Hybrid sensors

Most microhotplate-based chemical sensors have been realized as multi-chip solutions with separate transducer and electronics chips. One example includes a gas sensor based on a thin metal film [16]. Another example is a hybrid sensor system comprising a tin-oxide-coated microhotplate, an alcohol sensor, a humidity sensor and a corresponding ASIC chip (Application Specific Integrated Circuit) [17]. More recent developments include an interface-circuit chip for metal oxide gas sensors and the conccept for an on-chip driving circuitry architecture of a gas sensor array [18,19]. 

The decomposition of polymer molecules in wastewater samples can also be achieved by a hybrid sensor, which consists of a consortium of microorganisms and hydrolases together. As an example, the BOD of lactose-containing wastewater was determined by a hybrid sensor consisting of cells of the yeast Lipomyces kononenkoae and of the enzyme /1-galactosidase [50, 69]. It is also possible to use a column containing immobilized enzymes, which is inserted in the measuring device (e.g., an FIA) before the biosensor [52, 70]. Such a combination of an enzyme column, which contains a-amylase and amy-... 

Trichosporon cutaneum [89]. The principle of this method is based on the measurement of the rapid alteration (or acceleration) of respiration after addition of ammonium ions in the presence of glucose. The physiological background of this principle is probably the uptake of ammonium ions in connection with the respiration process. Finally, the combination of nitrifying bacteria with urease on a hybrid sensor also allows an amperometric determination of urea [93]. 

The contact problems are mitigated in the hybrid ion sensor by making the internal conductor shorter and shorter until it is more natural to talk about its thickness rather than its length. The material of this internal contact has not changed during this transition and neither has the electrochemistry at the interface. Thus, the only difference between the coated wire and the hybrid sensor is the length (or thickness) of the contact. We therefore skip it and go directly to the solid-state ISFET, in which the thickness of the internal contact is zero. In other words, the ion-selective membrane is placed directly at the input dielectric of the field-effect transistor (Fig. 6.20). 

E. Palecek, R. Kizek, L. Havran, S. Billova and M. Fotja, Electrochemical enzyme-linked immunoassay in a DNA hybridization sensor, Anal. Chim. Acta, 469 (2002) 73-83. 

K. Kerman, Y. Morita, Y. Takamura and E. Tamiya, Escherichia coli single-strand binding protein-DNA interactions on carbon nanotube-modified electrodes form a label-free electrochemical hybridization sensor, Anal. Bioanal. Chem., 381 (2005) 1114-1121. 

We now turn to the experimental method of measurement of Em. The potential on the membrane exterior is measured by an Ag AgCl or SCE reference electrode. The interior potential is very difficult to measure through a direct metal contact (only in some solid state and in hybrid sensors, Section 13.10) and one opts for another reference electrode called the internal reference. Thus a typical cell would be... 

It has been pointed out that nonspecific adsorption can also influence the results of hybridization sensors based on electroactive indicators. 

Scheme 4. Idealized sketches of gold surfaces for DNA hybridization sensors prepared by (a) chemisorption of thiolated ssDNA capture probes followed by chemisorption of a dilutor alkylthiol [168] and by (b) chemisorption of a mixed alkylthiol SAM followed by covalent attachment of an amino-terminated ssDNA capture probe (surface-bound carboxylic groups are activated by reaction with 0-(N-auccimrmdyl)-N,N,N, N -tetramethyluronium tetrafluoroborate in acetonitrile in the presence of N, A-diisopropylethylamine, [172]).

Acetylcholinesterase - Nanomaterials Hybrid Sensors for the Detection of Organophosphorous and Carbamate Pesticides... 

Since a large number of metabolic processes take place in HIS, HIS sensors generally appear less selective than enzyme electrodes. On the other hand, this property permits the detection of group effects (so-called complex variables), the measurement of whole classes of substances via their action on certain cell loci or on the metabolism as a whole, and the characterization of the biocatalytic systems themselves. The potentials of HIS sensors have been further expanded by the hybrid sensor technique, in which the substance to be determined is coupled to a given metabolic pathway by additional, coimmobilized enzymes. 

The following sections are devoted to HIS sensors which will be presented according to the increasing level of integration and illustrated by selected examples. Since hybrid sensors have been constructed with... 

Renneberg et al. (1984) described a microbial hybrid sensor for a-amylase assay. A membrane with coimmobilized B. subtilis cells and glucoamylase was attached to an O2 electrode. Starch and the a-amylase sample were added to the measuring cell. Low-molecular weight products of the a-amylase-catalyzed starch hydrolysis diffuse into the biocatalytic membrane where they are cleaved by glucoamylase to glucose, which is assimilated by the bacteria. The sensor responded linearly to a-amylase up to 1.5 U/ml. 

Fig. Ill, Schematic representation and measuring curve of the determination of the inhibitors, phosphate and fluoride, with a GOD-tissue slice hybrid sensor.

A limitation to this approach is that the organic waste water components are converted with different reaction velocities. Macromolecules, such as starch and proteins, are not indicated at all. This might be overcome by enzymatic sample pretreatment or by the use of hybrid sensors. 

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