Immobilization for sensor

Table I. Example of Immobilization for Sensor Applications (cont)... 

Immobilized Enzymes. The immobilized enzyme electrode is the most common immobilized biopolymer sensor, consisting of a thin layer of enzyme immobilized on the surface of an electrochemical sensor as shown in Figure 6. The enzyme catalyzes a reaction that converts the target substrate into a product that is detected electrochemicaHy. The advantages of immobilized enzyme electrodes include minimal pretreatment of the sample matrix, small sample volume, and the recovery of the enzyme for repeated use (49). Several reviews and books have been pubHshed on immobilized enzyme electrodes (50—52). 

We showed that these mesoporous silica materials, with variable pore sizes and susceptible surface areas for functionalization, can be utilized as good separation devices and immobilization for biomolecules, where the ones are sequestered and released depending on their size and charge, within the channels. Mesoporous silica with large-pore-size stmctures, are best suited for this purpose, since more molecules can be immobilized and the large porosity of the materials provide better access for the substrates to the immobilized molecules. The mechanism of bimolecular adsorption in the mesopore channels was suggested to be ionic interaction. On the first stage on the way of creation of chemical sensors on the basis of functionalized mesoporous silica materials for selective determination of herbicide in an environment was conducted research of sorption activity number of such materials in relation to 2,4-D. 

The instrument consists of a procesmg unit, reagents for Ugand immobilization, exchangeable sensor chips and a personal con uter for control and evaluation. 

The instrumentation for sensors based on absorption measurements can be designed on the traditional spectrophotometers by using a flowthrough cell for automatic sampling with the sensors mounted inside the flow-through cell shown in Fig. 20a.3. For remote optical sensing using optical fibers, the chromophores can be immobilized in reflective... 

D. C. Sundberg, Z. Zhujun, Y. Zhang, M. Wangbai, R. Russell, Z. M. Shakhsher, C. L. Grant, and W. R. Seitz, Poly(vinyl alcohol) as a substrate for indicator immobilization for fiber-optic chemical sensors, Anal. Chem. 61, 202-205 (1989). 

Stabilization of activated oxidoreductases on time scales of months to years has historically been challenging, and the lack of success in this regard has limited the industrial implementation of redox enzymes to applications that do not require long lifetimes. However, as mentioned in the Introduction, some possibility of improved stability has arisen from immobilization of enzymes in hydrophilic cages formed by silica sol—gels and aerogels, primarily for sensor applications.The tradeoff of this approach is expected to be a lowering of current density because... 

Figure 3.11 — (A) Immobilized peroxidase sensor. Glass-immobilized peroxidase is packed in the flow-cell shown. The plastic support plate fits the top surface of the photomultiplier chamber of the immunometer so as to support the vertically held flow-cell in front of the photomultiplier itself. (B) Flow system for hydrogen peroxide/ethanol determinations. For ethanol determinations, the immobilized alcohol oxidase column is inserted immediately after the injection valve (shown by the arrows). Luminol (62 /zM) and 4-iodophenoI (0.4 M) are dissolved in 200 mM borate buffer (pH 8.9) and pumped at a flow-rate of 0.8 mL/min. Phosphate buffer (10 mM, pH 7.0) is pumped at 1.6 ml/min. (Reproduced from [78] with permission of Elsevier Science Publishers).

Procedures ofMIP Immobilization for Electrochemical Sensor Fabrication... 

In order to obtain a ready-for-use sensor array, the probe was immobilized in a block copolymer matrix (polyacrylonitrile-co-polyacrylamide Hypan), which is completely penetrated by water if exposed to it [102], Prior to immobilization, the sensor membrane was cast onto an optically transparent ethyleneglycol-terephthalate polyester support (Mylar). The resulting sensor foil was glued on a black 96-microwell format matrix. The sensor arrays were analyzed by means of time-resolved RLI and PDI methods (see Sect. 2.1) with an optical set up as illustrated in Fig. 6 at an excitation wavelength of 405 nm. The ratiometric images resulted in similar calibration plots for both methods (Fig. 14). The limit of detection and the dynamic range of this sensor foil are comparable to those observed with [Eu(Tc)] in solution [103]. 

Enzyme micro-encapsulation is another alternative for sensor development, although in most cases preparation of the microcapsules may require extremely well-controlled conditions. Two procedures have usually been applied to microcapsule preparation, namely interfacial polymerization and liquid drying [80]. Polyamide, collodion (cellulose nitrate), ethylcellulose, cellulose acetate butyrate or silicone polymers have been employed for preparation of permanent micro capsules. One advantage of this method is the double specificity attributed to the presence of both the enzyme and the semipermeable membrane. It also allows the simultaneous immobilization of many enzymes in a single step, and the contact area between the substrate and the catalyst is large. However, the need for high protein concentration and the restriction to low molecular weight substrates are the important limitations to this approach. 

The covalent attachment of enzymes to water-insoluble carriers is usually the preferred immobilization method for sensor manufacturing. Obviously, the selected procedure should avoid the loss of enzymatic activity and keep the accessibility of the binding site to the substrate molecules. Unfortunately, this is usually not the case and due to the severe conditions of many of these procedures, major activity losses and/or changes on the substrate selectivity are produced during immobilization. Some authors have pointed out that the enzyme activity decreases approximately one fifth per formed bond [66]. 

Disadvantages. The few disadvantages relate to the incompatibility with alkanol type mediator solvents, the limited scope for covalent immobilization of sensor and mediator so as to prevent their leaching and poor adhesion to ISFET gates. 

Mediated Ehzyme Electrodes. Further improvements in the performance of immobilized enzyme sensors stem from the use of redox mediators which shuttle electrons from the redox centre of the enzyme to the surface of the indicator electrode according to the following reaction sequences depicted for glucose oxidase ... 

In recent years the electrochemistry of the enzyme membrane has been a subject of great interest due to its significance in both theories and practical applications to biosensors (i-5). Since the enzyme electrode was first proposed and prepared by Clark et al. (6) and Updike et al. (7), enzyme-based biosensors have become a widely interested research field. Research efforts have been directed toward improved designs of the electrode and the necessary membrane materials required for the proper operation of sensors. Different methods have been developed for immobilizing the enzyme on the electrode surface, such as covalent and adsorptive couplings (8-12) of the enzymes to the electrode surface, entrapment of the enzymes in the carbon paste mixture (13 etc. The entrapment of the enzyme into a conducting polymer has become an attractive method (14-22) because of the conducting nature of the polymer matrix and of the easy preparation procedure of the enzyme electrode. The entrapment of enzymes in the polypyrrole film provides a simple way of enzyme immobilization for the construction of a biosensor. It is known that the PPy-... 

Photo-immobilization for glycoengineering of sensors can be achieved in a controlled manner. 

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