Atomic Force Microscopy Force Research Articles

Exploring Nanomechanical Properties of Soot Particle Layers by Atomic Force Microscopy Nanoindentation

In this work, an experimental investigation of the nanomechanical properties of flame-formed carbonaceous particle layers has been performed for the first time by means of Atomic Force Microscopy (AFM). To this aim, carbon nanoparticles with different properties and nanostructures were produced in ethylene/air laminar premixed flames at different residence times. Particles were collected on mica substrates by means of a thermophoretic sampling system and then analyzed by AFM. An experimental procedure based on the combination between semi-contact AFM topography imaging, contact AFM topography imaging and AFM force spectroscopy has been implemented. More specifically, a preliminary topological characterization of the samples was first performed operating AFM in semi-contact mode and then tip-sample interaction forces were measured in contact spectroscopy mode. Finally, semi-contact mode was used to image the indented surface of the samples and to retrieve the projected area of indents. The hardness of investigated samples was obtained from the force–distance curves measured in spectroscopy mode and the images of intends acquired in semi-contact mode. Moreover, the Young’s modulus was measured by fitting the linear part of the retraction force curves using a model based on the Hertz theory. The extreme force sensitivity of this technique (down to nNewton) in addition to the small size of the probe makes it extremely suitable for performing investigation of mechanical properties of materials at the nanoscale. The experimental procedure was successfully tested on reference materials characterized by different plastic behavior, e.g., polyethylene naphthalate and highly oriented pyrolytic graphite. Both hardness and Young’s modulus values obtained from AFM measurements for different soot particle films were discussed.

Evidence for new enantiospecific interaction force in chiral biomolecules

Evidence for new enantiospecific interaction force in chiral biomolecules

An Investigation of the Adsorption of Xanthate on Bornite in Aqueous Solutions Using an Atomic Force Microscope

An atomic force microscope (AFM) was applied to study of the adsorption of xanthate on bornite surfaces in situ in aqueous solutions. AFM images showed that xanthate, i.e., potassium ethyl xanthate (KEX) and potassium amyl xanthate (PAX), adsorbed strongly on bornite, and the adsorbate bound strongly with the mineral surface without being removed by flushing with ethanol alcohol. The AFM images also showed that the adsorption increased with the increased collector concentration and contact time. Xanthate adsorbed on bornite in a similar manner when the solution pH changed to pH 10. The AFM force measurement results showed that the probe–substrate adhesion increased due to the adsorption of xanthate on bornite. The sharp “jump-in” and “jump-off” points on force curve suggest that the adsorbate is not “soft” in nature, ruling out the existence of dixanthogen, an oily substance. Finally, the ATR-FTIR (attenuated total reflection-Fourier-transform infrared) result confirms that the adsorbate on bornite in xanthate solutions is mainly in the form of insoluble cuprous xanthate (CuX) instead of dixanthogen. This xanthate/bornite adsorption mechanism is very similar to what is obtained with the xanthate/chalcocite system, while it is different from the xanthate/chalcopyrite system, for which oily dixanthogen is the main adsorption product on the chalcopyrite surface. The present study helps clarify the flotation mechanism of bornite in industry practice using xanthate as a collector.

Understanding the hetero-aggregation mechanism among sulfide and oxide mineral particles driven by bifunctional surfactants: Intensification flotation of oxide minerals

To promote the separation and enrichment efficiency of oxide minerals from the sulfide-oxide ores through froth flotation has become a challenging issue. In this paper, the driving role of S-[(2-hydroxyamino)-2-oxoethyl]-N,N-dibutyl-dithiocarbamate (HABTC) in facilitating the aggregation of copper sulfide and oxide minerals particles was explored through atomic force microscope (AFM) force measurements and the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory calculation. AFM imaging, contact angle measurement and surface energy computation showed that the self-assembly of HABTC on chalcopyrite and malachite improved the hydrophobicity of their surfaces and reduced their interaction free energies. AFM force measurements observed that the “jump in” attachment occurred at 8.6 ~ 12.5 nm during the approaching chalcopyrite towards malachite in HABTC solution, which was mainly attributed to the hydrophobic interaction between them. And the adhesion force between them in HABTC solution was much larger than that in OHA solution, being contributed to HABTC’s bridging role driven by its uncoordinated dithiocarbamate or hydroxamate groups to bond with the surface copper atoms of different minerals. The two-step hetero-aggregation among chalcopyrite and malachite particles sequentially-induced by HABTC’s nonpolar and bipolar groups built a carrier flotation platform where the floatability of malachite particles was significantly promoted by chalcopyrite particles with a superior hydrophobicity, resulting in an improved flotation recovery of malachite.

AFM Force Relaxation Curve Reveals That the Decrease of Membrane Tension Is the Essential Reason for the Softening of Cancer Cells.

Differences in stiffness constitute an extremely important aspect of the mechanical differences between cancer cells and normal cells, and atomic force microscopy (AFM) is the most commonly used tool to characterize the difference in stiffness. However, the process of mechanical characterization using AFM has been controversial and the influence of the membrane tension on AFM measurement results was often ignored. Here, a physical model involving a simultaneous consideration of the effects of the cell membrane, cytoskeleton network and cytosol was proposed. We carried out a theoretical analysis of AFM force relaxation curves, and as a result solved many of the remaining controversial issues regarding AFM-based mechanical characterization of cells, and provided a quantitative solution for the membrane tension measured using AFM indentation experiments for the first time. From the results of experiments on cells with different adherent shapes and different pairs of normal cells and cancer cells, we found additional force provided by membrane tension to be the main component of the force applied to the AFM probe, with decreased cell membrane tension being the essential reason for the greater softness of cancer cells than of normal cells. Hence, regulating membrane tension may become an important method for regulating the behavior of cancer cells.

In situ AFM detection of the stiffness of the in situ exposed cell nucleus

Biomechanical properties of the cell nucleus play critical roles in cell behaviors and functions. As one important biomechanical property, the stiffness (or Young's modulus) of the cell nucleus has been widely investigated by different techniques including atomic force microscopy (AFM). In most of previous studies, the stiffness of the nuclear region of an intact cell or the stiffness of the isolated nucleus was detected. In this study, we developed a strategy for in situ detecting the stiffness of the cell nucleus via AFM. The extranuclear components of adherent cells (endothelial cells) were in situ removed by Triton X-100 treatment and the bare, adherent nuclei were exposed for in situ AFM force measurement. We found that the nuclear regions of intact cells (5.59 ± 1.55 kPa) had a relatively higher average Young's modulus than the nonnuclear regions (1.47 ± 0.77 kPa) and that the in situ exposed nuclei (22.06 ± 7.29 kPa) were much stiffer than the nuclear regions of intact cells. This strategy is very simple and effective for detecting the stiffness of the cell nucleus and potentially is promising for a wide application.

Bulk chemical composition contrast from attractive forces in AFM force spectroscopy.

A key application of atomic force microscopy (AFM) is the measurement of physical properties at sub-micrometer resolution. Methods such as force–distance curves (FDCs) or dynamic variants (such as intermodulation AFM (ImAFM)) are able to measure mechanical properties (such as the local stiffness, kr) of nanoscopic heterogeneous materials. For a complete structure–property correlation, these mechanical measurements are considered to lack the ability to identify the chemical structure of the materials. In this study, the measured attractive force, Fattr, acting between the AFM tip and the sample is shown to be an independent measurement for the local chemical composition and hence a complete structure–property correlation can be obtained. A proof of concept is provided by two model samples comprised of (1) epoxy/polycarbonate and (2) epoxy/boehmite. The preparation of the model samples allowed for the assignment of material phases based on AFM topography. Additional chemical characterization on the nanoscale is performed by an AFM/infrared-spectroscopy hybrid method. Mechanical properties (kr) and attractive forces (Fattr) are calculated and a structure–property correlation is obtained by a manual principle component analysis (mPCA) from a kr/Fattr diagram. A third sample comprised of (3) epoxy/polycarbonate/boehmite is measured by ImAFM. The measurement of a 2 × 2 µm cross section yields 128 × 128 force curves which are successfully evaluated by a kr/Fattr diagram and the nanoscopic heterogeneity of the sample is determined.

A novel PRC signal drift reduction method for new developed SEM-based nanoindentation/nanoscratch instrument integrated with AFM

In-situ SEM nanoindentation and nanoscratch testing methods are commonly used for mechanical characterization and investigation of the deformation and failure mechanisms of coating materials with micro-to nano-scale thicknesses. However, existing SEM-based integrated nanoindentation and nanoscratch instruments have two main limitations. First, the measured mechanical properties of the coating materials at micro-to nano-scale thicknesses are highly sensitive to surface roughness. Second, the existing SEM-based instruments lack the capability to acquire the morphology of residual imprints in real-time after nanoindentation and nanoscratching. In this study, a novel SEM-based integrated nanoindentation, nanoscratch, and atomic force microscopy (AFM) instrument, namely, NMT-AFM was proposed, developed and fabricated. The self-sensing piezoresistive cantilever (PRC) was selected as the AFM force sensor owing to its miniaturization ability. However, the resistance of the PRC sensor fluctuated because of the electron irradiation from SEM, resulting in the continuous drift of the PRC signal during SEM imaging. To overcome this limitation, a mechanism of PRC signal drift inside SEM was analyzed for the first time, and a PRC signal drift reduction method was proposed based on the mechanism analysis. The experimental results indicated that the PRC signal drift was reduced to 2 nm in 2 min by applied external voltage value UA of 30 V to modified PRC, which proved the proposed mechanism of PRC signal drift during SEM imaging. Finally, the X–Y fine nanopositioner angle calibration test using AFM calibration chip VGRP-UM and the nanoindentation/nanoscratch characterizations of the TiAlSiN coating material were conducted.

Effect of quercetin on lipid membrane rigidity: assessment by atomic force microscopy and molecular dynamics simulations

Quercetin (3,3ʹ,4ʹ,5,7-pentahydroxyl-flavone) is a natural flavonoid with many valuable biological effects, but its solubility in water is low, posing major limitations in applications. Quercetin encapsulation in liposomes increases its bioavailability; the drug effect on liposome elastic properties is required for formulation development. Here, we quantify the effect of quercetin molecules on the rigidity of lipoid E80 liposomes using atomic force microscopy (AFM) and molecular dynamics (MD) simulations. AFM images show no effect of quercetin molecules on liposomes morphology and structure. However, AFM force curves suggest that quercetin softens lipid membranes; the Young modulus measured for liposomes encapsulating quercetin is smaller than that determined for blank liposomes. We then used MD simulations to interpret the effect of quercetin on membrane rigidity in terms of molecular interactions. The decrease in membrane rigidity was confirmed by the simulations, which also revealed that quercetin affects structural and dynamic properties: membrane thickness is decreased, acyl chains disorder is increased, and diffusion coefficients of lipid molecules are also increased. Such changes appear to be related to the preferential localization of quercetin within the membrane, near the interface between the hydrophobic core and polar head groups of the lipids.

AFM force spectroscopy as a powerful tool to address material design for biomedical applications. A review

Force Spectroscopy (FS), which is one of the operating modes of Atomic Force Microscope (AFM) technique proven to be useful in many biological and medical applications, such as cancer cells recognition. Currently, many scientific institutions carry on research on the Young’s modulus of individual cancer cells in order to detect the disease at an early stage of its development. As a result of the growing interest in the use of force spectroscopy to study cells’ mechanic, this review summarizes new applications of this method to study changes in the physical and chemical properties of cells under the influence of external stimuli of different origins. The work is divided into four research areas, in which the use of AFM force spectroscopy was used to explain phenomena occurring at the early stages of intracellular organization changes. Research areas presented in this manuscript focuses on detailed description of the effect of manifold external stimuli on cells, such as: (i) cell aging, (ii) active ingredients used in the cosmetics industry to improve skin condition, (iii) nanoparticles used in biomedicine, and (iv) micro- and nano-structures of topography on the surface of substrates used for cell cultures. This review is based on a critical analysis of the latest literature reports (seven of which were created with Author’s contribution) describing the use of force spectroscopy as an effective tool to study the mechanical properties of living cells.

Impacts of Smart Waters on Calcite–Crude Oil Interactions Quantified by “Soft Tip” Atomic Force Microscopy (AFM) and Surface Complexation Modeling (SCM)

The interactions of calcite surface with crude oil are influenced by the ionic chemistry of the injection water in “smart water” flooding; however, the molecular evidence of their influences is missing in the current studies. To address this issue, we measured the calcite–oil interactions in four kinds of smart waters by driving a droplet of crude oil (“soft tip”) toward and away from the calcite surface using atomic force microscopy (AFM). The force behaviors showed that the calcite–oil interactions were repulsive in smart waters containing low concentrations of Ca2+ ions, such as seawater (SW) and seawater with one-fourth of Ca2+ concentration (SWCa), which have produced extremely weak adhesions and works of adhesion in these two brines. In contrast, the force curves showed “jump-in” behavior in seawater with four times increased SO42– concentration (SWSO) or Mg2+ concentration decreased to one-fourth (SWMg) compared to SW, which has produced extremely large adhesions and works of adhesion. The underlying reasons for calcite–oil interactions in response to different smart waters were investigated by the method of surface complexation modeling (SCM), which combined a charge-distribution multisite ion complexation (CD-MUSIC) model and a basic Stern model (BSM) to describe the complex electrochemical reactions of calcite, smart water, and crude oil. The adhesion energies were calculated based on the surface complexations, which showed good agreements with the AFM force results. The SCM results suggested that the weak calcite–oil interactions in SW and SWCa solutions can be attributed to the mitigation of cation (Ca2+) bridging interactions owing to a deficiency in surface Ca2+ ions. The strong calcite–oil interactions in SWSO and SWMg solutions are caused by a dramatic increase of cation (Ca2+) bridging interactions due to an excess of surface Ca2+ ions. Overall, the attractions of calcite surface with crude oil follow the order SWCa < SW ≪ SWMg < SWSO, which infers that the oil molecules may be easily displaced from the calcite surface with smart waters containing low concentrations of Ca2+ and SO42– ions but high concentrations of Mg2+ ions.

Integrated Ca2+ flux and AFM force analysis in human iPSC-derived cardiomyocytes.

We developed a new approach for combined analysis of calcium (Ca2+) handling and beating forces in contractile cardiomyocytes. We employed human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from dilated cardiomyopathy (DCM) patients carrying an inherited mutation in the sarcomeric protein troponin T (TnT), and isogenic TnT-KO iPSC-CMs generated via CRISPR/Cas9 gene editing. In these cells, Ca2+ handling as well as beating forces and-rates using single-cell atomic force microscopy (AFM) were assessed. We report impaired Ca2+ handling and reduced contractile force in DCM iPSC-CMs compared to healthy WT controls. TnT-KO iPSC-CMs display no contractile force or Ca2+ transients but generate Ca2+ sparks. We apply our analysis strategy to Ca2+ traces and AFM deflection recordings to reveal maximum rising rate, decay time, and duration of contraction with a multi-step background correction. Our method provides adaptive computing of signal peaks for different Ca2+ flux or force levels in iPSC-CMs, as well as analysis of Ca2+ sparks. Moreover, we report long-term measurements of contractile force dynamics on human iPSC-CMs. This approach enables deeper and more accurate profiling of disease-specific differences in cardiomyocyte contraction profiles using patient-derived iPSC-CMs.

Cone-Paraboloid Transition of the Johnson-Kendall-Roberts-Type Hyperboloidal Contact.

Atomic force microscopy (AFM)-based nanoindentation technique has been widely used to investigate the mechanical properties of compliant specimens. When a sharp probe is indented into a soft and adhesive specimen, not only the rounded end of the probe but also the pyramidal base may be in contact. However, even in such a case, a contact model that assumes a paraboloidal tip geometry (the Hertz model or one of its expansions) is mainly employed to derive the mechanical properties; the error on the mechanical properties induced by the inaccurate tip geometry assumption has not been systematically clarified. Therefore, the focus of this work was put on quantifying this error with the assumption that the actual contact occurs between a hyperboloidal indenter and an elastic and adhesive sample surface. We demonstrated that the cone-paraboloid transition of the indentation curve is governed by a single parameter, A̅ = [4RE/3πw(1 - ν2)]1/3cotα, where E and ν are Young's modulus and Poisson's ratio of the specimen, respectively, R and α are the curvature radius and half-angle of the indenter, respectively, and w is the work of adhesion. Employing the general two-point method, we quantified the errors on elasticity and surface energy caused by the assumption of the paraboloidal and conical Johnson-Kendall-Roberts (JKR) models as functions of A̅ and the normalized load. AFM force measurements with cantilevers of different radii supported this A̅ dependency. These results showed unsuitable geometry assumption can give a large error, which is generally more serious than those caused by inappropriate choice of the adhesive interaction from the JKR and DMT (Derjaguin-Muller-Toporov). It can be said that the conical model gives a good approximation to a hyperboloidal contact when A̅<1 and so does the paraboloidal model when A̅>4. A̅ is expected to be an important index that validates the paraboloidal and conical approximation in a soft and adhesive contact.

Mechanical Characterization of Liposomes and Extracellular Vesicles, a Protocol.

Both natural as well as artificial vesicles are of tremendous interest in biology and nanomedicine. Small vesicles (<200 nm) perform essential functions in cell biology and artificial vesicles (liposomes) are used as drug delivery vehicles. Atomic Force Microscopy (AFM) is a powerful technique to study the structural properties of these vesicles. AFM is a well-established technique for imaging at nanometer resolution and for mechanical measurements under physiological conditions. Here, we describe the procedure of AFM imaging and force spectroscopy on small vesicles. We discuss how to image vesicles with minimal structural disturbance, and how to analyze the data for accurate size and shape measurements. In addition, we describe the procedure for performing nanoindentations on vesicles and the subsequent data analysis including mechanical models used for data interpretation.

Probing the Interaction Mechanism between Benzohydroxamic Acid and Mineral Surface in the Presence of Pb2+ Ions by AFM Force Measurements and First-Principles Calculations.

Probing the interaction mechanism between organic molecules and material surfaces in the presence of metal ions is of great importance in many fields, such as mineral flotation. The collectability of benzohydroxamic acid (BHA) to a spodumene (LiAl(SiO3)2) mineral surface during mineral flotation could be enhanced with the addition of metal ion activators-Pb2+ ions. Pb2+ ions could be added as either Pb-BHA complex formed by premixing Pb2+ ions and BHA molecules at a given ratio or sequential addition of Pb2+ ions and BHA molecules. However, the complete understanding of the interaction mechanisms (e.g., adhesion) between BHA and the spodumene mineral surface in the presence of Pb2+ ions remains very limited. In this study, atomic force microscopy (AFM) was used to measure the intermolecular forces between BHA and the spodumene mineral surface in aqueous solutions. A BHA model molecule, that is, N-hydroxy-4-mercaptobenzamide (MBHA), was synthesized to prepare a BHA-functionalized AFM probe for force measurements. Two model systems (i.e., a Pb-BHA complex interacting with the spodumene mineral surface (model I) and BHA with a Pb2+-activated spodumene surface (model II)) were investigated for comparing the role of Pb2+ in BHA-mineral adhesion. The adhesion measured for model I (23.7 mN/m) is much higher than that of model II (12.5 mN/m), as further supported by the adsorption energies obtained from density functional theory (DFT) calculations. The calculation results showed a higher adsorption energy for model I (∼188.58 kJ/mol) than model II (∼128.16 kJ/mol), which is due to the better spodumene flotation recovery for the Pb-BHA complex as a collector than the sequential addition of Pb2+ and BHA. This work provides useful information on the intermolecular interactions between chemical additives and mineral surfaces in complex mineral flotation processes, and the methodology can be readily extended to other related interfacial processes such as membrane technology, water treatment, oil production, and bioengineering processes.

Multiple-fibre interferometry setup for probe sample interaction measurements in atomic force microscopy

Atomic force microscopy (AFM) often relies on the assumption that cantilever bending can be described by simple beam theory and that the displacement of the tip can be evaluated from the cantilever angle. Some more advanced metrological instruments use free-space or fibre interferometers for measuring the position of the cantilever apex directly, thereby simplifying the metrology traceability chain. The next logical development, covering measurements of both the cantilever apex position and its deformation due to lateral forces acting during different AFM measurement regimes, is presented in this paper. It is based on using a set of closely packed fibre interferometers that can be used to determine localised bending of the cantilever at different positions along the cantilever. This can be used for detection of cantilever deformation beyond classical beam theory, and can yield both better understanding of sources of uncertainty in individual AFM force–distance measurements and more accurate scanning in constant height mode in high-speed AFM applications.

Non-contact microfluidic mechanical property measurements of single apoptotic bodies

BackgroundCells exchange information by secreting micro­ and nanosized extracellular vesicles (EVs), ranging from exosomes (30–100 nm) to apoptotic bodies (ABs, 1–5 μm). There is still much to understand about fundamental EV biological, physical, and chemical properties before clinical applications can be developed. EV mechanical properties have only been measured with atomic force microscopy (AFM) with its problematic adhesion and hard substrate effects. To understand EV mechanical behavior in less extreme mechanical conditions relevant to blood flow and many soft tissue environments, a non-contact measurement technique is needed. MethodsWe measured the mechanical properties of single microscale ABs derived from human blood plasma using non-contact microfluidics. EVs were gently stretched in extensional flow, similar to a traditional tensile test, and a linear mechanical model was applied to estimate mechanical stiffnesses from the observed stretching. ResultsThe effective shear elastic modulus of ABs in non-contact flow conditions is approximately 5.6 ± 0.5 Pa, 7 orders of magnitude lower than previously reported AFM-measured biological exosome stiffnesses and 200 times smaller than suspended cells. ConclusionsApoptotic bodies are very soft in fluid environments and exhibit lower effective stiffnesses than suspended cells. By measuring ABs in a natural fluid environment and low-force regime without hard probes and surfaces, we achieved closer agreement with linear mechanical theory and therefore more accurate stiffness measurements. General significanceAFM manufacturers and users should consider implementing new mechanical models to interpret AFM force indentation curves so that accurate extracellular vesicle mechanical properties can be extracted.

In situ multimodal imaging for nanoscale visualization of tribofilm formation

A third of the energy from fuel combustion in passenger car gasoline engines is lost due to friction. Carefully designed engine lubricants can recover some of these losses by reducing friction and wear by forming a nanometer-scale chemico-physico tribofilm between surfaces. Accordingly, attention has focused on developing oil formulations that form low-friction tribofilms. However, analyses of resultant tribofilms are typically conducted after tribo-tests with conventional characterization tools and do not offer insights into tribofilm formation and evolution, precluding information critical to tuning tribofilm properties. In this work, we developed a unique multimodal methodology based on Atomic Force Microscopy (AFM) with local probe heating for in situ tribological studies that activates friction modifiers and simultaneously captures the evolution of friction and surface roughness, with nanometer resolution. As a platform to demonstrate the ability of this methodology to visualize dynamics of tribofilm formation in situ, we apply it to molybdenum-based friction modifiers to distinguish key factors in their functionality and correlate nanoscale AFM and Friction Force Microscopy data to bench tribo-tests used in the industry. To decode the formation mechanisms observed in situ and underlying chemistry of tribofilms, we performed ab initio Molecular Dynamics (AIMD) simulations at comparable conditions. AIMD simulations confirmed both nanoscale and bench tribo-test results and showed deviations in molecular organization in tribofilms that are formed from different molybdenum-based friction modifiers that corroborates with surface functionality. With this innovative methodology, we demonstrate proof-of-principle in situ formation of molybdenum-based tribofilms directly on steel surfaces that could be applied generally to studying tribofilm formation.

Surfaces and Interfaces of Liquid Metal Core–Shell Nanoparticles under the Microscope

Eutectic gallium indium (EGaIn), a Ga-based liquid metal alloy holds great promise for designing next generation core-shell nanoparticles (CSNs). A shearing assisted ligand-stabilization method has shown promise as a synthetic method for these CSNs; however, determining the role of the ligand on stabilization demands an understanding of the surface chemistry of the ligand-nanoparticle interface. EGaIn CSNs have been created functionalized with aliphatic carboxylates of different chain length allowing a fundamental investigation on ligand stabilization of EGaIn CSNs. Raman and diffuse reflectance Fourier transform spectroscopies (DRIFTS) confirm reaction of the ligand with the oxide shell of the EGaIn nanoparticles. Changing the length of the alkyl chain in the aliphatic carboxylates (C2-C18) may influence the size and structural stability of EGaIn CSNs, which is easily monitored using atomic force microscopy (AFM). No matter how large the carboxylate ligand, there is no obvious effect on the size of the EGaIn CSNs, except the particle size got more uniform when coated with longer chain carboxylates. The AFM force distance (F-D) measurements are used to measure the stiffness of the carboxylate coated EGaIn CSN. In corroboration with DRIFTS analysis, the stiffness studies show that the alkyl chains undergo conformational changes upon compression.

In Situ High-Resolution AFM Imaging and Force Probing of Cell Culture Medium-Forming Nanogranular Surfaces for Cell Growth.

Utilizing cell culture medium to grow cells in vitro has been widely studied in the past decades and has been recognized as an acknowledged way for investigating cell activities. However, due to the lack of adequate observation tools, the detailed mechanisms regulating cell growth in cell culture medium are still not fully understood. In this work, atomic force microscopy (AFM), a powerful tool for observing native biological systems under near-physiological conditions with high resolution, was applied to reveal the nanogranular surfaces formed in cell culture medium in situ for promoting cell growth. First, AFM imaging of glass slides (glass slides were previously incubated in cell culture medium) in aqueous environment clearly visualized the cell culture medium-forming nanogranular surfaces on glass slides. By altering the incubation time of glass slides in cell culture medium, the dynamic formation of nanogranular surfaces was remarkably observed. Next, fluorescent labeling experiments of the cell culture medium-treated glass slides showed that bovine serum proteins were contained in the nanogranular surfaces. Further, the adhesive interactions between cells and nanogranular surfaces probed by AFM force spectroscopy and the cell growth experiments showed that cell culture medium-forming nanogranular surfaces promote cell attachment and growth. The study provides novel insights into nanotopography-regulated molecular mechanisms in cell growth and demonstrates the outstanding capabilities of AFM in addressing biological issues with unprecedented spatial resolution under aqueous conditions, which will have potential impacts on the studies of cell behaviors and cell functions.