On-chip EPR spectrometry of metalloproteins using superconducting lumped element resonators

We report electron paramagnetic resonance experiments performed on myoglobin hemeproteins using a chip hosting 6 superconducting lumped element resonators with resonance frequencies between 1.94 and 2.11 GHz. Successive layers of myoglobin were deposited onto the inductors of four of them using dip-pen nanolithography, a technique based on atomic force microscopy. A combination of atomic force and confocal microscopies estimated the number of protein molecules in each deposit, which ranges from 8.6 × 10¹¹ (one dip-pen layer) to 3.33 × 10¹² (four dip-pen layers). Two reference bulk samples were pipetted from the same solution onto the remaining two resonators. The microwave transmission of the device, measured at 11 mK, shows evidence of the coupling of protein spins to the photon excitations of all resonators. In particular, the resonance broadening measured as a function of magnetic field provides the spin resonance absorption spectrum. The analysis suggests that proteins tend to self-orient on the chip. It also allows estimating the single spin to single photon coupling strength, which is around 9 Hz. This high coupling value suggests that dip-pen nanolithography gives rise to a close to optimum interface between the molecules and the chip surface. The developed methodology combines an increase in sensitivity of at least three orders of magnitude with the ability to characterize multiple samples in a single experiment, opening the door to a highly sensitive multi-analyte detection technology.