Based Sensing

Such factors preclude quantitative measurements of intensities, or even intensity ratios. 

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Later on, such S-layer-based sensing layers were also used in the development of optical biosensors (optodes), where the electrochemical transduction principle was replaced by an optical one [97] (Fig. 10c). In this approach an oxygen-sensitive fluorescent dye (ruthenium(II) complex) was immobilized on the S-layer in close proximity to the glucose oxidase-sensing layer [97]. The fluorescence of the Ru(II) complex is dynamically quenched by molecular oxygen. Thus, a decrease in the local oxygen pressure as a result of... 

Ohuchi Y, Katayama Y, Maeda M (2001) Fluorescence-based sensing of protein kinase A activity using the dual fluorescent-labeled peptide. Anal Sci 17 il465—i 1467... 

Szmacinski, H. Lacowicz, J. R. Lifetime-based Sensing Using Phase-Modulation Fluorometry. In Fluorescent Chemosensor for Ion and Molecule Recognition. ACS Symposium Series 538, 1993. 

Optical sensors rely on optical detection of a chemical species. Two basic operation principles are known for optically sensing chemical species intrinsic optical property of the analyte is utilized for its detection indicator lor label) based sensing is used when the analyte has no intrinsic optical property. For example, pH is measured optically by immobilizing a pH indicator on a solid support and observing changes in the absorption or fluorescence of the indicator as the pH of the sample varies with time1 20. 

It is even possible to convert changes of the fluorophore emission intensity as a function of the analyte level into lifetime-based sensing devices using phase-sensitive detection. The technique is called dual lifetime... 

MEDICAL AND ENVIRONMENTAL APPLICATIONS OF SOL-GEL BASED SENSING DEVICES... 

As opposed to conventional analytical techniques, optical sensors and biosensors, particularly those employing absorption and fluorescence-based sensing materials potentially allow for measurement through transparent or semi-transparent materials in a non-destructive fashion4, 5> 9 10. Chemical sensor technology has developed rapidly over the past years and a number of systems for food applications have been introduced and evaluated with foods. 

Goh, J. B. Loo, R. W. Goh, M. C., Label free monitoring of multiple biomolecular binding interactions in real time with diffraction based sensing, Sens. Actuators B Chem. 2005, 106, 243 248... 

Yet the majority of cation sensors are mediator-based sensing. One type of optical sensor is based on the fluorescence quenching of fluor-ophore Rhodamine 6G by transition metals such as Co(II), Cr(III),... 

For the reaction-based sensing of anions, the most considerable success has been reached in cyanide sensing, basically due to the necessity for assessing this toxic species and because it can be easily addressed in water due to its high nucleophi-licity. For instance, the coumarin derivative 50 shows a dramatic fluorescence increase after cyanide-induced cyanohydrin reaction (Fig. 19), no response to... 

Fig. 37 (a) QD-based sensing of cocaine by the formation of a cocaine-aptamer supramolecular structure that triggers FRET and (b) time-dependent luminescence spectra of the system in the presence of cocaine. The inset shows a calibration curve for variable concentrations of cocaine and a fixed so observation time of 15 min. (c) Schematic of the FRET-based TNT sensor and (d) increase of the QD luminescence upon addition of TNT in the competitive assay format. (Reprinted with permission from [220, 221], Copyright 2009 Royal Society of Chemistry and 2005 American Chemical Society)... 

Tsagkatakis I, Peperv S, Retter R, Bell M, Bakker E (2001) Monodisperse plasticized poly (vinyl chloride) fluorescent microspheres for selective ionophore-based sensing and extraction. Anal Chem 73 6083-6087... 

Miranda OR, You CC, Phillips R, Kim IB, Ghosh PS, Bunz UHF, Rotello VM (2007) Array-based sensing of proteins using conjugated polymers. J Am Chem Soc 129 9856-9857... 

In principle, the problems of intensity-based sensing can be avoided using wavelength-ratiometric probes, i.e., fluorophores that display spectral changes in the absorption or emission spectrum on binding or interaction with the analytes (Figure 1.1). In this case, the analyte concentration can be determined independently of the probe concentration by the ratio of intensities at two excitation or two emission wavelengths. 

Lifetime-based sensing can be mostly insensitive to these real-world effects. This is because these factors are not expected to alter the rate at which the intensity decays (Figure 1.2, middle). In our opinion, phase-modulation sensing provides additional advantages (Figure 1.2, bottom). The instruments take advantage of radio-frequency... 

Figure 1.5. Noninvasive sensing of glucose. Within the optical window (650 nm-1300 nm), lifetime-based sensing by fluoresence is possible through the human skin.

H. Szmadnski and J. R. Lakowicz, lifetime-based sensing of magnesium (submitted for publication). 

Chapter 10 of this volume, entitled Lifetime-Based Sensing, by Henryk Szmacinski and Joseph R. Lakowicz is an excellent complement to the material presented in this section. 

In order to implement frequency domain based sensing systems capable of monitoring the temporal luminescence of sensors, in few seconds, data must be collected at multiple frequencies simultaneously. Single-frequency techniques have been used to make frequency domain measurements of luminescent decays. 14, 23 28) This approach is unsuitable for real-time applications since data must be acquired at several frequencies in order to precisely and accurately determine the temporal variables of luminescent systems. 1 Each frequency requires a separate measurement, which makes the single frequency approach too slow to monitor the evolution... 

J. R. Lakowicz, H. Szmacinski, and K. W. Berndt, Fluorescence lifetime-based sensing of blood gases and cations, Proc. SPIE 1648, 150-163, (1992). 

At present, most fluorescence sensors or assays are based on intensity measurements, i.e., intensity-based sensing, in which the intensity of the probe molecules change in response to the analyte of interest. Intensity-based methods are initially the easiest to implement because many fluorescent probes change intensity in response to analytes. These intensity changes can be due to changes in extinction coefficient due to probe ionization, changes in quantum yield of the probe on analyte binding, or due... 

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