Simply Modified Biosensorfor the Detection ofHuman IgG Bull. Korean Chem. Soc. 2009, Vol. 30, No. 7 1593 Simply Modified Biosensor for the Detection of Human IgG Based on Protein A- Modified Porous Silicon Interferometer Jaehyun Park, Youngdae Koh, Young Chun Ko「and Honglae Sohn* Department of Chemistry, Chosun University, Gwangju 501-759, Korea. *E-mail: hsohn@chosun.ac.kr ‘School ofLiberal Arts, Daebul University, Chonnam 526-702, Korea Received March 26, 2009, Accepted May 29, 2009 A biosensor has been developed based on induced wavelength shifts in the Fabry-Perot fringes in the visible reflection spectrum of appropriately derivatized thin films of porous silicon semiconductors. Porous silicon (PSi) was generated by an electrochemical etching of silicon wafer using two electrode configurations in aqueous ethanolic HF solution. Porous silicon displayed Fabry-Perot fringe patterns whose reflection maxima varied spatially across the porous silicon. The sensor system studied consisted of a mono layer of porous silicon modified with Protein A. The system was probed with various fragments of an aqueous Human Immunoglobin G (Ig G) analyte. The sensor operated by measurement of the Fabry-Perot fringes in the white light reflection spectrum from the porous silicon layer. Molecular binding was detected as a shift in wavelength of these fringes. Key Words: Porous silicon, Optical thickness, Biosensor, Human Immunoglobin G Introduction Recently, the worth noting properties of nano structured PSi in chemical and biological sensing should be reported.1-5 The PSi optical sensing features were based on the changes of its photonic properties, such as photoluminescence or reflec- tance, on exposure to the gaseous or liquid substances.6-7 Two main optical transduction methods for label-free biosensors were both optical interferometric methods based on inter- ferometers,8 evanescent wave devices,9 and grating couplers,10 and surface plasmon methods based on metal films,11 and colloids.12 However they required a tedious work and very complex preparation during analysis. One way to overcome this limit would be simple chemical modification of the PSi hydrogenated-surface in order to enhance the sensor selec- tivity through specific biochemical interactions. For applica- tions in high throughput drug discovery and disease diagnos- tics, label-free biosensors would be important due to the advantage of easy sample preparation.13-14 Especially, optical devices based on PSi attracted a great attention of the scientists.15-18 Chemical modification of PSi exhibited the modification of its physical, chemical, and electronic properties. Chemical or bio molecule was detected based on changes in the spectral interference pattern.19-20 The rugate PSi with reflection peaks in visible spectral range was recently demonstrated to be applied for the detection of different chemical species.21-23 PSi was employed as a large surface area matrix for immo- bilization of a variety of biomolecules including enzymes,24 DNA fragments, 25 and antibodies.26 In the present work, a simply modified biosensor for the detection of humman IgG based on protein A-modified PSi interferometer was reported. Fabry-Perot fringes were created by the reflections of illuminated white light on the air-PSi layer and the PSi-bulk silicon interface. The induced shift in the reflection pattern caused by the change in the refractive index of the medium upon molecular interactions of species in solution with immobilized receptor within the PSi matrix was used as a sensitive method for biomolecular sensing. The simple fabrication, optical characterization, and surface functionalization of a PSi were reported. For the application as a label-free biosensor, the binding interaction with protein was investigated. Experiment The PSi was prepared by an electrochemical etching of Si wafer (boron doped, polished on the <100> face, resistivity of 0.8 〜1.2 mQ-cm, Siltronix, Inc.). The etching solution con- sisted of a 1:1 (v/v) mixture of absolute ethanol (ACS reagent, Aldrich Chemicals) and aqueous 48% hydrofluoric acid (ACS reagent, Aldrich Chemicals). Galvanostatic etching was carried out in a Teflon cell using an electrode configuration. The current density and etching time were 300 mA/cm2 and 25 sec. The anodization current was supplied by Keithley 2420 high-precision constant current source controlled by a computer to allow the formation of PSi. To prevent the photo- generation of carriers, electrochemical etching was perform- ed the anodization in the dark. All samples were then rinsed several times with ethanol and dried under argon atmosphere prior to use. The silicon surface was predominant hydride-terminated after the etching procedure. This surface was sensitive to oxidation and hydrolysis upon exposure to aqueous solution. Thermally oxidized PSi samples were obtained by heat treat- ment in a furnace (Thermolyne F6270-26 furnace equipped with controller) using the following parameters: initial ramp rate, 5 oC/min to 300 oC, hold time, 3 h and passive cooling to ambient temperature. For functionalization of oxidized PSi, the oxidized PSi sample was refluxed in 0.9 mL of (3-aminopropyl)trimethoxy- silane (99%, Aldrich Chemicals) for 12 hrs. After func- tionalization with amine group, the sample was rinsed successively