The nitrogen-vacancy (NV) defect in diamond offers remarkable opportunities for quantum technologies. By taking advantage of the inherently quantum-friendly nature of diamond, a ‘solid state vacuum’, and its bio-friendly materials properties, we can make a variety of quantum-enabled bio-sensing devices. However the performance of these devices is fundamentally limited by the quality of the interface between the ideal diamond lattice and the real-world. I will present our efforts to address this relatively unexplored problem, through a combination of diamond synthesis and characterization capabilities, highlighting the complementarity between classic surface-science capabilities and our growing range of NV-based quantum sensing modalities. I will also describe a new approach to ESR and NMR at the diamond surface, and discuss how this affects our nascent efforts in nano-diamond biosensing.