Resonant Microcantilever Research Articles

Microgravimetric Thermodynamic Modeling for Optimization of Chemical Sensing Nanomaterials

On the basis of microgravimetric sensing data, an analytical modeling method is proposed for comprehensive evaluation and optimization of gas sensing or adsorbing related functional materials. Resonant microcantilever is loaded with the material to be evaluated for a gravimetric sensing experiment. With sensing isotherm curves obtained at different temperatures, key thermodynamic and kinetic parameters of the material, such as enthalpy ΔH°, Gibbs free energy, adsorption rate constant Ka, and coverage θ, etc., can be quantitatively extracted for optimal selection and design. On the basis of the gravimetric experiment, the modeling method is used on three sorts of trimethylamine sensing nanomaterials of mesoporous silica nanoparticles (MSNs). The COOH-functionalized material is clearly identified as the best sensing material among the three similar ones, thereby validating high accuracy of the proposed model. Broad applicability of the modeling method to other sensing materials and/or target gases is also experimentally confirmed, where sensing properties of a functionalized hyper-branched polymer to organophorous simulant of dimethyl methylphosphonate (DMMP) are still evaluated well. In addition to sensing materials, the gravimetric experiment-based modeling method can be expanded to other functional materials like moisture absorbents or detoxification agents. Water adsorbing experiment on KIT-5 mesoporous-silica is modeled, with the low -ΔH° value (i.e., low adsorption heat) result, indicating that the KIT-5 is a good adsorbent to humidity. Alternatively, the modeled high -ΔH° value (i.e., high reaction heat) shows promising usage of SBA-15 mesoporous-silica as detoxification material to hazardous organophorous chemicals. Therefore, the analytical modeling technology can be used for developing and evaluating new adsorbing materials for gas sensing, fixing, and detoxification applications.

Mesoporous Silica Nanoparticles (MSNs) for Detoxification of Hazardous Organophorous Chemicals

The study reports the effect of mesoporous silica nanoparticles (MSNs) on detoxification of toxic organophorous compounds. Based on gravimetric sensing experiment with resonant microcantilever, rapid adsorption of the organophorous simulant of dimethyl methylphosphonate (DMMP) onto MSNs is confirmed. The experimentally observed irreversible gravimetric-signal implies that substitution-reaction possibly occurs at the nanomaterial surface. By exploring a method of gravimetric detection at different temperatures to obtain two isotherms, high reaction-heat of 97.1 kJ mol(-1) is extracted that indicates strong chemical interaction. Characterizations with solid-state NMR and FT-IR to the MSNs are performed during the adsorption/interaction process, revealing that substitution-reaction exactly occurs. GC-MS analysis to the post-reaction vapor exhaust indicates that one or two methyl groups in a DMMP molecule can be substituted by hydrogen atom(s) through substitution-reaction with silanol group(s) of MSNs, thereby, destructing DMMP into two sorts of new molecules. With such comprehensive analyses, the destruction/detoxification mechanism is clearly identified. To evaluate the detoxification performance of the MSNs, real toxic of dichlorvos is experimentally examined, resulting in that organophosphate dichlorvos is detoxified into non-toxic dimethylphosphate. The low-cost and producible MSNs are promising for detoxification to organophorous compounds. Besides, the micro-gravimetric analysis method can be expanding for extensive researches on various functional materials.

Fabrication of Electrostatically Actuated 4H-SiC Microcantilever Resonators by Using n/p/n Epitaxial Structures and Doping-Selective Electrochemical Etching

We fabricated electrostatically actuated single-crystalline 4H-SiC microcantilever resonators. To realize a narrow gap between cantilevers and substrate, we etched a thin p-type SiC layer in n/p/n multilayer structure by doping-selective electrochemical etching. The resonant characteristics of the fabricated 4H-SiC microcantilevers were investigated under a vacuum condition. Electrostatic actuation of microcantilevers was successfully performed by applying 10 mVrms ac voltage with 20 mV dc bias. The quality factor of 4H-SiC microcantilevers was above 100,000, which is about ten times higher than the quality factor of Si cantilevers with the same structure. Resonant characteristics were almost identical for mechanical actuation and electrostatic actuation.

Real-time enzyme-digesting identification of double-strand DNA in a resonance-cantilever embedded micro-chamber

A novel direct identification of double-strand DNA is proposed by using real-time enzyme-digestion in a resonant-cantilever embedded microfluidic chip. The new gene-level detection method is expected to replace the conventional DNA-hybridization based gene-detection that suffers from not only nonspecific adsorption induced false-positives but also complicated single-strand DNA preparation and hybridization. Since a detected DNA chain features a unique cutting site for a certain restriction-enzyme, the accurately cut-off mass (representing the length of the digested segment) can be online recorded by the frequency-shift signal of the resonant micro-cantilever sensor. This enzyme-digestion technique is confirmed by experimental identification of the stx2 gene of E. coli O157:H7. The direct-PCR sample is directly analyzed by using our lab-made cantilever-embedded microfluidic-chip. The 3776 bp DNA is immobilized via biotin-streptavidin binding and the added mass is recorded by a frequency-decrease of 15.9 kHz within 10 min. Then, with EcoRV-enzyme digestion at the site of 2635 bp, the cut-off mass is real-time detected by a frequency-increase of 10.2 kHz within 6 min. The detected frequency-shift ratio of 15.9/10.2 = 64.2% is consistent with the length ratio between the cut-off fragment and the whole DNA chain (2635/3776 = 69.8%). Hence, the simple and accurate double-strand detection method is verified experimentally.

DC current measurement utilizing a resonant magnetically actuated piezoresistive microcantilever

This paper presents a novel current sensor by using a resonant piezoresistive microcantilever combined with magnetic actuation for the first time. An actuation power of several microwatts was necessary for stable self-oscillation. The sensor measured the DC current by obtaining the electrothermally induced resonant frequency shift of the microcantilever as a result of the Joule heating dissipated when the DC current flowed through the Wheatstone bridge on the microcantilever. Two theoretical models were established between the microcantilever's resonant frequency and the square of the DC current. The experimental results showed that the accuracy in the range of 0.5–5 mA (apart from 0.5 mA) using the two models was 2.60% and 1.00% with correlation coefficient value R > 0.999 in both cases, respectively. The current sensitivity of the sensor was about −3.983 Hz mA−2 in the range of 0.5–5 mA. To maintain stable results, the sensor chip should be sealed in vacuum and integrated into an oven-control system.

Development of an innovative microcantilever-based biosensor for 17β-estradiol detection in bovine muscles: preliminary results

17β-estradiol is the most powerful substance with estrogenic effect, commonly used as illegal growth promoter in livestock production. To avoid health risks for consumers, sensitive, reliable and low-cost methods for quantification of extremely low concentrations of such carcinogenic residues in food are needed. Antibody-immobilised microcantilever resonators were proposed as innovative biosensors able to quantify an adsorbed target mass thanks to a shift in resonance frequency. Furthermore, the quantification of masses on the order of few picograms has recently shown to be successfully achievable with very high precision. In this study, we analysed the performance of our microcantilever sensors using extracted samples of bovine muscle from experimental animals, containing variable concentrations of 17β-estradiol (HPLC-MS/MS tested). Preliminary data showed that treated animals are correctly revealed, exhibiting large negative frequency shifts. More experiments, though, are needed to obtain a correct quantification of 17β-estradiol concentration.

Cantilever Resonator With Integrated Actuation and Sensing Fabricated Using a Single Step Lithography

Micro- and nano-mechanical resonators have been proposed for a variety of applications ranging from mass sensing to signal processing. Often their actuation and/or detection involve external subsystems that are much larger than the resonator itself. We have designed a simple microcantilever resonator with integrated sensor and actuator, facilitating the integration of large arrays of resonators. This unique design can be manufactured with a low-cost fabrication process, involving just a single step of lithography. The bilayer cantilever of gold and silicon dioxide is used as piezoresistive sensor as well as thermal bimorph actuator. The ac current used for actuation and the dc current used for piezoresistive detection are separated in the frequency-domain using a bias-tee circuit configuration. The resonant response is measured by detecting the second harmonic of the actuation current using a lock-in amplifier.

Process-induced stress and hydrogen effects on monolithic integrated BiCMOS-MEMS resonators

In this article, the simultaneous fabrication of MEMS and Bi-CMOS components on a monolithic integrated micro-cantilever resonator is investigated. Surface micromachining method for cantilever fabrication has been merged with conventional CMOS process, and release of MEMS structure is conducted after CMOS process. We for the first time propose the additive effects of hydrogen and MEMS steps induced mechanical stress on the IC performance, and at the same time provide a re-adjusting method for integrated process design, in which the NMOS/PMOS threshold voltage is re-adjusted to symmetry by utilizing the MEMS-process-induced IC performance degradation. On-chip digital/analog Bi-CMOS circuits function properly as demonstrated. The proposed re-adjustment process, suitable for fabrications with a variety of design CMOS-MEMS integrated process flexibilities, has the potential to fabricate integrated devices (high quality factor variable capacitors, micromirror arrays and multi-axis gyroscopes etc.).

Porous-layered stack of functionalized AuNP–rGO (gold nanoparticles–reduced graphene oxide) nanosheets as a sensing material for the micro-gravimetric detection of chemical vapor

With oleylamine as a solvent and reducing agent, Au nanoparticles (AuNPs) are grown in situ on graphene oxide (GO) sheets. Thereafter, a porous-layered stack of AuNPs–GO nanosheets is formed by a gas-phase chemical reduction step. After the AuNPs are modified with 11-mercaptoundecanoic acid (11-MUA) to graft –COOH sensing groups to the amine, the functionalized AuNPs–rGO porous-layer stacked nanosheets are loaded onto a gravimetric resonant microcantilever for use as a mass sensing material. The rGO sheets serve as multi-layer nano-shelves to support and carry the functionalized AuNPs for gas adsorbing/sensing, while the AuNPs serve as nano-spacers between the rGO sheets to provide a high specific surface area for gas molecules accessing the material. The cantilever sensor experimentally exhibits a very rapid response to ppm-level trimethylamine (TMA) vapor, which is attributed to the novel sensing material. Compared with the hydrophilic AuNP–GO, the highly hydrophobic AuNP–rGO shows a much improved suppression to the noise from changes in environmental humidity. Featuring a rapid response, high sensitivity and good resistance to interference from environmental moisture, the novel sensing nanomaterial is promising in various chemical vapor detection applications.

Tuning the Sensitivity of a Metal-Based Piezoresistive Sensor Using Electromigration

We report a simple method to enhance the piezoresistive sensitivity of a gold film by more than 30 times and demonstrate it using a microcantilever resonator. Our method depends on controlled electromigration that we use to tune the resistance and sensitivity of the piezoresistive sensor. We attribute the enhancement in strain sensitivity to the creation of an inhomogeneous conduction medium at a predefined location by directed and controlled electromigration. We understand this phenomenon with tunneling-percolation model, which was originally hypothesized to explain nonuniversal percolation behavior of composite materials.

Micro-/Nanocombined Gas Sensors With Functionalized Mesoporous Thin Film Self-Assembled in Batches Onto Resonant Cantilevers

This paper reports a novel top-down/bottom-up combined resonant microcantilever chemical sensor, where the nanosensing material of a functionalized mesoporous thin film (MTF) is directly self-assembled on the sensing region of the integrated microcantilever. By using the batch-producible nano-on-micro construction technique, a large number of such sensors can be batch fabricated with uniform performance and low cost. More importantly, the sensing molecule terminals can be simultaneously constructed at the pore inner surface when the MTF is directly grown on the cantilever. With -NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -group-functionalized MTF directly grown onto the surface of the cantilever free end, the micro-/nanocombined gravimetric sensor has experimentally exhibited quick response and highly sensitive detection of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas.

Immunodetection of 17β-estradiol in serum at ppt level by microcantilever resonators

To date control strategies in detecting anabolic agents for promoting growth of food producing animals are mainly related to screening techniques based on immunochemical and physiochemical methods, whose major limit is represented by relative low analytical sensitivity. As a consequence, consumers are currently exposed to molecules with potential carcinogenic effects such as 17β-estradiol, the most powerful substance with estrogenic effect. Therefore, high analytical sensitivity screening and confirmatory methods are required, coupling easiness of use and efficiency. We here report on the immunodetection of 17β-estradiol in serum by antibody-immobilized microcantilever resonators, an innovative biosensing platform able to quantify an adsorbed target mass (such as cells, nucleic acids, biomolecules, etc.) thanks to a shift in resonance frequency. Our tool based on microcantilever resonator arrays has shown to be capable of discriminating treated and untreated animals, showing the ability of detecting traces of 17β-estradiol in serum at concentrations lower than the present accepted physiological serum concentration threshold value (40ppt) and commercial ELISA tests (25ppt). The method exhibits a limit of detection of 20ppt and a limited cross-reactivity with high concentrations (10ppb) of similar molecules (testosterone).

Development of a microcantilever-based immunosensing method for mycotoxin detection

Mycotoxins, such as aflatoxins and ochratoxin A, are presently considered as the most important chronic dietary risk factor, more than food additives or pesticide residues. Therefore, the serious health and economic consequences of mycotoxin contamination have created the need for rapid, sensitive, and reliable techniques to detect such dangerous molecules within foodstuffs.We here report on the development of an innovative immunosensing method for mycotoxin detection, based on antibody-immobilized microcantilever resonators, a promising label free biosensing technique. A considerable part of the work is devoted to show the effect on microcantilever resonance frequency of the composition of the incubation buffer, as well as of the washing and drying procedure. We show the feasibility of using microcantilever resonator arrays to effectively identify total aflatoxins and ochratoxin A, at low concentrations (3ng/mL and less than 6ng/mL, respectively), with relatively low uncertainty (about 10%) and good reproducibility for the same target concentration. Furthermore, the developed immunosensing method shows a limited cross-reactivity to different mycotoxins, paving the way to a highly specific technique, able to identify different mycotoxins in the sample. To our knowledge, this work represents the first example in literature of successfully immunodetection of low concentrations of multiple mycotoxins by microcantilever resonator arrays.

Chemo-Mechanical Joint Detection with Both Dynamic and Static Microcantilevers for Interhomologue Molecular Identification

The study presents a novel chemo-mechanical joint-sensing method to distinguish a certain molecule from its homologous chemicals, using both a resonant cantilever for gravimetric sensing and a static cantilever for surface-stress sensing. Homologous amines of trimethylamine (TMA, Me(3)N), dimethylamine (DMA, Me(2)NH), and monomethylamine (MMA, MeNH(2)) are herein used as model objects for investigation. The molecular identification is based on experimental characterizations on both molecule adsorbing capability (by the resonant cantilever) and intermolecular lateral interaction (by the static cantilever). The intensities of the two sets of sensing signals are expected to be in opposite sequence with each other, due to the complementary relationship among the interhomologue molecule structures, i.e., a molecule containing a greater number of methyl substituents must possess a fewer number of nonsubstituted hydrogens. On the basis of the proposed idea, ppm-level vapors of the three amines are sequentially detected by a resonant microcantilever to characterize the molecular adsorption speed and another static cantilever to characterize the intermolecular lateral attraction induced surface stress. From the experiment, a pair of opposite sequence in sensing-signal amplitude has indeed been obtained that verifies the proposed joint-sensing method. In addition, the two sensing signals both show a linear relationship with chemical concentration (at low-concentration range). Further comparison between the two sensing results can help to build a model to identify the molecule among a series of its homologous chemicals by eliminating the influence from concentration. Since a complementary relationship among homologous molecule structures widely exists, the dual-sensing method is promising in on-the-spot rapid molecular identification among homologous chemicals.

Characterization of Disturbances in Systems of Coupled Micro-Resonator Arrays

This paper describes a method for processing signals from an array of coupled resonators to detect perturbations due to external stimuli. This method is based on simultaneous monitoring of the relative changes in all eigenvalues of the characteristic equation and processing this information to quantify the amount of perturbations and to determine the resonators whose properties have changed. Since the method is based on the relative movements of eigenvalues, the variations in the absolute values of the eigenfrequencies do not strongly affect its accuracy. It will be shown that this technique is capable of detecting perturbations even if direct signals from some of the coupled resonators are not available. The role of coupling strength on the sensitivity of the system is also discussed. It is shown that it is possible to increase the sensitivity of the sensor system to perturbations by a large factor through proper selection of the coupling coefficient between the resonators. The proposed model is experimentally verified using an array of coupled micro-cantilever resonators fabricated in a standard micro-fabrication process.

Real‐Time Quantitative Monitoring of Specific Peptide Cleavage by a Proteinase for Cancer Diagnosis

Good vibrations: A bioassay using a resonant peptide-functionalized microcantilever enables the quantitative characterization of the proteolytic activity of membrane type-1 matrix metalloproteinase (MT1-MMP). In this assay, shifts in the frequency of the cantilever after specific proteolytic cleavage of the target peptides by MT1-MMP are measured (see picture).

Optimization of the performances of SU-8 organic microcantilever resonators by tuning the viscoelastic properties of the polymer

This paper describes a promising method to improve the performances of organic resonant microcantilevers. This method is based on the tuning of the viscoelastic properties of the material by controlling internally the temperature of the structure. Actuation of the SU-8 free-standing microstructures has been integrated by means of electromagnetic forces while a static resistive thermal effect, obtained by applying a DC current in a conducting path, has been used to control the temperature of the structure. Investigation of the electromagnetic and thermal influence on the static and dynamic behavior has shown that the thermal effect dominates the electromagnetic one in static mode and inversely in dynamic mode. As a result, by applying a constant heating of the structure, the viscoelastic properties of polymers have been tuned, and the quality factor of SU-8 microcantilevers has been thereby enhanced by a factor 2 in air and 2.25 in liquid.

Thermomechanical stability of ultrananocrystalline diamond

We have measured mechanical stiffness and dissipation in ultrananocrystalline diamond (UNCD) from 63 K to 450 K using microcantilever resonators in a custom ultrahigh vacuum (UHV) atomic force microscope. UNCD exhibits a temperature coefficient of modulus that is found to be extremely low: −26 ppm/K, which is close to the previously measured value of −24 ppm/K for single crystal diamond. The magnitude and the temperature dependence of dissipation are consistent with the behavior of disordered systems. The results indicate that defects, most likely at the grain boundaries, create the dominant contribution to mechanical dissipation. These measurements of modulus and dissipation versus temperature in this temperature range in UNCD establish the nanostructure’s effect on the thermomechanical stability and suggest routes for tailoring these properties.

Hyper-branched sensing polymer directly constructed on a resonant micro-cantilever for the detection of trace chemical vapor

A hyper-branched polymer is layer-by-layer self-assembled on a resonant micro-cantilever and, then, functionalized with sensing-terminals for the specific detection of the trace chemical vapor of dimethyl methylphosphonate (DMMP, a typical simulant for nerve agents). The hyper-branched polymer is directly constructed on the SiO2 surface of the cantilever via an A2 + B4 layer-by-layer route, where A2 and B4 are complementary interacting groups which undergo coupled linking. After modification with 4-(2-(4-(allyloxy)phenyl)-1,1,1,3,3,3-hexafluoropropan-2-yl)phenol (APHFPP) groups specific to DMMP, the high specific-surface-area hyper-branched polymer provides very dense sensing sites to adsorb a great number of DMMP molecules for micro-gravimetric detection. Moreover, the sensing polymer possesses a “more branches but fewer roots” configuration on the cantilever surface to depress the cross-talk effect caused by adsorption induced cantilever spring-stiffening. Experimental results indicate that, self-assembled with the hyper-branched sensing polymer, the resonant cantilevers exhibit rapid and reproducible detection of trace DMMP (with the detection limit lower than 7.2 ppb) and effectively depressed parasitic frequency-shift from the cantilever spring stiffening effect. In addition, the sensor features satisfactory selectivity in the presence of water and organic solvents. When an alternative sensing-group of 2-allylhexafluoroisopropanol (AHFIP) is modified on the hyper-branched architecture, the cantilever becomes specifically sensitive to trace explosive vapor. Therefore, the developed technique for the functionalization of hyper-branched polymer directly grown on a cantilever provides a widely usable micro/nano sensing-platform for the detection of trace chemical vapors.

Characteristics of laterally vibrating resonant microcantilevers in viscous liquid media

The characteristics of microcantilevers vibrating laterally in viscous liquid media are investigated and compared to those of similar microcantilevers vibrating in the out-of-plane direction. The hydrodynamic loading on the vibrating beam is first determined using a numerical model. A semi-analytical expression for the hydrodynamic forces in terms of the Reynolds number and the aspect ratio (beam thickness over beam width) is obtained by introducing a correction factor to Stokes’ solution for a vibrating plate of infinite area to account for the effects of the thickness. The results enable the effects of fluid damping and effective fluid mass on the resonant frequency and the quality factor (Q) to be investigated as a function of both the beam’s geometry and liquid medium’s properties and compared to experimentally determined values given in the literature. The resonant frequency and Q are found to be higher for laterally vibrating microcantilevers compared to those of similar geometry experiencing transverse (out-of-plane) vibration. Compared to transversely vibrating beams, the resonant frequency of laterally vibrating beams is shown to decrease at a slower rate (with respect to changes in viscosity) in media having higher viscosities than water. The theoretical results are compared to experimental data obtained for cantilevers completely immersed in solutions of varying aqueous percent glycerol. The increases in resonant frequency and Q are expected to yield much lower limits of detection in liquid-phase chemical sensing applications.