광학적 암호가 저장된 스마트 먼지의 센서응용

Abstract

Fluorescent molecules and quantum dots have been received much attention for use in high-throughput screening and bioassay applications. The development of new technology, which can achieve at nanometer scale, to build such a device has been great interests, because it is too complex to fabricate by using conventional lithographic method. Multistructured porous silicon (PSi) such as Rugate PSi has been recently investigated for use of its possible chemical and biological applications. Because of the tunable optical properties, PSi has been studied and applied material in recent years. Porous silicon is also an attractive material due to its high surface area, convenient surface chemistry, and optical signal transduction capability. DBR structured porous silicon having the photonic structure of a Bragg filter has been developed by applying a computer generated square current density waveform. Two types of DBR PSi (dodecyl-derivatized DBR PSi and oxidized DBR PSi) were prepared in this study shown in scheme 1. The resulting DBR PSi film has been removed from the silicon substrate by an applying of electropolishing current to obtain a free-standing DBR PSi. These freestanding films are very brittle and they can be fractured into few hundred nanometers to hundred micrometer-sized pieces in an ultrasonic bath. The freestanding DBR PSi films were then made into particles by ultrasonic fracture in an organic solution. Cross-sectional field emission electron microscope (FE-SEM) image of DBR smart particles indicates that the size of particles for is in the range from 40 to 50 micrometers. The surface image of smart particles indicates that the distribution of pore size is even and retains a good porosity with no destruction of porous structure during the ultrasonication. Photonic structure of DBR PSi results in a mirror with high reflectivity at 533 nm. The electrochemical process generates an optically uniform layer of porous silicon: the thickness and porosity of a given layer is controlled by the current density, the duration of the etch cycle, and the composition of the etchant solution. The reflection spectra of DBR PSi indicate that the full-with at half-maximum (FWHM) is of 18 nm which is narrower than that of fluorescent organic molecules or quantum dot. The surface modified DBR PSi is prepared by either thermal oxidation or thermal hydrosilylation. The oxidized DBR PSi sample has been prepared through the thermal oxidation at 300℃ for 5 min. The FT-IR spectra of the oxidized DBR PSi display absorption bands characteristic of silicon oxide, with vibrations assigned to Si-O stretching modes at 1200 cm-¹. The surface of DBR PSi has been also derivatized with 1-dodecene by the thermal hydrosilylation at 120℃ for 4 hr. FTIR spectra of the surface modified DBR PSi display absorption bands characteristic of the dodecyl group with vibrations assigned to C-H stretching modes at 2850-2960 cm-¹. Freshly etched DBR PSi containing hydrogen-terminated surface and dodecyl-terminated DBR PSi are hydrophobic characteristics on their surface, however the surface of oxidized DBR PSi is hydrophilic. The specificity of adsorption or microcapillary condensation at porous Si surfaces depends dramatically on the surface chemistry. Particles modified with dodecyl functionalities are hydrophobic materials and has a much greater affinity for hydrophobic versus hydrophilic analyte. Thus the particles exhibit the most red-shifted photonic feature for the hydrophobic analyte hexane (ca. 25 nm, 97 mmHg), but are quite insensitive to the methanol vapor (ca. 12 nm, 97 mmHg) at a partial pressure comparable to that of hydrophobic analyte hexane. However particles modified with hydroxyl group have a greater affinity for hydrophilic analyte and thus display in reverse order (ca. hexane; 15 nm, methanol; 24nm).