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A hybrid AFM/NMR probe technology combined AFM-coupled microcoils for NMR signal detection at the cellular scale. A commercial AFM probe was modified using nanotechnology fabrication processes to include a radiofrequency microcoil over the tip of the cantilever. Operation of the hybrid probe will include contact with the cell under investigation (using piezoelectric or other sensing feedback mechanisms) for combined biophysical (AFM) and biochemical (NMR) measurements.
AFM/NMR probe fabrication and assembly involved standard processing of commercially available AFM cantilevers. The probe fabrication process involved multiple steps: (a) a commercial AFM silicon cantilever was covered uniformly with 100 nm Parylene-C for electrical insulation; (b) gold was deposited with plasma sputtering and focused ion beam etching was performed for the definition of the NMR coil and connection pad geometry; (c)the probe was bonded to the custom-designed board through manual flip-chip alignment and bonding; (d) following epoxy underfill for securing the bond, the electrical connection was soldered, and the device was packaged in a glass vial for the insertion and the removal of the liquid samples (inset).
The transmission performance of the AFM/NMR probe was estimated using finite element analysis. Magnetic flux density due to the RF coil varied spatially on the plane through the center of the coil and perpendicular to the static magnetic field (a). Section AA′ indicates the vertical limit of the sensitive region of the coil as determined based on the coil radius; (b) magnitude of the field at the plane parallel to the coil at distance indicated by AA′. The axis of the static magnetic field is along y.
The AFM/NMR probe detection of NMR signals was demonstrated using (a) deuterium oxide (control) and (b) de-ionized water. The baseline of the spectra is not flat throughout the whole frequency range due to the contribution of the Parylene-C insulation layer. An internal reference signal was not used in this set of experiments.
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