High-Resolution Optical Imaging and Sensing Using Quantum Emitters in Hexagonal Boron-Nitride

Super-resolution microscopy has allowed optical imaging to reach resolutions well beyond the limit imposed by the diffraction of light. The advancement of super-resolution techniques is often an application-driven endeavor. However, progress in material science plays a central role too, as it allows for the synthesis and engineering of nanomaterials with the unique chemical and physical properties required to realize super-resolution imaging strategies. This aspect is the focus of this review. We show that quantum emitters in two-dimensional hexagonal boron nitride are proving to be excellent candidate systems for the realization of advanced high-resolution imaging techniques, and spin-based quantum sensing applications.


Super-Resolution Microscopy
Super-resolution microscopy (SRM) has expanded optical imaging to resolutions well beyond the diffraction limit of light. While traditional microscopy techniques can resolve lateral spot sizes ∼200-300 nm (and ∼500-700 nm, axially), super-resolution fluorescence microscopy can achieve resolutions of the order of ∼20-50 nm, and in some cases <10 nm [1][2][3]. The key of SRM is to render the fluorophores within the same diffraction region transiently discernible from one another for a short period of time. This is achieved through several strategies (briefly summarized below) that exploit the specific optical properties of photoluminescent materials, such as the non-linearity or stochastic nature of their emission.

Resolft
In reversible saturable/switchable optical linear fluorescence transitions (RESOLFT) microscopy, the super-resolved detection of individual emitters is achieved through non-linear optical excitation and emission schemes. RESOLFT microscopy generalizes the principles of techniques such as stimulated emission depletion (STED) and ground state depletion (GSD) microscopy [4,5]. These usually involve point-scanning strategies where the laser excitation and photoluminescence collection are designed ad hoc. A standard approach is to co-excite the sample with a torus-shaped beam overlapped to a confocal one to selectively induce "off" and "on" states in the fluorophores, and separate them in time/space as the beam is scanned across the sample. RESOLFT approaches can routinely achieve lateral resolutions of ∼40-80 nm.

Structured Illumination
In structured illumination microscopy (SIM) the sample is illuminated by high spatial-frequency patterns with a specific profile e.g., parallel lines [6]. Fluorophores are resolved beyond the diffraction limit as their emission combined with the patterned illumination generates large and detectable interference patterns. The incident pattern is applied in different orientations, and the super-resolution image is mathematically deconvolved from the interference signal. SIM methods can reach lateral and axial resolutions of ∼125 nm and 350 nm, respectively. Higher lateral resolutions of ∼50 nm can be achieved combining SIM with RESOLFT strategies, as for the case of nonlinear SIM [7] and instant SIM [8].

Stochastic Methods
Rather than through point-scanning or structured excitation, single-molecule localization microscopy (SMLM) achieves subdiffraction resolution via wide-field illumination. The emission of the fluorophores is photo-controlled as these are switched "on" and "off" by the laser. This is the case, for instance, for stochastic optical reconstruction microscopy (STORM) [9] and photoactivated localization microscopy (PALM) [10,11]. Different fluorophores within the (wide-field) excitation area are detected individually as their photo-blinking and/or -bleaching behavior render them optically active and inactive at different (random) times. Stochastic, state-switching detection is also the working principle of points accumulation for imaging in nanoscale topography (PAINT) [12]. PAINT exploits single fluorophores becoming optically "bright" as they (reversibly) bind to a target structure, which in turns makes them individually distinguishable from the "dark", unbound ones. Stochastic methods can generally achieve lateral resolutions of ∼20-25 nm with advanced setups reaching the ∼5 nm limit.

Off-State Microscopy
A particularly powerful SRM approach is minimal photon flux microscopy (MINFLUX) [3,13]. The method can operate both with scanning beam and standing wave microscopy arrangements. It relies on detecting the position of single fluorophores at the deep intensity minimum of e.g., a torusshaped excitation spot, as this is sequentially moved in space. The location of the emitter is inferred using a statistical maximum likelihood strategy that estimates the exact position based on where the fluorescence intensity is ∼zero (i.e., where the emitter is minimally excited at the center of the torus beam). MINFLUX microscopy can achieve lateral resolutions of ∼1-3 nm. Variants of MINFLUX include multiple off-state transitions for nanoscopy [14] and MINFIELD [15], which are designed to improve resolution, signal contrast and/or temporal sampling while reducing the light dose to the sample.

The Role of Material Science in SRM
Super resolution microscopy is an active field of research whose steady objective is to both develop new SRM approaches and design strategies to improve existing ones. Notably, this is not a mere application-driven endeavor. On one hand super-resolution microscopy has certainly being advanced as an indispensable characterization tool; for example in biology for monitoring cellular and subcellular processes [1], or in material science for studying complex materials [16]. On the other hand however, SRM has certainly been advanced by the progress made in material science and in the synthesis of nanomaterials with the unique chemical and physical properties required to realize super-resolution imaging strategies. This second aspect is the focus of this review. The goal is to present and discuss recent developments in high resolution imaging and sensing applications based on a specific two-dimensional material: hexagonal boron-nitride (hBN). We show that this material possesses distinctive physical and spin-optical properties that make it a desirable system for advanced realizations requiring the measurement of objects and quantities at the nanoscale.
This review is organized as per the following. The first part focuses on the hBN material and its properties. Emphasis is put on those most relevant for high resolution (bio)imaging and (bio) sensing such as the material's optical properties, the synthesis of hBN nanostructures as well as their toxicity, surface chemistry and functionalization. The second part discusses a selection of the most recent fundamental and practical realizations in superresolution microscopy and spin sensing based on quantum emitters in hBN.
Here, we focus specifically on hexagonal boron-nitride and its recent developments as a desirable material for high-resolution imaging and sensing. The material's properties are therefore presented with attention to the main key features required for ideal (bio)probes: 1) bright and photo-stable or controllable fluorescence, 2) small size (<10 nm), 3) non-toxicity, 4) roomtemperature operation, water solubility and ability to withstand pH variability, 5) controllable chemistry and high bindingspecificity. and nitrogen atoms are organized in atom-thin layers stacked over one another ( Figure 1A). Within each layer, B and N atoms are bound in a honeycomb hexagonal lattice (lattice constant 2.504 Å). The B-N bonds are sp [2]-hybridized, partially covalent and partially ionic, with the electron pairs more confined to the N atoms due to their high electronegativity.
The different atomic layers are instead connected (layer spacing 3.30-3.33 Å) mainly via weaker van der Waals forces [40]. This structure makes hBN a versatile material that can be produced using different methods and in a variety of nanostructures. The material has a large energy bandgap of ∼5-6 eV [41], which is the basis of some of its optical properties (Note that values of the bandgap have been reported in the wider range 3.6-7.1 eV [42], with the variability depending on hBN being mono-or multi-layered and on the stacking arrangement of its layers [43,44]).

Optical Properties
Hexagonal boron-nitride exhibits optical properties which are the foundations of several photoluminescence-based imaging and sensing applications. Being a wide-bandgap material, pristine hBN is nominally transparent in the visible spectral range (∼390-700 nm). It displays absorption and emission in the deep ultraviolet (∼210-220 nm) due to generation and recombination of free (∼215 nm) and bound excitons (∼227 nm) [45,46]. However, hBN generally hosts various type of crystalline defects: dislocations, grain boundaries, edges, vacancies and interstitial atoms [47][48][49][50][51][52][53]. Amongst these, atom-like defects consisting of complexes of vacancies and/or foreign atoms are of particular interest, as they give rise to characteristic photoluminescence emission in the visible and near infrared spectral range ( Figures 1A,B) [54, 55].
Besides a few exceptions [56-58], the exact chemical composition and structure of these defects are still unknown. Yet, these so-called color centers are widely utilized in both fundamental and practical realizations as they display properties that are ideal for photoluminescence-based applications ( Figures 1A-D [62,63], non-linear photo-physics compatible to super-resolution imaging [64,65], and addressable spin-dependent photo-emission [56]. Note also that due to the 2D nature of the host hBN material, the photon extraction efficiency from these atom-like emitters is comparatively high (∼10 6 counts/s at saturation)-as phenomena like Fresnel and total internal reflection, scattering and reabsorption are negligible [60]. This is ideal for photoluminescence-based applications as high collection efficiencies translate into high signal-to-noise ratio and thus better signal and/or temporal resolution.
Boron-nitride nanomaterials (BNNs) include zero-dimensional (0D) fullerenes and nanoparticles, one-dimensional (1D) nanotubes and nanoribbons, two-dimensional (2D) nanosheets and three-dimensional (3D) nanoporous BN. There are extensive reviews dedicated to the fabrication of BNNs [82,83], here we briefly summarize the methods developed to fabricate fluorescent nanoparticle-like (0D) objects, which are used in some of the SRM applications described below. Common nanofabrication approaches include the breaking down of large hBN microscopic crystals into nanosized hBN particles e.g., via ballmilling [84] or acoustic cavitation [85,86]. These methods can usually achieve high yields of hBN nanoparticles, but their size is relatively large (∼tens or ∼ hundreds of nm), especially for applications in bio-imaging and nanoscale sensing. Recently however, high yields of hBN nanoparticles less than 10 nm in size have been achieved via cryogenic-induced cracking of hBN [87,88] and by hydrothermal synthesis [89]. It should be noted that at this size the photostability of the emitters can be compromised-giving rise to photo-blinking and -bleaching-due to interaction with surface states and fluctuations in the local electric field. These are associated, for instance, with dangling bonds, other point-defects, or trapped charges, whose effect becomes more prominent as the particle size reduces [90][91][92][93]. These effects can partially be reduced with surface passivation strategies [94][95][96][97], but must be taken into account for assessing the reliability of the probes. In some cases however, they are central to achieve super-resolution imaging and are therefore a desirable feature.

Non-toxicity
Boron-nitride is considered a suitable material for biological and medical applications. Boron-nitride nanomaterials (BNNs) possess good biocompatibility [98][99][100][101] and high chemical and mechanical stability [102,103]. BNNs have been found not to inhibit cell growth or induce apostosis [104,105] and they have been shown to be up-taken by cells [99,106,107]. In fact, they have been used in practical realizations including as drug and gene carriers [108] and in cancer treatment studies [109]. For completeness, it should be noted that time-, dose-and celldependent cytotoxicity of BNNs has been reported in a small number of studies [108,110].

Functionalization
The chemical functionalization of boron-nitride nanomaterials (BNNs) is challenging as, in general, the network of B and N atom connected by sp [2]-hybridized bonds is highly stable and chemically inert. Nevertheless, several functionalization strategies have been proposed and realized, albeit with low overall yields ( Figure 1F) [111]. Note that both N and B can be utilized for functionalization whenever they are at an edge or at a lattice defect, where they give rise to dangling bonds. Conversely, reactions on the basal plane require opening of the B-N bonds. Both B and N must be terminated either by multiple functional groups (to balance the overall charge) or via a bridging bond. The B-N bond is partly covalent and partly ionic, which makes the nitrogen and boron atoms partially negatively and positively charged, respectively, due to their different electronegativity. As a result, B tends to bind to nucleophile (electron-donating) groups while N to electrophile (electronaccepting) ones. The list of functional groups successfully conjugated to BN nanomaterials include hydroxyl (-OH), amino (-NH 2 ), ether (-OR), amine (-NHR), arcyl (-COR), alkyl (-R), and halogen (-X) groups. The relevant functionalization methods can be found in dedicated reviews [111][112][113][114] and are summarized here, briefly.
Hydroxyl groups. Hydroxyl groups (-OH) are amongst the most fundamental functionalization groups for BNNs both for direct applications (matrix filling and bio-applications) and as base for more complex conjugations. The functionalization of BNNs-in particular BN nanotubes and nanosheets-with-OH groups is generally achieved via covalent bonding to B sites and has been realized through a series of alternative approaches including plasma treatment, hydrothermal reactions with NaOH, NaOH-assisted ball-milling, and reactions with H 2 O at a high temperature, or using reagents that can generate OH radicals [115][116][117][118][119][120][121].
Alkoxy groups. Alkoxy groups consisting of an alkyl group (carbon and hydrogen chain) bonded to an oxygen atom (-OR) can be functionalized directly onto BN surfaces e.g., by sonication in primary alcohols solvents [126]. Note that the-OR termination is advantageous for applications that require (alcoholysized) BN nanostructures to be readily dispersed in alcohol solvents.
Alkyl groups. Alkyl (-R) groups can attach to BN by forming B-C-N bridges or directly on B or N sites. This has been realized e.g., by carbene-assisted substitution and by reaction with a reductant solution and hexyl-containing compounds [127,128].
It is important to note that the ability to readily functionalize BN nanostructures-e.g., for stability in physiological environments and for target-specific labelling of subcellular structures-is only one of the aspects that must be considered for super-resolution imaging and sensing. Another critical factor to take into account is determining whether the presence of functionalized groups on the surface of the BN nanostructures alters their optical properties, possibly hindering their suitability as super-resolution probes. When nanostructures are smaller than a few nanometers, surface effects become relevant and the fluorescence of the hosted emitter can undergo intensity fluctuations, spectral diffusion, photoluminescence intermittency ("blinking") or even cease all together. These effects are not unique to hBN but rather universal and are due to the presence of surface states (e.g., dangling bonds, pointdefects, trapped charges and conjugated chemical species) which can drive the emitter in different charge states or generate random electric fields that destabilize the optical dipole moments of the emitters via spontaneous Stark shifts [93,136,137].
There are two main considerations to make in this regard. Firstly, these effects can be mitigated via specific surface passivation of the hBN nanoprobes. Suitable approaches are already available e.g., sol-gel coating via Stober reaction [138,139]. Besides, establishing whether certain functionalization groups deteriorate the optical properties of the BN nanoprobes is both feasible and experimentally testable. Secondly, in some cases the photo-instability of the probes is a desirable-in fact, necessary-feature. It allows for super-resolution imaging schemes based on the stochastic intermittency of individual, and otherwise unresolvable, emitters between "on" and "off" states, over time.

SUPER-RESOLUTION IMAGING WITH HBN
Standard, diffraction-limited microscopy using hexagonal boronnitride nanomaterials (BNNs) is well established. Yet, superresolution imaging has been realized only recently, thanks to the discovery and control of specific optical properties associated with quantum emitters in hBN (Table 1) [64,141].

Resolft
One of the first demonstration of super-resolution imaging in hBN was realized in 2018 [64]. The technique utilizes the nonlinear behavior of a specific class of hBN single-photon emitters (SPEs). The exact nature and chemical structure of these hBN emitters is unknown-the emission is likely due to transition between energy states in the hBN bandgap associated to atom-like defects in the lattice. Regardless, the non-linear behavior is observed consistently for an entire family of them with zerophonon line (ZPL) emission wavelengths >700 nm.
Upon excitation with a laser (at 675 nm or 708 nm in the study), the emitters produce a photoluminescence signal with a sharp ZPL (e.g., 785 nm). When co-excited with a second lowpower (∼30× less) laser at a shorter wavelength (532 nm), the emitters show a highly non-linear behavior characterized by an increase in photoluminescence (∼2×) and lowering (∼5×) of their saturation intensity. This non-linear behavior is attributed to the 532 nm laser repumping the system from a metastable "dark" state back to the excited "bright" state. The existence of photo-switchable dark/bright emission states is one of the main requirements for RESOLFT super-resolution microscopy. This unique photo-physical behavior can thus be harnessed to realize a variant of saturable, far-field, subdiffraction fluorescence nanoscopy, which is what was done in the study.
The technique has two notable advantages. Hexagonal boronnitride emitters are extremely robust against high-power illumination [143]. Whilst this is not a requirement for RESOLFT SRM, it is a desirable feature. For instance, both in STED and GSD the transition from the "bright" to the "dark" state is achieved by optical saturation-via stimulated emission depletion and ground-state depletion, respectively. This usually requires high excitation powers, which can lead to photobleaching of the emitters-a problem that has led to the development of off-state transitions nanoscopy methods. In addition, the issue of requiring high laser excitations for inducing saturation is further alleviated with this approach, as the second repumping laser effectively reduces the saturation intensity of the emitters. This is shown in the study where a negative-GSD-like scheme is utilized to assess the resolution of the technique (Figure 2A).
In the first experiment, a single torus beam (708 nm) is used as the excitation source. As the beam is scanned, the emitter is excited by a high-null-high intensity profile and produces a corresponding high-null-high emission pattern. In the confocal image, the emitter's location thus coincides with the center of the emission null (like in standard off-state transitions nanoscopy) and is readily extracted via mathematical deconvolution of the inverse image (Figure 2A, right). Sub-diffraction resolution is achieved as at higher excitation powers of the scanning torus beam, the high-null and null-high photoluminescence emission gradients become steeper. This effectively narrows the full width at half-maximum (FWHM) of the emission null according to the relation: where Δr is the resolution; I m is the maximum laser intensity in the periphery of the torus, and εI m is the minimum (null) intensity in the center. The quantity I s is the saturation intensity, while λ and n are the wavelength and the refractive index, respectively. The parameter β is the steepness of the point spread function which depends on the emitter's properties and on the periphery-to-minimum intensity gradient of the toroidal excitation beam. Equation 1 shows that the resolution Δr improves (Δr becomes smaller) when the ratio I s /I m is minimized. Generally, in other RESOLFT approaches, this is done by increasing the excitation laser intensity I m well above I s . This method, however, also allows to directly reduce I s via the re-pumping mechanism. This was demonstrated in a subsequent experiment of the study (Figure 2A, left and center panel), where a second torus beam (532 nm laser) was superimposed to the first one (708 nm laser). The addition of the 532 nm toroidal beam effectively reduces I s minimizing the ratio I s /I m , for any given intensity I m of the excitation torus at 708 nm. The highest resolution achieved using just the 708 nm excitation torus was (87 ± 10) nm, slightly worse than (63 ± 4) nm obtained with the two 708 nm and 532 nm torus beams overlapped. Both resolutions are far better than the ∼460 nm value of the confocal setup used in the study.
As a final point, it should be noted that this variant of SRM developed using hBN emitters could be extended to other classes of fluorophores, provided that they possess an analogous behavior characterized by the ability of the system to be repumped from the ("dark") metastable state to the excited ("bright") state.

Single-Molecule Localization Microscopy (SMLM)
Simultaneously to the approach described in §3.1, SRM was also realized in hBN monolayers using single-molecule localization microscopy [141]. The method exploits the on/off photoswitching behavior (blinking) of hBN emitters, which allows for their temporal-and thus spatial-separation, by capturing them when they are "bright/dark" alternately, one at a time ( Figure 2B).
Note that the photo-stability of hBN emitters depends on their local environment as, e.g., electric field fluctuations and nearby trap states can induce phenomena such as spectral diffusion and blinking [144,145]. In the case of hBN, photo-blinking is often more prominent for small (<10 nm) and thin (∼monolayer) nanoparticles/nanoflakes as the proximity to the surface increases the probability of local perturbations.
In the study, the sample consisted of a hBN monolayer. The material shows optically active emitters which are attributed to boron monovacancies V B (i.e., a missing B atom in the hBN lattice)-as deduced form transition electron microscopy (TEM) analysis. The blinking behavior of the emitters is attributed to photo-induced ionization and recombination between the neutral V 0 B and negatively charged V − B states. For SMLM to work, the sample must be spatially fixed with no drift over time. This can be achieved either through the use of a closedloop feedback-controlled stage or, like in this study, through post processing alignment using fiducial markers. The spatial resolution achieved in the study is 10.7 nm ( Figure 2B, right  panels).
The approach has a few desirable features. It utilizes wide-field illumination and detection of large sample areas (∼up to tens of μm) with fast acquisition (∼ms) and minimum damage to the sample thanks to the low power density (∼100 kW/m 2 ) of the excitation laser, compared to e.g., STED microscopy. It also allows for probing the chemical reactivity of the emitters. In the study this was done by submerging the sample in solutions of different pH and monitoring differences in fluorescence behavior. To this end, the technique was combined with balanced superresolution optical fluctuation imaging (bSOFI) [146], which utilizes higher-order statistics to increase resolution and image contrast-provided that the emitters display uncorrelated, stochastic fluctuations.
Almost at the same time, this SMLM technique developed for monolayer hBN was successfully implemented to investigate the optical properties of hBN nanoparticles as small as (3.0 ± 0.7) nm, produced by cryogenic exfoliation ( Figure 2C) [87]. The merit of the work was to demonstrate that hBN nanoparticles/nanoflakes possess a combination of optical and size properties that make them desirable for (bio-)imaging and sensing applications, either via SMLM or RESOLFT microscopy.
In a follow-up study [65], this SMLM technique was further improved to include the ability to perform spectral analysis of the emitters and thus producing super-resolved images containing spatial, spectral and temporal dynamics of the emitters ( Figure 2D). This was achieved by utilizing a calcium fluoride (CaF 2 ) filter in one of the detection paths ( Figure 2D, left). This dispersive element allowed for the simultaneous correlation of spatial and spectral mapping of each emitter-showcased in the study by clearly distinguishing two families of hBN emitters one displaying photoluminescence at ∼585 nm and the other at ∼640 nm ( Figure 2D, right panel).
A key feature of this multimodal SMLM method is the ability to correlate different characteristics and properties of the fluorophores, in time. For instance, the study shows the possibility to extract differences in blinking photo-kinetics for the two families of emitters and understand their physical origin (e.g., charge separation or ionization of the defects, as these follow different dynamics).
The versatility of this single-molecule localization microscopy technique was recently highlighted in a remarkable experiment that succeeded in imaging proton transfer phenomena in water, at an interface with single-charge resolution ( Figures 3A,B) [142].
The basic SMLM scheme is analogous to the ones described above. Multilayer hBN flakes were irradiated in low-power oxygen plasma to create a high density of surface defects. These defects are hypothesized to be V − B and show a considerably different fluorescence behavior when exposed to air or submerged in aqueous solutions of different pH. Monitoring the photoluminescence at ∼2 eV (∼620 nm) reveals that only a few emitters are luminescent in air (∼0.3 per 2 × 2 μm 2 area), while many more are when the sample is submerged in slightly acidic solutions (∼70 per 2 × 2 μm 2 area, in a 3.4 pH solution). The proposed explanation is that the V − B defects are optically dark but become active (emitting photons at ∼ 2 eV) as solvated protons H + compensate their charge through the reaction The SMLM technique thus allows monitoring-with high resolution, and over time-the occurrence of this protonation reaction in solution, at each one of the hBN defect sites.
Notably, SMLM can capture the specific hopping of protons between adjacent sites as these sites undergo "dark/bright" switching events corresponding to their V − B /V B H states. In the study, trajectories of single protons moving from site to site over distances up to ∼1 μm (imaging resolution ∼5-40 nm) were monitored over time (∼180 s). Different behaviors were observed, and it was possible to extract parameters such as diffusion coefficients and mean square displacement of the moving protons ( Figure 3B). The study was able to clearly show-at the single-molecule scale-the mechanism of desorption-limited transport of protons occurring in water at a solid/water interface between adjacent surface defects. It also underscores how the interface provides a preferential pathway for charge migration, which is relevant for several fields ranging from cellular transport and signaling, to catalysis and membrane dynamics. The study is a remarkable demonstration of the interfacial transport of protons in relation to the surrounding bulk water-a mechanism which, up to now, has been beyond reach due to the inability to measure directly the motion of single protons in water, with high-enough resolution.

Nonlinear Optical Microscopy
In the context of high-resolution imaging, nonlinear optical microscopy deserves a special mention as it has become a powerful tool not just for imaging but also for (2D) material characterization [147,148]. Nonlinear microscopy relies on multi-photon excitation of the fluorophores via virtual states, which produces a set of inherent benefits compared to traditional single-photon excitation [149,150]. These include better fluorophores-to-background contrast due to lower background autofluorescence [151], excitation through a favorable biological window (NIR wavelengths, 650-900 nm, characterized by lower absorption from hemoglobin and water) [152], as well as improved imaging resolution [153,154].
Nonlinear optical excitation of single-photon emitters (SPEs) in hexagonal boron nitride has been demonstrated [143]. In the study, individual atom-like defects with emission wavelength at 670 nm are excited either with a continuous laser at 532 nm or with a pulsed laser at 708 nm (pulses' duration 1 ps). The dependence of the SPEs' photon emission rate as a function of the excitation intensity reveals that excitation with the 708 nm pulsed laser takes place via a two-photon absorption process. In the experiment, large hBN flakes (size ∼200 nm) were used, but the technique can be readily extended to smaller hBN nanoparticles (size <10 nm). The results thus demonstrate the possibility of employing nonlinear microscopy of fluorescent hBN nanoprobes as a feasible high-resolution imaging strategy, complementary to superresolution microscopy.

BEYOND OPTICAL IMAGING
Beyond SRM, hexagonal boron-nitride is showing promise as the hardware material for another class of high-resolution sensing applications at the nanoscale. These are based on monitoring the spin state of individual hBN quantum emitters in response to changes in the surrounding environment. In general, optically active quantum defects can be separated in two categories: singlephoton emitters whose photoluminescence corresponds to transitions between well-defined quantum states, and so-called "spin centers" which exhibit coupling between the defects' intrinsic spin and their optical transitions. In other words, spin-centers display photoluminescence which is different depending on which spin state the emitter emits from. An archetype spin-center is the well-known nitrogen-vacancy (NV − ) center in diamond [155].
Spin-centers are sought after as potential qubits for solid-state, room-temperature quantum computation and information technologies [156], but also as ultra-sensitive nanoscale sensors [157]-which is of interest here. This is because their spin state can be manipulated and read out optically, while displaying extreme sensitivity to the local, surrounding environment.
The discovery of spin-centers in hBN is recent [56, 58], with the first identified one being the boron-vacancy center, V − B . The defect possesses distinguishable spin-optical transitions ( Figure 3C). Specifically, the V − B has a triplet (S 1) ground state of multiplicity m s 0 and m s ± 1, separated by a zerofield splitting (ZFS) energy D gs 3.46 GHz, in Planck's constant units h (Note that, for completeness, the degeneracy Optical excitation and cycling through the excited state of the defect 'spin-polarize' it in the m s 0 state, by preferentially populating it ( Figure 3C). Applying a microwave field resonant with the transitions m s 0↔ ± 1 allows manipulating the defect's spin state, which can then be read out, optically-as the excitation-decay cycle from the m s 0 ground state scatters more photons than the one from the m s ± 1 one. These features are the basis of optically detected magnetic resonance (ODMR) in which the spin state of individual point-like defects can be prepared, manipulated, and read out optically at room temperature ( Figure 3D). This is relevant for high resolution, quantum sensing applications as the ground state ZFS splitting (D gs , E gs ) can vary in response to external stimuli such as e.g., magnetic and electric fields, local chemistry or temperature. These quantities can therefore be measured directly using the V − B defect in hBN as nanoprobes, simply by measuring changes in photoluminescence.
There are several aspects that make spin-defects in hBN particularly attractive for quantum sensing applications. Unlike 3D semiconductor hosts, layered hBN are less prone to having unsaturated dangling bonds. These can act as fluctuating electron spins that generate magnetic noise and degrade the spin coherence-which is crucial for applications in quantum sensing [171].
The 2D nature of hBN confers it several additional, unique features. It allows for extremely high photon-extraction efficiencies. In fact, the out-coupling efficiency of hBN emitters is near-unity, as they are not surrounded by any high refractive index material and are not affected by Fresnel or total internal reflection, scattering and re-absorption [60]. This is desirable for nanoscale sensing applications based on photoluminescence detection, for it increases the signal-tonoise ratio and, thus, signal and/or temporal resolution.
Furthermore, the weak van der Waals forces between layers allow hBN to be readily transferred and integrated with other (hybrid) nanoscale systems made e.g., of 2D materials heterostructures [17,[172][173][174]. It also allows for the accurate placement of individual nanoprobe with respect to target areas or objects. This was demonstrated in a study that used a fluorescent hBN nanoparticle as an optical nanothermometer-placed deterministically onto a micro-circuit-to measure local temperature and potential hot spots in target areas of the device [175].
Another feature granted by the 2D nature of hBN is the potential close proximity between the emitter-sensors and the object or quantity to be sensed. This can be ∼(sub)nm, which is highly desirable as, for instance in magnetometry, dipolar magnetic fields decay as the third inverse power of the distance between sensor and spin [176], which makes their relative separation a critical limiting factor for the sensitivity of the technique.
It should be noted that the field of spin-based quantum sensing, whilst well established in 3D semiconductors such as diamond, is at its absolute infancy in 2D hBN. The spinaddressable properties of the V − B have been discovered only recently [56,57]. In fact, while it has been shown that V − B defects can be engineered in hBN via targeted irradiation, the isolation of individual defects is still beyond reach.
The research is certainly very active in the field. Shortly after the report of the V − B center, a second and different type of spindefect has been identified in hBN [58]. The defect is carbonrelated (possibly the negatively charged V B C − N ). It emits with a sharp zero-phonon line wavelength at ∼585 nm and is spin-active (S ≥ 1/2) with a detectable ODMR spectrum. Again, the defect has not been isolated at a single-center level (ODMR has only been observed in ensemble, currently), but there is optimism as emission at ∼585 nm from single centers is common in hBN.

CONCLUSION AND OUTLOOK
Two-dimensional materials have garnered a lot of attention in recent years, mostly because their low dimensionality and relative ease of fabrication have allowed the accomplishment of a whole suite of interesting demonstrations difficult to attain with traditional 3D bulk materials. Amongst 2D materials, hexagonal boron-nitride has become object of intense research since 2015, when quantum (single-photon) emission from atomlike defects has been identified [52]. While hBN color centers have-arguably-been trailing behind their well-established counterparts in e.g., diamond and silicon carbide, they are quickly becoming an active field of research in their own right. Their characteristic optical (and recently discovered) spin properties combined with the unique physico-chemical features granted by the low dimensionality of the hBN host make them ideal systems for several fundamental and practical applications-including in high resolution imaging and sensing.
In this review, we have highlighted the role hBN quantum emitters have recently played in the realization of novel superresolution imaging techniques (RESOLFT and SMLM), as well as their potential impact in advanced spin-based quantum sensing applications. Research geared towards exploiting hBN and its color centers for sensing and imaging is at its infancy. Future endeavors should focus on fully identifying the origin of the defects (besides the V − B ) and improving material and color centers engineering to move from fundamental demonstrations to practical realizations. In this regard, experiments such as observing single proton transport at the solid/water interface by hBN-based super-resolution microscopy are encouraging. They show that the material is offering novel capabilities that allow for the exploration of regimes inaccessible with alternative methods.

AUTHOR CONTRIBUTIONS
The author confirms being the sole contributor of this work and has approved it for publication.