Parallel accelerated electron paramagnetic resonance spectroscopy using diamond sensors

The nitrogen-vacancy (NV) center can serve as a magnetic sensor for electron paramagnetic resonance (EPR) measurements. Benefiting from its atomic size, the diamond chip can integrate a tremendous amount of NV centers to improve the magnetic-field sensitivity. However, EPR spectroscopy using NV ensembles is less efficient due to inhomogeneities in both sensors and targets. Spectral line broadening induced by ensemble averaging is even detrimental to spectroscopy. Here we show a kind of cross-relaxation EPR spectroscopy at zero field, where the sensor is tuned by an amplitude-modulated control field to match the target. The modulation makes detection robust to the sensor’s inhomogeneity, while zero-field EPR is naturally robust to the target’s inhomogeneity. We demonstrate an efficient EPR measurement on an ensemble of roughly 30000 NV centers. Our method shows the ability to not only acquire unambiguous EPR spectra of free radicals, but also monitor their spectroscopic dynamics in real time.

💡 Research Summary

This paper presents a novel method for performing electron paramagnetic resonance (EPR) spectroscopy using ensembles of nitrogen-vacancy (NV) centers in diamond, achieving a dramatic acceleration in measurement speed by enabling true parallel operation. While NV centers are excellent nanoscale magnetic sensors, using large ensembles for EPR has been inefficient due to two main inhomogeneities: spatial variations in the control microwave fields across the sensor array, and random orientations of target molecules, which broaden and obscure the combined spectral signal.

The authors’ innovative solution combines two key techniques. First, they perform measurements at zero external magnetic field. In this regime, the resonance frequencies of target electron spins depend only on their intrinsic hyperfine interactions, not their orientation. This makes the spectrum orientation-independent, allowing signals from all target molecules to add up coherently without line broadening. Second, they apply an amplitude-modulated continuous microwave drive to the NV sensors. The modulation frequency (f) effectively sets the energy level splitting of the sensors. By sweeping this frequency while proportionally adjusting the microwave amplitude (keeping a constant relative drive index κ), they scan for matching frequencies (ω) of the target spins. Critically, this scheme makes the detected resonance positions robust to inhomogeneities in the microwave drive strength across the NV ensemble; inhomogeneities affect only the signal amplitude, not the spectral line positions.

The experimental setup utilized a widefield microscope with a single-photon counter, monitoring approximately 30,000 near-surface NV centers within an area of about 2.3e-4 mm². The targets were nitroxide radicals (TEMPO derivatives) dispersed in a PMMA film on the diamond surface. The method successfully acquired clear zero-field EPR spectra for both 14N- and 15N-labeled TEMPO radicals. The observed peak positions (e.g., ~97 MHz for 14N-TEMPO and three peaks at ~27, 48, and 77 MHz for 15N-TEMPO) matched theoretical predictions perfectly, validating the technique’s accuracy.

The high efficiency of the method—achieving a signal-to-noise ratio of 25.4 in 30 minutes—enabled unprecedented real-time monitoring of spectroscopic dynamics. The researchers observed the gradual decay of the EPR signal from nitroxide radicals under continuous laser illumination over tens of hours. Concurrently, the spectral linewidth narrowed, indicating a reduction in radical concentration and thus dipolar broadening. This provided direct evidence for laser-induced quenching of the radicals. Further experiments established that the quenching rate (inverse of the signal decay time constant) depended linearly on the laser power density, confirming the photochemical nature of the process.

In conclusion, this work demonstrates a parallel-accelerated, zero-field EPR spectroscopy platform that is robust to sensor and target inhomogeneities. By effectively harnessing the power of NV ensembles, it transitions nanoscale EPR from a slow, single-spin detection technique to a practical tool capable of rapid spectral acquisition and real-time monitoring of chemical dynamics, opening new avenues for studying radical reactions and spin-labeled biomolecules.