A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. [1] Chemiresistors are a class of chemical sensors that rely on the direct chemical interaction between the sensing material and the analyte. [2] The sensing material and the analyte can interact by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: semiconducting metal oxides, some conductive polymers, [3] and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.
A basic chemiresistor consists of a sensing material that bridges the gap between two electrodes or coats a set of interdigitated electrodes. The resistance between the electrodes can be easily measured. The sensing material has an inherent resistance that can be modulated by the presence or absence of the analyte. During exposure, analytes interact with the sensing material. These interactions cause changes in the resistance reading. In some chemiresistors the resistance changes simply indicate the presence of analyte. In others, the resistance changes are proportional to the amount of analyte present; this allows for the amount of analyte present to be measured.
As far back as 1965 there are reports of semiconductor materials exhibiting electrical conductivities that are strongly affected by ambient gases and vapours. [4] [5] [6] However, it was not until 1985 that Wohltjen and Snow coined the term chemiresistor. [7] The chemiresistive material they investigated was copper phthalocyanine, and they demonstrated that its resistivity decreased in the presence of ammonia vapour at room temperature. [7]
In recent years chemiresistor technology has been used to develop promising sensors for many applications, including conductive polymer sensors for secondhand smoke, carbon nanotube sensors for gaseous ammonia, and metal oxide sensors for hydrogen gas. [2] [8] [9] The ability of chemiresistors to provide accurate real-time information about the environment through small devices that require minimal electricity makes them an appealing addition to the internet of things. [8]
Chemiresistors can be made by coating an interdigitated electrode with a thin film or by using a thin film or other sensing material to bridge the single gap between two electrodes. Electrodes are typically made of conductive metals such as gold and chromium which make good ohmic contact with thin films. [7] In both architectures, the chemiresistant sensing material controls the conductance between the two electrodes; however, each device architecture has its own advantages and disadvantages.
Interdigitated electrodes allow for a greater amount of the film's surface area to be in contact with the electrode. This allows for more electrical connections to be made and increases the overall conductivity of the system. [7] Interdigitated electrodes with finger sizes and finger spacing on the order of microns are difficult to manufacture and require the use of photolithography. [8] Larger features are easier to fabricate and can be manufactured using techniques such as thermal evaporation. Both interdigitated electrode and single-gap systems can be arranged in parallel to allow for the detection of multiple analytes by one device. [11]
Metal oxide chemiresistor sensors were first commercialized in 1970 [12] in a carbon monoxide detector that used powdered SnO2. However, there are many other metal oxides that have chemiresistive properties. Metal oxide sensors are primarily gas sensors, and they can sense both oxidizing and reducing gases. [2] This makes them ideal for use in industrial situations where gases used in manufacturing can pose a risk to worker safety.
Sensors made from metal oxides require high temperatures (200 °C or higher) to operate because, in order for the resistivity to change, an activation energy must be overcome. [2]
Metal oxide | Vapours |
---|---|
Chromium titanium oxide | H2S |
Gallium oxide | O2, CO |
Indium oxide | O3 |
Molybdenum oxide | NH3 |
Tin oxide | reducing gases |
Tungsten oxide | NO2 |
Zinc oxide | hydrocarbons, O2 |
In comparison to the other materials graphene chemiresistor sensors are relatively new but have shown excellent sensitivity. [14] Graphene is an allotrope of carbon that consists of a single layer of graphite. [15] It has been used in sensors to detect vapour-phase molecules, [16] [17] [18] pH, [19] proteins, [19] bacteria, [20] and simulated chemical warfare agents. [21] [22]
The first published report of nanotubes being used as chemiresistors was made in 2000. [23] Since then there has been research into chemiresistors and chemically sensitive field effect transistors fabricated from individual single-walled nanotubes, [24] bundles of single-walled nanotubes, [25] [26] bundles of multi-walled nanotubes, [27] [28] and carbon nanotube–polymer mixtures. [29] [30] [31] [32] It has been shown that a chemical species can alter the resistance of a bundle of single-walled carbon nanotubes through multiple mechanisms.
Carbon nanotubes are useful sensing materials because they have low detection limits, and quick response times; however, bare carbon nanotube sensors are not very selective. [2] They can respond to the presence of many different gases from gaseous ammonia to diesel fumes. [2] [9] Carbon nanotube sensors can be made more selective by using a polymer as a barrier, doping the nanotubes with heteroatoms, or adding functional groups to the surface of the nanotubes. [2] [9]
.
Many different nanoparticles of varying size, structure and composition have been incorporated into chemiresistor sensors. [33] [34] The most commonly used are thin films of gold nanoparticles coated with self-assembled monolayers (SAMs) of organic molecules. [35] [36] [37] [38] [39] The SAM is critical in defining some of the nanoparticle assembly’s properties. Firstly, the stability of the gold nanoparticles depends upon the integrity of the SAM, which prevents them from sintering together. [40] Secondly, the SAM of organic molecules defines the separation between the nanoparticles, e.g. longer molecules cause the nanoparticles to have a wider average separation. [41] The width of this separation defines the barrier that electrons must tunnel through when a voltage is applied and electric current flows. Thus by defining the average distance between individual nanoparticles the SAM also defines the electrical resistivity of the nanoparticle assembly. [42] [43] [44] Finally, the SAMs form a matrix around the nanoparticles that chemical species can diffuse into. As new chemical species enter the matrix it changes the inter-particle separation which in turn affects the electrical resistance. [45] [46] Analytes diffuse into the SAMs at proportions defined by their partition coefficient and this characterizes the selectivity and sensitivity of the chemiresistor material. [41] [47]
Conductive polymers such as polyaniline and polypyrrole can be used as sensing materials when the target interacts directly with the polymer chain resulting in a change in conductivity of the polymer. [8] [48] These types of systems lack selectivity due to the wide range of target molecules that can interact with the polymer. Molecularly imprinted polymers can add selectivity to conductive polymer chemiresistors. [49] A molecularly imprinted polymer is made by polymerizing a polymer around a target molecule and then removing the target molecule from the polymer leaving behind cavities matching the size and shape of the target molecule. [48] [49] Molecularly imprinting the conductive polymer increases the sensitivity of the chemiresistor by selecting for the target's general size and shape as well as its ability to interact with the chain of the conductive polymer. [49]
A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon.
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector. The sensitive biological element, e.g. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, etc., is a biologically derived material or biomimetic component that interacts with, binds with, or recognizes the analyte under study. The biologically sensitive elements can also be created by biological engineering. The transducer or the detector element, which transforms one signal into another one, works in a physicochemical way: optical, piezoelectric, electrochemical, electrochemiluminescence etc., resulting from the interaction of the analyte with the biological element, to easily measure and quantify. The biosensor reader device connects with the associated electronics or signal processors that are primarily responsible for the display of the results in a user-friendly way. This sometimes accounts for the most expensive part of the sensor device, however it is possible to generate a user friendly display that includes transducer and sensitive element. The readers are usually custom-designed and manufactured to suit the different working principles of biosensors.
Nanosensors are nanoscale devices that measure physical quantities and convert these to signals that can be detected and analyzed. There are several ways proposed today to make nanosensors; these include top-down lithography, bottom-up assembly, and molecular self-assembly. There are different types of nanosensors in the market and in development for various applications, most notably in defense, environmental, and healthcare industries. These sensors share the same basic workflow: a selective binding of an analyte, signal generation from the interaction of the nanosensor with the bio-element, and processing of the signal into useful metrics.
James Mitchell Tour is an American chemist and nanotechnologist. He is a Professor of Chemistry, Professor of Materials Science and Nanoengineering at Rice University in Houston, Texas.
Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nano-scale repeat distances between the different phases that make up the material.
Nanobatteries are fabricated batteries employing technology at the nanoscale, particles that measure less than 100 nanometers or 10−7 meters. These batteries may be nano in size or may use nanotechnology in a macro scale battery. Nanoscale batteries can be combined to function as a macrobattery such as within a nanopore battery.
Magnetic nanoparticles (MNPs) are a class of nanoparticle that can be manipulated using magnetic fields. Such particles commonly consist of two components, a magnetic material, often iron, nickel and cobalt, and a chemical component that has functionality. While nanoparticles are smaller than 1 micrometer in diameter, the larger microbeads are 0.5–500 micrometer in diameter. Magnetic nanoparticle clusters that are composed of a number of individual magnetic nanoparticles are known as magnetic nanobeads with a diameter of 50–200 nanometers. Magnetic nanoparticle clusters are a basis for their further magnetic assembly into magnetic nanochains. The magnetic nanoparticles have been the focus of much research recently because they possess attractive properties which could see potential use in catalysis including nanomaterial-based catalysts, biomedicine and tissue specific targeting, magnetically tunable colloidal photonic crystals, microfluidics, magnetic resonance imaging, magnetic particle imaging, data storage, environmental remediation, nanofluids, optical filters, defect sensor, magnetic cooling and cation sensors.
An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.
Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films.
Nanoarchitectures for lithium-ion batteries are attempts to employ nanotechnology to improve the design of lithium-ion batteries. Research in lithium-ion batteries focuses on improving energy density, power density, safety, durability and cost.
Carbon nanotube chemistry involves chemical reactions, which are used to modify the properties of carbon nanotubes (CNTs). CNTs can be functionalized to attain desired properties that can be used in a wide variety of applications. The two main methods of CNT functionalization are covalent and non-covalent modifications.
Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary (e.g. SiC, TiC), or ternary carbides, also known as MAX phases (e.g., Ti2AlC, Ti3SiC2). CDCs have also been derived from polymer-derived ceramics such as Si-O-C or Ti-C, and carbonitrides, such as Si-N-C. CDCs can occur in various structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded, and from highly porous to fully dense. Among others, the following carbon structures have been derived from carbide precursors: micro- and mesoporous carbon, amorphous carbon, carbon nanotubes, onion-like carbon, nanocrystalline diamond, graphene, and graphite. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas (up to more than 3000 m2/g). By varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the synthesis conditions, the average pore size control can be applied at sub-Angstrom accuracy. This ability to precisely tune the size and shapes of pores makes CDCs attractive for selective sorption and storage of liquids and gases (e.g., hydrogen, methane, CO2) and the high electric conductivity and electrochemical stability allows these structures to be effectively implemented in electrical energy storage and capacitive water desalinization.
Rodney S. "Rod" Ruoff is an American physical chemist and nanoscience researcher. He is one of the world experts on carbon materials including carbon nanostructures such as fullerenes, nanotubes, graphene, diamond, and has had pioneering discoveries on such materials and others. Ruoff received his B.S. in chemistry from the University of Texas at Austin (1981) and his Ph.D. in chemical physics at the University of Illinois-Urbana (1988). After a Fulbright Fellowship at the MPI fuer Stroemungsforschung in Goettingen, Germany (1989) and postdoctoral work at the IBM T. J. Watson Research Center (1990–91), Ruoff became a staff scientist in the Molecular Physics Laboratory at SRI International (1991–1996). He is currently UNIST Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST), and the director of the Center for Multidimensional Carbon Materials, an Institute for Basic Science Center located at UNIST.
Single-walled carbon nanohorn is the name given by Sumio Iijima and colleagues in 1999 to horn-shaped sheath aggregate of graphene sheets. Very similar structures had been observed in 1994 by Peter J.F. Harris, Edman Tsang, John Claridge and Malcolm Green. Ever since the discovery of the fullerene, the family of carbon nanostructures has been steadily expanded. Included in this family are single-walled and multi-walled carbon nanotubes, carbon onions and cones and, most recently, SWNHs. These SWNHs with about 40–50 nm in tubule length and about 2–3 nm in diameter are derived from SWNTs and ended by a five-pentagon conical cap with a cone opening angle of ~20o. Moreover, thousands of SWNHs associate with each other to form the ‘dahlia-like' and ‘bud-like’ structured aggregates which have an average diameter of about 80–100 nm. The former consists of tubules and graphene sheets protruding from its surface like petals of a dahlia, while the latter is composed of tubules developing inside the particle itself. Their unique structures with high surface area and microporosity make SWNHs become a promising material for gas adsorption, biosensing, drug delivery, gas storage and catalyst support for fuel cell. Single-walled carbon nanohorns are an example of the family of carbon nanocones.
Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.
Potential graphene applications include lightweight, thin, and flexible electric/photonics circuits, solar cells, and various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.
In materials and electric battery research, cobalt oxide nanoparticles usually refers to particles of cobalt(II,III) oxide Co
3O
4 of nanometer size, with various shapes and crystal structures.
In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.
Niveen M. Khashab is a Lebanese chemist and an associate Professor of chemical Sciences and engineering at King Abdullah University of Science and Technology in Saudi Arabia since 2009. She is a laureate of the 2017 L'Oréal-UNESCO Awards for Women in Science "for her contributions to innovative smart hybrid materials aimed at drug delivery and for developing new techniques to monitor intracellular antioxidant activity." She is also a fellow of the Royal Chemical Society, and a member of the American Chemical Society.
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.
{{cite journal}}
: CS1 maint: numeric names: authors list (link){{cite journal}}
: CS1 maint: numeric names: authors list (link)