Quantum sensors based on nitrogen-vacancy (NV) centers in diamond have emerged as promising tools for biological measurement, offering nanoscale access to magnetic, electric, and thermal signals under ambient conditions. Yet adapting these solid-state quantum systems to function reliably in the dynamic and complex environment of living matter remains a significant scientific and engineering challenge. This thesis presents a focused effort to bridge that gap—developing, engineering, and deploying diamond nanocrystals (DNCs) as practical, biocompatible quantum sensors capable of operating within cells and tissues. Chapter 2 addresses the fundamental limitations of DNCs as sensing materials, identifying surface-induced decoherence and charge instability as key sources of performance variability. Through spin characterization and material engineering, we verify a decoherence mechanism that has long limited the reproducibility of NV-based sensing and develop a silica-encapsulated core–shell architecture that suppresses surface spins, enhances charge-state stability, and improves optical signal quality. This engineered platform enables reproducible single-particle measurements and contributes broadly to the understanding and mitigation of surface-induced quantum decoherence. Chapter 3 turns to the biological interface, examining how surface chemistry influences colloidal stability, cytotoxicity, and immune activation. We show that the same core–shell structures that enhance quantum properties also reduce biological disruption and enable facile, high-density, and tunable surface functionalization. Chapter 4 explores how the biological environment influences the sensor, both as a noise source and as a sensing target. We find that shifts in intracellular conditions drive measurable charge transfer between the DNC and its surroundings, leading to electric field redistribution within the nanocrystal. Rather than treating this as noise, we harness this interaction to detect immune activation through quantum measurements. At the same time, we demonstrate that the core–shell structure suppresses these effects, enabling improved, artifact-free intracellular thermometry. These results yield both a novel biological sensing modality and a path to robust temperature readouts in live cells. Finally, Chapter 5 introduces tools and protocols to support the broader deployment of quantum sensors in biology, including methods for size-sorting and characterization of DNCs, wide-field signal acquisition, and the integration of ultrathin diamond membranes for chip-based nanoscale sensing. These developments begin to address challenges of scalability, reproducibility, and accessibility—shifting quantum biosensing from a frontier discipline toward a practical platform for biological discovery.