Sensors, absorption-type

The interaction phenomenon of the analyte with radiation is governed or modifies the principle on which light is transmitted through the waveguide. Depending on that interaction, we can differentiate three basic types of intrinsic optical sensors [82] refractive-index-type sensors, luminescence-based sensors and absorption-type sensors. 

Core-based optodes, such as hollow fibers, and direct and coating-based evanescent wave spectroscopic optodes are included in the classification of absorption-type sensors. 

The squaraine rotaxanes based on the macrocycle 16b exhibit intense NIR absorption and emission maxima, and it should be possible to develop them into molecular probes for many types of photonic and bioimaging applications. In contrast, the squaraine fluorescence intensity is greatly diminished when the dye is encapsulated with macrocycle 18. The fluorescence is restored when a suitable anionic guest is used to displace the squaraine dye from a pseudorotaxane complex, which indicates that the multicomponent system might be applicable as a fluorescent anion sensor. 

Titrations consist of the observation of one or several measures as a function of the addition of an appropriate reagent. Reagents are typically acids or bases or ligands in metal determinations. Measurements are typically pH and/or absorption spectra. We concentrate on the data analysis of these two types. It should be straightforward for the reader to adapt the algorithms to other observations. Currently, most titrations are done under computer control, either by commercial auto-titrators or by assemblies of burettes, sensors and vessels in the research laboratories. This is not crucial and the analysis of such a titration is essentially identical with the analysis of a manual titration. 

The sorbent materials used to construct this type of sensor are widely varied (ion exchangers, adsorbent solids, polymers) and are employed as particles (larger than 30 pm in order to avoid overpressure in the flow system) or films. Most of these sensors are optical and rely on absorption, reflectance or molecular fluorescence measurements. In order to ensure that the sensing microzone is fully compatible with the detector, the sorbent material used must be as transparent as possible (photometry) or give rise to no appreciable light scatter (fluorimetry) so that the baseline (resulting from passage of the carrier) may be as low as possible. 

Thus, sensors based on absorption (phase equilibrium) measure activity and, if only one type of sorption mechanism is involved, their response is logarithmic. 

This type of sensor utilizes a drastic increase in resistance at the point of dew condensation, since water absorption swells the polymers to counteract ohmic contact between carbon particles. This type sensor is now widely applied to humidity controling systems of video tape recorders or car windows. 

Table 1 Near-IR chromoionophores applied in bulk optodes. The absorption maxima, the type of matrix employed, the sensor configuration, the optical properties measured, the analyte and year of publication are listed... 

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