Determination of Kinetic Parameters within a Single Nonisothermal On-Flow Experiment by Nanoliter NMR Spectroscopy

Multinuclear 1D and 2D NMR with 19F-Photo-CIDNP hyperpolarization in a microfluidic chip with untuned microcoil

Nuclear Magnetic Resonance (NMR) spectroscopy is a most powerful molecular characterization and quantification technique, yet two major persistent factors limit its more wide-spread applications: poor sensitivity, and intricate complex and expensive hardware required for sophisticated experiments. Here we show NMR with a single planar-spiral microcoil in an untuned circuit with hyperpolarization option and capability to execute complex experiments addressing simultaneously up to three different nuclides. A microfluidic NMR-chip in which the 25 nL detection volume can be efficiently illuminated with laser-diode light enhances the sensitivity by orders of magnitude via photochemically induced dynamic nuclear polarization (photo-CIDNP), allowing rapid detection of samples in the lower picomole range (normalized limit of detection at 600 MHz, nLODf,600, of 0.01 nmol Hz1/2). The chip is equipped with a single planar microcoil operating in an untuned circuit that allows different Larmor frequencies to be addressed simultaneously, permitting advanced hetero-, di- and trinuclear, 1D and 2D NMR experiments. Here we show NMR chips with photo-CIDNP and broadband capabilities addressing two of the major limiting factors of NMR, by enhancing sensitivity as well as reducing cost and hardware complexity; the performance is compared to state-of-the-art instruments.

Near-digital amplification in paper improves sensitivity and speed in biplexed reactions

The simplest point-of-care assays are usually paper and plastic devices that detect proteins or nucleic acids at low cost and minimal user steps, albeit with poor limits of detection. Digital assays improve limits of detection and analyte quantification by splitting a sample across many wells (or droplets), preventing diffusion, and performing analyte amplification and detection in multiple small wells. However, truly digital nucleic acid amplification tests (NAATs) require costly consumable cartridges that are precisely manufactured, aligned, and operated to enable low detection limits. In this study, we demonstrate how to implement near-digital NAATs in low-cost porous media while approaching the low limits of detection of digital assays. The near-digital NAAT was enabled by a paper membrane containing lyophilized amplification reagents that automatically, passively meters and distributes a sample over a wide area. Performing a NAAT in the paper membrane while allowing diffusion captures many of the benefits of digital NAATs if the pad is imaged at a high spatial resolution during amplification. We show that the near-digital NAAT is compatible with a low-cost paper and plastic disposable cartridge coupled to a 2-layer rigid printed circuit board heater (the MD NAAT platform). We also demonstrate compatibility with biplexing and imaging with mobile phones with different camera sensors. We show that the near-digital NAAT increased signal-to-noise ratios by ~ 10×, improved limits of detection from above 10³ copies of methicillin-resistant Staphylococcus aureus genomic DNA to between 100 and 316 copies in a biplexed reaction containing 10⁵ copies of co-amplifying internal amplification control DNA, and reduced time-to-result from 45 min of amplification to 15-20 min for the positive samples.

Modern advancements in continuous-flow aided kinetic analysis

Although kinetic analysis has traditionally been conducted in a batch vessel, continuous-flow aided kinetic analysis continues to swell in popularity.

Quantification and Prediction of Imine Formation Kinetics in Aqueous Solution by Microfluidic NMR Spectroscopy

Quantitatively predicting the reactivity of dynamic covalent reaction is essential to understand and rationally design complex structures and reaction networks. Herein, the reactivity of aldehydes and amines in various rapid imine formation in aqueous solution by microfluidic NMR spectroscopy was quantified. Investigation of reaction kinetics allowed to quantify the forward rate constants k₊ by an empirical equation, of which three independent parameters were introduced as reactivity parameters of aldehydes (S_E , E) and amines (N). Furthermore, these reactivity parameters were successfully used to predict the unknown forward rate constants of imine formation. Finally, two competitive reaction networks were rationally designed based on the proposed reactivity parameters. Our work has demonstrated the capability of microfluidic NMR spectroscopy in quantifying the kinetics of label-free chemical reactions, especially rapid reactions that are complete in minutes.

Optimization of twin parallel microstrips based nuclear magnetic resonance probe for measuring the kinetics in molecular assembly in ultra-small samples

We present the design, fabrication, characterization, and optimization of a TPM (twin parallel microstrip)-based nuclear magnetic resonance (NMR) probe, produced by using a low-loss Teflon PTFE F4B high frequency circuit board. We use finite element analysis to optimize the radio frequency (RF) homogeneity and sensitivity of the TPM probe jointly for various sample volumes. The RF homogeneity of this TPM planar probe is superior to that of only a single microstrip probe. The optimized TPM probe properties such as RF homogeneity and field strength are characterized experimentally and discussed in detail. By combining this TPM based NMR probe with microfluidic technology, the sample amount required for kinetic study using NMR spectroscopy was minimized. This is important for studying costly samples. The TPM NMR probes provide high sensitivity to analysis of 5 µl samples with 2 mM concentrations within 10 min. The miniaturized microfluidic NMR probe plays an important role in realizing down to seconds timescale for kinetic monitoring.

Automated generation of photochemical reaction data by transient flow experiments coupled with online HPLC analysis

Competing homo- and crossdimerization reactions between coumarin and 1-methyl-2-quinolinone are investigated by transient continuous-flow experiments combined with online HPLC, enabling the generation and acquisition of large reaction data sets.

Monitoring Heterogeneously Catalyzed Hydrogenation Reactions at Elevated Pressures Using In-Line Flow NMR

We present a novel setup that can be used for the in-line monitoring of solid-catalyzed gas-liquid reactions. The method combines the high sensitivity and resolution of a stripline NMR detector with a microfluidic network that can withstand elevated pressures. In our setup we dissolve hydrogen gas in the solvent, then flow it with the added substrate through a catalyst cartridge, and finally flow the reaction mixture directly through the stripline NMR detector. The method is quantitative and can be used to determine the solubility of hydrogen gas in liquids; it allows in-line monitoring of hydrogenation reactions and can be used to determine the reaction kinetics of these reactions. In this work, as proof of concept we demonstrate the optimization of the Pd-catalyzed hydrogenation reactions of styrene, phenylacetylene, cyclohexene, and hex-5-en-2-one in a microfluidic context.

Non-destructive quantitation of hydrogen via mass-resolved neutron spectroscopy

This work introduces the use of mass-selective neutron spectroscopy as an analytical tool for the quantitative and non-destructive detection of hydrogen in bulk media. To this end, systematic measurements have been performed on a series of polyethylene standards of known thickness and density, in order to establish optimal data-acquisition protocols as well as associated limits of detection and quantitation. From this analysis, we conclude that state-of-the-art epithermal-neutron instrumentation enables the detection of aeral molar densities of bulk hydrogen in the μmol cm^-2 range. We also discuss potential improvements on the horizon, with a view to broadening the scope of the technique across chemistry, materials science, and engineering.

Inline Reaction Monitoring of Amine-Catalyzed Acetylation of Benzyl Alcohol Using a Microfluidic Stripline Nuclear Magnetic Resonance Setup

We present an in-depth study of the acetylation of benzyl alcohol in the presence of N, N-diisopropylethylamine (DIPEA) by nuclear magnetic resonance (NMR) monitoring of the reaction from 1.5 s to several minutes. We have adapted the NMR setup to be compatible to microreactor technology, scaling down the typical sample volume of commercial NMR probes (500 μL) to a microfluidic stripline setup with 150 nL detection volume. Inline spectra are obtained to monitor the kinetics and unravel the reaction mechanism of this industrially relevant reaction. The experiments are combined with conventional 2D NMR measurements to identify the reaction products. In addition, we replace DIPEA with triethylamine and pyridine to validate the reaction mechanism for different amine catalysts. In all three acetylation reactions, we find that the acetyl ammonium ion is a key intermediate. The formation of ketene is observed during the first minutes of the reaction when tertiary amines were present. The pyridine-catalyzed reaction proceeds via a different mechanism.

Digital microfluidics and nuclear magnetic resonance spectroscopy for in situ diffusion measurements and reaction monitoring

In recent years microcoils and related structures have been developed to increase the mass sensitivity of nuclear magnetic resonance spectroscopy, allowing this extremely powerful analytical technique to be extended to small sample volumes (<5 μl). In general, microchannels have been used to deliver the samples of interest to these microcoils; however, these systems tend to have large dead volumes and require more complex fluidic connections. Here, we introduce a two-plate digital microfluidic (DMF) strategy to interface small-volume samples with NMR microcoils. In this system, a planar microcoil is surrounded by a copper plane that serves as the counter-electrode for the digital microfluidic device, allowing for precise control of droplet position and shape. This feature allows for the user-determination of the orientation of droplets relative to the main axes of the shim stack, permitting improved shimming and a more homogeneous magnetic field inside the droplet below the microcoil, which leads to improved spectral lineshape. This, along with high-fidelity droplet actuation, allows for rapid shimming strategies (developed over decades for vertically oriented NMR tubes) to be employed, permitting the determination of reaction-product diffusion coefficients as well as quantitative monitoring of reactive intermediates. We propose that this system paves the way for new and exciting applications for in situ analysis of small samples by NMR spectroscopy.

High-resolution microstrip NMR detectors for subnanoliter samples

We present the numerical optimization and experimental characterization of two microstrip-based nuclear magnetic resonance (NMR) detectors. The first detector, introduced in our previous work, was a flat wire detector with a strip resting on a substrate, and the second detector was created by adding a ground plane on top of the strip conductor, separated by a sample-carrying capillary and a thin layer of insulator. The dimensional parameters of the detectors were optimized using numerical simulations with regards to radio frequency (RF) sensitivity and homogeneity, with particular attention given to the effect of the ground plane. The influence of copper surface finish and substrate surface on the spectral resolution was investigated, and a resolution of 0.8-1.5 Hz was obtained on 1 nL deionized water depending on sample positioning. For 0.13 nmol sucrose (0.2 M in 0.63 nL H₂O) encapsulated between two Fluorinert plugs, high RF homogeneity (A_810°/A_90° = 70-80%) and high sensitivity (expressed in the limit of detection nLOD_m = 0.73-1.21 nmol s^1/2) were achieved, allowing for high-performance 2D NMR spectroscopy of subnanoliter samples.

Pushing nuclear magnetic resonance sensitivity limits with microfluidics and photo-chemically induced dynamic nuclear polarization

Among the methods to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, small-diameter NMR coils (microcoils) are promising tools to tackle the study of mass-limited samples. Alternatively, hyperpolarization schemes based on dynamic nuclear polarization techniques provide strong signal enhancements of the NMR target samples. Here we present a method to effortlessly perform photo-chemically induced dynamic nuclear polarization in microcoil setups to boost NMR signal detection down to sub-picomole detection limits in a 9.4T system (400 MHz ¹H Larmor frequency). This setup is unaffected by current major drawbacks such as the use of high-power light sources to attempt uniform irradiation of the sample, and accumulation of degraded photosensitizer in the detection region. The latter is overcome with flow conditions, which in turn open avenues for complex applications requiring rapid and efficient mixing that are not easily achievable on an NMR tube without resorting to complex hardware.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique with an inherently low sensitivity. Here, the authors present a combination of microcoils with photo-chemically induced dynamic nuclear polarization to boost NMR sensitivity down to sub-picomole detection limits.

Illumination of Nanoliter-NMR Spectroscopy Chips for Real-Time Photochemical Reaction Monitoring

We report the use of a small-volume nuclear-magnetic-resonance (NMR)-spectroscopy device with integrated fiber-optics for the real-time detection of UV-vis-light-assisted chemical reactions. An optical fiber is used to guide the light from LEDs or a laser diode positioned safely outside the magnet toward the 25 nL detection volume and placed right above the microfluidic channel, irradiating the transparent back of the NMR chip. The setup presented here overcomes the limitations of conventional NMR systems for in situ UV-vis illumination, with the microchannel permitting efficient light penetration even in highly concentrated solutions, requiring lower-power light intensities, and enabling high photon flux. The efficacy of the setup is illustrated with two model reactions activated at different wavelengths.

Continuous Flow 1H and 13C NMR Spectroscopy in Microfluidic Stripline NMR Chips

Microfluidic stripline NMR technology not only allows for NMR experiments to be performed on small sample volumes in the submicroliter range, but also experiments can easily be performed in continuous flow because of the stripline's favorable geometry. In this study we demonstrate the possibility of dual-channel operation of a microfluidic stripline NMR setup showing one- and two-dimensional 1H, 13C and heteronuclear NMR experiments under continuous flow. We performed experiments on ethyl crotonate and menthol, using three different types of NMR chips aiming for straightforward microfluidic connectivity. The detection volumes are approximately 150 and 250 nL, while flow rates ranging from 0.5 μL/min to 15 μL/min have been employed. We show that in continuous flow the pulse delay is determined by the replenishment time of the detector volume, if the sample trajectory in the magnet toward NMR detector is long enough to polarize the spin systems. This can considerably speed up quantitative measurement of samples needing signal averaging. So it can be beneficial to perform continuous flow measurements in this setup for analysis of, e.g., reactive, unstable, or mass-limited compounds.

NMR reaction monitoring in flow synthesis

Recent advances in the use of flow chemistry with in-line and on-line analysis by NMR are presented. The use of macro- and microreactors, coupled with standard and custom made NMR probes involving microcoils, incorporated into high resolution and benchtop NMR instruments is reviewed. Some recent selected applications have been collected, including synthetic applications, the determination of the kinetic and thermodynamic parameters and reaction optimization, even in single experiments and on the μL scale. Finally, software that allows automatic reaction monitoring and optimization is discussed.

Interfacing digital microfluidics with high-field nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is extremely powerful for chemical analysis but it suffers from lower mass sensitivity compared to many other analytical detection methods. NMR microcoils have been developed in response to this limitation, but interfacing these coils with small sample volumes is a challenge. We introduce here the first digital microfluidic system capable of interfacing droplets of analyte with microcoils in a high-field NMR spectrometer. A finite element simulation was performed to assist in determining appropriate system parameters. After optimization, droplets inside the spectrometer could be controlled remotely, permitting the observation of processes such as xylose-borate complexation and glucose oxidase catalysis. We propose that the combination of DMF and NMR will be a useful new tool for a wide range of applications in chemical analysis.

Towards dial-a-molecule by integrating continuous flow, analytics and self-optimisation

The employment of continuous-flow platforms for synthetic chemistry is becoming increasingly popular in research and industrial environments. Integrating analytics in-line enables obtaining a large amount of information in real-time about the reaction progress, catalytic activity and stability, etc. Furthermore, it is possible to influence the reaction progress and selectivity via manual or automated feedback optimisation, thus constituting a dial-a-molecule approach employing digital synthesis. This contribution gives an overview of the most significant contributions in the field to date.