Sensing layer, conduction

In the case when the species is only consisted of amorphous phase, suitable value of sensing layer conductivity is not reaching. The presence of amorphous phase is essential in quantity, sufficient to provide chemisorption of detected gas, but not making difficult the charge transfer. The later should be provided by other highly conductive phases (C-In203). 

Fig. 2.3. Schematic view of a porous nanocrystaUine sensing layer with a one-dimensional representation of the energetic conduction band. A inter-grain band bending, eVs, occms as a consequence of smTace phenomena, and a band bending, eVc, occurs at the grain-electrode contact. Eb denotes the minimmn conduction band energy in the bulk tin oxide, and Ep is the Fermi-energy in the electrode metal...

A good example for a microanalytical device is the gas sensor array. The conductometric approach for gas sensing was favored during the last years using metal oxides or conductive polymers. Unfortunately such sensors are quite unspecific and therefore sensor arrays with modified sensing layers have to be used. The selectivity derives from a sophisticated data processing using neural networks. Complete gas analysis systems with microfluidic and data acquisition are now under development. 

As aheady described in Sections 1.1.4 and 1.1.6 (mobile or immobihzed) mediators and/or conducting polymers can also be employed to shuttle electrons between a redox enzyme and an electrode surface [11, 12, 16, 20, 21, 24, 25, 28, 270-273]. This approach is called mediated ET (see also Eigure 1.6). Efficient ET throughout the entire sensing layer is envisaged in order to avoid only the enzyme layer in close vicinity to the electrode surface contributing to the overall current signal. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

Therefore, the fluctuation of the conductance of the sensing layer can be written from... 

W (a) Conductance response of a SnOj gas sensing layer to ethanol (dashed line) and methanol (solid line) for temperature pulses of 275°C and 380°C (b). From Kunt et al. (1998). 

Cosnier et al. [166] has developed electropolymerizable materials of a dicarbazole-derivative functionalized by N-hydroxysuccinimide and pentafluorophenoxy groups [166]. The subsequent chemical functionalization of the poly(dicarbazole) film was easily performed by successive immersions in aqueous enzyme and mediator solutions. These derivatized, bioactive conducting polymer films were demonstrated as sensing layers for catechol. 

A number of other conducting polymer derivatives have been used for immobilizing enzymes and redox mediators for biosensor applications. Among these polymers are poly (tyramine) [167] and poly(l-(5-aminonaphthylethanoic acid) [168], which have accessible amine and carboxyl functionalities, respectively, for covalently linking to complementary groups on available amino acid residues of enzymes. Polypyrrole and polythiophene derivatives have also been widely exploited for covalent immobilization of enzymes, which then function as the sensing layers in efficient biosensors [169,170]. 

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