Absorbance-based sensors

Coleman J.T., Eastham J.F., Sepaniak M.J., Fiber optic based sensor for bioanalytical absorbance measurements. Anal. Chem. 1984 56 2246. 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Thus as in the titration curves in the solution, the response range of the pH indicators for a signal protonation process is about 2-3 pH units for absorbance-based measurements. Long-range pH sensing has to be achieved by using mixed dyes doped in solid support. While fluorescence sensors have a wider linearity because it is only related to one form of the indicator. 

What are the response principles for an absorbance-based optical sensor for hydrogen and other cations ... 

Luminescent evanescent wave-based sensors use optical fibers and planar waveguides [105,106] as fight-guiding structures, and they are more complex than the absorbance ones. However, such optodes have been satisfactorily applied to measure fluorescence of indicators or labels for the measurement of gas molecules, proteins or labeled antigen-antibody interactions as well as directly in solution [24,107] when immobilized in matrices [23,109]. 

Absorbance evanescent-based sensors are based on the absorption or dispersion of light outside the core. They rely on light attenuation in the evanescent field following the Beer-Lambert law (ATR sensors), but owing to the low intensity of the field, they offer poor sensitivity. This can be improved because the effective optical path length can be increased, especially when using optical fibers, capillary [62] or planar waveguides [114]. 

IWAOs have been designed to address some of the weak points concerning absorbance optochemical sensors based on bulk optodes. In order to reach a practical application level, new research is needed to overcome some signifi-... 

The measured signal from an in vivo fluorescence-based sensor depends on both the quantum yield of the fluorophore, which is the ratio of emitted photons to absorbed photons, and the absorption of the excited and emitted light by the surrounding tissue.12-15 While SWNT have a much lower quantum yield than many visible fluorophores, the most important factor for depth of implantation actually turns out to be the absorption coefficient of the surrounding medium.16,17 A one-dimensional absorption-fluorescence model can be used to compare the suitability of fluorophores for in vivo applications ... 

The quickest path to the fabrication of an optical sensor is to make a film that can be used in one of the commercially available film holders such as the one from Ocean Optics. Since absorbance-based probes require a differential measurement, they employ three fibres with source, signal and reference legs. The need for two spectroscopic channels is usually avoided by storing a reference spectrum. For luminescence measurements, the reference leg can be ignored or used to correct the spectra for variations in source intensity. A similar commercial probe was employed by Kriz and Mosbach [33]. 

Hales B., Burgess L., and Emerson S. (1997) An absorbance-based fiber-optic sensor for C02(aq) measurements in pore waters of seafloor sediments. Mar. Chem. 59, 51-62. 

A.D. McBrady, R. Chantiwas, A.K. Torgerson, K. Grudpan, R.E. Synovec, An absorbance-based micro-fluidic sensor for diffusion coefficient and molar mass determinations, Anal. Chim. Acta 575 (2006) 151. 

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