Sensing intensity-based

The various possible schemes for fluorescence sensing are summarized in Figure 1.1. At present, most fluorescence assays are based on the standard intensity-based methods, in which the intensity of the probe molecule changes in response to the analyte of interest. However, there has been the realization that lifetime-based methods possess intrinsic advantages for chemical sensing. (A more detailed description of... 

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. 

Continuous wave techniques do not offer the optimum use of luminescence for sensing applications. CW methods, also known as intensity-based techniques, have many inherent limitations. These limitations will be discussed later in the chapter. Many of the limitations of intensity-based methods can be overcome by using steady-state modulated excitation of the form... 

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... 

Figure 9-17. A novel oxygen-sensing coating based on combination of a luminescent dye molecule and an oxygen-binding cobalt porphyrin. Lower left Overlapping of absorption by the cobalt porphyrin with emission from the dye molecule. Lower right Stem-Volmer plots (JkJI normalized luminescence intensity).

Intensity-Based Sensing. There are a number of probes which display changes in inlensicy but do not display spectral shifts. Such probes include the calcium probes Calcium Green . Fluo-3, and Rhod-2. In these cases, the analyte concentration can be obtained finsn... 

Brillouin scattering occurs as a result of an interaction between the propagating optical signal and thermally acoustic waves present in the silica fibre giving rise to frequency-shifted components, similar to a Doppler effect. The acoustic velocity is directly related to the medium density and depends on both temperature and strain. As a result, the so-called Brillouin frequency shift carries information about the local temperature and strain of the fibre. Furthermore, Briflouin-based sensing techniques rely on the measurement of a frequency as opposed to Raman-based techniques that are intensity based. 

Wu C, Rong G, Xu J, Wang G (2011) An intensity-interrogated sensing technique based on porous silicon microcavity, 2011. In International conference on nanotechnology and biosensors (IPCBEE), Singapore, pp 39 3... 

For an optical probe that displays a wavelength shift in absorption or emission on binding, the analyte concentration may be determined from a ratio of intensities at two wavelengths. This is known as a wavelength-ratiometric approach and avoids some of the limitations mentioned above for intensity-based sensing, since, the measurements are independent of probe concentration. Absorption-, excitation-and emission-based ratiometric measurements are all possible. 

In most examples presented above, the sensing function is based on changes in the intensity of one transition, which can be heavily affected by the quantity of the luminophore, excitation power, and the drifts of the optoelectronic system. Thus, the comparison of the emission of different samples based on the detected intensity may lead to erroneous conclusions. Although the measurements of quantum yields and/or lifetime are affected neither by the intensity of the excitation source nor by the probe concentration, they require a relatively long time and the computational treatment. The utilizing of the ratio between the intensity of two transitions of the same luminescent material, instead of only one transition, can overcome the main drawbacks of the intensity-based measurements of only one transition. The ratiometric... 

In most of the luminescent MOFs, the sensing functionaUty is fulfilled based on the change of the luminescence intensity. The simple yet sophisticated ratiometric sensing approach is one of the breakthroughs on the exploration of functional luminescent MOF sensors. This approach makes the luminescent sensing independent of the concentration of the sample and of the drifts of the optoelectronic system including excitation source and detectors, thus overcoming the main drawbacks of the intensity-based measurements of only one transition. 

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