Luminescent sensors antennae

It should also be noted that, although the main interest in antennas comes from the search for an effective artificial photosynthesis, antennas may be relevant for any kind of application involving light-induced processes, such as luminescent sensors, photochemical reactions, and so on. The efficiency of any photoinduced process may actually be significantly enhanced by using suitable antennas, when the overall light energy incident on the system is considered. 

Lobnik A., Majcen N., Niederreiter K., Uray G., Optical pH sensor based on the absorption of antenna generated europium luminescence by bromothymolblue in a sol-gel membrane, Sensors and Actuators B 2001 74 200-206. 

Knall et al. have tested a series of europium complexes with antenna chromo-phores in different polymer matrices in terms of sensitivity, response time, and dynamic range for the sensing of water vapor. A copolymer containing a Eu " chelate that can be spin-cast as thin film on glass slides was developed [110]. Scheme 6 shows the chemical structure of the polymerizable precursor 29. This complex can be copolymerized with norbomene-2,3-dicarboxylic acid dimethyl ester by means of ring opening metathesis polymerization. The sensor spots respond to water vapor by reversible luminescence quenching, which can be analyzed by means of phase sensitive measurements of luminescence lifetime [110]. 

The luminescence intensity and decay time of EuCls in carboxymethyl cellulose membranes is decreased in presence of heavy metal ions like Cu" or Cu ", but also Cr " and Fe " exert a distinct quenching effect [111]. It is not likely that a sensor with high specificity can be prepared on the basis of LLCs that is free of interferences from other metal ions. However, an adequate choice of the ligand system may help to improve the selectivity of the response. Another approach uses a sol-gel technique to embed a complex of Eu " and silanized 2,6-pyridine-dicarboxylic acid as antenna in a silica network. This luminescent material can sense copper ion concentrations in water down to 50 pg but the sensor was not evaluated with respect to interferences of other metal ions or in environmental samples [112]. 

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]. 

In these lanthanide complexes, the multidentate Schiff-base ligands can stabilize Ln centers and act as antenna sensors for lanthanide luminescence. With the Ln + centers protected by the Schiff-base hgands from solvent and water molecules, those lanthanide complexes with multidecker and cage-like structures show improved luminescence properties. 

An example of a sensor system that does not incorporate a covalently linked chromophore has been reported by Gunnlaugsson and is essentially based upon a displacement assay. The system features a ternary Eu complex with bathophenathroline disodium sulfonate (BPS) as a co-ligand, which also provides the antenna the complex demonstrates classical Eu emission following irradiation at 278 nm. Addition of Fe to the ternary complex extricates the diimine ligand to form [Fe(BPS)3] " and thus liberates the antenna-free Eu complex, resulting in a dramatic drop in the luminescence intensity [39]. 

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