This application claims priority of the Swiss application 64/97, filed Jan. 14, 1997, the disclosure of which is incorporated herein by reference in its entirety.
The invention is related to process monitoring, especially a sensor system for the process monitoring technology, which allows the continuous and selective determination of various measured parameters.
Sensor systems are known and described by the state-of-the-art.
The most well known systems for continuous monitoring are FIA-systems (flow-injection analysis). said systems consist of an extensive fluidics-system having pumps and valves. They make use of traditional analytical procedures using reagents and, occasionally, biosensors or ion-selective electrodes (ISE) at the end of the flow-system as detectors.
The known sensor systems are solely defined and specified by the application of potentiometric or voltammetric (electrochemical determination of the variation in the oxygen partial pressure) or amperometric or optical measuring techniques. The company YSI (2700 SELECT) offers an instrument equipped with biosensors for continuous process monitoring. The sensor elements are inserted into a miniaturized measuring chamber, which is filled with the specimen and evacuated therefrom. The amperometric and voltammetric systems involve biosensors and miniaturized modified electrodes such as xe2x80x9cglassy carbonxe2x80x9d or carbon Pastexe2x80x3graphite electrodes [compare Kalcher, K.; Kauffmann, J.-M.; Wang, J.; Svancara, I.; Vytras, K.; Neuhold, C.; Yang, Z. Electroanalysis 7/1 (1995) 5-22 (Review)]. Publications involve especially arrays (serial switch of a multitude of identical sensors, being identical with respect to their principle of function, on a geometrically defined sensor surface) using miniaturized sensors, so-called solid-state electrodes and ISFETs (ion-selective field-effect transistors) respectively [see Schindler, J. G.,: Schindler, M. M.; Herna, K; Reisinger, E.; Kulmann, U.; Graef, R.; Lange, H. Biomed Tech 36/11 (1991) 271-280, 282-284. (HCA 116:136186.; van der Schoot, B. H.; Jeanneret, S.; van den Berg, A.; de Rooij, N. F. Sensors and Actuators B 15/1-3 (1993) 211-213; 1 Hoffmann, W.; Rapp, R.; Ache, H. J.;
Stolze, D.; Neuhaus, D.; Hofmann, D.; Freywald, K. H. Micro Total Anal. Syst., Proc. xcexc-TAS ""94 Workshop, 1st (1995), in van den Berg, A.; Bergveld, P. (Eds). Dordrecht: Kluwer, Netherlands.
These systems lack on the one hand the ruggedness, especially of the structured layer and of the reference system, on the other hand, the flexibility, to integrate novel developments. Depending on the fabrication process, they can be optimized for a limited selection and limited number of types of selective layers only. Therefore, the applications are considerably restricted. Modifications of micro-structured sensor platforms are rarely advantageous and only if extremely large numbers of pieces can be produced. On the other hand, due to their unsatisfactory ruggedness and long-term stability, miniaturized sensors are primarily marketed as xe2x80x9cone-wayxe2x80x9d sensors (i-STAT). This induces the production of waste of precious materials.
Continuous monitoring systems generally require the possibility of recalibration in order to guarantee a high reliability of the results. Nevertheless calibration-free systems were evaluated [Rumpf, G.; Spichiger, U. E.; Bxc3xchler, H.; Simon, W.; Anal. Sciences 8 (1992) 553-559.]
The selective determination of specific components of a mixture of substances is solely feasible upon applying so-called selectivity principles.
A large number of selectivity principles based on recognition molecules, such as selective ligands, or effective selectivity principles based on partition equilibria, are known [Fluka-catalogue xe2x80x9cSelectophore(trademark)xe2x80x9d, 1996]. The development of some of these selectivity principles such as the magnesium-, nitrite-, heavy metal-selective ligands and layers have been considerably developed [Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Talanta 41/6 (1994) 1001-1005; Spichiger, U. E.; Eugster, R.; Citterio, D.; Li, H.; Schmid, A.; Simon, W. In: S. Golf, D. Dralle, L. Vecchiet (Eds) xe2x80x9cMagnesium 1993xe2x80x9d London: John Libbey and Comp., 1993.]
Substance-selective ligands are dissolved in the bulk of a layer. Said layer or membrane often provides a satisfactory selectivity after a process of carefully optimizing the composition of the bulk medium. If such a layer is implemented into existing sensor elements by cast-in, gluing, photo-polymerization, and other polymerization procedures, a considerable loss in selectivity and sensitivity, respectively, has frequently to be accepted.
The process technique for casting layers and immobilizing of selectivity principles by photo-polymerization considerably limits the range of applications of selective layers and is not exploit the full potential of molecular recognition elements.
Despite of the large number of ligands and recognition elements, only few were incorporated into a layer and tested in analytical applications. Therefore, only single ligands were used in chemical sensors. The application of chemical sensors in analytical instruments has a fixed position only in medical applications since many years. Electrolytes are determined by automated instruments for daily routine, but also in xe2x80x9cbed-sidexe2x80x9d analyzers or peripheral analyzers for the physician""s lab, by using ion-selective sensors. The first instrument, which makes use of optical sensors, is on the market (OPTI, AVL Biosense Corp., Atlanta). The instrument is equipped with xe2x80x9cdisposable cartridgesxe2x80x9d and offers the parameters pO2, pCO2 and pH (based on fluorescence emission technology). In some instruments, a combination of biosensors (voltammetric or amperometric determination of the oxygen partial pressure, e.g. according to Olson, L. et al., Anal. Chim. Acta 224(1) (1989) 31 fe)) for lactate, glucose, peroxide, glutamate, creatinine combined with ion-selective electrodes are implemented. These combinations are based on miniaturized sensors making use of solid-phase transducers or semiconductor elements of type ISFET and ENFET (ion-selective-, enzymatic field effect transistors), where changes in the coulometric properties or the potential, respectively, at the gate are measured. Such a combination was marketed by xe2x80x9ci-STATxe2x80x9d as a disposable sensor. These sensors exhibit the disadvantages of sensors as discussed before such as unsatisfying ruggedness and large amounts of waste.
A company selling since many years an amperometric glucose sensor ExacTech(trademark) for self-monitoring, is MediSens Inc. (Waltham, Mass.) having a subsidiary in Basle; a coresponding instrument for industrial applications is the YSI glucose sensor of Yellow Springs (OH, USA). The sensors used in these instruments display several layers, which eventually provide for the selectivity of the biosensors. Each offered biosensor relies on the same electrode principle, which consists of the determination of the oxygen consumption by applying the traditional electrochemical oxygen electrode.
Since 1988, more than 1000 publications related to glucose biosensors have been published (see Gorton, L., Electroanalysis 7/1 (1995) 23-44). In contrast, optical sensors have hardly been introduced. The WPIDS (World Patent Index No) 95-256069 [34] describes the optical determination of straylight in living tissues, and WPIDS 93-213750 [26], the optical measurement of the blood hemoglobin content.
The objective of the present invention was therefore to provide a measuring system, especially for continuous process monitoring, which avoids the drawbacks of known sensor systems and sensors, and which is notably characterized by its high flexibility and ruggedness.
The intended objective has been achieved by providing a continuous, modular, especially multidimensional, and rugged measuring system, which is ideally operated without reagents, as well as by providing special modular sensing elements and recognition systems suited to such sensor elements.
For the purpose of a better comprehension of the invention, some basic terms shall be defined hereafter:
pluridimensional (multidimensional), modular, continuous measuring system; sensor element; half-cell; measuring module; reference, reference module.
A continuous measuring system based on sensing elements presumes such sensors being reversible or being regenerable within the desired time-period, which means that their signal and the quantity generated, respectively, adapt to the short-term variations of the specific characteristics of the specimen. The measuring system may enable the continuous analysis through batch mode as well as through the flow-through mode. The term xe2x80x9cmodularxe2x80x9d means that single sensors and sensor groups/-types/-procedures are implemented as sensor elements within single modules and may be combined as such in a system. The various sensing elements may be combined in rod-like modules, e.g. for reactor technology. Such modules are notably suitable for the flow-through. Characteristically, suitable dimensions of said modules allow to combine various measuring procedures within the same system. Therefore, the system becomes multidimensional, as the information describing a substance is yielded from orthogonal procedures. The term xe2x80x9cmultidimensionalxe2x80x9d describes the feature of the measuring system, which allows to combine sensor elements and measuring procedures in order to determine several substance in series or parallel. Two procedures, i.e. two different types of modules, may provide independent, i.e. orthogonal, information about the same substance and, therefore, increase the operational safety (e.g. an optical and potentiometric or optical and amperometric module). A module may also contain a half-cell or a reference element of a sensor element or more than a single sensor- or transducer element. Subsequently, where a specification shall be necessary, a module serving to measure will be termed xe2x80x9cmeasuring modulexe2x80x9d independent of its function as a complete element or as a half-cell only, whereas a reference element or reference half-cell will be termed xe2x80x9creference modulexe2x80x9d.
Ruggedness
Ruggedness generally describes the weak dependency of the generated quantity from minor variations of the parameters of the measuring system and the low susceptibility of a measuring technique or a -system to noise, including the long-term stability. The ruggedness may be further improved through automation of the analytical process and by automated recalibration.
Substance, target substance, target compound, analyte; group of target substances, background components.
A target substance or analyte is a substance or group of substances/class of compounds, which are involved in the process of yielding (physico-chemical) information. This information will mostly be used in order to quantify the substance in its medium and to yield a quantitative information, followed by decision making. The chemical sensing element or a group of elements is able to distinguish between the target substance or between groups of substances and the background components and are able to discriminate the background components.
Selective structured layer, selectivity principle.
Within the structured layer, the selectivity principle or recognition step, respectively, and the transducing step (see below) form a subsequent unit. The structured layer may consist of a single or of several layers, including layers having well-defined functions. They are referred to as a selective structured layer.
Selective recognition device; selectivity principle consisting of: recognition step; distribution equilibrium; partition; recognition component; reagent-free.
The recognition step leads to the typical selectivity of each sensor and sensor module, respectively. As a consequence of the selectivity principle, the target substance will be preferred against background substances. The selectivity principle may be based on distribution or partition equilibria and/or typical chemical interactions between target compound and at least one recognition component. The structured layer contains the relevant components of the chemical recognition process. As a rule, the selectivity principle may be reversible or regenerable within a time frame adapted to the continuous measuring system. The combination of these steps may be termed xe2x80x9cselective recognition devicexe2x80x9d.
Transducing Step
The transducing step involves a single step or a number of subsequent steps, which causes the generation of a quantifiable quantity upon inducing or triggering the selective recognition step and the selectivity principle.
Measured Quantity
The generation of the quantity results from the recognition process involving the specific selectivity principle, and from the subsequent transducing process. The measured quantity may be generated as a consequence of the changing partition equilibrium (potentiometry, IR-spectroscopy) or by coupling additives (e.g. an indicator) to the recognition step.
Information
After transducing and amplifying, the measured quantity is transferred to a data recording and processing system which generates the information on the target compound or on a group of target compounds. The measured quantity has to be referenced to a reference quantity in order to increase the reliability of the information. The information may consist in a qualitative but more frequently quantitative information, however it may contain validation and classification criteria.
The above defined terms are used in the following according to said defined meanings.
The modular system for continuous, reagent-free determination of a substance according to the invention is characterized by exhibiting at least two modules of identical or of different type for simultaneous yield of at least two informations, whereby each module contains at least one exchangeable (interchangeable) selective structured layer and a transducing device, whereby the layered structure enables a selective recognition step and allows for continuous monitoring. In a preferred embodiment of the measuring system, different modular types are combined, e.g. at least one ion-selective module and at least one module of another type etc. When combining three modules, one may contain a reference system. The reference system distinguishes itself from the measuring module in that it contains no selectivity principle or the selectivity principle being not active.
Further objects of the present invention are a process for the continuous determination of a substance, the use of the system as well as modules and recognition components that are adapted for the application within a measuring system. Specific embodiments are described in the dependent claims.
Reversible and also rapidly regenerable chemical sensors distinguish themselves through their capacity of continuos determination of a target component, the substance. In order to yield continuous information over a broad spectrum of substances, only FIA (flow injection analysis) has to be referenced as a comparable measuring technique. Contrary to chemical sensors, traditional chemical reactions using reagents are applied in FIA. The procedure distinguishes itself through a highly developed but cost-intensive Fluidics system.
The measuring system according to the invention is, contrary to existing measuring systems, distinguished by maximum flexibility, reagent-free operation and ruggedness against the characteristics of the specimen and against continuous long-term monitoring. Therefore, it may be applied for the monitoring of gases and volatile compounds. Through automation, the advantages according to the invention could be further improved. The flexibility is achieved, by the possibility of implementing basically all of the known sensor types, especially e.g. potentiometric and/or voltammetric measuring techniques may be combined with at least one other measuring technique, e.g. the amperometric and/or the optical one. The combination of such different measuring techniques makes it possible to provide a broad information base on one or several target substances. In addition, the combination of such different measuring techniques for identical target substances offers the greatest possible guarantee for reliable results of the measurements, especially for calibration-poor or -free systems. The selective layered structure, which is characteristic for each module and where both the recognition and transducing step occur, is forming the core of the sensor element. The ruggedness of the sensor element is guaranteed by optimizing the structure of the layer, by referencing, by automation, by the construction of the module and by the arrangement of the modules.
An essential feature of the system is therefore, that the selectivity principle, the selective structured layer, is exchangeable; it may be selected among a multitude of available layers and is, in view of a restricted lifetime, exchangeable. The selectivity principle, the selective layered structure, may be packaged cast on a support, and marketed in different sizes. Such a support may consist in the transducer simultaneously (e.g. planar wave-guide, planar or tube-like electrode, optical fiber, resonator, SAW-chip). The exchangeable selectivity principle may consequently be denoted as a chemically-active chip. The chemically active chip or chemochip is an important element of the measuring system. In addition, relevant features are that not only different sensors, but also various types of sensors may be connected as single modules in series or in any other arrangement. The arrangement in series is preferred in continuous flow monitoring (optimum throughput of specimens). The parallel arrangement is preferred for the determination of materials to be investigated that needs to be differently conditioned (pH, ionic strength, temperature etc.).
A module contains all the elements necessary as the basis of the selectivity principle, the molecular recognition step, the transducing step, the generation of the measured quantity and the transformation. As a consequence of its ruggedness, it is applicable repeatedly and continuously over an ex-tended-period of time. Owing to the special mode of implementation, the selectivity principle, the analyte-selective layer, may be calibrated with access from the side of the transducing element e.g. for measurements under sterile or aseptic conditions. This operation is feasible, as the transducing element is not necessarily inseparably connected to the structured layer.
The single module works reagent-free. Reagent-free operation means, that no assisting component is used as a reagent, which enables the chemical recognition step in addition to the selectivity principle. The modules according to the invention differ considerably from the FIA technique and similar continuous monitoring systems. Buffering, addition of a co-substrate or further conditioning agents are not defined as reagents and are, therefore, not excluded by the term reagent-free handling.
A single module may also consist of a single half-cell, another one can be the reference system, which is equally rugged and correspondingly long-term stable due to its high (buffer)-capacity. The combination of various autonomic sensor modules based on various function principles to one measuring system is named a multidimensional or pluridimensional system. Each module can be independent from another and contribute in an optimal way to the final information on the substance/s by an orthogonal contribution of information. The benefit for the user consists in the fact that he is not supposed to select a measuring technique, but he only decides on the analytical parameters and the target substances, respectively, he needs information and on the analytical performance that must be fulfilled. He may delegate to the supplier the decision concerning which measuring techniques, or sensor modules, to be combined under said circumstances. The same is valid for the decision on the number of measuring techniques and modules that have to be combined in order to achieve the given specifications and the highest possible operational security.
Experiences with various sensor technologies show that different principles exhibit different features, which are more or less suitable, but no single basic technology can cover a range of different applications with optimum performance. For instance, it was found that the optical measuring technique is especially attractive for the monitoring of dissolved oxygen (oxygen partial pressure in solution), and therefore is especially attractive to be used in modular sensors and sensor systems according to the present invention, owing to its simplicity and the quick response behavior. The optical measuring technique is therefore a valid alternative to electrochemical sensors. The optical measuring technique is in addition feasible for measurements of the sodium-, potassium- and calcium-, nitrite- and chloride activity as well as several uncharged target components and substrates such as alcohols, amines, glucose, humidity, lactate, etc. gases such as NOx, SO2, NH3 etc within the specimen directly.
Particularly suitable calibration-free assays are e.g. the decay-time (luminescence decay-time) according to Draxler, S.; Lippitsch, M. E.; Klimant, I.; Kraus, H.; Wolfbeis, O.; J. Phys. Chem. 99/10 (1995) 3162-3167, and the symmetric measuring technique according to Rumpf, G.; Spichiger, U. E.; Bxc3xchler, H.; Simon, W.; Anal. Sciences 8 (1992) 553-559. The symmetric maesuring techniques e.g. according to Haase, A. E.; xe2x80x9cInvestigation of the interaction between iono-selective liquid membranes and measured quantity in view of the continues monitoring of cations in bloodxe2x80x9d (Untersuchung der Wechselwirkung zwischen ionenselektiven Flxc3xcssigmembranen und Messgut im Hinblick auf die kontinuierliche Erfassung von Kationen im Blut). Diss ETH Nr. 10453 (1993) is adapted to prepare reference materials for the quality assessment of electrochemical determinations of the active molality of ions. Such reference materials/quality control specimens are not available world-wide.
It has now been found that through such a symmetric measuring technique or by using symmetric membranes showing optimum compatibility with a specimen, respectively, and by using a symmetric arrangement of the measuring cell, reference materials for determining the molal activity of ions e.g. for quality assessment can be prepared. Reference materials are e.g. aqueous solutions of lyopilized blood plasma or -serum.
Especially for measurements in the pharmaceutical branch, in medical care and food technology, it may be necessary or recommended to sterilize the modules. In this respect, current state-of-the-art techniques may be cited, e.g. sterilization is referred to Haase, A. E.; xe2x80x9cInvestigation of the interaction between iono-selective liquid membranes and measured quantity in view of the continues monitoring of cations in bloodxe2x80x9d (Untersuchung der Wechselwirkung zwischen ionenselektiven Flxc3xcssigmembranen und Messgut im Hinblick auf die kontinuierliche Erfassung von Kationen im Blut). Diss ETH Nr. 10453 (1993). X-ray sterilization of packed materials is feasible. In addition, it is necessary to avoid the contamination of measured specimens with toxic components. The toxicity of ISEs is described e.g. in Haase, A. E.; xe2x80x9cInvestigation of the interaction between iono-selective liquid membranes and measured quantity in view of the continues monitoring of cations in bloodxe2x80x9d (Untersuchung der Wechselwirkung zwischen ionenselektiven Flxc3xcssigmembranen und Messgut im Hinblick auf die kontinuierliche Erfassung von Kationen im Blut), Diss ETH Nr. 10453 (1993). Amperometric systems based on the mediator TTF/TCNQ (tetrathiafulvalene-p-tetracyanoquinodimethane) are generally viewed as being toxicologically harmless.