Christopher A. Tipple

Gold Nano-Structures for Transduction of Biomolecular Interactions into Micrometer Scale Movements

Microfabricated cantilevers, similar to those commonly used in scanning probe microscopies, have recently become increasingly popular as transducers in chemical and biological sensors. Surface stress changes that accompany intermolecular interactions on the cantilever surfaces offer an attractive means to develop new generations of microfabricated sensors and actuators that respond directly to chemical stimuli. In the present study, we demonstrate that interfacial molecular recognition events can be converted into mechanical responses much more efficiently when quasi 3-dimensional interfaces with nano-size features are used. Some of the particularly useful approaches to creating such interfaces are surface immobilization of gold nano-spheres and dealloying of co-evaporated Au:Ag films. Preliminary evaluation of these nanostructured surfaces was performed by measuring mechanical stresses generated by receptor modified nano-structures and smooth gold surfaces in response to gas-phase hydrocarbon compounds. The most efficient chemi-mechanical transduction was achieved when the cantilevers were modified with 50 to 75 nm thick dealloyed gold nanostrutures. Cantilevers of this type were selected for liquid phase experiments. These cantilevers were found to undergo several micron deflections upon adsorption of protein A and biotin-labeled albumin on nanostructured gold surfaces. Additional micrometer scale movements of the cantilevers were observed upon interaction of the surface bound bioreceptors with, respectively, immunoglobulin G and avidin from the aqueous phase.

Selectivity of chemical sensors based on micro-cantilevers coated with thin polymer films

In an effort to impart selectivity to chemical sensors based on micro-machined silicon cantilevers, thin films of polymeric chromatographic stationary phases (SP-2340 and OV-25) were applied to one side of the cantilever surface using a spin coating procedure. These coatings influenced the response of the micro-cantilever to vapor phase test analytes of varying chemical compositions. For the SP-2340 coated micro-cantilevers, the effects of both polymeric film thickness and cantilever structure thickness on response characteristics were investigated. Sensitivity improved as both film thickness and cantilever leg thickness were decreased. The selectivity, as indicated by differences in relative responses to the test analytes, were different for the two phases which differed significantly in polarity. The SP-2340 coated cantilevers exhibited response characteristics that are fairly similar to that expected for adsorption of the test analytes onto silica. Responses are shown to be proportional to analyte concentration. Response characteristics are shown to be consistent with predictions based on a gas chromatographic stationary phase classification scheme.

Enhanced Chemi-Mechanical Transduction at Nanostructured Interfaces

Interfacial molecular recognition processes can be converted into mechanical responses via modulation of surface stress. We demonstrate dramatic enhancement in this transduction when quasi 3-D interfaces with nano-size features are used. Microcantelever surfaces are modified with gold nanospheres or granular films and functionalized with macrocycle cavity and receptors. Deflections of these nanostructured cantilevers in response to vapor phase hydrocarbons are two orders of magnitude larger than with conventional smooth surfaces. Such a significant enhancements of surface stress changes resulting from intermolecular interactions at vapor- and liquid-solid interfaces offer an attractive means to develop novel nano-mechanical devices that respond directly and sensitively to chemical stimuli.

Photomechanical chemical microsensors

Abstract Recently, there has been an increasing demand to perform real-time in situ chemical detection of hazardous materials, contraband chemicals, and explosive chemicals. The advent of inexpensive mass produced MEMS (microelectromechanical systems) devices has enabled the use of various microstructures for chemical detection. For example, microcantilevers were found to respond to chemical stimuli by undergoing changes in their bending and resonance frequency even when a small number of molecules adsorb on their surface. In our present studies, we extended this concept by studying changes in both the adsorption-induced stress and photo-induced stress as target chemicals adsorb or desorb on the surface of microcantilevers. We demonstrate that photo-induced bending of microcantilevers depends on the number of absorbed molecules on their surface. On the other hand, microcantilevers that have undergone photo-induced bending will adsorb a different number of guest molecules. Depending on the photon wavelength and microcantilever material, the microcantilever can be made to bend by expanding or contracting a surface layer on one of its sides, unequally. Coating the surface of the microstructure with different materials can provide chemical specificity for the target chemicals. However, by choosing a handful of different photon wavelengths, tunable chemical selectivity can be achieved due to differentiated photo-induced response without the need for multiple chemical coatings. We will present and discuss our results on diisopropyl methyl phosphonate (DIMP), trinitrotoluene (TNT), two isomers of dimethylnaphthalene (DMN), tetrachloroethylene (TCE) and trichloroethylene (TRCE).

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.

Characterization of volatile, hydrophobic cyclodextrin derivatives as thin films for sensor applications

Abstract Successful development of chemical sensors and multi-sensor arrays critically depends on the availability of appropriate coatings. In the present studies, we synthesized hydrophobic and thermally stable cyclodextrin (CD) derivatives and, using a physical vapor deposition method, successfully deposited them on surfaces as thin films. 13 C NMR and FT-IR spectroscopy verified that while heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-methyl)cyclomaltoheptaose (DMe-CD) remained chemically intact after the deposition, heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-acetyl)cyclomaltoheptaose (DAc-CD) showed some variations. The hydrophobicity and refractive indices of the prepared thin films were determined using, respectively, contact angle measurements and ellipsometry. Selectivity patterns of the prepared CD films with respect to a series of analytes were defined using surface plasmon resonance. Because the chemical structure of the DAc-CD derivatives is compromised during the PVD process and any structural change may affect the selectivity of these thin films, additional attention should be given to the PVD process. In addition, thin films of the DMe-CD derivative were tested as protective coatings on silver island films for sensors based on surface enhanced Raman spectroscopy (SERS). The observed kinetics of SERS signals indicates that medium molecular weight analytes are able to rapidly diffuse through the CD films and reach the underlying silver islands.

Application of a high surface area solid-phase microextraction air sampling device: collection and analysis of chemical warfare agent surrogate and degradation compounds.

This work examines a recently improved, dynamic air sampling technique, high surface area solid-phase microextraction (HSA-SPME), developed for time-critical, high-volume sampling and analysis scenarios. The previously reported HSA-SPME sampling device, which provides 10-fold greater surface area compared to commercially available SPME fibers, allowed for an increased analyte uptake per unit time relative to exhaustive sampling through a standard sorbent tube. This sampling device has been improved with the addition of a type-K thermocouple and a custom heater control circuit for direct heating, providing precise (relative standard deviation ∼1%) temperature control of the desorption process for trapped analytes. Power requirements for the HSA-SPME desorption process were 30-fold lower than those for conventional sorbent-bed-based desorption devices, an important quality for a device that could be used for field analysis. Comparisons of the HSA-SPME device when using fixed sampling times for the chemical warfare agent (CWA) surrogate compound, diisopropyl methylphosphonate (DIMP), demonstrated that the HSA-SPME device yielded a greater chromatographic response (up to 50%) relative to a sorbent-bed method. Another HSA-SPME air sampling approach, in which two devices are joined in tandem, was also evaluated for very rapid, low-level, and representative analysis when using discrete sampling times for the compounds of interest. The results indicated that subparts per billion by volume concentration levels of DIMP were detectable with short sampling times (∼15 s). Finally, the tandem HSA-SPME device was employed for the headspace sampling of a CWA degradation compound, 2-(diisopropylaminoethyl) ethyl sulfide, present on cloth material, which demonstrated the capability to detect trace amounts of a CWA degradation product that is estimated to be less volatile than sarin. The rapid and highly sensitive detection features of this device may be beneficial in decision making for law enforcement, military, and civilian emergency organizations and responders, providing critical information in a contaminated environment scenario when time is of the essence.