Micromechanical Cantilever Research Articles

Surface Stress, Thickness, and Mass of the First Few Layers of Polyelectrolyte

The effects of surface stress and mass loading upon the adsorption of polyelectrolytes onto flexible silicon micromechanical cantilever sensors (MCSs) were studied in situ. A self-assembled monolayer of 2-mercaptoethylamine chloride (2-MEA) on gold was used to achieve single-side adsorption on the MCS. Such a preparation gave a positive surface potential, whereas a bare SiOx surface gave a negative surface potential. Wide scan X-ray photoelectron spectroscopy confirmed that the adsorption of polystyrenesulfonate (PSS) and polyallylamine hydrochloride (PAH) followed the general rule expected from the electrostatic interaction between the substrate and the polyelectrolyte, whereas the adsorption polyethyleneimine (PEI) did not. The adsorption of PAH on SiO(x) from a 3 mM water solution containing 1 M NaCl was associated with a deflection of the MCS toward the polyelectrolyte monolayer (tensile surface stress) owing to the hydrogen bonding between neighboring amino groups. Here, a surface stress change of 1.4 +/- 0.1 N/m was estimated. The adsorption of PSS from a 3 mM water solution containing 1 M NaCl on a 2-MEA surface induced a deflection of the MCS away from the polyelectrolyte layer (compressive stress), toward the SiO(x) side. Here, a surface stress change of 3.1 +/- 0.3 N/m was determined. The formation of a PAH layer on top of the PSS layer resulted in a deflection of the MCS toward the PAH layer. This indicated that the adjacent PSS layer was deswelling, corresponding to a surface stress change of 0.5 +/- 0.1 N/m.

Cantilever Micro-rheometer for the Characterization of Sugar Solutions.

The volume required for the rheological characterization of fluids can beminimized by using micromechanical cantilevers as viscosity sensors. Here, a simplemeasurement tool for the characterization of sugar solutions is proposed. The sensorconsists of a micromechanical cantilever as used in an atomic force microscopy which isintegrated into a closed fluid handling system. Fluid properties are derived from an analysisof the power spectral density of the fluctuations of the cantilever deflection signal. The dataacquisition system is operated with standard consumer computer components, which limitsthe costs for the hardware. Measurements with different sugar solutions indicate that thesensor system provides reliable viscosity values for sugar concentrations as they occur inbiological systems. The viscosities of the sugar solutions could be evaluated with an errorsmaller than 5 %.

Algorithm for analysis of periodic oscillations of structural systems with geometric nonlinearity

AbstractAn algorithm for analysis of periodic oscillations of forced elastic systems with geometric nonlinearity is presented in this paper. Modal decomposition of the solution, computation of periodic oscillations for every eigenmode and estimation of geometric nonlinearities using the method of initial deformations enable to construct a computational technique that can be very effective in computation of steady‐state periodic motions of slightly damped structures under periodic forcing. It is shown that the developed algorithm can be successfully exploited for the calculation of structural response of a micromechanical cantilever. Copyright © 2007 John Wiley & Sons, Ltd.

Readout of micromechanical cantilever sensor arrays by Fabry-Perot interferometry

The increasing use of micromechanical cantilevers in sensing applications causes a need for reliable readout techniques of micromechanical cantilever sensor (MCS) bending. Current optical beam deflection techniques suffer from drawbacks such as artifacts due to changes in the refraction index upon exchange of media. Here, an adaptation of the Fabry-Perot interferometer is presented that allows simultaneous determination of MCS bending and changes in the refraction index of media. Calibration of the instrument with liquids of known refraction index provides an avenue to direct measurement of bending with nanometer precision. Versatile construction of flow cells in combination with alignment features for substrate chips allows simultaneous measurement of two MCS situated either on the same, or on two different support chips. The performance of the instrument is demonstrate in several sensing applications, including adsorption experiments of alkanethioles on MCS gold surfaces, and measurement of humidity changes in air.

Multiwell micromechanical cantilever array reader for biotechnology

We use a multiwell micromechanical cantilever sensor (MCS) device to measure surface stress changes induced by specific adsorption of molecules. A multiplexed assay format facilitates the monitoring of the bending of 16 MCSs in parallel. The 16 MCSs are grouped within four separate wells. Each well can be addressed independently by different analyte liquids. This enables functionalization of MCS separately by flowing different solutions through each well. In addition, each well contains a fixed reference mirror which allows measuring the absolute bending of MCS. In addition, the mirror can be used to follow refractive index changes upon mixing of different solutions. The effect of the flow rate on the MCS bending change was found to be dependent on the absolute bending value of MCS. Experiments and finite element simulations of solution exchange in wells were performed. Both revealed that one solution can be exchanged by another one after 200 microl volume has flown through. Using this device, the adsorption of thiolated DNA molecules and 6-mercapto-1-hexanol on gold surfaces was performed to test the nanomechanical response of MCS.

Driven Localized Excitations in the Acoustic Spectrum of Small Nonlinear Macroscopic and Microscopic Lattices

Both bright and dark traveling, locked, intrinsic localized modes (ILMs) have been generated with a spatially uniform driver at a frequency in the acoustic spectrum of a nonlinear micromechanical cantilever array. Complementary numerical simulations show that a minimum density of modes, hence array size, is required for the formation of such locked smoothly running excitations. Additional simulations on a small 1D antiferromagnetic spin system are used to illustrate that such uniformly driven running ILMs should be a generic feature of a nanoscale atomic lattice.

Investigation into the Chemical Reactivity of Plasma-Deposited Perfluorophenyl Methacrylate Using Infrared Reflection Absorption Spectroscopy and Microcantilever Studies

Perfluorophenyl methacrylate (pp-PFM) was plasma-polymerized using low duty cycle conditions to yield a surface rich in active ester groups. The reactivity of this surface towards different primary amines was investigated using infrared reflection absorption spectroscopy (IRRAS) and micromechanical cantilever (MC) sensors. While IRRAS provided information on the chemistry, the MC sensors technique gave insights into volume changes induced by the reactions of the polymer film. We found that the volume change upon reaction was different for each of the amines studied and correlated this to the ability of the amines to diffuse into the polymer matrix. The changes observed can be related to reactions occurring either at the liquid–solid interface or to reactions occurring within the matrix of the polymer and appear to be related to the chemical structure of the amines.

Analyzing Gene Expression Using Combined Nanomechanical Cantilever Sensors

In diseases such as cancer or during viral infections gene expression is greatly altered. These changes in gene activity can be analysed at different levels of cellular activity, like transcription activation, transcription and translation. Currently, no simple method is available to detect all these biochemical signals simultaneously and rapidly. Micromechanical cantilever sensor array technology is applied, because it has the advantage that sample preconditioning like labelling and amplification is not required. Furthermore, DNA, RNA, protein or combinations thereof could be detected in parallel on a single cantilever array. With such a device, diagnosis and therefore treatment of diseases can be improved. Here we present successive detection of DNA hybridization and antigen using the same micromechanical cantilever sensor array.

Real time cantilever signal frequency determination using digital signal processing

We describe a digital signal processing method for high precision frequency evaluation of approximately sinusoidal signals based on a computationally efficient method. We demonstrate frequency measurement enabling sensitive measurement of the oscillatory force exerted on a micromechanical cantilever. We apply this technique to detection of the force signal arising in a micromechanically detected magnetic resonance force microscopy electron spin resonance signal. Our frequency detection measurements agree well with the theoretical noise analysis presented here, and we find that due to the excellent sensitivity of optical displacement detection, our sensitivity is limited only by the thermal displacement noise of the cantilever.

Micromechanical Cantilever Technique: A Tool for Investigating the Swelling of Polymer Brushes

Polymer brush coatings are well-known for their ability to tailor surface properties in a wide range of applications from colloid stabilization to medicine. In most cases, the brushes are used in solution. Consequently, efforts were expended to experimentally investigate or theoretically predict the swelling behavior of the brushes in solvents of different qualities. Here, we show that the micromechanical cantilever (MC) sensor technique is a tool to perform time-resolved physicochemical investigations of thin layers such as polymer brushes. Complementary to scattering techniques, which measure the thickness, the MC sensor technique provides information about changes in the internal pressure of the brushes during a swelling and deswelling process. We show that the kinetics of both swelling and deswelling are dependent on solvent quality. Comparing the measured data with its thickness evolution, which was calculated based on the Flory-Huggins theory, we found that only the first 10% of the thickness increase of the polymer brush results in a significant pressure increase inside the polymer brush layer.

A piezoresistive microcantilever magnetic-field sensor with on-chip self-calibration function integrated

A micromachined piezoresistive cantilever magnetometer, with a self-calibration function on-chip integrated is presented . When the cantilever is subjected to a magnetic field to be measured, the magnetic force will exert upon the magnetized nickel thin-film pattern that is located at the cantilever end. The magnetic force bends the micromechanical cantilever, which is further read out by an integrated piezoresistor. For realizing the self-calibration function, an aluminum spiral is integrated around the cantilever to provide an artificial magnetic field, when an electric current flows through the spiral coil. The artificial magnetic field can be used to drive the cantilever bending and causes a self-calibration output signal. With this on-chip self-calibration scheme, the detection of magnetic field can be immune to the long-term drift in remanence of the magnetized nickel pattern, thereby, improving the sensing stability. Bulk micromachining technologies are used to fabricate the sensors. The formed sensor is used for magnetic-field measurement, resulting in the piezoresistive sensitivity as 1.06×10−4/mT and the sensing resolution 4.58μT.

Simultaneous magnetic and electrostatic driving of microcantilevers

This paper shows that magnetic driving of cantilevers can be accompanied by a simultaneous, out-of-phase, electrical driving. As frequency increases, the increasing voltages on the solenoid not shielded properly enhance electrostatic forces acting on cantilevers placed in its neighbourhood. These forces drive efficiently cantilevers in air and also those immersed in water, although dielectric relaxations in the liquid reflecting its ability to react against the varying electric field, affect this later driving. This electrostatic contamination of the main driving (magnetic) degrades performances of feedback loops built around cantilevers, but it gives a simple reason for the high effectiveness of magnetic driving setups to excite high-frequency vibration modes in micromechanical cantilevers.

Thin film interference in the optomechanical response of micromechanical silicon cantilevers

The mechanical response of uncoated silicon microcantilevers is shown to modulate as a function of incident wavelength. Cantilever motion is measured interferometrically, using phase sensitive detection in response to a mechanically chopped excitation source. Thin film interference modeling shows that the fraction of absorbed light within the cantilever varies periodically over the range of 450–1000nm, in excellent agreement with the measurements. The results show that the optomechanical responsivity of these cantilevers can be tuned due to the effect via an appropriate selection of incident wavelength, incidence angle, lever thickness, and optical constants of the lever.

Resonance properties and mass sensitivity of monolithic microcantilever sensors actuated by piezoelectric PZT thick film

The PZT thick film cantilever devices fabricated via MEMS process have much attraction because they are appropriate for biological transducer or sensor, resulting from their large actuating force and relatively high sensitivity especially in liquid. By means of resonance behavior, theoretical calculation and experimental verification of the PZT thick film cantilever devices have not been studied before. Accordingly, we focused on the sensitivity analysis and interpretation of the PZT thick film cantilevers in this study. Especially, the investigation for mass sensitivity of the PZT thick film cantilever is of importance for physical, chemical and biological sensing application. The PZT thick film cantilever devices were constructed on Pt/TiO2/SiNX/Si substrates using screen printing method and MEMS process. The harmonic oscillation response (resonance frequency) was measured using an optical laser interferometric vibrometer. The effect of cantilever geometry on the resonance frequency change was investigated. Compared with the theoretical resonant frequency change by mass loading, the experimental resonant frequency change of the PZT micromechanical thick film cantilever shows a variation of less than 2%. Mass sensitivities are estimated to be 30.7, 57.1 and 152.0 pg/Hz for the 400 × 380 μm, 400 × 480 μm and 400 × 580 μm cantilever, respectively.

Monitoring the Formation of Self-Assembled Monolayers of Alkanedithiols Using a Micromechanical Cantilever Sensor

Using a micromechanical cantilever device, the surface stress induced during the growth of alkanedithiol (HS(CH2)nSH) monolayers on gold in solution is continuously monitored and reported. Adsorption of alkanedithiols of varying chain lengths is observed and compared to each other, as well as to the adsorption of hydroxyalkanethiols (HS(CH2)nOH) and alkanethiols (HS(CH2)nCH3). The results have revealed a significant change in surface stress on the basis of the chain length of the alkanedithiol. The long-chain (n > 10) alkanedithiol adsorption imposes a tensile stress on the gold-coated surface of the cantilever rather than the compressive stress exhibited by both alkanethiols and short-chain dithiols. Our results suggest a phenomenon in which the two thiols of the alkanedithiol adsorb onto the gold surface forming a loop inducing a tensile stress on the cantilever for long chain lengths. This study shows that micromechanical cantilever sensors can be very valuable tools in the exploration and characterization of self-assembled monolayers.

Swelling of cross-linked polystyrene spheres in toluene vapor

The swelling behavior of individual micron-sized polystyrene (PS) spheres in toluene vapor was studied via mass loading by means of micromechanical cantilever sensors. For 4%–8% cross-linked PS a mass increase of 180% in saturated toluene vapor was measured. The mass of the swollen PS sphere decreases with increasing exposure time to ultraviolet light. In addition, the swelling response is significantly different between the first and the second exposure to toluene vapor. This is attributed to the formation of a cross-linked shell at the surface of the PS spheres. Shape persistent parts were observed for locally irradiated PS spheres.

Multi-frequency response from a designed array of micromechanical cantilevers fabricatedusing a focused ion beam

We demonstrate arrays of cantilevers with different lengths, fabricated by focused ion beammilling. The arrays of oscillators generate a spectrum of different resonant frequencies, whereeach frequency correlates to the corresponding individual cantilever. The frequency responsefrom all the cantilevers is collected from a single measurement under the same environmentand conditions for the entire array. The mass response of the system generated the sameΔf/f0 for the cantilevers, within 0.1% accuracy. We denote the method MFSAC: multi-frequencysignal analysis from an array of cantilevers. The simultaneous detection of severalfrequencies in one spectrum has great benefits in mass sensor applications, offering thepossibility for true label-free detection.

Investigation of micromechanical cantilever sensors with microfocus grazing incidence small-angle x-ray scattering

Micromechanical cantilever arrays coated with polymer brushes are an alternative to traditional chemical sensors. A critical parameter for the sensor sensitivity is the coating quality, and small changes in the coating uniformity generate a different behavior of the sensors. Microfocus grazing incidence small-angle x-ray scattering proved to be a sensitive tool to characterize coating quality, enabling a structural and chemical coating analysis.

Thermal contributions to the bending of bimaterial cantilever sensors

The thickness dependent contribution of thermal expansion and surface potential to the thermal response behavior of polymer coated micromechanical cantilevers is investigated. Polystyrene films in a thickness range of 2–150nm are used as coatings on silicon micromechanical cantilever arrays. Depending on coating thickness, either thermal expansion or interfacial tension dominates the response. For thick films we obtain a thermal expansion coefficient of 2.3×10−5±1.0×10−5°C−1 for the polystyrene coating. Using this value a temperature coefficient of interfacial tension of −50×10−5±14×10−5Jm−2°C−1 is determined for the silicon oxide/polystyrene interface. This establishes a method to measure the temperature coefficient of interfacial tension at solid/solid interfaces.

A highly sensitive microsystem based on nanomechanical biosensors for genomics applications

Microcantilever-based biosensors are a promising tool to detect biomolecular interactions in a direct way with high accuracy. We show the development of a portable biosensor microsystem able to detect nucleic acid hybridization with high sensitivity. The microsystem comprises an array of 20 micromechanical cantilevers produced in silicon technology, a polymer microfluidic system for delivery of the samples, an array of 20 vertical cavity surface emitting lasers (VCSELs) with collimated beams thanks to an integrated microlens array, an optical coupling element to provide the optical path required, and chips with the photodetectors and the CMOS circuitry for signal acquisition and conditioning, capable of measuring the cantilever deflection with sub-nanometer resolution. Robust immobilization and regeneration procedures have been implemented for the oligonucleotide receptor sequences. In a further innovation, an optical waveguide cantilever transducer has been also developed in order to improve the final performance of the device. This has a number of advantages in terms of a simple optical geometry and improved sensitivity.