Ilaria Rea

Lensless light focusing with the centric marine diatom Coscinodiscus walesii.

In this work, we report on the light focusing ability exploited by the microshell of a marine organism: the Coscinodiscus wailesii diatom. A 100 microm spot size of a red laser beam is narrowed up to less than 10 microm at a distance of 104 microm after the transmission through the regular geometry of the diatom structure, which thus acts as a microlens. Numerical simulations of the electromagnetic field propagation show a good qualitative agreement with the experimental results. The focusing effect is due to the superposition of the waves scattered by the holes present on the surface of the diatom valve. Very interesting applications in micro-optic devices are feasible due to the morphological and biological characteristic of these unicellular organisms.

Photonic band gaps analysis of Thue-Morse multilayers made of porous silicon

Dielectric aperiodic Thue-Morse structures up to 128 layers have been fabricated by using porous silicon technology. The photonic band gap properties of Thue-Morse multilayers have been theoretically investigated by means of the transfer matrix method and the integrated density of states. The theoretical approach has been compared and discussed with the reflectivity measurements at variable angles for both the transverse electric and transverse magnetic polarizations of light. The photonic band gap regions, wide 70 nm and 90 nm, included between 0 and 30 degrees , have been observed for the sixth and seventh orders, respectively.

Periodic versus aperiodic: Enhancing the sensitivity of porous silicon based optical sensors

The authors have compared the sensitivities of resonant optical biochemical sensors, based on both periodic and aperiodic porous silicon structures, such as the Bragg and the Thue-Morse multilayer. The shifts of the reflectivity spectra of these devices on exposure to several chemical compounds have been measured: the aperiodic multilayer is more sensitive than the periodic one. Adopting a simple theoretical model of the optical response of devices, they inferred that the aperiodic structure provide a higher filling capability with respect to the periodic one, due to the lower number of interfaces.

Optical properties of diatom nanostructured biosilica in Arachnoidiscus sp: micro-optics from mother nature.

Some natural structures show three-dimensional morphologies on the micro- and nano- scale, characterized by levels of symmetry and complexity well far beyond those fabricated by best technologies available. This is the case of diatoms, unicellular microalgae, whose protoplasm is enclosed in a nanoporous microshell, made of hydrogenated amorphous silica, called frustule. We have studied the optical properties of Arachnoidiscus sp. single valves both in visible and ultraviolet range. We found photonic effects due to diffraction by ordered pattern of pores and slits, accordingly to an elaborated theoretical model. For the first time, we experimentally revealed spatial separation of focused light in different spots, which could be the basis of a micro-bio-spectrometer. Characterization of such intricate structures can be of great inspiration for photonic devices of next generation.

A Mechanochemical Approach to Porous Silicon Nanoparticles Fabrication

Porous silicon samples have been reduced in nanometric particles by a well known industrial mechanical process, the ball grinding in a planetary mill; the process has been extended to crystalline silicon for comparison purposes. The silicon nanoparticles have been studied by X-ray diffraction, infrared spectroscopy, gas porosimetry and transmission electron microscopy. We have estimated crystallites size from about 50 nm for silicon to 12 nm for porous silicon. The specific surface area of the powders analyzed ranges between 100 m2/g to 29 m2/g depending on the milling time, ranging from 1 to 20 h. Electron microscopy confirms the nanometric size of the particles and reveals a porous structure in the powders obtained by porous silicon samples which has been preserved by the fabrication conditions. Chemical functionalization during the milling process by a siloxane compound has also been demonstrated.

Fabrication and characterization of a porous silicon based microarray for label-free optical monitoring of biomolecular interactions

We have fabricated a microarray of porous silicon Bragg reflectors on a crystalline silicon substrate using a technological process based on standard photolithography and electrochemical anodization of the silicon. The array density is of 170 elements/cm2 and each element has a diameter of 200 μm. The porous silicon structures have been used as platform to immobilize an amino terminated DNA single strand probe. All fabrication steps have been monitored by spectroscopic reflectometry, optical and electron microscopy, and Fourier transform infrared spectroscopy. A label-free detection method has been employed to investigate the hybridization between micromolar DNA probe and its complementary target. Due to fast and low cost production, good reproducibility, and high quality optical features, this platform could be adopted also for other different microarray applications such as proteomics and medical diagnostics.

Porous Silicon Based Resonant Mirrors for Biochemical Sensing

We report on our preliminary results in the realization and characterization of a porous silicon (PSi) resonant mirror (RM) for optical biosensing. We have numerically and experimentally studied the coupling between the electromagnetic field, totally reflected at the base of a high refractive index prism, and the optical modes of a PSi waveguide. This configuration is very sensitive to changes in the refractive index and/or in thickness of the sensor surface. Due to the high specific area of the PSi waveguide, very low DNA concentrations can be detected confirming that the RM could be a very sensitive and label-free optical biosensor.

Porous silicon-based optical biochips

In this paper, we present our work on an optical biosensor for the detection of the interaction between a DNA single strand and its complementary oligonucleotide, based on the porous silicon (PSi) microtechnology. The crucial point in this sensing device is how to make a stable and repeatable link between the DNA probe and the PSi surface. We have experimentally compared some functionalization processes which modify the PSi surface in order to covalently fix the DNA probe on it: a pure chemical passivation procedure, a photochemical functionalization process, and a chemical modification during the electrochemical etching of the PSi. We have quantitatively measured the efficiency of the chemical bond between the DNA and the porous silicon surface using Fourier transform infrared spectroscopy (FT-IR) and light induced photoluminescence emission. From the results and for its intrinsic simplicity, photochemical passivation seems to be the most promising method. The interaction between a label-free 50 µM DNA probe with complementary and non-complementary oligonucleotides sequences has been also successfully monitored by means of optical reflectivity measurements.

Aminosilane functionalizations of mesoporous oxidized silicon for oligonucleotide synthesis and detection

Direct solid phase synthesis of peptides and oligonucleotides (ONs) requires high chemical stability of the support material. In this work, we have investigated the passivation ability of porous oxidized silicon multilayered structures by two aminosilane compounds, 3-aminopropyltriethoxysilane and 3-aminopropyldimethylethoxysilane (APDMES), for optical label-free ON biosensor fabrication. We have also studied by spectroscopic reflectometry the hybridization between a 13 bases ON, directly grown on the aminosilane modified porous oxidized silicon by in situ synthesis, and its complementary sequence. Even if the results show that both devices are stable to the chemicals (carbonate/methanol) used, the porous silica structure passivated by APDMES reveals higher functionalization degree due to less steric hindrance of pores.

The Pleurotus ostreatus hydrophobin Vmh2 and its interaction with glucans

Hydrophobins are small self-assembling proteins produced by fungi. A class I hydrophobin secreted by the basidiomycete fungus Pleurotus ostreatus was purified and identified. The pure protein is not water soluble, whereas complexes formed between the protein and glycans, produced in culture broth containing amylose, are soluble in water. Glycan structure matched to cyclic structures of alpha-(1-4) linked glucose containing from six to 16 monomers (cyclodextrins). Moreover, it was verified that not only pure cyclodextrins but also a linear oligosaccharide and even the simple glucose monomer are able to solubilize the hydrophobin in water. The aqueous solution of the protein-in the presence of the cyclic glucans-showed propensity to self-assembly, and conformational changes towards beta structure were observed on vortexing the solution. On the other hand, the pure protein dissolved in less polar solvent (60% ethanol) is not prone to self assembly, and no conformational change was observed. When the pure protein was deposited on a hydrophobic surface, it formed a very stable biofilm whose thickness was about 3 nm, whereas the biofilm was not detected on a hydrophilic surface. When the water-soluble protein-in the presence of the cyclic glucans-was used, thicker (up to 10-fold) biofilms were obtained on either hydrophilic or hydrophobic surfaces.