Optical anion sensors

Very recently Beer and co-workers have developed a surface-enhanced optical anion sensor based on gold nanoparticles [78]. Dodecanethiol-stablised gold nanoparticles were modified by ligand substitution with a disulphide-substituted zinc porphyrin 113 to provide 30 and 80 receptors per nanoparticle. Titration of both the free receptor and the modified nanoparticles with various... 

One approach to reach an effective anion sensor for biological anions involves the construction of optical anion sensors. Such a system generally... 

Ruthenium(ll) Polypyridyl Complexes as Optical Anion Sensors. 72... 

Another noteworthy example in which Cu(I) forms the basis of an optical anion sensor is 115, in which the metal complex acts both as a UV/vis signalling group and as a structural component dictating the topology of the urea anion-binding site [77]. The MLCT band within the Cu (phenanthroUne) complex at 282 nm is sensitive to halide ions, acetate and dihydrogenphosphate in 4 1 v/v THF/MeCN (a relatively low polarity solvent). However, in DMSO solution, only acetate and dihydrogenphosphate produced a UV/vis re-... 

K. Watanabe, E. Nakagawa, H. Yamada, H. Hisamoto and K. Suzuki, Lithium ion selective optical fiber sensor based on a novel neutral ion-ophore and a lipophilic anionic dye, Anal. Chem., 65(19) (1993) 2704-2710. 

This review will then consider optical sensors, together with details of their basic principles, construction and operation. Relative advantages and disadvantages compared to electronic sensors will also be discussed. Current applications of anion sensors within the field will be described together with a forward view towards some possible future technologies and applications. 

Anion sensors come in two main types that can give rise to either (1) an electrical or (2) an optical signal. These will be considered separately. 

This section attempts to present a broad review of this technique in the light of recent research on fiber-optic chemical sensors (FOCS). A discussion on the advantages and performance of pH optodes will be broached by considering the various applications planned and the resulting pH measurements, including titration and ionic strength. Sensors for low molecular weight electrolytes, particularly optodes for anions and cations in solution, are also considered. 

The first chapter by Bates and Gale provides an overview of the coordination of anions by synthetic organic hosts. The different organic functional groups used to bind anions are presented and this provides an introduction to the structural and electronic properties that hosts must have to recognise anionic guests. On the other hand, Bayly and Beer give a detailed account of the use of metal complexes as anion receptors. Besides the important structural features that metals can confer to receptors, their optical and redox properties make them attractive for the development of anion sensors. 

But the most smdied electropolymerizable porphyrins are the tetra-amino-and tetrahydroxyphenyl-substituted ones. Indeed, both of them were developed to elaborate electrodes having potentiometric responses to several kinds of anions, such as iodide for example . Co(II) tetrakis(p-hydroxyphenyl)porphyrin 53-based films were used for the elaboration of potentiometric and fibre optic pH sensors with minimal interferences from anions. Zn(II)-53-based films were also used and characterized for kinetics studies of hydrogen evolution at their surface . Zn(II)-53 as well as Pd(II)-53 and free base tetra(4-methyl pyridinium) porphyrin (H2-54) were electropolymerized on indium tin oxide substrates, leading thus to starting materials for the making of donor/acceptor... 

The second type of CO2 fiber-optic chemical sensor is constructed by using ion pairs consisting of a pH indicator anion and an organic quaternary cation. First, a pH indicator dye (DH) and a quaternary ammonium hydroxide (Q OH ) are entrapped into a proton-impermeable but CO2-permeable polymer membrane, which is then immobilized onto the fiber s surface. The mechanism of this CO2 sensor is based on the interaction between the dye molecules (DH) and the quaternary cations (Q+OH ) to form hydrated ion pairs (Q D XH2O). The hydrated ion pair is dissolved in the polymer, where it reacts with CO2 according to the following reaction ... 

These host mercuracarborand molecules were applied in Ion Selective Electrodes (ISE) and membrane formulations, as selective optical chloride sensors, as sensitive liquid/polymeric membrane electrodes for anions and as catalysts. ... 

Fiber optic chemical sensors (FOCS) have been reported for several different types of analytes such as ions, metals, pH, gases, enzymes, etc. These optical sensors can often be used as alternatives to electrochemical sensing devices. However, with a few exceptions (e.g., halides, cyanide, etc.), there is a lack of anion-selective FOCS. 

Tan S.S.S., Hauser P.C., Chaniotakis N.A., Suter G., Simon W., Anion-selective optical sensors based on a coextraction of anion-proton pairs into a solvent-polymeric membrane, Chimia 1989 43 257. 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

Similarly, an anionic chromophore HC can also be used for designing optical sensors for cations, in this case, no extra anionic site (R ) is needed to maintain charge neutrality during the sensing. 

Certain porphyrins have shown optical absorbance change when exposed to anions such as NO2, Cl-, I-, and SCN-, and optical sensors can be obtained accordingly. 

---