In situ electron paramagnetic resonance spectroscopy using single nanodiamond sensors

TL;DR

This paper introduces a robust zero-field EPR spectroscopy method using nanodiamond sensors with NV centers, enabling in situ molecular analysis in complex environments like living cells.

Contribution

It develops a generalized zero-field EPR technique with orientation-independent spectra using amplitude modulation, advancing in vivo EPR capabilities with nanodiamond sensors.

Findings

Successfully acquired zero-field EPR spectra of vanadyl ions in solution

Demonstrated orientation-robust EPR spectra with embedded nanodiamonds

Paved the way for in vivo EPR applications

Abstract

An ultimate goal of electron paramagnetic resonance (EPR) spectroscopy is to analyze molecular dynamics in place where it occurs, such as in a living cell. The nanodiamond (ND) hosting nitrogen-vacancy (NV) centers will be a promising EPR sensor to achieve this goal. However, ND-based EPR spectroscopy remains elusive, due to the challenge of controlling NV centers without well-defined orientations inside a flexible ND. Here, we show a generalized zero-field EPR technique with spectra robust to the sensor's orientation. The key is applying an amplitude modulation on the control field, which generates a series of equidistant Floquet states with energy splitting being the orientation-independent modulation frequency. We acquire the zero-field EPR spectrum of vanadyl ions in aqueous glycerol solution with embedded single NDs, paving the way towards \emph{in vivo} EPR.