Silicon Nitride AFM Tip Research Articles

Silanization of Sapphire Surfaces for Optical Sensing Applications.

Well-characterized silane layers are essential for optimized attachment of (bio)molecules enabling reliable chem/biosensor performance. Herein, binding properties and orientation of 3-mercaptopropyltrimethoxysilane layers at crystalline sapphire (0001) surfaces were determined by water contact angle measurements, infrared reflection absorption spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Infrared reflection absorption spectroscopy measurements suggest an almost perpendicular arrangement of the MPTMS molecules to the substrate surface. Adhesion force studies between a silicon nitride AFM tip and modified sapphire, gold, and silicon dioxide substrates were investigated by peak force tapping atomic force microscopy and used to define the silane binding properties on these surfaces. As expected, the Al-O-Si bond was determined to be responsible for the layer formation at the sapphire substrate surface.

Macro- and Nanotribological Properties of Graphite Tribofilms: Influence of the Sliding Interface

Macrotribological studies of microcrystalline graphite powder reveal a drastic decrease in the friction coefficient when the experiments are carried out in the presence of low-viscosity liquids. The friction reduction is attributed to the simultaneous presence of particles and liquid in the sliding contact, but the mechanisms involved remain unclear. In order to contribute to the understanding of liquid action in friction reduction mechanisms, nanoscale investigations of the tribofilms have been performed using lateral force microscopy. Attention is devoted to the nanostructure of the film surfaces and their nanofriction behavior using an atomic force microscope. The influence of the tip/sample interfaces on friction properties is investigated by using AFM tips constituted of different compounds (silicon, gold/chromium alloy, silicon nitride or carbon-covered AFM tip) and by performing the nanofriction tests in air or liquid environments. The results indicate that the friction reduction observed at macroscale is attributed neither to the lowering of the shear strength of the carbon/carbon interface in the presence of liquid nor to the nanostructure of the film surface. Collective liquid/particles effects inside the contact during sliding are probably involved.

Adhesion forces in AFM of redox responsive polymer grafts: Effects of tip hydrophilicity

The adherence between silicon nitride AFM tips and redox-active poly(ferrocenylsilanes) (PFS) grafts on gold was investigated by electrochemical AFM force spectroscopy. Before the adhesion measurements silicon nitride AFM probes were cleaned with organic solvents (acetone and ethanol) or piranha solution. Interestingly, these different AFM tip cleaning procedures drastically affected the observed adhesion forces. Water contact angle measurements on the corresponding AFM probe chips showed that piranha treatment resulted in a significant increase of AFM probe chip surface hydrophilicity compared to the organic solvent treatment. Obviously this hydrophilicity change caused drastic, even opposite changes in the tip-PFS adhesive force measurement upon electrode potential change to reversibly oxidize and reduce the PFS grafts. Our findings are of pivotal importance for AFM tip adhesion measurements utilizing standard silicon nitride AFM tips. Probe hydrophilicity must be carefully taken into consideration and controlled.

Evaluation of nanotribological behavior of amorphous polystyrene: The macromolecular weight effect

Tribological testing of polymers is of prime importance in many industrial applications. Silicon nitride AFM tips have been used to mimic the contact between amorphous polystyrene surfaces and a hard asperity, which is useful in understanding of how a multitude of asperities behave in a macroscopic contact. In this study, the adhesion force and the friction force of four PS molecular weights were measured and the average contact pressure was calculated by using the JKR contact theory. The nanotribological behavior of polystyrene showed a dependence on macromolecular weight with varying applied normal force and sliding velocity. The study indicates that the length of polymer chains noticeably influences the tribological behavior of amorphous polystyrenes. Mechanisms governing such behavior differences were ascribed to energy dissipating modes.

Quantitative Analysis of Human Internal Limiting Membrane Extracted from Patients with Macular Holes

We report the study of the morphology, topography, and adhesion properties of internal limiting membrane (ILM) from patients with macular holes. The quantitative analysis of human ILM could provide essential information toward the improvement of existing surgical instruments for more efficient and safer surgical removal of ILM. Imaging in air revealed the presence of globular structures in most of the samples analyzed which were coupled with fibrillar structures in some of the samples. Modification of silicon nitride AFM tips with oppositely charged functional groups showed changes in adhesion force at the membrane-tip interface. Defining the surface characteristics of the human ILM is an initial step in the development of improved surgical tools that may allow nontraumatic stripping of ILM during surgery.

Friction in ultra-thin conjunction of valve seals of pressurised metered dose inhalers

In many drug dispensing devices, such as syringes and inhalers, an elatomeric gasket is used to prevent the formulation from leaking from the chamber. During device actuation, the seal is subjected to friction, which in turn causes its deformation and can cause unintentional leakage, thus dose variability. Additionally, friction of seal is responsible for a host of potential problems such undue effort required for actuation and potential wear. The mechanism of friction generation in the seal conjunction is complex, arising from adhesion of rubber in contact with the moving interface, viscous action of a thin film of fluid and deformation of seal asperities. Therefore, the first step in understanding the conjunctional behaviour of rubber seals is a fundamental study of mechanisms of friction generation. A developed model can then be validated against measurements and used at a later stage to predict product performance, robustness and variability due to manufacturing tolerances. A friction model, based on the aforementioned mechanisms, for prediction of seal friction has been developed and validated against measured friction tests performed at both nano and component level scales. Pressure changes in the metered chamber have been taken into account in the model. Friction data are presented for nitrile rubber, using a silicon nitride AFM tip for nano-scale interactions and polybutylene terephthalate (PBT) for asperity interactions at a component level, where a traditional friction test apparatus is utilised. Reasonable agreement is found between measurements and model predictions for the nano-scale coefficient of friction of rubber against silicon nitride. Similarly, good agreement has been obtained for the mean coefficient of friction of rubber against PBT. It is found that dominant mechanism of friction is adhesion. For the typical asperity geometry of such seals and the usual low sliding speeds the other friction contributions, viscous shear and asperity deformation are negligible. However, this is not the case in many other seals or gaskets in other applications such as in hydraulic actuators.

Determining Surface Potential of the Bitumen‐Water Interface at Nanoscale Resolution using Atomic Force Microscopy

AbstractAtomic force microscopy (AFM) was used to measure the surface forces between a silicon nitride AFM tip and a deposited layer of Athabasca bitumen; the measurements were carried out in pure water (pH 6.0–6.5) and 1 mM KCl solution (pH 9). An AFM pyramidal‐shaped tip was moved stepwise using an operator‐controlled offset (10 nm per step) and the tip‐bitumen colloidal forces were measured at each location. Surface charge densities at the bitumen‐water interface were calculated from the measured colloidal forces using a theoretical model that combined both electrostatic and van der Waals forces for a conical tip‐flat substrate system. Fitted values of the bitumen surface charge density ranged from –0.002 to –0.004 C/m2 in water (pH 6.0–6.5), and –0.005 to –0.022 C/m2 in KCl solution (pH 9); the variation of local charge density along the bitumen surface appeared random. Bitumen surface potentials were also calculated from the surface charge densities using the Graham equation; the values ranged from –90 to –130 mV in water, and –45 to –110 mV in KCl solution. This study suggests the presence of bitumen surface domains of different surface charge densities/surface potentials. The domains are estimated to have characteristic sizes of 20 to 40 nm or less.

Choose Sides: Differential Polymer Adhesion

AFM-based single molecule desorption measurements were performed on surface end-grafted poly(acrylic acid) monolayers as a function of the pH of the aqueous buffer to study the adhesion properties of polymers that bridge two surfaces. These properties were found to depend on the adhesion forces of both surfaces in a differential manner, which is explained with a simple model in analogy to the Bell-Evans formalism used in dynamic force spectroscopy. The measured interaction forces between the poly(acrylic acid) chains and silicon nitride AFM tips depend on the grafting density of the polymer monolayers as well as on the contour length of the polymer chains. This study demonstrates that the stability of polymer bridges is determined by the adhesion strengths on both surfaces, which can be tuned by using pH-dependent polyelectrolyte monolayers.

Evidence for direct amelogenin–target cell interactions using dynamic force spectroscopy

Increasing evidence suggests that amelogenin, long held to be a structural protein of developing enamel matrix, may also have cell signaling functions. However, a mechanism for amelogenin cell signaling has yet to be described. The aim of the present study was to use dynamic chemical force spectroscopy to measure amelogenin interactions with possible target cells. Full-length amelogenin (rM179) was covalently attached to silicon nitride AFM tips. Synthetic RGD peptides and unmodified AFM tips were used as controls. Amelogenin-RGD cell binding force measurements were carried out using human periodontal ligament fibroblasts (HPDF) from primary explants and a commercially available osteoblast-like human sarcoma cell line as the targets. Results indicated a linear logarithmic dependence between loading rate and unbinding force for amelogenin-RGD target cells across the range of loading rates used. For RGD controls, binding events measured at 5.5 nN s-1 force loading rate resulted in a mean force of 60 pN. Values for amelogenin-fibroblast and amelogenin-osteoblast-like cell unbinding forces, measured at similar loading rates, were 50 and 55 pN, respectively. These data suggest that amelogenin interacts with potential target cells with forces characteristic of specific ligand-receptor binding, suggesting a direct effect for amelogenin at target cell membranes.

In Situ Atomic Force Microscopy Imaging of Adsorbed Block Copolymer Micelles

Atomic force microscopy has been used to study the in situ adsorption of an amphiphilic block copolymer from aqueous solution onto hydrophilic mica and hydrophobic silica substrates. Adsorption was studied as a function of concentrationclose to and below the bulk critical micelle concentration the copolymers adsorb as single chains or “premicelle aggregates”. As concentration is increased, micelles form into densely packed structures. At a given concentration, the ordering is higher for the strongly adsorbed micelles on silica. Competitive adsorption onto the hydrophilic silicon nitride AFM tip occurs. The substrate coverage of micelles on mica increases roughly linearly with scan time at low micelle concentration. Weak adsorption on mica was analyzed to provide a two-dimensional diffusion coefficient. The effect of tip oscillation frequency on the AFM images was investigatedhigher frequencies were required to image polymer aggregates on mica. Finally, force−distance curves provide information on the nano...

Deformation and wear of pyramidal, silicon-nitride AFM tips scanning micrometre-size features in contact mode

An experimental study was carried out, in order to investigate the deformation and wear taking place on pyramidal silicon-nitride AFM tips. The study focuses on the contact mode scanning of silicon features of micrometre-size. First the deformation and the mechanisms of wear of the tip during scanning are discussed. After that the results of an experiment showing both phenomena on a used AFM tip are presented. Both the damaged and the unused tip are shown on AFM and SEM images. Using these images the actual mechanisms of wear are determined. It is shown that adhesive wear, low cycle fatigue and plastic deformation take place on the tip.

Atomic force microscope measurements of the hardness and elasticity of peritubular and intertubular human dentin.

An atomic force microscope was used to measure the hardness and elasticity of fully-hydrated peritubular and intertubular human dentin. The standard silicon nitride AFM tip and silicon cantilever assembly were replaced with a diamond tip and stainless steel cantilever having significantly higher stiffness. Hardness was measured as the ratio of the applied force to the projected indentation area for indentations with depths from 10-20 nm. The sample stiffness was measured by imaging specimens in a force-modulated mode. Hardness values of 2.3 +/- 0.3 GPa and 0.5 +/- 0.1 GPa were measured for the peritubular and intertubular dentin, respectively. Stiffness imaging revealed that the elastic modulus of the peritubular dentin was spatially homogeneous; whereas, there was considerable spatial variation in the elasticity of the intertubular dentin. The atomic force microscope can be used to measure the mechanical properties of fully hydrated calcified tissues at the submicron level of spatial resolution, thus augmenting more traditional depth sensing probes.

Wear of the atomic force microscope tip under light load, studied by atomic force microscopy

The purpose of this paper was to study the wear process of the non-conductive microfabricated silicon nitride (Si 3N 4) AFM tip which occurs during apparently wearless light-loaded scanning of the smooth silicon surface. AFM tip shape and wear were imaged using an AFM tip of the same size and geometry. Study included the observation of initial and worn surfaces, detecting of probing tip destruction and worn area size calculations. Scanning of the flat silicon surface with relatively low nominal load of 10 nN resulted in wear of the tip without significant drop in the quality of AFM image, possibly, because of formation of rough surface with asperities, acting as a probing tip. Wear mechanisms of the tip were analyzed. Low-cycle fatigue process, resulting in surface fracture, was found the most probable wear mechanism of the Si 3N 4 AFM tip.

Tip-sample interactions in atomic force microscopy: I. Modulating adhesion between silicon nitride and glass

An adhesive interaction between a silicon nitride AFM tip and glass substrate in water is described. This adhesion is in the range 5-40 nN, of which a large component is likely to be due to hydrogen bonding between the silanol groups on both surfaces. The interaction can be modulated by a variety of buffers commonly used in biochemical and biological research, including sodium phosphate, tris(hydroxymethyl)aminomethane, glycine, and N-2-hydroxyethyl-piperazine N'-2-ethanesulfonic acid. Using these buffers it appears that there are effects of ion concentration, ion type and pH on the measured adhesion. Of the conditions examined, phosphate was most effective at reducing adhesion and could be used at concentrations as low as 10 mM at neutral pH. The results demonstrate that the chemical interactions between tip and sample can be modulated, and provide a basis for designing conditions for imaging and manipulating biological molecules and structures.