Alkylated a, ( 3 and y cyclodextrins have been incorporated in ion-selective electrodes as neutral ionophores and used for the detection of the tricyclic antidepressants imipramine, desipramine and trimipramine as their hydrochloride salts. Nemstian response and excellent potentiometric limits of detection were observed with each of the three alkylated cyclodextrins examined. Interference from 'serum' levels of Na', K+ and CaZf ions was very low. Interference from organic molecules such as glycine, L-histidine and nicotinamide was also low in most cases (see text for exceptions). Electrodes suitable for amperoinetric measurements were assembled by depositing alkylated cyclodextrins on screen printed electrode substrates. Inclusion of the tricyclic antidepressants in the cyclodextrin cavity aided by controlled potential electrolysis, allowed subnanomolar levels of the analyte to be monitored by square wave voltammetry. Mechanistic studies indicated that, by preconditioning the modified electrodes at approximately +0.6 V in phosphate buffer, the appropriate radical cation was formed, which, on further conditioning at approximately -0.2 V rapidly formed the dimer which possessed a lower oxidation potential of approximately +0.18 V. Monitoring the redox wave at 0.18 V afforded a powerful analytical handle for trace analysis of these tricyclic antidepressants. This redox wave is observable only in the presence of aqueous ghosphate which may be attributed to the formation of a hydrogen-bonded surface ion-pair including the substrate and a phosphate counterion.
The selective binding of charge diffuse alkyl and arylammonium ions relies upon multiple weak interactions with a complementary synthetic receptor. Using appropriately sized lipophilic cyclodextrin derivatives, the chemoselective binding of alkylammonium ions such as dopamine, acetyl choline, guanidine, and long chain cationic surfactants may be achieved allowing their selective detection by either potentiometric or amperometric methods of analysis. Enantioselectivity in the binding of chiral phydroxyarylammonium ions, such as propranolol, allows chiral sensors to be developed. The selective detection of various clinically important analytes, such as imipramine, lignocaine and creatinine has also been studied.The selective binding of charge dense metal ions is a relatively mature area of research that relies upon matching the donor atom, coordination number and geometric preferences of the cation with the structure of the ligand. For 'onium ions, such as R-NH3+, NMe4+ and the guanidinium cation, (NH2)3C+, the positive charge is delocalised over several atoms, giving a charge diffuse species. With the tetramethylammonium ion, for example, an approximate MNDO calculation indicates that 72% of the positive charge is distributed over the 12 peripheral hydrogen atoms. In seeking to bind such charge diffuse cations, therefore, particular attention must be paid to size complementarity and binding interactions to a synthetic receptor must involve multiple hydrogen bonding. Most of the published work relating to the complexation of tetraalkylammonium ions has tended to focus upon anionic cavitands in which electrostatic attraction probably dominates the binding (1,2,3). With the exception of certain nelectron rich macrocyclic ligands (4,5), most of the neutral ionophores that have been developed to bind ammonium ions only complex primary ammonium ions. Chiral crown ethers, for example, bind certain arylammonium ions enantioselectively (6,7,8), but the chemoselectivity is modest (severe interference from Na+ or K+) and N-alkylation compromises the N-H ... 0 hydrogen bonding. LiDoDhilic CyclodextrinsWe were attracted by the properties of lipophilic cyclodextrin derivatives (9,lO) for the electroanalysis of a broad spectrum of 'onium ions. Neutral, lipophilic ionophores are preferred for potentiometric analysis so that cation and pH interference may be minimised. Such compounds are finding widespread application in chiral GC and HPLC analysis, and the parent cyclodextrins form well defined 1 : 1 inclusion complexes with a variety of size-matched aryl guest species. Our initial work focused on the evaluation of peralkylated cyclodextrin derivatives such as 1,2,3 and 4 as enantioselective ionophores (1 1,12). These derivatives were characterised -paying particular attention to the precise degree of alkylation -by NMR, electrospray mass spectrometric and reductive depolymerisation methods. It is interesting to note that per-0-ethyl-P-cyclodextrin is almost totally insoluble in water but is perfectly soluble in organic ...
Abstract2,3,6-Trioctyl and 2,6-didodecyl-~-cyclodextrins have been incorporated in ion-selective electrodes as neutral ionophores for the detection of the local anesthetics procaine, prilocaine, lidocaine and bupivacaine hydrochlorides. Nernstian response and excellent limits of detection were observed (-log [C] 25.0). Interference from 'serum' levels of sodium, potassium and calcium (1 50.0 mmol dm-3 Na+, 1.26 mmol dm-3 Ca2+ 4.30mm0ldrn-~ K+ ), was negligible (-log K r ' ) (overall) 24.0). The major organic interferents were molecules of similar size and structure which caused reduced slope and drift in baseline potentials at 10 mmol dm-3 levels of concentrations. However, the interferences were substantially reduced when the interferent concentration was reduced by a factor of 10. Nernstian response was maintained in the presence of bovine serum albumin (BSA), serum (HS) and aI acid glycoprotein (AAG). A polyurethane, (Tecoflex SG80) membrane matrix was more biocompatible when compared to PVC as evidenced by lower @ (internal) shifts with the former.
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