Magnetic resonance imaging of chemical waves in porous media

MRI of chemical reactions and processes

As magnetic resonance imaging (MRI) can spatially resolve a wealth of molecular information available from nuclear magnetic resonance (NMR), it is able to non-invasively visualise the composition, properties and reactions of a broad range of spatially-heterogeneous molecular systems. Hence, MRI is increasingly finding applications in the study of chemical reactions and processes in a diverse range of environments and technologies. This article will explain the basic principles of MRI and how it can be used to visualise chemical composition and molecular properties, providing an overview of the variety of information available. Examples are drawn from the disciplines of chemistry, chemical engineering, environmental science, physics, electrochemistry and materials science. The review introduces a range of techniques used to produce image contrast, along with the chemical and molecular insight accessible through them. Methods for mapping the distribution of chemical species, using chemical shift imaging or spatially-resolved spectroscopy, are reviewed, as well as methods for visualising physical state, temperature, current density, flow velocities and molecular diffusion. Strategies for imaging materials with low signal intensity, such as those containing gases or low sensitivity nuclei, using compressed sensing, para-hydrogen or polarisation transfer, are discussed. Systems are presented which encapsulate the diversity of chemical and physical parameters observable by MRI, including one- and two-phase flow in porous media, chemical pattern formation, phase transformations and hydrodynamic (fingering) instabilities. Lastly, the emerging area of electrochemical MRI is discussed, with studies presented on the visualisation of electrochemical deposition and dissolution processes during corrosion and the operation of batteries, supercapacitors and fuel cells.

Spatially resolved chemical reaction monitoring using magnetic resonance imaging

Over the previous three decades, the use of MRI for studying dynamic physical and chemical processes of materials systems has grown significantly. This mini-review provides a brief introduction to relevant principles of MRI, including methods of spatial localization, factors contributing to image contrast, and chemical shift imaging. A few historical examples of (1) H MRI for reaction monitoring will be presented, followed by a review of recent research including (1) H MRI studies of gelation and biofilms, (1) H, (7) Li, and (11) B MRI studies of electrochemical systems, in vivo glucose metabolism monitored with (19) F MRI, and in situ temperature monitoring with (27) Al MRI. Copyright © 2015 John Wiley & Sons, Ltd.

A Synthetic Replicator Drives a Propagating Reaction-Diffusion Front

A simple synthetic autocatalytic replicator is capable of establishing and driving the propagation of a reaction-diffusion front within a 50 μL syringe. This replicator templates its own synthesis through a 1,3-dipolar cycloaddition reaction between a nitrone component, equipped with a 9-ethynylanthracene optical tag, and a maleimide. Kinetic studies using NMR and UV-vis spectroscopies confirm that the replicator forms efficiently and with high diastereoselectivity, and this replication process brings about a dramatic change in optical properties of the sample-a change in the color of the fluorescence in the sample from yellow to blue. The addition of a small amount of the preformed replicator at a specific location within a microsyringe, filled with the reaction building blocks, results in the initiation and propagation of a reaction-diffusion front. The realization of a replicator capable of initiating a reaction-diffusion front provides a platform for the examination of interconnected replicating networks under out-of-equilibrium conditions involving diffusion processes.

Chemical patterns in translating vortices: inter- and intra-cellular mixing effects

Stationary chemical patterns-flow distributed oscillations (FDOs)-are obtained when the Belousov-Zhabotinsky reaction is coupled with translating vortex flow in a Vortex Flow Reactor. For certain conditions, the FDOs are unstable with the observation of disappearing bands or complex patterns. The transitions between modes of pattern formation are reproduced in a modified Oregonator model consisting of two-zone cells connected in series. We show that increasing inter-cellular mixing of the outer zones results in a transition from FDO to absolute instabilities (AI) and increasing intra-cellular mixing between the core and outer zones can drive the reverse transition between modes (AI to FDO).

Imaging the Belousov-Zhabotinsky reaction in real time using an ion sensitive array

We show how an array of ion-sensitive-field-effect-transistors can be used to both spatially and temporally image the oscillating pH/ion waves produced by the Belousov-Zhabotinsky (BZ) reaction with high resolution.

Three-dimensional spiral waves in an excitable reaction system: initiation and dynamics of scroll rings and scroll ring pairs

We report experimental results on spiral and scroll waves in the 1,4-cyclohexanedione Belousov-Zhabotinsky reaction. The propagating concentration waves are detected by two-dimensional photometry and optical tomography. Wave pulses can disappear in front-to-front and front-to-back collisions. This anomaly causes the nucleation of vortices from collisions of three nonrotating waves. In three-dimensional systems, these vortices are scroll rings that rotate around initially circular filaments. Depending on reactant concentrations, the filaments shrink or expand indicating positive and negative filament tensions, respectively. Shrinkage results in vortex annihilation. Expansion is accompanied by filament buckling and bending, which is interpreted as developing Winfree turbulence. We also describe the initiation of scroll ring pairs in four-wave collisions. The two filaments are stacked on top of each other and their motion suggests filament repulsion.