Detection of a few metallo-protein molecules using color centers in nanodiamonds

Fluorescent Nanodiamonds for Quantum Sensing in Biology

Fluorescent nanodiamonds exhibiting outstanding optical and biocompatible properties are the subject of increased studies and attention in physics and biology. The nitrogen-vacancy center in diamonds with unique quantum properties at room temperature is sensitive to physical properties such as magnetic field, electric field, temperature, and pressure. By taking advantage of the NV center and high sensitivity that arises from the intrinsic quantum properties of spins in nanodiamonds, which are extensively employed in quantum sensing, bio-imaging, and bio-sensing. In this review, the selected topic mainly focuses on the surface functionalization of nanodiamonds and the recent progress in applying nanodiamonds as quantum sensors for intracellular orientation tracking, temperature sensing, and notably nuclear magnetic resonance and electron spin resonance applications.

Effects of Thermal Oxidation and Proton Irradiation on Optically Detected Magnetic Resonance Sensitivity in Sub-100 nm Nanodiamonds

In recent decades, nanodiamonds (NDs) have emerged as innovative nanotools for weak magnetic fields and small temperature variation sensing, especially in biological systems. At the basis of the use of NDs as quantum sensors are nitrogen-vacancy center lattice defects, whose electronic structures are influenced by the surrounding environment and can be probed by the optically detected magnetic resonance technique. Ideally, limiting the NDs' size as much as possible is important to ensure higher biocompatibility and provide higher spatial resolution. However, size reduction typically worsens the NDs' sensing properties. This study endeavors to obtain sub-100 nm NDs suitable to be used as quantum sensors. Thermal processing and surface oxidations were performed to purify NDs and control their surface chemistry and size. Ion irradiation techniques were also employed to increase the concentration of the nitrogen-vacancy centers. The impact of these processes was explored in terms of surface chemistry (diffuse reflectance infrared Fourier transform spectroscopy), structural and optical properties (Raman and photoluminescence spectroscopy), dimension variation (atomic force microscopy measurements), and optically detected magnetic resonance temperature sensitivity. Our results demonstrate how surface optimization and defect density enhancement can reduce the detrimental impact of size reduction, opening to the possibility of minimally invasive high-performance sensing of physical quantities in biological environments with nanoscale spatial resolution.

Quantum life science: biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, quantum biology, and quantum biotechnology

The emerging field of quantum life science combines principles from quantum physics and biology to study fundamental life processes at the molecular level. Quantum mechanics, which describes the properties of small particles, can help explain how quantum phenomena such as tunnelling, superposition, and entanglement may play a role in biological systems. However, capturing these effects in living systems is a formidable challenge, as it involves dealing with dissipation and decoherence caused by the surrounding environment. We overview the current status of the quantum life sciences from technologies and topics in quantum biology. Technologies such as biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, high-speed 2D electronic spectrometers, and computer simulations are being developed to address these challenges. These interdisciplinary fields have the potential to revolutionize our understanding of living organisms and lead to advancements in genetics, molecular biology, medicine, and bioengineering.

Electrochemical-Enhanced Charge State Modulation of Nitrogen-Vacancy Centers for Ultrasensitive Biodetection of MicroRNA-155

Sensitive and accurate miRNA detection is important in cancer diagnosis but remains challenging owing to the essential features of miRNAs, such as their small size, high homology, and low abundance. This work proposes a novel electrochemical (EC)-enhanced quantum sensor achieving quantitative detection of miRNA-155 with simultaneous EC sensing. Specifically, fluorescent nanodiamonds/MXene nanocomposites were synthesized and modified with dual-mode signal labels, enabling miRNA-155 concentration measurement via T1 relaxation time of nitrogen-vacancy (NV) centers and EC signals. Quantum sensing was enhanced via external voltage during the EC process, which modulated the negatively charged state of the NV centers, thereby improving the sensitivity and accuracy of miRNA-155 detection. EC sensing improved the accuracy and reliability of miRNA-155 detection while enhancing quantum sensing. The limit of detection (LOD) of the EC-enhanced quantum biosensor reached 10.0 aM, nearly 106 and 10 times lower than the reported LODs of a quantum sensor using bulk diamond and fluorescent sensors, respectively. The LOD of EC sensing was 2.6 aM, aligning with previous reports. The findings of the study indicated that quantum sensing combined with EC sensing can achieve ultrasensitive miRNA-155 detection with high accuracy and reliability, providing an advanced approach for early cancer diagnosis.

Controlled Formation of Silicon-Vacancy Centers in High-Pressure Nanodiamonds Produced from an "Adamantane + Detonation Nanodiamond" Mixture

Despite progress in the high-pressure synthesis of nanodiamonds from hydrocarbons, the problem of controlled formation of fluorescent impurity centers in them still remains unresolved. In our work, we explore the potential of a new precursor composition, a mixture of adamantane with detonation nanodiamond, both in the synthesis of nanodiamonds and in the controlled formation of negatively charged silicon-vacancy centers in such nanodiamonds. Using different adamantane/detonation nanodiamond weight ratios, a series of samples was synthesized at a pressure of 7.5 GPa in the temperature range of 1200-1500 °C. It was found that temperature around 1350 °C, is optimal for the high-yield synthesis of nanodiamonds <50 nm in size. For the first time, controlled formation of negatively charged silicon-vacancy centers in such small nanodiamonds was demonstrated by varying the atomic ratios of silicon/carbon in the precursor in the range of 0.01-1%.

Dynamics for High-Sensitivity Detection of Free Radicals in Primary Bronchial Epithelial Cells upon Stimulation with Cigarette Smoke Extract

Chronic obstructive pulmonary disease (COPD), the third leading cause of death worldwide, is caused by chronic exposure to toxic particles and gases, such as cigarette smoke. Free radicals, which are produced during a stress response to toxic particles, play a crucial role in disease progression. Measuring these radicals is difficult since the complex mixture of chemicals within cigarette smoke interferes with radical detection. We used a new quantum sensing technique called relaxometry to measure free radicals with nanoscale resolution on cells from COPD patients and healthy controls exposed to cigarette smoke extract (CSE) or control medium. Epithelial cells from COPD patients display a higher free radical load than those from healthy donors and are more vulnerable to CSE. We show that epithelial cells of COPD patients are more susceptible to the damaging effects of cigarette smoke, leading to increased release of free radicals.

Fluorescent Nanodiamonds for High-Resolution Thermometry in Biology

Optically active color centers in diamond and nanodiamonds can be utilized as quantum sensors for measuring various physical parameters, particularly magnetic and electric fields, as well as temperature. Due to their small size and possible surface functionalization, fluorescent nanodiamonds are extremely attractive systems for biological and medical applications since they can be used for intracellular experiments. This review focuses on fluorescent nanodiamonds for thermometry with high sensitivity and a nanoscale spatial resolution for the investigation of living systems. The current state of the art, possible further development, and potential limitations of fluorescent nanodiamonds as thermometers will be discussed here.

Quantum Sensing of Free Radical Generation in Mitochondria of Human Keratinocytes during UVB Exposure

Ultraviolet (UV) radiation is known to cause skin issues, such as dryness, aging, and even cancer. Among UV rays, UVB stands out for its ability to trigger problems within cells, including mitochondrial dysfunction, oxidative stress, and DNA damage. Free radicals are implicated in these cellular responses, but they are challenging to measure due to their short lifetime and limited diffusion range. In our study, we used a quantum sensing technique (T1 relaxometry) involving fluorescent nanodiamonds (FNDs) that change their optical properties in response to magnetic noise. This allowed us to monitor the free radical presence in real time. To measure radicals near mitochondria, we coated FNDs with antibodies, targeting mitochondrial protein voltage-dependent anion channel 2 (anti-VDAC2). Our findings revealed a dynamic rise in radical levels on the mitochondrial membrane as cells were exposed to UVB (3 J/cm2), with a significant increase observed after 17 min.

Quantum sensing of microRNAs with nitrogen-vacancy centers in diamond

Label-free detection of nucleic acids such as microRNAs holds great potential for early diagnostics of various types of cancers. Measuring intrinsic biomolecular charge using methods based on field effect has been a promising way to accomplish label-free detection. However, the charges of biomolecules are screened by counter ions in solutions over a short distance (Debye length), thereby limiting the sensitivity of these methods. Here, we measure the intrinsic magnetic noise of paramagnetic counter ions, such as Mn2+, interacting with microRNAs using nitrogen-vacancy (NV) centers in diamond. All-atom molecular dynamics simulations show that microRNA interacts with the diamond surface resulting in excess accumulation of Mn ions and stronger magnetic noise. We confirm this prediction by observing an increase in spin relaxation contrast of the NV centers, indicating higher Mn2+ local concentration. This opens new possibilities for next-generation quantum sensing of charged biomolecules, overcoming limitations due to the Debye screening.

Nanodiamonds: A Cutting-Edge Approach to Enhancing Biomedical Therapies and Diagnostics in Biosensing

Nanodiamonds (NDs) have garnered attention in the field of nanomedicine due to their unique properties. This review offers a comprehensive overview of NDs synthesis methods, properties, and their uses in biomedical applications. Various synthesis techniques, such as detonation, high-pressure, high-temperature, and chemical vapor deposition, offer distinct advantages in tailoring NDs' size, shape, and surface properties. Surface modification methods further enhance NDs' biocompatibility and enable the attachment of bioactive molecules, expanding their applicability in biological systems. NDs serve as promising nanocarriers for drug delivery, showcasing biocompatibility and the ability to encapsulate therapeutic agents for targeted delivery. Additionally, NDs demonstrate potential in cancer treatment through hyperthermic therapy and vaccine enhancement for improved immune responses. Functionalization of NDs facilitates their utilization in biosensors for sensitive biomolecule detection, aiding in precise diagnostics and rapid detection of infectious diseases. This review underscores the multifaceted role of NDs in advancing biomedical applications. By synthesizing NDs through various methods and modifying their surfaces, researchers can tailor their properties for specific biomedical needs. The ability of NDs to serve as efficient drug delivery vehicles holds promise for targeted therapy, while their applications in hyperthermic therapy and vaccine enhancement offer innovative approaches to cancer treatment and immunization. Furthermore, the integration of NDs into biosensors enhances diagnostic capabilities, enabling rapid and sensitive detection of biomolecules and infectious diseases. Overall, the diverse functionalities of NDs underscore their potential as valuable tools in nanomedicine, paving the way for advancements in healthcare and biotechnology.

Unraveling Eumelanin Radical Formation by Nanodiamond Optical Relaxometry in a Living Cell

Defect centers in a nanodiamond (ND) allow the detection of tiny magnetic fields in their direct surroundings, rendering them as an emerging tool for nanoscale sensing applications. Eumelanin, an abundant pigment, plays an important role in biology and material science. Here, for the first time, we evaluate the comproportionation reaction in eumelanin by detecting and quantifying semiquinone radicals through the nitrogen-vacancy color center. A thin layer of eumelanin is polymerized on the surface of nanodiamonds (NDs), and depending on the environmental conditions, such as the local pH value, near-infrared, and ultraviolet light irradiation, the radicals form and react in situ. By combining experiments and theoretical simulations, we quantify the local number and kinetics of free radicals in the eumelanin layer. Next, the ND sensor enters the cells via endosomal vesicles. We quantify the number of radicals formed within the eumelanin layer in these acidic compartments by applying optical relaxometry measurements. In the future, we believe that the ND quantum sensor could provide valuable insights into the chemistry of eumelanin, which could contribute to the understanding and treatment of eumelanin- and melanin-related diseases.

Quantum Sensing of Free Radical Generation in Mitochondria of Single Heart Muscle Cells during Hypoxia and Reoxygenation

Cells are damaged during hypoxia (blood supply deprivation) and reoxygenation (oxygen return). This damage occurs in conditions such as cardiovascular diseases, cancer, and organ transplantation, potentially harming the tissue and organs. The role of free radicals in cellular metabolic reprogramming under hypoxia is under debate, but their measurement is challenging due to their short lifespan and limited diffusion range. In this study, we employed a quantum sensing technique to measure the real-time production of free radicals at the subcellular level. We utilize fluorescent nanodiamonds (FNDs) that exhibit changes in their optical properties based on the surrounding magnetic noise. This way, we were able to detect the presence of free radicals. To specifically monitor radical generation near mitochondria, we coated the FNDs with an antibody targeting voltage-dependent anion channel 2 (anti-VDAC2), which is located in the outer membrane of mitochondria. We observed a significant increase in the radical load on the mitochondrial membrane when cells were exposed to hypoxia. Subsequently, during reoxygenation, the levels of radicals gradually decreased back to the normoxia state. Overall, by applying a quantum sensing technique, the connections among hypoxia, free radicals, and the cellular redox status has been revealed.

Quantum-enhanced diamond molecular tension microscopy for quantifying cellular forces

The constant interplay and information exchange between cells and the microenvironment are essential to their survival and ability to execute biological functions. To date, a few leading technologies such as traction force microscopy, optical/magnetic tweezers, and molecular tension-based fluorescence microscopy are broadly used in measuring cellular forces. However, the considerable limitations, regarding the sensitivity and ambiguities in data interpretation, are hindering our thorough understanding of mechanobiology. Here, we propose an innovative approach, namely, quantum-enhanced diamond molecular tension microscopy (QDMTM), to precisely quantify the integrin-based cell adhesive forces. Specifically, we construct a force-sensing platform by conjugating the magnetic nanotags labeled, force-responsive polymer to the surface of a diamond membrane containing nitrogen-vacancy centers. Notably, the cellular forces will be converted into detectable magnetic variations in QDMTM. After careful validation, we achieved the quantitative cellular force mapping by correlating measurement with the established theoretical model. We anticipate our method can be routinely used in studies like cell-cell or cell-material interactions and mechanotransduction.

Nano- and micro-crystalline diamond film structuring with electron beam lithography mask

Direct current plasma enhanced chemical vapor deposition (CVD) was employed to create polycrystalline diamond films from CH4/H2gaseous mixture at 98 mbar pressure and various substrate temperatures between 720 °C and 960 °C. The Si chips with patterns of periodic masked and open seeded zones were used as substrates. The mask free seeded areas evolved into polycrystalline diamond films after CVD process. The diamond crystallites of the films featured single crystal ordering individually with distinct cubic (100) or octahedral (111) facets on the film surfaces. Notably, specific growth conditions were determined for obtaining diamond films composed of the crystallites of nanometre and micrometre scale. These conditions are differing from those observed for non-pattern-prepared Si substrates. The nano-crystalline diamonds emerged within the 4.5-5 A current range, with growth conditions involving 3% CH4/H2mixture at 98 mbar. The micro-crystalline diamonds (MCDs) predominantly characterized by well-developed rectangular (100) crystal faces on the film surface were successfully grown with current settings of 5.5-6 A, under 3% CH4/H2mixture at 98 mbar. Furthermore, MCDs characterized by entirely crystalline (111) diamond faces forming CVD film surface were attained within a growth parameter range of 4.5-5.8 A, employing 3% CH4/H2mixture for certain samples, or alternatively, utilizing 5 A with a 1.5% CH4/H2mixture for others. Upon thorough evaluation, it was established that SiO2, TiO2, and Cr masks are well-suited materials for the planar patterning of both nano- and micro-crystalline diamond films, and the bottom-up approach can pave the way for the production of diamond planar structures through CVD, facilitated by electron beam lithography (EBL).

Robotic Vectorial Field Alignment for Spin-Based Quantum Sensors

Developing practical quantum technologies will require the exquisite manipulation of fragile systems in a robust and repeatable way. As quantum technologies move toward real world applications, from biological sensing to communication in space, increasing experimental complexity introduces constraints that can be alleviated by the introduction of new technologies. Robotics has shown tremendous progress in realizing increasingly smart, autonomous, and highly dexterous machines. Here, a robotic arm equipped with a magnet is demonstrated to sensitize an NV center quantum magnetometer in challenging conditions unachievable with standard techniques. Vector magnetic fields are generated with 1° angular and 0.1 mT amplitude accuracy and determine the orientation of a single stochastically-aligned spin-based sensor in a constrained physical environment. This work opens up the prospect of integrating robotics across many quantum degrees of freedom in constrained settings, allowing for increased prototyping speed, control, and robustness in quantum technology applications.

Nitrogen vacancy defects in single-particle nanodiamonds sense paramagnetic transition metal spin noise from nanoparticles on a transmission electron microscopy grid

Spin-active nanomaterials play a vital role in current and upcoming quantum technologies, such as spintronics, data storage and computing. To advance the design and application of these materials, methods to link size, shape, structure, and chemical composition with functional magnetic properties at the nanoscale level are needed. In this work, we combine the power of two local probes, namely, Nitrogen Vacancy (NV) spin-active defects in diamond and an electron beam, within experimental platforms used in electron microscopy. Negatively charged NVs within fluorescent nanodiamond (FND) particles are used to sense the local paramagnetic environment of Rb0.5Co1.3[Fe(CN)6]·3.7H2O nanoparticles (NPs), a Prussian blue analogue (PBA), as a function of FND-PBA distance (order of 10 nm) and local PBA concentration. We demonstrate perturbation of NV spins by proximal electron spins of transition metals within NPs, as detected by changes in the photoluminescence (PL) of NVs. Workflows are reported and demonstrated that employ a Transmission Electron Microscope (TEM) finder grid to spatially correlate functional and structural features of the same unique NP studied using NV sensing, based on a combination of Optically Detected Magnetic Resonance (ODMR) and Magnetic Modulation (MM) of NV PL, within TEM imaging modalities. Significantly, spin-spin dipole interactions were detected between NVs in a single FND and paramagnetic metal centre spin fluctuations in NPs through a carbon film barrier of 13 nm thickness, evidenced by TEM tilt series imaging and Electron Energy-Loss Spectroscopy (EELS), opening new avenues to sense magnetic materials encapsulated in or between thin-layered nanostructures. The measurement strategies reported herein provide a pathway towards solid-state quantitative NV sensing with atomic-scale theoretical spatial resolution, critical to the development of quantum technologies, such as memory storage and molecular switching nanodevices.

Recent Progress in Fluorescent Probes for Real-Time Monitoring of Glioblastoma

Treating glioblastoma (GBM) by resecting to a large extent can prolong a patient's survival by controlling the tumor cells, but excessive resection may produce postoperative complications by perturbing the brain structures. Therefore, various imaging procedures have been employed to successfully diagnose and resect with utmost caution and to protect vital structural or functional features. Fluorescence tagging is generally used as an intraoperative imaging technique in glioma cells in collaboration with other surgical tools such as MRI and navigation methods. However, the existing fluorescent probes may have several limitations, including poor selectivity, less photostability, false signals, and intraoperative re-administration when used in clinical and preclinical studies for glioma surgery. The involvement of smart fluorogenic materials, specifically fluorescent dyes, and biomarker-amended cell-penetrable fluorescent probes have noteworthy advantages for precise glioma imaging. This review outlines the contemporary advancements of fluorescent probes for imaging glioma cells along with their challenges and visions, with the anticipation to develop next-generation smart glioblastoma detection modalities.

Diamond Quantum Sensing Revealing the Relation between Free Radicals and Huntington's Disease

Huntington's disease (HD) is a well-studied yet rare disease caused by a specific mutation that results in the expression of polyglutamine (PolyQ). The formation of aggregates of PolyQ leads to disease and increases the level of free radicals. However, it is unclear where free radicals are generated and how they impact cells. To address this, a new method called relaxometry was used to perform nanoscale MRI measurements with a subcellular resolution. The method uses a defect in fluorescent nanodiamond (FND) that changes its optical properties based on its magnetic surroundings, allowing for sensitive detection of free radicals. To investigate if radical generation occurs near PolyQ aggregates, stable tetracycline (tet)-inducible HDQ119-EGFP-expressing human embryonic kidney cells (HEK PQ) were used to induce the PolyQ formation and Huntington aggregation. The study found that NDs are highly colocalized with PolyQ aggregates at autolysosomes, and as the amount of PolyQ aggregation increased, so did the production of free radicals, indicating a relationship between PolyQ aggregation and autolysosome dysfunction.

Mesoporous Polydopamine-Encapsulated Fluorescent Nanodiamonds: A Versatile Platform for Biomedical Applications

Fluorescent nanodiamonds (FNDs) are versatile nanomaterials with promising properties. However, efficient functionalization of FNDs for biomedical applications remains challenging. In this study, we demonstrate mesoporous polydopamine (mPDA) encapsulation of FNDs. The mPDA shell is generated by sequential formation of micelles via self-assembly of Pluronic F127 (F127) with 1,3,5-trimethyl benzene (TMB) and composite micelles via oxidation and self-polymerization of dopamine hydrochloride (DA). The surface of the mPDA shell can be readily functionalized with thiol-terminated methoxy polyethylene glycol (mPEG-SH), hyperbranched polyglycerol (HPG), and d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS). The PEGylated FND@mPDA particles are efficiently taken up by, and employed as a fluorescent imaging probe for, HeLa cells. HPG-functionalized FND@mPDA is conjugated with an amino-terminated oligonucleotide to detect microRNA via hybridization. Finally, the increased surface area of the mPDA shell permits efficient loading of doxorubicin hydrochloride. Further modification with TPGS increases drug delivery efficiency, resulting in high toxicity to cancer cells.

Detection of Paramagnetic Spins with an Ultrathin van der Waals Quantum Sensor

Detecting magnetic noise from small quantities of paramagnetic spins is a powerful capability for chemical, biochemical, and medical analysis. Quantum sensors based on optically addressable spin defects in bulk semiconductors are typically employed for such purposes, but the 3D crystal structure of the sensor inhibits sensitivity by limiting the proximity of the defects to the target spins. Here we demonstrate the detection of paramagnetic spins using spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material that can be exfoliated into the 2D regime. We first create negatively charged boron vacancy (V_B^-) defects in a powder of ultrathin hBN nanoflakes (<10 atomic monolayers thick on average) and measure the longitudinal spin relaxation time (T₁) of this system. We then decorate the dry hBN nanopowder with paramagnetic Gd³⁺ ions and observe a clear T₁ quenching under ambient conditions, consistent with the added magnetic noise. Finally, we demonstrate the possibility of performing spin measurements, including T₁ relaxometry using solution-suspended hBN nanopowder. Our results highlight the potential and versatility of the hBN quantum sensor for a range of sensing applications and make steps toward the realization of a truly 2D, ultrasensitive quantum sensor.

Advances in Stabilization and Enrichment of Shallow Nitrogen-Vacancy Centers in Diamond for Biosensing and Spin-Polarization Transfer

Negatively charged nitrogen-vacancy (NV^-) centers in diamond have unique magneto-optical properties, such as high fluorescence, single-photon generation, millisecond-long coherence times, and the ability to initialize and read the spin state using purely optical means. This makes NV^- centers a powerful sensing tool for a range of applications, including magnetometry, electrometry, and thermometry. Biocompatible NV-rich nanodiamonds find application in cellular microscopy, nanoscopy, and in vivo imaging. NV^- centers can also detect electron spins, paramagnetic agents, and nuclear spins. Techniques have been developed to hyperpolarize ¹⁴N, ¹⁵N, and ¹³C nuclear spins, which could open up new perspectives in NMR and MRI. However, defects on the diamond surface, such as hydrogen, vacancies, and trapping states, can reduce the stability of NV^- in favor of the neutral form (NV⁰), which lacks the same properties. Laser irradiation can also lead to charge-state switching and a reduction in the number of NV^- centers. Efforts have been made to improve stability through diamond substrate doping, proper annealing and surface termination, laser irradiation, and electric or electrochemical tuning of the surface potential. This article discusses advances in the stabilization and enrichment of shallow NV^- ensembles, describing strategies for improving the quality of diamond devices for sensing and spin-polarization transfer applications. Selected applications in the field of biosensing are discussed in more depth.

Nanoscale quantum sensing with Nitrogen-Vacancy centers in nanodiamonds - A magnetic resonance perspective

Nanodiamonds containing fluorescent Nitrogen-Vacancy (NV) centers are the smallest single particles, of which a magnetic resonance spectrum can be recorded at room temperature using optically-detected magnetic resonance (ODMR). By recording spectral shift or changes in relaxation rates, various physical and chemical quantities can be measured such as the magnetic field, orientation, temperature, radical concentration, pH or even NMR. This turns NV-nanodiamonds into nanoscale quantum sensors, which can be read out by a sensitive fluorescence microscope equipped with an additional magnetic resonance upgrade. In this review, we introduce the field of ODMR spectroscopy of NV-nanodiamonds and how it can be used to sense different quantities. Thereby we highlight both, the pioneering contributions and the latest results (covered until 2021) with a focus on biological applications.

Quantum sensors for biomedical applications

Quantum sensors are finding their way from laboratories to the real world, as witnessed by the increasing number of start-ups in this field. The atomic length scale of quantum sensors and their coherence properties enable unprecedented spatial resolution and sensitivity. Biomedical applications could benefit from these quantum technologies, but it is often difficult to evaluate the potential impact of the techniques. This Review sheds light on these questions, presenting the status of quantum sensing applications and discussing their path towards commercialization. The focus is on two promising quantum sensing platforms: optically pumped atomic magnetometers, and nitrogen-vacancy centres in diamond. The broad spectrum of biomedical applications is highlighted by four case studies ranging from brain imaging to single-cell spectroscopy.

Nonmonotonic Superparamagnetic Behavior of the Ferritin Iron Core Revealed via Quantum Spin Relaxometry

Ferritin is the primary storage protein in our body and is of significant interest in biochemistry, nanotechnology, and condensed matter physics. More specifically within this sphere of interest are the magnetic properties of the iron core of ferritin, which have been utilized as a contrast agent in applications such as magnetic resonance imaging. This magnetism depends on both the number of iron atoms present, L, and the nature of the magnetic ordering of their electron spins. In this work, we create a series of ferritin samples containing homogeneous iron loads and apply diamond-based quantum spin relaxometry to systematically study their room temperature magnetic properties. We observe anomalous magnetic behavior that can be explained using a theoretical model detailing a morphological change to the iron core occurring at relatively low iron loads. This model provides an L^0.35±0.06 scaling of the uncompensated Fe spins, in agreement with previous theoretical predictions. The necessary inclusion of this morphological change within the model is also supported by electron microscopy studies of ferritin with low iron content. This provides evidence for a magnetic consequence of this morphological change and positions diamond-based quantum spin relaxometry as an effective, noninvasive tool for probing the magnetic properties of metalloproteins. The low detection limit (ferritin 2% loaded at a concentration of 7.5 ± 0.4 μg/mL) also makes this a promising method for precision applications where low analyte concentrations are unavoidable, such as in biological research or even clinical analysis.

Relaxometry with Nitrogen Vacancy (NV) Centers in Diamond

Relaxometry is a technique which makes use of a specific crystal lattice defect in diamond, the so-called NV center. This defect consists of a nitrogen atom, which replaces a carbon atom in the diamond lattice, and an adjacent vacancy. NV centers allow converting magnetic noise into optical signals, which dramatically increases the sensitivity of the readout, allowing for nanoscale resolution. Analogously to T1 measurements in conventional magnetic resonance imaging (MRI), relaxometry allows the detection of different concentrations of paramagnetic species. However, since relaxometry allows very local measurements, the detected signals are from nanoscale voxels around the NV centers. As a result, it is possible to achieve subcellular resolutions and organelle specific measurements.A relaxometry experiment starts with polarizing the spins of NV centers in the diamond lattice, using a strong laser pulse. Afterward the laser is switched off and the NV centers are allowed to stochastically decay into the equilibrium mix of different magnetic states. The polarized configuration exhibits stronger fluorescence than the equilibrium state, allowing one to optically monitor this transition and determine its rate. This process happens faster at higher levels of magnetic noise. Alternatively, it is possible to conduct T1 relaxation measurements from the dark to the bright equilibrium by applying a microwave pulse which brings NV centers into the -1 state instead of the 0 state. One can record a spectrum of T1 at varying strengths of the applied magnetic field. This technique is called cross-relaxometry. Apart from detecting magnetic signals, responsive coatings can be applied which render T1 sensitive to other parameters as pH, temperature, or electric field. Depending on the application there are three different ways to conduct relaxometry experiments: relaxometry in moving or stationary nanodiamonds, scanning magnetometry, and relaxometry in a stationary bulk diamond with a stationary sample on top.In this Account, we present examples for various relaxometry modes as well as their advantages and limitations. Due to the simplicity and low cost of the approach, relaxometry has been implemented in many different instruments and for a wide range of applications. Herein we review the progress that has been achieved in physics, chemistry, and biology. Many articles in this field have a proof-of-principle character, and the full potential of the technology still waits to be unfolded. With this Account, we would like to stimulate discourse on the future of relaxometry.

Intracellular Relaxometry, Challenges, and Future Directions

Nitrogen vacancy (NV) centers change their optical properties on the basis of their magnetic surroundings. Since optical signals can be detected more sensitively than small magnetic signals, this technique allows unprecedented sensitivity. Recently, NV center-based relaxometry has been used for measurements in living cells with subcellular resolution. The aim of this Outlook is to identify challenges in the field, including controlling the location of sensing particles, limitations in reproducibility, and issues arising from biocompatibility. We further provide an outlook and point to new directions in the field. These include new diamond materials with NV centers, other defects, or even entirely new materials that might replace diamonds. We further discuss new and more challenging samples, such as tissues or even entire organisms, that might be investigated with NV centers. Then, we address future challenges that have to be resolved in order to achieve this goal. Finally, we discuss new quantities that could be measured with NV centers in the future.

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen-vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.

Functionalized Fluorescent Nanodiamonds for Simultaneous Drug Delivery and Quantum Sensing in HeLa Cells

Here, we present multifunctional fluorescent nanodiamonds (FNDs) for simultaneous drug delivery and free radical detection. For this purpose, we modified FNDs containing nitrogen vacancy (NV) centers with a diazoxide derivative. We found that our particles enter cells more easily and are able to deliver this cancer drug into HeLa cells. The particles were characterized by infrared spectroscopy, dynamic light scattering, and secondary electron microscopy. Compared to the free drug, we observe a sustained release over 72 h rather than 12 h for the free drug. Apart from releasing the drug, with these particles, we can measure the drug's effect on free radical generation directly. This has the advantage that the response is measured locally, where the drug is released. These FNDs change their optical properties based on their magnetic surrounding. More specifically, we make use of a technique called relaxometry to detect spin noise from the free radical at the nanoscale with subcellular resolution. We further compared the results from our new technique with a conventional fluorescence assay for the detection of reactive oxygen species. This provides a new method to investigate the relationship between drug release and the response by the cell via radical formation or inhibition.

Nanoscale MRI for Selective Labeling and Localized Free Radical Measurements in the Acrosomes of Single Sperm Cells

Free radicals play a major role in sperm development, including maturation and fertilization, but they are also linked to infertility. Since they are short-lived and reactive, they are challenging to detect with state of the art methodologies. Thus, many details surrounding their role remain unknown. One unknown factor is the source of radicals that plays a role in the sperm maturation process. Two alternative sources have been postulated: First, the NADPH-oxidase system embedded in the plasma membrane (NOX5) and second, the NADH-dependent oxidoreductase of mitochondria. Due to a lack of localized measurements, the relative contribution of each source for capacitation remains unknown. To answer this question, we use a technique called diamond magnetometry, which allows nanoscale MRI to perform localized free radical detection. With this tool, we were able to quantify radical formation in the acrosome of sperm heads. This allowed us to quantify radical formation locally in real time during capacitation. We further investigated how different inhibitors or triggers alter the radical generation. We were able to identify NOX5 as the prominent source of radical generation in capacitation while the NADH-dependent oxidoreductase of mitochondria seems to play a smaller role.

Insight into a Fenton-like Reaction Using Nanodiamond Based Relaxometry

Copper has several biological functions, but also some toxicity, as it can act as a catalyst for oxidative damage to tissues. This is especially relevant in the presence of H2O2, a by-product of oxygen metabolism. In this study, the reactions of copper with H2O2 have been investigated with spectroscopic techniques. These results were complemented by a new quantum sensing technique (relaxometry), which allows nanoscale magnetic resonance measurements at room temperature, and at nanomolar concentrations. For this purpose, we used fluorescent nanodiamonds (FNDs) containing ensembles of specific defects called nitrogen-vacancy (NV) centers. More specifically, we performed so-called T1 measurements. We use this method to provide real-time measurements of copper during a Fenton-like reaction. Unlike with other chemical fluorescent probes, we can determine both the increase and decrease in copper formed in real time.

Recent Developments of Nanodiamond Quantum Sensors for Biological Applications

Measuring certain quantities at the nanoscale is often limited to strict conditions such as low temperature or vacuum. However, the recently developed nanodiamond (ND) quantum sensing technology shows great promise for ultrasensitive diagnosis and probing subcellular parameters at ambient conditions. Atom defects (i.e., N, Si) within the ND lattice provide stable emissions and sometimes spin-dependent photoluminescence. These unique properties endow ND quantum sensors with the capacity to detect local temperature, magnetic fields, electric fields, or strain. In this review, some of the recent, most exciting developments in the preparation and application of ND sensors to solve current challenges in biology and medicine including ultrasensitive detection of virions and local sensing of pH, radical species, magnetic fields, temperature, and rotational movements, are discussed.

Detection of Few Hydrogen Peroxide Molecules Using Self-Reporting Fluorescent Nanodiamond Quantum Sensors

Hydrogen peroxide (H₂O₂) plays an important role in various signal transduction pathways and regulates important cellular processes. However, monitoring and quantitatively assessing the distribution of H₂O₂ molecules inside living cells requires a nanoscale sensor with molecular-level sensitivity. Herein, we show the first demonstration of sub-10 nm-sized fluorescent nanodiamonds (NDs) as catalysts for the decomposition of H₂O₂ and the production of radical intermediates at the nanoscale. Furthermore, the nitrogen-vacancy quantum sensors inside the NDs are employed to quantify the aforementioned radicals. We believe that our method of combining the peroxidase-mimicking activities of the NDs with their intrinsic quantum sensor showcases their application as self-reporting H₂O₂ sensors with molecular-level sensitivity and nanoscale spatial resolution. Given the robustness and the specificity of the sensor, our results promise a new platform for elucidating the role of H₂O₂ at the cellular level.

Applications of Fluorescent Nanodiamond in Biology

Fluorescent nanodiamond (FND) has emerged as a promising material in several multidisciplinary areas, including biology, chemistry, physics, and materials science. Composed of sp 3 ‐carbon atoms, FND offers superior biocompatibility, chemical inertness, a large surface area, tunable surface structure, and excellent mechanical characteristics. The nanoparticle is unique in that it comprises a high‐density ensemble of negatively charged nitrogen‐vacancy (NV − ) centers that act as built‐in fluorophores and exhibit a number of remarkable optical and magnetic properties. These properties make FND particularly well suited for a wide range of applications, including cell labeling, long‐term cell tracking, super‐resolution imaging, nanoscale sensing, and drug delivery. This article discusses recent applications of FND‐enabled developments in biology.

Silicon-Vacancy Nanodiamonds as High Performance Near-Infrared Emitters for Live-Cell Dual-Color Imaging and Thermometry

Nanodiamonds (NDs) with color centers are excellent emitters for various bioimaging and quantum biosensing applications. In our work, we explore new applications of NDs with silicon-vacancy centers (SiV) obtained by high-pressure high-temperature (HPHT) synthesis based on metal-catalyst-free growth. They are coated with a polypeptide biopolymer, which is essential for efficient cellular uptake. The unique optical properties of NDs with SiV are their high photostability and narrow emission in the near-infrared region. Our results demonstrate for the first time that NDs with SiV allow live-cell dual-color imaging and intracellular tracking. Also, intracellular thermometry and challenges associated with SiV atomic defects in NDs are investigated and discussed for the first time. NDs with SiV nanoemitters provide new avenues for live-cell bioimaging, diagnostic (SiV as a nanosized thermometer), and theranostic (nanodiamonds as drug carrier) applications.

Silicon-Vacancy Nanodiamonds as High Performance Near-Infrared Emitters for Live-Cell Dual-Color Imaging and Thermometry

Nanodiamonds (NDs) with color centers are excellent emitters for various bioimaging and quantum biosensing applications. In our work, we explore new applications of NDs with silicon-vacancy centers (SiV) obtained by high-pressure high-temperature (HPHT) synthesis based on metal-catalyst-free growth. They are coated with a polypeptide biopolymer, which is essential for efficient cellular uptake. The unique optical properties of NDs with SiV are their high photostability and narrow emission in the near-infrared region. Our results demonstrate for the first time that NDs with SiV allow live-cell dual-color imaging and intracellular tracking. Also, intracellular thermometry and challenges associated with SiV atomic defects in NDs are investigated and discussed for the first time. NDs with SiV nanoemitters provide new avenues for live-cell bioimaging, diagnostic (SiV as a nanosized thermometer), and theranostic (nanodiamonds as drug carrier) applications.

Quantum Sensing of Free Radicals in Primary Human Dendritic Cells

Free radicals are crucial indicators for stress and appear in all kinds of pathogenic conditions, including cancer, cardiovascular diseases, and infection. However, they are difficult to detect due to their reactivity and low abundance. We use relaxometry for the detection of radicals with subcellular resolution. This method is based on a fluorescent defect in a diamond, which changes its optical properties on the basis of the magnetic surroundings. This technique allows nanoscale MRI with unprecedented sensitivity and spatial resolution. Recently, this technique was used inside living cells from a cell line. Cell lines differ in terms of endocytic capability and radical production from primary cells derived from patients. Here we provide the first measurements of phagocytic radical production by the NADPH oxidase (NOX2) in primary dendritic cells from healthy donors. The radical production of these cells differs greatly between donors. We investigated the cell response to stimulation or inhibition.

Following Polymer Degradation with Nanodiamond Magnetometry

Degradable polymers are widely used in the biomedical fields due to non-toxicity and great biocompatibility and biodegradability, and it is crucial to understand how they degrade. These polymers are exposed to various biochemical media in medical practice. Hence, it is important to precisely follow the degradation of the polymer in real time. In this study, we made use of diamond magnetometry for the first time to track polymer degradation with nanoscale precision. The method is based on a fluorescent defect in nanodiamonds, which changes its optical properties based on its magnetic surrounding. Since optical signals can be read out more sensitively than magnetic signals, this method allows unprecedented sensitivity. We used a specific mode of diamond magnetometry called relaxometry or T1 measurements. These are sensitive to magnetic noise and thus can detect paramagnetic species (gadolinium in this case). Nanodiamonds were incorporated into polylactic acid (PLA) films and PLA nanoparticles in order to follow polymer degradation. However, in principle, they can be incorporated into other polymers too. We found that T1 constants decreased gradually with the erosion of the film exposed to an alkaline condition. In addition, the mobility of nanodiamonds increased, which allows us to estimate polymer viscosity. The degradation rates obtained using this approach were in good agreement with data obtained by quartz crystal microbalance, Fourier-transform infrared spectroscopy, and atomic force microscopy.

Charge state depletion nanoscopy with a nitrogen-vacancy center in nanodiamonds

The development of super-resolution imaging has driven research into biological labeling, new materials' characterization, and nanoscale sensing. Here, we studied the photoinduced charge state conversion of nitrogen-vacancy (NV) centers in nanodiamonds (NDs), which show the potential for multifunction sensing and labeling at the nanoscale. Charge state depletion (CSD) nanoscopy is subsequently demonstrated for the diffraction-unlimited imaging of NDs in biological cells. A resolution of 77 nm is obtained with 50 nm NDs. The depletion laser power of CSD nanoscopy is approximately 1/16 of stimulated emission depletion (STED) microscopy with the same resolution. The results can be used to improve the spatial resolution of biological labeling and sensing with NDs and other nanoparticles.

Quantum nanodiamonds for sensing of biological quantities: Angle, temperature, and thermal conductivity

Measuring physical quantities in the nanometric region inside single cells is of great importance for understanding cellular activity. Thus, the development of biocompatible, sensitive, and reliable nanobiosensors is essential for progress in biological research. Diamond nanoparticles containing nitrogen-vacancy centers (NVCs), referred to as fluorescent nanodiamonds (FNDs), have recently emerged as the sensors that show great promise for ultrasensitive nanosensing of physical quantities. FNDs emit stable fluorescence without photobleaching. Additionally, their distinctive magneto-optical properties enable an optical readout of the quantum states of the electron spin in NVC under ambient conditions. These properties enable the quantitative sensing of physical parameters (temperature, magnetic field, electric field, pH, etc.) in the vicinity of an FND; hence, FNDs are often described as “quantum sensors”. In this review, recent advancements in biosensing applications of FNDs are summarized. First, the principles of orientation and temperature sensing using FND quantum sensors are explained. Next, we introduce surface coating techniques indispensable for controlling the physicochemical properties of FNDs. The achievements of practical biological sensing using surface-coated FNDs, including orientation, temperature, and thermal conductivity, are then highlighted. Finally, the advantages, challenges, and perspectives of the quantum sensing of FND are discussed. This review article is an extended version of the Japanese article, In Situ Measurement of Intracellular Thermal Conductivity Using Diamond Nanoparticle, published in SEIBUTSU BUTSURI Vol. 62, p. 122–124 (2022).

Fluorescent Nanodiamonds for Detecting Free-Radical Generation in Real Time during Shear Stress in Human Umbilical Vein Endothelial Cells

Free-radical generation is suspected to play a key role in cardiovascular diseases. Another crucial factor is shear stress. Human umbilical vein endothelial cells (HUVECS), which form the lining of blood vessels, require a physiological shear stress to activate many vasoactive factors. These are needed for maintaining vascular cell functions such as nonthrombogenicity, regulation of blood flow, and vascular tone. Additionally, blood clots form at regions of high shear stress within a blood vessel. Here, we use a new method called diamond magnetometry which allows us to measure the dynamics of free-radical generation in real time under shear stress. This quantum sensing technique allows free-radical detection with nanoscale resolution at the single-cell level. We investigate radical formation in HUVECs in a microfluidic environment under different flow conditions typically found in veins and arteries. Here, we looked into free-radical formation before, during, and after flow. We found that the free-radical production varied depending on the flow conditions. To confirm the magnetometry results and to differentiate between radicals, we performed conventional fluorescent reactive oxygen species (ROS) assays specific for superoxide, nitric oxide, and overall ROS.

Spontaneous Light Emission Assisted by Mie Resonances in Diamond Nanoparticles

Spontaneous light emission is known to be affected by the local density of states and enhanced when coupled to a resonant cavity. Here, we report on an experimental study of silicon-vacancy (SiV) color center fluorescence and spontaneous Raman scattering from subwavelength diamond particles supporting low-order Mie resonances in the visible range. For the first time to our knowledge, we have measured the size dependences of the SiV fluorescence emission rate and the Raman scattering intensity from individual diamond particles in the range from 200 to 450 nm. The obtained dependences reveal a sequence of peaks, which we explicitly associate with specific multipole resonances. The results are in agreement with our theoretical analysis and highlight the potential of intrinsic optical resonances for developing nanodiamond-based lasers and single-photon sources.

Fluorescent nanodiamonds encapsulated by Cowpea Chlorotic Mottle Virus (CCMV) proteins for intracellular 3D-trajectory analysis

Long-term tracking of nanoparticles to resolve intracellular structures and motions is essential to elucidate fundamental parameters as well as transport processes within living cells. Fluorescent nanodiamond (ND) emitters provide cell compatibility and very high photostability. However, high stability, biocompatibility, and cellular uptake of these fluorescent NDs under physiological conditions are required for intracellular applications. Herein, highly stable NDs encapsulated with Cowpea chlorotic mottle virus capsid proteins (ND-CP) are prepared. A thin capsid protein layer is obtained around the NDs, which imparts reactive groups and high colloidal stability, while retaining the opto-magnetic properties of the coated NDs as well as the secondary structure of CPs adsorbed on the surface of NDs. In addition, the ND-CP shows excellent biocompatibility both in vitro and in vivo. Long-term 3D trajectories of the ND-CP with fine spatiotemporal resolutions are recorded; their intracellular motions are analyzed by different models, and the diffusion coefficients are calculated. The ND-CP with its brilliant optical properties and stability under physiological conditions provides us with a new tool to advance the understanding of cell biology, e.g., endocytosis, exocytosis, and active transport processes in living cells as well as intracellular dynamic parameters.

Toward Quantitative Bio-sensing with Nitrogen-Vacancy Center in Diamond

The long-dreamed-of capability of monitoring the molecular machinery in living systems has not been realized yet, mainly due to the technical limitations of current sensing technologies. However, recently emerging quantum sensors are showing great promise for molecular detection and imaging. One of such sensing qubits is the nitrogen-vacancy (NV) center, a photoluminescent impurity in a diamond lattice with unique room-temperature optical and spin properties. This atomic-sized quantum emitter has the ability to quantitatively measure nanoscale electromagnetic fields via optical means at ambient conditions. Moreover, the unlimited photostability of NV centers, combined with the excellent diamond biocompatibility and the possibility of diamond nanoparticles internalization into the living cells, makes NV-based sensors one of the most promising and versatile platforms for various life-science applications. In this review, we will summarize the latest developments of NV-based quantum sensing with a focus on biomedical applications, including measurements of magnetic biomaterials, intracellular temperature, localized physiological species, action potentials, and electronic and nuclear spins. We will also outline the main unresolved challenges and provide future perspectives of many promising aspects of NV-based bio-sensing.

Carbon Nanomaterials: Synthesis, Functionalization and Sensing Applications

Recent advances in nanomaterial design and synthesis has resulted in robust sensing systems that display superior analytical performance. The use of nanomaterials within sensors has accelerated new routes and opportunities for the detection of analytes or target molecules. Among others, carbon-based sensors have reported biocompatibility, better sensitivity, better selectivity and lower limits of detection to reveal a wide range of organic and inorganic molecules. Carbon nanomaterials are among the most extensively studied materials because of their unique properties spanning from the high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency fostering their use in sensing applications. In this paper, a comprehensive review has been made to cover recent developments in the field of carbon-based nanomaterials for sensing applications. The review describes nanomaterials like fullerenes, carbon onions, carbon quantum dots, nanodiamonds, carbon nanotubes, and graphene. Synthesis of these nanostructures has been discussed along with their functionalization methods. The recent application of all these nanomaterials in sensing applications has been highlighted for the principal applicative field and the future prospects and possibilities have been outlined.

Surface Modification of Fluorescent Nanodiamonds for Biological Applications

Fluorescent nanodiamonds (FNDs) are a new class of carbon nanomaterials that offer great promise for biological applications such as cell labeling, imaging, and sensing due to their exceptional optical properties and biocompatibility. Implementation of these applications requires reliable and precise surface functionalization. Although diamonds are generally considered inert, they typically possess diverse surface groups that permit a range of different functionalization strategies. This review provides an overview of nanodiamond surface functionalization methods including homogeneous surface termination approaches (hydrogenation, halogenation, amination, oxidation, and reduction), in addition to covalent and non-covalent surface modification with different functional moieties. Furthermore, the subsequent coupling of biomolecules onto functionalized nanodiamonds is reviewed. Finally, biomedical applications of nanodiamonds are discussed in the context of functionalization.

Sequential Bayesian experiment design for adaptive Ramsey sequence measurements

The Ramsey sequence is a canonical example of a quantum phase measurement for a spin qubit. In Ramsey measurements, the measurement efficiency can be optimized through careful selection of settings for the phase accumulation time setting, τ. This paper implements a sequential Bayesian experiment design protocol in low-fidelity Ramsey measurements, and its performance is compared to a previously reported adaptive heuristic protocol, a quantum phase estimation algorithm, and random setting choices. A workflow allowing measurements and design calculations to run concurrently largely eliminates computation time from measurement overhead. When precession frequency is the lone parameter to estimate, the Bayesian design is faster by factors of roughly 2 and 4 and 5 relative to the adaptive heuristic, random τ choices and the quantum phase estimation algorithm respectively. When four parameters are to be determined, Bayesian experiment design and random τ choices can converge to roughy equivalent sensitivity, but the Bayesian method converges 4 times faster.