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Polyaniline nanofibers are a high aspect form of
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 ...
, a
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
consisting of
aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...
monomers, which appears as discrete long threads with an average diameter between 30 nm and 100 nm. Polyaniline is one of the oldest known
conducting 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 main advantage of conductive polymers i ...
s, being known for over 150 years.Li, D.; Huang, J.; Kaner, R.B.; ''Acc. Chem. Res.'', 2009, 42 (1), 135–14

Polyaniline
nanofiber Nanofibers are fibers with diameters in the Nanometre, nanometer range (typically, between 1 nm and 1 μm). Nanofibers can be generated from different polymers and hence have different physical properties and application potentials. Examples ...
s are often studied for their potential to enhance the properties of polyaniline or have additional beneficial properties due to the addition of a
nanostructure A nanostructure is a structure of intermediate size between microscopic and molecular structures. Nanostructural detail is microstructure at nanoscale. In describing nanostructures, it is necessary to differentiate between the number of dimen ...
to the polymer. Properties that make polyaniline useful can be seen in the nanofiber form as well, such as facile synthesis, environmental stability, and simple
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
/ base doping/dedoping chemistry. These and other properties have led to the formation of various applications for polyaniline nanofibers as
actuator An actuator is a machine element, component of a machine that produces force, torque, or Displacement (geometry), displacement, when an electrical, Pneumatics, pneumatic or Hydraulic fluid, hydraulic input is supplied to it in a system (called an ...
s, memory devices, and
sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
s.


Synthesis

Methods for the
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
of polyaniline nanofibers seen in literature primarily include chemical oxidativepolymerization, interfacial synthesis, and "rapid mixing" methods.Kavitha, B.; kumar, K.S.; Narsimlu, N.; ''Indian J. Pure Appl. Phys.'', 2013, 51 (3), 207-20

Huang, J.; Kaner, R.B.; ''J. Am. Chem. Soc.'', 2004, 126 (3), 851–85

Huang, J.; Virji, S.; Weiller, B.H.; Kaner, R.B.; ''J. Am. Chem. Soc.'', 2003, 125 (2), 314–31

Huang, J.; ''Pure Appl. Chem.'', 2006, 78 (1), 15-2

Other less common methods include
nanofiber seeding Nanofiber Seeding is a process to control the bulk morphology of chemically synthesized conducting polymers. Typically, catalytic amount of nanofiber seeds are added in prior to onset of nanofiber seeding polymerization (reaction), where seeds are ...
,
electrosynthesis In electrochemistry, electrosynthesis is the synthesis of chemical compounds in an electrochemical cell. Compared to ordinary redox reactions, electrosynthesis sometimes offers improved selectivity and yields. Electrosynthesis is actively stud ...
,
electrospinning Electrospinning is a fiber production method that uses Electrostatics, electrical force (based on electrohydrodynamic principles) to draw charged threads of polymer solutions for producing nanofibers with diameters ranging from nanometers to mi ...
, and preforming polymerization in dilute aniline solutions.Chiou, N.R.; Epstein, A.J.; ''Adv. Mater.'', 2005, 17, 1679-168


Chemical oxidative polymerization

Chemical oxidative polymerization is a traditional and commonly used method for the polymerization of aniline in large quantities. When aniline is mixed with an oxidant in an acidic solution, polymerization will occur. The most important parameter to be controlled in this method for the synthesis of polyaniline nanofibers is the domination of Homogeneity and heterogeneity, homogeneous nucleation over heterogeneous
nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...
. Homogeneous nucleation describes when the nuclei are formed spontaneously in solution while heterogeneous nucleation describes when the nuclei are grown on other species. In the early stages of this polymerization, only nanofibers are formed since there are no heteronuclei available for heterogeneous nucleation. However, if the reaction is left uncontrolled, heterogeneous nucleation will begin to dominate as the polyaniline will preferentially grow on existing particles, leading to irreversible agglomeration. The reaction can be made to favor homogeneous nucleation throughout by increasing reaction speed, temperature of the reaction, and allowing the reaction to proceed without stirring. The diameter of the polyaniline nanofibers can be controlled with this method through choice of acid.
Hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
produces nanofibers with a diameter of about 30 nm, while
camphorsulfonic acid Camphorsulfonic acid, sometimes abbreviated CSA or 10-CSA is an organosulfur compound. Like typical sulfonic acids, it is a relatively strong acid that is a colorless solid at room temperature and is soluble in water and a wide variety of organic ...
and
perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. It is an oxoacid of chlorine. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxid ...
produce a diameter of 50 nm and 120 nm respectively. Under normal synthetic methods polyaniline derivatives, such as ones that are
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...
and
fluoro Fluorine is a chemical element; it has symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as pale yellow diatomic gas. Fluorine is extremely reactive as it reacts with all other elements except for the lig ...
substituted, do not exhibit a well-defined fibrous shape, however, in the presence of an aniline
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
nanofibers of certain derivatives can be synthesized. While the most common oxidant is
ammonium peroxydisulfate Ammonium persulfate (APS) is the inorganic compound with the formula (NH4)2S2O8. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used as a catalys ...
(APS), various others can be used. One study shows the use of potassium biiodate (KH(IO3)2) as an oxidant, claiming it to lead to polyaniline nanofibers that are longer, have higher
crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a large influence on hardness, density, transparency and diffusi ...
, and have higher
electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
.Rahy, A.; Yang, D.J.; ''Mater. Lett.'', 2008, 62, 4311-431


Interfacial synthesis

In interfacial synthesis, the polymerization happens at the interface between an
aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...
and an
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
layer. A typical reaction involves an aqueous solution of acid and oxidant and an organic layer of aniline together. This creates the reactive interface for polymerization to occur. As polymerization proceeds, the polyaniline nanofibers will diffuse into the water layer, leaving the reactive interface. This prevents overgrowth onto the existing wires, allowing for homogeneous nucleation to continue occurring. Conditions in the interfacial synthesis can be tuned, such as the type of acid used as well as the oxidant used.


Rapid mixing reactions

Polyaniline nanofibers can also be synthesized through "rapid mixing" reactions. This method attempts to prevent overgrowth that would compromise the nanofiber nature of the polymer by stopping the polymerization immediately after nanofibers have been formed. This is achieved by the rapid mixing of the monomer, aniline, and an initiator solution. At the start of the reaction, the initiator is consumed rapidly and completely depleted when the nanofibers are formed. Without initiator remaining, the synthesis of polyaniline is halted.


Applications


Monolithic actuators

Polyaniline nanofibers have been used in the creation of
monolithic A monolith is a monument or natural feature consisting of a single massive stone or rock. Monolith or monolithic may also refer to: Architecture * Monolithic architecture, a style of construction in which a building is carved, cast or excavated f ...
actuators.Baker, C.O.; Shedd, B.; Innis, P.C.; Whitten, P.G.; Spinks, G.M.; Wallace, G.G.; Kaner, R.B.; ''Adv. Mater.'', 2008, 20, 155-15

They can be used in this application due to their ability to be flash-welded. When exposed to
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
, polyaniline converts the absorbed energy directly into heat. In a polyaniline film, the heat is dispersed throughout the polymer. In polyaniline nanofibers, however, the heat is trapped within the individual fibers. Therefore, if the intensity of the light is great enough, it will cause the temperature of the nanofibers to rise rapidly, which causes them to weld together or burn. With a moderate flash intensity, the nanofibers will melt rapidly to form a smooth film. Using mask, welds in specific patterns can be made using this technique. In a thick enough sample of nanofibers, only the side exposed to the flash will be welded, creating an asymmetric film where one side remains intact as nanofibers while the other side is effectively
crosslinked In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
due to welding. These asymmetric films demonstrate rapid reversible actuation in the presence of acids and bases, in the form of bending and curling. The advantages polyaniline nanofiber asymmetric films have over other actuators include the ease of synthesis, large degree of bending, patternability, and no delamination. These actuators could be used in the development of microtweezers, microvalves, artificial muscles, chemical sensors, and patterned actuator structures.


Digital memory devices

Research has shown that polyaniline nanofibers can also be used to create nonvolatile plastic digital memory devices when decorated with various
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
, such as
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
,
nanoparticle A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...
s.Tseng, R.J.; Huang, J.; Ouyang, J.; Kaner, R.B.; Yang, Y.; ''Nano Lett.'', 2005, 5 (6), 1077–108

Gold nanoparticle Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. The colloid is coloured usually either wine red (for spherical particles less than 100  nm) or blue-purple (for larger spherical partic ...
s are grown inside dedoped polyaniline nanofibers using a redox reaction. A plastic composite film is placed between two electrodes, and an external bias is used to program ON-OFF states. The switching mechanism is thought to be caused through an interaction between the polyaniline nanofibers and the gold nanoparticles, where charge is transferred to the gold nanoparticles from the polyaniline nanofibers due to an induced
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
. Switching between the ON–OFF states has shown to be rapid, with times of less than 25ns. The retention time of these simple devices are on the order of days after
programming Program (American English; also Commonwealth English in terms of computer programming and related activities) or programme (Commonwealth English in all other meanings), programmer, or programming may refer to: Business and management * Program m ...
, and write–read–erase cycles have been demonstrated.


Chemical sensing applications

Polyaniline nanofibers have been shown to be incredibly successful as
chemical sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
s, as they perform better than conventional polyaniline films in numerous tests.Virji, S.; Huang, J.; Kaner, R.B.; Weiller, B.H.; ''Nano Lett.'', 2004, 4 (3), 491–49

This performance difference has been attributed to their high
surface area The surface area (symbol ''A'') of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the d ...
,
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
, and small diameters which enhance diffusion of materials through the nanofibers. Polyaniline nanofiber sensors function through a change in resistance. The polyaniline nanofiber film is placed on 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 a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
, where a
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...
flows through. The resistance of the electrode changes when the target interacts with the film, which allows the target to be detected. One study proposes the creation of
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
gas sensors using polyaniline nanofibers.Sadek, A.Z.; Wlodarski, W.; Kalantar-Zadeh, K.; Baker, C.; Kaner, R.B.; ''Sens. Actuators A'', 2007, 139, 53-5

It shows that both doped and dedoped polyaniline nanofibers can be used for detection of hydrogen gas through resistance changes, but the dedoped nanofibers were more stable and had better reproducibility. Another study shows the potential of polyaniline nanofibers as Nitrogen dioxide, NO2 gas sensors.Yan, X.B.; Han, Z.J.; Yang, Y.; Tay, B.K.; ''Sens. Actuators B'', 2007, 123, 107-11

NO2 gas acts as a strong oxidizing agent to the emeraldine form of polyaniline nanofibers, which causes resistance changes greater than three orders of magnitude at 100 ppm. Sensing targets can be expanded through adding materials to the polyaniline nanofibers. One study proposes polyaniline nanofiber composites with metal salts for the detection of
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
.Virji, S.; Fowler, J.D.; Baker, C.O.; Huang, J.; Kaner, R.B.; Weiller, B.H.; ''Small'', 2005, 1 (6), 624-62

Hydrogen sulfide is a weak acid that is dangerous at low ppm, but polyaniline nanofibers can only give a robust response to strong acids. Metal salts can react with hydrogen sulfate to form a metal sulfide precipitate and a strong acid. By combining metal salts and polyaniline nanofibers, detection of hydrogen sulfide can be performed. Another study decorated polyaniline nanofibers with gold nanoparticles to detect volatile
sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
compounds in expired
human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
breath.Liu, C.; Hayashi, K.; Toko, K.; Sens. Actuators B, 2012, 161, 504-50

These sensors can potentially be used in various breath analyses and also in disease diagnosis for
diseases A disease is a particular abnormal condition that adversely affects the structure or function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical conditions that are asso ...
with malodor biomarker gases.
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 (meteorology), precipitation, dew, or fog t ...
sensors have also been prepared using polyaniline nanofibers.Lin, Q.; Li, Y.; Yang, M.; Sens. Actuators B, 2012, 161, 967-97

These sensors were prepared through electrospinning of a
N,N-dimethylformamide Dimethylformamide, DMF is an organic compound with the chemical formula . Its structure is . Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with ...
solution of polyaniline nanofibers, poly(vinyl butyral) (PVB), and
poly(ethylene oxide) Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular wei ...
(PEO). These sensors were shown to have high sensitivity, with resistance changes of three orders of magnitude. Furthermore, the sensors showed good sensing linearity, fast response, small hysterics, and good repeatability.


References


See also

*
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 ...
*
Nanofiber Nanofibers are fibers with diameters in the Nanometre, nanometer range (typically, between 1 nm and 1 μm). Nanofibers can be generated from different polymers and hence have different physical properties and application potentials. Examples ...
{{DEFAULTSORT:Polyaniline Nanofibers Nanoparticles Organic polymers Conductive polymers Fibers