Single Nanoparticle Level Research Articles

High-efficiency and water-quenching-resistant Tb3+-based nanoparticles for single-particle imaging

Abstract The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase (β-phase) NaREF4 (RE = rare earth) is the most commonly used crystal structure for lanthanide-doped upconversion nanoparticles (UCNPs) owing to its high upconversion (UC) efficiency. In this work, we report, for the first time, that more efficient cooperative sensitization upconversion (CSU) can be achieved in cubic-phase (α-phase) NaREF4 UCNPs instead of their β-phase counterparts. With the passivation of an inert shell, the UC emission intensity of α-NaYbF4:Tb40%@CaF2 is 10.5 times higher than that of β-NaYbF4:Tb40%@NaYF4. We propose that the high-symmetry crystal structure of the α phase facilitates the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters, which are particularly beneficial for CSU. Moreover, we prove that such Tb3+-based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm−1) that existed in Tb3+ between its lowest emit-ting level (5D4) and next low-lying level (7F0). Finally, their potential application for single-nanoparticle imaging has also been demonstrated. As expected, the α-core-shell UCNPs measured at the single-nanoparticle level are estimated to be 9-fold brighter than their β-core-shell counterparts. Importantly, the α-NaYbF4:Tb40%@CaF2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm2).

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid.

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Single-Nanoparticle Coulometry Method with High Sensitivity and High Throughput to Study the Electrochemical Activity and Oscillation of Single Nanocatalysts.

To explore nanocatalysts with high electro-catalytic performance and less loading of precious metals, efforts have been made to develop electrochemical methods with high spatial resolution at the single nanoparticle level. Herein, a highly sensitive single-nanoparticle coulometry method is successfully developed to study the electrochemical activity and oscillation of single PtTe nanocatalysts. Based on microbattery reactions involving the formic acid electro-oxidation and the deposition of Ag on the single PtTe nanocatalyst surface, this method enables the transition from the undetectable sub-fA electric signal of the formic acid electro-oxidation into strong localized surface plasmon resonance scattering signal of Ag detected by dark-field microscopy. The lowest limiting current for a single nanocatalyst is found to be as low as 25.8 aA. Different trends of activity versus the formic acid concentration and types of activity of the single nanocatalyst have been discovered. Unveiled frequency-amplitude graph shows that the two electrochemical oscillation modes of low frequency with high amplitude and vice versa coexist in a single PtTe nanocatalyst, indicating the abundantly smooth surfaces and defects of nanocatalysts. This conducted study will open up the new avenue for further behavioral and mechanistic investigation of more types of nanocatalysts in the electrochemistry community.

Nanoscale characterization of the biomolecular corona by cryo-electron microscopy, cryo-electron tomography, and image simulation

The biological identity of nanoparticles (NPs) is established by their interactions with a wide range of biomolecules around their surfaces after exposure to biological media. Understanding the true nature of the biomolecular corona (BC) in its native state is, therefore, essential for its safe and efficient application in clinical settings. The fundamental challenge is to visualize the biomolecules within the corona and their relationship/association to the surface of the NPs. Using a synergistic application of cryo-electron microscopy, cryo-electron tomography, and three-dimensional reconstruction, we revealed the unique morphological details of the biomolecules and their distribution/association with the surface of polystyrene NPs at a nanoscale resolution. The analysis of the BC at a single NP level and its variability among NPs in the same sample, and the discovery of the presence of nonspecific biomolecules in plasma residues, enable more precise characterization of NPs, improving predictions of their safety and efficacies.

High Color-Purity Red, Green, and Blue-Emissive Core-Shell Upconversion Nanoparticles Using Ternary Near-Infrared Quadrature Excitations.

Development of multicolor-emitting upconversion nanoparticles (UCNPs) is of significant importance for applications in optical encoding, anti-counterfeiting, display, and bioimaging. However, realizing the orthogonal three-primary color (TPC) upconversion luminescence in a single nanoparticle remains a huge challenge. Herein, we have rationally designed core-multishell-structured NaYF4 UCNPs through regulating the dopant concentration, composition of luminescent layers, and shell position and thickness, which are capable of emitting red, green, and blue luminescence with high color purity in response to ternary near-infrared quadrature excitations (1560/808/980 nm). Moreover, their high color purity is well retained with varying excitation power densities. This orthogonal TPC emissions property of such UCNPs endows them with great promise in the field of security. As a proof-of-concept, we have demonstrated the feasibility of combining such UCNPs with MnO2 nanosheets for information encryption and decryption. This work not only offers a new way to achieve TPC upconversion luminescence at a single nanoparticle level but also broadens the scope of application for security protection.

Independent Size and Fluorescence Emission Determination of Individual Biological Nanoparticles Reveals that Lipophilic Dye Incorporation Does Not Scale with Particle Size.

Advancements in nanoparticle characterization techniques are critical for improving the understanding of how biological nanoparticles (BNPs) contribute to different cellular processes, such as cellular communication, viral infection, as well as various drug-delivery applications. Since BNPs are intrinsically heterogeneous, there is a need for characterization methods that are capable of providing information about multiple parameters simultaneously, preferably at the single-nanoparticle level. In this work, fluorescence microscopy was combined with surface-based two-dimensional flow nanometry, allowing for simultaneous and independent determination of size and fluorescence emission of individual BNPs. In this way, the dependence of the fluorescence emission of the commonly used self-inserting lipophilic dye 3,3'-dioctadecyl-5,5'-di(4-sulfophenyl)oxacarbocyanine (SP-DiO) could successfully be correlated with nanoparticle size for different types of BNPs, including synthetic lipid vesicles, lipid vesicles derived from cellular membrane extracts, and extracellular vesicles derived from human SH-SY5Y cell cultures; all vesicles had a radius, r, of ∼50 nm and similar size distributions. The results demonstrate that the dependence of fluorescence emission of SP-DiO on nanoparticle size varies significantly between the different types of BNPs, with the expected dependence on membrane area, r2, being observed for synthetic lipid vesicles, while a significant weaker dependence on size was observed for BNPs with more complex composition. The latter observation is attributed to a size-dependent difference in membrane composition, which may influence either the optical properties of the dye and/or the insertion efficiency, indicating that the fluorescence emission of this type of self-inserting dye may not be reliable for determining size or size distribution of BNPs with complex lipid compositions.

Colossal Magnetoelectric Effect in Core-Shell Magnetoelectric Nanoparticles.

Magnetoelectric coefficient values of above 5 and 2 V cm-1 Oe-1 in 20 nm CoFe2O4-BaTiO3 and NiFe2O4-BaTiO3 core-shell magnetoelectric nanoparticles were demonstrated. These colossal values, compared to 0.1 V cm-1 Oe-1 commonly reported for the 0-3 system, are attributed to (i) the heterostructural lattice-matched interface between the magnetostrictive core and the piezoelectric shell, confirmed through transmission electron microscopy, and (ii) in situ scanning tunneling microscopy nanoprobe-based ME characterization. The nanoprobe technique allows measurements of the ME effect at a single-nanoparticle level which avoids the charge leakage problem of traditional powder form measurements. The difference in the frequency dependence of the ME value between the two material systems is owed to the Ni-ferrite cores becoming superparamagnetic in the near-dc frequency range. The availability of novel nanostructures with colossal ME values promises to unlock many new applications ranging from energy-efficient information processing to nanomedicine and brain-machine interfaces.

Hydrogen Evolution Reaction Monitored by Electrochemiluminescence Blinking at Single-Nanoparticle Level.

Monitoring and characterization methods that provide performance tracking of hydrogen evolution reaction (HER) at the single-nanoparticle level can greatly advance our understanding of catalysts' structure and activity relationships. Electrochemiluminescence (ECL) microscopy is implemented for the first time to identify HER activities of single nanocatalysts and to provide a direction for further optimization. Here, we develop a novel ECL blinking technique at the single-nanoparticle level to directly monitor H2 nanobubbles generated from hollow carbon nitride nanospheres (HCNSs). The ECL ON and OFF mechanisms are identified being closely related to the generation, growth, and collapse of H2 nanobubbles. The power-law distributed durations of ON and OFF states demonstrate multiple catalytic sites with stochastic activities on a single HCNS. The power-law coefficients of ECL blinking increase with improved HER activities from modified HCNSs with other active HER catalysts. Besides, ECL blinking phenomenon provides an explanation for the low cathodic ECL efficiency of semiconductor nanomaterials.

Single-Nanoparticle Thermometry with a Nanopipette.

Thermal measurements at the nanoscale are key for designing technologies in many areas, including drug delivery systems, photothermal therapies, and nanoscale motion devices. Herein, we present a nanothermometry technique that operates in electrolyte solutions and, therefore, is applicable for many in vitro measurements, capable of measuring and mapping temperature with nanoscale spatial resolution and sensitive to detect temperature changes down to 30 mK with 43 μs temporal resolution. The methodology is based on local measurements of ionic conductivity confined at the tip of a pulled glass capillary, a nanopipettete, with opening diameters as small as 6 nm. When scanned above a specimen, the measured ion flux is converted into temperature using an extensive theoretical support given by numerical and analytical modeling. This allows quantitative thermal measurements with a variety of capillary dimensions and is applicable to a range of substrates. We demonstrate the capabilities of this nanothermometry technique by simultaneous mapping of temperature and topography on sub-micrometer-sized aggregates of thermoplasmonic nanoparticles heated by a laser and observe the formation of micro- and nanobubbles upon plasmonic heating. Furthermore, we perform quantitative thermometry on a single-nanoparticle level, demonstrating that the temperature at an individual nanoheater of 25 nm in diameter can reach an increase of about 3 K.

Highly-stable red-emissive photochromic nanoparticles based on a diarylethene-perylenebisimide dyad

Design and synthesis of highly-stable red-emissive photochromic nanoparticles (NPs) are essential tasks for the development of next-generation microscopic technologies. In this scope, we have prepared red-emissive photochromic NPs based on a diarylethene-perylenebisimide (DAE-PBI) dyad. In order to achieve both high fluorescence and outstanding photostability in the NP state, the fluorescence properties of PBI fluorophores having bulky substituents at the bay-positions were investigated in solution as well as in the NP state. The prepared DAE-PBI dyad shows bright red emission, complete ON-OFF contrast with nonlinear photoswitching response, and excellent photostability in the NP state. In addition, the fluorescence photoswitching property and the photostability at the single NP level was investigated under fluorescence microscope.

(Invited) Operando Optical Monitoring of Nanoparticle Electrodeposition

Electrodes, made of nanoparticles (NPs), are key components in electrochemistry to store and convert energy. The nanometer scale allows exalting surface properties and revealing, in addition to the composition, the importance of size, shape and NP organization on the kinetic and thermodynamic performance of devices. Therefore, the use of ultra-small objects and the study of their formation and reactivity appear as one of the best ways to meet the current energy challenges.Herein, electrodeposition, one of the most popular electrochemical methods for producing nanostructured electrodes, is coupled to an ultra-sensitive interferometric microscopy technique to probe operando the heterogeneity of electrochemical processes at the single NP level. The strategy is illustrated through the monitoring of the electrochemical growth of individual silver NPs, whose sizes can be tracked dynamically down to 10 nm in diameter. Optical images are further used to quantify the presence of metal nanoclusters at the early stages of the electrodeposition and to highlight galvanic reactions occurring between NPs with distinct surface free energies.As interferometric microscopy relies on refractive index changes, the technique can be applied to many kinds of nanomaterials and electrochemical experiments. For example, the optical coupling has proved to be useful for studying NP transformation.Figure. Schematic representation of the coupling and same location microscopy experiment with a scanning electron microscope.J.-F. Lemineur, J.-M. Noël, C. Combellas, D. Ausserré, F. Kanoufi. Angew. Chem. Int. Ed., 2018, 57,11998-12002.J.-F. Lemineur, J.-M. Noël, C. Combellas, D. Ausserré, F. Kanoufi. Faraday. Discuss., 2018, 210, 381-395. Figure 1

Regeneration of Reactive Pd Surfaces in Au-Pd Nanoparticles after Electrochemical Aging

One major hurdle in the development of efficient electrocatalysts is deterioration of activity, which can be caused by selective dissolution of metals [1]. The recent development of gold-palladium (Au-Pd) alloy nanoparticles outlines a contemporary strategy to impart increased stability in nanoparticles by combining a more electrochemically stable metal such as Au with lesser stable Pd [2, 3]. Unfortunately, the relative surface-reactivity of Au is poor, and as a result, there is an inevitable design constraint to form Pd-rich outer shell structure - a task that is particularly daunting considering the high dissolution tendency of Pd [4]. Here we present an electrochemical aging method that allows for increasing the Pd concentration at the surface of Au-Pd NPs by means of diffusion of Pd atoms from the particle-core to the shell. The method involves an electrochemical aging treatment in alkaline KOH media for which the choice of suitable upper vertex potential proved to be particularly very important.Au-Pd NPs synthesized by means of liquid-phase laser-ablation method [5] were used in this study, typically ranging in diameter between 14.5 nm to 22.6 nm and composition between Au/Pd: 20/80 to 80/20 ratios. The particle diameter remained homogeneous across the whole composition range, thanks to the size-selectivity of laser-ablation synthesis of alloy molar fraction series, in line with the literature [5]. This allows for excluding any size-effects on the electrochemical behaviour of alloy nanoparticles. The nominal Au/Pd compositions initially fed to the laser appeared to be well preserved down to single-nanoparticle level as confirmed by detailed analyses of the SEM/EDX and XPS spectroscopic data. Formation of Au/Pd solid-solution was also apparent from the estimation of the lattice-parameters using X-ray diffraction (XRD) that varied linearly in accordance with the change in Pd alloy content.The cyclic voltammograms recorded in 0.1M N2-saturated KOH revealed features characteristic of both Au and Pd surfaces, e.g., the reduction peaks at 0.98 V and 0.55 V vs RHE (respectively), which was reflective of a mixed Au and Pd surface-structure exposed to the electrochemical treatments. Importantly, both the peak-positions and the integrated surface-charge shifted in correspondence with the variable Pd content in the alloys, thus serving as the initial markers of the surface-state before further electrochemical treatments.Marked differences could be noted between the electrochemical behaviours in acid and the alkaline media, as well as the potential range and the number of cycles involved ( Figure 1 ). The initial H-desorption/adsorption peaks (A, B) and the PdO reduction peaks apparent at the onset of aging in 0.5M H2SO4 gradually disappeared with subsequent cycling (C1), along with the concomitant rise in the Au oxide reduction peaks (D1), indicative of gradually dissolution of Pd. Importantly, the lost Pd surface surprisingly regenerated (C2) with subsequent aging in 0.1M KOH media, particularly at sweeping potentials extending to very positive values. The regenerated Pd-rich surface is attributed to preferential diffusion of Pd from particle cores- to the surface aided by surface-reconstruction via a ‘place-exchange’ between adsorbed-oxygen and Pd. Detailed analyses of the XPS Pd-3d3/2 and Au-4f7/2 edges and the respective STEM-EDX elemental profiles are in good agreement with the changes of surface-structures proposed from the cyclic voltammetry measurements. All the alloys were simultaneously tested for oxygen reduction reaction (ORR).[1] Gasteiger, H. A., et al. (2005). Applied Catalysis B: Environmental, 56(1-2), 9-35.[2] Xu, J. B., et al. (2010). International Journal of Hydrogen Energy, 35(13), 6490-6500.[3] Sasaki, K., et al. (2010). Angewandte Chemie International Edition, 49(46), 8602-8607.[4] Rand, D. A. J., et al. (1972). Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 35(1), 209-218.[5] Reichenberger, S., et al. (2019). ChemCatChem, 11, 4489-4518. Figure 1

Light-Controlled Nanoparticle Collision Experiments.

Electrochemical monitoring of catalytically amplified collisions of individual metal nanoparticles (NP) with ultramicroelectrodes (UME) has been extensively used to study electrocatalysis, mass-transport, and charge-transfer processes at the single NP level. More recently, photoelectrochemical collision experiments were carried out with semiconductive NPs. Here, we introduce two new types of light-controlled nanoimpact experiments. The first experiment involves localized photodeposition of catalyst (Pt) on TiO2 NPs with a glass-sheathed carbon fiber simultaneously serving as the light guide and collector UME. The collisions of in situ prepared Pt@TiO2 NPs with the carbon surface produced blips of water oxidation current, while the activity of pristine TiO2 NPs was too low to yield measurable signal. In another experiment, collisions of catalytic (Ir oxide) NPs with the semiconductor (Nb doped n-type TiO2 rutile single crystal) electrode are monitored by measuring the photocurrent of water oxidation.

Needle‐Like Ferroelastic Domains in Individual Ferroelectric Nanoparticles

AbstractSuperior structural, physical, and electronic properties make ferroelectric nanocrystals essential in enabling a range of next‐generation devices. Ferroelectric responses are determined by crystal structure and domain morphology. The ability to reversibly displace, create, and annihilate elastic domains is critical to device applications. Although electric‐field control has been demonstrated for ferroelectric 180° surface domain walls and vortices, similar control of ferroelastic domains and domain boundaries within individual nanocrystals remains challenging. Using controlled external compressive and tensile axial stress, deterministic and reversible control of highly mobile ferroelastic domains and axial polarization in three dimensions is demonstrated. While many studies exist on ferroelastic domains in thin films and bulk, little is known about ferroelastic interactions at the single nanoparticle level, especially involving domain boundaries. Through combining Bragg coherent X‐ray diffractive imaging and Landau theory, strain gradients in individual BaTiO3 nanocrystals are shown to stabilize needle‐like ferroelastic twin domains. These domains are highly labile under applied axial stress, producing a locally enhanced electric polarization mediated by a ferroelectric phase transition. The efficacy of Bragg coherent X‐ray diffractive imaging in studying in operando domains in three dimensions is demonstrated, while synergy with theory provides a paradigm for domain boundary engineering and potential for nanoscale functional devices.

Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy.

Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.

Recent advances of plasmonic nanoparticle-based optical analysis in homogeneous solution and at the single-nanoparticle level.

Plasmonic nanoparticles with special localized surface plasmon resonance (LSPR) characters have been widely applied for optical sensing of various targets. With the combination of single nanoparticle imaging techniques, dynamic information of reactions and biological processes is obtained, facilitating the deep understanding of their principle and design of outstanding nanomaterials. In this review, we summarize the recently adopted optical analysis of diverse analytes based on plasmonic nanoparticles both in homogeneous solution and at the single-nanoparticle level. A brief introduction of LSPR is first discussed. Colorimetric and fluorimetric homogeneous detection examples by using different sensing mechanisms and strategies are provided. Single plasmonic nanoparticle-based analysis is concluded in two aspects: visualization of chemical reactions and understanding of biological processes. The basic sensing mechanisms and performances of these systems are introduced. Finally, this review highlights the challenges and future trend of plasmonic nanoparticle-based optical analysis systems.

Quantitative optical microspectroscopy, electron microscopy, and modelling of individual silver nanocubes reveal surface compositional changes at the nanoscale.

The optical response of metal nanoparticles is governed by plasmonic resonances, which depend often intricately on the geometry and composition of the particle and its environment. In this work we describe a method and analysis pipeline unravelling these relations at the single nanoparticle level through a quantitative characterization of the optical and structural properties. It is based on correlating electron microscopy with microspectroscopy measurements of the same particle immersed in media of different refractive indices. The optical measurements quantify the magnitude of both the scattering and the absorption cross sections, while the geometry measured in electron microscopy is used for numerical simulations of the cross section spectra accounting for the experimental conditions. We showcase the method on silver nanocubes of nominal 75 nm edge size. The large amount of information afforded by the quantitative cross section spectra and measuring the same particle in two environments, allows us to identify a specific degradation of the cube surface. We find a layer of tarnish, only a few nanometers thick, a fine surface compositional change of the studied system which would be hardly quantifiable otherwise.

A broadband aggregation-independent plasmonic absorber for highly efficient solar steam generation

A plasmonic solar absorber, featuring broadband light harvesting by manipulating the structural anisotropy at the single nanoparticle level, enables absorption over the entire solar spectrum.

Magnetic force fields of isolated small nanoparticle clusters.

The usage of magnetic nanoparticles (NPs) in applications necessitates a precise mastering of their properties at the single nanoparticle level. There has been a lot of progress in the understanding of the magnetic properties of NPs, but incomparably less when interparticle interactions govern the overall magnetic response. Here, we present a quantitative investigation of magnetic fields generated by small clusters of NPs assembled on a dielectric non-magnetic surface. Structures ranging from individual NPs to fifth-fold particulate clusters are investigated in their magnetization saturation state by magnetic force microscopy and numerical calculations. It is found that the magnetic stray field does not increase proportionally with the number of NPs in the cluster. Both measured and calculated magnetic force fields underline the great importance of the exact spatial arrangement of NPs, shedding light on the magnetic force field distribution of particulate clusters, which is relevant for the quantitative evaluation of their magnetization and perceptibly for many applications.

SQUID-on-tip with single-electron spin sensitivity for high-field and ultra-low temperature nanomagnetic imaging.

Scanning nanoscale superconducting quantum interference devices (nanoSQUIDs) are of growing interest for highly sensitive quantitative imaging of magnetic, spintronic, and transport properties of low-dimensional systems. Utilizing specifically designed grooved quartz capillaries pulled into a sharp pipette, we have fabricated the smallest SQUID-on-tip (SOT) devices with effective diameters down to 39 nm. Integration of a resistive shunt in close proximity to the pipette apex combined with self-aligned deposition of In and Sn, has resulted in SOTs with a flux noise of 42 nΦ0 Hz-1/2, yielding a record low spin noise of 0.29 μB Hz-1/2. In addition, the new SOTs function at sub-Kelvin temperatures and in high magnetic fields of over 2.5 T. Integrating the SOTs into a scanning probe microscope allowed us to image the stray field of a single Fe3O4 nanocube at 300 mK. Our results show that the easy magnetization axis direction undergoes a transition from the 〈111〉 direction at room temperature to an in-plane orientation, which could be attributed to the Verwey phase transition in Fe3O4.