Resonant Microcantilever Research Articles

In situ growth of noble metal nanoparticles on graphene oxide sheets and direct construction of functionalized porous-layered structure on gravimetric microsensors for chemical detection

Noble metal nanoparticles are directly and homogeneously grown onto graphene-oxide (GO) sheets in oleylamine. After the oleylamine is removed, the GO sheets are exfoliated by the nanoparticle pillars to further form hierarchical GO nanostructures with molecule accessible nanopores. With specific sensing-groups modified, the porous-layered nanostructure can be constructed onto resonant microcantilevers for chemical sensing.

Trace Gas Sensor Based on MEMS Cantilever Resonator

A chemical gas sensor for volatile organic compounds (VOCs) detection at trace level is proposed. In this paper, the development and demonstration of the sensor prototype are presented. The prototype is based on a microcantilever resonator that is fabricated from direct bonding silicon-on-insulator (SOI) wafer. The resonant cantilever employs integrated thermal driving and piezoresistive detecting units, and operates in a self-oscillation system. Polyethylenevinylacetate (PEVA) is deposited on top of the cantilever as gas sensitive layer through a spraying method. The responses of the prototype to relative humidity (RH) and six common VOCs: toluene, benzene, ethanol, acetone, hexane and octane have been tested. The PEVA-coated prototype has trace sensitivity to toluene, benzene, hexane and octane, while is insensitive to humidity. The experimental results provide confirmation that the microcantilever resonator is an excellent platform for chemical gas sensor.

A chelating-bond breaking and re-linking technique for rapid re-immobilization of immune micro-sensors

With high sensitivity and specificity to antigen, immune micro-sensors can be used in rapid detection of pathogenic microbial. This study proposes and develops a method for rapidly regeneration of antibody on a resonant micro-cantilever sensor. A nitrilotriacetic acid (NTA) derivative is synthesized with cystine and bromoacetic acid, then added with 2-mercaptoethanol to prepare a mixed self-assembled monolayer (SAM) on Au (111) surface of the cantilever. Ni²⁺ ions are thereafter chelated on the mixed SAM to form a breakable and re-linkable chelating-bond layer. Repeatable cycles of antibody immobilization and erasing are experimentally validated with a detectable marker of synthesized biotinylated poly peptides harboring six histidine residues (named as His-Bio). Two distinguished pathogenic microbial, Escherichia. coli O157:H7 and Bacillus Anthracis, are detected with the rapidly regenerated sensor. The E. coli O157:H7 sensor exhibits a three-time repeated detection to the 10³ CFU/ml concentration microbial. Then, an E. coli O157:H7 sensor is eluted with Tris-HCl (20 mM Tris, 150 mM NaCl, 0.1% Tween 20, pH = 3.0) and rapidly reconstructed into a B. Anthracis sensor by changing the re-immobilized antibody. The cantilever sensor no longer responses to E. coli O157:H7 even in a high concentration of 10⁷ CFU/ml. In contrast, the sensor is experimentally confirmed being resoluble to low concentration B. Anthracis at 10³ spores/ml level. The proposed fast regeneration method is promising in repeatedly or multi-target detection applications of micro/nano immune-sensors, e.g. the resonant micro-cantilevers.

High Sensitivity of Diamond Resonant Microcantilevers for Direct Detection in Liquids As Probed by Molecular Electrostatic Surface Interactions

Resonant microcantilevers have demonstrated that they can play an important role in the detection of chemical and biological agents. Molecular interactions with target species on the mechanical microtransducers surface generally induce a change of the beam's bending stiffness, resulting in a shift of the resonance frequency. In most biochemical sensor applications, cantilevers must operate in liquid, even though damping deteriorates the vibrational performances of the transducers. Here we focus on diamond-based microcantilevers since their transducing properties surpass those of other materials. In fact, among a wide range of remarkable features, diamond possesses exceptional mechanical properties enabling the fabrication of cantilever beams with higher resonant frequencies and Q-factors than when made from other conventional materials. Therefore, they appear as one of the top-ranked materials for designing cantilevers operating in liquid media. In this study, we evaluate the resonator sensitivity performances of our diamond microcantilevers using grafted carboxylated alkyl chains as a tool to investigate the subtle changes of surface stiffness as induced by electrostatic interactions. Here, caproic acid was immobilized on the hydrogen-terminated surface of resonant polycrystalline diamond cantilevers using a novel one-step grafting technique that could be also adapted to several other functionalizations. By varying the pH of the solution one could tune the -COO(-)/-COOH ratio of carboxylic acid moieties immobilized on the surface, thus enabling fine variations of the surface stress. We were able to probe the cantilevers resonance frequency evolution and correlate it with the ratio of -COO(-)/-COOH terminations on the functionalized diamond surface and consequently the evolution of the electrostatic potential over the cantilever surface. The approach successfully enabled one to probe variations in cantilevers bending stiffness from several tens to hundreds of millinewtons/meter, thus opening the way for diamond microcantilevers to direct sensing applications in liquids. The evolution of the diamond surface chemistry was also investigated using X-ray photoelectron spectroscopy.

Measurements of the phase transition and the average length of the density fluctuation under supercritical fluid using micromechanical resonators

This paper presents an original method to measure the phase transition and macroscopic density fluctuation of supercritical carbon dioxide using a resonant microcantilever beam. There is a significant reduction of the resonant frequency near the phase transition point where the density and viscosity of carbon dioxide exhibit large variations, while the vibration amplitude of the micromechanical resonator displayed a large shift up due to generation of microscopic density fluctuation. The average length of the density fluctuation of supercritical fluid could be calculated from this large shift of the vibration amplitude and the value is in the range of millimeter.

Resonant Microcantilevers for the Determination of the Loss Modulus of Thin Polymer Films

The increasing interest in polymer materials creates the need for accurate tools to characterize their mechanical properties. Due to energy dissipation in polymers during deformation, these materials exhibit viscoelastic behavior. Accurate determination of these viscoelastic properties and, more specifically, viscous losses, remains challenging and mainly unknown for thin polymer films. In this paper, a straightforward method to determine the loss modulus of organic materials using resonating microcantilevers has been developed. The extracted results for polyisobutylene show the variation of viscous losses over a large range of frequencies (7-350 kHz).

Contactless electromagnetic switched interrogation of micromechanical cantilever resonators

A principle for contactless interrogation of passive micromechanical resonator sensors is proposed. The principle exploits an external primary coil electromagnetically air-coupled to a secondary coil which is connected to a conductive path on the resonator. The interrogation periodically switches between interleaved excitation and detection phases. During the excitation phase the resonator is driven into vibrations, while in the detection phase the excitation signal is turned off and the decaying oscillations are contactless sensed. The principle advantageously avoids magnetic properties required to the resonator, thereby ensuring compatibility with standard silicon microfabrication processes. The principle has been implemented on a MEMS SOI microcantilever resonator sensor with mechanical resonant frequency of 10.186kHz and has been demonstrated to work over a distance of up to 1cm. Tests based on the deposition and evaporation of a water droplet have demonstrated the capability to sense physical and chemical quantities which affect either the resonant frequency or the quality factor of the resonator.

MEMS biosensor for detection of Hepatitis A and C viruses in serum

Resonant microcantilever arrays are developed for the purpose of label-free and real-time analyte monitoring and biomolecule detection. MEMS cantilevers made of electroplated nickel are functionalized with Hepatitis antibodies. Hepatitis A and C antigens at different concentrations are introduced in undiluted bovine serum. All preparation and measurement steps are carried out in the liquid within a specifically designed flowcell without ever drying the cantilevers throughout the experiment. Both actuation and sensing are done remotely and therefore the MEMS cantilevers have no electrical connections, allowing for easily disposable sensor chips. Actuation is achieved using an electromagnet and the interferometric optical sensing is achieved using laser illumination and embedded diffraction gratings at the tip of each cantilever. Resonant frequency of the cantilevers in dynamic motion is monitored using a self-sustaining closed-loop control circuit and a frequency counter. Specificity is demonstrated by detecting both Hepatitis A and Hepatitis C antigens and their negative controls. This is the first report of Hepatitis antigen detection by resonant cantilevers exposed to undiluted serum. A dynamic range in excess of 1000 and with a minimum detectable concentration limit of 0.1ng/ml (1.66pM) is achieved for both Hepatitis A and C. This result is comparable to labeled detection methods such as ELISA.

Study on Resonant Characteristics of Closed-Loop Controlled Microcantilever

A kind of double-layer Si-based microcantilever was fabricated using standard integrated circuit and micro-mechanical machining technology. It was thermally-excited and the signal was detected based on piezoresistance effect of semiconductors. Taking the advantage of FPGA, the digital closed-loop system on silicon resonant microcantilever was implemented. On this basis, the resonant characteristics like amplitude of output signal, the resonant frequency shift, the frequency stability and the quality factor of this kind of microcantilever were studied through theoretical analysis and experiment. This paper focus on the dependence of amplitude of output signal on the exciting power, bias voltage as well as the dependence of resonant frequency shift of output signal on the exciting voltage, bias voltage. Analysis about the result of the experiment was given, which showed that this silicon microcantilever is suitable for resonant microsensor, and provided some advices for the optimum design of the system. It laid the foundation for the development of a new resonant structure of silicon micro-cantilever sensors.

Proteomic technologies for the identification of disease biomarkers in serum: Advances and challenges ahead

Serum is an ideal biological sample that contains an archive of information due to the presence of a variety of proteins released by diseased tissue, and serum proteomics has gained considerable interest for the disease biomarker discovery. Easy accessibility and rapid protein changes in response to disease pathogenesis makes serum an attractive sample for clinical research. Despite these advantages, the analysis of serum proteome is very challenging due to the wide dynamic range of proteins, difficulty in finding low-abundance target analytes due to the presence of high-abundance serum proteins, high levels of salts and other interfering compounds, variations among individuals and paucity of reproducibility. Sample preparation introduces pre-analytical variations and poses major challenges to analyze the serum proteome. The label-free detection techniques such as surface plasmon resonance, microcantilever, few nanotechniques and different resonators are rapidly emerging for the analysis of serum proteome and they have exhibited potential to overcome few limitations of the conventional techniques. In this article, we will discuss the current status of serum proteome analysis for the biomarker discovery and address key technological advancements, with a focus on challenges and amenable solutions.

Functionalized Mesoporous Silica for Microgravimetric Sensing of Trace Chemical Vapors

Featuring a huge surface-to-volume ratio, synthesized SBA-15 mesoporous silica is functionalized by inner-channel-wall modification of sensing groups for highly specific chemical-vapor detection at trace level. With the developed sensing material loaded on resonant microcantilevers, the specifically adsorbed chemical-vapor molecules act as an added mass to shift the cantilever resonant frequency for gravimetric sensing signal readout. Two kinds of sensing materials for trinitrotoluene (TNT) and ammonia/amine are respectively prepared by inner-wall layer-by-layer grafting functionalization. By using hexafluoro-2-propanol-functionalized mesoporous silica (HFMS), experimental results show highly specific and rapid detection of TNT vapor, with a ppt-level detection limit; functionalized with a carboxyl (COOH) group, the mesoporous silica is loaded onto the cantilever resonating sensor that experimentally exhibits an ultrafine detection limit of tens of ppb to ammonia/amine gases.

Optimization Strategy for Resonant Mass Sensor Design in the Presence of Squeeze Film Damping

This paper investigates the design optimization of an electrostatically actuated microcantilever resonator that operates in air. The nonlinear effects of electrostatic actuation and air damping make the structural dynamics modeling more complex. There is a need for an efficient way to simulate the system behavior so that the design can be more readily optimized. This paper describes an efficient analytical approach for determining the optimum design for a microcantilever resonant mass sensor. One simple case is described. The sensor design is a square plate that is coated with a functional polymer and attached to the substrate with folded leg springs. The plate has a square hole in the middle to reduce the effect of squeeze film damping. With the analytical approach, the optimum hole size for maximum sensitivity is found.

Self-assembly and sensing-group graft of pre-modified CNTs on resonant micro-cantilevers for specific detection of volatile organic compound vapors

This paper reports MWCNT (multi-wall carbon nano-tube)-modified resonant micro-cantilever chemical sensors for detection of trinitrotoluene (TNT) vapor. The MWCNTs are pre-modified and then area-selectively self-assembled at the free-end gold pad of a micro-cantilever, in which a resonance-exciting heater and a signal-readout piezoresistive Wheatstone bridge are integrated. Featuring a high specific surface area, the MWCNTs are further functionalized with TNT-sensitive groups by grafting onto the sidewalls of the MWCNTs. To lower the non-specific absorption of water and other small organic molecules, the SiO2 surface of the micro-cantilever was also pre-treated for hydrophobicity and oleophobicity by self-assembling a monolayer of heptadecafluorodecyltrimethoxysilane. The results of our sensing experiments have shown a capability to rapidly detect ppb-level TNT vapor, and a high specificity of the functionalized groups to TNT molecules. The experiment has also confirmed a good long-term stability in detecting sensitivity.

Electrothermal Driving Microcantilever Resonator as a Platform for Chemical Gas Sensing

In the current research, the use of a micromachined cantilever resonator as a platform for chemical gas sensing was examined. The microcantilever resonator integrates an electrothermal driving unit and a piezoresistive detecting unit, and it is fabricated by direct bonding a silicon-on-insulator (SOI) wafer. With a particular polymer layer coated on the surface of the microcantilever, a gas sensor for volatile organic components (VOCs) detection can be realized. The operation mechanism provides the microcantilever resonator with integrated circuit (IC) compatibility in terms of both the fabrication process and operating voltage. The configuration of the microcantilever resonator can optimize the performance of the gas sensor. The SOI wafer provides a solution for the integrated fabrication of the microstructure, transducers, electronics, and the precise control of the resonator parameters. In this paper, the principles, design, analysis, process, and demonstration of the gas sensor based on the microcantilever resonator are presented. The experimental results provide confirmation that the polymer-coated microcantilever resonator has excellent performance with regard to VOC detection.

Position and mass determination of multiple particles using cantilever based mass sensors

Resonant microcantilevers are highly sensitive to added masses and have the potential to be used as mass-spectrometers. However, making the detection of individual added masses quantitative requires the position determination for each added mass. We derive expressions relating the position and mass of several added particles to the resonant frequencies of a cantilever, and an identification procedure valid for particles with different masses is proposed. The identification procedure is tested by calculating positions and mass of multiple microparticles with similar mass positioned on individual microcantilevers. Excellent agreement is observed between calculated and measured positions and calculated and theoretical masses.

Characterization of the gas sensors based on polymer-coated resonant microcantilevers for the detection of volatile organic compounds

The gas sensors based on polymer-coated resonant microcantilevers for volatile organic compounds (VOCs) detection are investigated. A method to characterize the gas sensors through sensor calibration is proposed. The expressions for the estimation of the characteristic parameters are derived. The effect of the polymer coating location on the sensor's sensitivity is investigated and the formula to calculate the polymer–analyte partition coefficient without knowing the polymer coating features is presented for the first time. Three polymers: polyethyleneoxide (PEO), polyethylenevinylacetate (PEVA) and polyvinylalcohol (PVA) are used to perform the experiments. Six organic solvents: toluene, benzene, ethanol, acetone, hexane and octane are used as analytes. The response time, reversibility, hydrophilicity, sensitivity and selectivity of the polymer layers are discussed. According to the results, highly sensitive sensors for each of the analytes are proposed. Based on the characterization method, a convenient and flexible way to the construction of electric nose system by the polymer-coated resonant microcantilevers can be achieved.

Contactless electromagnetic switched interrogation of micromechanical cantilever resonators

A principle for contactless interrogation of passive micromechanical resonator sensors is proposed, in which an external primary coil is electromagnetically air-coupled to a secondary coil connected to a conductive path on the resonator. The interrogation periodically switches between interleaved excitation and detection phases. During the excitation phase the resonator is driven into vibrations, while in the detection phase the excitation signal is turned off and the decaying oscillations are contactless sensed. The principle advantageously avoids magnetic properties required to the resonator, thereby ensuring compatibility with standard microfabrication processes. Results are reported on the successful implementation on a MEMS SOI microcantilever resonator working as a mass sensor for which the deposition and evaporation of a water droplet have been detected.

Feedback-induced phase noise in microcantilever-based oscillators

When electronic feedback is used around resonant microcantilevers, their native resonance frequency f 0 and that under feedback f FB = f 0 ± Δ f differ. This not only gives a wrong measurement of f 0 due to the feedback-induced error ± Δ f, but also degrades the performances of oscillating loops. The static frequency shift Δ f appearing in constant-gain loops due to feedback forces in phase with z( t) becomes a frequency noise when the loop gain changes randomly as in oscillating loops by the action of noise on the gain limiter of the loop. This is the source of phase noise this paper reveals for the first time that can be summarized by this sentence: the imperfect actuation of feedback loops in cantilever-based oscillators is a source of their phase noise. We show how to reduce this noise by a tight phase control in the loop gain to drive cantilevers by forces exactly in quadrature respect to their instantaneous displacement z( t).

Nanomechanical In Situ Monitoring of Proteolysis of Peptide by Cathepsin B

Characterization and control of proteolysis of peptides by specific cellular protease is a priori requisite for effective drug discovery. Here, we report the nanomechanical, in situ monitoring of proteolysis of peptide chain attributed to protease (Cathepsin B) by using a resonant nanomechanical microcantilever immersed in a liquid. Specifically, the detection is based on measurement of resonant frequency shift arising from proteolysis of peptides (leading to decrease of cantilever's overall mass, and consequently, increases in the resonance). It is shown that resonant microcantilever enables the quantification of proteolysis efficacy with respect to protease concentration. Remarkably, the nanomechanical, in situ monitoring of proteolysis allows us to gain insight into the kinetics of proteolysis of peptides, which is well depicted by Langmuir kinetic model. This implies that nanomechanical biosensor enables the characterization of specific cellular protease such as its kinetics.

Mechanical stiffness and dissipation in ultrananocrystalline diamond microresonators

We have characterized mechanical properties of ultrananocrystalline diamond (UNCD) thin films grown using the hot filament chemical vapor deposition (HFCVD) technique at $680\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, significantly lower than the conventional growth temperature of $\ensuremath{\sim}800\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The films have $\ensuremath{\sim}4.3%$ $s{p}^{2}$ content in the near-surface region as revealed by near edge x-ray absorption fine structure spectroscopy. The films, $\ensuremath{\sim}1\text{ }\ensuremath{\mu}\text{m}$ thick, exhibit a net residual compressive stress of $370\ifmmode\pm\else\textpm\fi{}1\text{ }\text{MPa}$ averaged over the entire 150 mm wafer. UNCD microcantilever resonator structures and overhanging ledges were fabricated using lithography, dry etching, and wet release techniques. Overhanging ledges of the films released from the substrate exhibited periodic undulations due to stress relaxation. This was used to determine a biaxial modulus of $838\ifmmode\pm\else\textpm\fi{}2\text{ }\text{GPa}$. Resonant excitation and ring-down measurements in the kHz frequency range of the microcantilevers were conducted under ultrahigh vacuum (UHV) conditions in a customized UHV atomic force microscope system to determine Young's modulus as well as mechanical dissipation of cantilever structures at room temperature. Young's modulus is found to be $790\ifmmode\pm\else\textpm\fi{}30\text{ }\text{GPa}$. Based on these measurements, Poisson's ratio is estimated to be $0.057\ifmmode\pm\else\textpm\fi{}0.038$. The quality factors $(Q)$ of these resonators ranged from 5000 to 16000. These $Q$ values are lower than theoretically expected from the intrinsic properties of diamond. The results indicate that surface and bulk defects are the main contributors to the observed dissipation in UNCD resonators.