Chemical trigger toward phase separation in the aqueous Al(III) system revealed

Mechanistic insights into the formation of hydroxides with unconventional coordination environments to achieve their cost-effective synthesis

Transition metal hydroxides, comprising metal-centered polyhedra, are abundant in nature and have a broad range of applications. Although the intercalation/deintercalation of polyhedra with unconventional coordination numbers (UCN) plays a pivotal role in their formation process and influences their chemical behavior, the detailed mechanism remains obscure. Here, taking the kinetically controlled growth of Co(OH)2 as a model system (where polyhedra with UCN refers to 4-coordinated tetrahedra), by combining in situ pH measurement and in situ ultraviolet-visible (UV-vis) spectroscopy, we tracked Co2+ tetrahedral intercalation/deintercalation with varying OH- concentrations, discovering that the initial Co2+ tetrahedral intercalation into the Co(OH)2 lattice is inevitable and its retention is influenced by the effective OH- concentration, challenging previous beliefs about the formation of Co(OH)2 and potentially other hydroxides. More importantly, an understanding of the intercalation mechanism would significantly contribute to optimization of the synthesis conditions of hydroxides with tunable coordination environments, which hold significant application potential, as evidenced by a proof-of-concept application in the oxygen evolution reaction.

Cations Induced Fluorescence Enhancement in Keggin-Al13 for Quantitative Detection

The Keggin-Al13 hydroxide clusters serve as pivotal models for elucidating molecular pathways in geochemical reactions. In this study, we presented a strategy aiming at the quantitative detection of ε-Al13 Keggin clusters using photoluminescent spectra. Specifically, we manipulate the electronic structure of ε-Al13 by introducing Na ions onto the ε-Al13 surface, encapsulated within a Na-O3 motif. The Na-ion-modified ε-Al13 (Na-ε-Al13) cluster demonstrates incredible photoluminescent qualities, with fluorescence excitation and emission peaks centered at 365 and 436 nm, respectively. In addition, the fluorescence intensities display a linear dependence on the concentrations of Na-ε-Al13, with a detection limit of 15.4 μM. This correlation facilitates the quantitative and precise determination of Na-ε-Al13 concentrations via fluorescence. Both experimental characterizations and theoretical calculations underscore the importance of decorated Na ions in regulating the electronic structure of the ε-Al13 cluster. Lastly, the influence of external anions/cations on the photoluminescent properties of ε-Al13 primarily mirrors modifications to the nonradiative decay process, which is regulated via electrostatic interactions. This work demonstrates an effective strategy for quantitative detection of the ε-Al13 Keggin clusters through photoluminescent spectra.

Unraveling the Structural Evolution of Aluminum Polyoxocations in Solution

The primary objective within the realm of aluminum solution chemistry is to elucidate the structural changes in aluminum polyoxocations under the influence of altered solution conditions. Notably, previous reports are primarily focused on specific types, such as aluminum monomers, species from the Keggin series, and the planar Flat-Al1315+ (F-Al13) cluster. As a result, there is a lack of comprehensive understanding of the remaining aluminum polyoxocations and their respective transformation pathways. In response to this lack, we adopt a combined experimental and theoretical approach to explore the spectral properties of aluminum polyoxocations. Specifically, we analyze infrared spectra, Raman spectra, and aluminum-27 nuclear magnetic resonance (27Al NMR) spectra. Notably, the changes in the spectral features originate from varying solution basicity levels. Through our findings, we can categorize the Al-O clusters into three primary groups: Al(H2O)63+ (Al1), ε-Keggin-[AlO4Al12(OH)24(H2O)12]7+ (ε-Al13), and 6-coordinated aluminum species. Notably, the Raman spectra exhibit prominent peak shifts at 559 and 595 cm-1, indicating the existence of Al3(1) intermediates during the transition from the Al monomer to the ε-Al13 cluster. Overall, this paper presents a comprehensive summary of the possible mechanisms that govern the formation of ε-Al13 from Al3(1), offering a clearer picture of the aluminum polyoxocation landscape and its dynamics under various solution conditions.

Organic Nucleation: Water Rearrangement Reveals the Pathway of Ibuprofen

The organic nucleation of the pharmaceutical ibuprofen is investigated, as triggered by the protonation of ibuprofen sodium salt at elevated pH. The growth and aggregation of nanoscale solution species by Analytical Ultracentrifugation and Molecular Dynamics (MD) simulations is tracked. Both approaches reveal solvated molecules, oligomers, and prenucleation clusters, their size as well as their hydration at different reaction stages. By combining surface-specific vibrational spectroscopy and MD simulations, water interacting with ibuprofen at the air-water interface during nucleation is probed. The results show the structure of water changes upon ibuprofen protonation in response to the charge neutralization. Remarkably, the water structure continues to evolve despite the saturation of protonated ibuprofen at the hydrophobic interface. This further water rearrangement is associated with the formation of larger aggregates of ibuprofen molecules at a late prenucleation stage. The nucleation of ibuprofen involves ibuprofen protonation and their hydrophobic assembly. The results highlight that these processes are accompanied by substantial water reorganization. The critical role of water is possibly relevant for organic nucleation in aqueous environments in general.

Laser-induced graphene electrochemical sensor for quantitative detection of phytotoxic aluminum ions (Al3+) in soils extracts

Aluminum in its Al3+ form is a metal that inhibits plant growth, especially in acidic soils (pH < 5.5). Rapid and accurate quantitative detection of Al3+ in agricultural soils is critical for the timely implementation of remediation strategies. However, detecting metal ions requires time-consuming preparation of samples, using expensive instrumentation and non-portable spectroscopic techniques. As an alternative, electrochemical sensors offer a cost-effective and minimally invasive approach for in situ quantification of metal ions. Here, we developed and validated an electrochemical sensor based on bismuth-modified laser-induced graphene (LIG) electrodes for Al3+ quantitative detection in a range relevant to agriculture (1-300 ppm). Our results show a linear Al3+ detection range of 1.07-300 ppm with a variation coefficient of 5.3%, even in the presence of other metal ions (Pb2+, Cd2+, and Cu2+). The sensor offers a limit of detection (LOD) of 0.34 ppm and a limit of quantification (LOQ) of 1.07 ppm. We compared its accuracy for soil samples with pH < 4.8 to within 89-98% of spectroscopic methods (ICP-OES) and potentiometric titration. This technology's portability, easy to use, and cost-effectiveness make it a promising candidate for in situ quantification and remediation of Al3+ in agricultural soils and other complex matrices.

Role of Water during the Early Stages of Iron Oxyhydroxide Formation by a Bacterial Iron Nucleator

Understanding the nucleation of iron oxides and the underlying hydrolysis of aqueous iron species is still challenging, and molecular-level insights into the orchestrated response of water, especially at the hydrolysis interface, are lacking. We follow iron(III) hydrolysis in the presence of a synthetic bacterial iron nucleator, which is a magnetosome membrane specific peptide, by using a constant pH titration technique. Three distinct hydrolysis regimes were identified. Interface-selective sum frequency generation (SFG) spectroscopy was used to probe the interfacial reaction and water in direct contact with the peptide. SFG data reveal that iron(III) species react quickly with interfacial peptides while continuously enhancing water alignment into the later stages of hydrolysis. The gradually aligning water molecules are associated with initially promoted (regimes I and II) and later suppressed (regime III) hydrolysis after the saturation of water alignment has occurred until regime II. These interfacial insights are crucial for understanding the early stage of iron oxide biomineralization.

Masking Strategy Constructed Metal Coordination Hydrogels with Improved Mechanical Properties for Flexible Electronic Sensors

Metal coordination hydrogels (MC-HGs) that introduce dynamically coordinate bonds together with metal ionic conduction have attracted considerable attention in flexible electronics. However, the traditional soaking method alleged to have technical scalability faces the challenge of forming MC-HGs with a "core-shell" structure, which undoubtedly reduces the whole mechanical properties and ionic stimulation responsiveness required for flexible electronics materials. Herein, a novel strategy referred to as "masking" has been proposed based on the theory of the valence bond and coordination chemistry. By regulating the masking agents and their concentrations as well as pairing mode with the metal ions, the whole mechanical properties of the resulting composites (MC-HGsMasking) show nearly double the values of their traditional soaking samples (MC-HGsSoaking). For example, the fracture stress and toughness of Fe-HGsMasking(SA, 5.0 g/L) are 1.55 MPa and 2.14 MJ/m3, almost twice those of Fe-HGsSoaking (0.83 MPa and 0.93 MJ/m3, respectively). Microstructure characterization combined with finite element analysis, molecular dynamics, and first-principles simulations demonstrates that the masking strategy first facilitating interfacial permeation of metal complexes and then effective coordination with functional ligands (carboxylates) of the hydrogels is the mechanism to strengthen the mechanical properties of composites MC-HGsMasking, which has the potential to break through the limitations of current MC-HGs in flexible electronic sensor applications.

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value-added chemicals, and metal-air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost-effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Crystal Nucleation and Growth of Inorganic Ionic Materials from Aqueous Solution: Selected Recent Developments, and Implications

In this review article, selected, latest theoretical, and experimental developments in the field of nucleation and crystal growth of inorganic materials from aqueous solution are highlighted, with a focus on literature after 2015 and on non-classical pathways. A key point is to emphasize the so far underappreciated role of water and solvent entropy in crystallization at all stages from solution speciation through to the final crystal. While drawing on examples from current inorganic materials where non-classical behavior has been proposed, the potential of these approaches to be adapted to a wide-range of systems is also discussed, while considering the broader implications of the current re-assessment of pathways for crystallization. Various techniques that are suitable for the exploration of crystallization pathways in aqueous solution, from nucleation to crystal growth are summarized, and a flow chart for the assignment of specific theories based on experimental observations is proposed.

Formation of Al30 aggregates and its correlation to the coagulation effect

Al₃₀ is the polycation with the highest degree of polymerization and surface charge in the currently known structural aluminum species. It shows excellent coagulation performance in water treatment process, and has the characteristics of wide application range of pH and dosage. pH value is one of the most important factors affecting the aggregation and coagulation process of Al₃₀, but the influence of Al₃₀ aggregation reaction on its coagulation effect is still unclear. Therefore, this article reports the deprotonation and aggregation reaction of Al₃₀ by adjusting the basicity (B) of the solution, particularly to further understand the coagulation mechanism of Al₃₀ under different conditions. The results showed that in the base titration process, when B < 2.86 in 0.01 M Al₃₀ solution as Al_T (the concentration of total Al), deprotonation and preliminary aggregation mainly occurred; when B > 2.86, the size of Al₃₀ aggregates (Al₃₀agg) increased rapidly, forming gels and gradually transforming into Al(OH)₃. In this process, in addition to the reduction of electrostatic repulsion induced by Al₃₀ deprotonation, the oligomers generated by the partial dissociation of Al₃₀ also play the role of bridging-connection. Under the experimental titration conditions, the Al₃₀agg always maintained a positive zeta potential. In addition, Al₃₀ can deprotonate and aggregate at lower pH, which is an important reason for its unique coagulation characteristics. The larger structure size of Al₃₀ also made it easy to form branched aggregates, so that it can play an effective role in a wider dosage range without destabilization of colloids. This study gives an insight in the advancement of coagulants and promotes the industrial application and commercialization of functional coagulants based on polyaluminum.