H.M. Widmer

Stacked modules for micro flow systems in chemical analysis: concept and studies using an enlarged model

Abstract A valveless flow-injection-analysis system incorporating a series of functional modules, provided by a set of flat elements that are stacked vertically to form a series of internal flow streams, has been developed. A dye is used for sample introduction and injection and the system is coupled to an optical detector. The samples undergo a minimal amount of dilution, retaining 96–97% of their original concentration. Good run-to-run reproducibility is achieved, as indicated by measured standard deviations of less than 1%. A calibration curve has been generated and produces a linear fit to the data. The use of micromachining techniques to produce the set of flat elements and therefore to manufacture large quantities of identical flow-injection systems is discussed.

A Silicon Flow Cell for Optical Detection in Miniaturized Total Chemical Analysis Systems

Abstract The applicability of silicon micromachining to the fabrication of a small-volume flow cell for UV-visible absorption detection is demonstrated. With volumes ranging from 1 to 100 nl and lengths of 1 and 5 mm, this type of cell has a long path length relative to its volume. Light is transported through the cell by means of a series of reflections, so that the optical path length may be increased to values beyond the actual cell length, depending on the input angle of the light. Preliminary experiments using a 1 mm, 15 nl cell to measure dye-containing solutions demonstrate an application of multireflection to the measurement of absorbance.

An integrated silicon thermopile as biosensor for the thermal monitoring of glucose, urea and penicillin☆

A new kind of calorimetric biosensor for the measurement of the heat (molar enthalpy change) of enzymatic reactions is presented. The device operates according to the Seebeck effect, the same principle on which thermocouples are based. The thermopile used in this work consists of an array of p-type silicon/aluminium strips integrated on a thin silicon membrane (5 microns). Its sensitivity is about 1 V output voltage per watt of heating power, corresponding to a temperature resolution in the order of 10(-5) K and a heating power resolution of some tenths of a mu W in the flow system used. Furthermore, this performance is obtained without any control of external temperature because of the high common-mode thermal noise rejection ratio of the thermopile. The universal technique of calorimetry combined with the specificity of biochemical reactions makes this biosensor very versatile, with a broad range of possible applications. Glucose oxidase together with catalase for the determination of glucose, urease and penicillinase for the monitoring of urea and penicillin G, respectively, were immobilized directly onto the back side of the thermopile. The sensor was operated in conjunction with flow injection analysis which, in addition to its traditional advantages, allows preconditioning of the samples. Thus, artefacts due to mixing effects were suppressed and interference caused by differences in ionic strength between sample and carrier was strongly decreased. Detection limits between 1 and 2 mM were reported in the flow injection conditions described.

Fiber-optic Atrazine immunosensor☆

Abstract Fiber-optic sensors based on the excitation of luminescent chromophores by the evanescent field associated with light guiding in an optical fiber can be used for highly sensitive and selective biochemical affinity assays. Due to the small penetration depth of the evanescent field into the medrium, the generation and detection of luminescence are restricted to the close proximity of the fiber core, i.e., fluorophores in solution beyond the evanescent field will not contribute to the emission signal. Evanescent wave sensors allow the binding of fluorophores to the sensor surface to be monitored in real-time mode. These advantages make this approach especially useful for the determination of substances in complex media, such as blood, river water or soil extracts. An evanescent-wave fiber-optic immunosensor for the detection of the herbicide Atrazine has been developed. In the competitive assay format chosen, fluorescein-labeled and nonlabeled Atrazine in solution compete for the binding sites of anti-Atrazine antibodies immobilized on the surface of the optical fiber. A signal reproducibility of better than 5% within the working range of the sensor (0.5–200 nM Atrazine concentration) is achieved. The sensor performance in complex media has been investigated using samples of surface water and soil extracts.

Fiber optic sensor for oxygen determination in liquids

Abstract A fiber optic sensor for the determination of oxygen in liquids is presented. Potential applications range from environmental analytics to medical diagnostics and process control. The sensing principle is based on dynamic luminescence quenching by oxygen. A ruthenium complex used as oxygen-sensitive dye is immobilized on an optical fiber by adsorption and subsequent coating with a membrane. Excitation of the Ru complex is performed both by the evanescent field associated with light guided in the optical fiber and by direct irradiation, dependent on the refractive index profile of the fiber/membrane/medium system. The emitted light is collected by the same fiber. The temperature-controlled sensor is exposed to a liquid stream of defined oxygen concentration. Oxygen is detectable in the range 0–800 Torr, with a resolution of 2 Torr in the 0–100 Torr range and of 2% for oxygen partial pressures above 100 Torr. Response times are of the order of 30 s. Generally, sensor signals are affected by sample medium. Optimized sensors exhibit a good performance, with no effects of protein adsorption or background fluorescence, even in serum.

Laser-based refractive-index detection for capillary electrophoresis: ray-tracing interference theory

The fringe pattern observed in a far field after a laser beam illuminates a fused silica capillary immersed in a refractive-index matching material and filled with an analyte fluid is exploited to develop a sensitive optical detector for capillary chemical analysis. The inner capillary interface splits the laser beam into a reflected beam fan and a refracted beam fan, which, on overlapping in the far field, lead to interferences. The intensity and the position of the fringes for capillaries with 250 microm >/= i.d. (inner diameter) >/= 25 microm are well reproduced by the presented model. The calculation predicts the fringe pattern for various beam/i.d. geometric configurations and is used to optimize the performance of the nanoliter-picoliter refractive-index on-column detection studied. It is found that the best contrast corresponds to a capillary that is illuminated with a beam waist of omega(0) ~ i.d./12, which is off-center focused with an offset of s ~ i.d./2. For a given interference pattern, the fringes that are found to be more sensitive to Deltan are those that appear near the optical axis but still retain high intensity and contrast. The sensitivity increases approximately linearly with the fringe number, and the maximal fringe number increases proportionally with the i.d.

Sensitivity and selectivity of surface acoustic wave sensors for organic solvent vapour detection

Abstract Surface acoustic wave (SAW) devices have been coated with 20 organic polymers of different chemical nature. Their sensitivities, selectivities and response times towards gaseous organic solvents such as toluene, acetone, ethanol, methanol and dichloromethane in dry air have been determined. The best sensitivities and selectivities are obtained for toluene vapours. In order to compensate for the lack of selectivity, a pattern-recognition technique (partial least-squares regression, PLS) is applied to sensor responses obtained from ternary mixtures of analytes. The predictive properties of the model are discussed.

Flow injection analysis and in-line biosensors for bioprocess control: a comparison

Miniaturization will unify the different approaches chosen for the application of biosensors in bioprocess control. The most versatile system, which in our opinion is flow injection analysis will be the method of choice for the introduction of biosensors in bioprocess control. A lot of experience will be gained for the future development of miniaturized total chemical analysis systems.

A model system for the development of an optical biosensor based on lipid membranes and membrane-bound receptors

Abstract The reproducible and easy immobilization of receptors on sensor surfaces is a prerequisite for the development of receptor-based fibre optic biosensors. Using a fused silica fiber as the transducer, binding processes of luminescently labeled ligands can be monitored by evanscent wave sensor (EWS) technology. The vesicle fusion technique was chosen for the immobilization of membrane-bound receptors in order to preserve their binding specificity and activity, by embedding them in an environment similar to a lipid bilayer. The results of initial studies of repetitive cycles of lipid layer deposition and removal, indicating good reproducibility of lipid layer formation on the fiber, are presented. Using the binding of fluorescently labeled streptavidin to a biotinylated lipid layer as a model system for receptor-ligand interaction, good sensitivity, combined with low non-specific binding were observed.

Implementing chemical sensors in industry: novel approaches

Abstract In the chemical industry, the need for real-time process and environmental control has elicited a great interest in chemical sensors. However, their successful introduction has not been achieved in spite of promising results at university laboratories. Ways of filling this gap will be demonstrated here. In many industrial applications, the measurand must be detected in the midst of many interfering substances which can be present in large excess. Because applications and their associated matrices vary widely, the sensing principle must be incorporated into a total analysis system (TAS), which will perform sample pretreatment, protect the sensor from aggressive matrices, and even provide a preliminary separation. This modular system is configured according to its exact application. Realized state-of-the-art TASs for process and environmental analytical applications as well as novel systems now containing chemical sensors will be presented.