Stefan Glöggler

Noninvasive nuclear magnetic resonance profiling of painting layers

In this work we demonstrate the potential of single-sided nuclear magnetic resonance (NMR) sensors to access deeper layers of paintings noninvasively by means of high-resolution depth profiles spanning several millimeters. The performance of the sensor in resolving painting structures was tested on models for which excellent agreement with microscopy techniques was obtained. The depth profiling NMR technique was used in situ to investigate old master paintings. The observation of differences in NMR relaxation times of tempera binders from these paintings and from artificially aged panels raises the possibility to differentiate between original and recently restored areas.

A Nanoparticle Catalyst for Heterogeneous Phase Para-Hydrogen-Induced Polarization in Water†

Para-hydrogen-induced polarization (PHIP) is a technique capable of producing spin polarization at a magnitude far greater than state-of-the-art magnets. A significant application of PHIP is to generate contrast agents for biomedical imaging. Clinically viable and effective contrast agents not only require high levels of polarization but heterogeneous catalysts that can be used in water to eliminate the toxicity impact. Herein, we demonstrate the use of Pt nanoparticles capped with glutathione to induce heterogeneous PHIP in water. The ligand-inhibited surface diffusion on the nanoparticles resulted in a (1) H polarization of P=0.25% for hydroxyethyl propionate, a known contrast agent for magnetic resonance angiography. Transferring the (1) H polarization to a (13) C nucleus using a para-hydrogen polarizer yielded a polarization of 0.013%. The nuclear-spin polarizations achieved in these experiments are the first reported to date involving heterogeneous reactions in water.

Para-hydrogen perspectives in hyperpolarized NMR

The first instance of para-hydrogen induced polarization (PHIP) in an NMR experiment was serendipitously observed in the 1980s while investigating a hydrogenation reaction (Seldler et al., 1983; Bowers and Weitekamp, 1986, 1987; Eisenschmid et al., 1987) [1-4]. Remarkably a theoretical investigation of the applicability of para-hydrogen as a hyperpolarization agent was being performed in the 1980s thereby quickly providing a theoretical basis for the PHIP-effect (Bowers and Weitekamp, 1986) [2]. The discovery of signal amplification by a non-hydrogenating interaction with para-hydrogen has recently extended the interest to exploit the PHIP effect, as it enables investigation of compounds without structural alteration while retaining the advantages of spectroscopy with hyperpolarized compounds [5]. In this article we will place more emphasis of the future applications of the method while only briefly discussing the efforts that have been made in the understanding of the phenomenon and the development of the method so far.

Thermal Maps of Gases in Heterogeneous Reactions

More than 85 per cent of all chemical industry products are made using catalysts, the overwhelming majority of which are heterogeneous catalysts that function at the gas–solid interface. Consequently, much effort is invested in optimizing the design of catalytic reactors, usually by modelling the coupling between heat transfer, fluid dynamics and surface reaction kinetics. The complexity involved requires a calibration of model approximations against experimental observations, with temperature maps being particularly valuable because temperature control is often essential for optimal operation and because temperature gradients contain information about the energetics of a reaction. However, it is challenging to probe the behaviour of a gas inside a reactor without disturbing its flow, particularly when trying also to map the physical parameters and gradients that dictate heat and mass flow and catalytic efficiency. Although optical techniques and sensors have been used for that purpose, the former perform poorly in opaque media and the latter perturb the flow. NMR thermometry can measure temperature non-invasively, but traditional approaches applied to gases produce signals that depend only weakly on temperature are rapidly attenuated by diffusion or require contrast agents that may interfere with reactions. Here we present a new NMR thermometry technique that circumvents these problems by exploiting the inverse relationship between NMR linewidths and temperature caused by motional averaging in a weak magnetic field gradient. We demonstrate the concept by non-invasively mapping gas temperatures during the hydrogenation of propylene in reactors packed with metal nanoparticles and metal–organic framework catalysts, with measurement errors of less than four per cent of the absolute temperature. These results establish our technique as a non-invasive tool for locating hot and cold spots in catalyst-packed gas–solid reactors, with unprecedented capabilities for testing the approximations used in reactor modelling.

Stacked planar micro coils for single-sided NMR applications.

This paper describes planar micro structured coils fabricated in a novel multilayer assembly for single-sided NMR experiments. By arranging the coils turns in both lateral and vertical directions, all relevant coil parameters can be tailored to a specific application. To this end, we implemented an optimization algorithm based on simulations applying finite element methods (FEMs), which maximizes the coils sensitivity and thus signal-to-noise ratio (SNR) while incorporating boundary conditions such as the coils electrical properties and a localized sensitivity needed for single-sided applications. Utilizing thin-film technology and microstructuring techniques, the planar character is kept by a sub-millimeter overall thickness. The coils are adapted to the Profile NMR-MOUSE® magnet with a homogeneous slice of about 200 μm in height and a uniform depth gradient of about 20T/m. The final design of a coil with 20 turns, separated in four layers with five turns each, and an outer dimension of 4×4 mm(2) is able to measure a sample volume almost five times smaller than that of a state-of-the-art 14×16 mm(2) Profile NMR-MOUSE® coil with the same SNR. This allows for volume-limited measurements with high SNR and enables different future developments. The minimal dead time of 4 μs facilitates further improvements of the SNR by echo adding techniques and the characterization of samples with short T2 relaxation times. Measurements on solid polymers like polyethylene (PE) and polypropylene (PP) with T2 components as short as 200 μs approve the overall beneficial coil properties. Furthermore the ability to perform depth profiling with microscopic resolution is demonstrated.

Online monitoring of intelligent polymers for drug release with hyperpolarized xenon.

Welcome to the guest zone: By combining hyperpolarized xenon and simple low-field NMR devices it is possible to obtain more control over hydrogels that show potential as drug delivery systems. An alternative way of polymer swelling-degree determination is demonstrated with real-time NMR analysis. An ideal region for solvent uptake can be defined in which the absorbed solvent molecules are completely confined in the nano-porous network of the hydrogel.

NMR Spectroscopy for Chemical Analysis at Low Magnetic Fields

This chapter addresses the limits of low-field NMR spectroscopy for chemical analysis and will answer the question of whether high-resolution NMR spectroscopy for chemical analysis of solutions can be achieved with magnetic fields much lower than 0.1 T without losing the chemical information which at high field is derived from the chemical shift and the indirect spin-spin or J-coupling. The focus is on two major issues. First, the thermal spin population differences given by the Boltzmann distribution are small at low field and so is the signal-to-noise-ratio when starting measurements from thermal equilibrium. Second, the possibility of identifying chemical groups is explored at low magnetic fields where the chemical shift can usually no longer be resolved.

Singlet order conversion and parahydrogen-induced hyperpolarization of 13C nuclei in near-equivalent spin systems.

We have demonstrated two radiofrequency pulse methods which convert the nuclear singlet order of proton spin pairs into the magnetisation of nearby 13C nuclei. These irradiation schemes work well in the near-equivalence regime of the three-spin system, which applies when the difference in the two 1H-13C couplings is much smaller than the 1H-1H coupling. We use pulse sequences to generate thermally polarized singlet states in a reproducible manner, and study the singlet-to-magnetisation transfer step. Preliminary results demonstrate a parahydrogen-enhanced 13C polarization level of at least 9%, providing a signal enhancement factor of more than 9000, using 50% enriched parahydrogen.

Aqueous Ligand-Stabilized Palladium Nanoparticle Catalysts for Parahydrogen-Induced 13C Hyperpolarization

Parahydrogen-induced polarization (PHIP) is a method for enhancing NMR sensitivity. The pairwise addition of parahydrogen in aqueous media by heterogeneous catalysts can lead to applications in chemical and biological systems. Polarization enhancement can be transferred from 1H to 13C for longer lifetimes by using zero field cycling. In this work, water-dispersible N-acetylcysteine- and l-cysteine-stabilized palladium nanoparticles are introduced, and carbon polarizations up to 2 orders of magnitude higher than in previous aqueous heterogeneous PHIP systems are presented. P13C values of 1.2 and 0.2% are achieved for the formation of hydroxyethyl propionate from hydroxyethyl acrylate and ethyl acetate from vinyl acetate, respectively. Both nanoparticle systems are easily synthesized in open air, and TEM indicates an average size of 2.4 ± 0.6 nm for NAC@Pd and 2.5 ± 0.8 nm for LCys@Pd nanoparticles with 40 and 25% ligand coverage determined by thermogravimetric analysis, respectively. As a step toward biological relevance, results are presented for the unprotected amino acid allylglycine upon aqueous hydrogenation of propargylglycine.

Real‐time Detection of Polymerization Reactions with Hyperpolarized Xenon at Low Magnetic Fields

For process control it is desirable to develop simple devices for studying polymerization reactions in real‐time and in‐situ. We are demonstrating an approach using NMR at fields as low as 35 G and hyperpolarized xenon, which allows us to observe polymerization reactions in real‐time. The investigated reaction is a free radical polymerization with the initiator azobisisobutyronitrile (AIBN) and the monomer methyl methacrylate (MMA). AIBN and MMA are mixed together in a sample tube under noble gas atmosphere, and the reaction is started by irradiation with UV light (360 nm). As the reaction goes on, xenon NMR spectra are acquired. They show increasing line broadening and a variation of the chemical shift depending on the state of polymerization. This observation gives rise to the idea that a single‐sided high resolution NMR sensor can be developed with which at least light induced polymerization reactions can be studied in‐situ and in real‐time.