Wideband low-noise optical beam deflection sensor with photothermal excitation for liquid-environment atomic force microscopy

Stability Enhancement by Hydrophobic Anchoring and a Cross-Linked Structure of a Phospholipid Copolymer Film for Medical Devices

Surface modification using zwitterionic 2-methacryloyloxyethylphosphorylcholine (MPC) polymers is one of the most reasonable ways to prepare medical devices that can suppress undesired biological reactions such as blood coagulation. Usable MPC polymers are hydrophilic and water soluble, and their surface modification strategy involves exploiting the copolymer structures by adding physical or chemical bonding moieties. In this study, we developed copolymers composed of MPC, hydrophobic anchoring moiety, and chemical cross-linking unit to clarify the role of hydrophobic interactions in achieving biocompatible and long-term stable coatings. The four kinds of MPC copolymers with cross-linking units, such as 3-methacryloxypropyl trimethoxysilane (MPTMSi), and four different hydrophobic anchoring moieties, such as 3-(methacryloyloxy)propyltris(trimethylsiloxy)silane (MPTSSi) named as PMMMSi, n-butyl methacrylate (BMA) as PMBSi, 2-ethylhexyl methacrylate (EHMA) as PMESi, and lauryl methacrylate as PMLSi, were synthesized and coated on polydimethylsiloxane, polypropylene (PP), and polymethyl pentene. These copolymers were uniformly coated on the substrate materials PP and poly(methyl pentene) (PMP), to achieve hydrophilic and electrically neutral coatings. The results of the antibiofouling test showed that PMBSi repelled the adsorption of fluorescence-labeled bovine serum albumin the most, whereas PMLSi repelled it the least. Notably, all four copolymers suppressed platelet adhesion similarly. The variations in protein adsorption quantities among the four copolymer coatings were attributed to their distinct swelling behaviors in aqueous environments. Further investigations, including 3D scanning force microscopy and neutron reflectivity measurements, revealed that the PMLSi coating exhibited a higher water intake under aqueous conditions in comparison to the other coatings. Consequently, all copolymer coatings effectively prevented the invasion of platelets but the proteins penetrated the PMLSi network. Subsequently, the dynamic stability required to induce shear stress was evaluated using a circulation system. The results demonstrated that the PMMMSi and PMLSi coatings on PMP and PP exhibited exceptional platelet repellency and maintained high stability during circulation. This study highlights the potential of hydrophobic moieties to improve hemocompatibility and stability, offering potential applications in medical devices.

Biophysical Properties of the Fibril Structure of the Toxic Conformer of Amyloid-β42: Characterization by Atomic Force Microscopy in Liquid and Molecular Docking

Alzheimer's disease is associated with the aggregation of the misfolded neuronal peptide, amyloid-β42 (Aβ42). Evidence has suggested that several reasons are responsible for the toxicity caused by the aggregation of Aβ42, including the conformational restriction of Aβ42. In this study, one of the toxic conformers of Aβ42, which contains a Glu-to-Pro substitution (E22P-Aβ42), was explored using atomic force microscopy and molecular docking to study the aggregation dynamics. We proposed a systematic model of fibril formation to better understand the molecular basis of conformational transitions in the Aβ42 species. Our results demonstrated the formation of amorphous aggregates in E22P-Aβ42 that are stem-based, network-like structures, while the formation of mature fibrils occurred in the less toxic conformer of Aβ42, E22-Aβ42, that are sphere-like flexible structures. A comparison was made between the biophysical properties of E22P-Aβ42 and E22-Aβ42 that revealed that E22P-Aβ42 had greater stiffness, dihedral angle, number of β sheets involved, and elasticity, compared with E22-Aβ42. These findings will have considerable implications toward our understanding of the structural basis of the toxicity caused by conformational diversity in Aβ42 species.

Molecular Scale Structure and Kinetics of Layer-by-Layer Peptide Self-Organization at Atomically Flat Solid Surfaces

Understanding the mechanisms of self-organization of short peptides into two- and three-dimensional architectures are of great interest in the formation of crystalline biomolecular systems and their practical applications. Since the assembly is a dynamic process, the study of structural development is challenging at the submolecular dimensions continuously across an adequate time scale in the natural biological environment, in addition to the complexities stemming from the labile molecular structures of short peptides. Self-organization of solid binding peptides on surfaces offers prospects to overcome these challenges. Here we use a graphite binding dodecapeptide, GrBP5, and record its self-organization process of the first two layers on highly oriented pyrolytic graphite surface in an aqueous solution by using frequency modulation atomic force microscopy in situ. The observations suggest that the first layer forms homogeneously, generating self-organized crystals with a lattice structure in contact with the underlying graphite. The second layer formation is mostly heterogeneous, triggered by the crystalline defects on the first layer, growing row-by-row establishing nominally diverse biomolecular self-organized structures with transient crystalline phases. The assembly is highly dependent on the peptide concentration, with the nucleation being high in high molecular concentrations, e.g., >100 μM, while the domain size is small, with an increase in the growth rate that gradually slows down. Self-assembled peptide crystals are composed of either singlets or doublets establishing P1 and P2 oblique lattices, respectively, each commensurate with the underlying graphite lattice with chiral crystal relations. This work provides insights into the surface behavior of short peptides on solids and offers quantitative guidance toward elucidating molecular mechanisms of self-assembly helping in the scientific understanding and construction of coherent bio/nano hybrid interfaces.

Ion-specific nanoscale compaction of cysteine-modified poly(acrylic acid) brushes revealed by 3D scanning force microscopy with frequency modulation detection

Stimuli-responsive polyelectrolyte brushes adapt their physico-chemical properties according to pH and ion concentrations of the solution in contact. We synthesized a poly(acrylic acid) bearing cysteine residues at side chains and a lipid head group at the terminal, and incorporated them into a phospholipid monolayer deposited on a hydrophobic silane monolayer. The ion-specific, nanoscale response of polyelectrolyte brushes was detected by using three-dimensional scanning force microscopy (3D-SFM) combined with frequency modulation detection. The obtained topographic and mechanical landscapes indicated that the brushes were uniformly stretched, undergoing a gradual transition from the brush to the bulk electrolyte in the absence of divalent cations. When 1 mM calcium ions were added, the brushes were uniformly compacted, exhibiting a sharper brush-to-bulk transition. Remarkably, the addition of 1 mM cadmium ions made the brush surface significantly rough and the mechanical landscape highly heterogeneous. Currently, cadmium-specific nanoscale compaction of the brushes is attributed to the coordination of thiol and carboxyl side chains with cadmium ions, as suggested for naturally occurring, heavy metal binding proteins.

Molecular insights on the crystalline cellulose-water interfaces via three-dimensional atomic force microscopy

Cellulose, a renewable structural biopolymer, is ubiquitous in nature and is the basic reinforcement component of the natural hierarchical structures of living plants, bacteria, and tunicates. However, a detailed picture of the crystalline cellulose surface at the molecular level is still unavailable. Here, using atomic force microscopy (AFM) and molecular dynamics (MD) simulations, we revealed the molecular details of the cellulose chain arrangements on the surfaces of individual cellulose nanocrystals (CNCs) in water. Furthermore, we visualized the three-dimensional (3D) local arrangement of water molecules near the CNC surface using 3D AFM. AFM experiments and MD simulations showed anisotropic water structuring, as determined by the surface topologies and exposed chemical moieties. These findings provide important insights into our understanding of the interfacial interactions between CNCs and water at the molecular level. This may allow the establishment of the structure-property relationship of CNCs extracted from various biomass sources.

Visualizing Molecular-Scale Adsorption Structures of Anti-freezing Surfactants on Sapphire (0001) Surfaces at Different Concentrations by 3D Scanning Force Microscopy

Anti-freezing surfactants form an adsorption layer at the solid-water interface to inhibit the nucleation and growth of ice. However, this mechanism has not been elucidated at the molecular scale because of the difficulties in visualizing such adsorption structures. In this study, we overcome this limitation by directly visualizing the three-dimensional (3D) adsorption structures of anti-freezing surfactants, hexadecyltrimethylammonium bromide (C₁₆TABs), on sapphire (0001) surfaces through 3D scanning force microscopy. We present molecularly resolved two-dimensional/3D images of the adsorption structures in solutions of 1, 10, and 100 ppm. At 1 ppm, the molecules form a monolayer with a flat-lying configuration. At 10 ppm, the molecular orientation is closer to the upright configuration, with a relatively large tilt angle. At 100 ppm, the molecules form a bilayer with almost upright configurations, providing excellent screening of the sapphire surface from water. Owing to the steric and electrostatic repulsion between adjacent molecular head groups, the surface of the bilayer exhibits relatively large fluctuations, inhibiting the formation of stable ice-like structures. The understanding of molecular-level mechanisms provides important guidelines for improving the design of anti-freezing surfactants for practical applications such as car coolants.

Polar zinc oxide surface in electrolyte solutions: an atomic view of reconstruction, hydration and surface states

The stabilization mechanism of the Zn-terminated (Zn-) ZnO(0001) surface in electrolyte solutions has been investigated by using atomic-resolution liquid-environment atomic force microscopy (AFM) and an electrochemical method. The electrochemically measured pH dependence of the flat band potential of the Zn-ZnO(0001) surface indicated the adsorption of OH groups onto the (0001) surface in the wide pH range of 1-13. Atomic-scale AFM images of the Zn-ZnO(0001) surface showed a well-ordered hydroxide superstructure in an alkaline solution but a disordered structure in an acidic solution, which is probably attributed to the rapid diffusion of the adsorbed OH groups. Furthermore, the density of the O-terminated step edge on the Zn-ZnO(0001) surface in an acidic solution was higher than that in an alkaline solution. From these findings, we concluded that the excess positive charges of the Zn-ZnO(0001) surface are compensated by the adsorbed OH groups and the O-terminated step edges. In acidic solutions, a higher density of the O-terminated step edge is required for charge compensation. In addition, it was found that potential-dependent reversible surface reconstruction occurs in the local transition area with disordered step orientation by electrochemical AFM. We concluded that the reconstruction compensates the excess surface charges of the local transition area which are induced and varied by potential-dependent local surface states.

Atomic force microscopy-A tool for structural and translational DNA research

Atomic force microscopy (AFM) is a powerful imaging technique that allows for structural characterization of single biomolecules with nanoscale resolution. AFM has a unique capability to image biological molecules in their native states under physiological conditions without the need for labeling or averaging. DNA has been extensively imaged with AFM from early single-molecule studies of conformational diversity in plasmids, to recent examinations of intramolecular variation between groove depths within an individual DNA molecule. The ability to image dynamic biological interactions in situ has also allowed for the interaction of various proteins and therapeutic ligands with DNA to be evaluated-providing insights into structural assembly, flexibility, and movement. This review provides an overview of how innovation and optimization in AFM imaging have advanced our understanding of DNA structure, mechanics, and interactions. These include studies of the secondary and tertiary structure of DNA, including how these are affected by its interactions with proteins. The broader role of AFM as a tool in translational cancer research is also explored through its use in imaging DNA with key chemotherapeutic ligands, including those currently employed in clinical practice.

High-Speed Atomic Force Microscopy of the Structure and Dynamics of Calcite Nanoscale Etch Pits

Calcite dissolution is initiated by the formation of a nanoscale etch pit followed by step edge propagation and hence strongly influenced by the interactions between surface diffusing ions and step edges. However, such atomic-scale dynamics are mostly inaccessible with current imaging tools. Here, we overcome this limitation by using our recent development of high-speed frequency modulation atomic force microscopy. By visualizing atomic-scale structural changes of the etch pits at the calcite surface in water, we found the existence of mobile and less-mobile surface adsorption layers (SALs) in the etch pits. We also found that some etch pits maintain their size for a long time without expansion, and their step edges are often associated with less-mobile SALs, suggesting their step stabilization effect.

Validity of point-mass model in off-resonance dynamic atomic force microscopy

The quantitative measurement of viscoelasticity of nano-scale entities is an important goal of nanotechnology research and there is considerable progress with advent of dynamic atomic force microscopy. The hydrodynamics of cantilever, the force sensor in AFM measurements, plays a pivotal role in quantitative estimates of nano-scale viscoelasticity. The point-mass (PM) model, wherein the AFM cantilever is approximated as a point-mass with mass-less spring is widely used in dynamic AFM analysis and its validity, particularly in liquid environments, is debated. It is suggested that the cantilever must be treated as a continuous rectangular beam to obtain accurate estimates of nano-scale viscoelasticity of materials it is probing. Here, we derived equations, which relate stiffness and damping coefficient of the material under investigation to measured parameters, by approximating cantilever as a point-mass and also considering the full geometric details. These equations are derived for both tip-excited as well as base-excited cantilevers. We have performed off-resonance dynamic atomic force spectroscopy on a single protein molecule to investigate the validity of widely used PM model. We performed measurements with AFMs equipped with different cantilever excitation methods as well as detection schemes to measure cantilever response. The data was analyzed using both, continuous beam model and the PM model. We found that both models yield same results when the experiments are performed in truly off-resonance regime with small amplitudes and the cantilever stiffness is much higher than the interaction stiffness. Our findings suggest that a simple PM approximation based model is adequate to describe the dynamics, provided care is taken while performing experiments so that the approximations used in these models are valid.

Multimodal real-time frequency tracking of cantilever arrays in liquid environment for biodetection: Comprehensive setup and performance analysis

We present a nanomechanical platform for real-time quantitative label-free detection of target biomolecules in a liquid environment with mass sensitivity down to few pg. Newly fabricated arrays of up to 18 cantilevers are integrated in a micromachined fluidic chamber, connected to software-controlled fluidic pumps for automated sample injections. We discuss two functionalization approaches to independently sensitize the interface of different cantilevers. A custom piezo-stack actuator and optical readout system enable the measurement of resonance frequencies up to 2 MHz. We implement a new measurement strategy based on a phase-locked loop (PLL), built via in-house developed software. The PLL allows us to track, within the same experiment, the evolution of resonance frequency over time of up to four modes for all the cantilevers in the array. With respect to the previous measurement technique, based on standard frequency sweep, the PLL enhances the estimated detection limit of the device by a factor of 7 (down to 2 pg in 5 min integration time) and the time resolution by more than threefold (below 15 s), being on par with commercial gold-standard techniques. The detection limit and noise of the new setup are investigated via Allan deviation and standard deviation analysis, considering different resonance modes and interface chemistries. As a proof-of-concept, we show the immobilization and label-free in situ detection of live bacterial cells (E. coli), demonstrating qualitative and quantitative agreement in the mechanical response of three different resonance modes.

The nano-scale viscoelasticity using atomic force microscopy in liquid environment

We measured viscoelasticity of two nanoscale systems, single protein molecules and molecular layers of water confined between solid walls. In order to quantify the viscoelastic response of these nanoscale systems in liquid environment, the measurements are performed using two types of atomic force microscopes (AFMs), which employ different detection schemes to measure the cantilever response. We used a deflection detection scheme, available in commercial AFMs, that measures cantilever bending and a fibre-interferometer based detection which measures cantilever displacement. The hydrodynamics of the cantilever is modelled using Euler-Bernoulli equation with appropriate boundary conditions which accommodate both detection schemes. In a direct contradiction with many reports in the literature, the dissipation coefficient of a single octomer of titin I27₈ is found to be immeasurably low. The upper bound on the dissipation coefficient is 5 × 10^-7 kg s^-1, which is much lower than the reported values. The entropic stiffness of single unfolded domains of protein measured using both methods is in the range of 10 mN m^-1. We show that in a conventional deflection detection measurement, the phase of the bending signal can be a primary source of artefacts in the dissipation estimates. It is recognized that the measurement of cantilever displacement, which has negligibly small phase lag due to hydrodynamics of the cantilever at low excitation frequencies, is better suited for ensuring artefact-free measurement of viscoelasticity compared to the measurement of the cantilever bending. Further, it was possible to measure dissipation in molecular layers of water confined between the tip and the substrate using fibre interferometer based AFM with similar experimental parameters. It confirms that the dissipation coefficient of a single I27₈ is below the detection limit of AFM. The results shed light on the discrepancy observed in the measured diffusional dynamics of protein collapse measured using Force spectroscopic techniques and single-molecule optical techniques.

Design and Fabrication of a High-Speed Atomic Force Microscope Scan-Head

A high-speed atomic force microscope (HS-AFM) requires a specialized set of hardware and software and therefore improving video-rate HS-AFMs for general applications is an ongoing process. To improve the imaging rate of an AFM, all components have to be carefully redesigned since the slowest component determines the overall bandwidth of the instrument. In this work, we present a design of a compact HS-AFM scan-head featuring minimal loading on the Z-scanner. Using a custom-programmed controller and a high-speed lateral scanner, we demonstrate its working by obtaining topographic images of Blu-ray disk data tracks in contact- and tapping-modes. Images acquired using a contact-mode cantilever with a natural frequency of 60 kHz in constant deflection mode show good tracking of topography at 400 Hz. In constant height mode, tracking of topography is demonstrated at rates up to 1.9 kHz for the scan size of 1μm×1μm with 100×100 pixels.

Atomic-scale structures and dynamics at the growing calcite step edge investigated by high-speed frequency modulation atomic force microscopy

We have investigated the calcite growth mechanism by directly imaging atomic-scale structural changes at the growing step edges with high-speed frequency modulation atomic force microscopy (HS-FM-AFM). We compared the results...

In situ formation of photoactive B-ring reduced chlorophyll isomer in photosynthetic protein LH2

Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.

Photothermal excitation efficiency enhancement of cantilevers by electron beam deposition of amorphous carbon thin films

In recent years, the atomic force microscope has proven to be a powerful tool for studying biological systems, mainly for its capability to measure in liquids with nanoscale resolution. Measuring tissues, cells or proteins in their physiological conditions gives us access to valuable information about their real ‘in vivo’ structure, dynamics and functionality which could then fuel disruptive medical and biological applications. The main problem faced by the atomic force microscope when working in liquid environments is the difficulty to generate clear cantilever resonance spectra, essential for stable operation and for high resolution imaging. Photothermal actuation overcomes this problem, as it generates clear resonance spectra free from spurious peaks. However, relatively high laser powers are required to achieve the desired cantilever oscillation amplitude, which could potentially damage biological samples. In this study, we demonstrate that the photothermal excitation efficiency can be enhanced by coating the cantilever with a thin amorphous carbon layer to increase the heat absorption from the laser, reducing the required excitation laser power and minimizing the damage to biological samples.

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy

This review critically summarizes the recent advances of the microcantilever-based force sensors for atomic force microscope (AFM) applications. They are one the most common mechanical spring-mass systems and are extremely sensitive to changes in the resonant frequency, thus finding numerous applications especially for molecular sensing. Specifically, we comment on the latest progress in research on the deflection detection systems, fabrication, coating and functionalization of the microcantilevers and their application as bio- and chemical sensors. A trend on the recent breakthroughs on the study of biological samples using high-speed atomic force microscope is also reported in this review.

Improvements in fundamental performance of in-liquid frequency modulation atomic force microscopy

In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ∼1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.

Tip dependence of three-dimensional scanning force microscopy images of calcite-water interfaces investigated by simulation and experiments

In this study, we have investigated the influence of the tip on the three-dimensional scanning force microscopy (3D-SFM) images of calcite-water interfaces by experiments and simulations. We calculated 3D force images by simulations with the solvent tip approximation (STA), Ca, CO3 and OH tip models. For all the 3D images, the z profiles at the surface Ca and CO3 sites alternately show oscillatory peaks corresponding to the hydration layers. However, the peak heights and spacings become larger when the mechanical stability of the tip becomes higher. For analyzing the xy slices of the 3D force images, we developed the extended STA (E-STA) model which allowed us to reveal the strong correlation between the hydration structure just under the tip and the atomic-scale force contrasts. Based on these understandings on the image features showing the strong tip dependence, we developed a method for objectively estimating the similarity between 3D force images. With this method, we compared the simulated images with the three experimentally obtained ones. Among them, two images showed a relatively high similarity with the image obtained by the simulation with the Ca or the CO3 tip model. Based on these agreements, we characterized the hydration structure and mechanical stability of the experimentally used tips. The understanding and methodology presented here should help us to derive accurate information on the tip and the interfacial structure from experimentally obtained 3D-SFM images.

Wideband Magnetic Excitation System for Atomic Force Microscopy Cantilevers with Megahertz-Order Resonance Frequency

Small cantilevers with a megahertz-order resonance frequency provide excellent sensitivity and speed in liquid-environment atomic force microscopy (AFM). However, stable and accurate oscillation control of a small cantilever requires the photothermal excitation, which has hindered their applications to the studies on photo-sensitive materials. Here, we develop a magnetic excitation system with a bandwidth wider than 4 MHz, enabling a light-free excitation of small cantilevers. In the system, a cantilever with a magnetic bead is driven by a magnetic field generated by a coil. In the coil driver, a differentiation circuit is used for compensating the frequency dependence of the coil impedance and keeping the current constant. By implementing several differentiation circuits with different frequency ranges, we enable to drive various cantilevers having different resonance frequencies with sufficient excitation efficiency. In contrast to the conventional coil driver with a closed-loop circuit, the developed one consists of an open-loop circuit and hence can be stably operated regardless of the coil design. With the developed system, atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency is demonstrated. This development should lead to the future applications of AFM with small cantilevers to the studies on various photo-sensitive materials and phenomena.

A molecular dynamics study proposing the existence of statistical structural heterogeneity due to chain orientation in the POPC-cholesterol bilayer

We performed molecular dynamics simulations of a lipid bilayer consisting of POPC and cholesterol at temperatures from 283 to 308K and cholesterol concentrations from 0 to 50% mol/mol. The purpose of this study was to look for the existence of structural differences in the region delimited by these parameters and, in particular, in a region where coexistence of liquid disordered and liquid ordered phases has been proposed. Our interest in this range of concentration and temperature responds to the fact that polyene ionophore activity varies considerably along it. Two force fields, CHARMM36 and Slipids, were compared in order to determine the most suitable. Both force fields predict non-monotonic behaviors consistent with the existence of phase transitions. We found the presence of lateral structural heterogeneity, statistical in nature, in some of the bilayers occurring in this range of temperatures and sterol concentrations. This heterogeneity was produced by correlated ordering of the POPC tails and not due to cholesterol enrichment, and lasts for tens of nanoseconds. We relate these observations to the action of polyenes in these membranes.

Array atomic force microscopy for real-time multiparametric analysis

Nanoscale multipoint structure-function analysis is essential for deciphering the complexity of multiscale biological and physical systems. Atomic force microscopy (AFM) allows nanoscale structure-function imaging in various operating environments and can be integrated seamlessly with disparate probe-based sensing and manipulation technologies. Conventional AFMs only permit sequential single-point analysis; widespread adoption of array AFMs for simultaneous multipoint study is challenging owing to the intrinsic limitations of existing technological approaches. Here, we describe a prototype dispersive optics-based array AFM capable of simultaneously monitoring multiple probe-sample interactions. A single supercontinuum laser beam is utilized to spatially and spectrally map multiple cantilevers, to isolate and record beam deflection from individual cantilevers using distinct wavelength selection. This design provides a remarkably simplified yet effective solution to overcome the optical cross-talk while maintaining subnanometer sensitivity and compatibility with probe-based sensors. We demonstrate the versatility and robustness of our system on parallel multiparametric imaging at multiscale levels ranging from surface morphology to hydrophobicity and electric potential mapping in both air and liquid, mechanical wave propagation in polymeric films, and the dynamics of living cells. This multiparametric, multiscale approach provides opportunities for studying the emergent properties of atomic-scale mechanical and physicochemical interactions in a wide range of physical and biological networks.

Selective oxidation of B800 bacteriochlorophyll a in photosynthetic light-harvesting protein LH2

Engineering chlorophyll (Chl) pigments that are bound to photosynthetic light-harvesting proteins is one promising strategy to regulate spectral coverage for photon capture and to improve the photosynthetic efficiency of these proteins. Conversion from the bacteriochlorophyll (BChl) skeleton (7,8,17,18-tetrahydroporphyrin) to the Chl skeleton (17,18-dihydroporphyrin) produces the most drastic change of the spectral range of absorption by light-harvesting proteins. We demonstrated in situ selective oxidation of B800 BChl a in light-harvesting protein LH2 from a purple bacterium Rhodoblastus acidophilus by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The newly formed pigment, 3-acetyl Chl a, interacted with the LH2 polypeptides in the same manner as native B800. B850 BChl a was not oxidized in this reaction. CD spectroscopy indicated that the B850 orientation and the content of the α-helices were unchanged by the B800 oxidation. The nonameric circular arrangement of the oxidized LH2 protein was visualized by AFM; its diameter was almost the same as that of native LH2. The in situ oxidation of B800 BChl a in LH2 protein with no structural change will be useful not only for manipulation of the photofunctional properties of photosynthetic pigment-protein complexes but also for understanding the substitution of BChl to Chl pigments in the evolution from bacterial to oxygenic photosynthesis.

Atomic- and Molecular-Resolution Mapping of Solid-Liquid Interfaces by 3D Atomic Force Microscopy

Hydration layers are ubiquitous in life and technology. Hence, interfacial aqueous layers have a central role in a wide range of phenomena from materials science to molecular and cell biology. A complete understanding of those processes requires, among other things, the development of very-sensitive and high-resolution instruments. Three-dimensional atomic force microscopy (3D-AFM) represents the latest and most successful attempt to generate atomically resolved three-dimensional images of solid-liquid interfaces. This review provides an overview of the 3D-AFM operating principles and its underlying physics. We illustrate and explain the capability of the instrument to resolve atomic defects on crystalline surfaces immersed in liquid. We also illustrate some of its applications to imaging the hydration structures on DNA or proteins. In the last section, we discuss some perspectives on emerging applications in materials science and molecular biology.

Visualizing charges accumulated in an electric double layer by three-dimensional open-loop electric potential microscopy

Charges accumulated in an electric double layer (EDL) play key roles in various interfacial phenomena and electronic devices. However, direct imaging of their spatial distribution has been a great challenge, which has hindered our nano-level understanding of the mechanisms of such interfacial phenomena and functions. In this study, we present direct imaging of charges accumulated at an electrode-electrolyte interface using three-dimensional open-loop electric potential microscopy (3D-OL-EPM). Conventional OL-EPM allows us to visualize two-dimensional potential distributions in liquid yet the zero of the measured potential is not well defined due to the influence of the long-range (LR) interaction between the cantilever and the sample. Here, we present practical ways to reduce such an influence by improving the equation for the potential calculation and subtracting the LR contribution estimated from a Z potential profile. These improvements enabled the calibration of the measured potential values with respect to the bulk solution potential. With these improvements, we visualized opposite charge accumulation behaviors on a polarizable and non-polarizable electrode with a varying electrode potential. Combining OL-EPM with a 3D tip scanning method, we also performed a 3D-OL-EPM measurement on a Cu fine wire and visualized the nanoscale distribution of the charges accumulated at the interface. Such real-space information on the charge distributions in an EDL should provide valuable insights into the mechanisms of interfacial phenomena and functions that are important in various academic and industrial research on electronic devices, electrochemistry, tribology and life sciences.

Quantitative comparison of wideband low-latency phase-locked loop circuit designs for high-speed frequency modulation atomic force microscopy

A phase-locked loop (PLL) circuit is the central component of frequency modulation atomic force microscopy (FM-AFM). However, its response speed is often insufficient, and limits the FM-AFM imaging speed. To overcome this issue, we propose a PLL design that enables high-speed FM-AFM. We discuss the main problems with the conventional PLL design and their possible solutions. In the conventional design, a low-pass filter with relatively high latency is used in the phase feedback loop, leading to a slow response of the PLL. In the proposed design, a phase detector with a low-latency high-pass filter is located outside the phase feedback loop, while a subtraction-based phase comparator with negligible latency is located inside the loop. This design minimizes the latency within the phase feedback loop and significantly improves the PLL response speed. In addition, we implemented PLLs with the conventional and proposed designs in the same field programmable gate array chip and quantitatively compared their performances. The results demonstrate that the performance of the proposed PLL is superior to that of the conventional PLL: 165 kHz bandwidth and 3.2 μs latency in water. Using this setup, we performed FM-AFM imaging of calcite dissolution in water at 0.5 s/frame with true atomic resolution. The high-speed and high-resolution imaging capabilities of the proposed design will enable a wide range of studies to be conducted on various atomic-scale dynamic phenomena at solid-liquid interfaces.

Selective Removal of B800 Bacteriochlorophyll a from Light-Harvesting Complex 2 of the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The selective removal of B800 bacteriochlorophyll (BChl) a from light-harvesting complex 2 (LH2) in purple photosynthetic bacteria is a clue about elucidation of the mechanism for the transfer of energy from these pigments to B850 BChl a and their roles in the LH2 protein structure. We demonstrated that the kinetics of the removal of B800 BChl a from two representative LH2 proteins derived from Phaeospirillum molischianum and Rhodoblastus acidophilus differed significantly, in contrast to the calculated binding enthalpy. These results may be interpreted as changes in the local structure near B800 BChl a with respect to the geometries of the original crystal structures upon removal of B800 BChl a. Despite the difficulty of removing B800 BChl a from molischianum-LH2, we prepared the molischianum-LH2 protein lacking B800 BChl a by combination of two detergents, n-dodecyl β-d-maltoside and n-octyl β-d-glucoside, under acidic conditions. Spectral and atomic force microscopy analyses indicated that the absence of B800 BChl a had little effect on the local structure in the vicinity of B850 BChl a and the circular arrangement in this protein. These results suggest that the hydrophobic domain near B850 BChl a is rigid and plays a major role in the structural formation of molischianum-LH2.

Resolving Point Defects in the Hydration Structure of Calcite (10.4) with Three-Dimensional Atomic Force Microscopy

It seems natural to assume that defects at mineral surfaces critically influence interfacial processes such as the dissolution and growth of minerals in water. The experimental verification of this claim, however, is challenging and requires real-space methods with utmost spatial resolution, such as atomic force microscopy (AFM). While defects at mineral-water interfaces have been resolved in 2D AFM images before, the perturbation of the surrounding hydration structure has not yet been analyzed experimentally. In this Letter, we demonstrate that point defects on the most stable and naturally abundant calcite (10.4) surface can be resolved using high-resolution 3D AFM-even within the fifth hydration layer. Our analysis of the hydration structure surrounding the point defect shows a perturbation of the hydration with a lateral extent of approximately one unit cell. These experimental results are corroborated by molecular dynamics simulations.

KPFM/AFM imaging on TiO2(110) surface in O2 gas

We have carried out high-speed imaging of the topography and local contact potential difference (LCPD) on rutile TiO₂(110) in O₂ gas by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We succeeded in KPFM/AFM imaging with atomic resolution at 1 frame min^-1 and observed the adsorbate on a hydroxylated TiO₂(110) surface. The observed adsorbate is considered to be oxygen adatoms (O_a), hydroperoxyls (HO₂), or terminal hydroxyls (OH_t). After adsorption, changes in the topography and the LCPD of the adsorbate were observed. This phenomenon is thought to be caused by the charge transfer of the adsorbate. This technique has the potential to observe catalytic behavior with atomic resolution.

Direct comparison between subnanometer hydration structures on hydrophilic and hydrophobic surfaces via three-dimensional scanning force microscopy

Hydration layers on heterogeneous substrates are characterized with subnanometer resolution using three-dimensional scanning force microscopy.

Self-assembled monolayers of sulfonate-terminated alkanethiols investigated by frequency modulation atomic force microscopy in liquid

A molecular-scale understanding of self-assembled monolayers (SAMs) of sulfonate-terminated alkanethiols is crucial for interfacial studies of functionalized SAMs and their various applications. However, such an understanding has been difficult to achieve because of the lack of direct information on these molecular-scale structures in real space. In this study, we investigated the structures of sulfonate SAMs of sodium 11-mercapto-1-undecanesulfonate (MUS) by frequency modulation atomic force microscopy (FM-AFM) in liquid. The subnanometer-resolution FM-AFM images showed that the single-component MUS SAM prepared in pure water had random surface structures. In contrast, the MUS SAM prepared in a water-ethanol mixed solvent showed periodic striped structures with a flat-lying conformation. The results suggest a significant solvent effect on molecular-scale structures of long-chain sulfonate SAMs. In addition, we investigated the molecular-scale structures of mixed SAMs of MUS and 11-mercapto-1-undecanol (MUO) with alkane chains of the same length. The FM-AFM images of the mixed SAMs showed clear phase separation between MUS SAM and MUO SAM domains. In the MUO SAM domains, the incorporated MUS molecules appeared as protrusions. The results obtained in this study provide direct structural information on long-chain sulfonate and mixed SAMs.

Influence of ions on two-dimensional and three-dimensional atomic force microscopy at fluorite-water interfaces

Recent advancement in liquid-environment atomic force microscopy (AFM) has enabled us to visualize three-dimensional (3D) hydration structures as well as two-dimensional (2D) surface structures with subnanometer-scale resolution at solid-water interfaces. However, the influence of ions present in solution on the 2D- and 3D-AFM measurements has not been well understood. In this study, we perform atomic-scale 2D- and 3D-AFM measurements at fluorite-water interfaces in pure water and a supersaturated solution of fluorite. The images obtained in these two environments are compared to understand the influence of the ions in solution on these measurements. In the 2D images, we found clear difference in the nanoscale structures but no significant difference in the atomic-scale contrasts. However, the 3D force images show clear difference in the subnanometer-scale contrasts. The force contrasts measured in pure water largely agree with those expected from the molecular dynamics simulation and the solvent tip approximation model. In the supersaturated solution, an additional force peak is observed over the negatively charged fluorine ion site. This location suggests that the observed force peak may originate from cations adsorbed on the fluorite surface. These results demonstrate that the ions can significantly alter the subnanometer-scale force contrasts in the 3D-AFM images.

Radiation pressure excitation of a low temperature atomic force/magnetic force microscope for imaging in 4-300 K temperature range

We describe a novel radiation pressure based cantilever excitation method for imaging in dynamic mode atomic force microscopy (AFM) for the first time. Piezo-excitation is the most common method for cantilever excitation, however it may cause spurious resonance peaks. Therefore, the direct excitation of the cantilever plays a crucial role in AFM imaging. A fiber optic interferometer with a 1310 nm laser was used both for the excitation of the cantilever at the resonance and the deflection measurement of the cantilever in a commercial low temperature atomic force microscope/magnetic force microscope (AFM/MFM) from NanoMagnetics Instruments. The laser power was modulated at the cantilever's resonance frequency by a digital Phase Locked Loop (PLL). The laser beam is typically modulated by ∼500 μW, and ∼141.8 nm_pp oscillation amplitude is obtained in moderate vacuum levels between 4 and 300 K. We have demonstrated the performance of the radiation pressure excitation in AFM/MFM by imaging atomic steps in graphite, magnetic domains in CoPt multilayers between 4 and 300 K and Abrikosov vortex lattice in BSCCO(2212) single crystal at 4 K for the first time.

Understanding 2D atomic resolution imaging of the calcite surface in water by frequency modulation atomic force microscopy

Frequency modulation atomic force microscopy (FM-AFM) experiments were performed on the calcite (10[Formula: see text]4) surface in pure water, and a detailed analysis was made of the 2D images at a variety of frequency setpoints. We observed eight different contrast patterns that reproducibly appeared in different experiments and with different measurement parameters. We then performed systematic free energy calculations of the same system using atomistic molecular dynamics to obtain an effective force field for the tip-surface interaction. By using this force field in a virtual AFM simulation we found that each experimental contrast could be reproduced in our simulations by changing the setpoint, regardless of the experimental parameters. This approach offers a generic method for understanding the wide variety of contrast patterns seen on the calcite surface in water, and is generally applicable to AFM imaging in liquids.

Highly compact and easy-to-use optical chip interferometer with picometric performances

In this paper, we present a compact, inexpensive, and easy-to-use optical chip interferometer based on the telecom integrated waveguide technology. The measurement evaluation is focused on the resolution and the noise level of the sensor. The power spectral density of 100 fm Hz^-1/2 @ 10 kHz is reached in static conditions. The same level is obtained with the standard Allan deviation for both short and long term measurements. Dynamic performances are also evaluated with sub-nanometer measurements made with piezoelectric systems. The potential bandwidth of the sensor is very high and is currently only limited by electronics (250 kHz).

Development of low temperature atomic force microscopy with an optical beam deflection system capable of simultaneously detecting the lateral and vertical forces

The atomic force microscopy (AFM) is a very important tool for imaging and investigating the complex force interactions on sample surfaces with high spatial resolution. In the AFM, two types of detection systems of the tip-sample interaction forces have been used: an optical detection system and an electrical detection system. In optical detection systems, such as optical beam deflection system or optical fiber interferometer system, both the lateral and the vertical tip-sample forces can be measured simultaneously. In electrical detection systems, such as qPlus or Kolibri sensors, either the lateral or vertical forces can be measured. Simultaneous measurement of the lateral and vertical interaction forces effectively allows investigation of force interactions because the force is a vector with magnitude and direction. In this study, we developed a low-temperature, frequency-modulation AFM using an optical beam deflection system to simultaneously measure the vertical and lateral forces. In this system, the heat sources, such as a laser diode and a current-to-voltage converter, for measuring the photocurrent of the four-segmented photodiode are located outside the observation chamber to avoid a temperature increase of the AFM unit. The focused optical beam is three-dimensionally adjustable on the back side of the cantilever. We demonstrate low-noise displacement measurement of the cantilever and successful atomic resolution imaging using the vertical and lateral forces at low temperatures.

Number density distribution of solvent molecules on a substrate: a transform theory for atomic force microscopy

Atomic force microscopy (AFM) in liquids can measure a force curve between a probe and a buried substrate. The shape of the measured force curve is related to hydration structure on the substrate. However, until now, there has been no practical theory that can transform the force curve into the hydration structure, because treatment of the liquid confined between the probe and the substrate is a difficult problem. Here, we propose a robust and practical transform theory, which can generate the number density distribution of solvent molecules on a substrate from the force curve. As an example, we analyzed a force curve measured by using our high-resolution AFM with a newly fabricated ultrashort cantilever. It is demonstrated that the hydration structure on muscovite mica (001) surface can be reproduced from the force curve by using the transform theory. The transform theory will enhance AFM's ability and support structural analyses of solid/liquid interfaces. By using the transform theory, the effective diameter of a real probe apex is also obtained. This result will be important for designing a model probe of molecular scale simulations.

A relationship between three-dimensional surface hydration structures and force distribution measured by atomic force microscopy

Hydration plays important roles in various solid-liquid interfacial phenomena. Very recently, three-dimensional scanning force microscopy (3D-SFM) has been proposed as a tool to visualise solvated surfaces and their hydration structures with lateral and vertical (sub) molecular resolution. However, the relationship between the 3D force map obtained and the equilibrium water density, ρ(r), distribution above the surface remains an open question. Here, we investigate this relationship at an interface of an inorganic mineral, fluorite, and water. The force maps measured in pure water are directly compared to force maps generated using the solvent tip approximation (STA) model and from explicit molecular dynamics simulations. The results show that the simulated STA force map describes the major features of the experimentally obtained force image. The agreement between the STA data and the experiment establishes the correspondence between the water density used as an input to the STA model and the experimental hydration structure and thus provides a tool to bridge the experimental force data and atomistic solvation structures. Further applications of this method should improve the accuracy and reliability of both interpretation of 3D-SFM force maps and atomistic simulations in a wide range of solid-liquid interfacial phenomena.

Efficiency improvement in the cantilever photothermal excitation method using a photothermal conversion layer

Photothermal excitation is a cantilever excitation method that enables stable and accurate operation for dynamic-mode AFM measurements. However, the low excitation efficiency of the method has often limited its application in practical studies. In this study, we propose a method for improving the photothermal excitation efficiency by coating cantilever backside surface near its fixed end with colloidal graphite as a photothermal conversion (PTC) layer. The excitation efficiency for a standard cantilever of PPP-NCHAuD with a spring constant of ≈40 N/m and a relatively stiff cantilever of AC55 with a spring constant of ≈140 N/m were improved by 6.1 times and 2.5 times, respectively, by coating with a PTC layer. We experimentally demonstrate high stability of the PTC layer in liquid by AFM imaging of a mica surface with atomic resolution in phosphate buffer saline solution for more than 2 h without any indication of possible contamination from the coating. The proposed method, using a PTC layer made of colloidal graphite, greatly enhances photothermal excitation efficiency even for a relatively stiff cantilever in liquid.

Visualizing Nanoscale Distribution of Corrosion Cells by Open-Loop Electric Potential Microscopy

Corrosion is a traditional problem but still one of the most serious problems in industry. To reduce the huge economic loss caused by corrosion, tremendous effort has been made to understand, predict and prevent it. Corrosion phenomena are generally explained by the formation of corrosion cells at a metal-electrolyte interface. However, experimental verification of their nanoscale distribution has been a major challenge owing to the lack of a method able to visualize the local potential distribution in an electrolytic solution. In this study, we have investigated the nanoscale corrosion behavior of Cu fine wires and a duplex stainless steel by in situ imaging of local corrosion cells by open-loop electric potential microscopy (OL-EPM). For both materials, potential images obtained by OL-EPM show nanoscale contrasts, where areas of higher and lower potential correspond to anodic areas (i.e., corrosion sites) and cathodic areas, respectively. This imaging capability allows us to investigate the real-time transition of local corrosion sites even when surface structures show little change. This is particularly useful for investigating reactions under surface oxide layers or highly corrosion-resistant materials as demonstrated here. The proposed technique should be applicable to the study of other redox reactions on a battery electrode or a catalytic material. The results presented here open up such future applications of OL-EPM in nanoscale electrochemistry.

An ultra-low noise optical head for liquid environment atomic force microscopy

The design considerations and eventual performance of a new, ultra-low noise optical head for dynamic atomic force microscopy (AFM) are presented. The head, designed specifically for the study of hydration layers and ion organization next to solid surfaces and biomolecules, displays an integrated tip-sample distance noise below 3 pm. The sensitivity of the optical beam deflection sensor, operating at frequencies up to 8.6 MHz (3 dB roll-off), is typically below 10 fm/√Hz, enabling utilization of high frequency cantilevers of low thermal noise for fundamental and higher mode imaging. Exceptional signal stability and low optical noise are achieved by replacing the commonly used laser diode with a helium-neon laser. An integral photothermal excitation of the cantilever produces pure harmonic oscillations, minimizing the generation of higher cantilever modes and deleterious sound waves characterizing the commonly used excitation by a piezoelectric crystal. The optical head is designed to fit on top of the widespread Multimode(®) (Bruker) piezo-tube and accommodate its commercial liquid cell. The performance of the new AFM head is demonstrated by atomic resolution imaging of a muscovite mica surface in aqueous solution.

Fabrication of electron beam deposited tip for atomic-scale atomic force microscopy in liquid

Recently, possibilities of improving operation speed and force sensitivity in atomic-scale atomic force microscopy (AFM) in liquid using a small cantilever with an electron beam deposited (EBD) tip have been intensively explored. However, the structure and properties of an EBD tip suitable for such an application have not been well-understood and hence its fabrication process has not been established. In this study, we perform atomic-scale AFM measurements with a small cantilever and clarify two major problems: contaminations from a cantilever and tip surface, and insufficient mechanical strength of an EBD tip having a high aspect ratio. To solve these problems, here we propose a fabrication process of an EBD tip, where we attach a 2 μm silica bead at the cantilever end and fabricate a 500-700 nm EBD tip on the bead. The bead height ensures sufficient cantilever-sample distance and enables to suppress long-range interaction between them even with a short EBD tip having high mechanical strength. After the tip fabrication, we coat the whole cantilever and tip surface with Si (30 nm) to prevent the generation of contamination. We perform atomic-scale AFM imaging and hydration force measurements at a mica-water interface using the fabricated tip and demonstrate its applicability to such an atomic-scale application. With a repeated use of the proposed process, we can reuse a small cantilever for atomic-scale measurements for several times. Therefore, the proposed method solves the two major problems and enables the practical use of a small cantilever in atomic-scale studies on various solid-liquid interfacial phenomena.

Improving image contrast and material discrimination with nonlinear response in bimodal atomic force microscopy

Atomic force microscopy has recently been extented to bimodal operation, where increased image contrast is achieved through excitation and measurement of two cantilever eigenmodes. This enhanced material contrast is advantageous in analysis of complex heterogeneous materials with phase separation on the micro or nanometre scale. Here we show that much greater image contrast results from analysis of nonlinear response to the bimodal drive, at harmonics and mixing frequencies. The amplitude and phase of up to 17 frequencies are simultaneously measured in a single scan. Using a machine-learning algorithm we demonstrate almost threefold improvement in the ability to separate material components of a polymer blend when including this nonlinear response. Beyond the statistical analysis performed here, analysis of nonlinear response could be used to obtain quantitative material properties at high speeds and with enhanced resolution.

Bimodal atomic force microscopy is a promising approach in obtaining high-quality image contrast and material property mapping. Here, the authors show that by considering nonlinear response in bimodal atomic force microscopy, significant improvements in image contrast and material discrimination can be achieved.

High-frequency multimodal atomic force microscopy

Multifrequency atomic force microscopy imaging has been recently demonstrated as a powerful technique for quickly obtaining information about the mechanical properties of a sample. Combining this development with recent gains in imaging speed through small cantilevers holds the promise of a convenient, high-speed method for obtaining nanoscale topography as well as mechanical properties. Nevertheless, instrument bandwidth limitations on cantilever excitation and readout have restricted the ability of multifrequency techniques to fully benefit from small cantilevers. We present an approach for cantilever excitation and deflection readout with a bandwidth of 20 MHz, enabling multifrequency techniques extended beyond 2 MHz for obtaining materials contrast in liquid and air, as well as soft imaging of delicate biological samples.

Note: High-speed Z tip scanner with screw cantilever holding mechanism for atomic-resolution atomic force microscopy in liquid

High-speed atomic force microscopy has attracted much attention due to its unique capability of visualizing nanoscale dynamic processes at a solid/liquid interface. However, its usability and resolution have yet to be improved. As one of the solutions for this issue, here we present a design of a high-speed Z-tip scanner with screw holding mechanism. We perform detailed comparison between designs with different actuator size and screw arrangement by finite element analysis. Based on the design giving the best performance, we have developed a Z tip scanner and measured its performance. The measured frequency response of the scanner shows a flat response up to ∼10 kHz. This high frequency response allows us to achieve wideband tip-sample distance regulation. We demonstrate the applicability of the scanner to high-speed atomic-resolution imaging by visualizing atomic-scale calcite crystal dissolution process in water at 2 s/frame.

Significant improvements in stability and reproducibility of atomic-scale atomic force microscopy in liquid

Recent advancement of dynamic-mode atomic force microscopy (AFM) for liquid-environment applications enabled atomic-scale studies on various interfacial phenomena. However, instabilities and poor reproducibility of the measurements often prevent systematic studies. To solve this problem, we have investigated the effect of various tip treatment methods for atomic-scale imaging and force measurements in liquid. The tested methods include Si coating, Ar plasma, Ar sputtering and UV/O₃ cleaning. We found that all the methods provide significant improvements in both the imaging and force measurements in spite of the tip transfer through the air. Among the methods, we found that the Si coating provides the best stability and reproducibility in the measurements. To understand the origin of the fouling resistance of the cleaned tip surface and the difference between the cleaning methods, we have investigated the tip surface properties by x-ray photoelectron spectroscopy and contact angle measurements. The results show that the contaminations adsorbed on the tip during the tip transfer through the air should desorb from the surface when it is immersed in aqueous solution due to the enhanced hydrophilicity by the tip treatments. The tip surface prepared by the Si coating is oxidized when it is immersed in aqueous solution. This creates local spots where stable hydration structures are formed. For the other methods, there is no active mechanism to create such local hydration sites. Thus, the hydration structure formed under the tip apex is not necessarily stable. These results reveal the desirable tip properties for atomic-scale AFM measurements in liquid, which should serve as a guideline for further improvements of the tip treatment methods.