Fluorescent sensors anions

Fabbrizzi L., Licchelli M., Taglietti A., The Design of Fluorescent Sensors for Anions Taking Profit from the Metal-Ligand Interaction and Exploiting Two Distinct Paradigms, J. Chem. Soc., Dalton Trans. 2003 3471-3479. 

The design of fluorescent sensors is of major importance because of the high demand in analytical chemistry, clinical biochemistry, medicine, the environment, etc. Numerous chemical and biochemical analytes can be detected by fluorescence methods cations (H+, Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Pb2+, Al3+, Cd2+, etc.), anions (halide ions, citrates, carboxylates, phosphates, ATP, etc.), neutral molecules (sugars, e.g. glucose, etc.) and gases (O2, CO2, NO, etc.). There is already a wide choice of fluorescent molecular sensors for particular applications and many of them are commercially available. However, there is still a need for sensors with improved selectivity and minimum perturbation of the microenvironment to be probed. Moreover, there is the potential for progress in the development of fluorescent sensors for biochemical analytes (amino acids, coenzymes, carbohydrates, nucleosides, nucleotides, etc.). 

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

The development of fluorescent probes for anion recognition has been very limited so far in comparison with those for cations. Most of the presently available methods of detection of anions based on fluorescence involve quenching, redox reactions, substitution reactions, ternary complex formation(15) and thus cannot be considered as recognition methods. For instance, the fluorescent sensors that are used for the determination of chloride anions in living cells are based on collisional quenching of a dye by halide ions 6-methoxy-iV-(sulfopropyl)quinoliniuni and... 

The PAH polymeric layer played an important role in our fluorescence sensor design. First, its positive charges enabled the deposition of anionic dextran that was labeled with the pH indicator fluorescein on the surface of the nanoparticles. More importantly, the PAH polymeric layer separated between the fluorescein molecules and the metal particle. In fact, the thickness of the polymeric layer was over 10 nm, which is larger than the Forster distance required for efficient energy transfer between the fluorophore and the metallic gold particles. 

As demonstrated in this chapter, the binding of metal ions to maclocyclic ligands (e.g., porphyrins) results in the change in both the thermodynamic and dynamic properties of ET reactions of metalloporphyrins. Excellent models of the photosynthetic reaction center were developed by the appropriate choice of combination of metal ions and macrocyclic ligands. The lifetimes of the CS states in models of photosynthetic reaction center composed of electron donors and acceptors also were controlled by binding of metal ions to radical anions of electron acceptor moieties in the electron donor-acceptor hnked molecules. The control of ET processes by coordination of metal ions to the dyads led us to develop a unique fluorescence sensor for the ion. The binding of metal ions to radical anions of electron acceptors results in acceleration of thermal ET reactions, which would otherwise be impossible to occur. Such effects of metal ions to enhance the ET... 

The unusual new [3,3]ferrocenophane 74 also acts as a selective fluorescent anion sensor—in this case for nitrate [52]. The protonated receptor is weakly fluorescent = 0.043) in CH2CI2 solution and on addition of nitrate the naphthalene-based emission at 354 nm is quenched (to

redox sensor for other anions and as a fluorescent sensor for group II cations. 

Fluorescent Sensors, p. 572 Ion-Selective Electrodes, p. 747 Organometallic Anion Receptors, p. 1006... 

In this article, we will describe different approaches to the design of fluorescence sensors for anions that exploit distinctive mechanism-based concepts. In most case, synthetic receptors are used for anion binding and induce specific responses from the appended fluorophores, leading to selective and sensitive sensing. 

Another class of transition metal-based anion fluorescence sensors is based on ruthenium(II) rrA(bipyridyl)... 

Anion-Directed Assembly, p. 51 Biosensors p. 115 Crown Ethers, p. 326 Electrochemical Sensors, p. 505 Fluorescence Sensing cf Anions, p. 566 Fluorescent Sensors, p. 572 Guanidium-Based Anion Receptors, p. 615 Hydrogen Bonding, p. 658... 

Biosensors, p. 115 DNA Nanotechnology, p. 475 Fluorescence Sensing of Anions, p. 566 Fluorescent Sensors, p. 572 Imaging and Targeting, p. 687 Luminescent Materials, p. 875 Photochemical Sensors, p. 1053 Supramolecular Photnchernistn, p. 1434... 

Fluorescence Sensing of Anions, p. 566 Fluorescent Sensors, p. 572 Luminescent Probes, p. 821... 

Small molecule anion sensors have been an active field of research for the last 20 years. Fluorescent sensors have received significant attention and have been reviewed exten-sively. They offer advantages such as high sensitivity and simple instrumentation. The incorporation of a fiuo-rophore such as the molecular clip, indolocarbazoles, and indoles described previously not only provide a reporter group but can also provide a structural element to the receptor. 

As mentioned above, the first fluorescent sensor for saccharides was reported by Yoon and Czamik." The internal charge transfer (ICT) sensor 1 consisted of a boronic acid fragment directly attached to anthracene. On addition of saccharide, it was noted that the intensity of the fluorescence emission for the 2-anthrylboronic acid 1 was reduced by 30%. This change in fluorescence emission intensity is ascribed to the change in electronics that accompanies rehybridization at boron. For boronic acid 1 (below its pA a). the nentral sp hybridized boronic acid displayed a strong flnorescence emission (above its pA a) and the anionic sp boronate displayed a reduction in the intensity of fluorescence emission. 

Figure 19 Examples of PET-based fluorescent sensors for anions. (Reproduced from Ref. 202. American Chemical Society, 1994.)...

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