Spatial Manipulation of Fluorescent Colloidal Nanodiamonds for Applications in Quantum Sensing

Nitrogen vacancy (NV) centers in diamonds are of current interest as quantum sensors, single photon sources, and biological nanomaterials due to their unique optical and spin properties, biocompatibility, and robust structure. Though NV center in diamonds demonstrates longer coherence time and has been used for more sensing and quantum operations compared to nanodiamonds (NDs), the prospect of selecting NDs with single, few, or multiple NV centers and moving the NDs to the location of interest makes NDs suitable for various applications, due to which there is a significant boost in the last decade to manipulate and position the NDs on the surface or to achieve dynamic control in the fluidic environment. This work covers some of the basics of the materials and optical properties of nanodiamonds and discusses the capabilities and challenges for practical applications, followed by a detailed review of the various static and dynamic manipulation methods of nanodiamonds in microfluidic environments (colloidal NDs).


Introduction
Fluorescent nanodiamonds (FNDs) are nanosized diamond particles engineered to emit light upon optical excitation.In recent years, FNDs have gained significant attention due to their unique optical properties and potential applications in various fields, including bioimaging, sensing, drug delivery, and quantum information science.These properties result from the combination of DOI: 10.1002/apxr.202300082 the diamond's inherent characteristics and the incorporation of fluorescent defects within the diamond lattice.The most well-known defect responsible for fluorescence in nanodiamonds is the nitrogen-vacancy (NV) center, where a nitrogen atom replaces a carbon atom adjacent to a vacant lattice site of the nanodiamond crystal lattice. [1]When excited with an appropriate energy source, typically a laser, the NV centers within the nanodiamond lattice emit fluorescence in the visible range.The NV centers offer several advantages compared to other fluorescent probes, such as quantum dots or organic dyes. [2]They exhibit exceptional photostability and long fluorescence lifetimes.The optical response can withstand long exposure to excitation sources while ensuring a stable and reliable fluorescence signal. [3]The NV center also has unique spin-related properties, which are crucial for various applications, such as quantum processing, quantum communication, and quantum sensing.Particularly, the relatively long coherence times compared to other quantum systems, even at ambient temperature and pressure, as well as high spatial resolution due to their nanoscale dimensions, make it suitable for quantum sensing.The FNDs are highly sensitive to changes in their local environment, such as temperature, electric and magnetic fields, pressure, and chemical composition.They can be used as extremely sensitive sensors to detect these nanoscale properties. [4]In addition to that, the relatively inert diamond core makes them suitable for sensing and imaging in biological systems with high sensitivity and resolution. [5]FNDs can be further engineered to be biocompatible thanks to their high surface area-to-volume ratio and the presence of abundant carbon atoms on the surface, allowing for easy surface functionalization and engineered interaction with the surroundings.For example, the carboxyl group (-COOH) on the ND surface can be attached by oxidative treatment or air or ozone purification. [6]Various chemical reactions can then be done with the ND-COOH complex, and the surface of ND can be further modified.[9] On the other hand, FNDs can be integrated with other nanomaterials and systems, such as superconducting circuits or optical resonators, to create hybrid quantum systems. [10]These combinations leverage the unique properties of both materials to enhance quantum control and information processing capabilities.
To fully harness the properties of the NV center, new techniques have been developed to manipulate nanodiamonds and position them in specific locations of interest. [11]Precise positioning of nanodiamonds is often required in many applications to target specific locations of interest for local sensing.For instance, in biomedical applications, nanodiamonds may need to be directed to specific cells or tissues for drug delivery, imaging purposes, or even for performing sensing measurements in specific locations within and around the cells. [12]Manipulation techniques can also be crucial in assembling nanodiamonds into desired structures, such as plasmonic and photonic structures for integrated quantum architecture or incorporating them into specific locations of microfluidic lab-on-chip devices, or optical signal processing chips, all of which can open exciting possibilities in electronics, optics, microfluidics, and quantum technologies.By controlling their positioning in a heterogeneous environment, nanodiamonds may be optimized for specific sensing tasks, such as detecting magnetic fields and temperature changes or measuring the local density of chemical analytes.Such manipulation techniques potentially enable the design and implementation of nanodiamond-based sensors with enhanced sensitivity, selectivity, and resolution.Further, precise manipulation of nanodiamonds (levitated in ultrahigh vacuum) also enhance or engineer control over their spins, which is vital for mass spectrometry, torque detection, and other sensing applications. [13]n this article, we briefly introduce the mechanism of quantum sensing, focusing on using fluorescent nanodiamonds as the platform.Subsequently, we review the different techniques for single as well as multiple ND manipulation, including optical and non-optical methods, explore their possible advantages and shortcomings, and discuss their potential applications by exploiting the capabilities of NDs for quantum sensing and technolo-gies.Finally, we discuss the challenges and opportunities associated with establishing a nanodiamond-based dynamic sensing scheme.

ND Physics Relevant to Quantum Sensing
NV center in diamond lattice consists of a substitutional nitrogen atom coupled with a vacancy defect in one of the adjacent sites in the diamond lattice.16][17] NV can exist in the negative (NV − ) or neutral state (NV 0 ).The negatively charged NV − center, the magneto optically active state referred to as NV center here, is useful for sensing and other applications in quantum technologies.The energy level diagram of a single NV center within the diamond structure is shown in Figure 1b.The ground state and excited state of the NV center have three distinct spin states, which can be represented as m s = −1, m s = 0, and m s = +1, with −1 and +1 states being degenerate, where m s represents the spin quantum number.When the NV center is excited by absorbing green light (≈532 nm), it can transition from the ground to the excited state.The NV center can either directly return to the ground state ( 3 E → 3 A 2 ), emitting a broadband red fluorescence signal with zero-phonon line (ZPL) at 637 nm or follow an alternative non-radiative ISC (intersystem crossing) pathway through singlet states, 1 A 1 → 1 E, emitting 1042 nm photon.The 1 E is a metastable state with a lifetime of ≈200 ns, while the 3 E state has a lifetime (t 3E ) of ≈10 ns. [18]he NV center in the m s = ±1 state of the excited state ( 3 E) has a higher probability (≈0.55) of taking the non-radiative pathway compared to the NV center in the m s = 0 state of the excited state (≈0.14). [18]Similarly, the transition probability of 1 E → m s = 0 is ≈1.2 times more probable than the 1 E → m s = ±1 states.Hence, following the non-radiative pathway, the NV center in the m s = ±1 state of the excited state transit to the m s = 0 state of the ground state.The ISC pathway results in the decrease of fluorescence of the NV in m s = ±1 states.Thus, the m s = 0 state and m s = ±1 state are referred to as the bright and dark states, respectively.In NV ensembles, after several cycles, the NV center in the m s = ±1 state eventually reaches the m s = 0 level of the ground state.This process, known as optical pumping, can achieve a polarization of 80%-90% with a laser pulse of 1-2 μs and laser intensity less than the saturation intensity, (hc)/(  t 3E ), of 0.3 MW cm −2 , where absorption cross-section,  = 1e −16 cm 2 , c is the speed of light,  = 532 nm is the optical wavelength, and h is Planck's constant. [19,20]he ground state has an energy gap, called zero-field splitting (ZFS) of 2.87 GHz at room temperature between the m s = 0 and m s = ±1 states.By optically pumping and applying a 2.87 GHz microwave, NV centers can be transferred from the m s = 0 to m s = ±1 states, resulting in a decrease in fluorescence.In the presence of a magnetic field along the NV axis, the m s = ±1 states split.Also, the ZFS is temperature-dependent, shifting to lower frequencies as the temperature increases.Therefore, the NV centers can function as magnetic field and temperature sensors.For sensing using NV centers, both a single NV and an ensemble of NV centers can be used.Single NV provides atomic scale resolution, while ensembles can achieve measurement sensitivity scaled by a factor of the square root of the number of NV centers.The basic technique for sensing using NV centers is continuous wave optically detected magnetic resonance (CW-ODMR).In this method, a laser continuously pumps the NV center in the ground state (m s = 0 level), while the microwave frequency is swept.When the microwave frequency matches the ZFS energy, a decrease in fluorescence occurs due to the transfer of NV centers from the m s = 0 to m s = ±1 levels.In the presence of a magnetic field, this dip splits into two for a single NV center, as the Zeeman effect causes the m s = ±1 states to become nondegenerate.Additionally, the dip position shifts due to changes in ZFS caused by temperature variations.Figure 1c illustrates the effect of temperature on ODMR for ensembles of NV centers, while Figure 1d depicts the split in ODMR for a single NV center in the presence of a magnetic field. [2]When using an ensemble of NV centers within a crystal, the ODMR dip can split into a maximum of eight dips, depending on the orientation of the magnetic field, as there are four possible orientations of the NV axis in the diamond crystal.This property finds applications in vector magnetometry. [21]The ZFS (D = 2.87 GHz) at room temperature changes by dD dT ≈ −74 KHz∕K due to the thermal expansion of the diamond lattice and electron-phonon interaction. [22]However, ZFS dependence is non-linear over a broader temperature range. [23,24]Due to the diamond lattice's inert structure, the NV center becomes a robust sensor with high spatial resolution.NDs containing NV centers provide a good platform for local measurements due to their small size and stable structure.For sensing applications, proximity to the source is important to increase the signal of the physical quantity to be measured.Hence, it necessitates the controlled manipulation of NDs and determines the measurement schemes for such a system.

Measurement Techniques
Here, we outline the quantum sensing schemes.Figure 2 shows the DC/AC measurement protocols used for sensing magnetic field and temperature using NV center in diamonds or nanodiamonds.The three time scales essential for NV spin manipulation are spin dephasing time (T * 2 ), spin coherence time (T 2 ) and spin-lattice relaxation time (T 1 ).NV spin also interacts with spins in its environment or from the surface.These spin impurities can cause dephasing and decoherence of the NV spin, named as T * 2 and T 2 , respectively.The source of these spin impurities is 13 C isotopes, electronic spins associated with Nitrogen donors, and other spins.NV spin can also interact with the lattice phonons, which causes spin flips and subsequent population decay by spin-lattice relaxation, referred to as T 1 .To manipulate NV spin, a microwave (MW) signal is applied continuously or in pulse sequences.Due to the continuous application of laser and microwave in CW-ODMR, Figure 2a, there is power broadening in the ODMR signal due to optical pumping (laser) and MW application.To remove power broadening and increase the sensitivity of the measurement, pump-probe techniques that use combinations of microwave pulses or pulse sequences are available.Figure 2b shows the Rabi pulse sequence in which MW frequency is matched to one of the transitions, m s = 0 → m s = + 1 or m s = 0 → m s = − 1, depending on the applied external magnetic field and its orientation with respect to the NV axis.A laser pulse (2 μs) is used to pump the NV center to m s = 0 level.MW is applied after the laser is switched off, and a short read-out laser pulse (300 ns) is used to detect the fluorescence.The same procedure is repeated, and fluorescence is measured as a function of MW pulse length, giving the Rabi oscillation from which the time duration for /2 and  pulses is estimated.To avoid power broadening, seen as lowered ODMR linewidth, pulsed ODMR is used, as shown in Figure 2c.The protocol is similar, except here the MW frequency is swept by keeping the duration of microwave pulse equal to  pulse, that is, T  , where T  is limited by dephasing time T * 2 . [27]To further improve the sensitivity of DC field measurement, one can use the Ramsey sequence, where a /2 pulse prepares the spin in an equal superposition of m s = 0, and one of the m s = ±1 states (Figure 2e).The spin is allowed to evolve for free precession time , in which it accumulates a phase depending on the applied field.A second /2 pulse is applied that projects the relative phase accumulated on the population difference of m s = 0, and one of the m s = ±1 states, which is measured optically using the read-out pulse. [28,29]This sequence is repeated by sweeping the free precession time , which is limited by the dephasing time T * 2 .These methods are limited to DC field measurements.For AC field sensing, the basic technique is Spin/Hahn echo sequences, where a  x or  y pulse is added in between (at /2 time) the two /2 pulses. [28,30] pulse functions as a refocusing pulse for the spin dephasing during the free precession time, Figure 2f, which makes the time  limited by decoherence time T 2 , hence increasing the sensitivity.The extension of the Hahn echo sequence is applying multiple refocusing pulses, known as dynamical decoupling sequences (DDS).It results in the extension of spin decoherence time with reduced bandwidth. [28,31]Figure 2d shows a spin relaxometry technique.NV center is polarized initially, and read pulse is applied after time  with MW switched off all the time.The spin population decays to a mixed state over a longitudinal relaxation time (T 1 ), which is affected by the presence of magnetic frequency noise or other paramagnetic impurities [32] and can be used for spinsensing applications. [33]However, the laser pulse used to spin polarize the NV − center can also ionize the NV − center to the NV 0 state when the concentration of an electron donor is high near the surface of NDs, which in turn alters the collected photoluminescence.To address this issue, a microwave resonant  pulse is modulated to transfer the population from m s = 0 to m s = +1 or m s = −1 state after the spin polarization pulse. [34,35]ulse sequences provide better sensitivity by increasing the coherence time effectively, thus providing higher sensitivity.[38] All these techniques can be applied to diamond substrates or static nanodiamonds.However, pulse techniques require B 0 (static bias field) and B 1 (field from the antenna) which define the resonance frequency and  pulse duration, respectively. [37]Rotation of ND will change  pulse duration, which will change the fluorescence measured with time, and measurement of the local magnetic field will be difficult.Further, the sensitivity of the measurements using these sequences and CW ODMR reduces for biological systems as the microwave power is less due to the distance between the microwave antenna and NDs.Also, microwave power has to be limited due to the potential damage that may be caused from microwave absorption by the cell media or the cells.A less stringent microwave power requirement is possible using concatenated continuous dynamical decoupling sequences for static NDs in cells. [39]Such measurement techniques and precise control over ND motion are beneficial to achieve more reliable and sensitive measurements with colloidal NDs.
The spin properties of NV centers in smaller NDs degrade due to increased interaction with surface spins. [32]This can be improved with better processing of ND's surface by oxidation and annealing methods.It has been shown that aerobic oxidation, compared to anaerobic triacid ND oxidizing, increases the T 2 1.4 times compared to the original ND, irrespective of their size. [40]D sensitivity can be increased by using an ensemble of NV centers in ND but with reduced spatial resolution.More NV centers increase the fluorescence intensity, and sensitivity becomes better by a factor of √ N, N is the number of NV defects.However, NDs with a high density of NV will also contain high levels of paramagnetic defects.This will decrease the coherence time and increase ODMR linewidth.[41] Hence, the optimal NV density and size of ND will be decided by the required application.[42]

Applications in Quantum Sensing and Nanophotonic Assembly
Nanodiamonds have unique properties that make them wellsuited for various sensing applications.Some of these application areas are summarized in Figure 3, in which the control over the maneuverability of the NDs has the potential to boost their sensing capabilities.Nanodiamonds containing nitrogenvacancy (NV) centers exhibit temperature-dependent fluorescence properties.This property makes them useful as nanoscale thermometers for measuring temperature variations in biological systems, nanomaterials, and cellular levels.Interestingly, NDs can be ingested/injected into cells or can be coated on the surface  [32] Copyright 2013, American Physical Society.b) Shows the effect of Ferritin on NDs, T1 measurement for free ND is shown, and the inset shows T1 measurement for ferritin-coated NDs, which is reduced compared to uncoated ND.Reproduced with permission. [55]Copyright 2013, American Chemical Society.c) pH calibration curve by taking the photoluminescence ratio -COOH and -OH functionalized NDs.Reproduced with permission. [65]Copyright 2023, IOP Publishing Ltd d) Top; Schematic of association between a nanodiamond and EGF receptor.Bright field image of C. elegans, bright field and selective imaging protocol image of the intestine of C. elegans, bottom (left to right), the circle shows the nanodiamond and orange line shows the outline of the intestine.Reproduced with permission. [66]Copyright 2020, American Chemical Society.e) Integration of NV-centers in nanodiamonds with nanophotonic circuits.Schematic of the device layout with PhC-cavity-coupled emitter system (center) that is optically accessible via independent waveguides (top), Scanning electron micrograph of Ta 2 O 5 cross-bar structure with PhC-cavity (middle), FDTD simulation showing PhC-cavity mode profile.Reproduced with permission. [10]Copyright 2020, American Chemical Society.f) Schematic of ND containing NV center covered by a hydrogen dense layer, immersed in a liquid PFPE analyte, NMR of 19F and 1H from NDs using an XY8-10 dynamical decoupling sequence.Reproduced with permission. [56]Copyright 2020, American Physical Society.g) Top, schematic of confocal microscopy from imaging fiber bundle (IFB) and the images captured from a facet and FND coated facet, bottom: ODMR captured at different positions between antenna wires at zero magnetic field, zero current, and at B = 3.8mT for different current.Reproduced with permission. [67]Copyright 2020, American Chemical Society.h) The Left image shows a fluorescence scan of cells (green: live cells and red: dead cells); the Bar graph shows ND temperature (circle in left image) when local heat is applied at two locations (crosses in image).Reproduced with permission. [68]Copyright 2013, Springer Nature.
with suitable molecules to promote uptake.Using such internalized FNDs, Kucko et al. first demonstrated nanoscale thermometry inside a living cell.Recently, thermometry measurements have been performed at selected locations of the intracellular space by manipulating optically trapped NDs. [12]The dependence of ZPL of ND-clusters on temperature was exploited for probing temperature.Due to the NDs' high biocompatibility, NDs stay inside the cell for a long duration.[45][46][47] For example, ND orientation tracking with respect to the direction of a known applied magnetic field for hours reveals important information for understanding membrane nano-mechanics and local viscosity. [48,49]The NV center is also highly sensitive to magnetic fields.The ODMR of FNDs is used to measure the magnitude and orientations of weak magnetic fields with high precision.Similarly, ND's fluorescence is affected by the distribution of local charges on their surface as the NV − state can transition to NV 0 state.This effect is used to sense processes that alter the charge distribution on the ND surface. [50]The state of NV center in NDs (i.e., NV 0 or NV − ) can be detected optically and electrically. [51,52]In addition to that, spins around the ND surface couple to the NV spin, which significantly affects the NV center's T 2 and T 1 time scales.Functional groups on the ND surface undergo protonation/deprotonation or H-bonding as pH changes.Hence, they have been used to sense local pH.[55] NDs can also be harnessed for localized nuclear magnetic resonance (NMR) measurements.This involves manipulating the NDs to bring them into proximity to the sample, enhancing the signal as the detectability of a few nuclei diminishes with distance.For example, Holzgrafe, Jeffrey, et al. use nanodiamonds to detect and distinguish 19 F and 1 H nuclear species in a sample volume of 20 3 nm 3 . [56]Additionally, NDs T1 and T2 measurements render them suitable as contrast agents in magnetic resonance imaging (MRI). [57]Due to low abundance and small gyromagnetic ratio of spin ½ 13C nuclei, detecting NDs directly in magnetic resonance imaging is challenging.[60] Therefore, the sensing measurements in fluidic environments using NDs can benefit by having maneuverability from one location to another and control over their orientation at a particular location.
Apart from these diverse measurements with such randomly dispersed diamond nanocrystals, the colloidal NDs may also be positioned at specific locations on surfaces, which are important for quantum sensing and achieving hybrid quantum photonic assemblies.For practical applications, control over NDs selection, positioning, and targeting is required on demand.FNDs with single or ensembles of NV or other defects, such as Silicon Vacancy (SiV) and germanium vacancy (GeV), are integrated with substrates or structures to make a quantum array platform for sensing or coupling defects to such structures.It is vital for quantum photonic devices to have NDs placed in the photonic nanostructured cavities precisely with nm or sub μm accuracy.Integrating NDs with photonic structures can be useful for quantum optical technologies by enhancing their single-photon emission rate and read-out speeds.Wolters et al. showed a 12.1 factor increase in the Purcell enhancement of fluorescence emission at ZPL of a single NV center in nanodiamond by coupling it to the surface of gallium phosphide crystal cavity. [61]In separate work, single NV centers in NDs were integrated with silver nanowires by manipulation using the tip of an atomic force microscope cantilever. [62]n enhancement of the spontaneous emission decay rate by a factor of 8.3 was observed.For large-scale applications of these integrated devices, one requires combining photonics structures and NDs in an array.Depending on the application, one requires selecting a single or few NV center NDs and manipulating them in 2D on the surface of the patterned substrate.Here, the challenge is to have control over a large number of NDs and pattern them on predefined structures or make an array of NDs on a substrate.ND's light-matter interaction with various substrates has potential applications in quantum communication, computation, and metrology, which is discussed extensively in other review articles. [63,64]

Reproducibility
Controlling the synthesis of diamond nano-crystallites with nanometer precision is fundamentally challenging due to their extreme and far-from-equilibrium production conditions.[71] Since there is no precise control of the NV center density and their proximity to the surface, NDs suffer from variability in their sensitivity.Factors such as local strain anisotropy and crystal impurities also lead to spin and optical properties variations.This inhomogeneity results in inconsistent measurements among different NDs.Further, the sensitivity of nanodiamonds to changes in temperature or magnetic fields can vary due to the variability in the ZFS values among individual NDs.In addition to that, spin-dephasing time (T * 2 ), spin coherence time (T 2 ) and spin-lattice relaxation time (T 1 ), which are crucial for quantum sensing, can be affected by factors such as surface interactions, lattice defects, impurities, and environmental conditions.For example, single NV in diamond, depending on the synthesis process used (HPHT, CVD, 12 C purified CVD), T * 2 can vary from 0.1 to 100 μs, T 2 (spin echo, dynamical decoupling) from 1 μs to 2 ms and T 1 from 1 to 10 ms. [36,21,29,72,73]he highest spin coherence time observed in a nanopillar (diameter 300-500 nm, length 1-2 μm) containing a single NV is 360 μs using Hahn Echo, extendable to 710 μs using advanced dynamical decoupling techniques. [74]Recent work from March et al. showed T * 2 = 2 μs, T 2 = 786 μs and T 1 = 4 ms for single NV in 12 C purified, polycrystalline, ball-milled nanodiamonds. [75]Relaxation time for commercially available nanodiamonds prepared by the HPHT process having size 100 nm is 100 μs or less, while T 2 can be of the order of a few μs for 45 nm size NDs. [76,49]It is crucial to calibrate and characterize each nanodiamond for accurate sensing.In the future, the fabrication process can be improved to have minimum inhomogeneity in NDs.However, controlling the defect density and their distance from the ND will still be challenging.Alternatively, one could make an array of NDs and separate the good ones by pick and place technique, which would be a time-consuming process.A more practical solution is to identify a ND during an experiment and then use the same ND for further measurements by manipulating it.A single but dynamic ND sensor can be useful for consistent and reproducible self-referenced measurements across different locations.

Thermal fluctuation
FNDs experience Brownian motion in fluidic environments, causing them to move and rotate randomly.Such thermal fluctuations intrinsic to colloidal systems pose challenges in making precise measurements.The randomized motion of such nanosized particles results in fluctuating fluorescence signal that leads to decreased sensitivity and increased noise in the measured signals.Figure 4a shows the schematic of the motion of ND in fluids with different viscosities (yellow, black curve), demonstrating the random displacement caused by Brownian motion.Due to this motion, NDs can move to a position of lower intensity or out of focus of the objective within the measurement time scale, resulting in fluorescence intensity fluctuations (as shown schematically in Figure 4a).Consequently, the signal-to-noise ratio decreases over time, ultimately compromising the sensitivity of the measurement.The experimental DC sensitivity for an infinitesimal magnetic field variation B of a measurement technique with integration time t is given by: for a given Δ (ODMR linewidth) and C (ODMR contrast) where I 0 is the number of photons collected. [36]Hence, I 0 depends on the collection optics and laser intensity.As mentioned earlier I 0 will change as ND moves to a different location in the illumination volume.We assume a Gaussian profile of the spatial distribution of laser intensity with power 100 mW, with maximum intensity I 0 at the focused spot and Δ = 10 MHz, C = 10%.Figure 4b illustrates the variation in the detectable change in the magnetic field (B) with measurement time, comparing scenarios with Brownian motion in increasingly viscous fluids to no motion (black curve).The schematic represents that the minimum detectable field initially improves with averaging but then worsens faster in a medium of lower viscosity due to a decrease or fluctuation in collected fluorescence signal from ND through the objective.Hence, the scope to apply more complex sensing sequences or averaging cycles that need more processing time to enhance the sensing resolution becomes largely limited.Therefore, such fundamental limitations imposed due to Brow-nian motion must be considered while designing experiments with nanodiamonds dispersed in fluids or gels.

Uncontrolled Rotation
The NV center is sensitive to the angle of the NV axis relative to the axis of the externally applied fields.The dependence of nanodiamond's ODMR with respect to known applied fields can reveal helpful information by analyzing the rotational dynamics of FNDs.For example, orientation tracking of FNDs located on the cell membrane can be correlated with cell metabolic activities. [49,77,78]Usually, such measurements are done using a confocal imaging setup and single particle tracking analysis of the diffusing nanodiamond.This requires a complicated analysis of the fields and trajectories.Hence, it may be useful to have precise control over the rotation of NDs on demand to do measurements in different directions.For example, by carefully selecting the orientation of an ND, information about the local magnetic field along different axes can be extracted from analyzing the fluorescence intensity and polarization.Particularly, the orientational control is important for colloidal FNDs that are free to rotate when dispersed in a media.Also, for a dynamically varying unknown field, individual FNDs must be positioned and rotated to perform a vectorial measurement, such as mapping magnetic fields in three dimensions.

Lack of Dynamic Control in 3D
The sensitivity of the NV center is highly restricted to the distance from the source.This limited range of sensitivity to external factors like magnetic fields or temperature changes might not cover all the variations that need to be detected in certain applications.Therefore, NDs need to be positioned appropriately to sense their environment effectively.Dynamic manipulation of nanodiamonds involves real-time control and movement of these nanoparticles within a fluidic or microscopic environment.One can rely on diffusion to reach the proximity of the target.For example, coating NDs in such a way that they get attached to specific organelles of biological cells is helpful.However, NDs passively diffusing in a fluidic environment is a slow and uncontrolled process.It requires a significant amount of time for passive NDs to reach the region of measurements in highly viscous and complex intracellular environments.Also, the ND sensor needs to be maneuvered to get a consistent measurement over a large area, such as mapping the variation of a physical parameter inside a living cell.So, to harness the full sensing potential of NDs, it will be advantageous to have control over the position of NDs.On the other hand, precisely positioning these FNDs is important for exploiting fundamental interactions and assembling quantum-photonic hybrid systems.As a simple approach, FNDs can be spin-coated or drop-cast on a predesigned substrate.Such an approach is inefficient, and getting a desired combination within the ensemble is problematic.

Colloidal ND Manipulation Schemes and Related Demonstrations
Manipulation of colloidal diamond crystals with dimensions typically ranging from a few nanometers to tens of nanometers  [79] Copyright 2012, National Academy of Science.b) Schematic of experimental trapping setup; gap antennas are on the coverslip with 2 μm separation.1064 nm laser, activates the plasmonic trap, and 532 nm laser, which excites NDs fluorescence, are superimposed.Reproduced with permission. [84]Copyright 2014, American Chemical Society.c) Illustration of an array of plasmon nanoantenna capable of trapping and manipulating NDs by low-frequency Electrothermoplasmonic tweezer (LFET), inset showing ND moving toward the laser spot.Reproduced with permission. [85]Copyright 2021, American Chemical Society.d) (right top)Metal nanoparticle surface charge modification by absorption of surfactant (CTAC(right bottom)), CTEC micelles formation, (left)Optical heating generates thermoelectric filed ET for trapping metal nanoparticle.Reproduced with permission. [86]Copyright 2018, Springer Nature.e) Trapping of single ND on 400 nm diameter silver nanowire and then release.Reproduced with permission. [87]Copyright 2023, Optica Publishing Group.f) (top left)Active colloidal tweezer(ACT) design with dielectric rod and plasmonic silver nanodisk, (top right) Dynamic manipulation of trapped particles with ACT.(bottom) manipulation 40 nm polystyrene particles by moving stage.Reproduced with permission. [93]Copyright 2019, Springer Nature.g) (top) Schematic of NDs trapping and release of NDs from mobile nano tweezer.(bottom) Helical microrobots, which can be propelled using a rotating magnetic field, Fluorescence image of NDs trapped by MNT and released.Reproduced with permission. [90]Copyright 2018, AAAS.
requires precise control and specialized techniques.It is important to note that the choice of manipulation method depends on the specific properties of the nanodiamonds and the desired application.Different techniques may be more suitable for specific scenarios, and a combination of methods might be employed to achieve precise manipulation.

Optical Control
Optical manipulation of fluorescent nanodiamonds involves using light-based techniques to capture, move, and control the position of NDs.One widely used method was optical trapping, also known as optical tweezing, which utilized focused infrared laser beams and high numerical aperture (NA) objectives to achieve precise manipulation of individual NDs.Stable optical traps in three dimensions were created by directing the laser beam toward the NDs and balancing scattering and gradient forces.The high refractive index of NDs helped them overcome thermal fluctuations, allowing them to be immobilized and moved by manipulating the laser beam or the microscope stage.Optical trapping has been successfully applied to capture and manipulate fluorescent NDs in various environments, including liquid, vacuum, and inside living cells.Researchers achieved 3D control of NDs, enabling spin manipulation and read-out of their ground-state ODMR transitions in an ensemble of NDs (see Figure 5a). [79]wever, the ODMR spectra in the presence of an applied magnetic field were broadened due to the random orientation of the trapped NDs with respect to the external field, resulting in a reduced magnetic field sensitivity of 50 μT/sqrt (Hz).In another study, trapping a single nanodiamond crystal with a size of 60-70 nm and a single NV center improved the signal-to-noise ratio, achieved better spatial resolution, and narrower ODMR line widths. [80]Counter-propagating trapping and excitation lasers were used to minimize scattering forces on the NDs dispersed in a 5:1 glycerol/water mixture.The researchers discovered that the NV axis remained fixed and could be adjusted by changing the polarization of light, enabling the application of trapped NDs to vector magnetometry.Despite the success of using focused laser beams for ND manipulation, optical trapping had practical limitations.The high optical power required for manipulating these nanoscopic particles could result in the quenching of fluorescence signals, which depended exponentially on laser intensity, leading to reduced ODMR contrast. [81,82]NDs were subject to Brownian motion in solution; therefore, low trapping power could introduce more noise and fluctuations in the optical readout.Additionally, collisions between NDs or interactions with the surrounding medium and entry and exit of NDs from the optical trap could impact the trap stability and positioning of the trapped NDs, affecting their manipulation and control.As an alternative, Russel et al. proposed and demonstrated the modulation of IR laser to trap NDs (100 nm) and measure their fluorescence while the IR laser was turned off. [83]Hence, PL intensity remained constant during the measurement, and spin decoherence effects of the trapping laser were removed, allowing estimation of the spinlattice relaxation time (T 1 ) of the order of milliseconds.Using optical trapping, Tianli Wu et al. showed that dispersed fluorescent NDs endocytosed inside living cells can be aggregated into microspheres using a scanning optical tweezing system (1064 nm laser) and used as intracellular temperature probes by using the dependence of Zero phonon line of NDs on temperature. [12]n this regard, plasmonic nanostructures under resonant illumination could realize trapping of NDs with at least two orders of magnitude lower optical intensities than laser tweezers.The intense interaction between light and matter near noble metallic nanostructures significantly amplified the optical intensity in the vicinity, resulting in subwavelength trapping volumes that provided much stronger confinement than conventional far-field methods.For instance, gold double nanorods with a narrow gap could trap 40 nm NDs (see Figure 5b) that were in proximity using only 3 mW of optical power. [84]The trapped NDs could be permanently attached to plasmonic hotspots with proper surface functionalization.Subsequently, when plasmonic nanostructures were illuminated with resonant excitation, they efficiently absorbed photon energy and generated heat.The localized thermal gradients, in combination with AC electric fields, could drive NDs toward the trapping location over a long distance (≈100 μm).The localized heating of the fluid around the illuminated plasmonic nanoantenna created gradients in permittivity and electrical conductivity.By applying a low-frequency AC electric field in the presence of these gradients, an electrical body force arose, exerting a drag force on suspended ND particles and rapidly transporting them to the heated plasmonic hot spots. [85]nce trapped, the NDs could be manipulated along the substrate by moving the laser spot or the microscope stage over the nano-patterned surface, as shown in Figure 5c.Alternatively, NDs in surfactant-modified solution could be covered with positively charged micelles, which generated a charge separation with other anions in solutions in response to the local thermal gradient. [86]s a result, an electric field toward the heat source was generated, creating an optothermoelectric force that trapped the positively charged particle (see Figure 5d).However, the presence of surfactants was essential in the solution, which dissociated into anions and positively charged micelles, facilitating trapping on the heated substrate.A similar approach in Figure 5e was applied, the Opto-thermo-electric (OTE) technique facilitated by surfactant, where the same 532 nm laser was used for resonant excitation of drop-casted Ag nanowires/Au nanoparticles for trapping and exciting NDs. [87]The force due to OTE is higher compared to optical trapping; hence less laser power is required.There will not be quenching of fluorescence compared to previous techniques, as no IR was used.
On the other hand, hybrid manipulation techniques have been proposed and demonstrated recently, offering not only low optical intensities but also 3D manipulation capabilities like conventional optical tweezers. [88]One such approach involved integrating plasmonic nanostructures with helical ferromagnetic nanostructures, resulting in a novel mobile nanotweezer.This nanotweezer could be remotely maneuvered through the bulk solution using a small rotating magnetic field while carrying target colloids under defocused optical illumination. [89][92] Another alternative method, demonstrated by Ghosh et al., involved active colloidal tweezers (ACTs) for optical trapping and maneuvering. [93]ACTs consisted of a silver nanodisk functioning as a plasmonic trap attached to the end of a silica microrod, as shown in Figure 5f.The presence of the large silica rod allowed manipulation using a low-power 1064 nm laser tweezer, which simultaneously created a strong trapping potential around the silver disk.Therefore, by simultaneously exploiting far-and near-field optical forces, ACTs could dynamically manipulate NDs inside a closed fluidic volume as well as on surfaces.Such hybrid plasmonic trapping approaches, along with optical and optothermal manipulation, provided great promise to exploit the sensing capabilities of NDs with very high precision and extend it to highly dynamic systems such as living biological entities, where properties could change locally over the nano-to micrometer scale.Additionally, the nano-positioning of NDs to photonic and plasmonic systems and subsequent integration with ubiquitous control opened up possibilities for futuristic quantum photonic devices.

Non-Optical Control
In addition to utilizing light and related effects, recent advancements showed remarkable potential in manipulating nanodiamonds (NDs) using non-optical methods.These methods were primarily nonperturbative techniques that did not interface with ND fluorescence properties and related sensing applications.One notable technique was the use of Anti Brownian Electrokinetic traps (ABEL traps) to confine NDs in three dimensions, as shown in Figure 6a.These traps employed a feedback mechanism that counteracted the Brownian motion of NDs in a solution by applying electrokinetic force. [94]Unlike traditional traps that created potential wells, ABEL traps generated a uniform field that guided the particles toward specific locations.However, this process was complex due to the need for intricate electrode design, fabrication, and trapping force calibration.Determining the precise relationship between the applied electric fields and particle positions added to the intricacy.
Recently, Kim et al. demonstrated an effective way to overcome the thermal fluctuations of NDs in a solution by physically tethering them to large TiO 2 microrods grown on polystyrene beads (see Figure 6b). [95]The microrods exhibited lower position fluctuations compared to the nanoscale NDs, enabling stable fluorescence measurements.In contrast to methods such as MNTs and ACTs, which relied on optical forces for ND attachment, this approach physically attached the NDs to polystyrene beads during fabrication.The researchers even utilized the ND fluorescence excitation laser to induce self-thermophoresis in the rod, resulting in controlled rotation of the entire assembly, including the attached ND.This rotational capability facilitated vectorial magnetometry, as the orientation of the NDs NV axis relative to the applied magnetic field could be tuned.These individual demonstrations and the previously discussed optical manipulation methods highlighted the potential for dynamic NDs to enable new applications at the single-particle level.Reproduced with permission. [94]Copyright 2014, American Chemical Society.b) Schematic of NV spin orientation in ND attached to a swimmer and corresponding SEM image, (top) ODMR of NV attached to the swimmer with a magnetic field of 8G applied along the y-axis and the swimmer rotated by 520 in the x-y plane with the same field(bottom).Reproduced with permission. [95]Copyright 2017, Wiley-VcH GmbH.c) SEM image of patterned ND clusters after attachment,(inset) ND self-assembly on electron beam deposited carbon seeds.Reproduced with permission. [96]Copyright 2016, Royal Society of Chemistry.d) Schematic showing electrostatic self-assembly process, Confocal image of 12 μm × 12 μm area(top right), Green and blue circles represent single ND and multiple NDs g(2) curves for the bright spot in scan image, blank frames represent insufficient signal.Reproduced with permission. [97]Copyright 2015, American Chemical Society.e) Illustration of electric field assisted surface absorption nano-patterning(EFSAP) of nanoparticles(top left), surface potential modification along nanopattern surface (top right).Reproduced with permission. [98]Copyright, Springer Nature.Schematic of a PMMA film deposited over a silicon dimer and a positively charged dot injected in the gap, AFM topography of fabricated structure(middle), AFM image of NDs assembled in the gap of dimers on charged dots (bottom).Reproduced with permission. [99]Copyright 2023, Royal Society of Chemistry.f) Illustration of Electrohydrodynamic printing of NDs using a DC voltage applied to back electrodes(left top), Fe-Sem image of patterned NDs cluster(right top).Reproduced with permission. [100]Copyright 2021, Wiley-VcH GmbH.g) (top) schematic of assembly of NDs into lithographically defined templates using capillary driven template-assisted self-assembly (TASA), PL Scan, and AFM image of assembled NDs(bottom).Reproduced with permission. [101]Copyright 2022, American Chemical Society.
However, a different manipulation approach was necessary when scalability and ordered assembly of multiple NDs over a large area were required.Researchers could modify the surface properties of NDs by attaching appropriate molecules or polymers, enabling manipulation using chemical or biochemical methods.For instance, NDs could be functionalized with carboxyl groups (COOH-) that can chemically bind to aminated locations on a substrate, allowing for positioning on pre-designed patterns, as shown in Figure 6c. [96]Similarly, Jiang et al. demonstrated electrostatic self-assembly of NDs in a regular array by applying opposite charges to the NDs and desired locations on the substrate (see Figure 6d). [97]This electrostatic assembly method could be further developed to enable the on-demand positioning of NDs at any desired location by applying a high-voltage electric field through an AFM nanotip, favoring highly efficient site-specific particle assembly, as shown in Figure 6e. [98,99]The number of particles assembled depended on the injection voltage at the trapping location, increasing linearly with the applied voltage.Additionally, pulsed DC voltage could be used to eject sub-attoliter volume droplets from a nanopipette containing NDs, which rapidly evaporated upon landing on the substrate, thereby printing the NDs. [100]The number of printed nanodiamonds could be controlled by adjusting printing parameters such as ND concentration and applied pulse length (see Figure 6f).While these assembly processes based on the electrostatic force were advantageous for the on-demand assembly of NDs, achieving single-particle resolution in assembly remained a challenge.
To address this challenge and enable large-scale statistical studies of individual, isolated nanodiamonds, Shulevitz et al. demonstrated a template-assisted approach in recent work. [101]n this method, individual NDs (≈40 nm in size) dispersed between a glass slide and the template surface were assembled.A motorized stage translated the nanodiamond aqueous dispersion across the template surface at a slow speed of about 3.5 μm s −1 , as depicted in Figure 6g.The template contained cylindrical traps with diameters ranging from 35 to 48 nm and a height of ≈62 nm.At the meniscus, an accumulation of particles occurred, and capillary forces drove the nanodiamonds into the trap sites, with each trap accommodating a single ND.Measurements revealed a wide distribution in charge states and spin lifetimes among the fluorescent NDs.Hence, novel manipulation schemes are necessary to perform consistent measurements using the same ND sensor for all measurements.Moreover, the precise positioning of individual NDs could facilitate large-scale statistical studies of emitter-nanostructure interactions, and high-resolution temperature mapping may enable the realization of chip-scale single photon sources and other large-area quantum devices. [102]n summary, recent developments in non-optical manipulation methods showed great promise for precisely positioning and manipulating nanodiamonds.These techniques offered new possibilities for single-particle applications and large-scale assembly, paving the way for advancements in various fields, including quantum technologies.

Conclusion
The active control over these nanoscale entities emerges as a cornerstone, opening up the possibility of harnessing their unique quantum properties with unprecedented precision and sophistication.In cases where passive sensing's limitations might be a concern, hybrid approaches that combine passive sensing with active manipulation and control can provide more comprehensive solutions.These methods allow researchers to dynamically manipulate nanodiamonds in a controlled manner, unlocking new possibilities in various fields ranging from quantum technology to biology and materials science.However, there is still some scope for improvements in regard to the futuristic applications of such dynamic ND sensors.We summarized the different limitations of the approaches as well as measurement techniques.In the present context, one of the big challenges is the active control of ND's motion and rotation (instead of a diffusing ND) in a complicated biological environment for sensing and nanobiotechnology.For photonics application, an ondemand large area integration of a single ND with a photonics structure that does not depend on NDs surface or substrate is a challenge that will require spatial and orientational (single NV aligned with dipole orientation of photonic structure) control.Greater control over the ND's inhomogeneity in reproducibility will benefit both fields.Then precise and on-demand manipulation of such NDs should open up new avenues in broad areas of sensing, nanobiotechnology, opto-mechanic study, developing scalable quantum photonic architectures.

Figure 2 .
Figure 2. Measurement techniques for measurement protocols for NV center.a) Continuous wave ODMR measurement where laser and microwave are ON, MW frequency is swept, and data is acquired continuously b)-f) Pulse measurement techniques: Laser pulse(2 μs) for initialization (optical pumping) of NV center and 300 ns pulse for read-out and corresponding MW pulses e) and f) sphere shows the Block sphere representation of spin states and evolution at different times. is the free precision time when MW and laser are OFF.

Figure 3 .
Figure 3. Application of NDs.a) Effect of increasing Gd3+  concentration on T1 on diamond nanocrystal, inset shows the decreasing T1 is also accompanied by decreasing T1 contrast.Reproduced with permission.[32]Copyright 2013, American Physical Society.b) Shows the effect of Ferritin on NDs, T1 measurement for free ND is shown, and the inset shows T1 measurement for ferritin-coated NDs, which is reduced compared to uncoated ND.Reproduced with permission.[55]Copyright 2013, American Chemical Society.c) pH calibration curve by taking the photoluminescence ratio -COOH and -OH functionalized NDs.Reproduced with permission.[65]Copyright 2023, IOP Publishing Ltd d) Top; Schematic of association between a nanodiamond and EGF receptor.Bright field image of C. elegans, bright field and selective imaging protocol image of the intestine of C. elegans, bottom (left to right), the circle shows the nanodiamond and orange line shows the outline of the intestine.Reproduced with permission.[66]Copyright 2020, American Chemical Society.e) Integration of NV-centers in nanodiamonds with nanophotonic circuits.Schematic of the device layout with PhC-cavity-coupled emitter system (center) that is optically accessible via independent waveguides (top), Scanning electron micrograph of Ta 2 O 5 cross-bar structure with PhC-cavity (middle), FDTD simulation showing PhC-cavity mode profile.Reproduced with permission.[10]Copyright 2020, American Chemical Society.f) Schematic of ND containing NV center covered by a hydrogen dense layer, immersed in a liquid PFPE analyte, NMR of 19F and 1H from NDs using an XY8-10 dynamical decoupling sequence.Reproduced with permission.[56]Copyright 2020, American Physical Society.g) Top, schematic of confocal microscopy from imaging fiber bundle (IFB) and the images captured from a facet and FND coated facet, bottom: ODMR captured at different positions between antenna wires at zero magnetic field, zero current, and at B = 3.8mT for different current.Reproduced with permission.[67]Copyright 2020, American Chemical Society.h) The Left image shows a fluorescence scan of cells (green: live cells and red: dead cells); the Bar graph shows ND temperature (circle in left image) when local heat is applied at two locations (crosses in image).Reproduced with permission.[68]Copyright 2013, Springer Nature.

Figure 4 .
Figure 4. Schematic representation of challenges associated with quantum sensing using FNDs.a) Schematic of NDs Brownian motion in fluid in a microfluidic chamber.Due to the Brownian motion, NDs move to different intensity locations of the excitation optical field or go out of focus of the objective, resulting in fluctuation in fluorescence signal.This is shown schematically (right) where fluorescence signal fluctuates for NDs in bulk but remains stable for static NDs.b) Effect of mean square displacement (MSD) of a 100 nm ND in media of increasing viscosity on the minimum magnetic field change detectable with the measurement time.The black curve shows the improvement of the minimum detectable field at the limit of infinite viscosity or no position fluctuation.c) Schematic showing the uncontrolled rotation of NDs in a fluidic environment.

Figure 5 .
Figure 5. Optical methods of manipulation of NDs a) Schematic of Optical trapping, excitation laser, and fluorescence measurements of NDs with coverslip and antenna for MW excitations.Reproduced with permission.[79]Copyright 2012, National Academy of Science.b) Schematic of experimental trapping setup; gap antennas are on the coverslip with 2 μm separation.1064 nm laser, activates the plasmonic trap, and 532 nm laser, which excites NDs fluorescence, are superimposed.Reproduced with permission.[84]Copyright 2014, American Chemical Society.c) Illustration of an array of plasmon nanoantenna capable of trapping and manipulating NDs by low-frequency Electrothermoplasmonic tweezer (LFET), inset showing ND moving toward the laser spot.Reproduced with permission.[85]Copyright 2021, American Chemical Society.d) (right top)Metal nanoparticle surface charge modification by absorption of surfactant (CTAC(right bottom)), CTEC micelles formation, (left)Optical heating generates thermoelectric filed ET for trapping metal nanoparticle.Reproduced with permission.[86]Copyright 2018, Springer Nature.e) Trapping of single ND on 400 nm diameter silver nanowire and then release.Reproduced with permission.[87]Copyright 2023, Optica Publishing Group.f) (top left)Active colloidal tweezer(ACT) design with dielectric rod and plasmonic silver nanodisk, (top right) Dynamic manipulation of trapped particles with ACT.(bottom) manipulation 40 nm polystyrene particles by moving stage.Reproduced with permission.[93]Copyright 2019, Springer Nature.g) (top) Schematic of NDs trapping and release of NDs from mobile nano tweezer.(bottom) Helical microrobots, which can be propelled using a rotating magnetic field, Fluorescence image of NDs trapped by MNT and released.Reproduced with permission.[90]Copyright 2018, AAAS.

Figure 6 .
Figure 6.Non-optical methods of manipulating NDs a) Schematic of ABEL trap in PDMS microfluidic cell.(inset) Position histogram of trapped NDs.Reproduced with permission.[94]Copyright 2014, American Chemical Society.b) Schematic of NV spin orientation in ND attached to a swimmer and corresponding SEM image, (top) ODMR of NV attached to the swimmer with a magnetic field of 8G applied along the y-axis and the swimmer rotated by 520 in the x-y plane with the same field(bottom).Reproduced with permission.[95]Copyright 2017, Wiley-VcH GmbH.c) SEM image of patterned ND clusters after attachment,(inset) ND self-assembly on electron beam deposited carbon seeds.Reproduced with permission.[96]Copyright 2016, Royal Society of Chemistry.d) Schematic showing electrostatic self-assembly process, Confocal image of 12 μm × 12 μm area(top right), Green and blue circles represent single ND and multiple NDs g(2) curves for the bright spot in scan image, blank frames represent insufficient signal.Reproduced with permission.[97]Copyright 2015, American Chemical Society.e) Illustration of electric field assisted surface absorption nano-patterning(EFSAP) of nanoparticles(top left), surface potential modification along nanopattern surface (top right).Reproduced with permission.[98]Copyright, Springer Nature.Schematic of a PMMA film deposited over a silicon dimer and a positively charged dot injected in the gap, AFM topography of fabricated structure(middle), AFM image of NDs assembled in the gap of dimers on charged dots (bottom).Reproduced with permission.[99]Copyright 2023, Royal Society of Chemistry.f) Illustration of Electrohydrodynamic printing of NDs using a DC voltage applied to back electrodes(left top), Fe-Sem image of patterned NDs cluster(right top).Reproduced with permission.[100]Copyright 2021, Wiley-VcH GmbH.g) (top) schematic of assembly of NDs into lithographically defined templates using capillary driven template-assisted self-assembly (TASA), PL Scan, and AFM image of assembled NDs(bottom).Reproduced with permission.[101]Copyright 2022, American Chemical Society.