Environmental sensors based on micromachined cantilevers with integrated read-out

Effect of nanometer surface morphology on surface stress and adsorption kinetics of alkanethiol self-assembled monolayers

Microcantilevers undergo quasi-static bending due to adsorption-induced stress when adsorption is confined to a single surface. Understanding the origin of surface stress is crucial for optimizing sensor performance. We have investigated the effect of surface morphology of gold-coated cantilevers on the adsorption characteristics of self-assembled monolayers of alkanethiols. Self-assembly of alkanethiols from liquid phase closely follows Langmuir-type kinetics up to a single monolayer assembly. Our results indicate that alkanethiol adsorption-induced surface stress is largely unaffected by surface roughness of the cantilever. Unlike prior reports that suggest surface roughness enhances adsorption-induced stress, we observe that nanometer-size roughness slightly decreases surface stress and adsorption kinetics.

MOSFET-Embedded Microcantilevers for Measuring Deflection in Biomolecular Sensors

A promising approach for detecting biomolecules follows their binding to immobilized probe molecules on microfabricated cantilevers; binding causes surface stresses that bend the cantilever. We measured this deflection, which is on the order of tens of nanometers, by embedding a metal-oxide semiconductor field-effect transistor (MOSFET) into the base of the cantilever and recording decreases in drain current with deflections as small as 5 nanometers. The gate region of the MOSFET responds to surface stresses and thus is embedded in silicon nitride so as to avoid direct contact with the sample solution. This approach, which offers low noise, high sensitivity, and direct readout, was used to detect specific binding events with biotin and antibodies.

A Forecast of Developments in Scanned Probe Microscopy

From direct atom manipulation and nano-fabrication, to single molecule sensing and probing energy landscapes, the tools of the nanotech age are here. Scanned probe microscopies (SPM) offer opportunities to directly interact with matter in native environments and their evolution shows no signs of slowing. How might this toolkit adapt for new and outstanding problems in science? Here some directions are entertained and potential developments explored.

Nanomechanical sensing of DNA sequences using piezoresistive cantilevers

A microfabricated cantilever with an internal piezoresistive component has been sensitized with thiol tethered ss-DNA strands and utilized for an in situ, label-free, highly specific, and rapid DNA detection assay. The generation of a differential surface stress onto the functionalized cantilever surface upon target recognition has allowed nanomechanical identification of 12-nucleotide complementary DNA probes with single base mismatch discrimination (sensitivity of 0.2 microM). Interestingly, utilization of an overhang extension distal to the surface enhanced the sensitivity to the 0.01 microM level. The cantilever was functionalized by inkjet printing technology. Replacing the capture probe with locked nucleic acid (LNA) resulted in a faster target probe capture kinetics compared to DNA-DNA hybridization. The capabilities of the piezoresistive cantilever indicate future ergonomic convenience via miniaturization alternative to the conventional laser-based detection method for portable on-site applications.

Novel electrical detection of label-free disease marker proteins using piezoresistive self-sensing micro-cantilevers

We report an electro-mechanical biosensor for electrical detection of proteins with disease markers using self-sensing piezoresistive micro-cantilevers. Electrical detection, via surface stress changes, of antigen-antibody (Ag-Ab) specific binding was accomplished through a direct nano-mechanical response of micro-fabricated self-sensing micro-cantilevers. A piezoresistive sensor measures the film resistance variation with respect to surface stress caused by biomolecules specific binding. When specific binding occurred on a functionalized Au surface, surface stress was induced throughout the cantilever, resulting in cantilever bending and resistance change of the piezoresistive layer. The cantilever biosensors were used for the detection of prostate specific antigen (PSA) and C-reactive proteins (CRP), which are a specific marker of prostate cancer and cardiac disease. From the above experiment, it was revealed that the sensor output voltage was proportional to the injected antigen concentration (without antigen, 10 ng/ml, 100 ng/ml, 1 microg/ml). PSA and CRP antibodies were found to be very specific for their antigens, respectively. This indicated that the self-sensing micro-cantilever approach is beneficial for detecting disease markers, and our piezoresistive micro-cantilever sensor system is applicable to miniaturized biosensor systems.

Optimised cantilever biosensor with piezoresistive read-out

We present a cantilever-based biochemical sensor with piezoresistive read-out which has been optimised for measuring surface stress. The resistors and the electrical wiring on the chip are encapsulated in low-pressure chemical vapor deposition (LPCVD) silicon nitride, so that the chip is well suited for operation in liquids. The wiring is titanium silicide which-in contrast to conventional metal wiring-is compatible with the high-temperature LPCVD coating process.

Enhancing chemi-mechanical transduction in microcantilever chemical sensing by surface modification

The use of chemically selective thin-film coatings has been shown to enhance both the chemical selectivity and sensitivity of microcantilever (MC) chemical sensors. As an analyte absorbs into the coating, the coating can swell or contract causing an in-plane stress at the associated MC surface. However, much of the stress upon absorption of an analyte may be lost through slippage of the chemical coatings on the MC surface, or through relaxation of the coating in a manner that minimizes stress to the cantilever. Structural modification of MC chemical sensors can improve the stress transduction between the chemical coating and the MC. Surfaces of silicon MC were modified with focused ion beam milling. Sub-micron channels were milled across the width of the MC. Responses of the nanostructured, coated MCs to 2,3-dihydroxynaphthalene and a series of volatile organic compounds (VOCs) were compared to smooth, coated MCs. The analytical figures of merit for the nanostructured, coated MCs in the sensing of VOCs were found to be better than the unstructured MCs. A comparison is made with a previously reported method of creating disordered nanostructured MC surfaces.

Hybridisation of short DNA molecules investigated with in situ atomic force microscopy

By introducing the complementary DNA (cDNA) strand to a molecular layer of short single stranded DNA (ssDNA), immobilised on a gold surface, we have investigated hybridisation between the two DNA strands through the technique of in situ atomic force microscopy (AFM). Before introduction of cDNA, the ssDNA molecular layer was modulated with the spacer molecule mercaptohexanol (MCH), which makes the ssDNA molecules more accessible for hybridisation. With in situ AFM, we have monitored the formation of a smooth, mixed molecular layer containing ssDNA and MCH. Furthermore, the hybridisation between the two DNA strands has been studied. Introduction of the cDNA strand resulted in an increase in smoothness and thickness of the molecular layer. Both the increase in order and thickness of the molecular layer can be expected if hybridisation occurs, since double stranded DNA molecules have a more rigid and elongated structure than ssDNA molecules.

An embedded polymer piezoresistive microcantilever sensor

We have developed a new type of chemical microsensor based on piezoresistive microcantilever technology. In this embedded polymer microsensor, a piezoresistive microcantilever is partially "embedded" into a polymeric material. Swelling of the polymer upon analyte exposure is measured as a simple resistance change in the embedded cantilever. Arrays of these sensors, each employing a different polymeric material, provide for the identification of a wide range of chemical vapor analytes. Advantages of this system over previous "surface" piezoresistive microcantilever chemical sensors include enhanced mechanical simplicity (no mechanical approach necessary), greater resistance to shock or movement, and lower cost.

Nanostructured microcantilevers with functionalized cyclodextrin receptor phases: self-assembled monolayers and vapor-deposited films

It is shown that the performance of microcantilver-based chemical sensors in a liquid environment is affected by altering cantilever surface morphology and receptor phase type and thickness. Self-assembled monolayers of thiolated beta-cyclodextrin (HM-beta-CD) and thin films of vapor-deposited heptakis (2,3-O-diacetyl-6-O-tertbutyl-dimethylsilyl)-beta-cyclodextrin (HDATB-beta-CD) were studied on smooth and nanostructured (dealloyed) gold-coated microcantilever surfaces. The dealloyed surface contains nanometer-sized features that enhance the transduction of molecular recognition events into cantilever response, as well as increase film stability for thicker films. Improvements in the limits of detection of the compound 2,3-dihydroxynaphthalene as great as 2 orders of magnitude have been achieved by manipulating surface morphology and film thickness. The observed response factors for the analytes studied varied from 0.02-604 nm/ppm, as determined by cantilever deflection. In general, calibration plots for the analytes were linear up to several hundred nanometers in cantilever deflections.

Complementary metal oxide semiconductor cantilever arrays on a single chip: mass-sensitive detection of volatile organic compounds

The sensing behavior of polymer-coated resonant cantilevers for mass-sensitive detection of volatile organic compounds was investigated. Industrial complementary metal oxide semiconductor (CMOS) technology combined with subsequent CMOS-compatible micromachining was used to fabricate a single-chip system comprising the transducers and all necessary driving and signal-conditioning circuitry. An analytical model was developed to describe the mass-sensing mechanism of polymer-coated resonant cantilevers. The model was validated by measurements of various gaseous analytes. As an exemplary application, the quantitative analysis of a binary mixture using an array of four cantilevers is described. Experimental results are given for the concentration prediction of a mixture of n-octane and toluene. Finally, it was established that the limit of detection achieved with cantilever sensors is comparable to that of other acoustic wave-based gas sensors.

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.

Scanning force microscopy in the applied biological sciences

Fifteen years after its invention, the scanning force microscope (SFM) is rooted deep in the biological sciences. Here we discuss the use of SFM in biotechnology and biomedical research. The spectrum of applications reviewed includes imaging, force spectroscopy and mapping, as well as sensor applications. It is our hope that this review will be useful for researchers considering the use of SFM in their studies but are uncertain about its scope of capabilities. For the benefit of readers unfamiliar with SFM technology, the fundamentals of SFM imaging and force measurement are also briefly introduced.

Chemical sensors and biosensors in liquid environment based on microcantilevers with amplified quality factor

A new technique is presented for bio/chemical sensors, based on microcantilevers, for detection in liquid environment. The low quality factor of the cantilever in liquid is increased up to three orders of magnitude by using Q-control. This enables AC detection that is immune to the long-term drift of the DC cantilever response in liquids, and to temperature variations. This technique has been applied for the detection of ethanol in aqueous solution by using the microbalance method, and for antibody/antigen recognition by the surface stress method. The results show the feasibility and very high sensitivity of these novel devices.