Microcantilever Sensors Research Articles

Impact of nano- and mesoscale particles on the performance of microcantilever-based sensors.

Microcantilever-based sensors comprise an emerging class of chemomechanical sensors. The crucial challenge for every new and promising sensing platform lies in its performance in complex mixtures. Since most biofluids are rich in particulates, we assessed the impact of particles in the liquid stream on the performance of microcantilever sensors operated in both deflection and resonance modes. For both detection modes, sensor response depends on the particle diameter, concentration, and velocity as well as the composition of a thin-film coating. The presence of particles in the fluid stream produce substantial scattering of the laser beam used to measure cantilever deflection. Thus, prior removal of particulate matter from biofluids is required for optimal performance of microcantilever-based biosensors.

Spatial optimization of an array of aligned microcantilever based sensors

The present work establishes the optimized spatial arrangement for an array of aligned microcantilever sensors placed inside a fluidic cell. The results of the present investigation show that the optimum spacing distance between the microcantilevers decreases as the fluidic cell height decreases. An increase in the flow Reynolds number is found to increase the optimum spacing distance. However, this increase is not noticeable at relatively small Reynolds numbers where the flow exhibits creep characteristics. Hydromagnetic effects are found to decrease the optimum spacing distance. The results of the present investigation show that the location of the microcantilevers from the wall of the fluidic cell also plays a significant role on the optimum distance between the microcantilevers. Finally, a correlation for the optimum spacing distance between the microcantilevers is obtained for various pertinent parameters. This correlation can be used to determine the minimum size of biochips that will produce similar flow conditions on each microcantilever, resulting in independent microcantilever detections for different analytes.

An anti E. coli O157:H7 antibody-immobilized microcantilever for the detection of Escherichia coli (E. coli).

A silicon microcantilever sensor was developed for the detection of Escherichia coli O157:H7. The microcantilever was modified by anti-E. coli O157:H7 antibodies on the silicon surface of the cantilever. When the aquaria E. coli O157:H7 positive sample is injected into the fluid cell where the microcantilever is held, the microcantilever bends upon the recognition of the E. coli O157:H7 antigen by the antibodies on the surface of the microcantilever. A negative control sample that does not contain E. coli O157:H7 antigen did not cause any bending of the microcantilever. The detection limit of the sensor was 1 x 10(6) cfu/mL when the assay time was < 2 h.

Fabrication and characterization of SiO 2 microcantilever for microsensor application

SiO 2 microcantilevers were developed to improve the sensitivity of microcantilever sensors. Silicon plasma dry etching technique was employed to release SiO 2 cantilever from bulk silicon at high rate using an ordinary 2 μm thickness of SiO 2 covered silicon wafer. The V-shaped microcantilever is 200 μm long, 25 μm wide, and 2 μm thick. The spring constant of the cantilevers was measured to be 0.104 N/m. Significant deflection amplitude of the microcantilever was observed upon exposure of low concentration of aminoethanethiol due to its low spring constant.

Viral detection using an embedded piezoresistive microcantilever sensor

The detection of aerosol-based vaccinia virus and solution-based vaccinia virus has been studied using a new type of microsensor design. In the embedded piezoresistive microcantilever (EPM) sensor, a tiny piezoresistive microcantilever is embedded or partially embedded into an active sensing material whose volumetric response to the presence of an analyte is recorded as a strain in the cantilever, and subsequent change in the resistance of the cantilever. In one set of experiments, a composite consisting of poly(ethylene oxide), or PEO, combined with vaccinia antibody was used as the active sensing material. In aerosol exposures, this sensor produced a distinct signal owing to vaccinia virus adsorption/incorporation. In addition, experiments using a piezoresistive microcantilever in direct contact with a pure antibody layer in solution were also performed to detect vaccinia virus in solution. Again, a clear and distinct analyte signals obtained.

Improved detection of thermally induced higher resonance modes and harmonics of a microcantilever

Driving a microcantilever externally may not be desirable in many sensor applications. Alternatively, it is possible to extract full spectral characteristics of the anharmonic thermal motion of a microcantilever naturally vibrating at ambient temperature. Present work aims at the detection of comparatively noise free higher resonance modes and harmonics of thermal vibration for sensor applications without externally induced vibration. In microcantilever sensor based experiments with optical detection of cantilever deflection, we demonstrate the problems associated with the conventional procedure of processing photodetector signal for resonating microcantilevers and describe improvements. It has been experimentally demonstrated that isolation of the dynamic component of a position sensitive photodetector signal from its static counterpart significantly improves the resolution and limit of detection of an instrument. Outputs from conventional and proposed methods have been compared with experiments performed in both ambient air and liquid environments. A very simple and cost-effective circuit design is presented.

Electron transfer behaviour of biological macromolecules towards thesingle-molecule level

Redox metalloproteins immobilized on metallic surfaces in contact with aqueousbiological media are important in many areas of pure and applied sciences. Redoxmetalloprotein films are currently being addressed by new approaches wherebiotechnology including modified and synthetic proteins is combined withstate-of-the-art physical electrochemistry with emphasis on single-crystal,atomically planar electrode surfaces, in situ scanning tunnelling microscopy(STM) and other surface techniques. These approaches have broughtbioelectrochemistry important steps forward towards the nanoscale andsingle-molecule levels. We discuss here these advances with reference to two specificredox metalloproteins, the blue single-copper protein Pseudomonas aeruginosaazurin and the single-haem protein Saccharomyces cerevisiae yeast cytochromec, anda short oligonucleotide. Both proteins can be immobilized on Au(111) bychemisorption via exposed sulfur-containing residues. Voltammetric, interfacialcapacitance, x-ray photoelectron spectroscopy and microcantilever sensor data,together with in situ STM with single-molecule resolution, all point to a coherentview of monolayer organization with protein electron transfer (ET) functionretained. In situ STM can also address the microscopic mechanisms for electrontunnelling through the biomolecules and offers novel notions such as coherentmulti-ET between the substrate and tip via the molecular redox levels.This differs in important respects from electrochemical ET at a singlemetal/electrolyte interface. Similar data for a short oligonucleotide immobilizedon Au(111) show that oligonucleotides can be characterized with comparabledetail, with novel perspectives for addressing DNA electronic conductionmechanisms and for biological screening towards the single-molecule level.

Adsorption and Interfacial Electron Transfer ofSaccharomycesCerevisiaeYeast CytochromecMonolayers on Au(111) Electrodes

We have studied the adsorption and electron-transfer dynamics of Saccharomyces cerevisiae (yeast) iso-1-cytochrome c adsorbed on Au(111) electrodes in aqueous phosphate buffer media. This cytochrome possesses a thiol group close to the protein surface (Cys102) suitable for linking the protein to gold without drastic protein unfolding. A comprehensive approach, based on linear sweep and differential pulse voltammetry, capacitance measurements, X-ray photoelectron spectroscopy (XPS), in situ scanning tunneling microscopy (STM), and microcantilever sensor (MCS) techniques has been used. The voltammetric data display a thiol reductive desorption signal corresponding to close to monolayer coverage. Reductive desorption is also reflected in a capacitance peak. Voltammetric signals from the heme group in both native and partially denatured states could also be detected. XPS shows clear Au−S bond formation, but this observation is not conclusive for aqueous buffer conditions, as the protein is extensively unfolded under ultrahigh vacuum conditions needed for XPS. In situ STM discloses clear sub-monolayer coverage to molecular resolution. Imaging is robust in a 0.2 V electrochemical potential range negative of the equilibrium potential of YCC, where the protein is electrochemically functional. The MCS data show tensile differential stress signals when YCC is adsorbed on a gold-coated MCS, with distinguishable adsorption phases in the time range from <102 s to several thousand seconds. Comprehensive approaches to the mapping of adsorbed functional redox metalloproteins toward the single-molecule level, such as in the present study, will be important in the construction of nanoscale devices for multifarious biological and environmental screening.

Adsorption kinetics and mechanical properties of thiol-modified DNA-oligos on gold investigated by microcantilever sensors

Immobilised DNA-oligo layers are scientifically and technologically appealing for a wide range of sensor applications such as DNA chips. Using microcantilever-based sensors with integrated readout, we demonstrate in situ quantitative studies of surface-stress formation during self-assembly of a 25-mer thiol-modified DNA-oligo layer. The self-assembly induces a surface-stress change, which closely follows Langmuir adsorption model. The adsorption results in compressive surface-stress formation, which might be due to intermolecular repulsive forces in the oligo layer. The rate constant of the adsorption depends on the concentration of the oligo solution. Based on the calculated rate constants a surface free energy of the thiol-modified DNA-oligo adsorption on gold is found to be −32.4 kJ mol −1. The adsorption experiments also indicate that first a single layer of DNA-oligos is assembled on the gold surface after which a significant unspecific adsorption takes place on top of the first DNA-oligo layer. The cantilever-based sensor principle has a wide range of applications in real-time local monitoring of chemical and biological interactions as well as in the detection of specific DNA sequences, proteins and particles.

Adsorption–desorption noise in micromechanical resonant structures

In this paper we analyze adsorption–desorption (AD) noise in resonant microelectromechanical structures. This noise is generated by instantaneous differences in the rates of adsorption and desorption of contaminant molecules to and from the resonator surface, which cause mass fluctuations and consequentially frequency fluctuations. Contrary to the usual statistical approach, we used the analogy between AD processes in resonant structures and generation–recombination processes in semiconductors. Starting from that general analogy and using the theory of Langmuir’s isotherm, since it is the most convenient for this case, we derived an exact expression for the AD fluctuations-induced phase noise for the case of one adsorption layer. Our numerical results show that AD induced phase noise is comparable to other noise sources and becomes dominant for very small resonator beam dimensions. The presented theory is applicable to the determination of the limiting performance of various micromechanical resonators and microcantilever sensors and also for the theoretical studying of AD processes in general. In addition, phase noise measurements and their comparison with the theory are convenient for the investigation of catalytic AD processes.

Investigation of adsorption and absorption-induced stresses using microcantilever sensors

The interaction between a vapor and a thin film adsorbed on one side of a bimaterial microcantilever produces differential stress, resulting in readily measurable curvatures of the cantilever structure. Depending upon the system studied, there exist two types of gas–solid interaction: bulk-like absorption and surface-like adsorption. The absorption of hydrogen into palladium results in film expansion whose magnitude is governed by hydrogen partial pressure. The bending of a bimaterial microcantilever (palladium/silicon) due to hydrogen absorption depends on the thickness of the palladium film and is reversible but rate limited by a surface barrier. In contrast, the stress induced by adsorption of mercury onto a bimaterial (gold/silicon) cantilever is irreversible at room temperature, is rate limited by surface coverage, and is independent of the gold–film thickness.

Manipulation and controlled amplification of Brownian motion of microcantilever sensors

Microcantilevers, such as those used in atomic force microscopy, undergo Brownian motion due to mechanical thermal noise. The root mean square amplitude of the Brownian motion of a cantilever typically ranges from 0.01–0.1 nm, which limits its use in practical applications. Here we describe a technique by which the Brownian amplitude and the Q factor in air and water can be amplified by three and two orders of magnitude, respectively. This technique is similar to a positive feedback oscillator, wherein the Brownian motion of the vibrating cantilever controls the frequency output of the oscillator. This technique can be exploited to improve sensitivity of microcantilever-based chemical and biological sensors, especially for sensors in liquid environments.

Sensor based on piezoresistive microcantilever technology

A new type of micromechanical sensor has been developed that incorporates piezoresistive microcantilevers, such as those used in scanning force microscope (SFM) instruments. In these devices, the microcantilever is in direct contact with an active sensing material, which may be a common organic polymer or a biologically active sensing layer. Upon analyte exposure, the cantilever directly measures the swelling or volume expansion experienced by an active sensing material. Individual microcantilever sensors may be incorporated into an array in which pattern recognition techniques are used to identify a wide variety of chemical or biological analytes. In addition, these devices are compact, lightweight, and require only simple, inexpensive support electronics.

Modification of micro-cantilever sensors with sol-gels to enhance performance and immobilize chemically selective phases

A chemical sensor based on the deflection of a surface modified silicon micro-cantilever is presented. A thin film of sol-gel was applied to one side of the micro-cantilever surface using a spin coating procedure. The sensor has been shown to give different responses to vapor phase analytes of varying chemical composition, as well as to varying concentrations of a given analyte. Ethanol, a highly polar molecule, exhibits a strong affinity for the polar sol-gel coating resulting in a large response; pentane, a non-polar hydrocarbon, shows very little response. The sol-gel coating has also been shown to function as a backbone for the immobilization of chemically selective phases on the cantilever surface. Reaction of the sol-gel film with chlorotriethoxysilane and subsequent capping of the remaining reactive surface silanols with hexamethyldisilizane increases the non-polar nature of the film. This results in an increase in the response of the sensor to non-polar analytes. The effects of film thickness and cantilever structure thickness on response were also investigated.

In situ scanning probe microscopy and new perspectives in analytical chemistry

The resolution of scanning tunnelling microscopy (STM) and other scanning probe microscopies is unprecedented but the techniques are fraught with limitations as analytical tools. These limitations and their relationship to the physical mechanisms of image contrast are first discussed. Some new options based on in situ STM, which hold prospects for molecular- and mesoscopic-scale analytical chemistry, are then reviewed. They are illustrated by metallic electro-crystallisation and -dissolution, and in situ STM spectroscopy of large redox molecules. The biophysically oriented analytical options of in situ atomic force microscopy, and analytical chemical perspectives for the new microcantilever sensor techniques are also discussed.

Infrared vision using uncooled micro-optomechanical camera

This letter presents the design, fabrication, and imaging results of an uncooled infrared (IR) camera that contains a focal plane array of bimaterial microcantilever sensors, and an optical readout technique that measures cantilever deflections in the nanometer range to directly project a visible image of the IR scene on the human eye or a visible camera. The results suggest that objects at temperatures as low as 100 °C can be imaged with the best noise-equivalent temperature difference (NEΔT) in the range of 10 K. It is estimated that further improvements that are currently being pursued can improve NEΔT to about 50 mK.

Self-excited piezoelectric PZT microcantilevers for dynamic SFM—with inherent sensing and actuating capabilities

Currently three crucial components are necessary for the dynamic SFM, they are the microcantilever, oscillator, and cantilever deflection sensor. We demonstrate a piezoelectric microcantilever made of PZT thin films for SFM. This PZT microcantilever can perform the same functions which have to be done by the three crucial components. It is able to excite itself when an ac voltage is applied to it, and actuate itself in z-directional displacement by a superimposed dc voltage, while force sensing is executed concurrently. Since the motional admittance can be derived from the piezoelectric current output when the PZT microcantilever is vibrated at resonance frequency, the motional admittance will vary as the change of vibrational amplitude. Thus the topography of the sample can be recorded as the trace of the difference between output signals of motional admittance. The piezoelectric coefficient d 31 of the PZT layer is about −58 pC/N. The dimensions of piezoelectric microcantilevers used are 125×50×3.8 μm. The actuating ability is estimated as 75 nm/V. The vertical resolution of the self-excited PZT microcantilever can be derived as 1.2 Å at the bandwidth of 125 Hz. Very clear images of a 1.0 μm pitch Au coated SiO 2 grating and an evaporated gold film on a smooth glass plate are obtained by an SFM using this self-excited force sensing PZT microcantilever. The invention of this smart PZT microcantilever endows the possibility of constructing a stand-alone micro SFM system and a multiprobe SFM system.