Patrick Ian Oden

Viscous drag measurements utilizing microfabricated cantilevers

The influence of viscous drag forces on cantilevers is investigated using standard atomic force microscope (AFM) cantilevers. Viscosity effects on several geometrically different cantilevers manifest themselves as variations in resonance frequencies, quality factors, and cantilever response amplitudes. With this novel measurement, a single cantilever can be used to measure viscosities ranging from η=10−2 to 102 g/cm s.

Comparison of techniques for enzyme immobilization on silicon supports

Abstract Enzyme immobilization onto silicon substrates has been investigated by five different coupling procedures. The methods included covalent coupling either through a metal link reagent or silane reagents containing pendant amino or epoxide linkers, an entrapment technique using a thin layer of gelatin, or an adsorption technique using poly- l -lysine. These immobilization procedures were evaluated using glucose oxidase and a simple spectrophotometric method employing Fenton’s reagent. Retention of enzyme activity and surface loading were assessed. The immobilization techniques were also evaluated by electron microscopy to characterize the evenness of the surface coatings. All of the covalent coupling procedures led to surface loadings, approaching 1 pmol mm −2 ; however, the surfaces appeared irregular on a microscopic scale. The poly- l -lysine adsorption technique provided the smoothest surface. With the exception of the entrapment technique, all immobilization procedures provided immobilized enzyme that retained >75% activity after several weeks of storage.

Multiple-input microcantilever sensors

A surface-micromachined micro-electro-mechanical-system (MEMS) process has been used to demonstrate multiple-input chemical sensing using selectively coated cantilever arrays. Cantilever motion due to absorption-induced stress was readout using a custom-designed, eight-channel integrated circuit. Combined hydrogen and mercury vapor detection was achieved with a palm-sized, self-powered module with spread-spectrum telemetry reporting.

Remote optical detection using microcantilevers

The feasibility of microcantilever‐based optical detection is demonstrated. Microcantilevers may provide a simple means for developing single‐element and multielement infrared sensors that are smaller, more sensitive, and lower in cost than quantum well, thermoelectric, or bolometric sensors. Here we specifically report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers, such as those used in atomic force microscopy. In this work, optical transduction techniques were used to measure microcantilever response to remote sources of thermal energy. The noise equivalent power at 20 Hz for room temperature microcantilevers was found to be approximately 3.5 nW/√Hz, with a specific detectivity of 3.6×107 cm Hz1/2/W, when an uncoated microcantilever was irradiated by a low‐power diode laser operating at 786 nm. Operation is shown to be possible from dc to kHz frequencies, and the effect of cantilever shape and the role of absorptive coatings are discussed. Finally, spectral response in the midinfrared is evaluated using both coated and uncoated microcantilevers.

Uncooled thermal imaging using a piezoresistive microcantilever

The operation of an uncooled, microcantilever‐based infrared (IR) imaging device is demonstrated. Bending of the microcantilever is a function of the IR radiation intensity incident on the cantilever surface. The infrared image of the source is obtained by rastering a microfabricated cantilever over the image formed at the focal plane of a concave mirror. The bending variation of the microcantilever, as it scanned the focal plane of the mirror, is used to construct an infrared image of the source in front of the mirror. The thermal image obtained by scanning a single element cantilever is presented.

Glucose biosensing using an enzyme-coated microcantilever

A microcantilever-based biosensor is described. The enzyme glucose oxidase was immobilized on a micromachined silicon cantilever containing a gold coating, such as those used for atomic force microscopy. Specific, quantifiable deflection of the derivatized cantilevers was observed in the presence of the appropriate analyte. An analysis of the reaction energetics and the expected thermal response of the cantilever indicates that cantilever deflection is not simply a result of reaction-generated heat. This deflection appears to result from surface induced stresses. The combination of a highly specific enzyme and the microcantilever platform provides a unique approach for quantifying enzyme substrates without the complication of sample labeling.

Remote infrared radiation detection using piezoresistive microcantilevers

A novel micromechanical infrared (IR) radiation sensor has been developed using commercially available piezoresistive microcantilevers. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress between the top layer (coating) and the substrate. The bending causes a change in the piezoresistance and is proportional to the amount of heat absorbed. The microcantilever IR sensor exhibits two distinct thermal responses: a fast one (<ms) and a slower one (∼10 ms). A noise equivalent power (at a modulation frequency of 30 Hz) was estimated to be ∼70 nW/Hz1/2. This value can be further reduced by designing microcantilevers with better thermal isolation that can allow microcantilevers to be used as uncooled IR radiation detectors.

Transient response of tapping scanning force microscopy in liquids

Tapping‐mode scanning force microscopy in liquids is usually accomplished by acoustic excitation of the cantilever because of the strong viscous damping. Contact of the tip with the sample surface results in a damping of the cantilever amplitude with an anharmonic response. This interaction is modeled as a viscous‐damped, one‐dimensional harmonic oscillator periodically perturbed by an exponential surface potential. Experimental results verify the validity of the model.

Studies of the electrical properties of large molecular adsorbates

The scanning tunneling microscope (STM) was used to investigate the conductivity of organic aggregates on gold electrodes. DNA fragments can form very stable aggregates in the presence of tris(hydroxymethyl)aminomethane buffer salt. With a submonolayer coverage, the tip can be moved back and forth between ‘‘clean’’ gold (in contact with the electrolyte) and an organic adsorbate patch. In these conditions, a systematic variation of the current voltage (I–V) characteristics indicates that the electrical characteristics are not dominated by a contaminant particle on the STM tip. The dependence of image contrast on tip bias was also studied over the range +0.3 to −0.3 V. It is found that: (a) the image contrast does not depend strongly on tip bias; (b) the I–V curves over what appears to be ‘‘clean’’ gold under the electrolyte are similar to those observed over clean gold maintained in an inert atmosphere; (c) the I–V curves over an adsorbate patch are diode‐like; (d) the I–V curves show no sign of a large vo...

Fabrication of large arrays of micron-scale magnetic features by selective area organometallic chemical vapor deposition

We demonstrate that it is possible to deposit a wide range of magnetic features, using photoassisted selective area organometallic chemical vapor deposition. Large arrays of identical micron‐scale Ni features were deposited on a Si(111) wafer by this method. Their magnetic properties were studied by alternating gradient force magnetometry as well as magnetic force microscopy. Our morphological and magnetic measurements show that the structures are spatially well defined, and the magnetic properties are related to the structural shapes of the features. This method can be adapted to the fabrication of smaller‐scale magnetic and electronic devices.