  Sensing of signals from biological processes, such as action potential
propagation in nerves, are essential for clinical diagnosis and basic
understanding of physiology. Sensing can be performed electrically by placing
sensor probes near or inside a living specimen or dissected tissue using well
established electrophysiology techniques. However, these electrical probe
techniques have poor spatial resolution and cannot easily access tissue deep
within a living subject, in particular within the brain. An alternative
approach is to detect the magnetic field induced by the passage of the
electrical signal, giving the equivalent readout without direct electrical
contact. Such measurements are performed today using bulky and expensive
superconducting sensors with poor spatial resolution. An alternative is to use
nitrogen vacancy (NV) centres in diamond that promise biocompatibilty and high
sensitivity without cryogenic cooling. In this work we present advances in
biomagnetometry using NV centres, demonstrating magnetic field sensitivity of
approximately 100 pT/$\sqrt{Hz}$ in the DC/low frequency range using a setup
designed for biological measurements. Biocompatibility of the setup with a
living sample (mouse brain slice) is studied and optimized, and we show work
toward sensitivity improvements using a pulsed magnetometry scheme. In addition
to the bulk magnetometry study, systematic artifacts in NV-ensemble widefield
fluorescence imaging are investigated.
