Surface Stress Changes Research Articles

Microcantilever arrays functionalised with spiropyran photoactive moieties as systems to measure photo-induced surface stress changes

Herein we investigate the feasibility of detecting photo-induced surface stress changes using the deflection response of cantilevers. For this purpose, silicon microcantilevers have been functionalised with spiropyran photochromic molecules, using both a monolayer and a polymeric brushes approach. Upon ultraviolet light irradiation, the spiropyran unit is converted to the merocyanine form due to the photo-induced cleavage of the Cspiro-O bond. The two forms of the molecule have dramatically different charge, polarity and molecular conformations. This makes spiropyrans an ideal system to study the correlation between photo-induced molecular changes and corresponding changes in surface stress. Our investigations include monitoring the changes in static cantilever deflection, and consequently, surface stress of the spiropyran functionalised cantilevers on exposure to ultraviolet light. Cantilever deflection data reveals that ultraviolet induced conformational changes in the spiropyran moiety cause a change in compressive surface stress and this varies with the type of functionalisation method implemented. The change in surface stress response from the spiropyran polymer brushes functionalised cantilevers gives an average surface stress change of 98 Nm−1 (n=24) while the spiropyran monolayer coated cantilevers have an average surface stress change of about 446 Nm−1 (n=8) upon irradiation with UV light.

Adsorption-Induced Surface Stresses of the Water/Quartz Interface: Ab Initio Molecular Dynamics Study.

Adsorption-induced deformation is expansion or contraction of a solid due to adsorption on its surface. This phenomenon is important for a wide range of applications, from chemomechanical sensors to methane recovery from geological formations. The strain of the solid is driven by the change of the surface stress due to adsorption. Using ab initio molecular dynamics, we calculate the surface stresses for the dry α-quartz surfaces, and investigate how these stresses change when the surfaces are exposed to water. We find that the nonhydroxylated surface shows small and approximately isotropic changes in stress, while the hydroxylated surface, which interacts more strongly with the polar water molecules, shows larger and qualitatively anisotropic (opposite sign in xx and yy) surface stress changes. All of these changes are several times larger than the surface tension of water itself. The anisotropy and possibility of positive surface stress change can explain experimentally observed surface area contraction due to adsorption.

Revisiting Bangham's law of adsorption-induced deformation: changes of surface energy and surface stress.

When fluids are adsorbed on a solid surface they induce noticeable stresses, which cause the deformation of the solid. D. H. Bangham and co-authors performed a series of experimental measurements of adsorption-induced strains, and concluded that physisorption causes expansion, which is proportional to the lowering of the surface energy Δγ. This statement is referred to as the Bangham effect or Bangham's law. However, it is known that the quantity that controls the deformation is actually the change in surface stress Δf rather than surface energy Δγ, but this difference has not been considered in the context of adsorption-induced deformation of mesoporous materials. We use the Brunauer-Emmett-Teller (BET) theory to derive both values and show the difference between them. We find the condition when the difference between the two vanishes, and Bangham's law is applicable; it is likely that this condition is satisfied in most cases, and prediction of strain based on Δγ is a good approximation. We show that this is the case for adsorption of argon and water on Vycor glass. Finally, we show that the difference between Δγ and Δf can explain some of the experimental data that contradicts Bangham's law.

(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface. Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth1, ΔF = Δfo + Δfpzc + Δh + Σcohtf (1) where Δfo is the intrinsic surface stress difference between the overlayer and the substrate, Δfpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣcohtf is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects2. Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au3, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu4. This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

Aptamer-based microcantilever biosensor for ultrasensitive detection of tumor marker nucleolin

We present an aptamer-based microcantilever biosensor for label-free detection of nucleolin. The sensor cantilevers in the microcantilever array were functionalized with nucleolin aptamer (AS1411) while the reference cantilevers were modified by 6-mercapto-1-hexanol (MCH) to eliminate environmental disturbances. The interaction between nucleolin and AS1411 induced surface stress changes, resulting in a differential deflection between sensor and reference cantilevers. The amplitude of differential cantilever deflection had a good linear relationship with the nucleolin concentration ranging from 10nM to 250nM with a correlation coefficient of 0.999. The detection limit was about 1.0nM, at a signal-to-noise ratio of 3. The aptamer-based microcantilever sensor demonstrated good selectivity and was facile, rapid, and reagentless. Our results show the potential for the application of microcantilever biosensor system as a powerful tool to detect tumor markers with high sensitivity and specificity.

Electrochemical Stress Development during CO and NO Oxidation on Pt

The adsorption and oxidation of CO on Pt electrodes is among the most studied electrochemical reactions due to its fundamental importance as a simple model system and practical importance as CO is a well-known poison in fuel cells.1 We use in situ electrochemical surface stress measurements to monitor the surface stress development during the oxidation of CO on Pt electrodes where adsorbed CO reacts with oxygen-containing species on the surface via Langmuir-Hinshelwood mechanism.1-2 The nature of adsorbed CO layer and the role of oxygenated species are studied in both acid and alkaline media. Further, the effect of dissolved CO is examined. Another strong adsorbate, NO, is oxidized on Pt surface and the surface stress change is monitored. Despite the similarity in the electronic structure, the oxidation of NO follows a different mechanism where the complete oxidation of adsorbed NO is very difficult due to surface oxide formation.3The different role of oxygenated species on the surface can be seen from the surface stress response.

Nanosensors for cancer detection.

Cancer is a major burden in today's society and one of the leading causes of death in industrialised countries. Various avenues for the detection of cancer exist, most of which rely on standard methods, such as histology, ELISA, and PCR. Here we put the focus on nanomechanical biosensors derived from atomic force microscopy cantilevers. The versatility of this novel technology has been demonstrated in different applications and in some ways surpasses current technologies, such as microarray, quartz crystal microbalance and surface plasmon resonance. The technology enables label free biomarker detection without the necessity of target amplification in a total cellular background, such as BRAF mutation analysis in malignant melanoma. A unique application of the cantilever array format is the analysis of conformational dynamics of membrane proteins associated to surface stress changes. Another development is characterisation of exhaled breath which allows assessment of a patient's condition in a non-invasive manner.

Characterization of a gold coated cantilever surface for biosensing applications.

Cantilever based sensors are a promising tool for a very diverse spectrum of biological sensors. They have been used for the detection of proteins, DNA, antigens, bacteria viruses and many other biologically relevant targets. Although cantilever sensing has been described for over 20 years, there are still no viable commercial cantilever-based sensing products on the market. Several reasons can be found for this – a lack of detailed understanding of the origin of signals being an important one. As a consequence application-relevant issues such as shelf life and robust protocols distinguishing targets from false responses have received very little attention.Here, we will discuss a cantilever sensing platform combined with an electrochemical system. The detected surface stress signal is modulated by applying a square wave potential to a gold coated cantilever. The square wave potential induces adsorption and desorption onto the gold electrode surface as well as possible structural changes of the target and probe molecules on the cantilever surface resulting in a measurable surface stress change. What sets this approach apart from regular cantilever sensing is that the quantification and identification of observed signals due to target-probe interactions are not only a function of stress value (i.e. amplitude), but also of the temporal evolution of the stress response as a function of the rate and magnitude of the applied potential change, and the limits of the potential change.This paper will discuss three issues that play an important role in future successful applications of cantilever-based sensing. First, we will discuss what is required to achieve a large surface stress signal to improve sensitivity. Second, a mechanism to achieve an optimal probe density is described that improves the signal-to-noise ratio and response times of the sensor. Lastly, lifetime and long term measurements are discussed.

Affinities and in-plane stress forces between glycopeptide antibiotics and biomimetic bacterial membranes

Understanding the molecular basis of interactions between antibiotics affecting bacterial cell wall biosynthesis and cellular membranes is important in rational drug design of new drugs to overcome resistance. However, a precise understanding of how bacteriostatic antibiotics effect action often neglects the effect of biophysical forces involved following antibiotic-receptor binding events. We have employed a combination of a label-free binding biosensor (surface plasmon resonance, SPR) and a force biosensor (in-plane stress cantilever), together with model membrane systems to study the complex interplay between glycopeptide antibiotics, their cognate ligands and different model membranes. Bacterial cell wall precursor analogue N-α-Docosanoyl-ε-acetyl-Lys-d-Alanine-d-Alanine (doc-KAA) was inserted into lipid layers comprised of zwitterionic or anionic lipids then exposed to either vancomycin or the membrane-anchored glycopeptide antibiotic teicoplanin. Binding affinities and kinetics of the antibiotics to these model membranes were influenced by electrostatic interactions with the different lipid backgrounds, in addition to ligand affinities. In addition, cantilever sensors coated with model membranes showed that planar surface stress changes were induced by glycopeptide antibiotics adsorption and caused compressive surface stress generation in a ligand-dependent manner.

Comparative Study of Microcantilever-Based Sensor for Biosensing Applications

Cantilever-based sensors have emerged as a promising label free detection technique, which have been used for high precision mass detection and biomolecular recognition. By surface functionalization, the cantilever can be modified specific to certain compounds detection. Molecules adsorbed to one side of the cantilever will deflect the cantilever due to changes in surface stress. Alternatively, minute mass changes can be detected by monitoring the resonant frequency change of the cantilever for high-precision mass detections. This work is dedicated to finite element (FE) 3Dstructural modeling of three layers micromechanical sensors in ANSYS 13.0 gives 3D model which are close to reality mathematical models. Material used in cantilever for different layers are silicon-dioxide, poly-silicon and nitride. The emphasis of the analysis is put on tile effects of the angle of inclination of the concentrated force upon the deformed shape, the load-deflection relationship stresses and strain for further analysis with a greater degree of accuracy. The model we made is three different model i.e. single layer microcantilevers, three layers microcantilever with same height and three layers with different height. In three layer the centre layer i.e. second layer, is piezoresistive layer that helps to calculate Characteristics i.e. deflection, deformation, stress and strain in the cantilever for the given applied force that can we used for future analysis for the detection of biomolecules in various biosensing application. Finally the comparison of all the three different model of cantilever according to their characteristics.

Heavy metal ion detection using a capacitive micromechanical biosensor array for environmental monitoring

Heavy metal ions are major contaminants in present day water supply. In this work the fabrication and evaluation of a DNAzyme functionalized capacitive micromechanical sensor array for the detection of lead ions is presented. The catalytic strand of a “8–17” DNAzyme can cleave a substrate DNA strand that has one ribonucleotide base, in the presence of Pb2+, dissociating the complex into three fragments. The DNAzyme strand is laser printed and immobilized on the sensor surface and hybridized with the substrate strand. When self-cleaving occurs, the surface stress changes, which is then registered as change in the capacitance of the device. The sensor was able to detect 10μM of Pb2+, whilst in a reverse procedure the re-hybridization of the immobilized catalytic strand with the substrate strand was demonstrated. The reactions were validated by tagging the catalytic strand with FAM (6-carboxyfluorescein), while the substrate strand was tagged with Dabcyl, a quencher.

Characterizing α-helical peptide aggregation on supported lipid membranes using microcantilevers.

We report the use of lipid membrane-coated microcantilevers to probe the interactions between phospholipid membranes and membrane-active peptides. This sensing method integrates two well-developed techniques: solid-supported lipid bilayers (SLBs) and microcantilever sensors. SLBs are prepared on the silicon dioxide surface of the microcantilevers using a vesicle fusion method. As molecules adsorb onto the surface of the microcantilever, the microcantilever bends due to the induced compressive or tensile stresses, which result from the surface free energy change. Real-time surface stress changes in the SLB due to interactions with small molecules can be detected by monitoring the deflection of the microcantilever. We investigate the mechanism for the interaction between SLBs and PEP1, a synthetic amphipathic peptide resembling a segment of the nonstructural protein (NS5A) of the hepatitis C virus. Initially, the PEP1 peptides adsorb onto the lipid membranes, and then at a critical concentration, the peptides begin to aggregate and form pores; finally, the peptides destabilize and induce solubilization of the supported membranes. The membrane-coated microcantilever sensor is capable of characterizing the kinetics and dynamics of this process with great sensitivity.

Surface stress variation as a function of applied compressive stress and temperature in microscale silicon

Surface stress has been shown to affect the mechanical properties of materials at or below the microscale. Surface-stress-induced dislocation activity at such length scales has been shown to be a major factor affecting the mechanical behavior of materials. Defect generation as a function of applied stress at the microscale has previously been measured experimentally and predicted using simulations. However, the change in surface stress in a material in response to externally applied stress as a function of temperature has not been explored experimentally. Such an investigation is presented in this work for the case of microscale silicon samples. In-situ nondestructive measurements of the applied compressive stress and the corresponding microscale surface stress were performed from room temperature to 100 °C. The applied stress was controlled by a nanomechanical loading system. Micro-Raman spectroscopy was used to measure the surface stress in-situ as the samples deformed under the applied uniaxial compressive stress. The surface stress was found to be lower than the applied stress at all temperatures. The difference between the surface stress and the applied stress became higher at higher temperatures indicating that surface relaxation was induced by the temperature increase. Based on the measured values and observed trends, an exponential Gaussian function is proposed to describe the stress as a function of surface depth.

Electrochemical Stress Measurements of Potential Induced Structural Changes in Sn Anodes for Li Ion Batteries

We report on electrochemical surface stress measurements of thin film Sn electrodes for Li-ion battery anodes at potentials <2.0 V vs. Li/Li+. In most anodes, mechanical changes are dominated and controlled by Li-host interactions. Graphite, Si, and a Au model system exhibit compressive stress resulting from Li insertion. Calculations support these experimental results. Anodes experience tensile stress resulting from Li removal. This tensile stress may create cracks, and cause capacity loss.In contrast, Sn surfaces exhibit major compressive and tensile surface stress changes even before Li insertion. These features originate in conversion reactions between SnOx on the electrode surface and Li forming Sn and LiOx. During the cathodic scan, an intense compressive feature at ca. 0.7 V vs. Li/Li+ is observed. A major tensile release at ca. 0.6 V vs. Li/Li+ follows this compressive feature. Changes in the amount of oxygen in the surface film affect the electrochemical stress measurement dramatically, while having a more modest impact on the voltammetry. The SnOx/Li conversion reaction impacts the ability of Sn and its alloys to serve as an anode material for a Li ion battery.We also report the results of matrix assisted laser desorption (MALDI) time of flight (TOF) mass spectrometry (MS) analysis of Sn electrodes. In a mixture of ethylene carbonate and dimethyl carbonate, long chain oligomers are observed following the first cycle. These oligomers are decomposed in the subsequent cycles showing that Sn surfaces form an unstable SEI. This decomposition results in oligomerized species, which are different from those formed at the end of the first cycle. We will discuss potential and solvent dependent oligomerization mechanisms and their effect on mechanical properties of Sn electrodes.

Intrinsic stress evolution during amorphous oxide film growth on Al surfaces

The intrinsic stress evolution during formation of ultrathin amorphous oxide films on Al(111) and Al(100) surfaces by thermal oxidation at room temperature was investigated in real-time by in-situ substrate curvature measurements and detailed atomic-scale microstructural analyses. During thickening of the oxide a considerable amount of growth stresses is generated in, remarkably even amorphous, ultrathin Al2O3 films. The surface orientation-dependent stress evolutions during O adsorption on the bare Al surfaces and during subsequent oxide-film growth can be interpreted as a result of (i) adsorption-induced surface stress changes and (ii) competing processes of free volume generation and structural relaxation, respectively.

ANSYS-Based Finite Element Analysis on TBM Disc Cutter

The purpose of the essay is to know the rock fragmentation mechanism of TBM and improve the efficiency of disc cutter. The method of simulation is applied. The cutter ring and cutter axis surface stress distribution are studied under different rocks and the change of them is analyzed under different penetrations. Under different rocks conditions, the cutter ring surface stress is affected due to the change of rock load; the same rock condition, it is changed along with the change of penetrations. But the cutter axis surface stress changes little. In different rocks conditions, the change of stress on the cutter ring surface is proportional to rock ultimate stress; the same rock condition, it is proportional to the penetration of the cutter ring. The results are significant in improving the working efficiency of disc cutter.

Structural modifications of gold thin films produced by thiol-derivatized single-stranded DNA immobilization

Recent experiments have reported an opposite sign of the differential surface stress produced on gold-coated cantilevers by a thiol-derivatized single-stranded DNA (SH-DNA) immobilization process. The sign of the surface stress depends on the method used to evaporate the gold thin film, being compressive (negative) or tensile (positive) for e-beam or resistively deposited gold, respectively. This study investigates the origin of this effect by means of a combination of x-ray diffraction and x-ray photoelectron spectroscopy. Both e-beam and resistively grown gold thin films are characterized to find the subtle differences responsible for this intriguing stress behaviour. Somewhat remarkably, these studies show a tight relation between the surface structure of the gold overlayer and the SH-DNA immobilization efficiency. The average grain size variation seems to correlate well with the differential surface stress triggered by the SH-DNA immobilization previously reported. These results suggest that the relation of the probe molecules with the surface structure must be considered to understand surface stress changes.

Microcantilevers Bend to the Pressure of Clustered Redox Centers

The redox-activated deflection of microcantilevers has attracted interest for nanoactuation and chemical sensing. Microcantilever sensors are devices that transduce (bio)chemical reactions into a quantifiable nanomechanical motion via surface stress changes. Despite promising applications in analytical science, poor signal-to-noise ratios and a limited understanding of the molecular origins of the surface stress changes that cause the observed deflections remain obstacles to cantilever-based sensing becoming an established (bio)detection method, such as surface plasmon resonance and electrochemistry. We use phase-separated, binary self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate and n-undecanethiolate as a model system to study the effect of the steric crowding of the redox centers on the surface stress change and cantilever deflection produced by the electrochemical oxidation of the surface-tethered ferrocene to ferrocenium. We correlate the measured surface stress change to the fraction of the clustered ferrocenyldodecanethiolate phase in the binary SAMs. The pairing of anions with the sterically crowded clustered ferroceniums induces a collective molecular reorientation which drives the cantilever deflection. The results provide fundamental insights into the response mechanism of microcantilever-based actuating and sensing technologies.

Label-free detection of kanamycin using aptamer-based cantilever array sensor

A label-free detection method of kanamycin using aptamer-based cantilever array sensor was developed. The cantilever array was composed of sensing cantilevers and reference cantilevers. This configuration allowed direct detection of individual cantilever deflections and subsequent determination of differential deflection of sensing/reference cantilever pair. The sensing cantilevers were functionalized with kanamycin aptamer, which was used as receptor molecules while the reference cantilevers were modified with 6-mercapto-1-hexanol (MCH) to eliminate the influence of environmental disturbances. The kanamycin–aptamer interaction induced a change in cantilever surface stress, which caused a differential deflection between the sensing and reference cantilever pair. The surface stress change was linear with kanamycin concentration over the range of 100μM–10mM with a correlation coefficient of 0.995. A detection limit of 50μM was obtained, at a signal-to-noise ratio of 3. The sensor also showed good selectivity against other antibiotics such as neomycin, ribostamycin and chloramphenicol. The facile method for kanamycin detection may have great potential for investigating more other molecules.

Evolution of surface stress during oxygen exposure of clean Si(111), Si(100), and amorphous Si surfaces

The evolutions of the surface stress of Si(111)-7 × 7, Si(100)-2 × 1, and a-Si surfaces upon oxygen exposure at pO2 = 1 × 10−4 Pa and room temperature have been investigated in a comparative manner using a specimen-curvature based technique. To this end, a generally applicable, dedicated set of experiments has been devised and performed to deduce and correct for the surface stress change owing to oxygen reaction(s) at the (poorly-defined) back face of the specimen only. On this basis, it could be demonstrated that exposure of clean Si(111)-7 × 7, Si(100)-2 × 1 and a-Si surfaces to pure oxygen gas results in compressive surface stress changes for all three surfaces due to the incorporation of oxygen into Si backbonds. The measured surface stress change decreases with decreasing atomic packing density at the clean Si surfaces, which complies well with the less-densily packed Si surface regions containing more free volume for the accommodation of adsorbed O atoms.