Microfabricated inserts for magic angle coil spinning (MACS) wireless NMR spectroscopy.

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

500 MHz ¹H NMR spectrum of 330 nl water spun at 110 Hz.

<p>The isotropic central peak is split in sidebands spaced at 110 Hz, i.e., 0.22 ppm at 500 MHz. The inset shows the central water peak which has a non-Gaussian shape due to the field inhomogeneities (see <i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042848#s4" target="_blank">Discussion</a></i> section).</p

Figure 1

<p>a) Microscope image of the microfabricated NMR insert. b) Profile of the electroplated on-chip capacitor. c) Schematic of the magic angle coil spinning (MACS) experimental set-up.</p

¹H NMR spectra of Drosophila pupae.

<p>The spectra was acquired with a total of three pupae in a 400 µm-microcoil coupled with a standard 7-mm MAS probe, spinning at 500 Hz for the single-pulse experiment, and at 371 Hz for the PASS experiment. The total experimental time for the PASS experiment is about 1 hour. Peak assignment (1) Lipid –CH₃ (2) fatty acid –(CH₂)_n (3) lactate CH₃ (4) Lipid –CH₂-CH₂-CO– (5) Lipid–CH = CH-CH₂-CH₂– (6) Lipid –CH₂-CH₂-CO–.</p

Successful tuning of the microfabricated NMR insert at 500 MHz.

<p>Successful tuning of the microfabricated NMR insert at 500 MHz.</p

Sensitivity of the microfabricated NMR insert.

<p>¹H spectrum at 700 MHz of a mixture of 99.9% D₂O and 0.1% H₂O.</p

Linewidth narrowing of solid-state adamantane spectrum for higher rotational speeds.

<p>NMR spectra at 500 MHz for two different spinning speeds: 0.9 ppm at 10 kHz vs. 1.5 ppm at 5 kHz.</p