Optical Ion Sensors

Optical sensors for ions use indicators, which exist in two different colors, depending on whether the analyte is bound to them. The use of colored indicators is one of the oldest principles of analytical chemistry, used extensively both in direct analytical spectroscopy and in so-called visual titrations. In their sensing application, the indicator is confined to the surface of the optical sensor or immobilized in the selective layer. In that sense, the oldest and most widespread optical sensor is a pH indicator paper, the litmus paper, which is commonly used for the rapid and convenient semiquantitative estimate of pH of solutions or for endpoint detection in acidobasic titrations. Its hi-tech counterpart is a pH optrode (the name of which is intentionally reminiscent of the pH electrode), which essentially does the same thing (Wolfbeis, 2004). The operation principles and limitations of ion optical sensors are common for all ions. 

The primary chemical interaction governing the operation of the pH optrode is the acid-base equilibrium of the indicator HA. 

Its dissociation constant Ka is defined by the equilibrium equation, which has been arranged such that it is usable for optical sensing  

In logarithmic form, it is known as the Henderson-Hasselbalch equation. 

The second term shows the log of the ratio of concentrations of the protonated HA and dissociated A form of the indicator, which are measured optically. If we assume that a hundredfold change of concentration of the absorbing molecule can be conveniently measured (i.e., the dynamic range belonging to one Ka equals 1 decade), which means that one indicator can cover two units of pH, by selecting a series of indicators with suitably spaced values of their dissociation constants, a pH optrode with broad dynamic range can be obtained. 

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The activity of water plays a dominating role in optical ion sensors in which the indicator dye is incorporated in a hydrophobic layer. Despite its hydrophobic label, such layers contain a finite, sometimes very significant concentration of water. In that respect, they behave as mixed organic-aqueous media with all the implications that affect the aforementioned acidity functions. In other words, the optical signal coming from such a selective layer is affected more by the degree of hydration than by the changes of ion activity in the solution (Janata, 1992). 

Chojnacki, P. Werner, T. Wolfbeis, O. S., Combinatorial approach towards materials for optical ion sensors, Microchim. Acta 2004,147, 87-92... 

Kawabata Y., Tahara R., Imasaka T., Ishibashi N., Fiber-optic potassium ion sensor using alkyl-acridine orange in plasticized poly(vinyl chloride) membrane, Anal. Chem. 1990 62 1528. 

Seitz W.R., Optical ion sensing, in Fiber optic chemical sensors and biosensors II (Wolfbeis O.S., ed.), CRC Press, Boca Raton, Florida, 1991. 

Rayss J., Sudolski G., Ion absorption in the porous sol-gel silica layer in the fibre optic pH sensor, Sens. Actuat B 2002 87 397-405. 

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. 

I.H.A. Badr and M.E. Meyerhoff, Highly selective optical fluoride ion sensor with submicromolar detection limit based on aluminum(III) octaethylporphyrin in thin polymeric film, J. Am. Chem. Sac., 127(15) (2005) 5318-5319. 

The potential applications of NIR OFCD determination of metal ions are numerous. The detection of metal contaminants can be accomplished in real-time by using a portable fiber optical metal sensor (OFMD). Metal probe applications developed in the laboratory can be directly transferred to portable environmental applications with minimal effort. The response time of the NIR probe is comparable to its visible counterparts and is much faster than the traditional methods of metal analysis such as atomic absorption spectroscopy, polarography, and ion chromatography. With the use of OFMD results can be monitored on-site resulting in a significant reduction in labor cost and analysis time. 

K. Suzuki, H. Ohzora, K. Tdida, K. Miyazaki, K. Watanabe, H. Inoue, and T. Shirai, Fibre-optic potassium ion sensors based on aneuttal ionophore and a novel lipophilic anionic dye, Anal Chim Acta 237, 155-164(1990). 

K. Suziki, K. Tohda, Y. Tanda, H. Ohzora, S. Nishihama, H. Inoue, and T. Shirai, Fiber-optic magnesium and calcium ion sensor based on a neutral carboxylic polyether antibiotic, Anal. Chem. 61, 382-384 (1989). 

Bifurcated optical fiber Sensor cell Ion-sensing beods... 

ILs, specifically [C4Qlm][Bp4], and [C4Cilm]Br, have also been employed as optical sensor matrices for the detection of gaseous and dissolved COj. Recently, Ertekin and coworkers developed a new optical COj sensor that is based on the spectrophotometric signal changes of the ion pair, bromothymol blue/tetraoctylammonium (BTB /[(Cg)4N] ) [17]. The authors report pK values... 

Figure 37. Principle of an optical Ca ion sensor based on a calmodulin (CaM)-me-diated Ca signaling pathway and surface plasmon resonance. ...

An example of an optical enzyme sensor (Arnold, 1985) in a bifurcated optical fiber is shown in Fig. 9.32. The bifurcated fiber delivers and collects light to and from the site of the enzymatic reaction. The enzyme, alkaline phosphatase (AP), catalyzes hydrolysis of p-nitrophenyl phosphate to p-nitrophenoxide ion which is being detected (A = 404 nm). 

Fig. 10.3 Identification and quantification of the interferant by a third-order sensor in which optical, ion-selective electrode, and chromatographic data are combined (a) pure standards (b) contaminated sample...

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

In parallel with the development of the EOID for focal plane mass spectrometers of the Mattauch-Herzog type, similar devices were developed for use with conventional sector-type mass spectrometers (15, 16, 17, 18). A schematic representative of this type detector, versus that implemented on a CEC type 21-490 single focusing mass spectrometer, is shown in Fig. 8. The main differences between these two applications of the EOID are a result of the differences in the ion optics of the two types of mass analyzers, as shown in Fig. 1. First, the detector of a sector type instrument resides outside the magnetic fringe field, thus eliminating the need for angling the primary ion sensors. 

Vasil ev V, Borisov SM. Optical oxygen sensor based on phosphorescent metal porphyrins immobilized in perfluorinated ion-exchange membrane. Sens Actuators 2002 B82 272-6. 

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