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