Ion optode

J. Janata, Ion optodes, Anal. Chem. 64,921A-927A (1992). 

Fig. 2 The model of ion extraction/ion exchange for an optode based on a neutral ionophore and a lipophilic cationic dye in relation to the ion-optode response mechanism i+ cation to be extracted, H+ proton, S neutral ionophore, R lipophilic cationic additive, D color-changeable dye. The subscripts o and w represent the organic phase and the water phase, respectively)...

NOj, cr, CIO4 VII. Ion Optodes (Ion-Selective Optical Sensors) polymer liquid membranes ... 

Ion Optodes The principles underlying the sophisticated complex polymer-based matrices for ISEs are the same as those used to design and construct ion-selective optical sensors, the optodes. Ion optodes have been developed for H, alkali metal ions, NH, Ca, NO, and CO . Numerous sensors for... 

Widmer, H. M. (1992) Ion-Selective Electrodes and Ion Optodes, Anal. Methods Instrum. 1, 60-72. 

Ion-selective electrodes are now well understood in terms of the underlying theory, and this has made it possible for new sensing principles to emerge that make use of the thousands of chemical receptors originally developed for ion-selective electrodes. One is the field of optical sensors, which has not been discussed here because it is outside the focus of this chapter. Such so-called bulk optodes do not require electrical connectivity between the sensing and detection unit and are therefore more easily brought into various shapes and sizes, including particle formats, which suit the need of modem chemical analysis. 

E. Bakker, P. Buhlmann, and E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics. Chem. Rev. 97, 3083-3132 (1997). 

E. Bakker and W. Simon, Selectivity of ion-sensitive bulk optodes, Anal. Chem., 64 (1992) 1805-1812. 

As in the case of bulk optodes, plasticizers can be added to modify the properties of polymers (e.g., gas permeability). Plasticizers are mainly used to design ion-sensitive nanobeads. 

Bychkova V, Shvarev A (2009) Fabrication of micrometer and submicrometer-sized ion-selective optodes via a solvent displacement process. Anal Chem 81 2325-2331... 

Wygladacz K, Radu A, Xu C, Qin Y, Bakker E (2005) Fiber-optic microsensor array based on fluorescent bulk optode microspheres for the trace analysis of silver ions. Anal Chem 77 4706 1712... 

In biomedical applications, the ranges of ion concentration are higher by several orders of magnitude. For instance, the abovementioned calcium probes for living cells cannot be used because the dissociation constant is so low that they would be saturated. Special attention is thus to be paid to the ionophore moiety to achieve proper selectivity and efficiency of binding. For instance, at present there is a need for a selective fluorescent probe for the determination of calcium in blood which could work in the millimolar range in aqueous solutions so that optodes with immobilized probes on the tip could be made for continuous monitoring calcium in blood vessels. 

In analytical chemistry, detection of metal ions is of major importance. In particular, the development of simple and reliable methods for continuous control in situ of metal ions in the environment is the object of much attention. For instance, the detection of lead, mercury, cadmium, and iron ions in sea water will be performed in the near future by optodes associated with suitable fluoroionophores, thus allowing continuous monitoring by instruments on ships. 

E. Bakker, M. Lerchi, T. Rosatzin, B. Rusterholz, and W. Simon, Synthesis and characterization of neutral hydrogen ion-selective chromoionophores for use in bulk optodes, Anal Chim Acta 278, 211-225 (1993). 

It has been widely accepted for many years that the LOD of an ISE in an unbuffered solution is at micromolar level. Interestingly, if a com-plexing agent is added into the sample and the concentration of the free primary ions is significantly lowered, the LOD is reduced sometimes to subnanomolar levels [35]. In addition, if halide ions are added to samples in which a silver-selective electrode is immersed, the electrode shows a decrease in potential indicating lowering of the activity of a silver at the sample/membrane phase boundary [36]. Moreover, ionophore-based optodes showed picomolar detection limits [37], even... 

Keywords Absorbance Antiresonant reflecting optical waveguide Core-based optodes Integration Ion-selective optodes... 

They are often plasticized PVC matrices, which occlude an ionophore as the key selective element, a chromoionophore or a fluoroionophore as the chemical-optical transducer and, sometimes, ionic additives to maintain electroneutrality. Such optodes follow ion-exchange mechanisms between the membrane and the aqueous solution and the analytical response originates from the ratio of the concentration of ions in the solution or from their product (Fig. 3). Moreover, selectivity is ruled by the ion distribution coefficients between both phases and by the formation constants of complexes within the membrane. 

The main advantages of using this approach are that response is not affected by the ionic strength of the solution and that the development of specific ion-selective optodes is easily adapted from their analogous well-studied ion-selective electrodes [32 - 34], because the same selective reagents and membrane constituents are employed. Most of the ion-selective optodes... 

Fig. 3 Basic composition of ion-selective bulk optodes and extraction mechanisms. This type of sensor follows an extraction equilibrium between the aqueous solution and the membrane and the signal is related to the analyte activity in the aqueous solution...

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

The response characteristics of an ion-selective optode rely on the formation constant of the ionophore-analyte complex and the acidity constant of the chromoionophore in the mediim. Therefore, for an ionophore of a specific target analyte, it would be useful to select a chromoionophore that gives the best response characteristics. 

The combination of keto cyanine dyes with commercial ionophores makes the design of ion-selective optodes feasible, and the use of the IWAO permits increased sensitivity using simple and low-cost equipment so that analysis of a target analyte can be performed in different application fields. 

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. 

A more sophisticated class of optical sensors with high selectivity towards ions are the ion-selective optodes (ISOs) [21], where the matrix (hydrophobic polymer such as PVC) contains a selective lipophilic ionophore (optically silent), a chromoionophore, a plasticizer and an anionic additive. The measurement principle is based on a thermodynamic equilibrium that controls the ion exchange (for sensing cations) or ion coextraction (for sensing anions) with the sample. The source of optode selectivity is a preferential interaction between the target ion and an ionophore. For a dye to act as a chromoionophore, it must... 

For measuring a significant change of the optical signal from the optode membrane, there must be a change in the bulk composition of the membrane to influence the chromoionophore. Simultaneously, electroneutrality within the bulk membrane phase must be maintained. Therefore, the mechanism of optode response must be either an ion-exchange of two anions or two cations, or must be a coextraction of two differently charged ions where one ion of the pair alters the optical properties of the ionophore, as shown in Fig. 1. 

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