Author's personal copy Available online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 279–284 High sensitivity photonic crystal biosensor incorporating nanorod structures for enhanced surface area Wei Zhang, Nikhil Ganesh, Ian D. Block, Brian T. Cunningham ∗ Nano Sensors Group, Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, 208 North Wright Street, Urbana, Illinois 61801, United States Received 7 February 2007; received in revised form 18 October 2007; accepted 12 November 2007 Available online 22 November 2007 Abstract The surface area of a photonic crystal biosensor is greatly enhanced through the incorporation of a porous TiO 2 film possessing the structure of nanorods into the device. The film is deposited by the glancing angle deposition technique in an e-beam evaporation system. The sensitivity of high surface area sensors is compared with sensors without the high surface area coating. Results for detection of polymer films, large proteins and small molecules indicate up to a four-fold enhancement of detected adsorbed mass density. © 2007 Elsevier B.V. All rights reserved. Keywords: Biosensor; Surface area; Photonic crystal; Nanostructure; Sensitivity; Glancing angle deposition 1. Introduction Label-free optical biosensors have emerged as important tools for pharmaceutical research, diagnostic testing, and envi- ronmental monitoring [1,2]. Development of sensor designs that enhance sensitivity is especially important because it allows detection of lower concentrations of analytes and detec- tion of small molecules with a higher signal-to-noise ratio. Recently, optical biosensors based on photonic crystals (PCs) have attracted widespread interest due to their ability to con- centrate light into extremely small volumes and to obtain very high local electromagnetic field intensities, resulting in reflec- tion/transmission properties that are tuned by the adsorption of chemical and biomolecular materials. These include one and two-dimensional surface PCs (also referred to as guided mode resonance filters) [3–6], three-dimensional opal and inverse opal structures [7,8], PCs with microcavities [9,10], and PC waveg- uides [11,12]. Because the response of a biosensor generally depends on the interaction of an analyte with a selective immobilized capture ligand on the sensor surface, increasing the surface area of the sensor generally improves the sensitivity through a higher den- ∗ Corresponding author. Tel.: +1 217 265 6291. E-mail address: bcunning@uiuc.edu (B.T. Cunningham). sity of the capture ligand. One commercially successful method for increasing ligand surface area has been the use of poly- mer hydrogels such as dextran [13,14] to extend the surface area for binding into a three-dimensional volume within the evanescent field region of surface plasmon resonance sensors. Other methods involve: (1) porous silicon-based structures by electrochemical etching [15–19], (2) coatings of nanostructured dielectrics by chemical vapor deposition [20,21], microcapillary pipetting [21,22], and sol–gel processes [23], and (3) coatings of high surface area polymers by electrophoretic deposition [21] and spray-on technique [24]. All these methods typically require complex procedures for deposition and/or functionaliza- tion using liquid-based processes, or high temperature annealing which is not suitable for plastic-based sensor structures. There- fore, enhancement of biosensor surface area using a simpler technique would be advantageous, especially if it would be a part of the sensor fabrication process, and performed inexpensively and uniformly over large areas. Glancing angle deposition (GLAD) [25] has been shown to be a method able to create thin films with very high porosities and surface-area-to-volume ratios. By orientating the incoming flux at an oblique angle, the self-shadowing effect during the deposition results in a porous film with a structure composed of isolated vertical nanorods. This technique has been used in various applications such as dielectric reflectors [26], optical fil- ters [27], liquid crystal displays [28], and substrates for surface 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.11.017