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Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17

many optical application. The PS Fabry-Perot film with two planer and parallel interfaces can produce high contrast optical fringes. Shift in the fringe occur when an analyte binds to the surfaces in the pores, providing a sensitive transduction modality [114]. Multilayer devices, like Bragg filters, can be prepared by periodically varying the current density during the etching process. Such multiplayer structures act as 1-D photonic crystals with reflectivity maxima that depends on the refractive index gradient and the periodicity of the superlattice. Porous silicon 1-D photonic crystals used as a label free optical sensor for detection of bacteria has been reported earlier [103].

The freshly etched PS surface is hydrogen terminated and hydrophobic in nature. Impurities like, carbon and fluorine are also found attached to the surface. The PS surface in unmodified form is unstable for sensor application and also very much fragile. For biosensor applications the PS surface needs to be stabilized. Different surface treatments have been reported to achieve a stable and hydrophilic surface [60, 96-97]. Mild oxidation removes the Si-H bonds which stabilizes and protects the surface. For biological molecules to be attached to the PS surface silanization or hydrosilanization treatment is to be done [94, 96-97].

The enormous medical application of silicon was recognized very recently. Researchers investigate PS material as a transducer in sensing systems [67, 68] because of its physical and structural properties. High sensitivity results have been obtained using PS by monitoring changes in optical properties, such as photoluminescence [69-70] and ellipsometry [71]. The special features of PS material which led to its applications in the sensor industry are large surface area within a small volume, controllable pore sizes, convenient surface chemistry and compatibility with conventional silicon microfabrication technologies [72]. Scientists used these properties to develop PS sensors to detect toxic gases, volatile organic compounds, explosives, DNA and proteins. Porous Silicon is a well known material for sensing layers in different gas and humidity sensors. It shows great effectiveness when combined with titanium, ceramics, composites, polymers and other materials, which are mainly used for biological implants.

The PS optical biosensors normally measure the change in the average refractive index of the device when a bioconjugation event takes place [67], because the immobilization of the probe and the target biological sample changes the effective refractive index of the PS surface, thus modifying the interference pattern on the output. In the case of label free optical biosensors, the biological probe is attached with a signaling material, which automatically transduces the hybridization effect into an optical signal. The label free optical detection of single strand of DNA (ssDNA) and its complementary (cDNA) conjugation is carried out on the PS chip by comparing the signals taken after the surface modification, then after probe immobilization on the chip surface and finally after its hybridization with the cDNA. In each step of the chip preparation, the optical path length changes which is recorded in the reflectivity spectrum [68-69]. Vicky Vamvakaki, et al. [113] developed PS DNA sensors, which can be used for label-free detection of oligonucleotides in DNA microarrays and microfabricated PS field effect sensors. Francia et al. [109] reported photoluminescence measurements for label-free optical porous silicon DNA sensors.

Singh, et al. [112] in their work, showed how PS films with good mechanical and optical properties can be effectively used for the biofunctionalization purpose for its possible application in immunosensors.

Measurement techniques of molecular binding interactions have been patented by Rauh-Adelmann and his coworkers [117], where ligands are immobilized within pores of a PS interaction region produced in a Si subtrate, after which analytes suspended in a fluid are flowed over the PS region. A large surface area with easily modified chemistry makes porous silicon an effective transducer for optical and electrical biosensing. Porous Silicon optical biosensors sensitivity and performance depend strongly on their nanomorphology and calculated as a function of the pore size [66].


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