The development of an NMR spectrometer for the precision determination of the ratio of the resonance frequencies of nuclei

A nuclear magnetic resonance (NMR) spectrometer for determining the resonance frequencies of various nuclei at a magnetic field induction of 2.142 T is described. The spectrometer enables the NMR frequency of a nucleus to be recorded simultaneously with the frequency of the signal from a comparison nucleus and enables the magnetic field to be stabilized using the NMR signal from a third nucleus. The ratios of the resonance frequencies of protons and of the lithium-7 nuclei: F(H₂O)F(⁷Li) = 2.573041789(3) are obtained for a solution of lithium chloride in water, and also the ratio for protons and deuterons of an isotopic solution of water: F(H₂O)/F(D₂O) = 6.514399603(2). Methods of determining the systematic errors which occur when there are quadratic gradients of the magnetic field in the sample when processing spectral numerical data matrices are discussed.     

A study of the oxidation curing mechanism of polycarbosilane fibre by solid-state high-resolution nuclear magnetic resonance

The chemical structure of oxidation-cured polycarbosilane fibres has been studied by IR and chemical analysis, but its structure has not been identified in detail. In this work, the molecular structure was examined by chemical analysis, and solid state¹³C and²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Si-O-Si, Si-O-C(I) and Si-O-C(II) bonds were formed by the oxidation curing process. The six chemical bonds (Si-C, Si-H, Si-Si, Si-O-Si, Si-O-C(I) and Si-O-C(II)) in oxidation-cured polycarbosilane were determined quantitatively, and the chemical structural model was shown. Solid state²⁹Si resolution NMR spectroscopy has proved to be a powerful tool for investigating the curing mechanism of oxidation-cured polycarbosilane fibres.     

Local structures of mesoporous bioactive glasses and their surface alterations in vitro: inferences from solid-state nuclear magnetic resonance

We review the benefits of using (29)Si and (1)H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy for probing the local structures of both bulk and surface portions of mesoporous bioactive glasses (MBGs) of the CaO-SiO(2)-(P(2)O(5)) system. These mesoporous materials exhibit an ordered pore arrangement, and are promising candidates for improved bone and tooth implants. We discuss experimental MAS NMR results from three MBGs displaying different Ca, Si and P contents: the (29)Si NMR spectra were recorded either directly by employing radio-frequency pulses to (29)Si, or by magnetization transfers from neighbouring protons using cross polarization, thereby providing quantitative information about the silicate speciation present in the pore wall and at the MBG surface, respectively. The surface modifications were monitored for the three MBGs during their immersion in a simulated body fluid (SBF) for intervals between 30 min and one week. The results were formulated as a reaction sequence describing the interconversions between the distinct silicate species. We generally observed a depletion of Ca(2+) ions at the MBG surface, and a minor condensation of the silicate-surface network over one week of SBF soaking.     

NMR characterization of silicon nitride: slurry homogeneity by T2-weighted proton imaging

The Si3N4/water slurries were studied by proton nuclear magnetic resonance (1HNMR) imaging for homogeneity. 1H nuclear spin echo signals from Si3N4/water slurries were observed by a (π/2)-τ-π-τ-echo pulse sequence. Bloch’s equations were used to calculate the spin-spin relaxation times (T2) from these echo intensities. The T2 for the protons in these slurries was measured to be 53.7±0.1 ms. The T2-weighted imaging technique utilizing (π/2)-τ-π-τ multiple pulse sequence was mixed with a “shape pulse” for radio frequency (RF)-excitation to detect nuclear spin echo signals for image construction. Sinc shape pulses were used to mix with both the (π/2) and the π pulses as a frequency carrier because of the mobility of water molecules in the slurry. The nuclear spin echo intensities were transformed into three-dimensional pictures by magnetic field gradients generated by coils along x, y and z-co-ordinates. Axial-section slices were taken to map the water distribution of the slurry in an NMR tube. A stable and well-dispersed Si3N4/H2O slurry, with ammonium polymethacrylate as dispersant, was observed for several hours. Agglomerization of this slurry was detected after 15 h of standing and NMR imaging shown in contour plots depicted clearly the location and the degree of agglomerization. The water distribution can also be presented in three dimensions by stack plotting of the water intensity profiles. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

Effects of sol aging on mesoporous silica thin films organization

Cetyltrimethylammonium bromide (CTAB) templated mesoporous silica thin films were deposited on glass slides by evaporation-induced self-assembly (EISA) process using a dip-coating method. The effects of sol aging on the mesophase structure of the thin films organization were investigated. Identification of the structures was accomplished by coupling X-ray diffraction (XRD) and transmission electron microscope (TEM) investigations, and corresponding sol was characterized by ²⁹Si solution state nuclear magnetic resonance (²⁹Si NMR) and UV-Vis spectrophotometer for studying the mesophase structure evolution. Results indicate that sol aging has great effects on the mesophase structure of the films organization, which includes degree of order and phase transformation of mesoporous silica thin films. To obtain a better understanding of the effects of sol aging on the mesophase structure, the theories of apparent mass fractal dimension and charge density matching were introduced to explain the self-assembly process.     

Methods for detecting silicones in biological matrixes.

Methods for analyzing for silicon and silicone in biological matrixes were developed. A silicone-specific technique involved microwave digestion of samples in acid solution to rapidly break down the biological matrix while hydrolyzing silicones to monomeric species. The resulting monomeric silanol species were then capped with trimethylsilyl groups, extracted into hexamethyldisiloxane, and analyzed by gas chromatography. In serum, positive identification of silicone species with detection limits below 0.5 microgram of Si/mL are possible with this technique. The technique is compared with a silicone-specific technique, 29Si NMR, and a non-silicone-specific technique, ICP-AES. 29Si NMR was far less sensitive, with a detection limit of only 64 micrograms of Si/mL in serum when analyzing for one compound with a single sharp resonance. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) has potentially lower detection limits, but the technique is not silicone-specific and suffers from species-dependent responses.     

Towards Single Biomolecule Imaging via Optical Nanoscale Magnetic Resonance Imaging

Nuclear magnetic resonance (NMR) spectroscopy is a physical marvel in which electromagnetic radiation is charged and discharged by nuclei in a magnetic field. In conventional NMR, the specific nuclei resonance frequency depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms. NMR is routinely utilized in clinical tests by converting nuclear spectroscopy in magnetic resonance imaging (MRI) and providing 3D, noninvasive biological imaging. While this technique has revolutionized biomedical science, measuring the magnetic resonance spectrum of single biomolecules is still an intangible aspiration, due to MRI resolution being limited to tens of micrometers. MRI and NMR have, however, recently greatly advanced, with many breakthroughs in nano‐NMR and nano‐MRI spurred by using spin sensors based on an atomic impurities in diamond. These techniques rely on magnetic dipole–dipole interactions rather than inductive detection. Here, novel nano‐MRI methods based on nitrogen vacancy centers in diamond are highlighted, that provide a solution to the imaging of single biomolecules with nanoscale resolution in‐vivo and in ambient conditions.     

Preparation and characterization of mesoporous lithium borosilicate material via the sol–gel process

The lithium borosilicate gels were prepared from the cohydrolysis of the tetraehtylorthosilicate (Si(C₂H₅O)₄) and triethylborate (B(C₂H₅O)₃) by using an ethanolic solution of tetradecyltrimethylammonium bromide (TTAB) as surfactant. The Li⁺ ions were introduced from an acidic solution of lithium carbonate (Li₂CO₃). Depending on the B/Si, Li/Si and TTAB/Si molar ratios at pH equal to 1 and at room temperature, monolithic and transparent colourless gels were obtained. The structure of the gel was investigated by infrared spectroscopy (IR), ²⁹Si, ¹¹B and ⁷Li solid-state magnetic resonance (MAS NMR) and by thermal analysis (DTA–TG). The results show the possibility of obtaining a borosilicate network via B–O–Si bonds in which Li⁺ ions were dispersed. The adsorption–desorption isotherms of the xerogel were characteristic of mesoporous materials. These materials may provide a greater free volume through which conducting ions can move.     

Alkali activation of reactive silicas in cements: in situ 29Si MAS NMR studies of the kinetics of silicate polymerization

In the highly alkaline environment of the cement paste of a concrete, a source of silica can potentially react in two ways. In the pozzolanic reaction, it can combine with free lime to generate additional calcium silicate hydrate binding phase. Alternatively, reaction with alkali to form a gel can occur; this gel may swell and degrade the concrete. ²⁹Si magic angle spinning (MAS) and cross-polarization (CP) MAS nuclear magnetic resonance (NMR) studies have been performed to determine the silicate connectivity in some model cement systems; ²⁹Si enrichment was utilized to enable a series of spectra to be acquired in situ from a single sample.The hydrate from pozzolanic reaction of lime with silica was similar to the hydrate formed around silica in blended pozzolanic cements, with a relatively high crystallinity and long silicate chains. In the absence of lime, silica reacted with an alkaline solution to produce a gel having a high degree of cross linking, and a range of silicate mobilities. Tricalcium silicate hydration was found to be accelerated significantly by high levels of alkali (KOH) in solution; the hydrate formed had shorter silicate chains and was more crystalline than that produced by reaction in pure water. Hydration in alkali solution of a model blended cement, comprising a mixture of tricalcium silicate and silica, gave rise to two products, a long chain calcium silicate hydrate (C-S-H) and an alkali silicate of low rigidity. The alkali silicate phase gradually polymerized; at later ages it underwent a phase change, although no crystalline phase appeared to be formed. Silicate exchange took place between the C-S-H and the alkali silicate phase at a slow rate.     

Calibration of a Van de Graaf accelerator and determination of the threshold of the reaction 45Sc(p,n)45Ti using a covariance analysis

The resonances of the ²⁷Al(p, γ)²⁸Si reaction are used to determine the relation between the proton energy and the frequency of a nuclear magnetic resonance (NMR) meter controlling the magnetic induction of a 90° bending magnet used for proton energy analysis.The experimental results and their covariance matrix have been used to obtain a calibration function by a least-squares fit. In addition the threshold and the Q value of the reaction ⁴⁵Sc(p, n)⁴⁵Ti have been determined including all uncertainties and correlations in the analysis.     

Silicon oxycarbide glasses derived from polymeric networks with different molecular architecture prepared by hydrosilylation reaction

We have prepared two polysiloxane networks with different molecular structures and similar compositions as suitable precursors of silicon oxycarbide glasses (SiOC), in order to understand the influence of the molecular architecture of the polymeric networks on the structure of these glasses. The structural evolution from the polymeric precursor to glasses was followed by solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and FTIR spectroscopies, X-ray diffraction, thermogravimetric analysis, and density measurements up to 1000 °C. The high- temperature behavior of SiOC glasses, up to 1600 °C, was studied by X-ray diffraction, and ²⁹Si MAS NMR spectroscopy. The formation of carbosilane bridges in the polymeric networks during the hydrosilylation reaction contributed to the generation of a larger amount of carbidic sites in the final products. An increase in the pyrolysis temperature led to a distribution of silicon sites and crystallization profiles that depended on the molecular architecture of the polymeric precursors.     

Solid-State Silicon NMR Quantum Computer

A solid-state quantum computer composed entirely of semiconductor silicon is proposed. Qubits are nuclear spins, I = 1/2, of ²⁹Si stable isotopes in the form of atomic chains embedded in a nuclear-spin free matrix of ²⁸Si stable isotopes. Each ²⁹Si nuclear spin in a chain can be accessed selectively with a different resonant frequency (rf) due to a large magnetic field gradient created by a nearby micromagnet, i.e., unitary operations needed for quantum computing can be performed by fine tuning of the rf. Ensemble readout of qubits from 10⁵ copies of the atomic chain is accomplished by magnetic resonance force microscopy.     

The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends

The C-S-H gels present in both water- and alkali-activated hardened pastes of white Portland cement/blast-furnace slag blends have been studied by solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and analytical transmission electron microscopy (TEM). Structural data are obtained by NMR for the semi-crystalline C-S-H gels in the alkali-activated systems and extended to the nearly amorphous gels in the water-activated systems by peak broadening; unambiguous chemical analyses are determined in the TEM. The following conclusions apply to both the semi-crystalline and nearly amorphous C-S-H gels: (1) aluminium substitutes for silicon at tetrahedral sites; (2) aluminium only substitutes for silicon in the central tetrahedron of pentameric silicate chains; (3) the results strengthen confidence in dreierkette-based models for the structure of C-S-H. Compositional similarities suggest that these conclusions will be true for OPC/slag blends, and possibly also for OPC/pulverized fuel ash blends indicating that the same structural model applies to C-S-H gels in a wide range of hardened cement pastes. This revised version was published online in November 2006 with corrections to the Cover Date.     

Geopolymerization reaction to consolidate incoherent pozzolanic soil

Pozzolanic material-based geopolymer has been proposed as a solving methodology to the geohazards, due to pozzolanic collapsible soils widely present in the South Italy. The geopolymer was synthesized from pozzolana material under activation of NaOH 10 M or slurry of NaAlO₂ in NaOH 10 M solution. The specimens were cured at 25 °C and 100% RH for different ageing times. The effect of the two activation methods on the properties of the geopolymer was investigated by means of X-ray diffraction, scanning electron microscopy (SEM), FTIR spectroscopy, nuclear magnetic resonance (²⁷Al and ²⁹Si NMR) and uniaxial compression tests. XRD, NMR and IR analysis indicate the geopolymer is generated by the dissolution of the silico-aluminate phases present in the pozzolana and the successive re-organization in amorphous and crystalline neo-formed phases. The spectroscopic evidences confirm that the 4-coordinated Al atoms present in the neat pozzolana and in the NaAlO₂ change their coordination state splitting between 6- and 4-coordinated atoms, modifying the traditional chemistry of polysialate formation. SEM results show the synthesized geo-polymer maintained the granular morphology of the pozzolana and the geo-polymeric reactions occurred mainly at the surface of pozzolana particulates. Furthermore, uniaxial strength data increase gradually upon the curing time, until 40 MPa for the specimens activated with the slurry system.     

Si complexes in calcium phosphate biomaterials

Silicon complexes in silicon doped calcium phosphate bioceramics have been studied using ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy with the objective of identifying the charge compensation mechanisms of silicon dopants. Three different materials have been studied: a multiphase material composed predominantly of a silicon stabilized α-tricalcium phosphate (α-TCP) phase plus a hydroxyapatite (HA) phase, a single phase Si-HA material and a single phase silicon stabilized α-TCP material. NMR results showed that in all three materials the silicon dopants formed Q¹ structures in which two silicate tetrahedra share an oxygen, creating an oxygen vacancy which compensated the substitution of two silicon for phosphorus. This finding may explain the phase evolution previously found where silicon stabilized α-TCP is found at low temperature after sintering.     

The solid state chemistry of metakaolin-blended ordinary Portland cement

The hydration of ordinary Portland cement (OPC) pastes containing 0 and 20% metakaolin was monitored by differential thermal analysis (DTA) and solid state magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The presence of hydrated gehlenite and a relative reduction in calcium hydroxide content of the metakaolin-blended OPC pastes observed by DTA are indicative of the pozzolanic reaction of metakaolin. An increase in the capacity of metakaolin-blended OPC pastes to exclude chloride ions from the pore electrolyte phase, via solid phase binding, has been reported. It is proposed that this increase in chloride binding capacity could be attributed to the participation of calcium aluminate species in the formation of Friedel's salt which would otherwise be engaged in the formation of hydrated gehlenite and other AFm phases. The accelerating effect of replacement additions of metakaolin has been shown by ²⁹Si NMR and was denoted by a comparative increase in the intensity of resonances arising from Q¹ and Q² species compared with that of Q⁰ species for metakaolin-blended specimens. The primary reactive centres of the pozzolan have been shown to be the 5-coordinate aluminium and amorphous silica. The spreading of the Q⁴ resonance of the amorphous silica of metakaolin through the Q³ and into the Q² and Q¹ regions of the NMR spectrum during pozzolanic reaction has been observed.     

Polymerization of alkaline-calcium-silicate hydrates obtained by interaction between alkali–silica solutions and calcium compounds. A 29Si nuclear magnetic resonance study

Alkali–silica solutions containing varying amounts of silica and alkali ions have been prepared. Solids were obtained by interaction of these solutions with calcium hydroxide, calcium chloride or calcium sulphate. A 29Si nuclear magnetic resonance study has been carried out on these products during the initial and final stages of the synthesis. Three major kinds of solids appear in the CaO–Na2O(K2O)–SiO2 phase diagram; a chain-like nanocrystalline solid, a plate-like amorphous solid, and a three-dimensional amorphous network solid. A relationship between the initial system (i.e., the initial solution) and the final system (i.e., the solid plus the residual solution) is established. The solid is found to have a higher degree of polymerization than the initial solution while the residual solution is mearly an alkali–silica solution without calcium ions.     

Thickness dependence of magnetic properties of Co90Fe10 nanoscale thin films

Ferromagnetic monolayers Co90Fe10 thin films with individual layer thicknesses 2, 6, and 8 nm were grown on thermally oxidized Si substrate and magnetic properties of these were investigated with Ferromagnetic resonance (FMR) technique at room temperature. The magnetoresistance (MR) of the samples were measured as a function of applied DC magnetic field and the thickness dependence of the MR was plotted. The FMR spectra were recorded for both parallel and perpendicular geometry. The X-band (9.5 GHz) FMR spectra and resonance field of samples were analyzed and fitted theoretically by using the Landau-Lifshits dynamic equation of motion for magnetization with the Bloch-Bloembergen type damping term. The computer programs have been written to extract the effective magnetization (M), g-values and spin-spin relaxation time (T2) fitting parameters. The thickness dependence of magnetic parameters has been obtained from experimental data by mean of a theoretical model.     

An NMR method of measuring the mass of magnetic beads in a lowdensity fluid [biomedical measurement]

A nuclear magnetic resonance (NMR) method of determining the mass of the magnetic particles present in a fluid by observing the relative frequency shift of an NMR signal is proposed. Experimental tests have been carried out to find the ratio of the mass of the beads to the total mass of the liquid with various densities by using this method. The method can be used to measure masses in a fluid containing a low concentration of magnetic particles