  Nitrogen vacancy (NV) color centers in diamond have emerged as highly
versatile optical emitters that exhibit room temperature spin properties. These
characteristics make NV centers ideal for magnetometry, which plays an
important role in chemical and biological sensing applications. The integration
of NV magnetometers with microfluidic systems could enable the study of
isolated chemical and biological samples in a fluid environment with high
spatial resolution. Here we demonstrate a method to perform localized
magnetometry with nanometer spatial precision using a single NV center in a
microfluidic device. We manipulate a magnetic particle within a liquid
environment using a combination of planar microfluidic flow control and
vertical magnetic actuation to achieve 3-dimensional manipulation. A diamond
nanocrystal containing a single NV center is deposited in the microfluidic
channels and acts as a local magnetic field probe. We map out the magnetic
field distribution of the magnetic particle by varying its position relative to
the diamond nanocrystal and performing optically resolved electron spin
resonance (ESR) measurements. We control the magnetic particle position with a
48 nm precision and attain a magnetic field sensitivity of 17.5
microTesla/Hz^1/2. These results open up the possibility for studying local
magnetic properties of biological and chemical systems with high sensitivity in
an integrated microfluidic platform.
