Ursula E. Spichiger-Keller

Ion- and substrate-selective optode membranes and optical detection modes

Abstract A brief overview covering the state-of-the-art of ion-selective bulk optode membranes and their characteristics is given in the first part. Detection principles involving attenuated total reflection (ATR) measurements and measurements of refractive-index changes are reviewed. In contrast to ion-selective optodes where an indicator is coupled to the ion-exchange reaction, chemical host-guest interactions for neutral analytes, where the chemically reactive ligand is chromogenic, are discussed. In the second part, an alcohol-selective optode membrane and its application in continuous ethanol monitoring is presented. The ethanol-sensitive membrane is used to monitor ethanol in the vapour phase of a bioreactor. Data derived from determining ethanol using the optode membrane are compared to ethanol concentrations in the liquid phase of the reactor filling by using a reference method (distillation and density measurements) simultaneously. The data are analysed by first-order linear regression (R2 = 0.935). The repeated use of a pair of optode membranes for six different fermentation runs shows, first, the reversible response behaviour of the membranes; secondly, reproducible starting values of the membrane absorbance at 305 nm wavelength; thirdly, a lifetime of the membranes of more than six days when analysing the vapour phase; fourthly, for each new batch of fermentation, a different but typical production rate of ethanol, which mirrored the production rate of the yeast cells, is monitored. This might well be the first demonstration of the feasibility of using a chemical sensor for continuous monitoring in biotechnology. References for preliminary studies on host-guest interactions of diols and oligools, such as catechol, fructose and glucose, with phenylboronic acid derivatives are provided.

Ionophores, ligands and reactands

Abstract The reversibility of a host–guest reaction is known to follow a thermodynamic description of the equilibration process. This thermodynamic description distinguishes the enthalpic and the entropic term. Electrostatic interactions between hard ions and a ligand have been shown to be mainly entropy-stabilized which means that the predominant conformer of the ion-ligand complex is more stable than the predominant conformer of the free solubilized ligand. On the other hand, in reversible chemical reactions based on covalent interactions, much higher potential energies are involved. These processes are assumed to be enthalpy-driven. Subsequently, the selectivity of both mechanisms, that involving ligands and that involving reactands, must be governed by fundamentally different rules. Electrostatic interactions are sensitive to the lipophilicity or polarity of a medium, whereas for reactands, it is primarily the partition equilibrium of the target analyte that is ruled by the solvent. The reactivity is less influenced by the solvent. Some aspects in the development of ion-selective electrodes and their thermodynamics are resumed. Analogous to sensors based on ligands or ionophores, the reversible chemical recognition of alcohols and amines by reactands and chromoreactands is described and compared to ion-selective complexation.

A highly sensitive NO2-selective optode membrane

Abstract A new NO 2 -sensitive polymeric optode membrane is introduced. It takes advantage of the reactivity of NO 2 with an aquacyanocobalt(III)-cobyrinate derivative. The detection mechanism leads to a protonation of the Nile Blue derivative ETH 5418, which changes its visible spectrum thereby. The polymer layer also contains the aquacyanocobalt(III)-cobyrinate derivative, which significantly reduces the response time and improves the operational lifetime of the sensor compared to a blank membrane. No cross-sensitivity to NO, CO, CO 2 and only a small interference by SO 2 was observed. A large variety of polymers and plasticizers were tested in order to achieve resistance to high temperature and to improve the lifetime of the sensor. Until now, the plasticized poly(vinylchloride) membranes still remain the material of choice in view of a good compromise between stability, response time and solubility of components.

Optical quantification of sodium, potassium, and calcium ions in diluted human plasma based on ion-selective liquid membranes

With the presented ion-selective optode membranes total ion concentrations of sodium, potassium, and calcium are measured reversibly with the same selectivity as in potentiometric systems. The optically sensing membranes combine the well established neutral carriers together with neutral so-called chromoionophores which change their absorption spectra in the VIS region upon complexation of hydrogen ions in the same membrane phase. Membranes are mounted in an optical flow-through cell with a volume of 300 (mu) l. All measurements are referred to a pair of blank-membranes in a reference cell to compensate for interferences induced by the sample. The response time is in the range of seconds. A good correlation to reference methods is achieved.

Chemically selective optode membranes and optical detection modes

The principle of optodes has been shown to be efficient. Optodes are based on conventional ion-selective liquid membranes coupled with an indicator as an optical transducing agent. Calcium, sodium, potassium, and ammonium were assayed in real samples as well as the biological relevant trace elements zinc and lead in aqueous solutions. However, some kind of drawbacks were observed which can be eliminated by an improved optical measuring technique. A considerable gain in sensitivity of the optical measurements is achieved for clinical analysis of total potassium concentrations in plasma by the evanescent wave technique (ATR). For the sodium optode the analytical error is shifted towards the allowable range. Furthermore, the adsorption of biological sample components at the surface of the PVC- membrane does not influence the optical signal.

Micellar bio-optode membranes

Ion-selective bulk optode membranes have been combined with enzymatic reactions for the determination of neutral analytes in clinical applications. A new development in membrane technology has been introduced, using reverse micelles to entrap the biocatalyst in bulk polymeric membranes. The use of such reverse micellar systems allows the design of a single layer biosensor where the recognition process as well as the chemical transduction into an optical signal take place in the same sensing layer. Urea-sensitive micellar bio-optode membranes have been realized; dynamic range, response behavior, long-term stability as well as the operational lifetime are discussed.