Surface combustion microengines based on photocatalytic oxidations of hydrocarbons at room temperature

The concept of a surface combustion microengine that is fuelled by volatile hydrocarbons at room temperature is demonstrated on a microcantilever covered with a thin layer of titanium oxide (TiO(2)). Exposing this microengine to ultraviolet (UV) radiation and hydrocarbon vapor produces controlled bending of the microcantilever as a result of differential stress produced by photocatalytic oxidation of organic molecules on the TiO(2) coating. Compared to the motion generated solely by UV radiation or hydrocarbon adsorption, the unique photocatalytic-mechanical effects in the presence of UV and hydrocarbon produce more work and exhibit fast response. The surface combustion based microengines would require less maintenance in minimally controlled field environment and could be potentially used in construction of miniature movable machines, conversion of solar and chemical energy to mechanical work, when extended to a large array of microcantilevers. We believe such microengines can be fuelled by a variety of molecules or mixtures due to the generally favorable photocatalytic reactivity of TiO(2), thus potentially offering a broad approach for mechanical work generation from multiple energy sources.     

Microcantilever-based sensors: effect of morphology, adhesion, and cleanliness of the sensing surface on surface stress

The surface stress response of micromechanical cantilever-based sensors was studied as a function of the morphology, adhesion, and cleanliness of the gold sensing surface. Two model systems were investigated: the adsorption of alkanethiol self-assembled monolayers at the gas-solid interface and the potential-controlled adsorption of anions at the liquid-solid interface. The potential-induced surface stress, on a smooth and continuous polycrystalline Au(111)-textured microcantilever in 0.1 M HClO4, is in excellent agreement with macroscopic Au(111) single-crystal electrode results. It is shown that ambient contaminants on the sensing surface dramatically alter the surface stress-potential response. This observation can be misinterpreted as evidence that for polycrystalline Au(111) microcantilever electrodes, surface stress is dominated by surface energy change. Results for anions adsorption on gold are in contrast to the gas-phase model system. We demonstrate that the average grain size of the gold sensing surface strongly influences the magnitude of the surface stress change induced by the adsorption of octanethiol. A 25-fold amplification of the change in surface stress is observed on increasing the average gold grain size of the sensing surface from 90 to 500 nm.     

Quantitative and label-free technique for measuring protease activity and inhibition using a microfluidic cantilever array

We report the use of a SiN x based gold coated microcantilever array to quantitatively measure the activity and inhibition of a model protease immobilized on its surface. Trypsin was covalently bound to the gold surface of the microcantilever using a synthetic spacer, and the remaining exposed silicon nitride surface was passivated with silanated polyethylene glycol. The nanoscale cantilever motions induced by trypsin during substrate turnover were quantitatively measured using an optical laser-deflection technique. These microcantilever deflections directly correlated with the degree of protease turnover of excess synthetic fibronectin substrate ( K M = 0.58 x 10 (-6) M). Inhibition of surface-immobilized trypsin by soybean trypsin inhibitor (SBTI) was also observed using this system.     

Molecular interactions in self-assembly monolayers on gold-coated microcantilever electrodes

An electrochemical microcantilever (EMC) was used to study the intermolecular interaction of self-assembly monolayers (SAMs) with different n-alkanethiols chain lengths (n = 0, 4, 6, 8, 12, 16) on a Au-coated microcantilever surface. Comparing potential cycling and steps in NaClO(4) solution within the same potential range, the deflection rate of bare microcantilevers is much smaller for the former which revealed that potential excitation, i.e. the surface charge, played the dominant role in driving the instant and large deflection of the bare microcantilever, while the smaller deflection amplitude of the former implied that adsorption of ClO(4)( - ) had an adverse effect on the potential-induced stress. Upon adsorption of SAMs, the deflection amplitude of the microcantilever under the potential step was much smaller than that of a bare microcantilever, and linearly decreased with the chain length increasing for n ≤ 8 (the linear correlation coefficient and the slope are 0.98 and about - 10.4 nm per CH(2) unit, respectively), following a transition (8 ≤ n ≤ 12) to a stable state (n ≥ 12). The decrease of deflection amplitude and faster decay of deflection rate of the SAMs modified microcantilever under the potential step implyed increasing compactness of the SAMs with longer chains.     

An energy criterion for fiber breakage in stressed matrix

Eshelby's method is used in this paper to evaluate approximately the energy released when a fiber, which is embedded in a stressed infinite matrix, is “cut” by the development of cracks in the matrix. The released energy can be considered as a measure of the toughness of a fiber against matrix cracks. As numerical examples, the energies released by breaking an embedded isotropic glass fiber and an embedded transversely isotropic carbon fiber are evaluated and the effects of the fiber geometry, material properties, as well as the stress field on the energies released are demonstrated.     

A Modified Multiscale Model for Microcantilever Sensor

In this paper, an existed model for adsorption-induced surface stress is modified with physical clarity, based on the equilibrium of force. In the proposed multiscale model, a four-atom system is used, instead of the existed three-atom system which did not consider the force equilibrium. By analyzing the force state of an atom, the thickness of the first layer atoms can be determined. Thus, the proposed model does not need to determine the layer-thickness by experiments or artificially. The results obtained from the proposed model agree very well with the experimental data. This paper is helpful to investigate the atomistic theory of the microcantilever sensor.     

Probing insertion and solubilization effects of lysolipids on supported lipid bilayers using microcantilevers

The interaction of surfactants with lipid membranes can result in composition change, area expansion, solubilization, or the formation of protrusion features of the membranes. Amphipathic surfactant molecules are simplified analogues to membrane-active drugs and peptides which are known for inserting into lipid bilayers; however, the effect of these amphipathic molecules on supported membranes is not well characterized. In this paper we explore the use of microcantilever sensors to quantify surfactants' effects on lipid membranes. We use microcantilevers which are coated with lipid membranes to probe the interactions between lysolipids and supported lipid bilayers (SLBs). In particular, we investigate the effects of four zwitterionic surfactants similar to phospholipids: lysolipids of different aliphatic chain lengths (lysophosphocholines, lysoPCs, 12:0, 14:0, 16:0, and 18:0) on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine-supported lipid bilayers. By monitoring the deflection of the microcantilevers, real-time free energy changes in the SLBs upon the addition of lysolipids can be detected. Additionally, the bending direction reveals whether the lysoPCs incorporate into or solubilize the SLB. When the bulk lysoPC concentration is less than its critical micelle concentration (CMC), we observe a compressive bending of the microcantilever, indicating adsorption to the SLB. Additionally, the change in surface stress is found to be proportional to the amount of membrane-bound lysoPCs. For bulk concentrations greater than the CMC, lysoPCs 12:0 and 14:0, there is tensile bending, indicating that the lysoPCs begin to solubilize and destroy the SLBs. Interestingly, this is not observed for lysoPCs with longer chain lengths. This new method of using microcantilevers for detecting and quantifying the surfactant insertion and solubilization of SLBs offers additional insights into the interactions between small amphipathic molecules and lipid membranes.     

Nanomechanical detection of cholera toxin using microcantilevers functionalized with ganglioside nanodiscs

The label-free detection of cholera toxin is demonstrated using microcantilevers functionalized with ganglioside nanodiscs. The cholera toxin molecules bind specifically to the active membrane protein encased in nanodiscs, nanoscale lipid bilayers surrounded by an amphipathic protein belt, immobilized on the cantilever surface. The specific molecular binding results in cantilever deflection via the formation of a surface stress-induced bending moment. The nanomechanical cantilever response is quantitatively monitored by optical interference. The consistent and reproducible nanomechanical detection of cholera toxin in nanomolar range concentrations is demonstrated. The results validated with such a model system suggest that the combination of a microcantilever platform with receptor nanodiscs is a promising approach for monitoring invasive pathogens and other types of biomolecular detection relevant to drug discovery.     

Label-free detection of DNA hybridization based on hydration-induced tension in nucleic acid films

The properties of water at the nanoscale are crucial in many areas of biology, but the confinement of water molecules in sub-nanometre channels in biological systems has received relatively little attention. Advances in nanotechnology make it possible to explore the role played by water molecules in living systems, potentially leading to the development of ultrasensitive biosensors. Here we show that the adsorption of water by a self-assembled monolayer of single-stranded DNA on a silicon microcantilever can be detected by measuring how the tension in the monolayer changes as a result of hydration. Our approach relies on the microcantilever bending by an amount that depends on the tension in the monolayer. In particular, we find that the tension changes dramatically when the monolayer interacts with either complementary or single mismatched single-stranded DNA targets. Our results suggest that the tension is mainly governed by hydration forces in the channels between the DNA molecules and could lead to the development of a label-free DNA biosensor that can detect single mutations. The technique provides sensitivity in the femtomolar range that is at least two orders of magnitude better than that obtained previously with label-free nanomechanical biosensors and with label-dependent microarrays.     

Magneto-Nanomechanical Array Biosensor for Ultrasensitive Detection of Oncogenic Exosomes for Early Diagnosis of Cancers

Exosomes are a class of nanoscale vesicles secreted by cells, which contain abundant information closely related to parental cells. The ultrasensitive detection of cancer-derived exosomes is highly significant for early non-invasive diagnosis of cancer. Here, an ultrasensitive nanomechanical sensor is reported, which uses a magnetic-driven microcantilever array to selectively detect oncogenic exosomes. A magnetic force, which can produce a far greater deflection of microcantilever than that produced by the intermolecular interaction force even with very low concentrations of target substances, is introduced. This method reduced the detection limit to less than 10 exosomes mL⁻¹. Direct detection of exosomes in the serum of patients with breast cancer and in healthy people showed a significant difference. This work improved the sensitivity by five orders of magnitude as compared to that of traditional nanomechanical sensing based on surface stress mode. This method can be applied parallelly for highly sensitive detection of other microorganisms (such as bacteria and viruses) by using different probe molecules, which can provide a supersensitive detection approach for cancer diagnosis, food safety, and SARS-CoV-2 infection.     

Surface-induced dissociation of protonated peptides: implications of initial kinetic energy spread.

Surface-induced dissociation (SID) has been used to produce daughter ion spectra of small protonated peptides generated by fast atom bombardment (FAB). The relative abundances of daughter ions depends critically upon the energy of the ion/surface collision. A wide array of decomposition processes may be observed using ELAB collision energies in the range 10-20 eV. At approximately 13-eV collision energy, the variety of decomposition processes is maximized for the small peptides studied; hence, maximum structural information may be deduced. Collisionally-activated dissociations (CAD) using argon gas and the identical protonated peptides could not produce as large an array of daughter ions in a constant condition experiment. An apparent contradiction is thereby posed because SID is known to produce a narrow distribution of ion internal energies relative to CAD. This apparent contradiction is resolved by considering the rather large kinetic energy spread of ions leaving the FAB source. For the SID process, this large initial kinetic energy distribution is converted into a significantly wider spread in center-of-mass collision energy, leading to a broader variety of decomposition processes (high and low energy) compared to CAD.     

A negative surface energy for alumina

The surface energy of a solid measures the energy cost of increasing the surface area. All normal solids therefore have a positive surface energy-if it had been negative, the solid would disintegrate. For this reason it is also generally believed that when certain ceramics can be found in a highly porous form, this is a metastable state, which will eventually sinter into the bulk solid at high temperatures. We present theoretical evidence suggesting that for theta-alumina, the surface energy is strongly dependent on the size of the crystallites, and that for some facets it is negative for thicknesses larger than approximately 1 nm. This suggests a completely new picture of porous alumina in which the high-surface-area, nanocrystalline form is the thermodynamic ground state. The negative surface energy is found to be related to a particularly strongly adsorbed state of dissociated water on some alumina surfaces. We also present new experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, that theta-alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.     

Nanomechanical detection of DNA melting on microcantilever surfaces

We observe surface stress changes in response to thermal dehybridization, or melting, of double-stranded DNA (dsDNA) oligonucleotides that are grafted on one side of a microcantilever beam. Changes in surface stress occur when one complementary DNA strand melts and diffuses away from the other, resulting in alterations of the electrostatic, counterionic, and hydration interaction forces between the remaining neighboring surface-grafted DNA molecules. We have been able to distinguish changes in the melting temperature of dsDNA as a function of salt concentration and oligomer length. This technique also highlights differences between surface immobilized and solution DNA melting dynamics, which allows us to better understand the stability of DNA on surfaces. The transduction of phase transitions into a mechanical signal is ubiquitous for DNA, making cantilever-based detection a widely useful and complementary alternative to calorimetric and fluorescence measurements.     

An internal-variable rod model for stress-induced phase transitions in a slender SMA layer. I: Asymptotic equations and a two-phase solution

In part I of this series of two papers, an internal-variable rod model is proposed to study the stress-induced phase transitions in a slender shape memory alloy (SMA) layer. To study the mechanical responses of SMAs, two independent energy functions are adopted: the Helmholtz free energy and the rate of mechanical dissipation. A phase state variable is introduced to describe the phase transition process. Starting from the 2-D governing system and by using the coupled series-asymptotic expansion method, one single equilibrium equation is derived, which involves the leading order term of the axial strain and the phase state functions. Further by using the phase transition criteria, the evolution laws of the phase state functions corresponding to the outer loop of the stress–strain response are derived. As a result, the governing ODEs for the purely loading and purely unloading processes are obtained, which are called the asymptotic rod equations. The two-phase solution of the asymptotic rod equations in an infinitely long layer is then constructed. An explicit solution for the phase volume fraction of the corresponding inhomogeneous deformation is deduced, which appears to be the first analytical expression for this important quantity in stress-induced phase transitions. The key parameters in terms of original material constants for the phase transitions and deformations are also identified.     

Polarization energy calculations in molecular crystals

The self-consistent polarization field (SCPF) and Fourier transform methods of calculating the polarization energy of excess charges are compared. The SCPF method is extended (i) to treat molecules as a set of submolecules rather than single points in the inner region around the charge, and (ii) to treat the outer region around the charge as an anisotropic dielectric continuum rather Ethan an isotropic one. The contribution to the polarization energy from the outer region depends on the average 〈? ^?1〉, where ? is the dielectric tensor. These extensions allow the SCPF method to be used for elongated molecules, with potential applications to various systems lacking translational symmetry.     

Impact of grinding aids on dry grinding performance,bulk properties and surface energy

In dry fine grinding processes the relevance of particle-particle interactions rises with increasing product fineness. These particle-particle interactions reduce the grinding efficiency and complicate the process control. The adsorption of grinding aid molecules on the product particle surface is a common measure to handle these effects. To ensure an efficient grinding aid application, the impacts of additives on particle and bulk properties, which influence the micro-processes inside the mill, need to be understood. Within this study the effects of several grinding aids on dry fine grinding of limestone in a laboratory vibration mill were investigated. Unlike in many other scientific works, the impacts of grinding aids were analyzed on different levels simultaneously: Grinding success and agglomerate size distributions were evaluated by wet and dry particle size measurements, respectively. Additionally, material coating on the grinding media, powder flowabilities and particle specific surface energies were measured. It was shown that all of the investigated grinding aids influence the grinding efficiency. However, the formation of agglomerates is not necessarily linked to the product fineness. Furthermore, a strong impact of certain grinding aids on the flowability of the product powder was determined. Thereby, the bulk flow behavior also determines the grinding result as it affects the stress mechanism inside the mill. Moreover, a direct relation between surface energy and powder flowability as well as agglomeration behavior could be demonstrated.     

Proteolytic activity monitored by fluorescence resonance energy transfer through quantum-dot-peptide conjugates

Proteases are enzymes that catalyse the breaking of specific peptide bonds in proteins and polypeptides. They are heavily involved in many normal biological processes as well as in diseases, including cancer, stroke and infection. In fact, proteolytic activity is sometimes used as a marker for some cancer types. Here we present luminescent quantum dot (QD) bioconjugates designed to detect proteolytic activity by fluorescence resonance energy transfer. To achieve this, we developed a modular peptide structure which allowed us to attach dye-labelled substrates for the proteases caspase-1, thrombin, collagenase and chymotrypsin to the QD surface. The fluorescence resonance energy transfer efficiency within these nanoassemblies is easily controlled, and proteolytic assays were carried out under both excess enzyme and excess substrate conditions. These assays provide quantitative data including enzymatic velocity, Michaelis-Menten kinetic parameters, and mechanisms of enzymatic inhibition. We also screened a number of inhibitory compounds against the QD-thrombin conjugate. This technology is not limited to sensing proteases, but may be amenable to monitoring other enzymatic modifications.     

Buckling-driven debonding of a carbon nanotube rope

Summary Debonding of a single CNT (carbon nanotube) from the midway or one end of a CNT rope is studied in this paper. The analysis of buckling-driven debonding of an individual CNT is based on an elastic beam model, while the local debonding growth, stability and arrest are explained by the energy release rate criterion. The debonding from a preloaded CNT rope is driven by excess energy released from the unbuckled to the buckled state of an individual CNT. Some interesting debonding behaviors are displayed for varying dimensions, compression strain and van der Waals (vdW) adhesive energy. The growth of the debonding may be stable, unstable or unstable growth followed by a stable growth when an initial debonding CNT rope is loaded.     

Thermal energy storage in supersaturated salt solutions

Some aqueous salt solutions can be supersaturated by cooling to temperatures well below their normal crystallization temperatures. Such a solution in this metastable state stores a substantial fraction of the heat of solution, and this energy can be released with an accompanying temperature rise upon allowing the mixture to crystallize to its equilibrium state. We have determined the thermal energy storage capacity for supersaturated aqueous solutions of sodium ethanoate and sodium thiosulphate over a range of concentrations and from 0°C up to the temperature at which the salt completely dissolves. A maximum storage capacity of 174 kJ kg^?1 at 30°C was observed for a sodium ethanoate solution in which the salt : water molar ratio was 1:4.     

Hygral stress in hardened cement paste

A specially constructed stress cell, permitting variation of degree of restraint, was used to measure the hygral stress produced in thin-walled hardened cement paste cylinders due to water absorption. The effects of porosity, relaxation and relative humidity on the hygral stress were investigated using a Portland and a Portland composite cement. It was found that capillary suction transports water to the gel pores causing an initial rapid stress development. This is followed by a gradual increase over ca. 3 days governed by the redistribution kinetics of water molecules in the cement gel. The hygral stress developed is proportional to the volume fraction of cement gel. The cement gel itself produces a uniaxial stress of ca. 8 MPa for degrees of restraint above ca. 80%. About 70% of the stress is caused by changes in surface energy at the gel particle/pore water interface; the remainder is due to the disjoining pressure. A change in surface energy of 0.17 J/m² was estimated based on measurements of specific surface and porosity. The development of hygral stress is also controlled by stress relaxation. This appears to be enhanced by the effect of the disjoining pressure which weakens bonds between gel particles to create a more mobile structure under stress.