Nanoscale spin detection of copper ions using double electron-electron resonance at room temperature

We report the nanoscale spin detection and electron paramagnetic resonance (EPR) spectrum of copper $({\mathrm{Cu}}^{2+})$ ions via double electron-electron (DEER) with single spins in diamond at room temperature low magnetic fields. measure unexpectedly narrow EPR resonances linewidths $\ensuremath{\sim}2\ensuremath{-}3$ MHz from copper-chloride molecules dissolved polylysine. also observe coherent Rabi oscillations hyperfine splitting ${\mathrm{Cu}}^{2+}$ ions, which could be used for dynamic nuclear polarization higher sensitivity detection. interpret analyze these observations using both Hamiltonian modeling numerical simulations predicted DEER response, obtain a sensing volume $\ensuremath{\sim}{(250\phantom{\rule{0.16em}{0ex}}\text{nm})}^{3}$. This work will open door copper-labeled measurements under ambient conditions biomolecules nanomaterials.