Published: June 29, 2011 r2011 American Chemical Society 6215 dx.doi.org/10.1021/ac2007762 | Anal. Chem. 2011, 83, 6215–6222 ARTICLE pubs.acs.org/ac Plasmonic Properties of the Multispot Copper-Capped Nanoparticle Array Chip and Its Application to Optical Biosensors for Pathogen Detection of Multiplex DNAs Do-Kyun Kim, † Seung Min Yoo, †,# Tae Jung Park,* ,†,‡ Hiroyuki Yoshikawa, § Eiichi Tamiya, § Jung Youn Park, ^ and Sang Yup Lee* ,†,# † BioProcess Engineering Research Center, Center for Systems & Synthetic Biotechnology, and Institute for the BioCentury, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Center for Nanobio Integration & Convergence Engineering, National Nanofab Center, 291 Daehak-ro, Yuseong-gu, Daejeon 305-806, Republic of Korea § Department of Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ^ Biotechnology Research Division, National Fisheries Research & Development Institute (NFRDI), 408-1 Sirang-ri, Gijang, Busan 619-705, Republic of Korea # Department of Chemical & Biomolecular Engineering (BK21), Department of Bio & Brain Engineering, Department of Biological Sciences, and Bioinformatics Research Center, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b S Supporting Information I n recent years, metallic nanostructures have become an inter- esting topic for many researchers because of their specific optical characteristics. The collective charge density oscillations of nanostructures are defined as localized surface plasmon resonance (LSPR). The LSPR is excited when electromagnetic radiation of incident light interacts with the free electrons of the nanostructures, which results in collective oscillations, leading to strong enhancements of the local electromagnetic fields sur- rounding the nanostructures. 1À3 Depending on the types of materials, sizes, and shapes of nanostructures, LSPR spectrum peaks have various sensing capacities for the interfacial refractive index (RI) and biomolecular interactions. 4À7 Metallic nanoshells, spherical nanoparticles (NPs) composed of a dielectric core and a concentric metal shell, are NPs whose plasmon resonant energies are particularly sensitive to geometry. 8 Coinage metallic nanoshells such as gold (Au) and silver (Ag) have been of particular interest for LSPR properties because they can support NP plasmon resonance in the ultra- violet, visible, and near-infrared regions of the spectrum, modifi- able by varying NP size and shape. 9À14 In coreÀshell type optical device applications, copper (Cu) may potentially be desirable over Au or Ag because of its compatibility with dielectric core- based processing. 15 Moreover, Cu nanoshells are of significant interest for potential large-scale or large-area applications relative to their Au and Ag counterparts because of the significantly reduced relative cost of the constituent metal. However, Cu is Received: March 28, 2011 Accepted: June 29, 2011 ABSTRACT: A localized surface plasmon resonance (LSPR)- based optical biosensor in connection with a multispot copper- capped nanoparticle array (MC-NPA) chip was proposed and developed. The copper (Cu) films, used as a shell, formed a “cap- like” layer on the top of the silica nanoparticles, used as a core, in an orderly fashion, to form the surface called a “Cu-capped nanoparticle array chip”. The plasmonic properties of this nanostructure type were initially investigated while controlling the shell thickness of the deposited Cu. Also, we quantified the sensitivity of MC-NPA chip to changes in bulk refractive index (RI). As a result of its LSPR properties, the MC-NPA chip displayed a sensitivity of 67.8 nm per RI unit, and the wavelength shift of the LSPR spectrum peak was sensitive to the RI of the surrounding bulk medium, such as the biomolecular layers. Using MC-NPA chips, multiplex sensing of target DNAs from reference bacteria and clinical samples was possible in a quantitative manner with a detection limit of 10 fM (50 zmol). The optical biosensor developed in this study represents a unique approach to performing LSPR that utilizes a simple and cost-effective optical setup with disposable chips.