparative purposes, (31)P NMR spectroscopy was applied too. It is shown that stability of Keggin anion is influenced both by pH and concentration of solution. The Keggin structure is stable in acidic solutions (pH<1.6) while defective Keggin structures are formed with further alkalization (up to pH5.6). Monolacunary anion PMo11O(39)(7-) is the main component in the pH region from 1.6 to 3.4. Further removal of molybdenyl species causes the appearance of other vacant Keggin structures such as PMo9O31(OH)(3)(6-) and PMo6O(25)(9-) at about pH4. At pH5.0, anion PMo6O(25)(9-) is the main species. In solutions with pH greater than 5.0, heteropolymolybdates disappear completely and isopolymolybdates Mo7O(24)(6-) and MoO(4)(2-) are formed in higher amounts. In more diluted solution of 0.005 M, the decomposition scheme of 12-molybdophosphoric acid solution with increasing of pH takes place without observation of significant amounts of Mo7O(24)(6-) species. If alkalinization is performed with 0.5 M instead of 5 M NaOH, there are no significant changes in the Raman spectra of solutions. It is shown that the spectra of evaporated samples may be used for the identification of molecular species in corresponding concentrated solutions. However, Raman spectra of dry residues of more diluted solutions differ from spectra of corresponding solutions due to the reactions performed during the process of drying and cannot be used for unambiguous identification of species in solution. Acidification of 0.05 M solution of Na2MoO4 shows that at pH>5.6, molybdate anion MoO(4)(2-) dominates, while in the pH range between 5.6 and 1, heptamolybdate anion Mo7O(24)(6-) is preferentially formed.