Crown ether sensors

A potassium-ion-selective, dendritic, fluorescing chemosensor, bearing three crown ether moieties in the periphery, shows a linear increase in fluorescence intensity with increasing potassium concentration (in acetonitrile). An important criterion for potassium chemosensors is their mode of action (selectivity) in the presence of large amounts of sodium. The tris-crown ether sensor shown in Fig. 8.16 is able to detect very small traces of potassium ions, even if large quantities of sodium ions are present in the same solution - such as in body fluids [55]. 

Gokel, G.W., Leevy, W.M. and Weber, M.E. (2004) Crown ethers sensors for ions and molecular scaffolds for materials and biological models, Chem. Rev. 104, 2723-2750. 

Design and synthesis of crown ethers and other macroheterocycles as highly selective ionophores for chemical ion sensors 98YGK291. 

Crown compounds/cryptands, 14 162 Crown ether motif, diagnostic sensors based on, 24 54-55 Crown ethers, 7 588 24 41... 

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

Marsella MJ, Swager TM (1993) Designing conducting polymer-based sensors - selective ionochromic response in crown-ether containing polythiophenes. J Am Chem Soc 115 12214-12215... 

OXIDIZABLE CATION SENSORS Ferrocene crown ether species... 

The electrochemical properties of ferrocene have been utilized by many workers in the field of electrochemical molecular recognition. Saji (1986) showed that the previously synthesized (Biernat and Wilczewski, 1980) ferrocene crown ether molecule (Fig. 3 [1]), whose binding properties had previously been studied only by nmr and UV/Vis techniques (Akabori et al., 1983), could be used as an electrochemical sensor for alkali metal cations involving a combination of through-space and through-bond interactions. 

Crown ethers [364] have proved to be an excellent choice as ionophores for the fabrication of ion sensors because of their ability to complex selectively a particular ion. The cadmium selective sensors have been fabricated from poly(vinyl chloride) (PVC) matrix membranes containing macrocyclic ionophores benzo-15-crown-5 [365], monoaza-18-crown-6 [366], dibenzo-24-crown-8 [367], dicyclohexano-18-crown-6 [368], 3,4 ll,12-dibenzo-l,6,... 

Selective complexation of cations by crown ethers 17 [lb, lg] and calixarenes 18 [If] depending on the rings size was proposed to be used in sensors. 

By bridging calixarenes 226 [9], forming their dimers like 227 with one or several bridges [10] or by combining them with crown ethers 228[11], calixarenes with several novel architectures and complexation behaviour have been obtained. Some of such systems have been proposed as prospective sensors. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Kimura used the cholesterol-substituted [15]crown-5 8 in ion sensing membranes. It was found that the addition of the crown ether affects the sensor properties, especially the ion specificity, and it was possible to obtain sensors with extremely high sensitivities [48]. The performance could be further increased by using perfluoroalkyl side chains instead of cholesterol [49]. 

The compounds presented in this section possess, in most cases, columnar phases as expected from their molecular shape. Aza crown ethers and conventional crowns offer a multitude of possibilities to add functional groups. Possible applications can be seen in the field of sensors or functional channels. Unfortunately, no applications have been reported yet. Addition of polymerizable groups might lead to functional membranes as shown in Scheme 40. 

The combination of a phthalocyanine ring with crown ether moieties and redox-active tetrathiafulvalenes gave compound 117 (Scheme 64) and was described by Zou as a good candidate for a redox-active Na+ sensor [133]. 

Crown ether-phthalocyanines 118 (n = 0, 1,2) (Scheme 65) were used as gas sensors for N02. They were found to be superior to the previously used materials... 

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

Scheme 65 Crown ether phthalocyanines 118 as sensor materials for NQ2...

A plastic sodium membrane is now predominantly based on a neutral carrier (ETH 2120) that ensures sufficient sensitivity, selectivity and lifetime for the sensor. Some other compounds such as neutral carriers ETH 157, 227, 4120, calixarenes, crown ethers and hemisphe-rands have been proposed. Anionic influence observed during measurements in undiluted urine may be circumvented by dilution of the sample. 

The main components of the membrane of the enantioselective, potentiometric electrode are chiral selector and matrix. Selection of the chiral selector may be done accordingly with the stability of the complex formed between the enantiomer and chiral selector on certain medium conditions, e.g., when a certain matrix is used or at a certain pH. Accordingly, a combined multivariate regression and neural networks are proposed for the selection of the best chiral selector for the determination of an enantiomer [17]. The most utilized chiral selectors for EPME construction include crown ethers [18-21], cyclodextrins [22-35], maltodextrins 136-421, antibiotics [43-50] and fullerenes [51,52], The response characteristics of these sensors as well as their enantioselectivity are correlated with the type of matrix used for sensors construction. 

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