The development of label-free optical biosensors could have a great impact on life sciences as well as on screening
techniques for medical and environmental applications. Peptide nucleic acid (PNA) is a nucleic acid analog in which the
sugar phosphate backbone of natural nucleic acid has been replaced by a synthetic peptide backbone, resulting in an
achiral and uncharged mimic. Due to the uncharged nature of PNA,
PNA-DNA duplexes show a better thermal stability
respect the DNA-DNA equivalents. In this work, we used an optical biosensor, based on the porous silicon (PSi)
nanotechnology, to detect PNA-DNA interactions. PSi optical sensors are based on changes of reflectivity spectrum
when they are exposed to the target analytes. The porous silicon surface was chemically modified to covalently link the
PNA which acts as a very specific probe for its ligand (<i>c</i>DNA).
Micro-total-analysis-systems and lab-on-chip are more than promises in lot of social interest applications such as clinical
diagnostic or environmental monitoring. There is an increasing demand of new and customized devices with better
performances to be used in very specific applications. Nanostructured Porous silicon is a functional material and a
versatile platform for the fabrication of integrated optical microsystems to be used in biochemical analysis. Our research
activity is focused on the design, the fabrication and the characterization of several photonic porous silicon based
structures, which are used in the sensing of specific molecular interactions. To integrate the porous silicon based optical
transducer in biochip devices we have modified standard micromachining processes, such as anodic bonding and photo-patterning,
in order to make them consistent to the utilization of biological probes.
The action of molecular interaction between a fluid and an adsorbent results in adsorption and wetting phenomena. However, the adsorbent is also submitted to the action of the molecular forces. In order to provide a large adsorption capacity, adsorbents with a large specific surface area are preferable. For this reason, for the study of adsorption phenomena, porous silicon is a material of great interest.
Wetting phenomena in porous silicon layers are experimentally investigated by Raman scattering. The experimental results prove a reversible blue-shift of Raman spectra of wetted porous silicon layers with isopropanol or ethanol with respect to unperturbed layers. We ascribe the shift to a compressive stress due to the increased lattice mismatch between the porous silicon layer and the bulk silicon substrate in wetting conditions. The use of two liquids having quite similar density and surface tension resulted, as expected, in quite comparable blue shift of the peak. This effect may be conveniently used in sensing applications of liquids on porous silicon layers.
In this work, we have compared the optical characteristic of two different photonic dielectric multilayers based on the porous silicon technology. We designed and realized two models devices: a Bragg mirror and the S<sub>6</sub> Thue-Morse sequence. Both the structures have the same thickness, the same porosity, and even the same number of the layers but differently spatially ordered. We demonstrate that the two arrangements of the layers influence not only the optical features of these interferometric devices but also their sensitivity when used as optical sensors. We have measured the change of the reflectivity spectra of the devices on exposure to several organic compounds. The experimental results demonstrated that the Thue-Morse aperiodic structure is more sensitive than the Bragg device due to a higher filling capability.
The interaction between an analyte and a biological recognition system is normally detected in biosensors by the
transducer element which converts the molecular event into a measurable effect, such as an electrical or optical signal.
Porous silicon microstructures have unique optical and morphological properties that can be exploited in biosensing. The
large specific surface area (even greater than 500 m<sup>2</sup>/cm<sup>3</sup>) and the resonant optical response allow detecting the effect of
a change in refractive index of liquid solutions, which interact with the porous matrix, with very high sensitivity.
Moreover, the porous silicon surface can be chemically modified to link the bioprobe which recognize the target
analytes, in order to enhance the selectivity and specificity of the sensor device. The molecular probe we used was
purified by an extremophile organism, <i>Thermococcus litoralis</i>: the protein is very stable in a wide range of temperatures
even if with different behavior respect to the interaction with the ligand.
In this work, an integrated optical microsystems for the continuous detection of flammable liquids has been fabricated
and characterized. The proposed system is composed of a the transducer element, which is a vertical silicon/air Bragg
mirror fabricated by silicon electrochemical micromachining, sealed with a cover glass anodically bonded on its top. The
device has been optically characterized in presence of liquid substances of environmental interest, such as ethanol and
isopropanol. The preliminary experimental results are in good agreement with the theoretical calculations and show the
possibility to use the device as an optical sensor based on the change of its reflectivity spectrum.