  Due to its superior coherent and optical properties at room temperature, the
nitrogen-vacancy (N-V ) center in diamond has become a promising quantum probe
for nanoscale quantum sensing. However, the application of N-V containing
nanodiamonds to quantum sensing suffers from their relatively poor spin
coherence times. Here we demonstrate energy efficient protection of N-V spin
coherence in nanodiamonds using concatenated continuous dynamical decoupling,
which exhibits excellent performance with less stringent microwave power
requirement. When applied to nanodiamonds in living cells we are able to extend
the spin coherence time by an order of magnitude to the $T_1$-limit of up to
$30\mu$s. Further analysis demonstrates concomitant improvements of sensing
performance which shows that our results provide an important step towards in
vivo quantum sensing using N-V centers in nanodiamond.
