Nitrogen-Vacancy Centers in Fluorescent Nanodiamonds: Emerging Applications from Healthcare Diagnostics to...

ConspectusNitrogen-vacancy (NV) centers in diamond are a unique class of quantum defects distinguished by their exceptional optical and magnetic properties, including bright photoluminescence, outstanding photostability, and optically addressable spin states. In nanoscale diamonds, commonly referred to as fluorescent nanodiamonds (FNDs), NV centers can exist in both neutral (NV⁰) and negatively charged (NV^-) states. These nanoparticles are chemically inert, highly biocompatible, and readily amenable to surface functionalization. Together, these attributes have established FNDs as powerful biological quantum probes since their introduction in 2005. Notably, the same optical and spin properties that enable their success in biological environments also underpin their utility in more demanding physical and engineering contexts. This Account highlights three emerging applications that arise from this shared foundation: immunodiagnostics, extreme ultraviolet (EUV) metrology, and semiconductor device analysis.First, we show that FNDs with NV^- centers are excellent fluorescent quantum reporters for quantitative immunoassays employing antibodies for specific antigen detection. A key limitation of traditional fluorescence-based immunoassays is the high background from substrates like nitrocellulose membranes, which significantly reduces detection sensitivity. This challenge is overcome by combining laser excitation with lock-in detection of magnetically modulated fluorescence from NV^- centers in FNDs to effectively eliminate background interference. The approach has enabled highly sensitive, high-throughput, quantitative, and rapid biomarker detection, marking a practically useful application of NV quantum defects in healthcare diagnostics.Second, FNDs with NV⁰ centers have emerged as a novel type of scintillator for EUV sensing and imaging. These carbon-based scintillators are fabricated into uniform, thin, and chemically stable films using electrospray deposition. They emit bright red fluorescence from NV⁰ centers when FNDs are exposed to EUV radiation at wavelengths of 13.5 nm. The scintillators are nonhygroscopic, photostable, and compatible with fiber-optic plates and sensors, allowing for integration into compact, high-resolution detection systems. These properties render them highly suitable for real-time, long-term imaging in the EUV and soft X-ray regimes, particularly for photolithographic applications.Third, FNDs with NV^- centers are quantum sensors capable of measuring temperature, magnetic, and electric fields at the nanoscale. These measurements are crucial for the design and evaluation of next-generation semiconductor devices. An innovative technique, termed FND-based lock-in photoluminescence thermography, has been developed to enable wide-field, real-time temperature mapping of actively operating devices such as bipolar junction transistors and field-effect transistors. The method achieves nanometer-scale spatial resolution and millisecond temporal resolution, yielding valuable insights into heat generation and dissipation processes in operando semiconductor devices.In summary, NV centers in FNDs constitute robust platforms for quantum sensing and metrology across a broad range of domains. From enhancing the sensitivity of immunodiagnostics to advancing EUV imaging and improving semiconductor thermal analysis, these quantum defects serve as transformative tools at the intersection of materials chemistry, biomedicine, and quantum technologies.