OPTICAL-MODE SENSING

In more recent work, the Swager group [845] described a chemosensor for TNT based on fluoroescence of a porous form of a phenylene/acetylene containing CP, shown in Eg. (17.31. 

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Lattice vibrations may be acoustic or optical in the former case the motion involves all the ions, in volumes down to that of a unit cell, moving in unison, while in the optical mode cations and anions move in opposite senses. Both acoustic and optical modes can occur as transverse or longitudinal waves. 

Microcavity ((AB)nB(AB)n) One or multiple optical modes Interferenee filters sensing Visible,... 

Guided modes for optical communication sensing Interference filters sensing microresonators, resonance peaks with high-quality factor in transmission spectra... 

Besides its widespread use for investigating the mechanism of redox processes, spectroelectrochemistry can be usefiil for analytical purposes. In particular, the simultaneous profiling of optical and electrochemical properties can enhance the overall selectivity of different sensing (30) and detection (31) applications. Such coupling of two modes of selectivity is facilitated by the judicious choice of the operating potential and wavelength. 

Apart from the optimisation of properties of sensing fibres themselves, the arrangement of optical hardware, i.e. the conditions of excitation and detection, play an important role influencing the selectivity and sensitivity of the sensor. As examples, the selective excitation (Figure 14) with the axial excitation or the detection with an output mode filter30 can be presented (Figure 15, Figure 16). 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

The design and implementation of a portable fiber-optic cholinesterase biosensor for the detection and determination of pesticides carbaryl and dichlorvos was presented by Andreou81. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol-gel layer that incorporated bromocresol purple, deposited on an inner disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. This method was successfully applied to the direct analysis of natural water samples (detection and determination of these pesticides), without sample pretreatment, and since the biosensor setup is fully portable (in a small case), it is suitable for in-field use. 

The unique field penetration into the liquid of a nonevanescent resonant mode like Tlij(7)0 is very promising for the sensing applications. To understand the origin of this behavior and further optimize the devices, a ray optical picture71 is presented. This type of resonant modes exist as if rays are bounced at the liquid/silica interface and confined in the liquid region as plotted in Fig. 8.32. From the viewpoint of ray optics, light is partially reflected (termed ray 1) and partially transmitted when it is... 

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