Fluorescence intensity, chemosensors

A Ca2+-specific fluorescent chemosensor 25 in aqueous buffer signals Ca2+ via a decrease in fluorescence intensity, whereas excess of Mg2+ ions has no effect on the emission [85]. This probe has limited solubility in aqueous solution after binding to Ca2+. A Zn2+ sensitive probe 26 showing different fluorescence responses depending on the complexation stoichiometry is described in [86],... 

In another approach, a fluorescent conjugated polymer was used as the material for the preparation of a chemosensor to detect 2,4,6-trinitrotoluene (TNT) and its related nitroaromatic compounds. To this end, microparticles, made of three-dimensionally cross-linked poly(l,4-phenylene vinylene) (PPV) via emulsion polymerization, were synthesized [61]. This material was chosen due to its high fluorescence intensity and sensitivity to changes in its microenvironment. The chemosensor was exposed to vapour containing different amounts of TNT and quenching of the polymer luminescence at 560 nm was observed after excitation at 430 nm. The dependence of the fluorescence signal in response to the analyte was described by a modified Stem-Volmer equation that assumes the existence of two different cavity types. The authors proposed the modified Stem-Volmer equation as follows ... 

A potassium-ion-selective, dendritic, fluorescing chemosensor, bearing three crown ether moieties in the periphery, shows a linear increase in fluorescence intensity with increasing potassium concentration (in acetonitrile). An important criterion for potassium chemosensors is their mode of action (selectivity) in the presence of large amounts of sodium. The tris-crown ether sensor shown in Fig. 8.16 is able to detect very small traces of potassium ions, even if large quantities of sodium ions are present in the same solution - such as in body fluids [55]. 

Ueno et al. also prepared the b i s (2 - n ap h t h y I s u I fe n y 1) - y - CD series in which the naphthyl moieties are very limited in their movement because the linker between naphthalene and CD is only sulfur [37], All isomers of 12(AB), 13(AC), 14(AD), and 15(AE) exhibit only monomer fluorescence due to the rigid linker. It means that the two naphthalene moieties cannot take face-to-face orientation because of the limited flexibility. Although the excimer cannot be used for sensing molecules, Ueno et al. found that the monomer fluorescence intensity increases with increasing guest concentration. Thus, this modified CD series can be used as chemosensors of a different type. 

Recently, Czamik et al. have reported the use of the acyclic protonated amine host 9 as a chemosensor of pyrophosphate. Typical fluorescence sensing methods rely on the ability of a complexed anion to quench the fluorophore. The fluorescence intensity of host 9, however, is actually increased upon complexation of anions and its 2200-fold selectivity of pyrophosphate over phosphate allows for real-time assay of pyrophosphate hydrolysis by inorganic pyrophosphatase.18... 

The possibility of measurement of Mg2+ concentration in a matrix complicated by alkali and alkaline earth metal ions has been explored by titration of 68 (5 x 10-5 M) in 1 1 MeOH/H20 solution (pH 7.2) containing Na+ (5 x 10 3 M), K+ (1 X 10-3 M), Ba2+ (1 x 10 3 M), Sr2 (1 x 1(T3 M), and Ca2+ (1 x 10-4 M). The titration was monitored via fluorescence.125 The fluorescence intensity reached a maximum at 1 equiv of Mg2+, indicating that fluorescent intensity could be directly correlated to Mg2+ concentration. The lack of interference from the other metal ions present can be a result of their lower binding constants with 68 and lower quantum yields of the charged complexes of 68 with Na+, K+, Ca2+, Sr2+, and Ba2+ at this pH. Thus, ligand 68 possesses characteristics of an efficient fluorescent chemosensor for Mg2+ and may find use in determining Mg2+ in biological samples and, if immobilized on a solid support, may be incorporated into sensory devices for measurement of Mg2+ concentrations in aqueous solutions. 

The magnitude of the signal generated by the sensor is normally proportional to the concentration of the analyte. Regarding practical applications, optical che-mosensors that monitor changes in fluorescence intensity, or to a lesser extent in optical absorption, are much more prevalent as compared to chemosensors that monitor changes in electrical conductivity or electrical current. 

Many chemosensor ensembles were reported by Ans-lyn and coworkers. Compound 14, with three gua-nidinium groups displayed in the same direction (Fig. 5). binds to the citrate tricarboxylate anion. Anionic fluorophore 5-carboxy-fluorescein 15 binds with modest affinity to this receptor and shows increased fluorescence intensity due to the decreased pKa of its phenol group. As citrate is added, the fluorophore is displaced from the receptor, and the emission intensity diminishes. The same fluorophore was used in another sensing ensemble for inositol-1,4,5-triphosphate (IP3), an important second messenger involved in many cell regulatory functions. The fluorophore bound to positively charged hexa-gua-nidinium receptor 16 is displaced by added IP3, and the fluorescence intensity decreases. 

Determination of the binding constants confirmed the enantioselec-tivity. For instance, at pH 5.6, in the presence of o-tartaric acid, the fluorescence enhancement for Rfi]-2 and (S,S)-2 are 9.05- and 3.61-fold, respectively. The binding constants are log AT = 5.78 and 4.20, respectively. At pH 2.5, the fluorescence intensity of (Rfi]-2 and (5,S)-2 increased by 3.31-and 1.49-fold, respectively, whereas the a-methylbenzylamine-based chiral bis-boronic acid chemosensors hardly give any fluorescence enhancement (Figure 6.11). 

The principle of the action of fluorescent enolimine/ketoenamine chemosensors is usually based on two fundamentally different effects - CHEF (Chelation-Enhanced Fluorescence) and CHEQ (Chelation-Enhanced Fluorescence Quenching). The former is connected with hindering the enolimine C=N bond rotation by complexation with cations. As a result, the fluorescence intensity of compounds 23 (Figure 10.15) is selectively and significantly enhanced by adding of, for example, Cd " " cations [92, 93]. This enhancement was attributed to the formation of a 1 1 complex in which the rotation around the acyclic C=N bond is frozen. 

Figure 2. Fluorescence intensity as a function of time during the pyrophosphatase-catalyzed hydrolysis of PPi to 2Pi, as monitored using chemosensor 3.

The new type of chemosensor 27 is much more sensitive to the adamantane and borneol derivatives (Fig. 11). The shape of these guests is comparatively spherical and their size matches the j6-CD cavity more closely. The response of 27 to these guests is a large increase in the fluorescence intensity. It is noteworthy that 27 is not sensitive to bile acids, although bile acids are strongly bound by the native j6-CD. 

There are two primary reasons why a fluorescent chemosensor with visible emission would be desirable, in addition to its inherent aesthetic appeal. First, it simplifies qualitative experimental measurement the human eye is sensitive enough to detect extremely small changes in intensity (the limit of visual detection by a dark-adjusted eye has been... 

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