5-Axis CNC Micromilling for Rapid, Cheap, and Background-Free NMR Microcoils

Abstract

The superior mass sensitivity of micro-coil technology in Nuclear Magnetic Resonance (NMR) Spectroscopy provides potential for the analysis of extremely small mass-limited samples such as eggs, cells, and tiny organisms. For optimal performance and efficiency, the size of the micro-coil should be tailored to the size of the mass-limited sample of interest, which can be costly as mass-limited samples come in many shapes and sizes. Therefore, rapid and economic micro-coil production methods are needed. One method with great potential is 5-axis Computer Numerical Control (CNC) micro-milling, commonly used in the jewelry industry. Most CNC milling machines are designed to process larger objects and commonly have a precision >25 µm (making the machining of common spiral micro-coils, for example, impossible). Here, a 5-axis MiRA6 CNC milling machine, specifically designed for the jewelry industry, with a 0.3 µm precision was used to produce working planar micro-coils, microstrips, and novel micro-sensor designs, with some tested on the NMR in less than 24 hours after the start of the design process. Sample wells could be built into the micro-sensor and could be machined at the same time as the sensors themselves, in some cases leaving a sheet of Teflon as thin as 10 µm between the sample and sensor. This provides the freedom to produce a wide array of designs and demonstrates 5-axis CNC micro-milling as a versatile tool for the rapid prototyping of NMR micro-sensors. This approach allowed the experimental optimization of a prototype microstrip for the analysis of two intact adult Daphnia magna organisms. In addition, a 3D volume slotted tube resonator was produced that allowed for 2D 1 H-13 C NMR of D. magna neonates and exhibited 1 H sensitivity (nLOD ꙍ 600 = 1.49 nmol s 1/2 ) close to that of double striplines, which themselves offer the best compromise between concentration and mass sensitivity published to date.

Comparing the Potential of Helmholtz and Planar NMR Microcoils for Analysis of Intact Biological Samples

“…The preceding comparison provides an estimation of relative sensitivity; however, in order to obtain absolute values, the approach outlined by Finch et al is employed . Here, 150 mM sodium acetate is acquired under identical conditions to the previously reported studies. , Finch et al recorded an SNR of 4200 for a 2 μL sample at 300 MHz which converted to an absolute sensitivity of nLOD ω 600 = 1.57 nmol s 1/2 . Here, the Helmholtz microcoil obtained an SNR of 1244 under the same experimental conditions with 70 nL in the active volume at 500 MHz.…”

“…Microcoil NMR is a more economical and accessible approach for very small samples, which can increase mass sensitivity by thousands of times over a 5 mm room temperature probe, − and thus holds great potential for the study of mass limited complex biological samples. ,, Due to the small coil diameter, only small amounts of sample are required, decreasing costs and improving the filling factor. Furthermore, the increase in mass sensitivity potentially permits the study of single organisms/eggs or considerable reductions in tissue sample requirements.…”

Evaluation of double‐tuned single‐sided planar microcoils for the analysis of small 13C enriched biological samples using 1H‐13C 2D heteronuclear correlation NMR spectroscopy

“…Micro-coils offer many advantages for smaller masslimited samples. Recently, it was shown that they can be milled from PCB materials for only a few dollars [34] and clipped to a universal probe body. This allows the size and shape of intact samples to be matched to a custom coil improving filling factor.…”

“…[14,30] The coil was mounted to a dual channel 1 H- 13 C probe customized with a dual tuned circuit and platform to which microcoils can be mounted via pins as shown in Figure 1a and described elsewhere. [34]…”

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

“…10,11 Microcoils have been investigated with various geometries, e.g. from solenoids, 9 to helmholtz, 12 planar spiral, 13,14,15 stripline 16,17 and microslot. 18 Most of the microcoils are relatively cheap, and often can be designed, and in some cases even manufactured, in-house.…”