We present the viability of obtaining the particle size and surface coverage in a monolayer of polystyrene
particles adsorbed on a glass surface from optical coherent reflectance data around the critical angle in an
internal reflection configuration. We have found that fitting a CSM to optical reflectivity curves in an
internal reflection configuration around the critical angle with a dilute random monolayer of particles
adsorbed on the surface can in fact provide the particle's radius and surface coverage once the particles
are sufficiently large.
PSi microcavity (PSiMc) is characterized by a narrow resonance peak in the optical spectrum that is very sensitive to
small changes in the refractive index. We report that the resonant optical cavities of PSi structures can be used to
enhance the detection of labeled fluorescent biomolecules. Various PSi configurations were tested in order to compare
the optical response of the PSi devices to the capture of organic molecules. Morphological and topographical analyses
were performed on PSiMc using Atomic Force (AFM) and Scanning Electron (SEM) microscopies. The heterogeneity in
pores lengths resulting from etching process assures a better penetration of larger molecules into the pores and sensor
sensitivity depends on the pore size. Molecular detection is monitored by the successive red shifts in the reflectance
spectra after the stabilization of PSiMc with
3-aminopropyltriethoxysilane (APTES). The glucose oxidase was cross linked into the PSiMc structures following a silane-glutaraldehyde (GTA) chemistry.
Porous silicon (PSi) nanostructures have remarkable optical properties that can be used for biosensing applications. In
this paper we report first on the fabrication of heavily doped p-type PSi with pore diameters in the range of 400-4000
nm. The nonspecific and specific binding of the Glucose Oxidase protein (GOX) was then studied onto the PSi mirrorlike
substrate. Adsorption of GOX was tuned by the pH of the protein solution (pI = 4.2) depending of the surface
charge. PSi matrixes were first stabilized by thermal oxidation and GOX adsorption was performed once directly on the
oxidized PSi surface, and also on previously functionalized PSi surfaces. In the latter case the GOX was coupled to the
PSi via the S-H group of the 3-(mercaptopropyl)trimethoxysilane (MPTS). The silane-GOX and GOX interactions on
the PSi surface were monitored by the Fourier Transformed Infrared spectra that display characteristic bands of the
linked molecules. The interference spectrum shows a large blue shift in the Fabry-Perot interference pattern caused by
the change in the refractive index of the medium implying a decrease in the effective optical thickness. Quantitative
analysis shows that chemically modified PSi samples admit approximately 24% of GOX. Activity assay proved that the
protein preserves its catalyst properties under these adsorption conditions.
Reflection and transmission of the light in a random medium are composed by coherent and incoherent waves. The coherent one can be modeled as interacting with a medium with effective optical coefficients. In a random dilute suspension, the coherent wave travels in a medium with an effective index of refraction given by the van de Hulst formula. This effective index is, in general, complex. The imaginary part takes into account the loss of the coherent wave due to scattering. Internal reflection, due to random particles in suspension defines a critical angle determined by the
effective index of refraction of the particles in suspension. The curve of reflectivity is smoothed near the critical angle by
the imaginary part of the effective index of refraction. One can show that the diffuse component of the reflection tends to zero at the critical angle. In this work, laser reflectometry near the critical angle is used to study particle adsorption on a flat surface. We monitored the adsorption of polystyrene particles with positive and negative charge in suspension. This method allows the direct measuring of reflectivity and its angle derivative on the prism surface where is formed the film.