Luminescent probes luminescence intensity

An important class of luminescence sensors are those based on the decrease of luminescence intensity and lifetime of the probes as function of analyte concentration. Assume that the probe intensity decays by a single exponential with an unquenched lifetime tq. If quenching occurs only by a dynamic (collisional) mechanism, then the ratio to/t is equal to Fq/F and is described by the classic Stern-Volmer equation... 

The minimum prerequisite for generation of upconversion luminescence by any material is the presence of at least two metastable excited states. In order for upconversion to be efficient, these states must have lifetimes sufficiently long for ions to participate in either luminescence or other photophysical processes with reasonably high probabilities, as opposed to relaxing through nonradiative multiphonon pathways. The observed decay of an excited state in the simplest case scenario, as probed for example by monitoring its luminescence intensity I, behaves as an exponential ... 

Interestingly enough there is another sharp drop in luminescence intensity at the transition to the liquid-crystalline phase 215). This is shown in Fig. 45. In the latter phase there is an orientation of the phthalocyanine molecules that is more favorable for migration, so that more quenching centers are reached. The transition shows hysteris (Fig. 45) and coincides with thermodynamic measurements. Therefore the luminescence is used to probe the crystalline to liquid-crystalline transition. A further analysis yielded an estimation of the number of phthalocyanine molecules in the stack. 

A similar unique effect of PMA on the photophysics of RuCbpy) is observed at pH 5, for example both the lifetime and luminescence intensity of RuCbpyjj show maxima at pH of about 5. The luminescence of the probe also exhibits a blue spectral shift at this particular pH compared to other pH. The change in the photophysical properties are due to binding of RuCbpyjj " " into a partially coiled or swollen polymer PMA at pH 5. The binding is electrostatic in nature and the ligands of the organometallio complex probe are quite restricted in a hydrophobic environment, so that unlike more mobile systems such as water or a stretched polymer, complete relaxation of the excited state is not achieved. Hence, the lifetime and the yield of luminescence increase accordingly and the emission spectra show a blue shift.(42)... 

Figure 1. (A) Chemiluminescence spectra of la induced by sup oxide in phosphate buffe at 25 °C with various probe concentrations, and (B) plots of the ratio of the superoxide-triggered luminescence intensity, for la in Mops buffer at 25 °C.

Results of the superoxide-induced chemiluminescence at a probe concentration of 1.0 pM, are summarized in Fig. 1. Probe B showed green-luminescence intensity that was 26 times that of FCLA, which was also the highest luminescence intensity in this present study. At probe concentrations of less than 1.0 pM, the ratio of the superoxide-dependent chemiluminescence intensity to the background... 

General measurement of chemiluminescence intensities and spectra. To a mixture of 20 mmol/L Mops / 0.2 mol/L KC1 (pH 7.2, 0.1 mL), 0.3 mmol/L hypoxanthine (0.1 mL), and probe solution was added xanthine oxidase (XOD) at 20 °C. Luminescence intensity and spectra were measured for 1 min using a LumiFISpectroCapture AB1850 (Atto Corporation, Tokyo). 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

Examples of europium complexes 30-32 that have been applied in temperature sensors or dual pressure- and temperature-sensitive paints are listed in Table 5. The respective ligand structures are shown in Scheme 7. The temperature dependency is quantified as average luminescence intensity temperature coefficient 7 [%/°C]. Usually, it is determined in a temperature range from 1 to 40 or 50°C. These examples exceed the intensity temperatiue coefficients of other established temperature sensitive probes such as ruthenium(II)-tris-(l,10-phenanthroline) [121]. Generally, the lifetime temperature coefficients are significantly lower. This indicates that thermal quenching of the triplet state of the antenna chromophore plays an important role. Due to the narrow emission band of europium complexes at 615 nm even triple sensors for temperature, oxygen, and pH are achievable [122]. 

Water-soluble CdTe nanorods modified with thioglycolic acid and cysteine have been used as a fluorescence probe for the determination of vitamin Bi (Li et al. 2010). The size-dependent luminescence of the nanorods was studied in this work. A synthesized nanorod of short length was not luminous, even in the presence of vitamin Bi, but longer nanorods emitted strong light at a wavelength of 665 nm in the presence of vitamin Bi. The degree of enhancement of the luminescence intensity was proportional to the concentration of vitamin Bi. The LOD was achieved at micromolar scale. The method was applied to determine vitamin Bi in a commercial tablet and vitamin B complex. 

The interaction of a luminescent probe with an analyte is expected to induce several effects, among them being modifications of the luminescent intensity, lifetime, etc. Positive as well as negative effects can be recorded. In other words, one can monitor a quenching or, on the contrary, the appearance of the luminescence of the probe. 

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