Photonic crystal optical biosensor incorporating structured low-index porous dielectric

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Abstract

The sensitivity of a photonic crystal optical biosensor is greatly enhanced through the incorporation of low refractive index porous dielectric material into the device structure. In this work, computer models are used to predict the reflectance spectra and sensitivity performance of a one-dimensional photonic crystal biosensor. A manufacturable replication method is demonstrated that can produce a low-index dielectric periodic surface structure with a 550 nm period over large surface areas. The sensitivity of porous glass biosensors is characterized and compared with sensors incorporating non-porous polymer material. Results for detection of proteins, polymer layers, and bulk liquids indicate up to a four-fold sensitivity increase.

Introduction

Label-free optical biosensors based upon surface structured photonic crystals have recently been demonstrated as a highly sensitive method for performing a wide variety of biochemical and cell-based assays [1]. The device structure is designed to reflect only a narrow band of wavelengths when illuminated with white light at normal incidence, where positive shifts of the reflected peak wavelength value (PWV) indicate the adsorption of detected material on the sensor surface [2]. By spatially confining incident photons at the resonant wavelength, a high optical field is generated at the sensor surface that extends a short distance into a test sample, much like an evanescent field. The high degree of spatial confinement of resonant photons within the device structure leads to a strong interaction between the structure and adsorbed biomaterial, and to the ability to perform high resolution imaging of protein and cell attachment [3].

Previously, photonic crystal optical biosensors have been fabricated from continuous sheets of plastic film using a process in which the periodic surface structure is replicated from a silicon master wafer using a UV-cured polymer material [4]. This patterned polymer is subsequently coated with a high refractive index TiO2 layer that is generally thinner than the height of the surface structure. Such devices have been demonstrated for a wide variety of biochemical and cell-based assays, with a mass density sensitivity resolution less than 0.1 pg/mm2 and a large dynamic range enabling single cell detection [5]. In general, optimization of the device sensitivity requires increasing the interaction of the electromagnetic field intensity distribution with the biological material deposited atop the photonic crystal surface. Therefore, selection of optical materials and design of the surface structure topology should be aimed at extending the electromagnetic field profile from the interior regions of the photonic crystal (where they cannot interact with adsorbed material) to the region adjacent to the photonic crystal that includes the liquid test sample. In this work, we demonstrate that the substitution of an extremely low refractive index material for the surface structure within the photonic crystal biosensor has the desired effect of substantially increasing detection sensitivity.

We used rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) simulations to predict the resonant wavelength and bulk refractive index sensitivity of a one-dimensional surface photonic crystal biosensor. The device incorporates a low-index (n = 1.17) nanoporous dielectric surface structure in place of the polymer (n = 1.39) surface structure reported previously. We use a soft contact embossing method to create a surface-structured low-index porous film on glass substrates with a depth and period that are identical to the previous polymer structures to enable a side-by-side sensitivity comparison. The sensitivity of porous glass biosensors is compared to nonporous polymer biosensors through methods that characterize sensitivity to bulk refractive index and surface-adsorbed material. Finally, a protein binding assay comparison is performed to demonstrate sensor stability and the ability to functionalize the device for selective detection.

Section snippets

Computer simulation

The polymer and porous glass sensors were modeled and simulated using two software packages. First, a 2-D diffraction grating analysis tool (GSOLVER) employing the RCWA algorithm provides a quick and simple method for initial sensor modeling. Second, FDTD (Lumerical) provides a much more versatile and powerful tool that can calculate any field component at any temporal or spectral location for an arbitrary optical device illuminated by an arbitrary source [6]. FDTD was used to verify RCWA

Computer simulation

RCWA and FDTD simulations both indicated that replacement of the patterned UV-cured polymer of previous devices with a material of lower refractive index would produce a two-fold increase in the bulk shift coefficient. The resonant wavelength of the porous glass sensor immersed in DI H2O was predicted by RCWA to be 844.3 nm with a full-width at half-maximum (FWHM) of approximately 2 nm, as shown in Fig. 2. Simulation predicts further improvements in the bulk shift coefficient with slight

Discussion

The photonic crystal biosensor is designed to couple electromagnetic energy to biological material deposited upon its surface from a liquid test sample. While the device itself consists of a low refractive index surface structure and a high refractive index dielectric coating, the liquid test sample that fills in the surface structure must also be considered an integral part of the sensor—and the only dynamic component that can induce a change of resonant wavelength. The motivation for

Conclusion

We have demonstrated a novel photonic crystal biosensor incorporating a surface-patterned low-index material that exhibits up to a four-fold sensitivity increase over similar sensors that use a patterned higher-index polymer. Computer simulations provided accurate predictions of the porous glass sensor behavior. Several experiments explored differing measures of sensitivity by introducing a bulk index change, generating both a single protein monolayer as well a multilayer polymer stack, and

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. 0427657. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors gratefully acknowledge SRU Biosystems for providing financial support for this work and Honeywell Electronic Materials for the donation of Nanoglass® low-k porous dielectric material. The

Ian D. Block received a BS in Electrical & Computer Engineering from Cornell University in 2004, and a MS in Electrical Engineering from the University of Illinois at Urbana-Champaign in 2005. He is currently working towards a PhD under the direction of Dr. Brian Cunningham at the University of Illinois. The focus of his research is the design and characterization of enhanced sensitivity photonic crystal biosensors.

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Ian D. Block received a BS in Electrical & Computer Engineering from Cornell University in 2004, and a MS in Electrical Engineering from the University of Illinois at Urbana-Champaign in 2005. He is currently working towards a PhD under the direction of Dr. Brian Cunningham at the University of Illinois. The focus of his research is the design and characterization of enhanced sensitivity photonic crystal biosensors.

Leo Li-Ying Chan is a graduate research assistant at the University of Illinois at Urbana-Champaign in the Nano Sensors Group directed by Dr. Brian T. Cunningham. His research focuses on the characterization of photonic crystal optical biosensors and the optimization of small molecule biodetection using this platform. Before joining Dr. Cunningham's group, Leo Chan served as an undergraduate research at Keck Graduate Institute: Claremont, California, where he worked on the application of free solution electrophoresis to DNA finger printing. He earned his BS and MS in Electrical and Computer Engineering with a minor in Biomedical Engineering from the University of Illinois at Urbana-Champaign, where he is currently pursuing a PhD.

Brian T. Cunningham (PhD) is an Associate Professor of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign, where he is the director of the Nano Sensors Group. His group focuses on the development of photonic crystal-based transducers, plastic-based fabrication methods, and novel instrumentation approaches for label-free biodetection. Prof. Cunningham is a founder and the Chief Technical Officer of SRU Biosystems (Woburn, MA), a life science tools company that provides high sensitivity plastic-based optical biosensors, instrumentation, and software to the pharmaceutical, academic research, genomics, and proteomics communities. Prior to founding SRU Biosystems in June, 2000, Dr. Cunningham was the Manager of Biomedical Technology at Draper Laboratory (Cambridge, MA), where he directed R&D projects aimed at utilizing defense-related technical capabilities for medical applications. In addition, Dr. Cunningham served as Group Leader for MEMS Sensors at Draper Laboratory, where he directed a group performing applied research on microfabricated inertial sensors, acoustic sensors, optical switches, microfluidics, tissue engineering, and biosensors. Concurrently, he was an Associate Director of the Center for Innovative Minimally Invasive Therapy (CIMIT), a Boston-area medical technology consortium, where he led the Advanced Technology Team on Microsensors. Before working at Draper Laboratory, Dr. Cunningham spent 5 years at the Raytheon Electronic Systems Division developing advanced infrared imaging array technology for defense and commercial applications. Dr. Cunningham earned his BS, MS, and PhD degrees in Electrical and Computer Engineering at the University of Illinois. His thesis research was in the field of optoelectronics and compound semiconductor material science, where he contributed to the development of crystal growth techniques that are now widely used for manufacturing solid state lasers, and high frequency amplifiers for wireless communication.

1

These authors contributed equally to this work.

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