  We demonstrate nuclear magnetic resonance (NMR) spectroscopy of
picoliter-volume solutions with a nanostructured diamond chip. Using optical
interferometric lithography, diamond surfaces were nanostructured with dense,
high-aspect-ratio nanogratings, enhancing the surface area by more than a
factor of 15 over mm^2 regions of the chip. The nanograting sidewalls were
doped with nitrogen-vacancy (NV) centers so that more than 10 million NV
centers in a (25 micrometer)^2 laser spot are located close enough to the
diamond surface (5 nm) to detect the NMR spectrum of 1 pL of fluid lying within
adjacent nanograting grooves. The platform was used to perform 1H and 19F NMR
spectroscopy at room temperature in magnetic fields below 50 mT. Using a
solution of CsF in glycerol, we demonstrate that 4 +/- 2 x 10^12 19F spins in a
1 pL volume, can be detected with a signal-to-noise ratio of 3 in 1 s
integration. This represents nearly two orders of magnitude improvement in
concentration sensitivity over previous NV and picoliter NMR studies.
