J.-P. Ramseyer

A chemical sensor based on a microfabricated cantilever array with simultaneous resonance-frequency and bending readout

Abstract We present a chemical sensor based on a microfabricated array of eight silicon cantilevers actuated at their resonance-frequency and functionalized by polymer coatings. The operating principle relies on transduction of chemical or physical processes into a mechanical response. After exposure to analyte vapor, analyte molecules diffuse into the cantilever coating, which begins to swell. Jointly with the mass increase, a change of interfacial stress between coating and cantilever occurs, resulting in a bending of the cantilevers. Our setup allows the simultaneous detection of cantilever oscillation and bending of eight cantilevers by time-multiplexed optical beam deflection readout. The ac component of the cantilever response is demodulated, and the cantilever resonance-frequency is tracked by a custom-built phase-locked loop. By filtering out the ac component (oscillation), the dc signal (bending) is extracted, yielding information on mass as well as surface stress changes simultaneously. Detection results of water, primary alcohols, alkanes and perfumes are presented.

Label-free protein assay based on a nanomechanical cantilever array

We demonstrate continuous label-free detection of two cardiac biomarker proteins (creatin kinase and myoglobin) using an array of microfabricated cantilevers functionalized with covalently anchored anti-creatin kinase and anti-myoglobin antibodies. This method allows biomarker proteins to be detected via measurement of surface stress generated by antigen–antibody molecular recognition. Reference cantilevers are used to eliminate thermal drifts, undesired chemical reactions and turbulences from injections of liquids by calculating differential deflection signals with respect to sensor cantilevers. The sensitivity achieved for myoglobin detection is below 20 µg ml−1. Both myoglobin and creatin kinase could be detected independently using cantilevers functionalized with the corresponding antibodies, in unspecific protein background. This approach permits the use of up to seven different antigen–antibody reactions simultaneously, including an additional thermomechanical and chemical in situ reference. Applications lie in the field of early and rapid diagnosis of acute myocardial infarction.

A cantilever array-based artificial nose

We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.

An artificial nose based on a micromechanical cantilever array

A novel chemical sensor based on a micromechanical array of silicon cantilevers is presented. Chemical reactions are transduced by sensitization of cantilevers with coatings such as metals, self-assembled monolayers, or polymers into a mechanical response. This is read out using an optical beam-deflection technique by a sequential readout scheme. Reference cantilever sensors permit subtraction of background signals (differential measurement). Coating of each cantilever sensor with a different sensitive layer allows operation of the array-device as a new form of chemical nose. Detection of hydrogen, primary alcohols, natural flavors, and water vapor is demonstrated. We show that the magnitude of sensor response is proportional to the amount of analyte present.

Higher modes of vibration increase mass sensitivity in nanomechanical microcantilevers

We evaluated the potential and limitations of resonating nanomechanical microcantilevers for the detection of mass adsorption. As a test system we used mass addition of gold layers of varying thickness. Our main findings are: (1) A linear increase in mass sensitivity with the square of the mode number—a sensitivity increase of two orders of magnitude is obtained from mode 1 to mode 7 with a minimum sensitivity of 8.6 ag Hz −1 μm −2 and mass resolution of 0.43 pg at mode 7 for a 1 μm thick cantilever. (2) The quality factor increases with the mode number, thus helping to achieve a higher sensitivity. (3) The effective spring constant of the cantilever remains constant for deposition of gold layers up to at least 4% of the cantilever thickness. (Some figures in this article are in colour only in the electronic version)

Resonating modes of vibrating microcantilevers in liquid

A study of nanomechanical cantilevers vibrating at various resonating modes in liquid is presented. Resonant frequency spectrum with 16 well resolved flexural modes is obtained. The quality factor increased from 1 at mode 1 to 30 at mode 16. The theoretical estimate of eigenfrequency using the Elmer–Dreier model [F.-J. Elmer and M. Dreier, J. Appl. Phys. 81, 12 (1997)] and Sader’s extended viscous model [C. A. Van Eysden and J. E. Sader, J. Appl. Phys. 101, 044908 (2007)] matched well with the experimental data. The apparent mass of the liquid comoved by the oscillating cantilevers decreased asymptotically with mode number.

Capacitive micromachined ultrasonic transducers for chemical detection in nitrogen

The authors present the prototype of a chemical sensor using a capacitive micromachined ultrasonic transducer array. Each element in the array consists of a large number of resonating membranes connected in parallel. A five-channel oscillator circuit operates at the resonant frequency around 6MHz in this prototype. The surface of the elements in the array is coated by polymers such as polyallylamine hydrochloride, polyethylene glycol, and polyvinyl alcohol to detect different chemicals. By measuring shift in oscillation frequencies due to the mass-loading effect, analytes, e.g., water and isopropanol, with concentrations around 20ppbv (parts per 109 by volume) range can be detected.

The nanomechanical NOSE

We present a novel chemical sensor based on a microfabricated array of silicon cantilevers. Individual cantilevers are sensitized for the detection of analytes using metal coatings. Analyte molecules chemisorbing or physisorbing on the cantilever coating and chemical reactions produce a change in interfacial stress between analyte molecules and cantilever. This leads to a nanomechanical response of the cantilever, i.e. bending. The bending is read out using a time-multiplexed optical beam-deflection technique. From magnitude and temporal evolution of the bending, quantitative information on analyte species and concentration is derived. Here, we demonstrate the detection of ethene and water vapor with such a nanomechanical nose.

Digital processing of multi-mode nano-mechanical cantilever data

Nanomechanical sensors based on cantilever technology allow the measurement of various physical properties. Here we present a software for the comprehensive analysis of such data. An example for the combined measurement of mass and surface stress is presented.

An Electronic Nose Based on A Micromechanical Cantilever Array

We present a novel chemical sensor based on a micromechanical array of silicon cantilevers sensitized for the detection of analytes using cantilever coatings such as metals, self-assembled monolayers, or polymers. Chemical reactions are transduced into a mechanical response and read out using an optical beam-deflection technique. Detection of primary alcohols, natural flavors, and water vapor is demonstrated.