Sensor coated with different

Figure 6. Characteristic patterns of response of an array of six sensors coated with different phthalocyanines to various vapor exposures. The maximum concentration was 100 ppm for ammonia, sulfur dioxide, and DMMP and 1000 ppm for water, ethanol, air and benzene. The central metal of each phthalocyanine is shown at the top.

Microsensors have the potential for selective GC detectors and also as remote sensors when combined in arrays often referred to as electronic noses . Promising microsensors include surface acoustic wave (SAW) detectors normally coated with different semi-selective polymeric layers and microelectromechanical systems (MEMS) including microcantilever sensors. The hope is that, in the future, hundreds of such microcantilevers, coated with suitable coatings, may be able to achieve sufficient selectivity to provide a cost-effective platform for detecting explosives in the presence of potentially interfering compounds in real environments. This array of... 

Kings sorption detector could detect moisture to 0.1 ppm and hydrocarbons such as xylene to 1 ppm. The sensors consisted of Pz crystals coated with different coatings (of varying selectivities), and their interaction with different analytes were monitored. The coatings used were based on gas chromatography (GC) stationary phases. An imcoated crystal was also incorporated as a reference. 

The use of sensor arrays should also play an important role in the future. They could be used to simultaneously give an accurate account of many different analytes and an overall result at once. For example, an array of Pz sensors coated with specific antibodies against different freshwater cyanobacterial toxins could be used to give an overall toxicity indication and also a specific result on the concentration and type of each toxin present. Incorporation of miniaturised flow cells and crystals, such as the excellent system developed by Michalzik et al. [152,153], could allow real-time monitoring directly in solution. 

A electronic nose based on an array of eight quartz microbalance-based (QCM) sensors coated with modified MPPs (5,10,15,20-tetraphenylporphyrin) was used for apple aroma measurements [13], The response of each QCM sensor was modeled with Brunauer-Emmett-Teller (BET) adsorption isotherms. By means of multivariate analysis on all sensor responses, the different compounds could be discriminated well and quantified accurately. This calibration protocol can be used to characterize the sensors for the vapors of complex mixtures. 

Traces of VOCs in human breath can be a symptom of diabetes, kidney failure, asthma or breast cancer. VOCs were mainly detected by mass spectroscopy with a ppb detection limit. The most advanced VOC sensor based on SWCNTs for medical application was introduced by Peng et al. (2008a, 2008b). This device is designed as an array of ten SWCNT chemiresistors coated with different non-polymeric organic layers. Since the responses of each chemiresistor are different, a principal component analysis of the measurement allows the direct discrimination between a healthy patient and a patient with cancer or renal diseases. The same authors have also highlighted the dramatic effect of humidity on the sensor response and have proposed coupling the device with a water extractor (Fig. 10.8). 

There are different types of commercially available AFM tips. We mostly use tips from two different groups. In the first are silicon-based tips, which can be functionalized in two step procedures. The first step involves the introduction of functional groups, which can serve as non-specific chemical sensors on their own. The second step adds specificity to them by binding desired molecules to these anchoring points, which serve as efficient sensors of desired species. In the second group are tips, which are coated with different coatings. 

In Figures 15 and 16 are reported the responses in Hz of QCM sensors coated with cavitands 37-42 to different analytes at the same concentration. In both series the responses increase with increasing the chain length of the substituents at the lower rim, in particular between Ci and Q. 

The key to the successfid use of QCM in liquids is the development of sensitive and stable sensor coatings on the oscillating surface. The material of choice must be insoluble in water with no swelling behaviour. We studied the responses of the QCM sensors coated with cavitands 38-39 and 41-42 toward several different analytes (Figure 18), and compared the resulting sensor signals with their partition coefficients log Poet (octanol/water) as function of their hydrophobicity (Table 3)/... 

The previously mentioned quantities are completely general, and their importance holds for any kind of sensor. For chemical sensors an additional parameter of great importance is the selectivity. The selectivity defines the capability of a sensor to be sensitive only to one quantity rejecting all the others. In case of physical sensors, the number of quantities is limited to a dozen and the selectivity can be achieved in many practical applications. For chemical sensors, it is important to consider that the number of chemical compounds is of millions and that the structural differences among them may be extremely subtle. With these conditions the selectivity of chemical sensor can be obtained only in very limited conditions. Lack of selectivity means that the sensor responds with comparable intensity to different species and with such a sensor it is not possible to deduce any reliable information about the chemical composition of the measured sample. Selectivity is a straightforward requisite for analytical systems where sensors and its related measurement technique are addressed to the detection of individual compounds. As mentioned in the previous section, selectivity is not found in olfactory receptors. As a consequence, artificial olfaction systems are not based on individual selective sensors, but on sensors whose selectivity can be oriented towards molecular families, or better, towards interaction mechanisms. Figure 22.5 shows a typical selectivity map related to an array of quartz microbalances (see next section) coated with different metalloporphyrins based on the same macrocycle (tetraphenyl-porphyrin) but with different metal atoms. Figure 22.5 depicts well the concept of combinatorial selectivity, namely each compounds is identified by a unique sensitivity pattern that makes possible the identification. 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

As an example of the use of array methodology to study chemical sensor properties let us consider the thirteen molecular structures reported in Figure 5. To investigate the sensing properties of these molecules we studied the behaviour of the response of thickness shear mode resonators (TSMR) sensors, each coated with a molecular film, to different concentration of various volatile compounds (VOC). Analyte compounds were chosen in order to have different expected interaction mechanisms. 

Fig. 4.10. Measured relative temperature differences between the temperatiu e sensors T2 to and T. The microhotplate was coated with a nano-crystalline Sn02 droplet...

Direct detection of Staphylococcal enterotoxins B (SEB) is illustrated in Fig. 14 which shows binding of SEB to the wavelength-modulated SPR sensor surface coated with respective antibodies for five different SEB concentrations". Figure 15 shows the sensor response to binding after 30-minute SEB incubation and initial binding rate as a function of SEB... 

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