Sensors for Anions

As discussed in the introduction to this chapter, the lanthanides are normally found in their 3-1- oxidation sate and as such are strong Lewis acids. Consequently, these ions have high affinity for anions hydroxide is one particular example, which leads to the precipitation of 

Eu or Tb ions, or by using a mixture of two Tb ions, which were found to bind to the 

The above examples have focussed on the sensing of anions by direct association with the lanthanide centre and the advantages in this design approach have been discussed. Usually such systems give rise to enhancements of the lanthanide emission and/or allow for the monitoring of the sensing event by observing the ratios of several transitions. The drawback 

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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. 

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. 

Cortina, M., Duran, A., Alegret, S., and Valle, M. A. (2006). Sequential injection electronic tongue employing the transient response from potentiometric sensors for anion multidetermination. Anal. Bioanal. Chem. 385(7), 1186-1194. 

Nomura Y, Ikebukuro K, Yokoyama K, Takeuchi T, Arikawa Y, Ohno S, Karube I (1994) A novel microbial sensor for anionic surfactant determination. Anal Lett 27 3095 - 3108... 

Solid-state sensors for anionic surfactants can be constructed by using polyaniline as sensing membrane [107,108], and by using polypyrrole as ion-to-electron transducer in combination with plasticized PYC as sensing membranes [53,66]. The sensors may be applied for the determination of dodecylsulfate in, e.g., mouth-washing solution and tap water [107], and for the determination of dodecylbenzenesulfonate in detergents [66,108]. Solid-state surfactant sensors allow a sample rate of 30 samples/h, when applied in flow-injection analysis [53]. 

Ferrocene appended 2,5-diamidopyrole receptors were subsequently developed in order to produce electrochemical sensors for anions (.Figure 7).8 The anion complexation of these compounds was reported through the results of both H NMR titration and cyclic voltammetry techniques. 

Other workers have used pyrilium cations [137] as sensors for anions such as ATP or sulphate. Charge-transfer complexes between calixpyrroles and chlo-... 

Cryptate 72, in which the aryl spacer of 71 is replaced with a furanyl unit, acts a colorimetric sensor for anions. UV-vis titrations in aqueous solution gave log K values for the 1 1 halide/receptor adducts of 3.98 for chloride, 3.01 for bromide and 2.39 for iodide. X-ray diffraction studies confirm that bromide is held between the two copper atoms. Under the same conditions 72 also interacts strongly with azide (log K=4.7) and thiocyanate (log X=4.28) anions. This receptor is interesting because of its lack of selectivity compared to 71. The complex appears to be able to expand and contract its bite length in order to accommodate anions of various sizes. 

Fabrizzi L, Lichelli M, Rabaioli G, Taglietti A. The design of luminescent sensors for anions and ionizable analytes. Coord Chem Rev 2000 205 85-108. 

Castellano et al.221 reported the formation of a luminescence lifetime-based sensor for cyanide and other counterions using Ru11 diimines possessing MLCT excited states with the anion recognition capabilities of 2,3-di(l//-2-pyrrolyl)quinoxaline (DPQ). Using time-resolved photoluminescence decay, its viability as a lifetime-based sensor for anions has been tested. There were significant changes to the UV-vis and steady-state emission properties after the addition of several ions (e.g., fluoride, cyanide, and phosphate). 

Gale. P.A. Hursthouse. M.B. Light, M.E. Sessler, J.L. Wamner, C.N. Zimmerman. R.S. Ferrocene-substituted calix[4]pyrole A new electrochemical sensor for anions involving CH... anion hydrogen bonds. Tetrahedron Lett. 

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. 

Gale. P.A. Chen. Z. Drew. M.G.B. Heath. J.A. Beer. P.D. Lower-rim ferrocenyl substituted calixarenes New electrochemical sensors for anions. Polyhedron 1998. 17, 405. 

A. Preparation of Porphyrin-Based Potentiometric Sensors for Anions.252... 

A. PREPARATION OF PORPHYRIN-BASED POTENTIOMETRIC SENSORS FOR ANIONS... 

From the above examples, one can see that metal-anion interactions have proven to be very useful in the development of sensors for anionic species, especially biomolecules containing the phosphate group. However, sensors with higher selectivity and sensitivity for similar or other biologically important targets are still very much needed. 

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

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