Host-guest chemistry detection

Fig. 10 Artificial ion channel with staphylococcal y.-hemolysin (ocHL) with a molecular adaptor, a) Schematic of the otHL pore showing pCD loged in the lumen of the channel with adapters. pCD and hepta-6-sulfato-P-cyclodextrin. b) Detection of organic molecules by stochastic sensing. The pore contains a noncovalent pCD adapter, which is capable of carrying out host-guest chemistry while lodged in the lumen. Upper trace, promethazine middle trace, imipramine loner trace, mixture of promethazine (100. uM), and...

A discussion of the state of the art of ISEs for every target ion for which such sensors have been developed would have been well beyond the scope of this chapter. However, we hope that this chapter gives the interested reader an overview of state-of-the-art ionophore-based potentiometry that goes beyond a superficial treatment and illustrates what developers of ISEs need to consider when they want to take advantage of host-guest chemistry to fabricate highly selective sensors. As some of the examples show, analytical chemists have developed a variety of techniques for the optimization of ionophore-doped membranes. This has led to sensors with remarkably high selectivities and extraordinarily low detection limits. 

Calix[4]arene-based fluorescent chemodosimeters 21a and b (Fig. 28.6) in the cone conformation bearing two anthryl groups and a 1,2,4-oxadiazole moiety have been developed for the detection of Fe " ions [48]. The strong fluorescence of 21a at 440 nm is quenched by addition of Fe " and Cu " whereas 21b was selective for Fe " only. This is due to the oxidation of the phenolic OH groups of calix[4]arene to form phenoxy radicals upon binding with Cu " ion and PET of oxadiazole-derivatized anthracene. The addition of excess Fe " ions to probe 21a and 21b leads to the initial oxidation of calix[4]quinone, and the 3-(9-anthryl)-l,2,4-oxadiazolyl groups are then further oxidized to mono- and bis-oxanthrones. These oxidative transformations of calix[4]arene and anthracene are potentially useful in host guest chemistry. 

Cahxarenes are one of the most interesting scaffolds for molecular recognition in host guest chemistry. The versatihty of calixarene macrocycles includes modification of their upper and lower rims, different conformation behavior, and hydrophobic cavity for the inclusion of guest molecules. The conformational preference of calixarene derivatives depends on the substitution at the upper and lower rim. Numerous attractive receptors have been developed for the detection of various guest molecules. The molecular recognition can be characterized by various... 

Only few investigations have been published on the gas-phase ion chemistry of host-guest complexes of calixarenes. With the advent of ESI mass spectrometry, especially when combined with ion-trap and FT-ICR mass spectrometry, this field has started to be developed. Binding selectivities of alkali metal ions to cahxarene-based crown ethers and open-chain ethers have been studied , the inclusion of neutral guests into the protonated resorcarene-based cavitand hosts by gas-phase ion-molecule reactions with amines have been studied and the formation of capsules from various calixarene tetraether derivatives and alkylammonium ions as ionic guests (notably enabling their detection by mass spectrometry) have been described recently . ... 

The concept of the host-guest complex is a the heart of supramolecular chemistry. It is the formation of this chemical entity that allows us to extract specific chemical species and sense others. Complex formation can be detected through the changes in numerous parameters such as color, fluorescence, NMR spectra, mass spectra, and electrochemical response. The complex can be imaged by... 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

The detection of neutral guests by redox-active hosts will be another challenge for the future of this area of chemistry. We have suggested one way in which this could be achieved via a proposed interference pathway. Time will tell whether these goals are met. 

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