Specificity sensor arrays

Another way for BOD estimation is the use of sensor arrays [37]. An electronic nose incorporating a non-specific sensor array of 12 conducting polymers was evaluated for its ability to monitor wastewater samples. A statistical approach (canonical correlation analysis) showed a linear relationship between the sensor responses and BOD over 5 months for some subsets of samples, leading to the prediction of BOD values from electronic nose analysis using neural network analysis. 

A sensor array where different haptens are immobilized at well-defined areas on a plain glass surface has been developed [66], Using an automated flow injection system it was possible to incubate all areas on the chip with analytes, specific antibodies, secondary HRP-labeled antibodies, and CL substrate. Measurement of the light output via imaging performed with a CCD device allowed determination of the analytes present in the sample on the basis of the spatial localization of the CL signal. 

Dye-doped polymeric beads are commonly employed in different formats (Fig. 5), namely as water-dispersible nanosensors, labels and in composite materials (DLR-referenced and multianalyte sensors, sensor arrays, magnetic materials, etc.). The sensing properties of the dye-doped beads are of little or no relevance in some more specific materials, e.g., the beads intended for photodynamic therapy (PDT). The different formats and applications of the beads will be discussed in more detail in the following section, and the relative examples of sensing materials will be given. 

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]. 

Progress in the application of sensor arrays to gas analysis will be made through increasingly independent data channels using novel combinations of sensors and operating modes. Computational approaches will be modified to suit specific types of sensor arrays and to make economical use of computational space for portable instrument applications. The primary challenges of the near future will be to solve the "needle-in-the-haystack" problem and to proceed to complex mixture analysis using a plurality of sensor responses. 

Given their non-specific nature, the electronic nose and electronic tongue sensor arrays can only perform yes or no tests inside the set of product. Contrary to traditional analytical methods, the electrochemical sensor responses do not need and do not provide information on the nature of the compounds under investigation, but only on digital fingerprint of the typical food products. 

Figure 4.2 Graphical recording of oxygen levels measured from several different oxygen electrodes of an implanted planar sensor array. The oxygen levels inhaled by the subject were varied, and despite the fact that the sensors were fabricated to precise specifications and the output current calibration was adjusted, the oxygen levels measured across tissue distances of 1-2 mm varied considerably.

Fig. 7 Multispot sensor arrays composed of nanoporous pigment microspheres, exhibiting different responses to aliphatic amines. By combination of different pigment particles, fingerprint patterns specific for each amine can be obtained. Reprinted, with permission, from [78]. Copyright (2008) American Chemical Society...

Fig. 8 a, b. a PCA classification of lignocellulose hydrolysates from pine, spruce, aspen and birch using a combination of MOS, MOSFET and CP sensors, b Prediction of the ferment-ability of the same hydrolysates expressed as specific ethanol production rate using ANNs with topologies adapted to the sensor array (from [34] with permission of ACS)... 

Nature has developed recognition systems that are able to discriminate on the basis of highly specific molecule-molecule interactions generating a unique signal. Alternatively, nature utilises arrays of less specific sensors to collect information that is deciphered using pattern recognition processes carried out in the brain. Both approaches have also been pursued using ICPs in the development of synthetic sensors. 

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