Magic Angle Spinning NMR Research Articles

A roadmap to high-resolution standard microcoil MAS NMR spectroscopy for metabolomics.

Current microcoil probe technology has emerged as a significant advancement in NMR applications to biofluids research. It has continued to excel as a hyphenated tool with other prominent microdevices, opening many new possibilities in multiple omics fields. However, this does not hold for biological samples such as intact tissue or organisms, due to the considerable challenges of incorporating the microcoil in a magic-angle spinning (MAS) probe without relinquishing the high-resolution spectral data. Not until 2012 did a microcoil MAS probe show promise in profiling the metabolome in a submilligram tissue biopsy with spectral resolution on par with conventional high-resolution MAS (HR-MAS) NMR. This result subsequently triggered a great interest in the possibility of NMR analysis with microgram tissues and striving toward the probe development of "high-resolution" capable microcoil MAS NMR spectroscopy. This review gives an overview of the issues and challenges in the probe development and summarizes the advancements toward metabolomics.

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

In the pursuit of controlling the propensity of Cu-mordenite (MOR) for the selective oxidation of CH4, we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12-rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as 1H magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu2+ species, previously established as the active sites in the selective oxidation of CH4.

Structure and Conductivity Correlation in NASICON Based Na3Al2P3O12 Glass: Effect of Na2SO4

Identifying the factors influencing the movement of sodium cations (Na+) in glasses accelerates the possible options of glass-based solid electrolyte materials for their applications as a promising electrolyte material in sodium-ion batteries. Nevertheless, due to the poor correlation between the structure and conductivity in glass materials, identifying the factors governing the conductivity still exists as a challenging task. Herein, we have investigated the DC-conductivity variations by correlating the structure and conductivity in sodium superionic conductor (NASICON) based Na3Al2P3O12 (NAP) glass (mol%: 37.5 P2O5—25.0 Al2O3—37.5 Na2O) due to the successive substitution of Na2SO4 for Al2O3. Structural variations have been identified using the Raman and magic-angle spinning nuclear magnetic resonance (MAS-NMR) (for 31P, 23Na, and 27Al nuclei) and conductivity measurements have been done using the impedance spectroscopy. From the ac-conductivity spectra, the correlations between mean square displacement (MSD) and dc-conductivity and between the Na+ concentration and dc-conductivity have also been evaluated. Raman spectra reveal that the increase in the Na2SO4 concentration increases the number of isolated SO42− sulfate groups that are charge compensated by the Na+ cations in the NAP glass. MAS-NMR spectra reveal that the increase in Na2SO4 concentration increases the concentration of non-bridging oxygens and further neither S-O-P nor S-O-Al bonds are formed. Impedance spectroscopy reveals that, at 373 K, the DC conductivity of the NAP glass increases with increasing the Na2SO4 up to 7.5 mol% and then decreases with the further increase. In the present study, we have shown that the mobility of sodium cations played a significant role in enhancing the ionic-conductivity. Further, we have shown that inter-ionic Coulombic interactions and the structural modification with the formation of SO42− units significantly influence the critical hopping length < R2 (tp)> of the sodium cations and consequently the mobility and the ionic conductivity. The present study clearly indicates that, based on the compositions, glass materials can also be treated as strong-electrolyte materials.

Efficient Pulsed Dynamic Nuclear Polarization with the X-Inverse-X Sequence.

Pulsed dynamic nuclear polarization (DNP) is a promising new approach to enhancing the sensitivity of high-resolution magic-angle spinning (MAS) NMR. In pulsed DNP, the transfer of polarization from unpaired electrons to nuclei (usually 1H) is induced by a sequence of microwave pulses. Enhancement factors of the thermal 1H polarization are expected to be independent of the magnetic field, and sample heating by absorption of microwave irradiation will be strongly reduced. The development of DNP pulse sequences is still in its infancy. Of the two basic sequences in existence, NOVEL and TOP DNP, the former is, due to an extremely high power requirement, incompatible with high-resolution MAS NMR, while the latter displays a relatively slow transfer of polarization from electrons to 1H. We introduce here a new pulse sequence for DNP of solids, termed X-inverse-X (XiX) DNP. In experiments at 1.2 T, XiX DNP produces, compared to TOP DNP, a 2-fold higher gain in sensitivity. Our data suggest that a faster transfer of polarization from electrons to 1H is behind the superior performance of XiX DNP. Numerical simulations and experiments indicate that microwave pulse lengths can be chosen across a broad range, without loss of efficiency. These findings are a substantial step toward the implementation of pulsed DNP at high magnetic fields.

High-Entropy Polyanionic Lithium Superionic Conductors

High-entropy ceramics are attracting large interest because of their unique materials properties. Nevertheless, the effect of entropy on the lithium transport remains largely elusive. Here, we report, for the first time, about medium- and high-entropy polyanionic lithium superionic conductors crystallizing in the F–43m space group and adopting the so-called argyrodite structure. The Li6PS5[Cl0.33Br0.33I0.33], Li6P[S2.5Se2.5][Cl0.33Br0.33I0.33], and Li6.5[Ge0.5P0.5][S2.5Se2.5][Cl0.33Br0.33I0.33] materials were structurally characterized using complementary synchrotron and neutron scattering techniques in combination with 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. We show that, in contrast to other high-entropy ceramics, an unequal distribution of elements over the respective crystallographic sites occurs in these materials. Using electrochemical impedance spectroscopy (EIS) and 7Li pulsed field gradient (PFG) NMR spectroscopy, we demonstrate that introducing entropy (compositional disorder) marginally affects the room-temperature ionic conductivity (∼10–3 S cm–1) but instead lowers the activation energy for conduction to 0.22 eV. Our results emphasize the possibility of increasing entropy in polyanionic materials, thereby opening up compositional space for the search of Li-ion conductors with unprecedented properties.

Solid-state NMR spectroscopy insights for resolving different water pools in alginate hydrogels

Alginate hydrogels are versatile self-assembling biocompatible materials with diverse biomedical and food industrial applications, which includes uses in encapsulation, (drug) delivery and tissue engineering. Hydrogel formation requires cross-linking, which for alginates is often done with calcium ions that engage in specific interactions with the polysaccharide carboxylic acid groups. Water molecules also hydrate these alginate groups and fill macropores within the hydrogels, with implications for both mechanical properties and cargo encapsulation. Understanding these aspects of hydrogels requires the observation and characterization of the hydrogel waters, how they engage the alginate, and fill the macropores. Here we employed solid-state NMR (ssNMR) spectroscopy to detect and study water molecules in re-hydrated alginate hydrogels. 1H, 2H, and 13C magic angle spinning (MAS) NMR and relaxation measurements were combined to observe both water and alginate. Two different water phases were detected that vary upon gradual (re)hydration of the alginate hydrogels. These water pools differ in their chemical shifts and NMR relaxation properties, reflecting hydration waters directly associated with the carbohydrate polymers alongside dynamic waters in the macropores. Thus, the ssNMR detects the water-filled macropore water pools and how they vary upon calcium cross-linking. We also observe how calcium cross-linking selectively immobilizes the α-guluronate monosaccharides, but leaves the β-mannuronate units more flexible and prone to selective re-hydration. Thus, these ssNMR experiments can be used to probe cross-linking and hydration of alginate hydrogels, with implications for our understanding of design parameters that tune their performance in (drug) delivery and other food industrial applications.

Quantitative analysis of C-(K)-A-S-H-amount and hydrotalcite phase content in finely ground highly alkali-activated slag/silica fume blended cementitious material

For the development of CO2-reduced binders for structural engineering and for predicting their performance, knowledge of the amount formed in weight fractions of the reaction products is essential. This paper presents a method to specify the hydration products of an alkali-activated slag/silica fume blended cementitious material by weight. The 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to determine the C-A-S-H phases was coupled with energy dispersive analysis (EDX) technology to determine the bound K/Si ratio. Furthermore, the 27Al MAS NMR was used to define the amount of hydrotalcite. With the help of the developed model, the hydration phases of alkali-activated slag/silica fume blended cementitious material in dependency of the activator can be calculated. The results of the mass ratios deduced by NMR with coupled EDX was in agreement with the loss of ignition measured by thermogravimetric analysis (TGA). Consequently, the hydration products can be self-checked.

A Metabonomic View on Wilms Tumor by High-Resolution Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy.

Pediatric cancer NMR-metabonomics might be a powerful tool to discover modified biochemical pathways in tumor development, improve cancer diagnosis, and, consequently, treatment. Wilms tumor (WT) is the most common kidney tumor in young children whose genetic and epigenetic abnormalities lead to cell metabolism alterations, but, so far, investigation of metabolic pathways in WT is scarce. We aimed to explore the high-resolution magic-angle spinning nuclear magnetic resonance (HR-MAS NMR) metabonomics of WT and normal kidney (NK) samples. For this study, 14 WT and 7 NK tissue samples were obtained from the same patients and analyzed. One-dimensional and two-dimensional HR-MAS NMR spectra were processed, and the one-dimensional NMR data were analyzed using chemometrics. Chemometrics enabled us to elucidate the most significant differences between the tumor and normal tissues and to discover intrinsic metabolite alterations in WT. The metabolic differences in WT tissues were revealed by a validated PLS-DA applied on HR-MAS T2-edited 1H-NMR and were assigned to 16 metabolites, such as lipids, glucose, and branched-chain amino acids (BCAAs), among others. The WT compared to NK samples showed 13 metabolites with increased concentrations and 3 metabolites with decreased concentrations. The relative BCAA concentrations were decreased in the WT while lipids, lactate, and glutamine/glutamate showed increased levels. Sixteen tissue metabolites distinguish the analyzed WT samples and point to altered glycolysis, glutaminolysis, TCA cycle, and lipid and BCAA metabolism in WT. Significant variation in the concentrations of metabolites, such as glutamine/glutamate, lipids, lactate, and BCAAs, was observed in WT and opened up a perspective for their further study and clinical validation.

Confinement Promotes Hydrogen Bond Network Formation and Grotthuss Proton Hopping in Ion-Conducting Block Copolymers

Proton-conducting polymer membranes frequently contain nanoscale structural and ion-conducting phases. In addition to enhancing the mechanical properties, this mesoscale structure often leads to a significant increase in ion dynamics; however, the molecular underpinnings behind this phenomenon are not well understood. Here, a model proton-conducting polymeric ionic liquid (PIL) block copolymer is shown to have conductivity up to an order of magnitude higher than an analogous homopolymer. Variable temperature 1H solid-state magic angle spinning (MAS) NMR spectroscopy reveals that confinement in the block copolymer PIL decreases the segmental motion of the PIL and increases the structural rigidity of the ionic functional groups. This increased structural rigidity is found to dramatically increase the connectivity of the hydrogen bonding network in the block copolymer PIL according to double quantum–single quantum 1H MAS NMR. This nanoscale restructuring leads to a significant increase in Grotthuss proton hopping dynamics in the block copolymer PIL compared to the homopolymer.

Quantitative 1H NMR method for analyzing primaquine diphosphate in active pharmaceutical ingredients

Thermal analysis, Fourier Transform IR, the isotropic chemical shift of 1H NMR in different solvents, their temperature dependence and spin-lattice relaxation time constant (T1), solution 1D and 2D NMR, and solid-state 13C and 31P NMR (magic angle spinning NMR) were employed to obtain full information and elucidate the structures of primaquine diphosphate (PQD) samples used for quality controlling malaria medicines. Additionally, a simple, rapid, specific, and reliable quantitative method (qNMR) was developed to determine the PQD level in the raw material of active pharmaceutical ingredients (APIs). The method was developed using ethylene carbonate (EC) as the internal standard and dimethylsulfoxide-d6 (DMSO-d6) as the NMR solvent. For the API qNMR, 1H NMR signals at 3.82 and 1.22 ppm were used. The qNMR methodology, through the linearity, range, LOD and LOQ, stability, precision, robustness, and accuracy, was validated within the requirements of guidelines. The accuracy of the qNMR was evaluated by comparing it to a pharmacopeial HPLC technique and there were no statistical differences (p > 0.05). The proposed qNMR method authentically supports and endorses the current pharmacopoeial methods used for determining the PQD content.

Correlating Sulfur Solubility with Short-to-Intermediate Range Ordering in the Structure of Borosilicate Glasses

The present study focuses on understanding the structural dependence of sulfur (as SO3) solubility in Na2O–B2O3–SiO2 glasses over a broad composition space exhibiting a rich variety of structural associations. The focus is on probing the impact of the (1) structural role of Na+, (2) degree of polymerization, and (3) network former connectivity (formation of Si–O–B and B–O–B linkages) on sulfur solubility in borosilicate glasses. By employing a suite of state-of-the-art characterization techniques, including inductively coupled plasma-optical emission spectroscopy (ICP-OES), magic-angle spinning (MAS) NMR spectroscopy, and Raman spectroscopy, an inverse correlation has been established between the sulfur solubility and the degree of network polymerization in the glasses across the investigated composition space. While the modification of SO3 leading to the formation of SO42– can successfully compete with the network depolymerization mechanisms (i.e., creation of nonbridging oxygens) and induce repolymerization in the glass network, the BO3 → BO4 conversion facilitated by the charge compensation by alkali cations takes precedence. The results from this study, when extended to more complex borosilicate-based glass compositions, will not only add to our understanding of the fundamental science controlling the sulfur solubility in borosilicate-based real-world nuclear waste glasses but will also form a basis for the development of nonempirical quantitative structure–property relationship (QSPR)-based predictive models.

Evaluation of three different remineralizing agents on artificially demineralized enamel lesions: Using scanning electron microscopy-energy dispersive X-ray and magic-angle spinning nuclear magnetic resonance - An in vitro study.

Demineralization can be arrested or reversed when remineralization agents are applied to incipient carious or noncavitated carious lesions. A large number of therapeutic agents, including nonfluoridated products, have been developed to promote enamel remineralization. This study aims to evaluate the efficacy of different remineralizing agents on artificially demineralized enamel lesions. The present in vitro study was conducted on 75 sound premolars divided into three groups of normal, demineralized (n = 15 each), and remineralized teeth (n = 45). The remineralized teeth were further subdivided into three groups (n = 15) as remineralized with 2% sodium fluoride (NaF), 2% NaF, and Psoralea corylifolia (bakuchi) and white mineral trioxide aggregate. Specimens of each group were treated with the above-mentioned remineralizing agents and then subjected to Vickers hardness number (VHN), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), and magic-angle spinning nuclear magnetic resonance (MAS-NMR) for further evaluation. The test results showed significantly the highest VHN and the emission peak of elements under the EDX test, such as calcium, phosphorous, oxygen, and fluorine with remineralized with NaF + bakuchi. MAS-NMR spectra showed fluorine and phosphorous peak in a group with NaF + bakuchi indicative of the increase in remineralization. NaF + bakuchi showed effective results in VHN, SEM-EDX, and MAS-NMR with no antagonist interaction. Thus, P. Corylifolia presents an advantage in enhancing remineralization and inhibiting demineralization for early carious lesions and can be used as a herbal extract for effective reduction in pathogenic bacteria.

STRUCTURAL ENVIRONMENT OF FLUORIDE IONS IN APATITES OF CHEMICAL COMPOSITION AND ORIGIN: 19F MAS NMR STUDY

Apatites of markedly different chemical composition and origin were studied using 19F magic-angle spinning nuclear magnetic resonance. Synthetic carbonate fluorapatites (CFAp) containing 2.6 to 4.7 wt% F, 0 to 4 wt% CO2 and 0 to 1 wt% Na2О, hydroxylfluorapatite (OH:F ≈ 1:1) containing about 3 wt% Y (Y-HFAp), natural REE-apatites, and CFAp and Y-HFAp heated at temperature from 700 to 1000 oC were researched. The spectra of apatites with isomorphic substitutions show the signals (chemical shift δ) caused by fluoride ions in fluorapatite structure and, possibly, near defects in Ca sites (from −102.5 to −100 ppm), near water molecules (H2Os) incorporated in the channels (about −96.5 ppm), and one or two signals with δх from −91 to −86 ppm. The spectra of synthetic CFAp and Y-НFAр heated up to 900 oС and original natural REE-apatites show two components, δх1 and δх2 shifted with 2 − 3 ppm, in this range. It is shown that the component δx2 is new, it hasn’t been observed in the spectra previously. Signals in the δх range are caused by Fх ions (up to 12% F) whose structural environment is different substantially from the "ideal" fluorapatite structure. It is found that the contents of Fх ions and CO2 in synthetic CFAp correlate linearly. It is shown that Fx (Fх1) ions can occupied sites in the channels near single vacancies Ca, and Fх2 ions — near double vacancies, Ca and anionic those in the channels, in CFAp with the F content not higher than stoichiometric and Y-НFАр. These vacancies can form through different heterovalent isomorphism mechanisms such as РО43− → СО32− and/or Са2+ → M3+ (М = REE, Al, Fe), vacancies in the channels of heated apatites with partial substitutions F → H2Os, OH — through dehydration and/or dehydroxylation.

Spectroscopic evaluation of UVI-cement mineral interactions: ettringite and hydrotalcite.

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U LIII-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U LIII-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.

Heterogeneous parahydrogen induced polarization on Rh-containing silicalite-1 zeolites: effect of the catalyst structure on signal enhancement

Parahydrogen-induced polarization (PHIP) on Rh-containing silicalite-1 catalysts is studied using both liquid-state and in situ magic angle spinning NMR techniques and the catalyst structure effect is revealed.

A study combining magic-angle spinning NMR and small-angle X-ray scattering on the interaction in the mixture of poly(benzyl methacrylate) and ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide.

A mixture of poly(benzyl methacrylate) (PBnMA) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]) exhibits lower-critical-solution-temperature (LCST)-type phase separation. An investigation combining magic-angle spinning NMR spectroscopy and small-angle scattering was performed to gain new insights into the interaction between PBnMA and the ionic liquid. The molecular mobility and the solute-solvent interaction in the system were investigated using 1H high-resolution magic-angle spinning NMR. Applying a magic-angle spinning frequency of 2 kHz allowed identifying the PBnMA peaks, which were not observed by conventional solution-state NMR. The peaks of [C2mim]+ almost coincided in the presence and absence of PBnMA, indicating the decoupling of the bulk solvent and polymer. The conformational state of PBnMA in [C2mim][NTf2] was investigated using small-angle X-ray scattering (SAXS). The pair distribution functions of PBnMA chains calculated from SAXS profiles suggest that PBnMA adopts a random coil conformation upon dissolution in [C2mim][NTf2]. The combined study clarifies the decoupled low mobility of polymers with a random coil conformation. It is considered that the specific decoupled low mobility is one of the origins of the decoupling conductivity of [C2mim][NTf2] in a matrix polymer. In addition, an increase in temperature induced a downfield shift and broadening of the [C2mim]+ peaks, suggesting that a larger amount of [C2mim]+ was bound to the PBnMA chains even at temperatures approaching the LCST.

1H detection and dynamic nuclear polarization–enhanced NMR of Aβ1-42 fibrils

Several publications describing high-resolution structures of amyloid-β (Aβ) and other fibrils have demonstrated that magic-angle spinning (MAS) NMR spectroscopy is an ideal tool for studying amyloids at atomic resolution. Nonetheless, MAS NMR suffers from low sensitivity, requiring relatively large amounts of samples and extensive signal acquisition periods, which in turn limits the questions that can be addressed by atomic-level spectroscopic studies. Here, we show that these drawbacks are removed by utilizing two relatively recent additions to the repertoire of MAS NMR experiments-namely, 1H detection and dynamic nuclear polarization (DNP). We show resolved and sensitive two-dimensional (2D) and three-dimensional (3D) correlations obtained on 13C,15N-enriched, and fully protonated samples of M0Aβ1-42 fibrils by high-field 1H-detected NMR at 23.4 T and 18.8 T, and 13C-detected DNP MAS NMR at 18.8 T. These spectra enable nearly complete resonance assignment of the core of M0Aβ1-42 (K16-A42) using submilligram sample quantities, as well as the detection of numerous unambiguous internuclear proximities defining both the structure of the core and the arrangement of the different monomers. An estimate of the sensitivity of the two approaches indicates that the DNP experiments are currently ∼6.5 times more sensitive than 1H detection. These results suggest that 1H detection and DNP may be the spectroscopic approaches of choice for future studies of Aβ and other amyloid systems.

Backbone Torsion Angle Determination Using Proton Detected Magic-Angle Spinning Nuclear Magnetic Resonance.

Protein torsion angles define the backbone secondary structure of proteins. Magic-angle spinning (MAS) NMR methods using carbon detection have been developed to measure torsion angles by determining the relative orientation between two anisotropic interactions—dipolar coupling or chemical shift anisotropy. Here we report a new proton-detection based method to determine the backbone torsion angle by recoupling NH and CH dipolar couplings within the HCANH pulse sequence, for protonated or partly deuterated samples. We demonstrate the efficiency and precision of the method with microcrystalline chicken α spectrin SH3 protein and the influenza A matrix 2 (M2) membrane protein, using 55 or 90 kHz MAS. For M2, pseudo-4D data detect a turn between transmembrane and amphipathic helices.

Metal-Free Pyrene-Based Conjugated Microporous Polymer Catalyst Bearing N- and S-Sites for Photoelectrochemical Oxygen Evolution Reaction.

The development of an efficient, sustainable, and inexpensive metal-free catalyst for oxygen evolution reaction (OER) via photoelectrochemical water splitting is very demanding for energy conversion processes such as green fuel generators, fuel cells, and metal-air batteries. Herein, we have developed a metal-free pyrene-based nitrogen and sulfur containing conjugated microporous polymer having a high Brunauer-Emmett-Teller surface area (761 m2 g−1) and a low bandgap of 2.09 eV for oxygen evolution reaction (OER) in alkaline solution. The π-conjugated as-synthesized porous organic material (PBTDZ) has been characterized by Fourier transform infrared spectroscopy (FT-IR), solid-state 13C (cross-polarization magic angle spinning-nuclear magnetic resonance) CP-MAS NMR, N2 adsorption/desorption analysis, field-emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) experiments. The material acts as an efficient catalyst for photoelectrochemical OER with a current density of 80 mA/cm2 at 0.8 V vs. Ag/AgCl and delivered 104 µmol of oxygen in a 2 h run. The presence of low bandgap energy, π-conjugated conducting polymeric skeleton bearing donor heteroatoms (N and S), and higher specific surface area associated with inherent microporosity are responsible for this admirable photoelectrocatalytic activity of PBTDZ catalyst.

Preparation of geopolymer based on lunar regolith simulant at in-situ lunar temperature and its durability under lunar high and cryogenic temperature

Lunar base construction is of great importance for deep space exploration, and the establishment of lunar pavements for the movement of machinery and transportation of materials is essential to improve construction efficiency. Lunar regolith as an in-situ resource has been demonstrated to be used as raw materials to prepare geopolymers for lunar pavements. In this paper, geopolymers based on lunar regolith simulant with different concentrations of alkali activator were synthesized using the natural high-temperature of the lunar surface. Then, the durability of the resulting geopolymer under lunar high and cryogenic temperature was investigated. The flexural and compressive strength was tested, and the microstructure was characterized using Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy (SEM-EDS), 29Si Magic Angle Spinning-Nuclear Magnetic Resonance (29Si MAS-NMR), and Mercury Intrusion Porosimetry (MIP). The results of the curing section showed that the geopolymer with 8 mol/L sodium hydroxide (NaOH) generated high strength and dense structure, and the period of 420 h–492 h of a lunar day corresponding to a temperature variation from 84.5 °C to 33.5 °C in 72 h was suitable for preparation, with 5.7 MPa and 31.2 MPa 72-h flexural and compressive strength. Three test points were selected on the lunar 30° latitude temperature curve, corresponding to before and after the cryogenic attack, and after high temperature again to investigate the durability of the geopolymer, and the degradation was found noticeable. The flexural strength decreased about 49% and 70%, and the compressive strength decreased about 15% and 18% after the cryogenic temperature and high temperature again, respectively. Microscopic observations revealed that the geopolymer structure was significantly granulated with obvious cracks and increased porosity. In addition, the formation of the zeolitic phase was unexpectedly found, leading to interfacial cracks and mechanical strength reduction of the geopolymer. This paper will be beneficial to explore the evolution of properties of lunar pavement construction materials in the lunar environment.