Membrane covered sensors

For completeness it should be mentioned that some of the theoretical conclusions for SECMIT are analogous to earlier treatments for the transient and steady-state response for a membrane-covered inlaid disk UME, which was investigated for the development of microscale Clark oxygen sensors [62-65]. An analytical solution for the steady-state diffusion-limited problem has also been proposed [66,67]. 

An alternative approach places the Pt electrodes directly on a flexible polymer carrier [80]. Eh Lilly developed a three-electrode transducing system based on a polyimide carrier with electroadsorbed enzyme and a highly oxygen-permeable membrane covering the sensor. Such a system was tested in vivo and published results seem encouraging although the company cancelled the project. 

While most gas sensors rely on potentiometric detection, the important oxygen probe is based on amperometric measurements. In particular, membrane-covered oxygen probes based on the design of Clark et al. (105) have found acceptance for many applications. The sensor is based on a pair of electrodes immersed in an electrolyte solution and separated from the test solution by a gas-permeable hydrophobic membrane (Fig. 6.22). The membrane is usually... 

Gas pressures in vacuum applications are usually either recorded via membrane transducers, systems that monitor the gas density via partial ionisation of the gas or sensors that make use of the fact that the thermal conductivity or diffusivity of a gas is pressure dependent. The first type of transducer is sensitive to the total gas pressure while the other methods yield gas dependent signals. In terms of application properties such as the response time of the sensor, the sensitivity and the pressure range that the sensor covers are important technical specifications. The response of a membrane pressure sensor to a step-like pressure change is essentially an exponential function characterized by a relaxation time r for a MKS transducer, type Baratron 220 [1], r was determined to be 0.227 s (see Fig. 1), the actual pressure and the value as recorded by the transducer therefore do not match within the error bars given for the sensor until more than a second passed. 

Entrapment methods of immobilization of bioreceptors utilized the lattice structure of particular base material. They include such methods as entrapment behind the membrane, covering the active surface of biosensors, entrapment within a self-assembled monolayers on the biosensor surface, as well as on freestanding or supported bilayer lipid membranes, and also entrapment within a polymeric matrix membranes, or within bulk material of sensor. All these mentioned methods are widely employed in design of biosensors. The essential condition of success of these methods of immobilization is preservation of sufficient mobility of substrate or products of biochemical reaction, involved in sensing mechanism, as matrix may act as a barrier to mass transfer with significant implications for... 

The Fiber optic immnnosensors are the second major generation of biosensors. Many of the biosensors are a mini version of the classical spectrophotometry. The most freqnently nsed is the fiber optic chain or membrane, covered with a biological element. The optical biosensors are only optical sensors where the element to be detected is of biological origin. This provides greater specificity for the analyte. 

Membrane-covered optochemical sensors (optodes) with O2 sensitive or pH sensitive fluorescence indicators (e.g. pyrene butyric acid or hydroxypyrene trisulfonic acid) have been coupled with different enzyme reactions, such as the conversion of glucose, lactate, ethanol, or xanthine, and with antigen-antibody couples (Opitz and Lubbers, 1987). 

Colorimetric and fluorimetric NH3 sensors contain mixtures of pH indicators having suitable dissociation constants at the tip of the fiber bundle. The measuring solution is separated from this indicator layer by an NH3 gas-permeable membrane covered by an immobilized de-aminating enzyme, e.g. urease (Wolfbeis, 1987 Arnold, 1987). The fluorimetric indication of NADH has been used in optical biosensors for lactate, pyruvate, and ethanol, where the respective dehydrogenase is immobilized at the tip of an optical NADH sensor (Arnold, 1987 Wangsa and Arnold, 1988). 

At low substrate concentration the sensitivity of kinetically controlled sensors increases linearly with Umax. Consequently, the application of several identical enzyme layers one over the other enhances the measuring signal. When the amount of enzyme becomes sufficiently high as to provide complete substrate conversion the system passes over to diffusion control. Under these conditions a decrease of the diffusion resistance by decreasing the layer thickness results in an increased sensitivity. Nevertheless, a membrane-covered enzyme electrode is only 10 to 50% as sensitive as a bare electrode for an analogous electrode-active substance. 

The polyethylene membrane covered an electrochemical enzymatic biosensor. The construction scheme of the sensor is displayed in Fig. 25. The electrochemical basic electrodes consist of a platinum anode and a silver-silver chloride cathode. A thermally sensitive layer of glucose oxidase was localized directly on the anode surface and was covered by a 30-pm-thick hydrophobic polyethylene membrane. The biosensor for the detection of the glucose concentration is based on an enzyme (glucose oxidase)-cat-alyzed reaction of the analyte (glucose) and the subsequent electrochemical detection of a reaction product (electron current). A stoichiometric surplus of the coreactant oxygen has to be guaranteed. This is realized by the polyethylene membrane (membrane with analyte door ) controlling the glu-... 

The principle of the fiber-optic pH sensor led Vurek et al. [133] to devise a CO2 sensor. Instead of coupling the pH indicator dye to an insoluble polymer, a simple isotonic solution of salt, hydrogencarbonate, and dye was used, which was covered with a C02-permeable silicone-rubber membrane. The sensor s performance was demonstrated in vivo. Similarly, a fluorescein-based C02-sensitive system was reported by Hirschfeld et al. [134]. 

This article will concern itself only with devices that involve a chemical or biochemical transduction mechanism to generate the analytical information, with the processes occurring in a membrane or layer attached to the probe in such a manner that the analytical information can be accessed electronically from the outside world. This covers sensors that are for single use and for continuous monitoring because the basic chemistry and sensor configuration used are very similar for a particular application. Hence, the article does not cover techniques such as open-cell Fourier transform infrared or remote fiber spectroscopy, which can be used to sense the chemical nature of the environment without involving the use of a bona fide sensor. 

Appropriate protective membrane covering the electrode or/and modification of the electrode surface permit the operation of voltammetric sensors independently of the solution variables. This feature plays a critical role especially in reactions occurring in suspensions, emulsions or in biofluids (cf. paragraph 7.3). The application of voltammetry and polarography is discussed in monographs such as [3] or [140]. The problems of electrochemical detection techniques in clinical, biotechnological and environmental applications for the biosciences are dealt with in [141]. 

Membrane covered amperometric sensors are also suitable for determining gases. If a disc-shaped working electrode radius is less than about 90 membrane thicknesses, then the edge current causes more than 1% exhibition of the one-dimensional (1-D, planar) component of the current, which is proportional to the electrode area [122]. The 1-D mass flux, JiD, can be calculated from ... 

To date, the majority of enzyme-based potentiometric sensors do not involve detection with an ionophore-doped selective membrane and fall outside of the scope of this chapter. The same is also true for most Severinghaus-type gas sensors, where a gas-permeable membrane covers an inner solution in which the gaseous analyte is determined with an ISE. Most Severinghaus-type electrodes use a pH-sensitive glass electrode to monitor the pH of this inner filling solution. However, ammonia has been detected indirectly with an ammonium-selective ionophore-based ISEs upon protonation in that inner solution, and the use of other ionophore-based ISEs for the more selective detection both in enzyme-based and Severinghaus-type ISEs is readily conceivable. 

---