Kurt Seiler

Theoretical aspects of bulk optode membranes

Abstract The principles of optodes, which are based on a reversible mass transfer of analyte from the sample into the bulk of the sensing layer, have been elucidated in detail. It is demonstrated that the sensor response of many different types of bulk optode membranes described in the literature can be related to the same formalism. A universal description is given that is independent of the type of the optical transduction process and can be based on absorption (transmission or attenuated total reflection mode) or on fluorescence. The theoretical performance of bulk optodes based on plasticized PVC is described in terms of selectivity, response time, dynamic measuring range, sensitivity and lifetime, and whatever a comparison with experimental data is given. The geometrical arrangement and optimization of the membrane composition for a set of specific requirements are discussed in detail. By means of the spin-on technique, a simple manufacture of very thin optode membranes is described. With this technique, highly homogeneous membranes of thickness from 0.5 to to μm can be produced in only a few seconds.

Carriers for chemical sensors: Design features of optical sensors (optodes) based on selective chromoionophores

Egstract-Macrocyclic and non-macrocyclic ion-selective carriers containing structural elements that heavily change their optical properties in the UV / VIS region upon ion complexation have been introduced earlier as chromoionophores and fluoroionophores, respectively. Here we present different membrane morphologies resulting in a successful design of corresponding sensor membranes with an optical transduction of chemical signals, i.e. for optical measurements of concentrations resp. activities. Workable examples of novel optodes based on chromoionophores are presented, and their response behavior is discussed in more detail.

Reversible optical sensing membrane for the determination of chloride in serum

Abstract An optical sensor membrane for the determination of chloride is described. It incorporated trioctyltin chloride as an ionophore and a chromoinophore in a plasticized PVC matrix. The concentration of chloride is indicated by co-extraction of chloride and hydrogen ions into the bulk of the membrane yielding large absorbance changes which can easily be measured in the visible spectral range. The performance of this sensor and its application to chloride determinations in human plasma are reported.

Characterization of an optode membrane for zinc(II) incorporating a lipophilized analogue of the dye 4-(2-pyridylazo)resorcinol

A chemical optical sensing system (optode) for ZnII based on a lipophilized metal ion indicator dye [1-octadecyloxy-4-(2-pyridylazo)resorcinol; ETH 2464] dissolved in a plasticized poly(vinyl chloride) membrane has been developed. In the measuring range from 1 × 10–6 to 3 × 10–3 mol dm–3 Zn2+(pH 4.8), the absorbance response shows a good correlation with the theoretically derived formulae. The optode membranes were found to reach 95% of the final signal within 5 min (t95). When exposed to a continuous sample flow (1 ml min–1), the optode membrane did not lose a perceptible amount of the indicator over a period of 10 h. The selectivity of the optode membrane for the response towards Zn2+ ions in pH-buffered solutions is described.

DEVELOPMENT OF AN ETHANOL-SELECTIVE OPTODE MEMBRANE BASED ON A REVERSIBLE CHEMICAL RECOGNITION PROCESS

Abstract Lipophilic trifluoroacetophenone derivatives incorporated in plasticized PVC membranes are able to selectively extract water and alcohols from the sample solution into the organic membrane phase, reversibly forming hydrates and hemi-acetals, respectively. Since this is accompanied by a change in the absorption spectrum of the acetophenone isologue, the chemical recognition process can directly be translated into an optical signal. With N -acetyl- N -dodecyl-4-trifluoroacetylaniline (ETH 6022) as the electrically neutral, lipophilized carrier ethanol can be determined from 0.5 to 35% (v/v) in aqueous solutions. The calibration curve for different ethanol-water mixtures shows a good correlation with the mathematically derived formalism and thus confirms the theoretically expected behavior. Besides high reproducibility of the optical signals, very short response times of less than 30 s were realized. The optode membrane presented exhibits a preference for ethanol compared to water by a factor greater than 11. The selectivities for several primary alcohols, such as methanol, ethanol, 1-propanol and 1- n -butanol, are comparable, but isopropanol and tert. -butanol are rejected by a factor of about 10. The alcohol concentration in different beverages was determined to evaluate the reliability of the system. The values obtained for wine, beer and different spirits show an excellent correlation with those obtained by a conventional approach involving distillation and density measurements. A residual standard deviation of ± 0.27% (v/v) over the 0.7–40% (v/v) range was found.

Enzymatic biosensor for urea based on an ammonium ion-selective bulk optode membrane

Abstract A urea biosensor with the enzyme urease entrapped in a photo-cross-linked polymer is described. The enzyme layer is placed on top of an NH + 4 -selective optode membrane the ionophore nonactin and a hydrogen ion-selective chromoionophore. The immobilization was carried out under very mild conditions, so that the properties of the optode membrane remained unchanged. The short-term repeatability, dynamic response, selectivity and the long-term stability of the sensor and the influence of the buffer concentration are discussed.

NEW SENSORS BASED ON CARRIER MEMBRANE SYSTEMS: THEORY AND PRACTICE

ABSTRACT Ion-selective carriers containing structural elements that heavily change their optical properties in the UV / VIS region upon ion complexation have been introduced earlier as chromoionophores and fluoroionophores, respectively, and corresponding membrane sensors for an optical transduction of chemical signals (optodes for the measurement of concentrations resp. activities) have been unraveled very recently. In the present contribution, we set forth and discuss in detail the basic principles, the capabilities of measurements, the theoretical vs. experimental response curves, the selectivity behavior, the absorbance range, the response time, the life time, and the geometrical aspects of novel optodes based on chromoionophores. Whenever possible, a comparison to corresponding ion-selective electrodes is given.

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.