Ionophore optical sensors

Heterocycles as ligands in ionophores for potentiometric and optical sensors 98CRV1593. 

Ozawa S., Hauser P.C., Seiler K., Tan S.S., Morf W.E., Simon W., Ammonia-gas-selective optical sensors based on neutral ionophores, Anal. Chem. 1991 63 640. 

The optical sensors are composed of ion-selective carriers (ionophores), pH indicator dyes (chromoionophores), and lipophilic ionic additives dissolved in thin layers of plasticized PVC. Ionophores extract the analyte from the sample solution into the polymer membrane. The extraction process is combined with co-extraction or exchange of a proton in order to maintain electroneutrality within the unpolar polymer membrane. This is optically transduced by a pH indicator dye (chromoionophore)10. 

P. Buhlmann, E. Pretsch, and E. Bakker, Carrier-based ion-selective electrodes and bulk optodes. 2. Ionophores for potentiometric and optical sensors. Chem. Rev. 98, 1593-1687 (1998). 

R.D. Johnson and L.G. Bachas, Ionophore-based ion-selective potentiometric and optical sensors. Anal. Bioanal. Chem. 376, 328-341 (2003). 

Y. Qin, S. Peper, A. Radu, A. Ceresa, and E. Bakker, Plasticizer-free polymer containing a covalently immobilized Ca2+-selective ionophore for potentiometric and optical sensors. Anal. Chem. 75, 3038—... 

E. Wang, L. Ma, L. Zhu and C.M. Stivanello, Calcium optical sensors based on lipophilic dichlorofluorescein anionic dye and calcium organo-phosphate ionophore or neutral carriers, Anal. Lett., 30(1) (1997) 33-44. 

I.H. Badr, R.D. Johnson, M. Diaz, M.F. Hawthorne and L.G. Bachas, A selective optical sensor based on [9]mercuracarborand-3, a new type of ionophore with a chloride complexing cavity, Anal. Chem., 72(18) (2000) 4249-4254. 

Figure 5.21 — Mechanisms involving an optical change in flow-through metal ionophore-based sensors. The ionophore (I), indicator (In) and hydrophobic counter-ion (X) are in the lipophilic phase (shaded area), which can be a solid (1), a layer (2) or a membrane (3-6). The analyte (A) can be a cation (1-4), an anion (6) or a neutral species (5). The dotted arrow indicates the origin of the optical change, which is always related to the indicator. For details, see text.

For this reason, the most studied application has been the use of optically active pH indicators as H+-selective ionophores to develop pH-selective optical sensors [38]. First, the optical and analytical characteristics in PVC membranes of a series of novel neutral pH indicators, a class of keto cyanine dyes [144], were studied in a conventional configuration. The absorption maxima of the dyes are located in the NIR region, which is very appropriate for applications using telecommunications components and the membranes show good performance during calibrations in the pH range 3-8.5. 

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... 

The fact that the organotin carriers were proven to be anion selective in their neutral form provided the required information for the development of an optical sensor. Based on the coextraction principle, an optical sensor membrane for the determination of chloride was described. For this, tri-n-octyltin chloride was used as the ionophore, and together with the appropriate pH sensitive chromoionophore, allowed for the development of an optical sensor system for the monitoring of chloride levels in blood and serum. 

H. Yanagi, T. Sakaki and T. Ogata, Development of high-performance ion sensors based on the functions of crown ethers and synthetic bilayer membranes, Nippon Kagaku Kaishi, 1999, 1999, 629 P. Izatt, E. Pretsch and E. Bakker, Carrier-based ion-selective electrodes and bulk optodes. 2. Ionophores for potentiometric and optical sensors, Chem. Rev., 1998, 98, 1593 S. Yajima and K. Kimura, Recent trends in ionsensing research, Bunseki Kagaku Anal. Chem.), 2000, 49, 279 etc. 

Because ISE responses do not depend on any type of ion current or flux, they are much less affected by adsorption of contaminants onto the sensor membrane than most other electrochemical and optical sensors. As long as adsorbed contaminants do not completely cover the ISE membrane, they have no effect on the measured response. However, in contrast to sensors based on solid sensing phases, such as metals in voltammetric sensors or inorganic salts in solid-state ISEs, extraction of hydrophobic sample components into the hydrophobic sensing membranes of ionophore-doped ISEs can result in the deterioration of the ISE selectivity. This is of particular concern in the case of long-term measurements in biological samples. 

Systems capable of effecting the detection of lanthanide and actinide ions using electrochemical methods have generally consisted of a receptor or ionophore immobilized within a membrane to form an ion-selective electrode (ISE). This electrode is then pertmbed when a cation guest is boimd. On the other hand, optical sensors or optodes are based on a molecule that is either free standing or attached to a polymer matrix. In both cases, the expectation is that an optical response will be produced when the system is exposed to a particular anal5de. Needless to say, for either approach it is beneficial to have a receptor that is well tuned for the lanthanides and actinides. In practice, this has translated to the use of macrocycles rich in O, N, S, or P donor atoms as will be clear from the summary of recent work provided in this article. This article focuses on ISE- and optode-based approaches to lanthanide and actinide cation sensing. 

Lindner E, Horvath M, Toth K, Pungm E, Bitter L Agai B et al (1992) Zinc selective ionophores for potentiometric and optical sensors. Anal Lett 25 453—470... 

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Carbonate ionophore I [ETH 6010] (heptyl 4-trifluoroacetylbenzoate) [129476-47-7] M 316.3, b 170°/0.02 Torr, d 0.909. Purified by flash chromatography (2g of reagent with 30g of Silica Gel 60) and eluted with EtOAc/hexane (1 19). The fractions that absorbed at 260nm were pooled, evapd and dried at room temp (10.3 Torr). The oily residue was distd in a bubbled-tube apparatus (170°/0.02 Torr). Its IR (CHCI3) had peaks at 1720, 1280, 940cm and its sol in tetrahydrofuran is 50mg/0.5mL. It is a lipophilic neutral ionophore selective for carbonate as well as being an optical humidity sensor. [Anal Chim Acta 233 41 1990.]... 

The fourth type of mediator-based cation optical sensing is using potential sensitive dye and a cation selective ionophore doped in polymer membrane. Strong fluorophores, e.g. Rhodamine-B C-18 ester exhibits differences in fluorescence intensity because of the concentration redistribution in membranes. PVC membranes doped with a potassium ionophore, can selectively extract potassium into the membrane, and therefore produce a potential at the membrane/solu-tion interface. This potential will cause the fluorescent dye to redistribute within the membrane and therefore changes its fluorescence intensity. Here, the ionophore and the fluorescence have no interaction, therefore it can be applied to develop other cation sensors with a selective neutral ionophore. 

An analogous system to the cation sensors is the use of optically nonactive anion ionophores together with a pH-sensitive chromophore for the anion sensing. In this system, simultaneous extraction of H+ and anion X- from the aqueous solution into an organic phase results in protonation of the chromophore anion C, e.g. 

Ceresa A, Qin Y, Peper S, Bakker E (2003) Mechanistic insights into the development of optical chloride sensors based on the [9]mercuracarborand-3 ionophore. Anal Chem 75 133-140... 

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). 

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