Rotiroti, Lucia (2008) BIOCHIP AND LAB-ON-CHIP FOR GENOMIC AND PROTEOMIC APPLICATIONS. [Tesi di dottorato] (Unpublished)

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There is no other inorganic material that brings together such much intriguing features like porous silicon (PSi) for people working in the optical sensing field. Porous silicon was unexpectedly discovered in the late fifties when chemists attempted to electropolish silicon wafers with an electrolyte containing hydrofluoric acid to get better electrical contacts in the first integrated electronic circuit fabrications. The acid dissolution left a sponge-like nanocrystalline silicon structure on the wafer. Today, the electrochemical etching is a standard method to fabricate nanostructured porous silicon: a proper choice of the applied current density, the electrolyte composition, and the silicon doping allow precise control over the morphology and, consequently, on the physical and chemical properties of the porous silicon structure. Computer controlled electrochemical etching processes are exploited for the realization of porous silicon films of controlled thickness and porosity (defined as the percentage of void in the silicon volume). Nanoporous, mesoporous and macroporous structures can be achieved, with pore size ranging from few nanometers up to microns. Moreover, since the etching process is self-stopping, it is possible to fabricate with a single run process multilayer stacks made of single layers of different porosity. The dielectric properties of each PSi layer, and in particular its refractive index n, can be namely modulated between those of crystalline silicon (n = 3.54, porosity = 0) and air (n=1, porosity = 100 %); so that alternating high and low porosity layers, lot of photonic structures, such as Fabry-Perot interferometers, omni-directional Bragg reflectors, optical filters based on microcavities, and even complicated quasi-periodic sequences can be simply realized. The other key feature for a transducer material is the large area and the chemistry of its surface: the porous silicon exhibits a very reactive hydrogenated specific area of the order of 200 – 500 m2 cm-3, which assures an effective interaction with several adsorbates. The porous silicon optical sensing features are based on the changes of its photonic properties, such as photoluminescence or reflectance, on exposure to the gaseous or liquid substances. Of course, these interactions are not specific. Therefore, the porous silicon hydrogenated surface has to be chemically or physically modified in order to achieve the sensing selectivity through specific biochemical interactions. The biosensor reliability strongly depends on the functionalization process: how simple, homogenous and repeatable it can be. The substitution of the superficial Si-H bonds with Si-C ones or Si-OC guarantees a much more stable surface interface from the thermodynamic point of view. Several functionalization strategies were investigated. Testing and demonstrating the porous silicon capabilities as a useful functional material in the immobilization of biological probes and in the optical transduction of biochemical interactions is only the first action in the realisation of an optical biochip based on this nanostructured material. In this case, all the fabrication processes should be compatible with the utilisation of biological probes and the feasibility of such devices must be proven. This means that the standard integrated circuit micro-technologies should be modified and adapted to this new field of application. The porous silicon low cost technology could thus provide a link between the conventional CMOS technology and the photonic devices in the realization of the so-called smart sensors and biochips. A very strong interdisciplinary approach is required to match and resolve all the technological problems. The purpose of this thesis was to design and fabricate different resonant optical structures, integrate in simple lab-on-chip devices based on the porous silicon nanotechnology, and it was focused on optical biosensing applications of social interest, such as environmental monitoring and biomedical diagnostics.

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