Michael J. Sepaniak

Cantilever transducers as a platform for chemical and biological sensors

Since the late 1980s there have been spectacular developments in micromechanical or microelectro-mechanical (MEMS) systems which have enabled the exploration of transduction modes that involve mechanical energy and are based primarily on mechanical phenomena. As a result an innovative family of chemical and biological sensors has emerged. In this article, we discuss sensors with transducers in a form of cantilevers. While MEMS represents a diverse family of designs, devices with simple cantilever configurations are especially attractive as transducers for chemical and biological sensors. The review deals with four important aspects of cantilever transducers: (i) operation principles and models; (ii) microfabrication; (iii) figures of merit; and (iv) applications of cantilever sensors. We also provide a brief analysis of historical predecessors of the modern cantilever sensors.

Surface-Enhanced Raman Spectroscopy Substrates Created via Electron Beam Lithography and Nanotransfer Printing

The development of quantitative, highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates requires control over size, shape, and position of metal nanoparticles. Despite the fact that SERS has gained the reputation as an information-rich spectroscopy for detection of many classes of analytes, in some isolated instances down to the single molecule detection limit, its future development depends critically on techniques for nanofabrication. Herein, an unconventional nanofabrication approach is used to produce efficient SERS substrates. Metallic nanopatterns of silver disks are transferred from a stamp onto poly(dimethysiloxane) (PDMS) to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via electron beam lithography (EBL) of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated onto the stamp to overcome the adhesive nature of the EBL resist and to function as a releasing layer. Subsequently, Ag is physically vapor deposited onto the stamp at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by scanning electron microscopy. Transferred Ag nanodisk-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. There are observed optimal conditions involving both Ag and cyclodextrin thickness. The SERS response of metallic nanodisks of various shapes and sizes on the original stamp is compared to the corresponding nTP created substrates with similar trends observed. Limits of detection for crystal violet and Mitoxantrone are approximately 10(-8) and 10(-9) M, respectively. As an innovative feature of this approach, we demonstrate that physical manipulation of the PDMS post-nTP can be used to alter morphology, e.g., to change internanodisk spacing. Additionally, stamps are shown to be reusable after the nTP process, adding the potential to scale-up regular morphology substrates by a stamp-and-repeat methodology.

Bile salt surfactants in micellar electrokinetic capillary chromatography. Application to hydrophobic molecule separations

Abstract Bile salt surfactants are used in the micellar electrokinetic capillary chromatography (MECC) separation of various hydrophobic compounds. The use of methanol in the mobile phase allows the separation of previously intractable compounds including polyaromatic hydrocarbons. The effects of methanol on critical micelle concentration is investigated for sodium dodecyl sulfate (SDS) and the bile salt sodium cholate. It is determined that the unique structure of the bile salt micelle is much more tolerant to the addition of organic solvents than SDS, thereby increasing the scope of applications of MECC to include hydrophobic compounds.

Antibody-Based Fiberoptics Biosensor for the Carcinogen Benzo(a)pyrene

A new antibody-based fiberoptics biosensor was used to detect the important carcinogen benzo(a)pyrene (BaP). The fiberoptics sensor utilizes anti-BaP antibodies covalently bound to its tip. A helium cadmium laser was used as the excitation source to induce fluorescence from BaP conjugated to the bound anti-BaP antibodies. The fiberoptics device can detect 1 femtomole of BaP in a 5-μL sample drop.

Use of Native and Chemically Modified Cyclodextrins for the Capillary Electrophoretic Separation of Enantiomers

Capillary zone electrophoresis is shown to be useful for the separation of select enantiomers via the use of mobile phases containing cyclodextrins (CDs). Both native and chemically modified CDs are utilized herein. Parameters important in achieving enantiomeric separations are CD type, concentration, and mobile phase pH. In addition, rapid (1 minute) enantiomeric separations are demonstrated. Experimental parameters important in attaining short analysis times are discussed.

Parameters affecting reproducibility in capillary electrophoresis.

It is well known that poor quantitative reproducibility substantially limits the practical implementation of capillary electrophoresis (CE) separations in chemical analysis. The principal sources of variance in observed peak areas are irreproducible flow rate, which influences on‐column detector response, and inconsistent injection volume or amount. An overview of studies by researchers to address the reproducibility issue will be presented. In addition, current efforts in our laboratory to assess sources of quantitative variance for separations of dansylated amino acids using an automated CE system are presented and related when appropriate to the body of existing knowledge on this important topic. A comparison of different injection methods (hydrostatic vs. electrokinetic) and approaches (e.g., high vs. low pressure), the effect of random changes in electroosmotic flow (EOF) due to air bubbles in the CE capillary, and choice of certain peak integration parameters in terms of peak area reproducibility are presented. Under optimum conditions relative standard deviation (RSD) values in raw peak area are typically 2.0%. With nonoptimum conditions (e.g., with air bubbles in capillary), RSD values can substantially degrade. However, normalizing with retention times, internal standards, or observed electrophoretic current produces RSD values in a range of 1.4—2.3%.

Determination of adriamycin and daunorubicin in urine by high-performance liquid chromatography with laser fluorometric detection

A separation and detection scheme is presented for the determination of the antitumor drugs adriamycin and daunorubicin in human urine. Separation is accomplished by reversed-phase high-performance liquid chromatography and the drugs are detected down to the low picogram level by laser excited fluorescence using a unique fiber optic based flow-cell. Excellent detector selectivity and linearity are reported, and some of the factors influencing the performance of the detector are discussed. Possible extension of the procedure to other biologically important determinations are mentioned.

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.

Laser-Based Fluorimetric Detection Schemes for the Analysis of Proteins by Capillary Zone Electrophoresis

Abstract Presented herein are three laser-based fluorimetric detection schemes for the analysis of proteins by capillary zone electrophoresis (CZE). Experimental parameters are discussed for high peak efficiency along with detection considerations. Detection of proteins is performed using the native fluorescence of the protein, precolumn labeling and on-column labeling with arylaminonaphthalene-sulfonates. Minimum detectable concentrations are presented as well as a discussion of the merits of each method.

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.