Photobleaching Effect

Photobleaching of fluorophores in fluorescent detection can affect quantitative measurement. The extent of photobleaching can be estimated by the photochemical lifetime (x). This parameter can be determined by measuring the fluorescence signal (/ ) of the fluorophore as a function of residence time (/). The parameter 

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We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

Freshwater photobleaching effects in surface waters of lakes ... 

Figure 18. Evolution of the indices of refraction in the plane of the LBK (Pj.ioi 156 monolayers) structure n, n ), and in the perpendicular direction (n ) under different conditions. The first column refers to the sample before any irradiation (New), and the columns labelled UV or B refer, respectively, to the sample after UV (360 nm) and blue-light (450 nm) irradiations. The high anisotropy shown in the columns labelled New or B indicates a highly optically anisotropic LBK structure. The columns labelled UV show a much less optically anisotropic structure. The evolution of the mean refractive index n under successive UV and blue-light irradiations suggests that the azo-molecules are switched between the two conformations (e.g., cis and trans) without a photobleaching effect.

Together with the Banin group, the authors of Refs [10-12] carried out optical spectroscopy investigations on some of the cluster molecules obtained ] 14-16]. These materials were treated as the molecular limit of the bulk semiconductor CdSe, and issues such as oscillator strength, steady-state and time-resolved photoluminescence and photoluminescence excitation were addressed. In addition, emission-mediating vibrational modes were detected, and photobleaching effects observed. 

Due to fluorescence photobleaching, the detector signal is dependent on the flow rate. The flow rate dependence, however, is not a problem in these experiments since the analysis is performed at a constant flow rate. In fact, the photobleaching effect can be exploited to monitor the extremely low flow rates, which cannot be measured directly. A complete discussion of the detector s flow rate dependence is described elsewhere [18]. 

Lifetime. As already mentioned above, some rugged solid-state sensors have lifetimes of several years. Certain sensors can also be regenerated when their function begins to deteriorate. The shortest lifetimes are exhibited by biosensors. Ion-selective electrodes and optical sensors based on membrane-bound recognition molecules often lose their ability to function by a leaching-out effect. In optical sensors the photobleaching effect may also reduce the lifetime to less than a year. On the other hand, amperometric cells work well for many years, albeit with restricted selectivities. 

Quantum dots (QDs) were developed in the early 1970s. These atomic clusters are luminescent nanometer-scale (1.5-12 nm) heterostructures, containing from a few hundred to a few thousand atoms of a semiconductor material (CdSe, CdS or InP and InAs). They can be coated with an additional semiconductor shell (e.g. zinc sulphide) to improve their optical properties such as their brightness, and the photostability of the material since, in the core-shell QDs, photobleaching effects are strongly reduced [29]. 

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