Ion-selective Optrodes

A sensor for halides, Ch, Br and I , using fluorescence quenching of either acridinium or quinidinium-based fluorescent reagents covalently bound to a glass support via carbodiimide has been described. Other examples are sensors for Na, K+ and Ca.  

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Y. Kawabata, T. Kamichika, T. Imasaka, and N. Ishibashi, Ion-selective optrode using hexadecyl-acridine orange attached on poly(vinyl chloride) membrane, Anal. Chem, 62, 2054-2055 (1990). 

Figure 5.25 — Flow-through ion-selective optrode based on a multilayer lipidic membrane prepared by the Langmuir-Blodgett method. (A) Cross-sectional view of the composite six-layer membrane (four layers of arachidic acid/ valinomycin covered by an arachidic acid and rhodamine dye bilayer). (B) Optical arrangement integrated with the sensor, which is connected to a flow system. LS light source Ml and M2 excitation and emission monochromator, respectively FI and F2 primary filters M mirror LB lipid-sensitive membrane in a glass platelet FC flow-cell A amplifier D display P peristaltic pump. (Reproduced from [107] with permission of the Royal Society of Chemistry).

H. He, H. Li, G. Mohr, B. Kovacs, T. Wemer, and O. S.Wolfbeis, Novel type of ion-selective fluorosensor based on the inner filter effect An optrode for potassium, Anal. Chem, 65, 123-127... 

As was found for electrochemical-based sensors, problems can occur with peeling of the membrane element from its substrate. PVC is not the only membrane used for optrodes other polymers include polyurethane, which displayed improved adhesion [125]. Ambrose and Meyerhoff [126] incorporated ion-selective species into thin liquid films of decyl methacrylate on glass and then photopolymerised the methacrylate, with simultaneous crosslinking and covalent attachment of the polymer to its substrate to give substantially improved lifetimes. 

Key words Calixarenes, ion-selective electrodes, sensors, optrodes. 

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. 

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