High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors

Abstract

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

Multinuclear nanoliter one-dimensional and two-dimensional NMR spectroscopy with a single non-resonant microcoil

“…(iii) The capability of the RF pulse to produce the oscillating magnetic field B 1 depends upon the transmitted current and not, as usually stated, the transmitted power. B 1 is in fact proportional to the current and inversely proportional to the sample-to-coil distance 27 . The current passed through the unmatched BBC set-up is sufficient to produce a very usable B 1 field in the sample across the full RF range.…”

Silicon cylinder spiral coil for nuclear magnetic resonance spectroscopy of nanoliter samples

“…The simulated numbers are derived from Eqs. [2]- [5] and from the effective coil resistances R c,eff that are obtained from electromagnetic simulations of the entire resonator circuit which includes the circuit model of the coil (Fig. 8) and those supplied by the capacitor vendors.…”

“…As described in (1) and in other publications (2)(3)(4)(5), the main challenge of using microcoils is the low spectral resolution. Commonly, the linewidth LW is compromised by a nonhomogeneous distribution of the static magnetic field (B 0 -field) within the sample.…”

Fabrication of Left‐Handed Metal Microcoil from Spiral Vessel of Vascular Plant

“…[ 7 ] Techniques of lithography or precision machining have afforded precise fabrication of desired microcoils within small margin of error. [8][9][10][11][12] For nanohelix fabrication, electron-beam-induced deposition [ 13 ] is one of invaluable methods. These techniques are not targeting the subject of mass-production and less accessible to researchers because of equipment reliance.…”