Jesús Álvarez

Birefringent porous silicon membranes for optical sensing

In this work anisotropic porous silicon is investigated as a material for optical sensing. Birefringence and sensitivity of the anisotropic porous silicon membranes are thoroughly studied in the framework of Bruggeman model which is extended to incorporate the influence of environment effects, such as silicon oxidation. The membranes were also characterized optically demonstrating sensitivity as high as 1245 nm/RIU at 1500 nm. This experimental value only agrees with the theory when it takes into consideration the effect of silicon oxidation. Furthermore we demonstrate that oxidized porous silicon membranes have optical parameters with long term stability. Finally, we developed a new model to determine the contribution of the main depolarization sources to the overall depolarization process, and how it influences the measured spectra and the resolution of birefringence measurements.

Phase-Sensitive Detection for Optical Sensing With Porous Silicon

We report on a photonic sensor with an ultralow limit of detection (LoD) based on a phase interrogation readout scheme together with an anisotropic porous silicon (PSi) membrane. First, the fabrication of porous free-standing membranes from medium doped (100) surface oriented silicon, with pore diameters suitable for the infiltration of biomolecules, around 50 nm, is reported. Then, the phase interrogation scheme for characterizing the PSi membranes is presented whose results show that while volumetric sensitivity increases with the membrane thickness, the resolution in the birefringence measurements decrease dramatically due to depolarization effects. The best LoD was found to be equal to 6.25 ×10-6 RIU, from the thinnest 10-μm-thick membrane. Finally, the thermooptic coefficient of the 10 μm membrane was measured in an aqueous environment and shown to be equal to 8×10-4 rad/°C.

Highly-sensitive anisotropic porous silicon based optical sensors

The modeling, fabrication and characterization of PSi fabricated from both (110) and (100) surface oriented silicon for optical sensing is thoroughly reported. First, based on the generalized Bruggeman method, the birefringence and sensitivity of the fabricated membranes were calculated as a function of the fabrication parameters such as porosity and pore sizes; and external effects, such as the pores surface oxidation. Thereafter we report on the fabrication of PSi membranes from (110) and (100) surface oriented silicon with pore sizes in the range of 50 - 80 nm, and the characterization of their birefringence using a polarimetric setup. Their sensitivities were determined by filling the pores with several liquids having different refractive index. As a result, sensitivities as high as 1407 nm/RIU were obtained for the (110) samples at a 1500 nm wavelength and 382 nm/RIU for the (100) samples at the same wavelength.

Real-time polarimetric biosensing using macroporous alumina membranes

We report the first demonstration of real-time biosensing in free standing macroporous alumina membranes. The membranes with their 200 nm diameter pores are ideal candidates for biosensing applications where fast response times for small sample volumes are needed as they allow analytes to flow through the pores close to the bioreceptors immobilized on the pores walls. A bulk refractive index sensitivity of 5.2x10-6 refractive index units was obtained from signal responses to different concentrations of NaCl solutions flowing through the pores. Finally, after functionalizing the alumina pore surfaces with an epoxysilane and then spotting it with β-Lactoglobulin protein, the interactions between the β-lactoglobulin and rabbit anti-β-lactoglobulin, as well as the interaction between the rabbit anti-β-lactoglobulin and a secondary antibody anti-rabbit Immunoglobulin G were monitored in real-time.

Bowtie plasmonic nanoantenna arrays for polarimetric optical biosensing

We report on the first polarimetric plasmonic biosensor based on arrays of bowtie nanoantennas. Using the Finite Element Method (FEM) the phase retardation between the components of light polarized parallel and perpendicular to the axis of the nanoantennas is studied. After optimizing them for high volumetric sensitivity at a wavelength of 780 nm, sensitivities ~5 rad/RIU are obtained, corresponding to a detection limit ~10-7 RIU when using the polarimetric readout platform. Surface sensitivity values resulted from studies of phase retardation changes from a coverage of bioreceptors and analytes.

A polarimetric sensor based on nanoporous free standing membranes

A polarimetric sensor with state of the art sensitivity is developed using free standing porous silicon membranes. The use of an optimized etching receipt greatly reduces the pore roughness. Depolarization factors are thus limited and material birefringence is increased. Free standing membranes are fabricated in n-type substrates and characterized both from the optical and structural point of view. The proposed approach is fully CMOS compatible and can therefore pave the way to the development of cheap microarray that exploits multiplexing capabilities while keeping the amount of analyte required by the analysis down to the microliter level.

A real time immunoassay in alumina membranes

To date, photonic biosensing with porous membranes has produced slow responses and long sensing times, due to the narrow (less than 100 nm) closed end pores of the membranes used. Recently, polarimetry was used to demonstrate analyte flow through, and real time biosensing in, free-standing porous alumina membranes. Here, we demonstrate how an improved functionalization technology, has for the first time enabled a real-time immunoassay within a porous membrane with a total assay time below one hour. With the new approach, we show a noise floor for individual biosensing measurements of 3.7 ng/ml (25 pM), and a bulk refractive index detection limit of 5×10-6 RIU, with a standard deviation of less than 5%. The membranes, with their 200 nm pore diameter enabling targeted delivering of analytes to bioreceptors immobilized on the pore walls, therefore provide a route towards rapid and low cost real-time opto-fluidic biosensors for small sample volumes.