Optically Detected Magnetic Resonance of Fluorescent Nanodiamonds Separated in Microdroplets: Implications for Sensing Electromagnetic Fields, Temperatures, and Chemical Potential

Fluorescence nanodiamonds (FNDs) containing negatively charged nitrogen vacancy (NV^–) centers promise sensors for electromagnetic fields, temperatures, and chemical potential in sub-micro- and nanoscales. Nevertheless, handling separative FND particles for such an objective is quite challenging. We report in this paper an approach to separate FNDs into trapped microdroplets for optically detected magnetic resonance (ODMR). We form microscale aqueous droplets on a droplet-based microfluidic chip for separating and trapping FNDs. We estimate the average encapsulated FND population through the proportion of FND-containing droplets in the total generated droplets. These droplets are separately trapped in 80 × 80 μm² trap cells. We developed a home-built detection system for exciting fluorescence and detecting ODMR for FND particles entrapped in an individual droplet. To the best of our knowledge, this work is the first for the ODMR detection of FNDs trapped in separative microdroplets. In this initiative study, we obtain ODMR spectra of the contrast of 0.07 to 0.15%. The zero-field splitting (ZFS) of the electron spin triplet of the nano NV ensemble is extracted from the ODMR spectra with the standard deviations among 0.2–0.5 MHz. The technique we develop here presents a potential platform separating FNDs in microdroplets for single-particle analysis, delivery, and traceable measurements of electromagnetic fields, temperatures, and chemical potentials.