Microfabricated Inserts for Magic Angle Coil Spinning (MACS) Wireless NMR Spectroscopy

Abstract

This article describes the development and testing of the first automatically microfabricated probes to be used in conjunction with the magic angle coil spinning (MACS) NMR technique. NMR spectroscopy is a versatile technique for a large range of applications, but its intrinsically low sensitivity poses significant difficulties in analyzing mass- and volume-limited samples. The combination of microfabrication technology and MACS addresses several well-known NMR issues in a concerted manner for the first time: (i) reproducible wafer-scale fabrication of the first-in-kind on-chip LC microresonator for inductive coupling of the NMR signal and reliable exploitation of MACS capabilities; (ii) improving the sensitivity and the spectral resolution by simultaneous spinning the detection microcoil together with the sample at the “magic angle” of 54.74° with respect to the direction of the magnetic field (magic angle spinning – MAS), accompanied by the wireless signal transmission between the microcoil and the primary circuit of the NMR spectrometer; (iii) given the high spinning rates (tens of kHz) involved in the MAS methodology, the microfabricated inserts exhibit a clear kinematic advantage over their previously demonstrated counterparts due to the inherent capability to produce small radius cylindrical geometries, thus tremendously reducing the mechanical stress and tearing forces on the sample. In order to demonstrate the versatility of the microfabrication technology, we have designed MACS probes for various Larmor frequencies (194, 500 and 700 MHz) testing several samples such as water, Drosophila pupae, adamantane solid and LiCl at different magic angle spinning speeds.

Capacitor re‐design overcomes the rotation rate limit of MACS resonators

“…The crycooling of the entire device offered the added advantage of improved SNR. Badilita et al implemented a micro‐resonator in which a micro‐coil was wirebonded onto a capacitor formed on a chip. The entire assembly had a footprint of 2 mm × 2 mm.…”

“…A very specific field where the utility of a smaller footprint has been exploited, is the field of MACS NMR . Magic angle spinning (MAS) rotates the sample at tens of kHz around an axis inclined at the magic angle, that is, 54.74° with respect to the direction of the static magnetic field, , removing multiple sources of spectral broadening from the NMR spectrum, for example, anisotropic chemical shift interactions, magnetic dipolar interactions, or first‐order quadrupolar interactions .…”

“…This limitation originates from the reported micro‐resonator designs, which are a combination of a micro‐coil and a suitable capacitor, tuned to the target Larmor frequency. In different implementations of the MACS technique, the capacitor is either a discrete element soldered to the ends of the micro‐coil, manufactured on‐chip, or stems from the parasitic capacitance between the windings of the coil …”

Fluid flow dynamics in MAS systems

“…For very small sample volumes (nanoliter to microliter), Janssen et al [54] and Yamauchi and Asakura [55] brought forward microMAS. Also for small sample volumes and improved filling factors, Sakellariou, Jacquinot, and Badilita, et al [56][57][58] introduced Magic Angle Coil Spinning (MACS), where a tuned circuit with a micro-solenoidal coil is placed inside the rotor and is spinning while inductively coupled to a static outer coil in a conventional probe circuit. For specific MAS samples that need to be sealed in vacuum, under a special atmosphere or high-pressure, or sample that require injection of a gas flow inside the spinning MAS rotor, solutions for MAS rotors and turbines accommodated to these purposes have been worked out [59][60][61][62][63][64][65][66].…”

Fabrication of three-dimensional micro-Rogowski coil based on femtosecond laser micromachining

“…The microwire could meet the growing requirements of highly integrated microcircuit, which would improve the integration level of the microdevices and reduces the size of the microsystems effectively. The microcoil sensors are usually used as inductors to enhance radio-frequency (RF) response signals and created magnetic fields to improve imaging capability in magnetic resonance imaging (MRI) systems and various types of advanced electromagnetic microsensors and microactuators [4][5][6].…”