Photoswitchable radicals as reporter spins for quantum sensing with spin defects in diamond

TL;DR

The paper addresses the challenge of distance- and surface-related limitations in NV-based nanoscale magnetometry by introducing photoreduced dye-derived radicals as optically addressable, surface-localized reporter spins that can be generated and read out via DEER. Using Alexa Fluor 488, the authors demonstrate long-lived radical formation under 488 nm illumination in the presence of MEA, confirmed by cw-EPR and evidenced by accelerated NV $T_1$ relaxation in ensembles. They realize single-NV DEER measurements showing a resonance at $\\omega_{radical} \,\\approx \,652\\,\\mathrm{MHz}$ under $B \,\\approx \,233\\,\\mathrm{G}$ with a linewidth near $20\\,\\mathrm{MHz}$, and emphasize the need to incorporate finite $T_s$ in modeling; time-dependent data reveal heterogeneity and density fluctuations of the radical reporters, quantified via a density estimator $\\hat{\\sigma}$. The work paves the way for correlative nanoscale magnetic and optical imaging and suggests future avenues toward single-molecule ESR and molecular-qubit architectures using surface-bound, photoswitchable dye radicals, potentially enabling broader sensing modalities beyond the diamond surface.

Abstract

The rapid decay of target signal strength with distance from the sensor presents a key challenge in nanoscale magnetic sensing with nitrogen-vacancy (NV) centers in diamond, limiting both sensitivity and spatial resolution. Here we introduce a strategy to overcome this limitation by using radical anions formed from rhodamine-derived dyes as reporter spins localized to the diamond surface. These radicals, generated through photoreduction, are optically identifiable and stable on timescales exceeding an hour. We experimentally demonstrate their coherent manipulation and detection using single, shallow NV centers for readout. We observe heterogeneity in the local magnetic environments of the photoactivated spins from site to site, likely due to variations in inter-radical couplings across our measurements. Looking forward, our approach enables correlative nanoscale magnetic and optical imaging, and opens new pathways toward single-molecule magnetic resonance studies.

Figures (4)

• Figure 1: Dye-derived radicals as reporter spins. (A) Energy-level diagram of Alexa Fluor 488 depicting photoreduction of dye triplet state to form radical anions. (B) Room-temperature cw-EPR spectra of an aqueous solution of 400-$\upmu$M Alexa Fluor in 100 mM MEA at pH 9.6 after irradiation with 488-nm light for 5 minutes. (C) Relaxometry measurements using a dense, shallow ensemble of NVs, with (red) and without (blue) photoactivated dye present in the solution above. Solid lines are bi-exponential fits to the data. (D) Illustration of Alexa Fluor 488 molecules attached to the diamond surface forming radicals upon 488-nm photoexcitation in the presence of MEA. The inset shows a confocal image of the dye-functionalized diamond surface after burning a hole with the laser (fluorescence units arbitrary).
• Figure 2: Sensing radical reporter spins using a single, shallow NV center. (A) DEER pulse sequence. Two pulses of duration $T_s$ and frequency $\omega_{radical}$ are applied to drive the reporter spins, one in each half of the NV spin echo sequence. The change in magnetic field at the NV center caused by the reversal of the radical spins disrupts the NV spin echo, resulting in a dip in the NV coherence. (B) DEER spectrum as measured by a single NV center positioned below a photoactivated patch of dye/radical. Recorded at a constant applied field of 233 G, with $T_s$ fixed to 100 ns and $\tau =$ 900 ns. Blue curve is Lorentzian fit.
• Figure 3: Results of single-NV DEER measurements with frequency fixed to the Larmor frequency of the electron and $T_s$ varied. (A-D) Examples of DEER curves measured on covalently-functionalized diamond. (E) Example of DEER curve measured at diamond surface beneath a thin layer of PMMA containing Alexa Fluor 488. Additional data for this sample preparation are given in Fig. S9. In each of (A-E), the solid blue lines are intended to guide the eye and depict smoothed data averaged over a 5-point running window. (F) Results of control experiments on clean diamond surface.
• Figure 4: Time-dependent DEER measurements. (A) A loss of DEER signal amplitude, likely due to conversion of the radicals back to the fluorescent state. Data are from the same measurement as in Fig. \ref{['fig:Fig3']}D. (B) An apparent gain of DEER signal amplitude over time, perhaps due to prolonged exposure to 532-nm laser used to initialize and readout NV. Data are from same measurement as in Fig. \ref{['fig:Fig3']}B.