Chemical sensor array

There exist several types of chemical sensor arrays including electronic, optical, acoustic wave, and potentiometric devices.

[3] Signal processing in the brain of individual array component responses of olfactory receptors results in pattern recognition for discrimination of a particular scent.

[10][11][12] The method of data analysis chosen depends on a variety of factors including sensing parameters, desired use of the information (quantitative or qualitative), and the method of detection which can be classified under four major types of chemical sensor array: electronic, optical, acoustic wave, and electrochemical sensor arrays.

[1][5] In the 1990s, several researchers at the University of Warwick created the first cross-reactive (non-selective) metal-oxide semiconductor sensor array integrated with pattern recognition software for sensing and distinguishing organic vapors, including acetone, ethanol, methanol, and xylene, in multianalyte mixtures.

These microdevices have shown promise with lowered limits of detection and enhanced ability to distinguish volatile organic compounds and carbon monoxide with arrays containing different numbers of device, and these systems also reduce the amount of sensor material with thin films of metal-oxides.

[14] Sensitivity of sensors has also been shown to be influenced by changing the ratio of the metal within each device and data processing utilized least square analysis.

[12] Another example of metal-oxide semiconductors is arrays of metal-oxide semiconductor field effect transistors (MOSFET), which consist of a catalytically active gate metal (such as palladium) over a silicon dioxide layer on a p-type silicon base with n-doped channels adjacent to the gate, and they have been used to sense hydrogen, ammonia, and ethanol.

As a result, conductive polymers are a promising alternative to metal-oxide semiconductors because a greater number of sensors with different functionalities may be used to design a more robust array tailored for specific applications.

Monomer identity, polymerization conditions, and device fabrication methods impact both the morphological and chemical properties of conductive polymers, which also contributes to the greater variety of possible array components which may be designed.

[1] One example of these is metal-oxide nanowires coated in thin films of MOFs, which have been reported to have enhanced sensing performance over sensors made with the individual materials.

Generally, optical sensors probe chemical interactions with light through a variety of quantifiable methods including absorbance, diffraction, fluorescence, refraction, and scattering.

These include redox active chromo- and fluorophores which undergo specific color changes at different applied potentials.

[3] A‘push-pull’mechanism of electron density through these systems through intermolecular interactions results in augmentation of their dipole moments between ground and excited states, which manifests as observable changes to optical transition.

[28] Unlike the materials used in electronic chemical sensor arrays, in which direct interaction between the sensing material and an analyte leads to signal transduction as a change in conductivity or voltage, fabrication of colorimetric sensor arrays requires consideration of both analyte-substrate interaction and transduction of the optical signal.

[29] One method for colorimetric sensor array fabrication involves preparation of microspheres by suspending dyes into an inert, and transparent matrix.

[3] These microplate arrays enable colorimetric analysis of complex mixtures in a variety of phases with applications in identification of toxic industrial chemicals using cross-reactive nanoporous pigments,[30] cancer diagnosis using an array of gold nanoparticle-green fluorescent proteins,[31] and development and assessment of combinatorial libraries of metal-dye complexes as sensors themselves.

[1] Modification of wave devices with materials such as micromachined metal-oxide cantilevers coated in polymer films enable enhanced detection of mixtures of volatile organic compounds as well as hydrogen gas and mercury vapor.

[35] A chemical sensor array of surface-modified quartz crystal microbalances with a variety of materials including copper phthalocyanine, single- and multi-walled carbon nanotubes was shown as a promising electronic nose for gas sensing when machine learning algorithms were employed for data processing.

[37] Semipermeable membrane materials allows for electrodes to be made into sensors through their ability to selectively oxidize or reduce target analytes.

[1] One example includes, the use of an array of semipermeable membrane sensors made from potentiometric polymers like poly(vinyl chloride) have demonstrated their ability to monitor nitrate, nitrite, and ammonium concentrations in aqueous solution.

[38] Both voltametric and potentiometric methods have been developed, and this technique is an active area of research not only for multianalyte analysis of aqueous solutions such as cerebrospinal fluid, but also differentiation of redox products in electrochemical reactions.

[26][37] There exists a diversity of well-understood, and emerging research focused on developing chemical sensor arrays for a variety of applications.

Analytical devices integrated with a chemical sensor array have been proposed as diagnostic tests for cancer, bacterial infections[39] based on fingerprint analysis of exhaled breath, as well as for food and product quality control.

Figure 1. The design and inspiration for many chemical sensor arrays is one or more of the five senses such as smell or taste. As depicted here, the process by which sensor array data is utilized may be broken down into similar steps as odor biological odor detection: 1. acquiring a signal, 2. processing the signal, 3. comparing the signal to what is already known, and 4. producing a response.
Figure 2. Overview of the principles underlying colorimetric and fluorometric sensor arrays. 1. Array of several colorimetric and/or fluorometric sensors constructed. 2-3. Exposure of the array to particular analytes allows fingerprint identification of components. 4. Multicomponent analysis of a mixture may be achieved with pattern recognition of known fingerprints. This process if another generalization of Figure 1. Adapted from Figure created by Askim and coauthors. [ 3 ]
Figure 3. The Handheld Electronic Nose (HEN) employs a chemical sensor array to assess tea fermentation to enable optimization of tea preparation and quality.
Figure 4 . Diagram of Cyranose 320 electronic nose employing an array of 32 black carbon-polymer for detection of eye infection causing bacteria. Image provided by Cyranose Sciences Inc. [ 27 ]