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arXiv:2608.09434

Birefringent Biomineral Microcarriers Stabilise Multimodal Nanodiamond Quantum Sensing in Liquids

T. Amro, M. Attrash, A. Droby, A. Ushkov, R. Malkinson, P. Penshin, A. Hen, V. Bobrovs, P. Ginzburg, H. Barhum, N. Bar-Gill

quant-phSubmitted Aug 10, 2026• Announced Aug 11, 2026
v1 · Aug 10, 2026v2 · Aug 17, 2026
arXivHTMLPDF

Abstract

Mobile nanodiamond quantum sensors in liquids are limited by Brownian rotation, variable photon collection, and perturbations introduced by optical trapping. Here, we assemble 40-nm nitrogen-vacancy nanodiamonds at the surface of porous, birefringent vaterite microspherulites. This carbonate biomineral provides a polarization-addressable body frame, anisotropic emission redistribution, and a proton-active thermodynamic interface. Under 976-nm optical trapping, the hybrids retain their spin resonance and longitudinal relaxation, with the resonance contrast varying by less than 7 percent and the resonance centre shifting by approximately 1 MHz at 0.8 W. Zeeman-split resonances enable magnetic-field sensing from 0 to 0.8 mT, with a response metric of 78-144 microT per square root Hz. In buffered cell-culture medium, a 10.7 microM nominal proton-equivalent dose shortens T1 from 23.4 +/- 2.3 to 9.0 +/- 1.2 microseconds, corresponding to concentration and pH sensitivities of 6.46 microM per square root Hz and 6.54 mpH per square root Hz, respectively, in DMEM. By contrast, a 500-fold higher proton dose in ethanol produces a substantially weaker spin response, highlighting the role of the carbonate-rich interface. We develop a grand-canonical charge-regulation model coupling proton chemical potential to interfacial switching kinetics within the NV spectral window, capturing medium-dependent dynamic proton transduction. By integrating orientation stabilisation, optical manipulation, magnetic sensing, and interfacial chemical response, this carbonate-rich hybrid platform establishes a versatile approach to multimodal quantum sensing in complex biological liquids.

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•Jan 1, 2018•0 citations
Mobile nanodiamond quantum sensors in liquids are limited by Brownian rotation, variable photon collection, and perturbations introduced by optical trapping. Here, we assemble 40-nm nitrogen-vacancy nanodiamonds at the surface of porous, birefringent vaterite microspherulites. This carbonate biomineral provides a polarization-addressable body frame, anisotropic emission redistribution, and a proton-active thermodynamic interface. Under 976-nm optical trapping, the hybrids retain their spin resonance and longitudinal relaxation, with the resonance contrast varying by less than 7 percent and the resonance centre shifting by approximately 1 MHz at 0.8 W. Zeeman-split resonances enable magnetic-field sensing from 0 to 0.8 mT, with a response metric of 78-144 microT per square root Hz. In buffered cell-culture medium, a 10.7 microM nominal proton-equivalent dose shortens T1 from 23.4 +/- 2.3 to 9.0 +/- 1.2 microseconds, corresponding to concentration and pH sensitivities of 6.46 microM per square root Hz and 6.54 mpH per square root Hz, respectively, in DMEM. By contrast, a 500-fold higher proton dose in ethanol produces a substantially weaker spin response, highlighting the role of the carbonate-rich interface. We develop a grand-canonical charge-regulation model coupling proton chemical potential to interfacial switching kinetics within the NV spectral window, capturing medium-dependent dynamic proton transduction. By integrating orientation stabilisation, optical manipulation, magnetic sensing, and interfacial chemical response, this carbonate-rich hybrid platform establishes a versatile approach to multimodal quantum sensing in complex biological liquids.
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.
Thanks to their versatility, nitrogen-vacancy (N-VVV) centers in nanodiamonds have been widely adopted as nanoscale sensors. However, their sensitivities are limited by their short coherence times relative to N-VsV\mathrm{s}Vs in bulk diamond. A more complete understanding of the origins of decoherence in nanodiamonds is critical to improving their performance. Here we present measurements of fast spin relaxation on qutrit transitions between the energy eigenstates composed of the ${m}_{s}=|\ifmmode\pm\else\textpm\fi{}1⟩$ states of the N-${V}^{\ensuremath{-}}$ electronic ground state in approximately 40-nm nanodiamonds under ambient conditions. For frequency splittings between these states of 20 MHz or less the maximum theoretically achievable coherence time of the N-VVV spin is approximately 2 orders of magnitude shorter than would be expected if the N-VVV spin is treated as a qubit. We attribute this fast relaxation to electric field noise. We observe a strong falloff of the qutrit relaxation rate with the splitting between the states, suggesting that, whenever possible, measurements with N-VsV\mathrm{s}Vs in nanodiamonds should be performed at moderate axial magnetic fields (>60>60>60 G). We also observe that the qutrit relaxation rate changes with time. These findings indicate that surface electric field noise is a major source of decoherence for N-VsV\mathrm{s}Vs in nanodiamonds.
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.
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.
Nitrogen-vacancy (NV) centres are exceptional solid-state quantum sensors with optically addressable spins. With room-temperature operability and biocompatibility, functionalised NV centres in nanodiamonds facilitate atomic-scale tracking and real-time monitoring of physical and chemical properties within living systems [1]. Conventionally, these systems work by addressing stochastically-positioned defects with a complex bulk-optic microscope [2] or using a single scanning probe, limited in scalability and the ability to capture spatial and temporal information simultaneously.
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.