Biosensors and Bioelectronics 26 (2011) 2413–2418 Contents lists available at ScienceDirect Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios Label free sub-picomole level DNA detection with Ag nanoparticle decorated Au nanotip arrays as surface enhanced Raman spectroscopy platform Hung-Chun Lo a , Hsin-I Hsiung b , Surojit Chattopadhyay c,∗ , Hsieh-Cheng Han b , Chia-Fu Chen d,∗∗ , Jih Perng Leu a , Kuei-Hsien Chen b,e , Li-Chyong Chen b,∗∗∗ a Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan b Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan c Institute of Biophotonics, National Yang-Ming University, 155, sec-2, Li-Nong Street, Taipei 112, Taiwan d Department of Materials Science and Engineering, Ming-Dao University, Changhua 523, Taiwan e Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan article info Article history: Received 6 September 2010 Accepted 11 October 2010 Available online 16 October 2010 Keywords: Biosensor Surface enhanced Raman spectroscopy DNA Surface plasmon Nanostructure abstract Label free optical sensing of adenine and thymine oligonucleotides has been achieved at the sub-picomole level using self assembled silver nanoparticles (AgNPs) decorated gold nanotip (AuNT) arrays. The plat- form consisting of the AuNTs not only aids in efficient bio-immobilization, but also packs AgNPs in a three dimensional high surface area workspace, assisting in surface enhanced Raman scattering (SERS). The use of sub-10 nm AgNPs with optimum inter-particle distance ensures amplification of the chemically specific Raman signals of the adsorbed adenine, thymine, cytosine and guanine molecules in SERS exper- iments. High temporal stability of the Raman signals ensured reliable and repeatable DNA detection even after three weeks of ambient desk-top conservation. This facile architecture, being three dimensional and non-lithographic, differs from conventional SERS platforms. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Surface enhanced Raman scattering (SERS) has developed into a frontline tool for chemical analysis and sensing (Aroca, 2006), since its discovery (Fleischmann et al., 1974), owing to an improved scattering cross-section over the conventional spontaneous Raman process and its efficacy at room temperatures. The enhancement of the Raman scattering cross-section (>10 6 ) is believed to be due to a surface plasmon aided stronger electro magnetic coupling at the analyte site (Moskovits, 1985; Nie and Emory, 1997). However, another school of thought attributes the enhancement to a chemi- cal charge transfer effect, though it could only predict a theoretical enhancement of the Raman scattering cross-section to the tune of 10 2 (Moskovits, 1985). Apart from the range of organic and inor- ganic molecules successfully detected using SERS, the technique is becoming increasingly important for bio-sensing (Tripp et al., 2008; Vo-Dinh et al., 2010). Fodor et al. (1985) used a pulsed ultraviolet (UV) laser to enhance the signals from the mononucleotide of ade- nine (A), thymine (T), cytosine (C) and guanine (G) with limited ∗ Corresponding author. Tel.: +886 2 28267909; fax: +886 2 28235460. ∗∗ Corresponding author. Tel.: +886 4 8876660x1030. ∗∗∗ Corresponding author. Tel.: +886 2 33665249; fax: +886 2 23655404. E-mail addresses: sur@ym.edu.tw, suroch@gmail.com (S. Chattopadhyay), cfchen@mdu.edu.tw (C.-F. Chen), chenlc@ntu.edu.tw (L.-C. Chen). success. Silver nanoparticle (AgNP) aided SERS was well estab- lished for the detection of the nonfluorescent bio-species, such as DNA and fluorescent biomarkers (Brolo et al., 2004; Faulds et al., 2004, 2005; Kneipp et al., 1997; Krug et al., 1999; Stokes et al., 2007; Sun et al., 2007; Wei and Xu, 2007; Zhang et al., 2005). How- ever, Ag did not participate as well as Au in bio-immobilization. For this purpose, gold (Au) plays a major role in establishing the Au–S (gold–sulphur) bond to efficiently immobilize the thiolated bio-species on the substrate, but lost out on the enhancement, and hence sensitivity, of the SERS process against Ag. Conventional detection techniques, such as fluorescence, yielded detection lim- its of 10 -13 M from dye labeled-DNA sequences dispersed in AgNP containing buffer solution (Faulds et al., 2004, 2005; Kneipp et al., 1997; Zhang et al., 2005). However, those are indirect detection schemes in the solution phase. The use of Au nanostructures to obtain high immobilization was also employed to detect labeled- DNA sequences through SERS (Brolo et al., 2004; Krug et al., 1999; Stokes et al., 2007; Sun et al., 2007). These detection schemes were limited by the complex sample preparation and relatively low enhancement factors compared to AgNP, as discussed before (Stokes et al., 2007). For increased reliability, measurements using solid substrates were also reported (Brolo et al., 2004; Wei and Xu, 2007). A recent development, tip enhanced Raman spectroscopy (TERS), has demonstrated detection of low concentration non- fluorescent analytes such as DNA (Becker et al., 2008; Domke et al., 0956-5663/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2010.10.022