Shear Force Microscopy Research Articles

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy.

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Matched impedance amplifier design for shear-force acoustic near-field microscopy

Efficient detection of the acoustic signal constitutes the most critical aspect in Shear-force Acoustic Near-field Microscopy, for reliably characterize the dynamic response of confined fluids under shear stress. The near-field acoustic emission from the fluid is monitored as the probe gradually approaches to, and subsequently retracts from, the substrate. Herein we report a 7dB improvement in signal-to-noise sensitivity in detecting the acoustic emission from the fluid trapped between a flat sample and the apex of a tapered probe (the latter attached to a quartz tuning fork of 32 kHz resonance frequency). The new design capitalizes on the inherent capacitance of the SANM acoustic sensor (comprising a pile of piezoelectric plates) and a proper matching inductor/capacitance combination to, altogether connected in a tank-circuit fashion, optimize the sensor’s response at 32 kHz. A detailed construction of the circuit amplifier, as well as detailed frequency response bandwidth and noise characterization, is included herein.

Monitoring near-field acoustic emission from confined water under shear interaction with blunt tips

Mesoscopic water adhered to a surface at ambient conditions is typically reported to behave as sticky glue. Still, Shear-force Acoustic Near-field Microscopy (SANM), complemented with the Whispering Gallery Acoustic Sensing (WGAS) technique, has demonstrated the ability of a water meniscus, formed at the nanometer-sized gap between a sharp probe and a flat surface, to remain flexible enough for emitting near-field acoustic waves while being subjected to shear interactions exerted by a laterally oscillating probe. To gain insights on the meniscus formation process, as well as to better understand its viscoelastic response, purposely blunt probes (composed of multiple sharp asperities) are used. The experimental results suggest the stochastic formation and break of water bridges at multiple asperities on the probe, each providing an energy dissipation channel from the lateral motion of the probe to the fluid. Despite heavily damping the lateral motion of the probe, the ability of the large fluid meniscus to emit acoustic wave is recovered by properly increasing the amplitude of the lateral oscillations of the probe. Overall, monitoring the behavior of large meniscus adhered to a blunt probe allows to infer the behavior of smaller volume meniscus adhered to sharp probes.

Acoustic monitoring humidity effects on the formation of water bridges at sharp contacts

Shear-force Acoustic Near-field Microscopy (SANM) and Whispering Gallery Acoustic Sensing (WGAS) are used to evaluate the role of humidity on the formation and rupture of water meniscus bridges that form between a laterally oscillating tapered probe and a flat substrate. SANM monitors the near-field acoustic wave emitted by the meniscus, while WGAS monitors acoustically the probe’s amplitude of oscillation, as a function of the probe-sample distance. In the instances reported here, the SANM/WGAS measurements are performed in frequency modulation modality, which allows to discriminate the elastic and inelastic components of the probe-fluid-substrate interaction. The response signal traces, acquired when the probe first approaches and then retracts from the sample at 56% relative humidity (or below), revealed absence of hysteresis; this result persists when performing the same experiment a few days later. However, as the humidity increases (different trials from 56% to 67% are included) a notorious hysteresis is observed between the approach and retraction traces. The results indicate that, as the probe approaches the substrate, a higher humidity promotes an increase of stochastic instabilities in the probe-fluid-substrate interactions (reflected in higher damping effects on the probe, and instabilities in the probe-sample distance feedback control). However, the elastic response of the water meniscus prevails over damping effect, as revealed by the greater acoustic emission despite a decrease in the probe’s amplitude. These measurements are relevant to investigations on the dynamic behavior of fluids near hydrophobic/philic substrates, and wetting properties of solids and nanotribology phenomena in general.

Shear‐Force Photoacoustic Microscopy: Toward Super‐resolution Near‐Field Imaging (Laser Photonics Rev. 16(12)/2022)

Super-Resolution Near-Field Imaging In article number 2200296, Byullee Park, Moongyu Han, Hongyoon Kim, Junsuk Rho, and Chulhong Kim developed a lens-free shear-force photoacoustic microscopy system using a tapered fiber and a shear force mechanism. This is the first proof-of-concept study for near-field generation in photoacoustic microscopy toward super-resolution imaging.

Role of acoustic phonon transport in near- to asperity-contact heat transfer

Acoustic phonon transport is revealed as a potential radiation-to-conduction transition mechanism for single-digit nanometer vacuum gaps. To show this, we measure heat transfer from a feedback-controlled platinum nanoheater to a laterally oscillating silicon tip as the tip-nanoheater vacuum gap distance is precisely controlled from a single-digit nanometer down to bulk contact in a high-vacuum shear force microscope. The measured thermal conductance shows a gap dependence of ${d}^{\ensuremath{-}5.7\ifmmode\pm\else\textpm\fi{}1.1}$ in the near-contact regime, which is in good agreement with acoustic phonon transport modeling based on the atomistic Green's function framework. The obtained experimental and theoretical results suggest that acoustic phonon transport across a nanoscale vacuum gap can be the dominant heat transfer mechanism in the near- and asperity-contact regimes and can potentially be controlled by an external force stimuli.

Linear scattering off a dynamically controlled nanosphere-mirror plasmonic antenna on a fiber taper.

A quartz-tuning-fork shear-force microscope was used to demonstrate the gap size dependency of the resonance frequency for a nanosphere-mirror plasmonic antenna. The nanosphere was mounted at the end of a fiber taper scanning probe. A semi-transparent silicon film mirror was used to couple evanescent fields from incident light with the plasmonic antenna using an inverted optical microscope. The plasmon resonance spectra were acquired with a 0.4 nm-step gap size tuning resolution, and were confirmed by finite-difference time-domain simulations. The proposed technique provides a dynamic approach to tuning and detecting distance-dependent localized surface plasmon resonance with a sub-nanometer step resolution.

High spatial resolution imaging of biological tissues using nanospray desorption electrospray ionization mass spectrometry.

Mass spectrometry imaging (MSI) enables label-free spatial mapping of hundreds of biomolecules in tissue sections. This capability provides valuable information on tissue heterogeneity that is difficult to obtain using population-averaged assays. Despite substantial developments in both instrumentation and methodology, MSI of tissue samples at single-cell resolution remains challenging. Herein, we describe a protocol for robust imaging of tissue sections with a high (better than 10-μm) spatial resolution using nanospray desorption electrospray ionization (nano-DESI) mass spectrometry, an ambient ionization technique that does not require sample pretreatment before analysis. In this protocol, mouse uterine tissue is used as a model system to illustrate both the workflow and data obtained in these experiments. We provide a detailed description of the nano-DESI MSI platform, fabrication of the nano-DESI and shear force probes, shear force microscopy experiments, spectral acquisition, and data processing. A properly trained researcher (e.g., technician, graduate student, or postdoc) can complete all the steps from probe fabrication to data acquisition and processing within a single day. We also describe a new strategy for acquiring both positive- and negative-mode imaging data in the same experiment. This is achieved by alternating between positive and negative data acquisition modes during consecutive line scans. Using our imaging approach, hundreds of high-quality ion images were obtained from a single uterine section. This protocol enables sensitive and quantitative imaging of lipids and metabolites in heterogeneous tissue sections with high spatial resolution, which is critical to understanding biochemical processes occurring in biological tissues.

Comparison of methods for measuring shear force and sarcomere length and their relationship with sensorial tenderness of longissimus muscle in beef

Slice shear force (SSF) and laser diffraction, considered faster methodologies, for measuring beef instrumental tenderness and sarcomere length, were compared with reference methodologies Warner-Bratzler shear force (WBSF) and phase contrast microscopy. Striploin samples (n = 74) were analyzed for pH, sarcomere length, instrumental tenderness, myofibrillar fragmentation index, and sensorial tenderness. Pearson's correlation measured the association of meat evaluation methods with residual analysis of the multivariate analysis of variance model. The n-dimensional profile to evaluated methods was presented by biplot to identify the behavior of the correlation between the methods (variables). There was moderate correlation between SSF and WBSF (r = .63; p < .01) and both presented moderate correlation with sensorial tenderness (r = - .62 and -.55, to SSF and WBSF, respectively; p < .01). However, WBSF was more efficient to classify samples as tender (68%) than SSF (47%), comparing with sensorial tenderness (80%). There was a moderate correlation for laser and microscopy for sarcomere length (r = .57; p < .01). Sarcomeres were shorter when measured by laser than microscopy. Either with low correlation coefficients, sarcomere measured by laser (r = .29; p < .05) presented higher correlation with sensorial tenderness than with microscopy (r = .22; p < .10). Results highlighted that SSF was faster and easier to run, while WBSF was more appropriate to classify samples by sensorial tenderness grades. Laser diffraction is more suitable to explain effects on tenderness; however, microscopy revealed results of sarcomere length that were more realistic, once laser can underestimate sarcomeres.

Surface Morphologies in Ultra-short Pulsed Laser Processing of Stainless-Steel at High Repetition Rate

Stainless-steel is ablated with femtosecond laser pulses at high repetition rate. A multi-pass, high spatial overlap laser scanning strategy is applied in order to cope with the requirements for large-scale machining of high aspect ratio structures. Topography of the processed surfaces is analyzed via Shear Force Microscopy scans, with the main aim to investigate morphology changes as a function of process parameters. Quantitative assessment of local height variations enables a detailed investigation of the produced features. Depending on the process parameters, in particular on laser fluence and repetition rate, a transition from small islands to large bumps is observed, explained in terms of feature coalescence.

Instrumentation-level improvements in shear-force near-field acoustic microscopy

The recently introduced Shear-force Near-field Acoustic Microscopy (SANM) brings a new sensing mechanism to the scanning probe microscopy family. SANM’s ability to simultaneously monitor, in real time, several physical sample’s responses presents some challenges for ensuring optimal operation; namely, avoid “cross-talk” among the multiple signals, address the compensation of thermal drift to ensure reproducibility of the measurements, and measuring the typical low-level signals obtained from nanometer-sized tested regions. Here, several improvements relevant to SANM, but valid for SPM in general, are addressed. i) The probe’s coarse approach is performed via stepper motors, which are controlled either by a computer software interface, or simply by a user-friendly RF remote control. ii) The inherent mechanical drift of the stage is evaluated first (by monitoring the feedback voltage that acts on the sample to maintain the probe-sample distance constant), and then automatically compensated (via linear interpolation) in the immediate subsequent probe’s approach/retraction measurements. iii) To determine the absolute position of the substrate relative to the probe, the probe-substrate contact current is measured with circuitry properly shielded from eventual electrical ground loops. Buffer amplifiers are used to drive the quartz tuning fork (that holds the probe) and to measure the AC and DC tunneling currents between the tip and sample. iv) To improve the signal to noise ratio, a Kalman filter is implemented into the SANM’s field-programmable gate array board, which processes the signals in real time. Finally, v) to simplify the operation of the microscope, an intuitive LabVIEW host program is developed to control the whole system and offer the user a visualization of the data in real time.

Probe damage evaluation in frequency-modulation shear-force acoustic near-field microscopy

Shear-force acoustic near-field microscopy (SANM) and Whispering Gallery Acoustic Sensing have recently been introduced as a tandem system to characterize the viscoelastic response of fluids confined between two solid-boundaries in relative oscillatory lateral motion. SANM uses a) a laterally oscillating tapered probe (attached to a quartz tuning fork QTF) as one of the trapping boundaries, and b) an acoustic sensor (attached to the other flat-substrate boundary) that independently monitors the fluid’s acoustic emission. On the other and, WGAS is another technique that uses an acoustic transducer (attached to the frame holding the probe) to monitor the probe’s lateral motion amplitude. Steps towards improving the standing of the SANM/WGAS system as a nanometrology tool include i) to integrate frequency modulation methods, to thus be able to discriminate the elastic from the inelastic component of the probe-fluid interactions, and ii) evaluate the integrity and robustness of the probe to endure such surface interactions, since eventual deformation of the probe affect the reproducibility of the measurements. Here we report using the WGAS signal (or alternatively the QTF electrical response) to control the probe’s motion in frequency modulation modality. In addition, systematic evaluation of eventual probe damage (acquiring scanning electron micrograph before and after the probe exerts one approach/retraction trip to/from the substrate) are also presented. The non-monotonic behavior of the fluid’s acoustic emission (first increasing and then decreasing) as the probe approaches the sample—which occurs before the probe touches the surface, as revealed by the lack of damaged observed on the probe—constitutes one of the main findings reported here. The acoustic signal provides clues on the fluid’s dynamic response when subjected to nanoscale confinement.

Combining atomic force microscopy and shear-force acoustic near-field microscopy to characterize confined mesoscopic fluids

An atomic force microscopy (AFM) cantilever is integrated into to a quartz tuning fork (QTF) to probe the viscoelastic properties of mesoscopic fluid layers confined between two solid surfaces under shear. Two procedures to fabricate the AFM/QTF probe are described herein. In the first, a nano-manipulator is used to transport a commercially available afm cantilever from its chip holder to the edge of a QTF tine. In the second, an afm cantilever is fabricated at the edge of the QTF tine itself. In both cases we exploit the capabilities of a dual-beam system (focused ion beam/scanning electron microscope), equipped with Omni-Probe nano-manipulator and a Gas Injection System (GIS). The new device improves the ability of shear-force acoustic near-field microscopy (SANM) to monitor the constraining normal and damping shear forces exerted by the solid boundaries, concurrently with the acoustic emission from the trapped fluid.

Finite element method analysis of whispering gallery acoustic sensing

Whispering Gallery Acoustic Sensing (WGAS) has recently been introduced as a sensing feedback mechanism to control the probe-sample separation distance in scanning probe microscopy that uses a quartz tuning fork as a sensor (QTF-SPM) [1]. WGAS exploits the SPM supporting frame as a resonant acoustic cavity to monitor the nanometer-sized amplitude of the QTF oscillations. Optimal WGAS sensitivity depends on attaining an exact match between the cavity’s frequency peak response and the TF resonance frequency. However, two aspects play against this objective: i) the unpredictable variability of the TF resonance frequency (upon attaching a SPM-probe to one of its tines), and ii) cavities of arbitrary geometry tend to display complicated (multiple peaks) frequency response, making difficult to identify which cavity dimension control which peak. Practical matching frequency procedures are needed then to operate the Shear-force Acoustic Near-field Microscopy (SANM) more efficiently. As a first step, here we undertake finite-element method (FEM) analysis to find out cavities of simple frequency response and, ideally, easy frequency tuning ability. Based on previous results we focus our studies in analyzing the frequency response of conical cavities within a range around the 32 kHz operating frequency. To first validate our numerical simulation studies, we reproduce the experimental results obtained from a specific conical cavity. Then we proceed to simulate the response of cavities of slightly different geometries, and investigate the dependence on the young modulus, poison ratio, and slight changes in dimensions. This initial success encourages to undertake studies of cavities having more sophisticated geometries.

Confined Fluid Analyzed with Near-field Acoustic Detection

Measurement of the damping and elastic interactions between two solids interfaces (one being the apex of a tapered probe that is attached to one tine of a quartz tuning fork while the other is a flat substrate) under relative lateral oscillatory motion are reported. The solid boundaries are separated by a nanometer sized gap, and emphasis is placed on the role played by the mesoscopic fluid trapped in between. The measurements were implemented using two new acoustic techniques that have been integrated into a tuning fork based scanning probe microscope; the whole metrology system offers sub-nanometer precision for controlling the probe’s vertical position, as well as thermal drift correction. One of the acoustic techniques, Shear-force Acoustic Near-field Microscopy (SANM), monitors the acoustic emission from the trapped fluid subjected to shear stress, while Whispering Gallery Acoustic Sensing (WGAS) monitors the oscillation amplitude of the QTF. Additionally, the elastic and damping components of the probe-fluid interaction forces are decoupled by operating the SANM in frequency modulation mode, while the eventual contact between the probe and substrate is tracked by monitoring the electrical current signal. Results involving interaction in the 0 to 5 nN regime are reported. The measurements reveal a strong correlation between the monotonic shift of the probe’s resonance frequency and the near-field acoustic emission from the trapped fluid. The SANM signal, then, accounts as one of the elastic energy dissipation channels involved in surface interactions at ambient conditions.

Microstructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy.

Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS2 ) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable the fourth-order elastic constants of monolayer WS2 to be acquired experimentally. The results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.

Towards Laser-Textured Antibacterial Surfaces

Escherichia coli and Staphylococcus aureus bacterial retention on mirror-polished and ultrashort pulse laser-textured surfaces is quantified with a new approach based on ISO standards for measurement of antibacterial performance. It is shown that both wettability and surface morphology influence antibacterial behavior, with neither superhydrophobicity nor low surface roughness alone sufficient for reducing initial retention of either tested cell type. Surface structures comprising spikes, laser-induced periodic surface structures (LIPSS) and nano-pillars are produced with 1030 nm wavelength 350 fs laser pulses of energy 19.1 μJ, 1.01 μJ and 1.46 μJ, respectively. SEM analysis, optical profilometry, shear force microscopy and wettability analysis reveal surface structures with peak separations of 20–40 μm, 0.5–0.9 μm and 0.8–1.3 μm, average areal surface roughness of 8.6 μm, 90 nm and 60 nm and static water contact angles of 160°, 119° and 140°, respectively. E. coli retention is highest for mirror-polished specimens and spikes whose characteristic dimensions are much larger than the cell size. S. aureus retention is instead found to be inhibited under the same conditions due to low surface roughness for mirror-polished samples (Sa: 30 nm) and low wettability for spikes. LIPSS and nano-pillars are found to reduce E. coli retention by 99.8% and 99.2%, respectively, and S. aureus retention by 84.7% and 79.9% in terms of viable colony forming units after two hours of immersion in bacterial broth due to both low wettability and fine surface features that limit the number of available attachment points. The ability to tailor both wettability and surface morphology via ultrashort pulsed laser processing confirms this approach as an important tool for producing the next generation of antibacterial surfaces.

Direct manipulation of metallic nanosheets by shear force microscopy.

Micro/nanomanipulation is a rapidly growing technology and holds promising applications in various fields, including photonic/electronic devices, chemical/biosensors etc. In this work, we present that shear force microscopy (ShFM) can be exploited to manipulate metallic nanosheets besides imaging. The manipulation is realized via controlling the shear force sensor probe position and shear force magnitude based on our homemade ShFM system under an optical microscopy for in situ observation. The main feature of the ShFM system is usage of a piezoelectric bimorph sensor, which has the ability of self-excitation and detection. Moreover, the shear force magnitude as a function of the spring constant of the sensor and setpoint is obtained, which indicates that operation modes can be switched between imaging and manipulation through designing the spring constant before experiment and changing the setpoint during manipulation process, respectively. We believe that this alternative manipulation technique could be used to assemble other nanostructures with different shapes, sizes and compositions for new properties and wider applications.

High Spatial Resolution Imaging of Mouse Pancreatic Islets Using Nanospray Desorption Electrospray Ionization Mass Spectrometry.

Nanospray Desorption Electrospray Ionization mass spectrometry imaging (nano-DESI MSI) enables ambient imaging of biological samples with high sensitivity and minimal sample pretreatment. Recently, we developed an approach for constant-distance mode MSI using shear force microscopy to precisely control the distance between the sample and the nano-DESI probe. Herein, we demonstrate the power of this approach for robust imaging of pancreatic islets with high spatial resolution of ∼11 μm. Pancreatic islets are difficult to characterize using traditional mass spectrometry approaches due to their small size (∼100 μm) and molecular heterogeneity. Nano-DESI MSI was used to examine the spatial localization of several lipid classes including phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyelin (SM), phosphatidylinositol (PI), and phosphatidylserine (PS) along with fatty acids and their metabolites (e.g., prostaglandins) in the individual islets and surrounding tissue. Several lipids were found to be substantially enhanced in the islets indicating these lipids may be involved in insulin secretion. Remarkably different distributions were observed for several pairs of Lyso PC (LPC) and PC species differing only by one double bond, such as LPC 18:1 vs LPC 18:0, PC 32:1 vs PC 32:0, and PC 34:2 vs PC 34:1. These findings indicate that minor variations in the fatty acid chain length and saturation have a pronounced effect on the localization of PC and LPC species in pancreatic islets. Interestingly, oxidized PC species observed experimentally were found to be specifically localized to pancreatic islets. These PCs are potential biomarkers for reactive oxygen species in the islets, which could be harmful to pancreatic beta cells. The experimental approach presented in this study will provide valuable information on the heterogeneity of individual pancreatic islets, which is difficult to assess using bulk characterization techniques.

Atomic Force Microscopy Based Tip-Enhanced Raman Spectroscopy in Biology.

Most biological phenomena occur at the nanometer scale, which is not accessible by the conventional optical techniques because of the optical diffraction limitation. Tip-enhanced Raman spectroscopy (TERS), one of the burgeoning probing techniques, not only can provide the topography characterization with high resolution, but also can deliver the chemical or molecular information of a sample beyond the optical diffraction limitation. Therefore, it has been widely used in various structural analyses pertaining to materials science, tissue engineering, biological processes and so on. Based on the different feedback mechanisms, TERS can be classified into three types: atomic force microscopy based TERS system (AFM-TERS), scanning tunneling microscopy based TERS system (STM-TERS) and shear force microscopy based TERS system (SFM-TERS). Among them, AFM-TERS is the most widely adopted feedback system by live biosamples because it can work in liquid and this allows the investigation of biological molecules under native conditions. In this review, we mainly focus on the applications of AFM-TERS in three biological systems: nucleic acids, proteins and pathogens. From the TERS characterization to the data analysis, this review demonstrates that AFM-TERS has great potential applications to visually characterizing the biomolecular structure and crucially detecting more nano-chemical information of biological systems.