Available online at www.sciencedirect.com Biosensors and Bioelectronics 23 (2008) 939–944 A universal biosensing platform based on optical micro-ring resonators A. Ramachandran, S. Wang ∗ , J. Clarke ∗∗ , S.J. Ja, D. Goad, L. Wald, E.M. Flood, E. Knobbe, J.V. Hryniewicz 1 , S.T. Chu 1 , D. Gill 1 , W. Chen 1 , O. King 1 , B.E. Little 1 Nomadics Inc., 1024 S. Innovation Way, Stillwater, OK 74074, United States Received 5 June 2007; received in revised form 6 September 2007; accepted 17 September 2007 Available online 21 September 2007 Abstract The use of optical micro-ring resonators as a platform for quantitative and qualitative biosensing applications was explored. Vertically coupled, high refractive index micro-ring resonators, used as sensing elements, were fabricated on silicon chips by photolithographic techniques. An optical reader system consisting of a near-infrared broad band light source and an optical spectrum analyzer were employed for data acquisition. Micro-ring resonator surfaces were modified with specific target receptors, including antibodies and single-stranded DNA oligonucleotides. The system was successfully used for label-free, specific, and rapid detection of whole bacterial cells, proteins and nucleic acids. © 2007 Elsevier B.V. All rights reserved. Keywords: Micro-ring resonators; Biosensor; Label-free quantitative detection; Optical resonators; Whole cell detection; Antibody; Antigen; Nucleic acid; Bacteria; IgG; IgM 1. Introduction Rapid and simple sensing and detection of bioanalytes are of immense significance for environmental monitoring, food safety, and diagnostic applications. In recent years, different sen- sor platforms exhibiting enhanced detection capabilities, based on potentiometric, amperometric, magnetic, and optical trans- ducers have been developed (Lazcka et al., 2007). Herein, we report the use of micro-ring resonators as a universal platform for label-free bioanalyte detection. Micro-ring resonators are made using dielectric materials, and they can be fabricated by standard silicon wafer processes similar to those used for manufacture of integrated circuit (IC) chips. Other dielectric structures, for example, micro- spheres, already have been well-studied for their use as optical resonators and waveguides (Matsko and Ilchenko, 2006). ∗ Corresponding author at: ICx Nomadics, 1024 S. Innovation Way, Stillwater, OK 74074, United States. Tel.: +1 405 372 9535; fax: +1 405 372 9537. ∗∗ Corresponding author at: ICx Nomadics, 2260 NW Kinderman Drive, Cor- vallis, OR 97330, United States. Tel.: +1 405 372 9535; fax: +1 405 372 9537. E-mail addresses: shaopeng.wang@icxt.com (S. Wang), jean.clarke@icxt.com (J. Clarke). 1 Currently at PLC division of Infinera Corporation, Annapolis Junction, MD 20701, United States. Micro-spheres, made of optically dense materials such as fused quartz, internally reflect light that is propagating within the sphere, thereby spatially constraining the light to travel around the sphere circumference until the light is absorbed or scattered by material imperfections. This phenomenon, known as whis- pering gallery mode (WGM) resonance, has been investigated for biosensing applications (Vollmer et al., 2002, 2003; White et al., 2005; Zhu et al., 2006). Similar resonance patterns have also been recognized in other types of optical resonators, such as micro-cylinders, micro-disks, and micro-rings (Deng et al., 2004; Smotrova et al., 2006; Zeng et al., 1999). Micro-ring resonators are versatile structures that have been used in optical communication circuits as wavelength filters, switches and frequency converters (Little et al., 1997; Ibrahim et al., 2003). Their utility also has been recognized for biosens- ing applications, and demonstrated using generic avidin–biotin interactions (Yalcin et al., 2006; Chao et al., 2006). Micro-ring resonators comprise ring-shaped waveguide structures (micro- rings) optically coupled to one or more linear waveguides, patterned on a planar surface (Little et al., 2000)(Fig. 1). They can be fabricated by photolithographic techniques and are amenable to very large scale integration (VLSI) (Tan et al., 2002). Several design formats of micro-ring resonators have been reported (Ibrahim et al., 2003; Chu et al., 1999; Yanagase et al., 2002; Little et al., 2004). Micro-ring resonators offer sev- 0956-5663/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2007.09.007