High-resolution friction force microscopy under electrochemical control

Utilising Commercially Fabricated Printed Circuit Boards as an Electrochemical Biosensing Platform

Printed circuit boards (PCBs) offer a promising platform for the development of electronics-assisted biomedical diagnostic sensors and microsystems. The long-standing industrial basis offers distinctive advantages for cost-effective, reproducible, and easily integrated sample-in-answer-out diagnostic microsystems. Nonetheless, the commercial techniques used in the fabrication of PCBs produce various contaminants potentially degrading severely their stability and repeatability in electrochemical sensing applications. Herein, we analyse for the first time such critical technological considerations, allowing the exploitation of commercial PCB platforms as reliable electrochemical sensing platforms. The presented electrochemical and physical characterisation data reveal clear evidence of both organic and inorganic sensing electrode surface contaminants, which can be removed using various pre-cleaning techniques. We demonstrate that, following such pre-treatment rules, PCB-based electrodes can be reliably fabricated for sensitive electrochemical biosensors. Herein, we demonstrate the applicability of the methodology both for labelled protein (procalcitonin) and label-free nucleic acid (E. coli-specific DNA) biomarker quantification, with observed limits of detection (LoD) of 2 pM and 110 pM, respectively. The proposed optimisation of surface pre-treatment is critical in the development of robust and sensitive PCB-based electrochemical sensors for both clinical and environmental diagnostics and monitoring applications.

In situ friction study of Ag Underpotential deposition (UPD) on Au(111) in aqueous electrolyte

The electrodeposition of silver on Au(111) was investigated using lateral force microscopy (LFM) in Ag+ containing sulfuric acid. Friction force images show that adsorbed sulfate forms3 × 7 R 19 . 1 ∘ structure (θ s u l f a t e = 0 . 2 ) on Au(111) prior to Ag underpotential deposition (UPD) and ( 3 × 3 R 30 ∘ ) structure (θ s u l f a t e = 0 . 33 ) on a complete monolayer or bilayer of Ag. Variation of friction with normal load shows a non-monotonous dependence, which is caused by increasing penetration of the tip into the sulfate adlayer. In addition, the friction force is influenced by the varying coverage and mobility of Ag atoms on the surface. Before Ag coverage reaches the critical value, the deposited silver atoms may be mobile enough to be dragged by the movement of AFM tip. Possible penetration of the tip into the UPD layer at very high loads is discussed as a model for self-healing wear. However, when the coverage of Ag is close to 1, the deposited Ag atoms are tight enough to resist the influence of the AFM tip and the tip penetrates only into the sulfate adlayer.

Static and dynamic calibration of torsional spring constants of cantilevers

A method for calibrating the dynamic torsional spring constant of cantilevers by directly measuring the thermally driven motion of the cantilever with an interferometer is presented. Random errors in calibration were made negligible (<1%) by averaging over multiple measurements. The errors in accuracy of ±5% or ±10% for both of the cantilevers calibrated in this study were limited only by the accuracy of the laser Doppler vibrometer (LDV) used to measure thermal fluctuations. This is a significant improvement over commonly used methods that result in large and untraceable errors resulting from assumptions made about the cantilever geometry, material properties, and/or hydrodynamic physics of the surroundings. Subsequently, the static torsional spring constant is determined from its dynamic counterpart after careful LDV measurements of the torsional mode shape, backed by finite element analysis simulations. A meticulously calibrated cantilever is used in a friction force microscopy experiment that measures the friction difference and interfacial shear strength (ISS) between graphene and a silicon dioxide AFM probe. Accurate calibration can resolve discrepancies between different experimental methods, which have contributed to a large scatter in the reported friction and ISS values in the literature to date.

Estimation of interaction energy and contact stiffness in atomic-scale sliding on a model sodium chloride surface in ethanol

Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal load up to 38 nN, while above it a sudden increase by almost one order of magnitude was observed. Comparing this to previous results suggests that considerable atom rearrangements may occur in the contact region, although the (100) surface structure is preserved by ethanol-assisted diffusion of Na and Cl ions.

Atomic-Scale Sliding Friction on Graphene in Water

The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick-slip is almost identical to that found in ultrahigh vacuum. Furthermore, they show that water plays a purely stochastic role in sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely of the operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high-vacuum measurements.

Dynamic shear force microscopy of viscosity in nanometer-confined hexadecane layers

Hexadecane exhibits pronounced molecular layering upon confinement to gaps of a few nanometer width which is discussed for its role in boundary lubrication. We have probed the mechanical properties of the confined layers with the help of an atomic force microscope, by quasi-static normal force measurements and by analyzing the lateral tip motion of a magnetically actuated torsional cantilever oscillation. The molecular layering is modeled by a oscillatory force curve and the tip approach is simulated assuming thermal equilibrium correlations in the liquid. The shear response of the confined layers reveals gradually increasing stiffness and viscous dissipation for a decreasing number of confined layers.

Effects of molecule anchoring and dispersion on nanoscopic friction under electrochemical control

The application of electric fields is a promising strategy for in situ control of friction. While there have recently been many experimental studies on friction under the influence of electric fields, theoretical understanding is very limited. Recently, we introduced a simple theoretical model for friction under electrochemical conditions that focused on the interaction of a force microscope tip with adsorbed molecules whose orientation was dependent on the applied electric field. Here we focus on the effects of anchoring of the molecules on friction. We show that anchoring affects the intensity and width of the peak in the friction that occurs near a reorientation transition of adsorbed molecules, and explain this by comparing the strength of molecule-molecule and molecule-tip interactions. We derive a dispersion relation for phonons in the layer of adsorbed molecules and demonstrate that it can be used to understand important features of the frictional response.

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.

Probing fibronectin-antibody interactions using AFM force spectroscopy and lateral force microscopy

The first experiment showing the effects of specific interaction forces using lateral force microscopy (LFM) was demonstrated for lectin-carbohydrate interactions some years ago. Such measurements are possible under the assumption that specific forces strongly dominate over the non-specific ones. However, obtaining quantitative results requires the complex and tedious calibration of a torsional force. Here, a new and relatively simple method for the calibration of the torsional force is presented. The proposed calibration method is validated through the measurement of the interaction forces between human fibronectin and its monoclonal antibody. The results obtained using LFM and AFM-based classical force spectroscopies showed similar unbinding forces recorded at similar loading rates. Our studies verify that the proposed lateral force calibration method can be applied to study single molecule interactions.

Stick-slip behaviour on Au(111) with adsorption of copper and sulfate

Several transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick-slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × √7 superstructure on the copper monolayer. Stick-slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick-slip resolution persists at all normal forces investigated.

Nanoscopic friction under electrochemical control

We propose a theoretical model of friction under electrochemical conditions focusing on the interaction of a force microscope tip with adsorbed polar molecules whose orientation depends on the applied electric field. We demonstrate that the dependence of friction force on the electric field is determined by the interplay of two channels of energy dissipation: (i) the rotation of dipoles and (ii) slips of the tip over potential barriers. We suggest a promising strategy to achieve a strong dependence of nanoscopic friction on the external field based on the competition between long-range electrostatic interactions and short-range chemical interactions between tip and adsorbed polar molecules.

Indentation-formed nanocontacts: an atomic-scale perspective

One-to-one comparisons between indentation experiments and atomistic modelling have until recently been hampered by the discrepancy in length scales of the two approaches.

Stochastic noise in atomic force microscopy

Having reached the quantum and thermodynamic limits of detection, atomic force microscopy (AFM) experiments are routinely being performed at the fundamental limit of signal to noise. A critical understanding of the statistical properties of noise leads to more accurate interpretation of data, optimization of experimental protocols, advancements in instrumentation, and new measurement techniques. Furthermore, accurate simulation of cantilever dynamics requires knowledge of stochastic behavior of the system, as stochastic noise may exceed the deterministic signals of interest, and even dominate the outcome of an experiment. In this article, the power spectral density (PSD), used to quantify stationary stochastic processes, is introduced in the context of a thorough noise analysis of the light source used to detect cantilever deflections. The statistical properties of PSDs are then outlined for various stationary, nonstationary, and deterministic noise sources in the context of AFM experiments. Following these developments, a method for integrating PSDs to provide an accurate standard deviation of linear measurements is described. Lastly, a method for simulating stochastic Gaussian noise from any arbitrary power spectral density is presented. The result demonstrates that mechanical vibrations of the AFM can cause a logarithmic velocity dependence of friction and induce multiple slip events in the atomic stick-slip process, as well as predicts an artifactual temperature dependence of friction measured by AFM.

Retrofitting an atomic force microscope with photothermal excitation for a clean cantilever response in low Q environments

It is well known that the low-Q regime in dynamic atomic force microscopy is afflicted by instrumental artifacts (known as "the forest of peaks") caused by piezoacoustic excitation of the cantilever. In this article, we unveil additional issues associated with piezoacoustic excitation that become apparent and problematic at low Q values. We present the design of a photothermal excitation system that resolves these issues, and demonstrate its performance on force spectroscopy at the interface of gold and an ionic liquid with an overdamped cantilever (Q < 0.5). Finally, challenges in the interpretation of low-Q dynamic AFM measurements are discussed.

Atomic force microscopy in viscous ionic liquids

Extracting quantitative information from amplitude-modulation atomic force microscopy (AM-AFM) in viscous ionic liquids is difficult because existing theory requires knowledge of the cantilever natural frequency, which cannot be measured in the absence of a resonance peak. We present a new model that describes cantilever dynamics in an overdamped medium (Q < 0.5) and derive the theory necessary to extract the stiffness and damping in highly viscous liquids. The proposed methodology is used to measure the solvation layers of an ionic liquid at a gold electrode.

The noise of coated cantilevers

In atomic force microscopy, cantilevers with a reflective coating are often used to reduce optical shot noise for deflection detection. However, static AFM experiments can be limited by classical noise and therefore may not benefit from a reduction in shot noise. Furthermore, the cantilever coating has the detrimental side-effect of coupling light power fluctuations into true cantilever bending caused by time-varying thermal stresses. Here, we distinguish three classes of noise: detection, force, and displacement noise. We discuss these noises with respect to cantilever coating in the context of both static and dynamic AFM experiments. Finally, we present a patterned cantilever coating which reduces the impact of these noises.

Switching atomic friction by electrochemical oxidation

Friction between the sliding tip of an atomic force microscope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomic-scale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only above a load threshold at which wear of the surface is initiated. In contrast, irregular stick-slip motion and a linear increase of friction with load are observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions.

Atomic force microscope imaging and force measurements at electrified and actively corroding interfaces: challenges and novel cell design

We report the design of an improved electrochemical cell for atomic force microscope measurements in corrosive electrochemical environments. Our design improvements are guided by experimental requirements for studying corrosive reactions such as selective dissolution, dealloying, pitting corrosion, and∕or surface and interface forces at electrified interfaces. Our aim is to examine some of the limitations of typical electrochemical scanning probe microscopy (SPM) experiments and in particular to outline precautions and cell-design elements, which must necessarily be taken into account in order to obtain reliable experimental results. In particular, we discuss electrochemical requirements for typical electrochemical SPM experiments and introduce novel design features to avoid common issues such as crevice formations; we discuss the choice of electrodes and contaminations from ions of reference electrodes. We optimize the cell geometry and introduce standard samples for electrochemical AFM experiments. We have tested the novel design by performing force-distance spectroscopy as a function of the applied electrochemical potential between a bare gold electrode surface and a SAM-coated AFM tip. Topography imaging was tested by studying the well-known dealloying process of a Cu(3)Au(111) surface up to the critical potential. Our design improvements should be equally applicable to in situ electrochemical scanning tunneling microscope cells.

Optical detection system for probing cantilever deflections parallel to a sample surface

To date, commercial atomic force microscopes have been optimized for measurements of forces perpendicular to the sample surface. In many applications, sensitive parallel force measurements are desirable. These can be obtained by positioning the cantilever with its long axis perpendicular to the sample: the so-called pendulum geometry. We present a compact optical beam deflection system which solves the geometrical constraint problems involved in focusing a light beam onto a cantilever in the pendulum geometry. We demonstrate the performance of the system on measurements of forces imparted by a muscle myofibril, which is in-plane to a high-magnification objective of an optical microscope.

Exploiting cantilever curvature for noise reduction in atomic force microscopy

Optical beam deflection is a widely used method for detecting the deflection of atomic force microscope (AFM) cantilevers. This paper presents a first order derivation for the angular detection noise density which determines the lower limit for deflection sensing. Surprisingly, the cantilever radius of curvature, commonly not considered, plays a crucial role and can be exploited to decrease angular detection noise. We demonstrate a reduction in angular detection shot noise of more than an order of magnitude on a home-built AFM with a commercial 450 μm long cantilever by exploiting the optical properties of the cantilever curvature caused by the reflective gold coating. Lastly, we demonstrate how cantilever curvature can be responsible for up to 45% of the variability in the measured sensitivity of cantilevers on commercially available AFMs.