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.
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]
Overview
Definition
Sensor array components are individual sensors, which are selected based on their individual sensing properties (ie. 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
Urinalysis, a portmanteau of the words ''urine'' and ''analysis'', is a panel of medical tests that includes physical (macroscopic) examination of the urine, chemical evaluation using urine test strips, and microscopic examination. Macroscopic ...
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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
) can interfere with sensor performance.[ Chemical sensor arrays employ ]pattern recognition
Pattern recognition is the automated recognition of patterns and regularities in data. It has applications in statistical data analysis, signal processing, image analysis, information retrieval, bioinformatics, data compression, computer graphic ...
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
An audition is a sample performance by an actor, singer, musician, dancer or other performer. It typically involves the performer displaying their talent through a previously memorized and rehearsed solo piece or by performing a work or piece ...
, gustation, olfaction, somatosensation, and vision
Vision, Visions, or The Vision may refer to:
Perception Optical perception
* Visual perception, the sense of sight
* Visual system, the physical mechanism of eyesight
* Computer vision, a field dealing with how computers can be made to gain und ...
—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 receptor
Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give ri ...
s.[ 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
An electronic nose is an electronic sensing device intended to detect odors or flavors. The expression "electronic sensing" refers to the capability of reproducing human senses using sensor arrays and pattern recognition systems.
Since 1982, rese ...
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
Principal component analysis (PCA) is a popular technique for analyzing large datasets containing a high number of dimensions/features per observation, increasing the interpretability of data while preserving the maximum amount of information, and ...
(PCA), least square analysis, and more recently training of neural network
A neural network is a network or neural circuit, circuit of biological neurons, or, in a modern sense, an artificial neural network, composed of artificial neurons or nodes. Thus, a neural network is either a biological neural network, made up ...
s and utilization of machine learning
Machine learning (ML) is a field of inquiry devoted to understanding and building methods that 'learn', that is, methods that leverage data to improve performance on some set of tasks. It is seen as a part of artificial intelligence.
Machine ...
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 polymer
Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymer ...
s, nanomaterials
*
Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale).
Nanomaterials research takes a materials science-based approach to n ...
, 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 ]capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a ...
s and transistors
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
; these materials have a resistance which can be altered through physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
Overview
The fundamental interacting force of physisorption is Van der Waals force. Even ...
or chemisorption
Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated at the adsorbent surface. Examples include macroscopic phenomena that can be very obvious, like cor ...
of target molecules and thus a measurable signal as a change in electrical current, capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized a ...
, or voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
.[
]
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
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
.[ Although most known for their use in ]carbon monoxide detector
A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) gas to prevent carbon monoxide poisoning. In the late 1990s Underwriters Laboratories changed the definition of a single station CO ...
s, 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
, mottoeng = Mind moves matter
, established =
, type = Public research university
, endowment = £7.0 million (2021)
, budget = £698.2 million (202 ...
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
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscible wi ...
, ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...
, methanol, and xylene
In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...
, 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
Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" ...
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 compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s 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
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself nam ...
) over a silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
layer on a p-type silicon base with n-doped channels adjacent to the gate, and they have been used to sense hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
, and ethanol.[ These MOSFETs through adsorbed-analyte modulating the semiconductor gate ]work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
, 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
Polyacetylene (IUPAC name: polyethyne) usually refers to an organic polymer with the repeating unit . The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups. This compound ...
, polythiophene
Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have alkyl substituents ...
, and polyaniline
Polyaniline (PANI) is a conducting polymer and organic semiconductor of the semi-flexible rod polymer family. The compound has been of interest since the 1980s because of its electrical conductivity and mechanical properties. Polyaniline is one ...
, and others may be made conductive through processes including chemical doping
In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic ...
.[ 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
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.
Polar molecules must contain one or more pol ...
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 compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s; 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
*
Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale).
Nanomaterials research takes a materials science-based approach to n ...
such as graphene
Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure. , carbon nanotube
A scanning tunneling microscopy image of a single-walled carbon nanotube
Rotating single-walled zigzag carbon nanotube
A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers.
''Single-wall carbon nan ...
s, 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 compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s.
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
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The und ...
, ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
, and nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...
. 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
Efforts have been made to overcome the specific limitations of different classes of materials suited for use in electronic chemical sensor arrays by combining sensors fabricated with different materials into one array.[ 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. ]Carbon black
Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...
-polymer blends have also shown enhanced analyte discrimination and array-element signals to afford enhanced detection of volatile organic compounds both across a variety of classes, as well as within the same class.
Molecularly imprinted polymer A molecularly imprinted polymer (MIP) is a polymer that has been processed using the molecular imprinting technique which leaves cavities in the polymer matrix with an affinity for a chosen "template" molecule. The process usually involves initiati ...
s have also been integrated into array formats and shown utility as the imprinting process enables molecularly imprinted polymer arrays to be tailors to specific analytes.
Optical/colorimetric chemical sensor arrays
Separate from electronic chemical sensor arrays are optical chemical sensor arrays which probe chemical interactions between target analytes and a sensing material with light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
(ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
, visible, infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
). Generally, optical sensors probe chemical interactions with light through a variety of quantifiable methods including absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
, diffraction, fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
, refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
, and scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
.[ Generally, fluorescence sensors show greater sensitivity than other optical methods.][ Optical sensors consist of a light source, wavelength filter(s), a sample, and a detector, with variations in sensor design based on the method used.][ Similar to the electronic nose, optical chemical sensor arrays have been categorized under the umbrella topic of optoelectronic nose and similarly operate by developing fingerprints for specific compounds and using pattern recognition to identify those components in mixture. Figure 2. shows the principles underlying colorimetric and fluorometric sensor arrays. Chemical interactions with dyes result in changes to light being detected in an optical sensor.
Optical sensors require selective interaction with analytes and two components are required: a probe material, and a chromo- or ]fluorophore
A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with ...
.[ Cross-reactive optical and fluorescence arrays require strategic consideration of molecular interactions between probes and analytes. Much like electrical chemical sensor arrays, optical chemical sensor arrays face challenges in sensing in the presence of competing analytes such as water.][ Consideration of host-guest interactions allows an array to probe a variety of molecular features because integration of ‘promiscuous sensors’ (non-selective) such as optically active polymers permit non-discriminate sensing of a variety of compounds primarily based on ]hydrophobicity
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
, and so-called ‘monogamous’ sensors with exclusive binding to a particular analyte (much like a lock-and-key design) will enhance specificity and applicability of a colorimetric sensor array. Regardless of the type of sensing probe, there are five major types of intermolecular interaction
An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction
or repulsion which act between atoms and other types of neighbouring particles, e.g. ...
which lead to a measurable colorimetric change to a material.[
]
Brønsted-Lowry acid-base interactions in colorimetric chemical sensor arrays
Brønsted-Lowry acid-base interactions such as those of dyes commonly used as pH indicator
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hen ...
s are one of the earliest methods for colorimetric sensing. Since the early 20th century, natural dyes such as 7-hydroxyohenoxazone (litmus
Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. It is a purple dye that is extract ...
) and anthocyanin
Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical co ...
oxonium dye have been used both as pH indicators and colorimetric sensors.[ Many other chromophores with Brønsted-Lowry acid-base functionality have been developed such ]azo dye
Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C- ...
s, nitrophenols, phthaleins
Phthalein dyes are a class of dyes mainly used as pH indicators, due to their ability to change colors depending on pH. They are formed by the reaction of phthalic anhydride with various phenols. They are a subclass of triarylmethane dyes.
Common ...
, and sulfophthaleins.[ The Brønsted-Lowry acid-base functionality of these chromophores relates to specific chemical moieties within their structures and their corresponding pKa. Color changes resulting from ]protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
/deprotonation
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
events may be broadly defined as intermolecular interactions with an acid or base of a particular strength and/or concentration.[
]
Lewis acid-base interactions in colorimetric chemical sensor arrays
While Brønsted-Lowry acid-base interactions are sensitive to a broad range of compounds, Lewis acid and base interactions comprise some of the most sensitive set of intermolecular interactions relevant to colorimetric chemical sensor arrays.[ The selectivity of Lewis acid and base interactions in chemical sensing are underscored by the fact that the most pungent of odors arise from Lewis bases (]thiol
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...
s, phosphine
Phosphine ( IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotti ...
s, amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...
s) and the metal cation-containing olfactory receptor
Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give ri ...
s utilized to sense them at some of the lowest concentrations of all molecular motifs in biology use Lewis acid receptors.[ Lewis acid dyes (namely metals cations with an open-coordination site) are used in biological olfaction for sensing.][ As such, Lewis acids such as metalloporphyrins are of particular interest to researchers developing colorimetric sensor because of their strong Lewis acid-base interactions.][
]
Other interactions in colorimetric chemical sensor arrays
File:Cyranose 320 Labelled.jpg
A variety of other reversible molecular interactions have been shown to produce color changes upon interaction with analytes. These include redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
active chromo- and fluorophores which undergo specific color changes at different applied potentials.[ There also exists a variety of dyes such as merocyanine and azobenzene which show color changes based on the polarity of their environment.][ A‘push-pull’mechanism of ]electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial ...
through these systems through intermolecular interactions results in augmentation of their dipole moments between ground and excited state
In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers t ...
s, which manifests as observable changes to optical transition
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
.[ ]Nanomaterials
*
Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale).
Nanomaterials research takes a materials science-based approach to n ...
development has allowed for surface modification of certain dyes (especially redox active dyes) to afford high sensitivity due to larger surface area-to-volume ratio resulting for more active sites for analyte interaction with dyes.
Colorimetric chemical sensor array fabrication
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. One method for colorimetric sensor array fabrication involves preparation of microspheres by suspending dyes into an inert, and transparent matrix. These microspheres are then incorporated into fiber optics
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...
.[ Other methods for fabricating colorimetric sensor arrays include printing of array fluor- and colorimetric dyes (either directly or in a nanoporous matrix) onto various substrates including ]paper
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre e ...
, silica gel
Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other ...
, or porous polymer membranes.[
Inclusion of ]digital imaging
Digital imaging or digital image acquisition is the creation of a digital representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imply or include ...
and or illumination of optical chemical sensor array elements allows for rapid, real-time signal transduction of colorimetric data measurements in real-time of colorimetric and fluorescent data from microsphere or plated sensors.[ Detectors can process specific wavelengths of light, or employ RGB image processing programs to analyze data obtained from direct imaging of a sensor array.][ Much like electronic chemical sensor arrays, optical chemical sensor arrays are being miniaturized using ]microfabrication
Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" ...
techniques to increase the applicability. Recent advancements in optical chemical sensor arrays have resulted in sensor arrays being directly integrated into flatbed scanners and mobile electronics such as smart phones (through microplate fabrication).[ 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, ]cancer diagnosis
Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
using an array of gold nanoparticle-green fluorescent proteins, and development and assessment of combinatorial libraries of metal-dye complexes as sensors themselves.
Other types of chemical sensor arrays
Although less common, there are two other classifications of devices with demonstrated functionality as chemical sensor arrays. These include wave devices and electrochemical sensors.
Wave devices as chemical sensor arrays
There are several major types of wave devices including acoustic wave devices, thickness shear mode resonators (TSM), and quartz crystal microbalance A quartz crystal microbalance (QCM) (also known as ''quartz microbalance'' (QMB), sometimes also as ''quartz crystal nanobalance'' (QCN)) measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. Th ...
s. These devices oscillate at known frequencies and their frequencies of oscillation are modulated by changes in the mass of the device. These devices may be modified with the plurality of the materials already discussed as being useful materials in chemical sensor array.[ All of these materials are marked by the broad compatibility of their intermolecular interactions as well as selective interactions to a variety of compounds, which when combined together allow for fingerprint detection of compounds in mixtures.][
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
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, a ...
and mercury vapor. Bulk and surface acoustic wave devices have used in higher order sensors in which the sensing material gives rise to multiple modes for signal transduction, such as electrical and optical; additionally the same wave devices have also been used to create virtual chemical sensor arrays, in which data from one sensor component is further processed. 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.
Electrochemical sensor arrays
Another class of devices usable in chemical sensor arrays are electrodes. Commonly, electrochemical-based sensors are referred to as electronic tongue
The electronic tongue is an instrument that measures and compares tastes. As per the IUPAC technical report, an “electronic tongue” as analytical instrument including an array of non-selective chemical sensors with partial specificity to diffe ...
s. Surface modification of an electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
in a multielectrode system allows for targeting of specific molecular interactions.[ ]Semipermeable membrane
Semipermeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecule ...
materials allows for electrodes to be made into sensors through their ability to selectively oxidize or reduce target analytes.[ 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
Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...
, nitrite
The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also re ...
, and ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternar ...
concentrations in aqueous solution. 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
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the bra ...
, but also differentiation of redox products in electrochemical reactions.[
]
Examples of chemical sensor arrays with real-world uses
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
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bl ...
, bacterial infections based on fingerprint analysis of exhaled breath, as well as for food and product quality control. A few examples include:
* Clinical trial of a chemical sensor array device made with gold nanoparticles linked with different organic ligands capable of detecting COVID-19
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease quickl ...
infections.
* The Handheld Electronic Nose (HEN) commercial chemical sensor array device which enables optimization of the fermentation of tea, see Figure 3.
* The WOLF eNose is a commercially available system of chemical sensor arrays using both electronic and colorimetric sensors for the detection of volatile organic compounds, and it has been employed for detection of urinary tract infection
A urinary tract infection (UTI) is an infection that affects part of the urinary tract. When it affects the lower urinary tract it is known as a bladder infection (cystitis) and when it affects the upper urinary tract it is known as a kidne ...
-causing bacteria.
*The Cyranose 320 Electronic Nose is a commercially available chemical sensor array fabricated from 32 black carbon-polymer sensors capable of identifying six bacteria that cause eye infections with 96% accuracy, see Figure 4.
References
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Sensors
Emerging technologies
Materials science
Arrays