Optical Microring Resonator Sensors with Selective

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Abstract: Optical microresonator sensors with surface customization using chemically selective ... pollutants in water, including benzene, toluene, and xylenes. 2.
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Optical Microring Resonator Sensors with Selective Membrane Surface Customization Sang-Yeon Cho*, Gary Dobbs**, Nan Marie Jokerst*, Boris Mizaikoff**, Tray Cooper* *


129 Hudson Hall, Electrical and Computer Engineering Department, Duke University, Durham, NC 27708, USA 901 Atlantic Drive, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA

Abstract: Optical microresonator sensors with surface customization using chemically selective membranes have been demonstrated for the first time. The ethylene/propylene copolymer membrane enriches o-xylene representing an organic contaminant from water, which was sensed by the microresonator. ©2007 Optical Society of America OCIS Codes: Sensors (130.6010), Integrated Optics Devices (130.3120)

1. Introduction Portable, customizable, low cost chip scale integrated sensing systems will transform the next generation of health diagnostics, environmental monitoring, and security. These next generation miniaturized sensor systems could be comprised of arrays of different sensors that are individually customized and integrated together onto the same substrate to collect multivariate bio/chem data localized in time and space, offering discrimination and low false positives through advanced signal processing of the overlapping multivariate data. The challenge for these integrated sensor systems is to integrate arrays of customized transducers. Good candidates for these systems are optical sensors, since they are highly sensitive, and are simple to integrate in a planar format. Optical sensors currently being studied include microresonators, Mach-Zehnder interferometers [1], surface plasmon sensors [2], and grating sensors [3]. Microresonator sensors (including microrings and microdisks) can be fabricated in an integrated array format, and are attractive because they have a highly sensitive (abrupt) spectral response around resonance, are compact in size, simple to fabricate in arrays using vertical coupling, and can be surface customized for targeted biochemical sensing. While sensor selectivity is notoriously poor for index of refraction sensors such as microresonators, appropriate surface customization of the microresonator sensors, where the index of refraction of the surface customization material changes in response to an analyte, enables discrimination. Selective polymer membranes, which provide either chemical binding or selective permeability for target chemicals, [4, 5] can be used for surface customization of optical microresonators. In fact, variation of the surface customizations at multiple microresonators comprised in a chip scale array is an enticing prospect for generating overlapping signatures that can be deconvoluted with appropriate signal processing strategies. In this paper, vertically coupled microring sensors with a chemically selective polymer membrane surface coating have been demonstrated for the first time. The exemplary surface customization used in this paper is an ethylene/propylene copolymer (E/Pco), which has preferential enrichment properties (as evidenced in published mid-infrared sensor data [4]) for common organic pollutants in water, including benzene, toluene, and xylenes. 2. Sensor Fabrication and Characterization Chemically selective optical microring resonator sensors were fabricated using polymer microring resonators with a chemically selective E/Pco membrane coating. A polymer material (SU8 2002TM) from Microchem was used as the waveguiding layer of the demonstrated sensor. To fabricate a vertically coupled microring resonator, a 2 μm thick layer of SU8 was spin coated on a 3 μm thick SiO2/Si substrate. Next, 4 μm wide input/output optical waveguides were photopatterned in the SU8 using UV photolithography. A 0.2 nm thick PMMA layer was spin coated on top of the fabricated SU8 input/output optical waveguides as a middle cladding layer. A 2 μm thick SU8 microring with a 300 μm radius was fabricated on top of the middle cladding layer. The fabricated SU8 microring was vertically aligned with the input/output optical waveguides with a 0.2 nm thick PMMA acting as the gap between the input/output waveguides and the microring. To achieve chemical and mechanical stability, the SU8 layers were thermally cured at 160 oC for 10 min [6]. The chemically selective polymer membrane material was prepared by dissolving 0.4258 g of commercially available E/Pco (ethylene:propylene = 60:40) granular polymer (Aldrich) in 20 mL of toluene. The prepared E/Pco solution was spin coated at 300 rpm onto the surface of the fabricated SU8 microring to create a chemically selective membrane. This membrane enables selective diffusion of the target analyte (o-xylene in these experiments) to the sensor surface for detection. The measured average thickness of the E/P-co coating was 560 nm. Figure 1a shows a

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schematic diagram of the measurement setup and the fabricated microring resonator sensor with the chemically selective E/Pco thin film coating (inset). Surface changes due to selective diffusion of o-xylene into the membrane at the vertically coupled microring sensor results in changes of the effective refractive index of the resonant modes in the microring resonator. These changes introduce a resonant wavelength shift in the spectral response of the microring resonator sensor. To demonstrate the response of microfabricated chemically selective microring resonator sensors, the output spectrum utilizing an E/Pco membrane as chemically selective coating was monitored as a function of time for a solution of 10 ppm o-xylene in deionized (DI) water. The o-xylene sample was prepared by spiking 10 µL of xylene with a calibrated pipette (accuracy between 9.84 and 10.16 µL) into 1 L of DI water while stirring the solution at 1600 rpm. To prevent any undesired evaporation of o-xylene from the prepared solution during the sensor measurement, the entire chemical flow system was encapsulated by a macro scale liquid reservoir on top of the fabricated microring resonator sensors with input and output tubing connected to a syringe for sample delivery to the sensor. The output spectrum of the fabricated microring resonator sensors was measured with a broad band amplified spontaneous emission source and an optical spectrum analyzer. An input optical beam from a tunable laser was coupled into the input/output waveguide through a single mode optical fiber at λ=1550 nm. A linear polarizer was used to selectively launch a linearly polarized beam into the microring resonator. Figure 1b shows the output spectrum change as a function of time corresponding to the diffusion of o-xylene into the membrane after exposing the E/Pco membrane coated sensor to the 10 ppm o-xylene sample. First, the output spectrum of the fabricated sensor was measured each minute for 12 minutes with DI water, and no resonant shift was noted. Then, the output spectrum of the sensors was monitored and recorded as a function of time after introducing 10 ppm of o-xylene solution on top of the sensor (membrane) surface. As shown in Figure 1b, the entire output spectrum shifted after approximately 2 min, and remained unchanged after this shift as long as the o-xylene solution was present. To investigate the recovery characteristics of the fabricated sensor, the output spectrum was measured again after exposing the sensor to DI water. As shown in Figure 1b, the output spectrum returned to the baseline after approx. 4 h. Repeated measurements of 10 ppm of o-xylene resulted in the same spectral shift (within 10%) at 2 min, and recovery was observed from separate measurement with identical concentration of o-xylene.

(a) (b) Figure 1. (a) A schematic diagram of the measurement setup for the fabricated chemically selective optical microring resonator sensor (inset), (b) Output spectrum of the fabricated chemically selective sensors for 10 ppm of o-xylene in DI water and pure DI water monitored with time. 3. References 1. 2. 3. 4. 5. 6.

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