Magnetic Modulation of NV Fluorescence for Paramagnetic Analyte Detection

The fluorescence intensity of NV centers is sensitive to magnetic fields. This behavior results from the electronic spins of NV centers, which emit more or less photons depending on their spin state projection. Magnetic fields (B-fields) change the spin state projections of NV electronic spins and alter the number of emitted photons. The change in fluorescence of NV centers in the presence of an external magnetic field as compared to zero (or lower) magnetic field is the magnetically modulated fluorescence contrast of the NV center(s) (Fig. 1). Oscillating magnetic fields will therefore result in oscillating NV fluorescence intensity.

The oscillatory response of NV fluorescence is exploitable for use of established phase-sensitive signal processing methods (e.g., lock-in amplification), removing contributions of non-modulated background fluorescence noise. This provides greatly enhanced sensitivity.

Paramagnetic analytes such as Gd3+, Fe3+, and reactive oxygen species (ROS) generate fluctuating magnetic fields due to their unpaired electron spins. This fluctuating magnetic noise interacts with NV centers and alters their fluorescence contrast. Therefore, NV centers can be used to detect for the presence of paramagnetic analytes, which are important in, for instance, cellular metabolic pathways. NV centers incorporated into biocompatible nanoscale diamond particles can be useful tools for the study of intracellular metabolism or paramagnetic analyte generating chemical reactions. For instance, Fig. 2 shows a fluorescence contrast measurement over time in the presence of an enzyme generating superoxide. When the enzyme is suppressed, no change in signal is observed.

Fig. 1. Schematic of magnetic field induced fluorescence contrast of NV centers. Application of an oscillating magnetic field will cause oscillations of the NV fluorescence signal intensity. Phase-sensitive detection methods can then be used to remove background fluorescence contributions. The fluorescence intensity of the NV in the presence of a magnetic field as compared to the absence of a magnetic field results in a fluorescence contrast. Paramagnetic analytes alter the magnitude of this contrast.