  Nitrogen-vacancy centers in diamond are a promising platform for nanoscale
nuclear magnetic resonance sensing. Despite significant progress towards using
NV centers to detect and localize nuclear spins down to the single spin level,
NV-based spectroscopy of individual, intact, arbitrary target molecules remains
elusive. NV molecular sensing requires that target molecules are immobilized
within a few nanometers of NV centers with long spin coherence time. The inert
nature of diamond typically requires harsh functionalization techniques such as
thermal annealing or plasma processing, limiting the scope of functional groups
that can be attached to the surface. Solution-phase chemical methods can be
more readily generalized to install diverse functional groups, but they have
not been widely explored for single-crystal diamond surfaces. Moreover,
realizing shallow NV centers with long spin coherence times requires highly
ordered single-crystal surfaces, and solution-phase functionalization has not
yet been shown to be compatible with such demanding conditions. In this work,
we report a versatile strategy to directly functionalize C-H bonds on
single-crystal diamond surfaces under ambient conditions using visible light.
This functionalization method is compatible with charge stable NV centers
within 10 nm of the surface with spin coherence times comparable to the state
of the art. As a proof of principle, we use shallow ensembles of NV centers to
detect nuclear spins from functional groups attached to the surface. Our
approach to surface functionalization based on visible light-driven C-H bond
activation opens the door to deploying NV centers as a broad tool for chemical
sensing and single-molecule spectroscopy.
