Fingerprint sensors

Simple bar code readers or fingerprint sensors may also be introduced into household appliances in the next decade. Visionaries even expect the introduction of displays in the kitchen or bathroom within the next decade. 

The fingerprint sensor market also demands systems for detecting fake fingers on the sensor, and a bioimpedance-based solution for spoof detection was described by Martinsen et al. (2007). Their system is based on the simultaneous measurement of skin impedance at different depths, and the use of multivariate models to classify the fingers as living or fake. 

Considerable corrosion monitoring is carried out utilising invasive methods, i.e. where the corrosion sensor is required to penetrate the pipe or vessel wall. Avoidance of penetration using non-invasive methods (thin layer activation, ultrasonics, radiography and magnetic fingerprinting) is receiving considerable developmental attention. 

N. (2004) Maturity discrimination of snake frmt (Salacca edulis Reinw.) cv. Pondoh based on volatiles analysis using an electronic nose device eqmpped with a sensor array and fingerprint mass spectrometry. Flavour Fragrance J. 19 44-50. 

Some analytes, such as riboflavin (vitamin B2)16 and polycyclic aromatic compounds (an important class of carcinogens), are naturally fluorescent and can be analyzed directly. Most compounds are not luminescent. However, coupling to a fluorescent moiety provides a route to sensitive analyses. Fluorescein is a strongly fluorescent compound that can be coupled to many molecules for analytical purposes. Fluorescent labeling of fingerprints is a powerful tool in forensic analysis.17 Sensor molecules whose luminescence responds selectively to a variety of simple cations and anions are available.18 Ca2+ can be measured from the fluorescence of a complex it forms with a derivative of fluorescein called calcein. 

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. 

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...

Many other important application areas of chemometrics have been the subject of reviews and are too numerous to list here. A sampling of reviews in this category illustrates the breadth and diversity of chemometrics application areas. A review of applications in smart sensors [54] describes how chemometrics is an important enabling technology for the development of smart and reliable sensor systems. A review of environmental forensics [55] describes how numerical methods are critical in the process of identifying the chemical fingerprints of complex contaminant sources in environmental systems. Often, multiple sources are present at different geographic sites. By use of appropriate chemometric methods, these mixtures of different sources can be mathematically resolved to identify them and map their temporal and spatial distributions. 

Recently, the fabrication of a sensor array in a chip by covalent attachment of fluorescent monolayers on the walls of glass microchannels has been demonstrated. The monolayer array (confined in a multichannel chip) is prepared by parallel synthesis of monolayers functionalized with different fluorophore-ligand pairs, resulting in a sensing chip, which is able to generate a fingerprint of the network with a single fluorescence snapshot for different analytes.63... 

The interaction between a chemical sensor and the molecules it detects is a dynamic stochastic process. The resultant fluctuations carry a stochastic fingerprint . Conventional sensing methods measure average values and thereby ignore the stochastic component of the signal these sensors are several orders of magnitude less sensitive than the nose of a canine or even a human. 

The artificial intelligence systems to which sensor arrays are coupled supply the closest likeness to the human olfactory system. Some of the recent theories on olfaction require that the human nose has only relatively few types of receptor, each with low specificity. The activation of differing patterns of these receptors supplies the brain with sufficient information for an odour to be described, if not recognized. As a consequence of this belief, the volatile chemical-sensing systems commercially available only contain from 6 to 32 sensors, each having relatively low specificity. Statistical methods such as principal component analysis, canonical discriminant analysis and Euclidian distances are used for mapping or linked to artificial neural nets as an aid to classification of the odour fingerprints . 

Conducting polymer sensors can be operated either to quantitatively measure the concentration of a target vapor species or to qualitatively analyze a complex mixture of vapors. For single vapors, the detection limits can be in the low-ppm region. Exposure to a mixture of vapors results in a unique pattern of responses, which is usually deciphered using standard chemometric techniques. The pattern can be used like a fingerprint to identify certain products, or to establish the quality of foodstuffs, wines, perfumes, etc. The electronic nose has similar components as the natural nose this is illustrated in Figure 1.15. 

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