Array sensor system configuration

Figure 9. Four SAW array sensor system configuration.

We further addressed the use of the nucleic acids as biopolymers for the formation of supramolecular structures that enable the electronic or electrochemical detection of DNA. Specifically, we discussed the use of aptamer/low-molecular-weight molecules or aptamer/protein supramolecular complexes for the electrical analysis of the guest substrates in these complexes. Also, nucleic acid-NPs hybrid systems hold a great promise as sensing matrices for the electrical detection of DNA in composite three-dimensional assemblies. While sensitive and selective electrochemical sensors for DNA were fabricated, the integration of these sensor configurations in array formats (DNA chips) for the multiplexed analysis of many DNAs can also be envisaged. 

Fig. 2. The principle configuration of an electronic nose system where the analyte mixture is contacted with a chemical sensor array that produces raw data which subsequently are treated with a pattern recognition algorithm that delivers the predicted result...

Today, several companies sell commercial electronic nose systems with their favorite sensor configurations [20-22]. The commercial systems have the drawback that the types of sensors used in the array cannot be changed. If these configurations are not the appropriate ones for the analytes to be measured, it becomes necessary to combine different commercial instruments. Alternatively, a research instrument may be used. 

Even before the introduction of miniaturized biosensor arrays, however, some systems able to simultaneously measure glucose and lactate had been reported in the literature. One example is provided by Osborne et al. [157], who described plastic film carbon electrodes fabricated in a split-disk configuration and then modified to obtain a dual biosensor. They achieved a continuous monitoring of these metabolites by placing the dual electrode in a thin-layer radial flow cell coupled to a microdialysis probe. The stability of the sensors was sufficient for short-term in vivo experiments in which the crosstalk, i.e., the percentage of current measured by one biosensor but due to product generated by the partner biosensor, was acceptable for an in vivo application. 

Miniaturization and utilization of biocompatible materials for construction have allowed the electrochemical sensors to be successfully used for in vivo measurements. Usually, they are arranged as sensor detection systems in an array.316 Because spectrometric methods cannot be reliably applied for in vivo measurements, their uncertainty cannot be compared with that obtained by in vivo measurements using electrochemical sensors. The main problems for in vivo measurements are the sterilization of sensors, dimension of sensors (usually cannot exceed a nanometer magnitude order), and their geometric configuration. The calibration of sensors for in vivo measurements is also a problem because high-quality standards are necessary. 

Water had a relatively weak effect on the sensor array. This is helpful if we are trying to detect substances that are much more reactive than water. In fact, it might be possible to configure the sensor array to study flavor notes in aqueous substances such as coffee or tea. If, on the other hand, humidity measurement is desired, then the presence of more volatile constituents in the system may mask any response to humidity. 

What would constitute the ideal MS-based e-nose system It would likely (a) incorporate a more sensitive technique than static headspace to deliver volatiles to the sensor array (b) be less expensive than most e-nose instruments currently on the market (c) be easy to use and (d) provide results in less than 10 minutes per sample. An optimum instrument configuration would also allow the same instrument to be used in a rapid e-nose mode but would also permit investigation of sample anomalies using conventional GC/MS methods. Such an instrument could be used as a rapid screening tool and also as a research tool for uncovering further chemical information about suspect samples. Switching from one mode to the other should not require any hardware modification or even instrument shutdown to change columns. 

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