A Polarization Isolation Method for High-Sensitivity, Low ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008

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A Polarization Isolation Method for High-Sensitivity, Low-Cost On-Chip Fluorescence Detection for Microfluidic Lab-on-a-Chip Ansuman Banerjee, Andrea Pais, Ian Papautsky, and David Klotzkin

Abstract—The trend in medical equipment is toward compact and integrated low-cost medical test devices. Fluorescence-based assays are used to identify specific pathogens through the presence of dyes, but typically require specialized microscopes and narrowband optical filters to extract information. We present a novel, high-sensitivity, cost-effective, cross-polarization scheme to filter out excitation light from a fluorescent dye emission spectrum. This concept is demonstrated using an inverted microscope fitted with a halide lamp as the excitation source and an organic photo voltaic (organic photodiode) cell as the intensity detector. The excitation light is linearly polarized and used to illuminate a microfluidic device containing a 1 L volume of dye dissolved in ethanol. The detector is shielded by a second polarizer, oriented orthogonally to the excitation light, thus reducing the magnitude of the detector photocurrent by about 25 dB. The signal due to fluorescence emission light, which is randomly polarized, is only attenuated by about 3 dB. As proof-of-principle, the fluorescence signal from the dyes Rhodamine 6G (emission wavelength of 570 nm) and Fluorescein (emission wavelength 514 nm) are measured in a dilution series with resulting emission signal being detected by an organic photodiode. Both dyes were detectable down to concentrations of 10 nM. This suggests that an integrated microfluidic device, with an organic photodiode and an organic light emitting excitation source and integrated polarizers, could be fabricated to realize compact and economical lab-on-a-chip devices for point-of-care diagnostics and on-site analysis. Index Terms—Biological systems, fluidics, fluorescence, microelectromechanical devices, photoluminescence.

I. INTRODUCTION

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ICROFLUIDIC analysis systems are a rapidly emerging field. The past few years have seen great advances in lab-on-a-chip (LOC) technologies for genomic, proteomic, and enzymatic analysis [1]. The trend has been moving towards integrated, portable, and disposable microfluidic systems that can be used for point-of-care medicine. By integrating different functional units for reaction, detection, and separation into a microfluidic channel network, it could become feasible to realize

Manuscript received May 27, 2007; accepted July 11, 2007. This work was supported in part by the National Science Foundation under Award 0428600 and Award 0725812 and in part by the Institute for Nanoscale Science and Technology, University of Cincinnati, Cincinnati, OH. The associate editor coordinating the review of this paper and approving it for publication was Dr. Richard Fair. The authors are with the Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, OH 45221 USA (e-mail: banerjaa @email.uc.edu; [email protected]; [email protected]; david. [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2008.918961

one microchip comprising all features of a complete lab that could be used to perform complex reactions and analyses [2]. Fluorescence is one of biotechnology’s essential analytical and diagnostic methods, and commonly used in LOC systems. Fluorescence occurs when a fluorophore or quantum dot relaxes to its ground state after being excited. In biotechnology applications, fluorophores are used to label antibodies which tag molecules of interest in order to visualize their presence. Typically, the detection is done by illumination in a fluorescence microscope with a set of optical filters to observe the narrow emission spectra of the fluorescent dye. Fluorescence analysis is attractive for biomedical applications as many biochemistry protocols incorporate fluorescence labels [3]. Integrating fluorescence detection on-chip would enable numerous point-of-care applications [4]. However, a fully-integrated fluorescence assay LOC has not yet been realized, primarily because of the bulk of components necessary for characterization. These components typically include the excitation light source and the specialized microscope with filter to suppress the excitation light signal and observe only the emission signal. The primary problem in going from microscope-based analysis with filters to an integrated quantitative analysis with a photodiode-like detector is a practical method to shield the detector from the excitation light signal, which is typically much stronger than the dye emission signal. The problem becomes even more significant for bio applications requiring detection of analytes in the nanomolar range. Many different approaches have been used to overcome this problem. Optical long-pass spectral filters have been embedded in a PDMS channel [5]. However, this approach does not allow the filter to be fabricated with commonly used hybrid PDMS—glass microfluidic devices, thereby increasing the distance between the microchannel and the detector and resulting in inefficient collection of the fluorescence signal. [6]. Another approach demonstrated recently has been the use of integrated multilayer interference filters on top of the detector [7], [8]. These filters must be specifically matched to the particular dye, and as such, each integrated system has to be manufactured for a certain dye. Integrated systems which have the excitation light and emission light oriented orthogonally have also been tried [9], but are more complex to fabricate than filter-based, columnar systems. Several different designs for integrated microanalytical devices have been published. For high-sensitivity applications, a photomultiplier tube has been used as a photodetector in integrated fluidic channels having a detection limit of 100 fM [10].

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Integrated silicon photodiodes have also been used for monitoring chemiluminescent reactions in microfluidic systems [11], with detection limits of 3 M, or for fluorescence measurements [12], with detection limits of 1 M. Of all the schemes found in literature, the best results were obtained for a PDMS microfluidic device with micro avalanche photodiode arrays, exhibiting a detection limit of 25 nM [13]. These integrated systems, however, necessarily have much lower sensitivities than bulk systems, which can have higher excitation intensities and larger analyte volume. For bulk systems such as laser-induced fluorescence, the limits of detection is quite low; detectable values of 200 fM for Rhodamine 6G using PMT tubes [14] have been reported. Nevertheless, a much more desirable approach for low cost, compactness, and ease of integration is to use integrated organic photodiodes, which offers a simple route to integrated device fabrication. To date, the best result reported for detection with organic photodiode has been 1 M using Rhodamine 6G dye [12]. In this paper, we describe a novel, inexpensive approach to filtering out excitation light from the emission signal suitable for development into an integrated, high-sensitivity, low-cost, truly compact LOC quantitative fluorescence analysis device. Using cross-polarization, we have demonstrated a compact and inexpensive fluorescence detection system by integrating an organic photodiode fabricated on a glass slide with a PDMS microfluidic device for intensity-based light detection. The excitation light is polarized, with a second polarizer orientated at 90 between the sample and the detector. This approach practically eliminates the “noise” signal due to the leakage excitation light on the optical intensity detector, thereby significantly enhancing the signal-to-noise (S/N) ratio. The assembled system can be used for any dye, even if the emission and the excitation signals overlap in wavelength. Using this approach, we have been able to detect dye concentrations down to 10 nM, which is an improvement of several orders of magnitude over the previously published reports with systems using organic photodetctors [15]. The organic photodiode detector can be fabricated directly onto the glass slide which is the substrate for the microfluidic device. Although organic photodiode detectors have been previously used to detect emission from chemiluminescent reactions [16], this is the first demonstration of the use of an organic photodiode as detector in a microfluidic fluorescence-based system. This “proof-of-principle” demonstrates that simple intensity detectors can be used to detect reasonably low concentrations without the need for spectral filtering of the output using polarization-based filtering. II. EXPERIMENTAL SECTION A. Experimental Setup The cross-polarization scheme is illustrated as a potential monolithic integrated device on glass in Fig. 1(a) with green OLED as source, PDMS microchannel containing the dye detection area, two sets of polarizers, and an organic photodiode as detector. The overall experimental setup is shown in Fig. 1(b). Edmund Optics NT45-667 polarizing film was used to polarize the light. A PDMS microfluidic chip was sandwiched between two cross-aligned polarizers. Polarizer 2,

located above the microfluidic chip, filters out the excitation light and permits only the emission signal to pass through to the organic photodiode detector. Here, a metal halide lamp (100 W) of an inverted fluorescence microscope (Olympus IX 71) was used as an excitation source. In our experiments, a HiQ TRITC filter cube (Chroma), comprised of an exciter filter (510–550 nm), a barrier filter ( 590 nm) and a dichroic mirror (570 nm), was used with Rhodamine 6G dye. A FITC filter cube (Chrome), comprised of an exciter filter (480 nm), a barrier filter ( 505 nm), and a diachronic mirror (535 nm) was used with Fluorescein dye. The emission light was detected by an organic photodiode cell directly bonded on top of microchannel with polarizer 2 using UV-cured epoxy (Loctite 3321). A Keithley 2400 Source meter was used to supply bias voltage and measure the photocurrent. The Keithley source meter was connected to a personal computer via a General Purpose Information Bus (GPIB) card and was controlled using LabView 7.0 (National Instruments). B. Organic Photodiode Fabrication Indium tin oxide (ITO) coated glass wafers were purchased from Delta-Technologies. The ITO layer was about 170 nm thick, with typical sheet resistances of 30–60 . The wafers were patterned by wet etching of ITO in a 3:1 mixture of HCl: HNO at 55 C for 3 min. The patterned ITO glass was sonicated in acetone for 30 min, rinsed with fresh acetone, methanol and deionized water, and dried with N gas. A 100 nm thick layer of 3, 4-polyetylenedioxythiopene-polystyrenesulfonate (PEDOT: PSS from Baytron) was then spin-coated on the patterned ITO glass and cured at 93 C for 30 min. Multilayer thermal evaporation was then performed on the substrate. CuPC C LiF Al were evaporated layer by layer up to thicknesses of 20, 60, 1, and 100 nm, respectively, in the same order (Fig. 2) [17], [18]. The Al strips were evaporated through a shadow mask to form devices in a cross-linked configuration. The device was stored under vacuum until use. Fig. 3(a) is a photograph of the top view of the organic photodiode showing a 3 2 array of devices. C. PDMS Microchannel Fabrication First, 3-inch silicon wafers were cleaned with acetone, methanol, and DI water for 3 min each, followed by a 10 min cleaning in sulfuric peroxide (7:3 mixture of H SO H O ). Next, wafers were rinsed with DI water for a , H O NH F few minutes and dipped in BOE (3:3:1 HF) for 15 s to remove any native oxide layer from the surface, and finally again rinsed with DI water. Wafers were dehydrated on a hotplate at 150 C for 20 min and allowed to cool to room temperature before spin coating them with the photoresist. Wafers were coated with a negative photoresist SU-8 2075 (MicroChem Corporation, MA) to define 55 m deep microchannels. Following spin coating, wafers were transferred to a leveled surface for 10 min to allow the photoresist to relax. The prebake was performed on a leveled hot plate at 65 C for 10 min and at 95 C for 45 min. Wafers were permitted to air cool to room temperature. The resist was exposed for 35 s at 5 mW/cm using an I-line (365 nm) high-pass filter. Glycerin L was used during the exposure to reduce any

BANERJEE et al.: A POLARIZATION ISOLATION METHOD FOR HIGH-SENSITIVITY, LOW-COST ON-CHIP FLUORESCENCE DETECTION

Fig. 1. (a) Integrated organic excitation/detection system for a bilayer CuPc=C the organic photodiode and the halide source of the inverted microscope.

Fig. 2. Schematic of ITO/CuPc/C60/LiF/Al multilayer organic photovoltaic cell fabricated by thermal evaporation.

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detector. (b) Experimental setup for testing the cross-polarization scheme with

diffraction effects that may arise due to surface roughness. Post exposure bake was carried out on a hotplate at 65 C for 5 min and at 95 C for 10 min. The wafers were developed in the SU-8 developer (MicroChem Corporation, MA), rinsed with isopropyl alcohol to confirm the development, and blown dry using N . Following development, patterned wafers were descummed in O plasma (20 sccm, 13.56 MHz) for 2 to 3 min at 300 W to clean the wafer off any residual photoresist. Polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning), chosen due to its low autofluorescence [19], was mixed at a ratio of base to curing agent, degassed, and then 10:1

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Fig. 3. Photographs of (a) 3 2 array of organic photodiodes and (b) PDMS microfluidic device with 55 m deep 200 m wide channel and 4 mm diameter microwell. Fig. 5 Comparison of I-V curves for 1 mM of Rhodamine 6G, background current, and dark current measured with the organic photodiode.

Fig. 4. Responsivity curve for the organic photodiode illustrating three distinct peaks in the visible range.

poured over the SU-8 molds to achieve 1 mm thickness. Patterned wafers were pretreated with Sigmacote (SL-2, SigmaAldrich) to facilitate release of PDMS. The PDMS was cured at 80 C for 2.5 h on a leveled hotplate, cut, and peeled from the masters with a sharp razor blade or scalpel. Input/output ports were carefully pierced using a 2 mm (OD) punch. PDMS glass microscope slides molds were bonded to clean using a corona discharge wand [20] and 2 h on a leveled hotplate at 85 C. Fig. 3(b) illustrates the final device containing a microwell 4 mm in diameter, with 100 m wide input channels and 200 m wide mixing channel. III. RESULTS AND DISCUSSION A. Organic Photodiode Characterization Light from a broadband source was filtered through a spectrometer and focused onto the CuPc C photodector, and the responsivity (photocurrent/optical power versus wavelength) was measured and is shown in Fig. 4. The responsivity curve exhibits three peaks, centered at 530 nm (0.005 A/W), 570 nm (0.008 A/W), and at 610 nm (0.014 A/W). The Rhodamine 6G dye used in this work has a peak emission wavelength of 566 nm which overlaps with the peak at 570 nm of the photodetector response. Similarly, Fluorescein dye has a peak emission wavelength of 514 nm, near the peak at 530 nm in the organic photodiode response. However, the peak intensity at 570 nm is higher than the peak intensity at 530 nm and, consequently, the photocurrent response for Rhodamine 6G was expected to be

higher than that of Fluorescein. The highest intensity peak is at 610 nm, indicating that red-shifted dyes such as Texas Red (peak emission wavelength 615 nm) and Alexa 568 (peak emission wavelength 610 nm) may yield higher photocurrent response than Rhodamine 6G and Fluorescein. Current-voltage curves of the organic photodiode were plotted under dark and full illumination conditions. The voltage was swept from 1 V to 1 V and the corresponding photocurrent was obtained. Fig. 5 compares the photoresponse of Rhodamine 6G having concentration of 1 mM with the background current and dark current when the voltage was varied from 1 V to 1 V. The shift in current curves represents the signal due to the dye. Since the maximum shift was observed at 0 V bias, the device was kept under 0 V bias when used as a detector. B. Polarization Isolation To characterize the response of the organic photodiode cell and the degree of isolation from the excitation light source, the polarization angle versus the photocurrent was measured using polarized excitation light as input, while rotating the second polarizer from 0 to 360 . Edmund Optics NT45-667 polarizer film used in this work has a nominal extinction ratio of 2700 at 550 nm [21]. The photocurrent measured by the OPD was reduced from 5 A to 19 nA (25 dB) when the two polarizers were rotated from parallel to orthogonal orientation. This extinction level compares favorably with the usage of integrated background filter to remove background light, where intensity reductions of 20 dB have been reported [14]. In the future, these film polarizers may be replaced by wire grid polarizers [22] for a fully integrated microfluidic LOC system. Thin-film interference filters may be used to provide an even greater degree of isolation, but they must be specialized to the particular dye. However, polarization-based isolation works for any dye and is suitable for integration into low-cost, microfluidic systems, especially for multianalyte sensor design. When in operation, two other effects also improve the S/N ratio and reduce the background signal further. First, the excitation is also largely absorbed by the dye and fluid in the microchannel. Second, the responsivity of the OPD to Rhodamine

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Fig. 6. Photocurrent versus concentration of Rhodamine 6G tested for parallel and orthogonal orientation of the polarizers and detected using the organic photodiode.

6G at 570 nm is about twice that of the responsivity to excitation light at 530 nm. These two effects combined with the polarizer allow the system to show clear signal dependence versus concentration at very low concentrations. To illustrate the efficacy of the polarizers in bringing out the fluorescence signal, the OPD photocurrent at 0 V applied bias was measured for a dilution series of Rhodamine 6G (concenM to M) for polarizers aligned trations ranging from parallel and perpendicular to each other (Fig. 6). Ethanol was used to flush the microfluidic channel between experiments and to establish a baseline signal before each concentration measurement. When the polarizers are arranged in parallel, the light incident on the photodetector is mostly excitation light, as the intensity of the excitation light overshadows the emission signal from the dye by many orders of magnitude. With increase in dye concentration, the photocurrent decreases due to the increased absorption and scattering of the excitation light by the dye molecules in the microchannel. With crossed polarization, the excitation light photocurrent signal is reduced by 25 dB and the photocurrent is proportional to the concentration of the dye. With this setup, dye concentrations down to 10 nM can be measured, which represents a substantial improvement in detection limit over the results previously reported in literature incorporating an organic photodetector [12], [20]. This demonstrates how this polarizer approach can bring the dye signal clearly out of the noise due to excitation light. These polarizers do, however, reduce the overall signal intensity. The polarizer in front of the excitation light source absorbs about 50% of the light used for exciting the dye and the filter polarizer in front of the detector absorbs about 50% of the signal light. C. Concentration Measurements The OPD photocurrent as a function of dye concentration was M to measured in a dilution series of concentrations from M for both Rhodamine 6G and Fluorescein with crossed polarizers. The results are shown in Fig. 7. For the same concentrations of Rhodamine 6G and Fluorescein, the photocurrent response for Rhodamine 6G was always higher than Fluorescein,

Fig. 7. (a) OPD response for a 10 nM to 1 M Rhodomine 6G dilution series. Inset illustrates the OPD photocurrent at low dye concentrations on a linear scale. (b) OPD response versus Rhodamine 6G concentrations recorded measured approximately one week apart with reassembled setups. (c) S/N ratio versus concentration for both runs, with the noise equal to the background current on that day.

which is attributed to the 37% higher responsivity of the organic photodiode at 570 nm versus 530 nm. To clearly show the dependence of photocurrent on concentration the inset box illustrates dependence of photocurrent on concentration over the M to M on a linear scale. more useful range from

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The simple measurement of two dyes illustrates the flexibility of this polarization isolation scheme and its application to dyes of many different emission wavelengths, which is important for multianalyte sensor design. The detection limit is given by the strength of single over the baseline noise or background signal. Two types of baseline signals were measured; dark current (with no light on the detector) and background current (with no dye in the ethanol liquid volume, but conditions otherwise the same as measurement conditions). The dark current was measured to be 13 nA, and the background current approximately 19 nA. The background current is higher than the dark current due to excitation light leaking through the polarizers from the excitation source and PDMS autofluorescence, which are independent of dye concentration. In this work, we considered the noise signal to be the background current as the photoresponse is composed of the dye signal and the background signal. The S/N ratio for Rhodamine 6G varies from 3 at 1 mM to 1.5 at 10 nM. For Fluorescein, the S/N ratio varies from 2.8 at 1 mM to 1.3 at 10 nM. The S/N level increases with concentration, as the signal increases, while the background remains constant. The absolute limit of detection (LOD) is theoretically reached when the photocurrent becomes almost equal to the background ). Other investigators reported LODs from current (i.e., S N at the lowest detection limit for a mi1 M with S N crofluidic device with p-i-n photodiode [13] to a 10 M with for a microfluidic device with an organic photodiode S N [17], [23]. In our system, concentrations of 10 nM can be clearly seen above the background signal, where the S/N is 1.5. The CuPc organic photodiode is better suited for integration with microfluidic fluorescence-based LOCs with dyes like Rhodamine 6G than a broadband device such as silicon photodetector or the previously reported polymer photodiodes. Although silicon has a much greater responsivity ( 0.4 A/W), it detects the excitation light as well as the emission light; this organic photodiode is more sensitive above 530 nm, making it less sensitive to excitation light in the green for dyes which emit in the orange-red (like Rhodamine 6G). For high-sensitivity detection, it is more critical to reduce the background signal rather than to increase the absolute value of the photocurrent. This is one of the reasons that such low detection limits were recorded in this work. The OPD photocurrent is sublinear over the large concentration range used in this work; similar results have been reported by others [12]. This is due to the fluorescence intensity that is described by Beer–Lambert’s law [24]. The first step in generating fluorescence signal is the absorption of excitation light by the dye, with the absorption coefficient being proportional to the concentration , and specific absorption per molecule as

on the solid angle subtended by the detector, and including the 50% of the signal absorbed by the polarizer). The two efficiency terms are not expected to be dependent on concentration. The exponential term is approximately linear for low concentrations. Across the several decades of concentration range characterized here, the nonlinearity of the response can be seen clearly. The overall shape of the curve is consistent with Beer–Lambert’s law across the concentration range, and further analysis is ongoing. To illustrate the run-to-run variability of the detection scheme, Fig. 7(b) shows OPD response versus Rhodamine 6G concentration data recorded approximately one week apart with reassembled setups. There is a clear systematic difference due to the variations in the setup (particularly, the amount of light coupling into the detector), which leads to different amount of photocurrent response. Fig. 7(c) illustrates the S/N ratio versus concentration for both runs, with the noise equal to the background current on that day. For both runs, the S/N ratio is consistent with concentration, with a value about 1.5 at a concentration of 10 nM. Fabricating the OPD and the microfluidic chip together in a single integrated device will yield consistent light coupling and, thus, background signal, alleviating photocurrent variability. IV. CONCLUSION In this work, we have demonstrated a novel cross-polarization scheme to filter out excitation light from the emission spectra of dyes used for bioanalysis in microfluidic LOC systems and demonstrated detection limits of 10 nM. While this represents a significant improvement over results previously reported with organic detectors, modulating the excitation light and using lock-in detection techniques may lower the detection limit even further. Another possibility to increase the S/N ratio is to incorporate colored glass filters in addition to the cross-polarization scheme. By attenuating the excitation light only, this should reduce the background signal level and, consequently, increase the detection limit. In the future, multianalyte fluorescence detection may be possible by integrating multiple photodetectors having different color sensitivities on a single chip. This opens the door to integrated high-sensitivity disposable microfluidic LOCs with organic excitation and detection devices for inexpensive fluorescence-based analysis. Such a platform for both organic electronics and microfluidic systems is compatible, robust, and low-cost. Integration of the detection system into the microfluidic LOCs makes the system portable and feasible for point-of-care medical diagnostics or on-site environmental analysis. ACKNOWLEDGMENT

(1)

The authors thank Dr. M. Cahay, K. Garre, A. Bhagat, and E. Peterson of the Department of Electrical and Computer Engineering, University of Cincinnati, for assistance with the fabrication processes.

is the quantum yield of the dye (nearly 100%), and where is the “detection efficiency.” The detection efficiency consists of the inherent responsivity of the OPD (about 1% photons converted to current, or 0.01 A/W), and the fraction of emitted dye signal that is coupled to the detector (a few percent, based

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BANERJEE et al.: A POLARIZATION ISOLATION METHOD FOR HIGH-SENSITIVITY, LOW-COST ON-CHIP FLUORESCENCE DETECTION

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Ansuman Banerjee received the B.E. degree from Jadavpur University, Kolkata, India, in 2001, and the M.S. degree from Virginia Commonwealth University, Richmond, VA, in 2004, both in electrical engineering. Currently, he is working towards the Ph.D. degree in electrical engineering at the University of Cincinnati, Cincinnati, OH. He has worked at Eastman Kodak in the area of large area organic displays in 2006. His current research interests include organic electronics, microelectromechanical systems (MEMS), and novel nanodevices.

Andrea Pais received the B.E. degree from Anna University, Chennnai, India, in 2005, and the M.S. degree from the University of Cincinnati, Cincinnati, OH, in 2007, both in electrical engineering. She is currently working towards the Ph.D. degree in electrical engineering at the University of Florida, Gainesville, FL. Her research interests include BioMEMS, optical microelectromechanical systems (MEMS), and organic electronics.

Ian Papautsky received the Ph.D. degree in bioengineering from the University of Utah, Salt Lake City. He is an Associate Professor with the Department of Electrical and Computer Engineering, University of Cincinnati. His research focuses on applying microelectromechanical systems (MEMS) technologies to biology, medicine, and environmental systems. His research interests include polymer microfabrication technologies, sensors for point-of-use/point-of-care applications in environmental and biomedical engineering, polymer micro/nanofluidic systems, and plastic biochips/lab-on-a-chip systems.

David Klotzkin received the B.S. degree in electrical engineering from Rennselaer Polytechnic Institute, Troy, NY, in 1988, the M.S. degree in materials science from Cornell University, Ithaca, NY, in 1994, and the M.S. and Ph.D. degrees in electrical engineering from the University of Michigan, Ann Arbor, in 1997 and 1998, respectively. His research interests are in optoelectronics devices including semiconductor lasers, waveguide amplifiers, organic optoelectronics, and photonic-crystal-based planar lightwave circuits. His industrial experience includes three years at IBM Corporation from 1988 to 1991, and several years of semiconductor laser design for telecommunications applications at various companies, including Lucent Technologies and Agere Systems. In 2002, he joined the Electrical, Computer Engineering and Computer Science Departments at the University of Cincinnati.