A chemical sensor array is a sensor architecture with multiple sensor components that create a pattern for analyte detection from the additive responses of individual sensor components. There exist several types of chemical sensor arrays including electronic, optical, acoustic wave, and potentiometric devices. These chemical sensor arrays can employ multiple sensor types that are cross-reactive or tuned to sense specific analytes.
Overview
Definition Sensor array components are individual sensors, which are selected based on their individual sensing properties (i.e. method of detection, specificity for a particular class of analyte and molecular interaction). Sensor components are chosen to respond to as many analytes as possible; so, while the sensitivity and selectivity of individual sensor components vary, the sensors have an additive effect by creating a nonselective fingerprint for a particular analyte when combined into an array architecture. Recognition of fingerprints enables detection of analytes in mixtures. Chemical sensor arrays differ from other multianalyte tests such as a urinalysis stick assay which utilizes multiple, specific sensor materials for targeted detection of analytes in a mixture; instead, chemical sensor arrays rely on cross-reactivity of individual sensor components to generate fingerprints based on the additive responses of sensor components to the target analyte.
Comparison to other chemical sensors Single sensor devices sense target analytes based on physical, optical, and electronic properties. Some sensors contain specific molecular targets to afford strong and specific binding with a particular analyte; however, while this approach is specific, complex mixture impact sensor performance. Several of these complex mixtures include odors and vapors exhaled from the lungs. Individual chemical sensors often utilize controlled sensing environments, and variations in ambient conditions (e.g., temperature and humidity) can interfere with sensor performance. Chemical sensor arrays employ pattern recognition of combinatorial responses of cross-reactive sensor components to enable sensing of a diverse array of mixtures in a variety of conditions. Chemical sensor arrays are often noted as mimicking the five senses—audition, gustation, olfaction, somatosensation, and vision—because the combinatorial responses to the different array components of a particular analytes create fingerprints for specific analytes or mixtures using both targeted molecular interactions and pattern recognition.
History The history of chemical sensor arrays is closely linked with the development of other chemical sensor technologies, with research in the area of electronic chemical sensors picking up in the 1960s with the demonstration of metal-oxide semiconductor sensors capable of sensing analyses such as oxygen. Humans are capable of identifying and discerning between an estimated 10,000 scents or more, while only possessing 400 olfactory receptors. Signal processing in the brain of individual array component responses of olfactory receptors results in pattern recognition for discrimination of a particular scent. One of the design aims of many chemical sensor arrays is to mimic the performance of olfaction to design an electronic nose integrated with a variety of materials. Combining chemical sensor arrays with pattern recognition methods mimics biological sensory recognition methods. See Figure 1. Commercially available electronic nose systems exist and are used in the food industry for quality control. Current research efforts demonstrate the introduction of the electronic nose principle into environmental monitoring and medicine both as commercial instruments as well as in consumer-grade wearable electronic devices. At the center of chemical sensor arrays is the principle that different analytes will interact differently with a variety of materials. As such, any sort of material may be used in a sensor array, so long as it responds differently to different analytes or mixtures. From this idea, cross-reactive sensor arrays have been the focus of chemical sensor array development for their broad compatibility with the compounds as components of mixtures.
Array signal processing The signal(s) coming from an array sensor must be processed and compared with already-known patterns. Many techniques are useful in processing array data including principal component analysis (PCA), least square analysis, and more recently training of neural networks and utilization of machine learning for pattern development and identification. Machine learning has been a more recent development for generation and recognition of patterns for chemical sensor array data. 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.
Electronic chemical sensor arrays The first type of chemical sensor array relies on modulation of an electronic signal for signal acquisition. This type of chemical sensor array often utilizes a semiconductive material such as metal-oxide semiconductors, conductive polymers, nanomaterials, or framework materials such as metal-organic and covalent-organic frameworks. One of the simplest device architectures for an electronic chemical sensor is a chemiresistor, and other architectures include capacitors and transistors; these materials have a resistance which can be altered through physisorption or chemisorption of target molecules and thus a measurable signal as a change in electrical current, capacitance, or voltage.
Metal-oxide semiconductors in electronic chemical sensor arrays Metal-oxide semiconductors were first reported in the 1960s as a chemiresistor sensor for single-analyte detection of organic vapors. The first commercially available chemiresistive sensors utilized metal-oxide semiconductors for the detection of carbon monoxide. Although most known for their use in carbon monoxide detectors, metal-oxide semiconductors are capable of sensing other analytes through strategic tuning of their composition. The high operating temperature required to operate these sensors make these semiconductors inefficient and cross-reactive particularly with water.
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. This electronic nose system was known as the Warwick Nose, and combined commercially available tin- and silicon-oxide semiconductors into an array format for gas sensing, see Figure 2. Current efforts are advancing the format of metal-oxide semiconductor arrays using microfabrication techniques to enable smaller array designs and integration of signal processing components into each array component. 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. 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.
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. These MOSFETs through adsorbed-analyte modulating the semiconductor gate work function, which causes changes in voltage across the device. MOSFETs are highly tunable but remain limited by their cross-reactivity, and high operating temperatures.
Intrinsically conductive polymers in electronic chemical sensor arrays Several intrinsically conductive polymers of interest include polyacetylene, polythiophene, and polyaniline, and others may be made conductive through processes including chemical doping. The principle chemistry underlying the electronic sensing mechanism of conductive polymers is modulation of the conductivity of these polymers upon changes to their physical structure (swelling) resulting from interactions with analytes (mainly through absorption). An advantage of using conductive polymers in sensor arrays is that there is synthetic access of a vast library of polymers. 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. The limitations of conductive polymer arrays are similar to those of single sensor analogs in that the signal transduction pathways through the polymer material are poorly understood and both struggle to sense non-polar species due to minimal adsorption to the polymer. Several commercially available systems are available and are used in food analysis and sensing of volatile organic compounds; however, progress to advance chemiresistive sensor arrays utilizing conductive polymers has decreased as other materials and sensing methods have been developed.
Nanomaterials in electronic chemical sensor arrays Development of novel nanomaterials such as graphene, carbon nanotubes, and 2D and 3D framework materials have been reported as new classes of materials for applications in electronic chemical sensor arrays. For graphene and carbon nanotubes, surface functionalization via covalent or non-covalent modification, and edge site defects are utilized as sites for host-guest interactions. One such example is single-walled carbon nanotubes modified with various metalloporphyrins to enable discrimination of volatile organic compounds.
Conductive framework materials in electronic chemical sensor arrays Conductive framework materials have similar mechanisms for sensing; however these materials may be designed with installed active sites tuned for a specific molecular interaction. Bimetallic metallophthalocyanine metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have shown promise in single device chemiresistors at sensing hydrogen sulfide, ammonia, and nitric oxide. The development of these materials as chemiresistors allows for strategic design of arrays capable of targeted molecular interactions, which can be employed to develop array components tailored to sensing specific compounds. Computational research of several MOFs has also focused on optimizing which combinations of MOFs are best suited for sensing particular components in various mixtures. The focus on curation of framework array components demonstrated the opportunity to design robust sensor arrays experimentally and computationally.
Mixed-material electronic chemical sensor arrays