V. Torres-Costa

Optical biosensors based on semiconductor nanostructures.

The increasing availability of semiconductor-based nanostructures with novel and unique properties has sparked widespread interest in their use in the field of biosensing. The precise control over the size, shape and composition of these nanostructures leads to the accurate control of their physico-chemical properties and overall behavior. Furthermore, modifications can be made to the nanostructures to better suit their integration with biological systems, leading to such interesting properties as enhanced aqueous solubility, biocompatibility or bio-recognition. In the present work, the most significant applications of semiconductor nanostructures in the field of optical biosensing will be reviewed. In particular, the use of quantum dots as fluorescent bioprobes, which is the most widely used application, will be discussed. In addition, the use of some other nanometric structures in the field of biosensing, including porous semiconductors and photonic crystals, will be presented.

Optical gas sensing properties of thermally hydrocarbonized porous silicon Bragg reflectors

In the present work, porous silicon (PS) based Bragg reflectors are fabricated, and the reactive PS surface is passivated by means of thermal carbonization (TC) by acetylene decomposition. The gas sensing properties of the reflectors are studied with different gas compositions and concentrations. Based on the results it can be concluded that thermally carbonized Bragg reflectors provide an easy and inexpensive means to produce chemically stable high quality PS reflectors with good gas sensing properties, which differ from those of unpassivated PS reflectors.

Biomedical applications of nanostructured porous silicon: a review

Current fabrication and characterization techniques allow the development of nanostructured systems with controlled size, shape and composition. Additionally, modifications can be made to these nanosystems to better suit their integration with biological systems, leading to such interesting properties as enhanced aqueous solubility, biocompatibility or bio-recognition. The particular morphology and overall properties of nanostructured porous silicon allow the use of this material in the fields of drug delivery, eye diseases, tumor imaging, and tissue engineering.

Porous silicon-cyclodextrin based polymer composites for drug delivery applications

One of the main applications of porous silicon (PSi) in biomedicine is drug release, either as a single material or as a part of a composite. PSi composites are attractive candidates for drug delivery systems because they can display new chemical and physical characteristics, which are not exhibited by the individual constituents alone. Since cyclodextrin-based polymers have been proven efficient materials for drug delivery, in this work β-cyclodextrin-citric acid in-situ polymerization was used to functionalize two kinds of PSi (nanoporous and macroporous). The synthesized composites were characterized by microscopy techniques (SEM and AFM), physicochemical methods (ATR-FTIR, XPS, water contact angle, TGA and TBO titration) and a preliminary biological assay was performed. Both systems were tested as drug delivery platforms with two different model drugs, namely, ciprofloxacin (an antibiotic) and prednisolone (an anti-inflammatory), in two different media: pure water and PBS solution. Results show that both kinds of PSi/β-cyclodextrin-citric acid polymer composites, nano- and macro-, provide enhanced release control for drug delivery applications than non-functionalized PSi samples.

RBS characterization of porous silicon multilayer interference filters

Porous silicon (PS) has great potential in optical applications due to its tunable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indexes can be used as interference filters for applications in optoelectronics. In the present work, Rutherford backscattering spectroscopy (RBS) measurements and optical characterization have been carried out on PS multilayer stacks consisting of alternate low-porosity/high-porosity layers to determine their compositional profile, homogeneity, and overall optical behavior. In addition, RBS has been used for the first time to determine the porosity profile of this kind of structures. The experimental results show a constant indepth composition among alternate layers, revealing the good homogeneity of the multilayer structures. Neither porosity nor oxidation degree gradient were observed.

Porous silicon multilayer stacks for optical biosensing applications

Porous silicon (PS) multilayer stacks were developed for their use as interference filters in the visible range. The optical behavior of these structures was previously simulated by the use of a computational program, from which the optical constants and thickness of the individual PS layers were determined. The possibility of using these structures as biosensors has been explored, based on the significant changes in the reflectance spectra before and after exposing the PS multilayer to proteins (antibodies). In particular, it is shown that there is a notably reduction of reflectance from PS structures when this material is exposed to polyclonal mouse antibodies. Thus, the experimental results open the possibility of developing biosensors based on the variation of the shape and/or position of the optical or photoluminescent spectrum from PS.

Optical constants of porous silicon films and multilayers determined by genetic algorithms

Porous silicon (PS) can be optically described as a homogeneous mixture of air, silicon, and, eventually, silicon dioxide, which explains the great tunability of its refractive index. This behavior makes porous silicon a very interesting material for the development of interference filters for use in optoelectronic applications. For the accurate design of such filters, the precise determination of the optical constants of PS is mandatory. However, since PS is a dispersive and absorbing material, determination of its complex refractive index is not trivial. In the present work, a genetic algorithm has been used to precisely determine the complex refractive index and thickness of thin films from their reflectance spectra in the visible wavelength range. This algorithm was applied to porous silicon. The precise spectral values of the refractive index obtained for PS layers of different characteristics have been used to design and fabricate interference filters with a predetermined optical behavior. In additi...

Effective passivation of porous silicon optical devices by thermal carbonization

Nanostructured porous silicon (PS) optical filters have been proposed for their use in biological and chemical sensing applications. PS, however, presents a reactive surface that must be adequately passivated in order to achieve the required chemical stability mandatory for sensing applications. In the present work, thermal carbonization (TC) by acetylene decomposition is shown to provide effective passivation of a PS internal surface. Thermally carbonized PS optical filters are stable even in strong oxidizing environments such as absolute ethanol. Moreover, it is shown that the TC process, as opposed to more commonly used oxidation treatments, has only minor effects on the optical properties of PS. Thus, the optical performance of PS interference filters is preserved after the carbonization process. In addition, the hydrophilicity of the PS device surface can be adjusted by setting the appropriate TC process temperature. These results show that it is possible to produce low-cost, reusable, chemically sta...

Surface Functionalization of Nanostructured Porous Silicon by APTS: Toward the Fabrication of Electrical Biosensors of Bacterium Escherichia coli

Nanostructured porous silicon (nanoPS) basically consists in a network of silicon nanocrystals with high specific surface. Its intrinsic high surface reactivity makes nanoPS a very suitable material for the development of biosensors. In this work, the surface of nanoPS was functionalized by the use of (3-aminopropyl)triethoxysilane solutions in toluene. Escherichia coli (E. coli) antibodies were subsequently immobilized on the functionalized surfaces. Finally, fragments of this bacterium, which are specifically recognized by the antibodies, were immobilized. Moreover, devices with a metal/nanoPS/semiconductor/metal structure were fabricated aiming at the electrical biosensing of E. Coli bacterium. The experimental results showed a strong variation of the current as a function of the presence/absence of bacterium E. Coli and surface concentration.

Silicon-based photonic crystals fabricated using proton beam writing combined with electrochemical etching method

A method for fabrication of three-dimensional (3D) silicon nanostructures based on selective formation of porous silicon using ion beam irradiation of bulk p-type silicon followed by electrochemical etching is shown. It opens a route towards the fabrication of two-dimensional (2D) and 3D silicon-based photonic crystals with high flexibility and industrial compatibility. In this work, we present the fabrication of 2D photonic lattice and photonic slab structures and propose a process for the fabrication of 3D woodpile photonic crystals based on this approach. Simulated results of photonic band structures for the fabricated 2D photonic crystals show the presence of TE or TM gap in mid-infrared range.