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Polymers are used in many applications and are often documented on their tensile strength and physical properties. However, polymers have been around for many years and are one of the oldest chemical industries in existence. Because of this, there are many different types of polymers that have been synthesized to perform different functions. One of these is a class of polymers known as conductive polymers which can be used in sensor-based applications.
Most polymers have insulating properties. There are, however, a class of polymers that buck the norm and have conductive properties—although in most cases, many of these polymers exhibit properties more akin to a semiconductor than that of a material that is fully conductive. Conductive properties are important for sensor applications because they are a fundamental part of the sensing mechanism. In most sensors, when a stimulus is detected (be it a strain, binding of a molecule, or a change in temperature), the conductivity and electrical current of the active material changes and this acts as the measurable response which is detected by the electronics in the sensor. So, conductivity properties are important for sensor applications.
Traditional Polymer Sensors
Bulk polymers have made up most of the polymer-based sensors to date and often have a response time of the order of seconds, so they are useful for a lot of applications. The organic and long-chain makeup of polymers means that there are many different ways that the polymer can sense a change in its local environment, with the most common sensing mechanisms involving redox reactions, ion adsorption, and desorption mechanisms, volume and weight changes, chain conformational changes, or charge transfer processes. Aside from their sensing abilities, conductive polymers also possess many of the benefits that more common polymers have, such as a low-temperature synthesis and processing, large-area manufacture, flexibility, and low cost, all of which have contributed to their realization is sensors.
Many of the traditional materials used for polymer-based sensors are specific bulk polymers, such as polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), and Poly(3,4-ethylenedioxythiophene) (PEDOT). These materials have been at the forefront of polymer sensors for years because each of them has specific properties that are beneficial for sensor applications. For example, PPy has low oxidation potential and good biocompatibility, PANI has very inexpensive monomer building blocks, PTh has many different useful derivatives, and PEDOT has good optical transparency and is soluble in water.
Many of the polymers used are created in either a powder or film form by chemical and electrochemical polymerization methods. However, given that sensors need to be accurate and precise, the polymerization processes used need to create polymers with a precise structure and morphology. This is easier said than done but needs to be considered as many polymers are insoluble in a lot of solvents and there are many which don’t possess specific properties (such as thermoplastic properties).
The Shift to Polymeric Nanomaterials
Because the conductivity properties of bulk polymers are limited, there has been a shift in recent years to using nano-sized polymeric materials because they have enhanced conductivity properties over their bulk counterparts. The use of nano-sized polymers has also enabled polymer-based sensors to become smaller, have more complex architectures, and possess a more sensitive surface— which is based on the increased surface area ratio compared to bulk polymeric materials and the faster absorption/desorption kinetics on the nanomaterial surface.
The one main drawback (on polymeric nanomaterials as a whole) is that they are less stable than their bulk counterparts. Because of this, the development of polymer nanostructures for sensors has been quite slow compared to other nanomaterial developments, and the only reason they are now being considered an option is because of the development of advanced fabrication technologies that can better (i.e. precisely) control the size and morphology of the nanostructured polymers. This has been achieved in recent years by combining polymerization methods with templating mechanisms, where the templates aid in the specific and controlled growth of the nanostructured polymer.
In terms of the sensors which can utilize polymer materials—both bulk and nano-sized—there are many different types of sensors that have been trialed, although most have them have been used for sensing chemicals or biomolecules. On the side of chemical sensing, their chemical inertness makes them ideal for detecting toxic gases and volatile organic compounds (VOCs), as well as being used to detect alcohol, humidity, and specific chemical anions (electrochemical potentiometric sensor). On the biosensor side of things, their biocompatibility means that they can be used to detect a wide range of biomolecules, including enzymes, antibodies, nucleic acids, and proteins, as well as being able to detect cells. Polymer-based sensors are also being explored for other sensing applications, with the most promise being shown in mechanical sensors (strain gauges, etc).
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