Structural changes,thermodynamic properties, 1H magic angle spinning NMR,and 14N NMR of (NH4)2CuCl4·2H2O

The structural changes and thermodynamic properties of (NH₄)₂CuCl₄·2H₂O were studied by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. In addition, the chemical shift, line width, and spin-lattice relaxation time of the crystals were also investigated by ¹H magic angle spinning nuclear magnetic resonance (MAS NMR), focusing on the role of NH₄ and H₂O near the phase transition temperature. The change at T_C2 (=406 K) and T_C3 (=437 K) seems to be a chemical change caused by thermal decomposition rather than a physical change such as a structural phase transition. The changes in the temperature dependence of these data near T_C2 are related to variations in the environments surrounding NH₄ and H₂O. The ¹⁴N NMR spectrum is also measured in order to investigate local phenomena related to the phase transition.

The structural changes and thermodynamic properties of (NH₄)₂CuCl₄·2H₂O were studied by differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, and nuclear magnetic resonance (NMR).     

Thermodynamics of the double sulfates Na2M2+(SO4)2·nH2O (M = Mg,Mn, Co,Ni, Cu,Zn, n = 2 or 4) of the blödite–kröhnkite family

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials. These sulfates belong structurally to the blödite (n = 4) and kröhnkite (n = 2) family and the M cations considered in this work were Mg, Mn, Co, Ni, Cu, Zn. Using a combination of calorimetric methods, we have measured enthalpies of formation and entropies of these phases, calculated their Gibbs free energies (Δ_fG°) of formation and evaluated their stability with respect to Na₂SO₄, simple sulfates MSO₄·xH₂O, and liquid water, if appropriate. The Δ_fG° values (all data in kJ mol⁻¹) are: Na₂Ni(SO₄)₂·4H₂O: −3032.4 ± 1.9, Na₂Mg(SO₄)₂·4H₂O: −3432.3 ± 1.7, Na₂Co(SO₄)₂·4H₂O: −3034.4 ± 1.9, Na₂Zn(SO₄)₂·4H₂O: −3132.6 ± 1.9, Na₂Mn(SO₄)₂·2H₂O: −2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to Na₂SO₄, MSO₄·mH₂O and H₂O(l). Na₂Ni(SO₄)₂·4H₂O is stable with respect to Na₂SO₄, NiSO₄ and H₂O(l) by a significant amount of ≈50 kJ mol⁻¹ whereas Na₂Mn(SO₄)₂·2H₂O is stable with respect to Na₂SO₄, MnSO₄ and H₂O(l) only by ≈25 kJ mol⁻¹. The values for the other blödite–kröhnkite phases lie in between. When considering the stability with respect to higher hydrates, the stability margin decreases; for example, Na₂Ni(SO₄)₂·4H₂O is still stable with respect to Na₂SO₄, NiSO₄·4H₂O and H₂O(l), but only by ≈20 kJ mol⁻¹. Among the phases studied and chemical reactions considered, the Na–Ni phase is the most stable one, and the Na–Mn, Na–Co, and Na–Cu phases show lower stability.

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials.     

Nuclear quantum effect and H/D isotope effect on Cl· + (H2O)n → HCl + OH·(H2O)n−1 (n = 1–3) reactions

Cl· + (H₂O)_n → HCl + OH·(H₂O)_n−1 (n = 1–3) reactions are fundamental and important ones in atmospheric chemistry. In this study, we focused on the nuclear quantum effect (NQE) of the hydrogen nucleus on these reactions with the aid of the multicomponent quantum mechanics (MC_QM) method, which can directly take account of NQE of light nuclei. Our study reveals that the NQE of the hydrogen nucleus lowers the activation barriers of the reactions and enhances the catalytic effects of second and third water molecules. In particular, we find that (i) the NQE of the proton removes the activation barrier of the reverse reaction of HCl + OH· → Cl· + H₂O, and (ii) the catalytic effect of the third water molecule appears in only our MC_QM calculation. We also analyze the H/D isotope effects on these reactions by using the MC_QM method.

Cl·+ (H₂O)_n → HCl + OH(H₂O)_n−1 (n = 1–3) reactions have been investigated using multicomponent quantum mechanics method, which can take account of the nuclear quantum effect of proton and deuteron.     

Up-conversion white emission and other luminescence properties of a YAG:Yb2O3·Tm2O3·Ho2O3@SiO2 glass-nanocomposite

We report on a glass-nanocomposite material consisting of yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystals co-doped with Yb³⁺, Tm³⁺ and Ho³⁺ ions as well as entrapped into a SiO₂ xerogel. This 94YAG·5Yb₂O₃·0.8Tm₂O₃·0.2Ho₂O₃@SiO₂ (abbr. YAG:YbTmHo@SiO₂) nanocomposite material has been prepared by sol–gel procedure. Its structure and morphology has been characterized by means of X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques as well as energy dispersive X-ray (EDX), X-ray photoelectron (XPS) and luminescence spectroscopies. The luminescent glass-nanocomposite exhibited an up-conversion effect under λ_exc = 980 nm and emission when excited under 355 nm in steady-state conditions. Then time-resolved luminescence emission was observed, when the sample was excited at 290 and 355 nm by a pulse laser. Average decay times for the SiO₂ matrix and for some transitions of the Tm³⁺ and Ho³⁺ dopants present in the YAG:YbTmHo@SiO₂ material have been evaluated. The luminescent nanocomposite when excited under 290 or 355 nm wavelengths in both conditions emits blue light. However, the nanocomposite is promising as a single-source white-light phosphor owing to its up-conversion luminescence under 980 nm excitation. Such optical features make the studied material an alternative phosphor.

We present a glass-nanocomposite of the type YAG:YB³⁺, Tm³⁺, Ho³⁺@SiO₂ as an alternative white phosphor based on up-conversion effect.     

Facile synthetic routes for photocatalytic Pb3(BTC)2·H2O coordination polymers

Herein, we report on the successful synthesis of photocatalytic Pb₃(BTC)₂·H₂O polymers via different methods including the surfactant-assisted hydrothermal method, ultrasonic method and reflux method. As the crystal growth is subjected to preparation atmosphere, changes in reaction conditions do not alter the crystal structures of products, but vary their morphology. High ultraviolet-light-driven photocatalytic abilities are attributed to the stable Pb₃(BTC)₂·H₂O, and the effective productions of h⁺ and ˙OH on the catalysts.

Pb₃(BTC)₂·H₂O was fabricated by hydrothermal, ultrasonic and reflux methods. The results indicated that different reaction conditions have a great impact on the photocatalytic performance of the products.     

Intrinsic poorly-crystallized Fe5O7(OH)·4H2O: a highly efficient oxygen evolution reaction electrocatalyst under alkaline conditions

As the bottleneck of electrochemical overall water splitting, the oxygen evolution reaction (OER) needs efficient catalysts to lower the required overpotential. Electrocatalysts with an amorphous form are highly active but suffer with low structural stability. Poorly crystallized materials with activity like amorphous forms, while maintaining the mechanical robustness of crystalline forms, are expected to be ideal materials. Towards this direction, we, for the first time, developed low-crystalline Fe₅O₇(OH)·4H₂O as an excellent OER electrocatalyst with an overpotential of 269 mV, in order to drive a current density of 100 mA cm⁻² in a 1.0 M KOH environment, and this outperforms most of the reported Fe-based electrocatalysts. Notably, its activity can be maintained for at least 100 hours. A one-pot synthesis for the poorly-crystallized material using one of the most abundant metal elements to obtain effective OER catalysis will provide great convenience in practical applications.

A novel low-crystallized Fe₅O₇(OH)·4H₂O electrocatalyst was fabricated and it demonstrates excellent OER activity, outperforming most of the reported Fe-based electrocatalysts.     

Mechanochemical approach to synthesize citric acid-soluble fertilizer of dittmarite (NH4MgPO4·H2O) from talc/NH4H2PO4 mixture

In this study, a citric acid-soluble fertilizer of dittmarite (NH₄MgPO₄·H₂O) was synthesized by balling talc with NH₄H₂PO₄. The effects of ball milling speed and milling time on the dissolution rates of N, P and Mg in deionized water and 2% citric acid were explored. Characterization technologies such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG) and Scanning electron microscopy (SEM) were applied to test the prepared samples. In water, the prepared dittmarite was changed into struvite (NH₄MgPO₄·6H₂O) with almost no N, P or Mg release, while the dissolution rates of nutrient elements reached almost 100% in 2% citric acid. The proposed work presented a facile and environmentally friendly method to produce CASF with high agricultural and ecological value.

Dittmarite synthesis by a mechanochemical route for application as a citric acid-soluble fertilizer.     

Expanding pentafluorouranates: hydrothermal synthesis and characterization of β-NaUF5 and β-NaUF5·H2O

Two new sodium uranium(iv) pentafluorides were synthesized from uranium dioxide, HF, and NaF using mild hydrothermal conditions. β-NaUF₅·H₂O has greater lattice energy than previously-known α-NaUF₅·H₂O and possesses lower symmetry with the latter compound being orthorhombic, whereas β-NaUF₅·H₂O is monoclinic. Trigonal β-NaUF₅ also possesses different connectivity between the [UF_n] building units than the α-phase, with higher symmetry and greater lattice energy than orthorhombic α-NaUF₅. The single crystal absorption spectra of these compounds are also reported and compared.

Two new sodium uranium(iv) pentafluorides eliminate the necessity of a copper catalyst in the synthesis of alkali metal uranium fluorides.     

Aspect ratio-controlled preparation of α-CaSO4·0.5H2O from phosphogypsum in potassium tartrate aqueous solution

In this study, a simple and efficient strategy is developed to synthesize rod-shaped α-CaSO₄·0.5H₂O crystals with tunable aspect ratio from industrial phosphogypsum only in potassium tartrate aqueous solution at a low temperature. Industrial phosphogypsum can be effectively converted into rod-shaped α-CaSO₄·0.5H₂O crystals with the assistance of potassium tartrate, and the aspect ratio of α-CaSO₄·0.5H₂O crystals gradually decreases from 52 : 1 to 1 : 1 with increasing the concentration of potassium tartrate. The formation process of the rod-shaped α-CaSO₄·0.5H₂O crystals in this system involves the dissolution of CaSO₄·2H₂O and nucleation of α-CaSO₄·0.5H₂O crystals. The tartrate ions from potassium tartrate in this system preferentially bind to (001) and (002) facets of α-CaSO₄·0.5H₂O crystals, inhibiting the growth of α-CaSO₄·0.5H₂O crystals along the c-axis and controlling its morphology and aspect ratio.

The conversion of industrial gypsum to rod-shaped α-CaSO₄·0.5H₂O crystals with tunable aspect ratio in a H₂O system is realized with potassium tartrate.     

Electrochemical characterization of and theoretical insight into a series of 2D MOFs, [M(bipy)(C4O4)(H2O)2]·3H2O (M = Mn (1), Fe (2), Co (3) and Zn (4)), for chemical sensing applications

The electrochemical sensing applications of a series of water-stable 2D metal–organic framework (MOF)-modified screen-printed carbon electrodes (SPCEs) are reported. The MOF materials in this study are [M(bipy)(C₄O₄)(H₂O)₂]·3H₂O, in which bipy = 4,4′-bipyridine and M = Mn, Fe, Co and Zn. The MOF materials are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), showing that the MOFs have a layer-by-layer rod structure with a smooth surface. We use the nitrofurazone molecule as a probe to investigate the influence of the metal ions of MOFs on electrochemical sensing ability. Cyclic voltammetry demonstrated that the Mn-MOF electrode of interest delivered stronger signals than that of other electrodes. Through first-principles calculations, we also revealed that the change in the spin polarization of divalent metal ions passing from the free ion state to the MOF environment appeared to be significantly correlated with the enhancement in the peak response current. The theoretical and experimental results consistently indicate that Mn-MOF has the smallest bandgap and good sensitivity among these MOF materials. Accordingly, we proposed a simple model to illustrate this observation and disclosed the importance of the electron configuration of the transition metal constructing the MOF materials used in improving electrochemical sensing applications.

Framework-to-metal charge transfer of the MOF materials results in enhancing electrochemical sensing ability to nitrofurazone.     

The A·T(rWC)/A·T(H)/A·T(rH) ↔ A·T*(rwWC)/A·T*(wH)/A·T*(rwH) mutagenic tautomerization via sequential proton transfer: a QM/QTAIM study

In this study for the first time we have revealed by QM and QTAIM calculations at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of QM theory the novel routes of the mutagenic tautomerization of three biologically important A·T DNA base pairs – reverse Watson–Crick A·T(rWC), Hoogsteen A·T(H) and reverse Hoogsteen A·T(rH) – followed by their rebuilding into the wobble (w) A·T*(rw_WC), A·T*(w_H) and A·T*(rw_H) base mispairs by the participation of the mutagenic tautomers of the DNA bases (denoted by asterisk) and vice versa, thus complementing the physico-chemical property of the canonical A·T(WC) Watson–Crick DNA base pair reported earlier (Brovarets'' et al., RSC Adv., 2015, 5, 99594–99605). These non-dissociative tautomeric transformations in the classical A·T(rWC), A·T(H) and A·T(rH) DNA base pairs proceed similarly to the canonical A·T(WC) DNA base pair via the intrapair sequential proton transfer with shifting towards major or minor grooves of DNA followed by further double proton transfer along the intermolecular H-bonds and are controlled by the plane symmetric and highly stable transition states – tight ion pairs formed by the A⁺ nucleobase, protonated by the N1/N7 nitrogen atoms, and T⁻ nucleobase, deprotonated by the N3H imino group. Comparison of the estimated populations of the tautomerised states (10⁻²¹ to 10⁻¹⁴) with similar characteristics for the canonical A·T(WC) DNA base pair (10⁻⁸ to 10⁻⁷) leads authors to the conclusion, that only a base pair with WC architecture can be a building block of the DNA macromolecule as a genetic material, which is able for the evolutionary self-development. Among all four classical DNA base pairs, only A·T(WC) DNA base pair can ensure the proper rate of the spontaneous point errors of replication in DNA.

We discovered tautomeric wobbling of the classical A·T DNA base pairs. This data evidence, that only a base pair with Watson–Crick architecture can be a building block of the DNA macromolecule as a genetic material, which is able for the evolutionary self-development.     

Preparation of a Bi6O5(OH)3(NO3)5·2H2O/AgBr composite and its long-lasting antibacterial efficacy

A novel Bi₆O₅(OH)₃(NO₃)₅·2H₂O/AgBr (6535BBN/AgBr) composite with long-lasting antibacterial efficacy was prepared. The microstructure of the composite was characterized. AgBr nanoparticles (NPs) were sandwiched in 6535BBN nanosheets (NSs) or loaded on their surfaces. The utilization of 6535BBN as carriers contributed to the long-term lasting antibacterial activity of the composite after storage in water or 0.9% NaCl. The antibacterial activity was evaluated by inhibition zones against E. coli. The inhibition zone diameters of 6535BBN/AgBr stored in water for 0 h, 8 h, 16 h, and 48 h were measured as 22.50, 21.71, 20.43, and 20.29 mm, respectively. The activity of the composite after storage in water for 48 h remained 90.2% of that in the beginning. After storing in 0.9% NaCl for 16 h, the activity was determined to be 90.1% of that in the beginning. In comparison with the rapid decrease in the antibacterial activity of pure AgBr, the slow reduction of 6535BBN/AgBr after storage indicates long-lasting efficacy. The excellent dispersion states of 6535BBN/AgBr powders after storage in solutions were revealed, and the positive relationship between the dispersion state and its long-lasting antibacterial activity was suggested. Based on the unique load-on-carrier (LOC) structure, the long-lasting antibacterial performance was promoted by the synergy of the sharp-edge-cutting effect of 6535BBN NSs, prolonged ROS antibacterial effect, and restrained sterilization effects of silver ions caused by their slow release.

A novel Bi₆O₅(OH)₃(NO₃)₅·2H₂O/AgBr (6535BBN/AgBr) composite with long-lasting antibacterial efficacy was prepared.     

Effect of Cu2+ on the nucleation kinetics and crystallization of rod-shaped CaSO4·2H2O in aqueous solution

In this study, a simple and efficient strategy is developed to synthesize rod-like CaSO₄·2H₂O (DH) crystals with tunable aspect ratio in aqueous solution using Cu²⁺ as modifier. The aspect ratio and length of the DH crystals are effectively reduced to 5.7 : 1 and about 35 μm in the presence of Cu²⁺, respectively. The interfacial tension value (γ) in the aqueous solution is improved significantly with the assistance of Cu²⁺, yet the nucleation rate (J) of the DH crystal is decreased sharply. The interfacial tension value (γ) in the aqueous solution is improved and the nucleation rate (J) of the DH crystal is drastically decreased due to the introduction of Cu²⁺, leading to the induction time of the DH crystallization being extended from 4 min to 25 min. The diversification of morphology for the DH crystals is incited by the changes of nucleation kinetics and Cu²⁺ incorporated into the crystal lattice, affecting the crystal growth habit, and finally controlling the growth of DH crystals in aqueous solution.

The induction time of DH crystals is extended by the introduction of Cu²⁺.     

Coordination environment evolution of Co(ii) during dehydration and re-crystallization processes of KCoPO4·H2O towards enhanced electrocatalytic oxygen evolution reaction

Development of efficient and stable electrodes for electrocatalytic oxygen evolution reaction (OER) is essential for energy storage and conversion applications, such as hydrogen generation from water splitting, rechargeable metal–air batteries and renewable fuel cells. Alkali metal cobalt phosphates show great potential as OER electrocatalysts. Herein, an original electrode design strategy is reported to realize an efficient OER electrocatalyst through engineering the coordination geometry of Co(ii) in KCoPO₄·H₂O by a facile dehydration process. Experimental result indicated that the dehydration treatment is accompanied by a structural transformation from orthorhombic KCoPO₄·H₂O to hexagonal KCoPO₄, involving a concomitant coordination geometry evolution of Co(ii) from octahedral to tetrahedral configuration. More significantly, the local structural evolution leads to an advantageous electronic effect, i.e. increased Co–O covalency, resulting in an enhanced intrinsic OER activity. To be specific, the as-produced KCoPO₄ can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Thus, this present research provides a new way of developing alkali metal transition-metal phosphates for efficient and stable electrocatalytic oxygen evolution reaction.

Coordination environment evolution of potassium cobalt phosphate towards enhanced electrocatalytic oxygen evolution reaction.     

Genomic DNA-mediated formation of a porous Cu2(OH)PO4/Co3(PO4)2·8H2O rolling pin shape bifunctional electrocatalyst for water splitting reactions

Among the accessible techniques, the production of hydrogen by electrocatalytic water oxidation is the most established process, which comprises oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, we synthesized a genomic DNA-guided porous Cu₂(OH)PO₄/Co₃(PO₄)₂·8H₂O rolling pin shape composite structure in one pot. The nucleation and development of the porous rolling pin shape Cu₂(OH)PO₄/Co₃(PO₄)₂·8H₂O composite was controlled and stabilized by the DNA biomolecules. This porous rolling pin shape composite was explored towards electrocatalytic water oxidation for both OER and HER as a bi-functional catalyst. The as-prepared catalyst exhibited a very high OER and HER activity compared to its various counterparts in the absence of an external binder (such as Nafion). The synergistic effects between Cu and Co metals together with the porous structure of the composite greatly helped in enhancing the catalytic activity. These outcomes undoubtedly demonstrated the beneficial utilization of the genomic DNA-stabilised porous electrocatalyst for OER and HER, which has never been observed.

Among the accessible techniques, the production of hydrogen by electrocatalytic water oxidation is the most established process, which comprises oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).     

Nano- and micro-structural control of WO3 photoelectrode films through aqueous synthesis of WO3·H2O and (NH4)0.33WO3 precursors

Nano- and micro-structured tungsten trioxide (WO₃) photoelectrode films were prepared through an aqueous solution route. WO₃ precursor layers were deposited on glass substrates through heterogeneous nucleation from (NH₄)₁₀W₁₂O₄₁ aqueous solutions at 50–60 °C. The crystal phase of the precursors changed from WO₃·H₂O to (NH₄)_0.33WO₃ with increasing (NH₄)₁₀W₁₂O₄₁ concentration (x), which involved a morphological change from micron-scale plates to nano-scale fine particles. The WO₃·H₂O and (NH₄)_0.33WO₃ layers were thermally converted to the monoclinic WO₃ phase. The fine-particle WO₃ films obtained from (NH₄)_0.33WO₃ layers showed a better photoanodic performance in the UV range below 350 nm, which was attributed to the larger surface area arising from the porous structure. On the other hand, platy-particle WO₃ films were obtained from WO₃·H₂O layers, which exhibited strong light scattering in the visible range, and resulted in an enhanced photoanodic response at wavelengths above 375 nm.

Nano- and micro-structured WO₃ films obtained from WO₃·H₂O and (NH₄)_0.33WO₃ precursors prepared by an aqueous route showed a good photoanodic performance depending on their morphologies.     

SrTi(IO3)6·2H2O and SrSn(IO3)6: distinct arrangements of lone pair electrons leading to large birefringences

Three new iodates SrTi(IO₃)₆·2H₂O, (H₃O)₂Ti(IO₃)₆, and SrSn(IO₃)₆ have been synthesized via a facile hydrothermal method. The three compounds have zero-dimensional crystal structures composed of one [MO₆]⁸⁻ (M = Ti, Sn) octahedron connected with six [IO₃]⁻ trigonal pyramids. However, the particular coordination of Sr²⁺ cations results in distinct arrangements of lone pair electrons in an [IO₃]⁻ trigonal pyramid, which leads to large birefringences. More importantly, this work enriches the species crystal chemistry for [M(IO₃)₆]²⁻ (M = Ti, Sn) clusters-containing iodates.

The distinct arrangements of [IO₃]⁻ trigonal pyramids lead to larger birefringences in SrTi(IO₃)₆·2H₂O and SrSn(IO₃)₆ than that in (H₃O)₂Ti(IO₃)₆.     

Enhanced photocatalytic activity and ultra-sensitive benzaldehyde sensing performance of a SnO2·ZnO·TiO2 nanomaterial

The synthesis of a ternary SnO₂·ZnO·TiO₂ nanomaterial (NM) by a simple co-precipitation method and its potential applications as an efficient photocatalyst and chemical sensor have been reported. The synthesized nanomaterial was fully characterized by XRD, SEM, EDS, XPS, FTIR, AFM and photoluminescence studies. This nanomaterial exhibited enhanced efficiency in photo-catalysis of Methyl Violet 6b (MV) dye degradation. The observed photocatalyst efficiency of the SnO₂·ZnO·TiO₂ nanomaterial was 100% under UV light at pH 9. Moreover, it lost around 12% efficiency over five reuses. The PL properties with changing excitation energy were also reported. Glassy carbon electrode (GCE) was modified with the SnO₂·ZnO·TiO₂ nanomaterial by an efficient electrochemical technique to develop a chemical sensor for selective benzaldehyde. Hazardous benzaldehyde was carefully chosen as a target analyte by a selectivity study; it displays a rapid response towards the SnO₂·ZnO·TiO₂/Nafion/GCE sensor probe in electrochemical sensing. It also shows superb sensitivity, an ultra-low detection limit, long-term stability, and very good repeatability and reproducibility. In this study, a linear calibration plot was obtained for 0.1 nM to 1.0 mM aqueous benzaldehyde solutions, with a sensitivity value of 4.35 nA μM⁻¹ cm⁻² and an exceptionally low detection limit (LOD) of 3.2 ± 0.1 pM (S/N = 3). Hence, a chemical sensor modified with SnO₂·ZnO·TiO₂/GCE may be a promising sensor in the determination of toxic chemicals in the environmental and healthcare fields.

Schematic of a GCE fabricated with SnO₂·ZnO·TiO₂ NMs/Nafion/GCE using a conducting Nafion binder and its electrochemical response as a benzaldehyde sensor.     

A concise synthesis of (±)-7-O-galloyltricetiflavan

(±)-7-O-galloyltricetiflavan (1a) was synthesized successfully in five steps from the commercially available trihydroxyacetophenone (2) and trimethoxybenzoyl chloride (3). The flavone 4a was prepared in a one-pot reaction and it gave hex-O-methylflavan 6 followed by acylation and reduction. However, the demethylation of flavan 6, 5-O-acetylflavan 10 and 5-O-phenylacetylflavan 11 by BBr₃ gave all the hydrolyzed fragments 7 and 8 as the major products. By contrast, in the same condition, hept-O-methylflavan 9 could provide the desired product (±)-7-O-galloyltricetiflavan (1a) in 91% yield. The additional 5-O-B-Br₂ complex may stabilize the ester bond during the demethylation process.

We report the first total synthesis of (±)-7-O-galloyltricetiflavan (1a) in five steps as well as an interesting discovery during the demethylation process.

(−)-7-O-Galloyltricetiflavan (1, Fig. 1) was isolated from a methanolic extract of the leaves of Pithecellobium clypearia by Ooi V. and coworkers in 2006.¹ It is a catechin-like compound without an OH substituent at C-3, and it shows good antiviral activities against respiratory syncytial virus (RSV), influenza A (H1N1) virus, coxsackie B3 (Cox B3) virus and herpes simplex virus type 1 (HSV-1) as well as anti-inflammatory and anti-allergic activities.^2–5 To date, the preparation of (−)-7-O-galloyltricetiflavan (1) still requires extraction and purification of plant material, and only a few synthetic examples of this type of flavan have been reported.^6,7 Herein we report the first total synthesis of (±)-7-O-galloyltricetiflavan (1a) in five steps as well as an interesting discovery during the demethylation process.Open in a separate windowFig. 1The structure of 7-O-galloyltricetiflavan (1).The synthesis of (±)-7-O-galloyltricetiflavan (1a) was started from the preparation of the flavone derivative 4a as shown in Scheme 1. Buckle''s group reported an efficient one-pot synthesis of flavones by the treatment of 2-hydroxyacetophenones with the corresponding aroyl chloride in wet K₂CO₃/acetone (1% w/w water),⁸ but the reaction proceeded very slowly because the trihydroxyacetophenone (2) was insoluble in acetone. With water-toluene as the solvent, in the presence of K₂CO₃ and tetrabutylammonium hydrogen sulfate,⁹ the reaction could provide flavone 4a in 30% yield and 3-acylated product 4b in 50% yield in one-pot in about two hours. Many efforts to improve the yield of 4a failed, but 4b could be converted into 4a by hydrolysis in 50% yield.¹⁰ Afterwards, acylation of 4a with trimethoxybenzoyl chloride (3) and K₂CO₃ gave 7-O-galloylflavone 5 in 91% yield, which was then reduced to the flavan 6 by hydrogenation with palladium on carbon as the catalyst for 3 days in 62% yield.¹¹Open in a separate windowScheme 1The synthesis of intermediates of (±)-7-O-galloyltricetiflavan (1a).When flavan 6 was treated with BBr₃ in dichloromethane at −40 °C or −78 °C,¹² the desired product 1a was generated in only 3% yield (based on HPLC-MS analysis), accompanied with 4-O-methyl gallic acid (7) and flavan 8 as the major products, indicating the ester bond of hex-O-methylflavan 6 is highly unstable under acidic conditions (Scheme 1).Then, 5-O-methylflavan 9, 5-O-acetylflavan 10 and 5-O-phenylacetylflavan 11 were prepared as substrates to explore if they provided different results (Scheme 2). Similarly, 5-O-acetylflavan 10 and 5-O-phenylacetylflavan 11 were not tolerated under these reaction conditions, which gave the hydrolyzed products 7 and 8 as major products. In contrast, when flavan 9 was treated with BBr₃ in dichloromethane at −40 °C to room temperature, the desired product (±)-7-O-galloyltricetiflavan (1a) was generated in 91% yield and no hydrolyzed product was detected after 24 h. The structure of (±)-7-O-galloyltricetiflavan (1a) were confirmed by ¹H NMR, ¹³C NMR, and HR-MS spectrum, and they are consistent with the literature''s report.¹Open in a separate windowScheme 2The demethylation of flavan derivatives.We presumed that when BBr₃ was added to the additional 5-O-methyl group to form the 5-O-B-Br₂ complex, it may stabilizes the ester bond of 7-phenolic hydroxyl group. By contrast, the 5-O-acetyl or 5-O-phenylacetyl groups was more easily hydrolyzed and could not help stabilize the ester bond.In conclusion, (±)-7-O-galloyltricetiflavan (1a) was synthesized successfully in five steps from commercial available trihydroxyacetophenone (2) and trimethoxybenzoyl chloride (3). Flavone 4a was prepared in a one-pot reaction and it gave hex-O-methylflavan 6 followed by acylation and reduction. However, the demethylation of flavan 6 by BBr₃ gave the hydrolyzed fragments 7 and 8 as major products. Similarly, neither 5-O-acetylflavan 10 nor 5-O-phenylacetylflavan 11 could provide the desired product. In contrast, hept-O-methylflavan 9 could give the desired product (±)-7-O-galloyltricetiflavan (1a) in 91% yield. The additional 5-O-B-Br₂ complex may stabilize the ester bond during the demethylation process. Our method could also provide an efficiently pathway to prepare other 7-O-acylated flavans.     

Cold sintering of YBa2Cu3O7−δ

Cold sintering is a sintering technique which enables ceramic powders to be densified at greatly reduced temperatures compared to traditional solid state techniques, which often require temperatures in excess of 1000 °C. These temperatures often preclude the exploitation of size or orientational effects in ceramics as these are lost during heating. One such effect is the orientation of the crystallographic c axis in YBa₂Cu₃O_7−δ (YBCO) which can be controlled through applied pressure. This effect is of interest for increasing critical current density which is highly dependent on the orientation of the a–b (CuO₂) planes within the ceramic. Using cold sintering, we demonstrate that dense YBCO can be created at 180 °C (vs. 1000 °C using solid state) and demonstrate that the likely sintering mechanism is mediated by the cracking which occurs in YBCO when exposed to water. In addition, the ceramics produced show and retain the orientational effect, representing a unique opportunity to study the effect on critical current density. We show that the intergranular critical current when the a–b planes are parallel to the applied field is around 15% higher than when perpendicular.

Cold sintered superconducting YBa₂Cu₃O_7−δ densified at 180 °C shows enhanced critical current densities by exploiting grain alignment created during pressing.