Aditya Rawal

Strongly bound citrate stabilizes the apatite nanocrystals in bone

Nanocrystals of apatitic calcium phosphate impart the organic-inorganic nanocomposite in bone with favorable mechanical properties. So far, the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Here we show that the apatite surfaces are studded with strongly bound citrate molecules, whose signals have been identified unambiguously by multinuclear magnetic resonance (NMR) analysis. NMR reveals that bound citrate accounts for 5.5 wt% of the organic matter in bone and covers apatite at a density of about 1 molecule per (2 nm)2, with its three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.

Shifting paradigms: development of high-efficiency biochar fertilizers based on nano-structures and soluble components

Many biochars have a complex carbon lattice structure with aromatic and aliphatic domains, acidic and basic groups, vacancies, metallic and non-metallic elements, and free radicals. Biochars also have separate mineral oxide, silicate and salt phases, and small and large organic molecules. In the rhizosphere, such constituents can be involved in chemical and biological processes along a soil–microbe–plant continuum, including nutrient cycling, metal chelation and stabilization, redox reactions, and free radical scavenging. It is hypothesized that the greater the amount of these nanoparticles and dissolved components, the greater will be plant and microbial responses. We provide suggestions for developing low-dose, high-efficiency biochar–nanoparticle composites, as well as initial field trial results and detailed characterization of such a biochar–fertilizer composite, to highlight the potential of such biochars.

Molecular silicate and aluminate species in anhydrous and hydrated cements.

The compositions and molecular structures of anhydrous and hydrated cements are established by using advanced solid-state nuclear magnetic resonance (NMR) spectroscopy methods to distinguish among different molecular species and changes that occur as a result of cement hydration and setting. One- and two-dimensional (2D) solid-state (29)Si and (27)Al magic-angle spinning NMR methodologies, including T(1)-relaxation-time- and chemical-shift-anisotropy-filtered measurements and the use of very high magnetic fields (19 T), allow resonances from different silicate and aluminate moieties to be resolved and assigned in complicated spectra. Single-pulse (29)Si and (27)Al NMR spectra are correlated with X-ray fluorescence results to quantify the different crystalline and disordered silicate and aluminate species in anhydrous and hydrated cements. 2D (29)Si{(1)H} and (27)Al{(1)H} heteronuclear correlation NMR spectra of hydrated cements establish interactions between water and hydroxyl moieties with distinct (27)Al and (29)Si species. The use of a (29)Si T(1)-filter allows anhydrous and hydrated silicate species associated with iron-containing components in the cements to be distinguished, showing that they segregate from calcium silicate and aluminate components during hydration. The different compositions of white Portland and gray oilwell cements are shown to have distinct molecular characteristics that are correlated with their hydration behaviors.

Understanding and controlling organic-inorganic interfaces in mesostructured hybrid photovoltaic materials.

The chemical compositions and structures of organic-inorganic interfaces in mesostructurally ordered conjugated polymer-titania nanocomposites are shown to have a predominant influence on their photovoltaic properties. Such interfaces can be controlled by using surfactant structure-directing agents (SDAs) with different architectures and molecular weights to promote contact between the highly hydrophobic electron-donating conjugated polymer species and hydrophilic electron-accepting titania frameworks. A combination of small-angle X-ray scattering (SAXS), scanning and transmission electron microscopy (SEM, TEM), and solid-state NMR spectroscopy yields insights on the compositions, structures, and distributions of inorganic and organic species within the materials over multiple length scales. Two-dimensional NMR analyses establish the molecular-level interactions between the different SDA blocks, the conjugated polymer, and the titania framework, which are correlated with steady-state and time-resolved photoluminescence measurements of the photoexcitation dynamics of the conjugated polymer and macroscopic photocurrent generation in photovoltaic devices. Molecular understanding of the compositions and chemical interactions at organic-inorganic interfaces are shown to enable the design, synthesis, and control of the photovoltaic properties of hybrid functional materials.

A new NMR method for determining the particle thickness in nanocomposites, using T2,H-selective X{1H} recoupling

A new nuclear magnetic resonance approach for characterizing the thickness of phosphate, silicate, carbonate, and other nanoparticles in organic-inorganic nanocomposites is presented. The particle thickness is probed using the strongly distant-dependent dipolar couplings between the abundant protons in the organic phase and X nuclei (31P, 29Si, 13C, 27Al, 23Na, etc.) in the inorganic phase. This approach requires pulse sequences with heteronuclear dephasing only by the polymer or surface protons that experience strong homonuclear interactions, but not by dispersed OH or water protons in the inorganic phase, which have long transverse relaxation times T2,H. This goal is achieved by heteronuclear recoupling with dephasing by strong homonuclear interactions of protons (HARDSHIP). The pulse sequence alternates heteronuclear recoupling for approximately 0.15 ms with periods of homonuclear dipolar dephasing that are flanked by canceling 90 degrees pulses. The heteronuclear evolution of the long-T2,H protons is refocused within two recoupling periods, so that 1H spin diffusion cannot significantly dephase these coherences. For the short-T2,H protons of a relatively immobile organic matrix, the heteronuclear dephasing rate depends simply on the heteronuclear second moment. Homonuclear interactions do not affect the dephasing, even though no homonuclear decoupling is applied, because long-range 1H-X dipolar couplings approximately commute with short-range 1H-1H couplings, and heteronuclear recoupling periods are relatively short. This is shown in a detailed analysis based on interaction representations. The algorithm for simulating the dephasing data is described. The new method is demonstrated on a clay-polymer nanocomposite, diamond nanocrystals with protonated surfaces, and the bioapatite-collagen nanocomposite in bone, as well as pure clay and hydroxyapatite. The diameters of the nanoparticles in these materials range between 1 and 5 nm. Simulations show that spherical particles of up to 10 nm diameter can be characterized quite easily.

Mineral-Biochar Composites: Molecular Structure and Porosity.

Dramatic changes in molecular structure, degradation pathway, and porosity of biochar are observed at pyrolysis temperatures ranging from 250 to 550 °C when bamboo biomass is pretreated by iron-sulfate-clay slurries (iron-clay biochar), as compared to untreated bamboo biochar. Electron microscopy analysis of the biochar reveals the infusion of mineral species into the pores of the biochar and the formation of mineral nanostructures. Quantitative (13)C nuclear magnetic resonance (NMR) spectroscopy shows that the presence of the iron clay prevents degradation of the cellulosic fraction at pyrolysis temperatures of 250 °C, whereas at higher temperatures (350-550 °C), the clay promotes biomass degradation, resulting in an increase in both the concentrations of condensed aromatic, acidic, and phenolic carbon species. The porosity of the biochar, as measured by NMR cryoporosimetry, is altered by the iron-clay pretreatment. In the presence of the clay, at lower pyrolysis temperatures, the biochar develops a higher pore volume, while at higher temperature, the presence of clay causes a reduction in the biochar pore volume. The most dramatic reduction in pore volume is observed in the kaolinite-infiltrated biochar at 550 °C, which is attributed to the blocking of the mesopores (2-50 nm pore) by the nonporous metakaolinite formed from kaolinite.

Exploiting stable radical states for multifunctional properties in triarylamine-based porous organic polymers

Redox-active porous organic polymers (POPs) have enormous potential in applications ranging from electrocatalysis to solar energy conversion. Exploiting the different electronic states offers exciting prospects for controlling host–guest chemistry, however, this aspect of multifunctionality has to date, remained largely unexplored. Here, we present a strategy for the development of multifunctional materials with industrially sought-after properties. A series of hydrophobic POPs containing redox-active triarylamines linked by ethynyl (POP-1), 1,4-diethynylphenyl (POP-2) and 4,4′-diethynylbiphenyl (POP-3) bridges have been synthesised and characterised by NMR and EPR spectroscopy, as well as spectroelectrochemistry and computational modelling. The facile electrochemical or chemical oxidation of the POPs generate mixed-valence radical cation states with markedly enhanced adsorption properties relative to their neutral analogues, including a 3-fold improvement in the H2 uptake at 77 K and 1 bar, and an increase in the isosteric heat of adsorption for CO2.

Origins of saccharide-dependent hydration at aluminate, silicate, and aluminosilicate surfaces

Sugar molecules adsorbed at hydrated inorganic oxide surfaces occur ubiquitously in nature and in technologically important materials and processes, including marine biomineralization, cement hydration, corrosion inhibition, bioadhesion, and bone resorption. Among these examples, surprisingly diverse hydration behaviors are observed for oxides in the presence of saccharides with closely related compositions and structures. Glucose, sucrose, and maltodextrin, for example, exhibit significant differences in their adsorption selectivities and alkaline reaction properties on hydrating aluminate, silicate, and aluminosilicate surfaces that are shown to be due to the molecular architectures of the saccharides. Solid-state 1H, 13C, 29Si, and 27Al nuclear magnetic resonance (NMR) spectroscopy measurements, including at very high magnetic fields (19 T), distinguish and quantify the different molecular species, their chemical transformations, and their site-specific adsorption on different aluminate and silicate moieties. Two-dimensional NMR results establish nonselective adsorption of glucose degradation products containing carboxylic acids on both hydrated silicates and aluminates. In contrast, sucrose adsorbs intact at hydrated silicate sites and selectively at anhydrous, but not hydrated, aluminate moieties. Quantitative surface force measurements establish that sucrose adsorbs strongly as multilayers on hydrated aluminosilicate surfaces. The molecular structures and physicochemical properties of the saccharides and their degradation species correlate well with their adsorption behaviors. The results explain the dramatically different effects that small amounts of different types of sugars have on the rates at which aluminate, silicate, and aluminosilicate species hydrate, with important implications for diverse materials and applications.

Redox tunable viologen-based porous organic polymers

The use of an organic donor–acceptor polymer containing a viologen electron acceptor and triarylamine electron donor as a platform in the development of multifunctional materials is presented. The highly robust porous organic polymer (POP) system allows for exploration of the interplay between electronic and host–guest interactions in the synthesized polymers, POP-V1, which contains a redox-active triarylamine core and POP-V2, which contains a redox-inactive benzene core, where each of the redox states present can be reversibly accessed. The degree of charge transfer in addition to the H2 and CO2 gas adsorption properties of the polymer are able to be tuned as a function of the electronic state which has important implications for the potential applications of these polymers in optical, electrochromic and solar cell devices.

Unifying Design Strategies in Demosponge and Hexactinellid Skeletal Systems

Biological systems are well known for their ability to construct remarkably complex and mechanically robust skeletal structures from a great diversity of minerals. One such example, silica, is widely used in the synthesis of skeletal elements (spicules) within the phylum Porifera (the sponges). As a result, members of this diverse group have served as useful model systems for analysis of the dynamic processes of biosilicification and for investigating structure function relationships in their often hierarchically ordered skeletal systems. This article describes in detail the skeletal diversity within the two silica-forming sponge classes, the Demospongiae and the Hexactinellida, and through the use of several representative examples, discusses the mechanical consequences of the various modes of construction implemented as well as the potential evolutionary pressures that resulted in their observed structural complexity.