The use of optical tweezers as a tool to facilitate nondestructive nanoscale sensing has been a growing area of research, particularly in the biological sciences. The nitrogen-vacancy (NV) center in diamond has attracted significant interest in this area due to the array of sensing modalities available and the biocompatibility of the material itself. Many of the diamond sensing modalities rely on the measurement and characterization of the NV spin. Recent work has demonstrated the utility of the spin–lattice relaxation time ( T 1 ) of NV centers in nanodiamond for nanoscale magnetic sensing and spectroscopy. Here, we demonstrate spin relaxometry with optically trapped nanodiamonds. The all-optical sensing protocol we developed eliminates the spin decoherence effects of the trapping laser and can determine spin–lattice relaxation times on the order of ms. Moreover, the protocol requires relatively low trapping powers <50 mW, making it particularly applicable to biological systems.
The use of optical tweezers as a tool to facilitate nondestructive nanoscale sensing has been a growing area of research, particularly in the biological sciences. The nitrogen-vacancy (NV) center in diamond has attracted significant interest in this area due to the array of sensing modalities available and the biocompatibility of the material itself. Many of the diamond sensing modalities rely on the measurement and characterization of the NV spin. Recent work has demonstrated the utility of the spin–lattice relaxation time ( T 1 ) of NV centers in nanodiamond for nanoscale magnetic sensing and spectroscopy. Here, we demonstrate spin relaxometry with optically trapped nanodiamonds. The all-optical sensing protocol we developed eliminates the spin decoherence effects of the trapping laser and can determine spin–lattice relaxation times on the order of ms. Moreover, the protocol requires relatively low trapping powers <50 mW, making it particularly applicable to biological systems.
The nitrogen-vacancy (NV) centre in diamond gained popularity as a probe for nanoscale sensing applications in research thanks to the array of sensing modalities available and the bio-compatibility of the material itself, however the utility of NV sensing is limited by the lack of suitable strategies to control them spatially. By confining single nanodiamonds using an optical tweezers (OT) we are able to combine the sensing opportunities of the NV centre with the precision control of position and orientation afforded by OT. This work is an investigation of the interaction of the trapping laser with the spin-based photoluminescence of the NV centre, further it is a demonstration of an all-optical sensing protocol which eliminates the spin depolarisation effects of the trapping laser and allows for NV spin relaxometry in an optically trapped nanodiamond. This relaxometry protocol can determine spin lattice relaxation times on the order of ms and requires relatively low trapping powers < 50 mW, making it particularly applicable to biological systems.
Quantum sensing using nitrogen-vacancy (NV) centers in diamond has emerged as a powerful platform for detecting ultra-low magnetic fields under ambient conditions. Owing to their long spin coherence times, optical addressability, and compatibility with aqueous environments, it has found widespread applications in biosensing and bio-imaging. This review presents the fundamental principles and recent advances in NV-based quantum magnetometry for biosensing applications, with a particular focus on measurements in aqueous medium and at cellular and molecular length scales. We discuss the underlying spin physics of NV centers and highlight two primary detection modalities: optically detected magnetic resonance (ODMR) and T1 relaxometry-based sensing and how these approaches aids in the detection of both static magnetic fields and dynamic magnetic noise arising from biological processes. The review explores key application areas, including nanoscale nuclear magnetic resonance (NMR), monitoring of neural activity, detection of abnormal or rogue cells using NV-based platforms etc. In addition, strategies for enhancing sensitivity, such as surface functionalization of nanodiamonds, femtosecond (fs) laser-written photonic structures, and integration with microfluidic and lab-on-chip systems have also been discussed in depth. We also address the critical challenges, including surface-induced decoherence, charge-state instability, and signal-to-noise limitations in biofluids associated with NV-based biosensing applications. Finally, we outline future prospects, highlighting how NV-based magnetic biosensing provides a promising pathway for translating quantum sensing technologies into practical biomedical applications.
Optically addressable solid-state spin defects have emerged as powerful multimodal quantum sensors, with nitrogen-vacancy (NV) centers in bulk diamond providing benchmark quantum control and sensitivity under ambient conditions. Embedding such defects in nanodiamonds (NDs) extends these capabilities to mobile probes capable of accessing complex biological and nanoscale environments. Reduced dimensions, however, introduce constraints beyond volumetric spin impurities, notably enhanced lattice strain and surface-induced noise sources, which shorten NV spin relaxation times (T1 and T2) and destabilize the NV charge state, as well as resulting in pronounced particle-to-particle variability in NDs typically produced by top-down approaches. These effects complicate both sensing performance and the quantitative interpretation of multimodal signals in realistic environments. This article provides a structured perspective on the physical mechanisms by which material properties constrain NV behavior in NDs, together with mitigation strategies that shape the robust use of these mobile quantum sensors for biosensing and nanoscale science.
Quantum relaxometry is one of the most successful applications of nitrogen-vacancy (NV) centers in diamond and, more broadly, solid-state spin qubits, enabling ultrasensitive detection of magnetic noise and paramagnetic species via measurements of the spin-lattice relaxation time T1. Conventional pulsed protocols, however, probe T1 efficiently only over a limited temporal range, which restricts the scope and throughput of the technique. Here we introduce a continuous-wave quantum relaxometry protocol that operates in the frequency domain. By measuring the frequency response of the optically detected magnetic resonance signal under low-frequency microwave amplitude modulation, we extract T1 from the characteristic response time of the spin system. The method enables efficient T1 measurements spanning more than three orders of magnitude -- directly demonstrated from 60 μs to 200 ms in our experiments -- across a broad temperature range and under substantial ensemble inhomogeneity. We further show that this protocol enables quantitative relaxometry-based sensing in nanodiamonds, achieving a substantial speed-up over the pulsed methods and offering a practical approach to optimizing nanodiamond size for enhanced sensitivity.
The NV defect center in diamond forms a pseudo-atomic quantum system with discreet optically excitable transitions between ground and excited states in the gap between valence and conduction bands, making the NV center a deep-level defect center in diamond. For the negatively charged NV center (NV-), both the ground and excited states are spin triplets (S=1) and coupling between optical and spin states provides unique opportunities for optical detection of magnetic resonance (ODMR) and quantum metrology under ambient conditions. As an atomically sized point defect that is stable in single nanodiamonds as small as a few nanometers, the NV center enables spin based imaging of temperature, electric fields, magnetic fields and mechanical strain with nanoscale spatial resolution. Made of carbon, nanodiamonds are also ideal nanoparticles for use in biological systems, exhibiting extremely low cytotoxicity, no photo bleaching and exceptional contrast in transmission electron and light microscopy. With the ability to act as both a passive, non perturbing sensor (e.g. a diamagnetic material used for sensing magnetic fields) and a connection point for two way, direct coupling of far field optics to NV− spins and indirectly to the highly localized neighborhood of atoms, electronic states and phonon modes in the local vicinity, NV diamond provides a route to nanoscale field sensing and spin based optical microscopy. These sensors are also stable in nanodiamonds. For nanoparticles, the reduction in particle diameter dramatically enhances the proportion of the material that is surface exposed, enhancing any surface related properties. Nitrogen Vacancy centers in nanodiamonds provide another example of material properties that vary with particle size. In chapter 5, evidence that modulation of nanodiamonds fluorescent emissions by electrical perturbations in their surrouding environment allows transduction of local electrodynamics into a far field optical signal capable of mapping action potentials in cardiomyocytes in presented. The unifying theme for all of the work presented in this dissertation is to enable the use of magnetic resonance signals and interactions for imaging and sensing in previously inaccessible regimes. The specific focus is on methods and techniques to enable nanoscale physical sensing and functional imaging in live tissues, with the ultimate aim to lay the predicate for the combination of both without compromise.
Nanoscale quantum probes such as the nitrogen-vacancy (NV) center in diamonds have demonstrated remarkable sensing capabilities over the past decade as control over fabrication and manipulation of these systems has evolved. The biocompatibility and rich surface chemistry of diamonds has added to the utility of these probes but, as the size of these nanoscale systems is reduced, the surface chemistry of diamond begins to impact the quantum properties of the NV center. In this work, we systematically study the effect of the diamond surface chemistry on the quantum coherence of the NV center in nanodiamonds (NDs) 50 nm in size. Our results show that a borane-reduced diamond surface can on average double the spin relaxation time of individual NV centers in nanodiamonds when compared to thermally oxidized surfaces. Using a combination of infrared and X-ray absorption spectroscopy techniques, we correlate the changes in quantum relaxation rates with the conversion of sp 2 carbon to C–O and C–H bonds on the diamond surface. These findings implicate double-bonded carbon species as a dominant source of spin noise for near surface NV centers. The link between the surface chemistry and quantum coherence indicates that through tailored engineering of the surface, the quantum properties and magnetic sensitivity of these nanoscale systems may approach that observed in bulk diamond.