Magnetic Resonance Microscopy. Группа авторов

has not been as dramatic as in macro-scale NMR.¹ This observation can be explained by considering first, the microfluidic NMR community is certainly small compared with the NMR community as a whole; and second, several technical challenges must be addressed to bring hyperpolarization to the micro-scale (material interfaces, scaling effects, integration, etc.). Nevertheless, all versions of NMR hyperpolarization have been explored at small scales.

Hyperpolarized gases offer profiling of both physical structures and chemical space. Using a remote sensing approach, both Xe and H₂ gases with enhanced polarization levels were used together with microfluidic systems to characterize on-chip gas flow profiles [64] and chemical micro-reactor systems [65]. Production of Xe at polarization levels approaching 10% at the micro-scale using microfabricated hyperpolarization cells significantly enhances portability together with local production for microfluidic applications [66,67]. To access the chemical space, the hyperpolarized gas must typically be brought into solution. At the macro-scale this can be achieved by bubbling a sample and pressuring the tube with the gas prior to the NMR experiment. Under microfluidic conditions a bubbling procedure is somewhat disastrous – bubbles easily disrupt microfluidic flow and destroy NMR spectral resolution and thus must be either managed by fluidic design, or preferably avoided altogether. This has been addressed by using membrane contacting systems (high-field examples in Figure 1.12), enabling bubble-free parahydrogen transport from the gas to solution phase. This has been leveraged for flow chemistry using both hydrogenative [52] and nonhydrogenative [51,53] PHiP experiments, with picomole sensitivities achieved under high-magnetic-field conditions.

Figure 1.12 Membrane-contacting devices for high-field microfluidic parahydrogen-induced polarization (PHiP). (A) Drawing of the contact chip (top), and overview of the assembly (bottom). The region of the sample chamber slides into a micro-stripline detector. (B) Photograph of the gas contacting and NMR detector assembly (left) and schematic of the exploded assembly (right). The fluidic insert is compatible with a micro Helmholtz detector, with liquid lines in blue and gas lines in orange. [51] Lorenzo Bordonali et al. (2019), figure 01[p.03]/with permission from Royal Society of Chemistry.

DNP has made enormous advances in academic spectroscopy and imaging research, even to the point of commercialization and clinical studies [68]. Leveraging this potential for microfluidic-compatible MR microscopy has only begun, first taking root primarily in metabolic monitoring. Jeong et al. [69] used the relatively long-lived ¹³C enhanced state of pyruvate and lactate to monitor cancer cell metabolism. The hyperpolarized pyruvate was added to a microcoil accommodating 2 µl of sample and subsequently moved to a 1.05-T NMR magnet for measurement. Metabolic flux was measured for a population of only 104 cells, three orders of magnitude better than what was typically required. Mompéan et al. [70] used the photo-inducible version of DNP (photo-chemically induced DNP, photo-CIDNP) on a micro-NMR flow device to enable sub-picomole sensitivity of a 1-µl sample at 9.4 T. Uberrück et al. [71] demonstrated liquid-state Overhauser DNP (ODNP) at 0.342 T, while Kiss [72] performed ODNP at 0.5 T using a fully integrated microsystem approach (Figure 1.13). A key challenge to be faced with microfluidic-compatible DNP-based hyperpolarization at high magnetic fields is handling high-frequency microwaves for electron excitation. This is alleviated in the case of photo-CIDNP where microwaves are not necessary, at the cost of limited applicability. This challenge is general, and is a focus for the entire high-field DNP community.

      Figure 1.13 Photograph of a fully integrated Overhauser dynamic nuclear polarization (ODNP) probe head, designed to operate in a palm-held 0.5-T permanent magnet. [72] Sebastian Kiss (2019)/figure 05.26 [p.125]/with permission from University of Freiburg.

      1.5 Conclusions

MR microscopy can benefit greatly from engineering approaches to bridge the gap between the sample and the spectrometer, especially in the application areas of cell and small organism biology, and (electro) chemistry. In this chapter we considered the contribution that microengineering can make, covering custom resonators and sample holders, which provide more ideal SNR and sample conditions, thus facilitating increased experimental flexibility. In essence, Faraday induction NMR detection has arrived at the LOD, so research work is currently mainly focusing on system issues, dealing with complex samples, achieving better field shimming, hyphenation of measurement modalities, and hyperpolarization of the spin population, and of course achieving spectrometer agility and proximity. Certainly, quantum sensors may provide more sensitivity in the future, but also require proximity of the detection system to the spin of interest, which for many applications is tantamount to becoming invasive. It is also not yet clear how hyperpolarization can be made less invasive, or less poisonous. The future will see small detectors becoming easier to use, not least because of better integration into the NMR workflow and equipment. Already some micro-NMR and hyperpolarization startups are pushing the field in this direction.

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