Malic and tartaric acid are the primary acids in wine grapes.
The acids in wine are an important component in both winemaking and the finished product of wine. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeast during fermentation and protecting the wine from bacteria. The measure of the amount of acidity in wine is known as the “titratable acidity”
or “total acidity”, which refers to the test that yields the total of
all acids present, while strength of acidity is measured according to pH,
with most wines having a pH between 2.9 and 3.9. Generally, the lower
the pH, the higher the acidity in the wine. There is no direct
connection between total acidity and pH (it is possible to find wines
with a high pH for wine and high acidity). In wine tasting,
the term “acidity” refers to the fresh, tart and sour attributes of the
wine which are evaluated in relation to how well the acidity balances
out the sweetness and bitter components of the wine such as tannins. Three primary acids are found in wine grapes: tartaric, malic, and citric acids. During the course of winemaking and in the finished wines, acetic, butyric, lactic, and succinic acids can play significant roles. Most of the acids involved with wine are fixed acids with the notable exception of acetic acid, mostly found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. Sometimes, additional acids, such as ascorbic, sorbic and sulfurous acids, are used in winemaking.
Tartaric
While normally clear, tartaric crystals (pictured) can be dyed the color of the wine in which it has been saturated.
Tartaric acid is, from a winemaking perspective, the most important in wine due to the prominent role it plays in maintaining the chemical stability of the wine and its color and finally in influencing the taste of the finished wine. In most plants, this organic acid is rare, but it is found in significant concentrations in grape vines.
Along with malic acid, and to a lesser extent citric acid, tartaric is
one of the fixed acids found in wine grapes. The concentration varies
depending on grape variety and the soil content of the vineyard. Some varieties, such as Palomino, are naturally disposed to having high levels of tartaric acids while Malbec and Pinot noir generally have lower levels. During flowering, high levels of tartaric acid are concentrated in the grape flowers and then young berries. As the vine progresses through ripening, tartaric does not get metabolized through respiration
like malic acid, so the levels of tartaric acid in the grape vines
remain relatively consistent throughout the ripening process.
Less than half of the tartaric acid found in grapes is free standing, with the majority of the concentration present as potassium acid salt. During fermentation, these tartrates bind with the lees,
pulp debris and precipitated tannins and pigments. While some variance
among grape varieties and wine regions exists, generally about half of
the deposits are soluble in the alcoholic
mixture of wine. The crystallization of these tartrates can happen at
unpredictable times, and in a wine bottle may appear like broken glass,
though they are in fact harmless. Winemakers will often put the wine
through cold stabilization, where it is exposed to temperatures below freezing to encourage the tartrates to crystallize and precipitate out of the wine, or electrodialysis which removes the tartrates via a membrane process.
Malic
Riesling from cool climate wine regions, such as the Rheingau in Germany will have more malic acid and green apple notes than wines from warmer regions.
Malic acid, along with tartaric acid, is one of the principal organic acids found in wine grapes. It is found in nearly every fruit and berry plant, but is most often associated with green (unripe) apples, the flavor it most readily projects in wine. Its name comes from the Latinmalum
meaning “apple”. In the grape vine, malic acid is involved in several
processes which are essential for the health and sustainability of the
vine. Its chemical structure allows it to participate in enzymatic
reactions that transport energy throughout the vine. Its concentration
varies depending on the grape variety, with some varieties, such as Barbera, Carignan and Sylvaner, being naturally disposed to high levels. The levels of malic acid in grape berries are at their peak just before veraison, when they can be found in concentrations as high as 20 g/L. As the vine progresses through the ripening stage, malic acid is metabolized in the process of respiration, and by harvest,
its concentration could be as low as 1 to 9 g/L. The respiratory loss
of malic acid is more pronounced in warmer climates. When all the malic
acid is used up in the grape, it is considered “over-ripe” or senescent. Winemakers must compensate for this loss by adding extraneous acid at the winery in a process known as acidification.
Malic acid can be further reduced during the winemaking process through malolactic fermentation or MLF. In this process, bacteria convert the stronger malic acid into the softer lactic acid; formally, malic acid is polyprotic (contributes multiple protons, here two), while lactic acid is monoprotic (contributes one proton), and thus has only half the effect on acidity (pH); also, the first acidity constant (pKa) of malic acid (3.4 at room temperature)
is lower than the (single) acidity constant of lactic acid (3.86 at
room temperature), indicating stronger acidity. Thus after MLF, wine has
a higher pH (less acidic), and a different mouthfeel.
The bacteria behind this process can be found naturally in the winery, in cooperages, which make oak
wine barrels that will house a population of the bacteria or they can
be introduced by the winemaker with a cultured specimen. For some wines,
the conversion of malic into lactic acid can be beneficial, especially
if the wine has excessive levels of malic acid. For other wines, such as
Chenin blanc and Riesling, it produces off flavors in the wine (such as the buttery smell of diacetyl)
that would not be appealing for that variety. In general, red wines are
more often put through MLF than whites, which means a higher likelihood
of finding malic acid in white wines (though notable exceptions, such
as oaked Chardonnay, are often put through MLF).
Lactic
Chardonnay
is often put through malolactic fermentation when it is being oaked,
such as via oak chips as pictured. The softer, milky lactic acid helps
contribute to a creamier mouthfeel in the wine.
A much milder acid than tartaric and malic, lactic acid is often associated with “milky” flavors in wine and is the primary acid of yogurt and sauerkraut. It is produced during winemaking by lactic acid bacteria (LAB), which includes three genera: Oenococcus, Pediococcus and Lactobacillus.
These bacteria convert both sugar and malic acid into lactic acid, the
latter through MLF. This process can be beneficial for some wines,
adding complexity and softening the harshness of malic acidity, but it
can generate off flavors and turbidity in others. Some strains of LAB can produce biogenicamines, such as histamine, tyramine and putrescine, which may be a cause of red wine headaches in some wine drinkers. Winemakers wishing to control or prevent MLF can use sulfur dioxide to stun the bacteria. Racking the wine quickly off its lees
will also help control the bacteria, since lees are a vital food source
for them. The winemakers must also be very careful of what wine barrels
and winemaking equipment to which the wine is exposed, because of the
bacteria's ability to deeply embed themselves within wood fibers. A wine
barrel that has completed one successful malolactic fermentation will
almost always induce MLF in every wine stored in it from then on.
Citric
While very common in citrus fruits, such as limes, citric acid
is found only in very minute quantities in wine grapes. It often has a
concentration about 1/20 that of tartaric acid. The citric acid most
commonly found in wine is commercially produced acid supplements derived
from fermentingsucrose
solutions. These inexpensive supplements can be used by winemakers in
acidification to boost the wine's total acidity. It is used less
frequently than tartaric and malic due to the aggressive citric flavors
it can add to the wine. When citric acid is added, it is always done
after primary alcohol fermentation has been completed due to the
tendency of yeast to convert citric into acetic acid. In the European Union, use of citric acid for acidification is prohibited, but limited use of citric acid is permitted for removing excess iron and copper from the wine if potassium ferrocyanide is not available.
Acetic
Acetic acid is a two-carbon
organic acid produced in wine during or after the fermentation period.
It is the most volatile of the primary acids associated with wine and is
responsible for the sour taste of vinegar. During fermentation, activity by yeast cells naturally produces a small amount of acetic acid. If the wine is exposed to oxygen, Acetobacter bacteria will convert the ethanol
into acetic acid. This process is known as the “acetification” of wine
and is the primary process behind wine degradation into vinegar. An
excessive amount of acetic acid is also considered a wine fault. A taster's sensitivity to acetic acid will vary, but most people can detect excessive amounts at around 600 mg/L.
Ascorbic
Ascorbic acid, also known as vitamin C, is found in young wine grapes prior to veraison, but is rapidly lost throughout the ripening process. In winemaking, it is used with sulfur dioxide as an antioxidant,
often added during the bottling process for white wines. In the
European Union, use of ascorbic acid as an additive is limited to
150 mg/L.
The smell of crushed Pelargonium geranium leaves is a sign that a wine has a wine fault derived from sorbic acid.
Butyric
Butyric acid is a bacteria-induced wine fault that can cause a wine to smell of spoiled Camembert or rancid butter.
Sorbic
Sorbic acid is a winemaking additive used often in sweet wines as a preservative against fungi,
bacteria and yeast growth. Unlike sulfur dioxide, it does not hinder
the growth of the lactic acid bacteria. In the European Union, the
amount of sorbic acid that can be added is limited — no more than
200 mg/L. Most humans have a detection threshold of 135 mg/L, with some
having a sensitivity to detect its presence at 50 mg/L. Sorbic acid can
produce off flavors and aromas which can be described as “rancid”. When
lactic acid bacteria metabolize sorbates in the wine, it creates a wine
fault that is most recognizable by an aroma of crushed Pelargonium geranium leaves.
Succinic
Succinic acid
is most commonly found in wine, but can also be present in trace
amounts in ripened grapes. While concentration varies among grape
varieties, it is usually found in higher levels with red wine grapes.
The acid is created as a byproduct of the metabolization of nitrogen by yeast cells during fermentation. The combination of succinic acid with one molecule of ethanol will create the esterethyl succinate which contributes to a mild fruity aroma in wines.
Effects
Making
A wine with high pH and low acidity like Carménère(pictured) will have more bluish color notes than a wine with high acidity.
Acidity is highest in wine grapes just before the start of veraison, which ushers in the ripening period of the annual cycle of grape vines. As the grapes ripen, their sugar levels increase and their acidity levels decrease. Through the process of respiration, malic acid is metabolized by the grape vine. Grapes from cooler climate wine regions
generally have higher levels of acidity due to the slower ripening
process. The level of acidity still present in the grape is an important
consideration for winemakers in deciding when to begin harvest. For wines such as Champagne and other sparkling wines, having high levels of acidity is even more vital to the winemaking process, so grapes are often picked under-ripe and at higher acid levels.
In the winemaking process, acids aid in enhancing the effectiveness of sulfur dioxide
to protect the wines from spoilage and can also protect the wine from
bacteria due to the inability of most bacteria to survive in low pH
solutions. Two notable exceptions to this are Acetobacter and the lactic acid bacteria. In red wines, acidity helps preserve and stabilize the color of the wine. The ionization of anthocyanins is affected by pH, so wines with lower pH (such as Sangiovese-based wines) have redder, more stable colors. Wines with higher pH (such as Syrah-based
wines) have higher levels of less stable blue pigments, eventually
taking on a muddy grey hue. These wines can also develop a brownish
tinge. In white wines, higher pH (lower acidity) causes the phenolics in the wine to darken and eventually polymerize as brown deposits.
Winemakers will sometimes add acids to the wine (acidification)
to make the wine more acidic, most commonly in warm climate regions
where grapes are often harvested at advanced stages of ripeness with
high levels of sugars, but very low levels of acid. Tartaric acid is
most often added, but winemakers will sometimes add citric or malic
acid. Acids can be added either before or after primary fermentation.
They can be added during blending or aging, but the increased acidity
will become more noticeable to wine tasters if added at this point.
Tasting
The acidity in wine is an important component in the quality and
taste of the wine. It adds a sharpness to the flavors and is detected
most readily by a prickling sensation on the sides of the tongue and a
mouth-watering aftertaste. Of particular importance is the balance of
acidity versus the sweetness of the wine (the leftover residual sugar)
and the more bitter components of the wine (most notably tannins but
also includes other phenolics).
A wine with too much acidity will taste excessively sour and sharp. A
wine with too little acidity will taste flabby and flat, with less
defined flavors.
A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solutions and salts of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate. When citrate trianion is part of a salt, the formula of the citrate trianion is written as C6H5O73– or C3H5O(COO)33–.
Natural occurrence and industrial production
Lemons, oranges, limes, and other citrus fruits contain high concentrations of citric acid.
Citric acid occurs in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes
have particularly high concentrations of the acid; it can constitute as
much as 8% of the dry weight of these fruits (about 47 g/L in the
juices[12]).[a] The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes; these values vary within species depending upon the cultivar and the circumstances under which the fruit was grown.
Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.
Industrial-scale citric acid production first began in 1890 based
on the Italian citrus fruit industry, where the juice was treated with
hydrated lime (calcium hydroxide) to precipitate calcium citrate, which was isolated and converted back to the acid using diluted sulfuric acid. In 1893, C. Wehmer discovered Penicilliummold could produce citric acid from sugar. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian Citrus exports.
In 1917, American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers, and the pharmaceutical company Pfizer began industrial-level production using this technique two years later, followed by Citrique Belge in 1929. In this production technique, which is still the major industrial route to citric acid used today, cultures of Aspergillus niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, molasses, hydrolyzed corn starch, or other inexpensive, carbohydrate solution. After the mold is filtered out of the resulting suspension, citric acid is isolated by precipitating
it with calcium hydroxide to yield calcium citrate salt, from which
citric acid is regenerated by treatment with sulfuric acid, as in the
direct extraction from citrus fruit juice.
In 1977, a patent was granted to Lever Brothers
for the chemical synthesis of citric acid starting either from aconitic
or isocitrate (also called alloisocitrate) calcium salts under high
pressure conditions; this produced citric acid in near quantitative
conversion under what appeared to be a reverse, non-enzymatic Krebs cycle reaction.
Although industrial-scale production of citric acid by chemical
synthesis or extraction from citrus fruits are both feasible,
fermentation by molds (and sometimes yeasts) is almost exclusively the
only method actually practiced.
Global production was in excess of 2,000,000 tons in 2018. More than 50% of this volume was produced in China. More than 50% was used as an acidity regulator
in beverages, some 20% in other food applications, 20% for detergent
applications, and 10% for applications other than food, such as
cosmetics, pharmaceuticals, and in the chemical industry.
Chemical characteristics
Speciation diagram for a 10-millimolar solution of citric acidCitric acid monohydrate, crystallized by evaporation from solution at room temperature.
Citric acid is known to occur as a monohydrate, though the anhydrous form can be obtained by crystallization from hot water.
Water can be driven off the monohydrate to produce the anhydrate
by heating to around 80 °C, though this can also occur at ambient
temperatures slowly over time by efflorescence at humidities in range of ~50% or less.
Citric acid dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 °C. It decomposes with loss of carbon dioxide above about 175 °C.
Citric acid is a triprotic acid, with pKa values, extrapolated to zero ionic strength, of 3.128, 4.761, and 6.396 at 25 °C. The pKa of the hydroxyl group has been found, by means of 13C NMR spectroscopy, to be 14.4. The speciation diagram shows that solutions of citric acid are buffer solutions
between about pH 2 and pH 8. In biological systems around pH 7, the two
species present are the citrate ion and mono-hydrogen citrate ion. The
SSC 20X hybridization buffer is an example in common use. Tables compiled for biochemical studies are available.
Conversely, the pH of a 1 mM solution of citric acid will be about 3.2. The pH of fruit juices from citrus fruits
like oranges and lemons depends on the citric acid concentration, with a
higher concentration of citric acid resulting in a lower pH.
Acid salts of citric acid can be prepared by careful adjustment of the pH before crystallizing the compound. See, for example, sodium citrate.
The citrate ion forms complexes with metallic cations. The stability constants for the formation of these complexes are quite large because of the chelate effect.
Consequently, it forms complexes even with alkali metal cations.
However, when a chelate complex is formed using all three carboxylate
groups, the chelate rings have 7 and 8 members, which are generally less
stable thermodynamically than smaller chelate rings. In consequence,
the hydroxyl group can be deprotonated, forming part of a more stable
5-membered ring, as in ammonium ferric citrate, [NH+4]5Fe3+(C6H4O4−7)2·2H2O.
Citric acid can be esterified at one or more of its three carboxylic acid groups to form any of a variety of mono-, di-, tri-, and mixed esters.
Citrate is an intermediate in the citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, a central metabolic pathway for animals, plants, and bacteria. In the Krebs cycle, citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to form citrate. Citrate then acts as the substrate for aconitase and is converted into aconitic acid.
The cycle ends with regeneration of oxaloacetate. This series of
chemical reactions is the source of two-thirds of the food-derived
energy in higher organisms. The chemical energy released is available under the form of Adenosine triphosphate (ATP). Hans Adolf Krebs received the 1953 Nobel Prize in Physiology or Medicine for the discovery.
Other biological roles
Citrate can be transported out of the mitochondria and into the cytoplasm, then broken down into acetyl-CoA for fatty acid synthesis, and into oxaloacetate. Citrate is a positive modulator of this conversion, and allosterically regulates the enzyme acetyl-CoA carboxylase, which is the regulating enzyme in the conversion of acetyl-CoA into malonyl-CoA
(the commitment step in fatty acid synthesis). In short, citrate is
transported into the cytoplasm, converted into acetyl-CoA, which is then
converted into malonyl-CoA by acetyl-CoA carboxylase, which is
allosterically modulated by citrate.
High concentrations of cytosolic citrate can inhibit phosphofructokinase, the catalyst of a rate-limiting step of glycolysis.
This effect is advantageous: high concentrations of citrate indicate
that there is a large supply of biosynthetic precursor molecules, so
there is no need for phosphofructokinase to continue to send molecules
of its substrate, fructose 6-phosphate, into glycolysis. Citrate acts by augmenting the inhibitory effect of high concentrations of ATP, another sign that there is no need to carry out glycolysis.
Citrate is a vital component of bone, helping to regulate the size of apatite crystals.
Applications
Food and drink
Powdered citric acid being used to prepare lemon pepper seasoning
Because it is one of the stronger edible acids, the dominant use of
citric acid is as a flavoring and preservative in food and beverages,
especially soft drinks and candies. Within the European Union it is denoted by E numberE330. Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements. Citric acid has 247 kcal per 100 g. In the United States the purity requirements for citric acid as a food additive are defined by the Food Chemicals Codex, which is published by the United States Pharmacopoeia (USP).
Citric acid can be added to ice cream as an emulsifying agent to
keep fats from separating, to caramel to prevent sucrose
crystallization, or in recipes in place of fresh lemon juice. Citric
acid is used with sodium bicarbonate in a wide range of effervescent formulae, both for ingestion (e.g., powders and tablets) and for personal care (e.g., bath salts, bath bombs, and cleaning of grease).
Citric acid sold in a dry powdered form is commonly sold in markets and
groceries as "sour salt", due to its physical resemblance to table
salt. It has use in culinary applications, as an alternative to vinegar
or lemon juice, where a pure acid is needed. Citric acid can be used in food coloring to balance the pH level of a normally basic dye.
Cleaning and chelating agent
Structure of an iron(III) citrate complex
Citric acid is an excellent chelating agent, making metals soluble by binding to them. It is used to remove and discourage the buildup of limescale from boilers and evaporators. It can be used to treat water, which makes it useful in improving the
effectiveness of soaps and laundry detergents. By chelating the metals
in hard water,
it lets these cleaners produce foam and work better without need for
water softening. Citric acid is the active ingredient in some bathroom
and kitchen cleaning solutions. A solution with a six percent
concentration of citric acid will remove hard water stains from glass
without scrubbing. Citric acid can be used in shampoo to wash out wax
and coloring from the hair. Illustrative of its chelating abilities,
citric acid was the first successful eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s. In the 1950s, it was replaced by the far more efficient EDTA.
Cosmetics, pharmaceuticals, dietary supplements, and foods
Citric acid is used as an acidulant
in creams, gels, and liquids. Used in foods and dietary supplements, it
may be classified as a processing aid if it was added for a technical
or functional effect (e.g. acidulent, chelator, viscosifier, etc.). If
it is still present in insignificant amounts, and the technical or
functional effect is no longer present, it may be exempt from labeling
<21 CFR §101.100(c)>.
Citric acid is an alpha hydroxy acid and is an active ingredient in chemical skin peels.
Citric acid is commonly used as a buffer to increase the solubility of brown heroin.
Citric acid is used as one of the active ingredients in the production of facial tissues with antiviral properties.
Citric acid is used as an odorless alternative to white vinegar for fabric dyeing with acid dyes. It can enhance the mordanting process, crosslinking fabrics and dyes through an esterification reaction.
Sodium citrate is a component of Benedict's reagent, used for both qualitative and quantitative identification of reducing sugars.
Citric acid can be used as a lower-odor stop bath as part of the process for developing photographic film. Photographic developers are alkaline, so a mild acid is used to neutralize and stop their action quickly, but commonly used acetic acid leaves a strong vinegar odor in the darkroom.
Citric acid is an excellent solderingflux, either dry or as a concentrated solution in water. It should be removed
after soldering, especially with fine wires, as it is mildly corrosive.
It dissolves and rinses quickly in hot water.
Alkali citrate
can be used as an inhibitor of kidney stones by increasing urine
citrate levels, useful for prevention of calcium stones, and increasing
urine pH, useful for preventing uric acid and cystine stones.
Synthesis of other organic compounds
Citric acid is a versatile precursor to many other organic compounds. Dehydration routes give itaconic acid and its anhydride. Citraconic acid can be produced via thermal isomerization of itaconic acid anhydride. The required itaconic acid anhydride is obtained by dry distillation of citric acid. Aconitic acid can be synthesized by dehydration of citric acid using sulfuric acid:
Although a weak acid, exposure to pure citric acid can cause adverse
effects. Inhalation may cause cough, shortness of breath, or sore
throat. Over-ingestion may cause abdominal pain and sore throat.
Exposure of concentrated solutions to skin and eyes can cause redness
and pain. Long-term or repeated consumption may cause erosion of tooth enamel.
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, and favoured by massive cost decreases in graphene
production.
Medicine
Researchers in 2011 discovered the ability of graphene to accelerate the osteogenic differentiation of human mesenchymal stem cells without the use of biochemical inducers.
In 2015 researchers used graphene to create biosensors with epitaxial graphene on silicon carbide. The sensors bind to 8-hydroxydeoxyguanosine (8-OHdG) and is capable of selective binding with antibodies. The presence of 8-OHdG in blood, urine and saliva is commonly associated with DNA damage. Elevated levels of 8-OHdG have been linked to increased risk of several cancers. By the next year, a commercial version of a graphene biosensor was
being used by biology researchers as a protein binding sensor platform.
In 2016 researchers revealed that uncoated graphene can be used
as neuro-interface electrode without altering or damaging properties
such as signal strength or formation of scar tissue. Graphene electrodes
in the body are significantly more stable than electrodes of tungsten
or silicon because of properties such as flexibility, bio-compatibility
and conductivity.
Tissue engineering
Graphene has been investigated for tissue engineering. It has been
used as a reinforcing agent to improve the mechanical properties of
biodegradable polymeric nanocomposites for engineering bone tissue
applications. Dispersion of low weight % of graphene (≈0.02 wt.%) increased in
compressive and flexural mechanical properties of polymeric
nanocomposites. The addition of graphene nanoparticles in the polymer matrix lead to
improvements in the crosslinking density of the nanocomposite and better
load transfer from the polymer matrix to the underlying nanomaterial
thereby increasing the mechanical properties.
Contrast agents, bioimaging
Functionalized and surfactant dispersed graphene solutions have been designed as blood pool MRIcontrast agents. Further, iodine and manganese incorporating graphene nanoparticles have served as multimodal MRI-computerized tomograph (CT) contrast agents. Graphene micro- and nano-particles have served as contrast agents for photoacoustic and thermoacoustic tomography. Graphene has also been reported to be efficiently taking up cancerous
cells thereby enabling the design of drug delivery agents for cancer
therapy. Graphene nanoparticles of various morphologies such as graphene nanoribbons, graphene nanoplatelets and graphene nanoonions are non-toxic at low concentrations and do not alter stem cell
differentiation suggesting that they may be safe to use for biomedical
applications.
Polymerase chain reaction
Graphene is reported to have enhanced PCR by increasing the yield of DNA product. Experiments revealed that graphene's thermal conductivity
could be the main factor behind this result. Graphene yields DNA
product equivalent to positive control with up to 65% reduction in PCR
cycles.
Devices
Graphene's modifiable chemistry, large surface area per unit volume,
atomic thickness and molecularly gateable structure make
antibody-functionalized graphene sheets excellent candidates for
mammalian and microbial detection and diagnosis devices. Graphene is so thin that water has near-perfect wetting transparency which is an important property particularly in developing bio-sensor applications. This means that a sensor coated in graphene has as much contact with an
aqueous system as an uncoated sensor, while remaining protected
mechanically from its environment.
Energy of the electrons with wavenumber k in graphene, calculated in the Tight Binding-approximation. The unoccupied (occupied) states, colored in blue–red (yellow–green), touch each other without an energy gap exactly at the above-mentioned six k-vectors.
Integration of graphene (thickness of 0.34 nm) layers as nanoelectrodes into a nanopore can potentially solve a bottleneck for nanopore-based single-molecule DNA sequencing.
On November 20, 2013, the Bill & Melinda Gates Foundation awarded $100,000 'to develop new elastic composite materials for condoms containing nanomaterials like graphene'.
In 2014, graphene-based, transparent (across infrared to
ultraviolet frequencies), flexible, implantable medical sensor
microarrays were announced that allow the viewing of brain tissue hidden
by implants. Optical transparency was greater than 90%. Applications
demonstrated include optogenetic activation of focal cortical areas, in vivo imaging of cortical vasculature via fluorescence microscopy and 3D optical coherence tomography.
Drug delivery
Researchers at Monash University
discovered that a sheet of graphene oxide can be transformed into
liquid crystal droplets spontaneously—like a polymer—simply by placing
the material in a solution and manipulating the pH. The graphene
droplets change their structure in the presence of an external magnetic
field. This finding raises the possibility of carrying a drug in
graphene droplets and releasing the drug upon reaching the targeted
tissue by making the droplets change shape in a magnetic field. Another
possible application is in disease detection if graphene is found to
change shape at the presence of certain disease markers such as toxins.
A graphene 'flying carpet' was demonstrated to deliver two anti-cancer drugs sequentially to the lung tumor cells (A549 cell) in a mouse model. Doxorubicin (DOX) is embedded onto the graphene sheet, while the molecules of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) are linked to the nanostructure via short peptide
chains. Injected intravenously, the graphene strips with the drug
payload preferentially concentrate to the cancer cells due to common
blood vessel leakage around the tumor. Receptors on the cancer cell membrane bind TRAIL and cell surface enzymes
clip the peptide thus release the drug onto the cell surface. Without
the bulky TRAIL, the graphene strips with the embedded DOX are swallowed
into the cells. The intracellular acidic environment promotes DOX's
release from graphene. TRAIL on the cell surface triggers the apoptosis while DOX attacks the nucleus. These two drugs work synergistically and were found to be more effective than either drug alone.
The development of nanotechnology and molecular biology has
provided the improvement of nanomaterials with specific properties which
are now able to overcome the weaknesses of traditional disease
diagnostic and therapeutic procedures. In recent years, more attention has been devoted to designing and the
development of new methods for realizing sustained release of diverse
drugs. Since each drug has a plasma level above which is toxic and below
which is ineffective and in conventional drug delivery, the drug
concentration in the blood rises quickly and then declines, the main aim
of an ideal drug delivery system (DDS) is to maintain the drug within a
desired therapeutic range after a single dose, and/or target the drug
to a specific region while simultaneously lowering the systemic levels
of the drug. Graphene–based materials such as graphene oxide (GO) have considerable
potential for several biological applications including the development
of new drug release system. GOs are an abundance of functional groups
such as hydroxyl, epoxy, and carboxyl on its basal surface and edges
that can be also used to immobilize or load various biomolecules for
biomedical applications. On the other side, biopolymers have frequently
been used as raw materials for designing drug delivery formulations
owing to their excellent properties, such as non-toxicity,
biocompatibility, biodegradability
and environmental sensitivity, etc. Protein therapeutics possess
advantages over small molecule approaches including high target
specificity and low off target effects with normal biological processes.
Human serum albumin (HSA) is one of the most abundant blood proteins.
It serves as a transport protein for several endogenous and exogenous
ligands as well as various drug molecules. HSA nanoparticles have long
been the center of attention in the pharmaceutical industry due to their
ability to bind to various drug molecules, high storage stability and
in vivo application, non–toxicity and antigenicity, biodegradability,
reproducibility, scale–up of the production process and a better control
over release properties. In addition, significant amounts of drugs can
be incorporated into the particle matrix because of the large number of
drug binding sites on the albumin molecule. Therefore, the combination of HSA-NPs and GO-NSs could be useful for
reducing the cytotoxicity of GO-NSs and the enhancement of drug loading
and sustained drug release in cancer therapy.
Biomicrorobotics
Researchers demonstrated a nanoscale biomicrorobot (or cytobot) made
by cladding a living endospore cell with graphene quantum dots. The
device acted as a humidity sensor.
Testing
In 2014 a graphene based blood glucose testing product was announced.
Biosensors
Graphene based FRET biosensors can detect DNA and the unwinding of DNA using different probes.
Graphene has a high carrier mobility, and low noise, allowing it to be used as the channel in a field-effect transistor. Unmodified graphene does not have an energy band gap, making it unsuitable for digital electronics. However, modifications (e.g. Graphene nanoribbons) have created potential uses in various areas of electronics.
Transistors
Both chemically controlled and voltage controlled graphene transistors have been built.
Graphene-based transistors could be much thinner than modern silicon devices, allowing faster and smaller configurations.
Graphene exhibits a pronounced response to perpendicular external electric fields, potentially forming field-effect transistors (FET), but the absence of a band gap fundamentally limits its on-off conductance ratio to less than ~30 at room temperature. A 2006 paper proposed an all-graphene planar FET with side gates. Their devices showed changes of 2% at cryogenic temperatures. The first
top-gated FET (on–off ratio of <2) was demonstrated in 2007. Graphene nanoribbons may prove generally capable of replacing silicon as a semiconductor.
A patent for graphene-based electronics was issued in 2006. In 2008, researchers at MIT Lincoln Lab produced hundreds of transistors on a single chip and in 2009, very high frequency transistors were produced at Hughes Research Laboratories.
A 2008 paper demonstrated a switching effect based on reversible
chemical modification of the graphene layer that gives an on–off ratio
of greater than six orders of magnitude. These reversible switches could
potentially be employed in nonvolatile memories. IBM announced in December 2008 graphene transistors operating at GHz frequencies.
In 2009, researchers demonstrated four different types of logic gates, each composed of a single graphene transistor. In May 2009, an n-type transistor complemented the prior p-type graphene transistors. A functional graphene integrated circuit was demonstrated—a complementary inverter consisting of one p- and one n-type transistor. However, this inverter suffered from low voltage gain. Typically, the
amplitude of the output signal is about 40 times less than that of the
input signal. Moreover, none of these circuits operated at frequencies
higher than 25 kHz.
In the same year, tight-binding numerical simulations demonstrated that the band-gap induced in graphene bilayer field effect
transistors is not sufficiently large for high-performance transistors
for digital applications, but can be sufficient for ultra-low voltage
applications, when exploiting a tunnel-FET architecture.
In February 2010, researchers announced graphene transistors with
an on-off rate of 100 gigahertz, far exceeding prior rates, and
exceeding the speed of silicon transistors with an equal gate length.
The 240 nm devices were made with conventional silicon-manufacturing equipment. According to a January 2010 report, graphene was epitaxially grown on SiC in a quantity and with quality suitable for mass production of integrated circuits. At high temperatures, the quantum Hall effect could be measured. IBM built 'processors' using 100 GHz transistors on 2-inch (51 mm) graphene sheets.
In June 2011, IBM researchers announced the first graphene-based wafer-scale integrated circuit, a broadband radio mixer. The circuit handled frequencies up to 10 GHz. Its performance was
unaffected by temperatures up to 127 °C. In November researchers used 3D
printing (additive manufacturing) to fabricate devices.
In 2013, researchers demonstrated graphene's high mobility in a
detector that allows broad band frequency selectivity ranging from the
THz to IR region (0.76–33 THz) A separate group created a terahertz-speed transistor with bistable
characteristics, which means that the device can spontaneously switch
between two electronic states. The device consists of two layers of
graphene separated by an insulating layer of boron nitride a few atomic layers thick. Electrons move through this barrier by quantum tunneling.
These new transistors exhibit negative differential conductance,
whereby the same electric current flows at two different applied
voltages. In June, an 8 transistor 1.28 GHz ring oscillator circuit was described.
The negative differential resistance experimentally observed in
graphene field-effect transistors of conventional design allows for
construction of viable non-Boolean computational architectures. The
negative differential resistance—observed under certain biasing
schemes—is an intrinsic property of graphene resulting from its
symmetric band structure. The results present a conceptual change in
graphene research and indicate an alternative route for graphene
applications in information processing.
In 2013 researchers created transistors printed on flexible
plastic that operate at 25 gigahertz, sufficient for communications
circuits and that can be fabricated at scale. The researchers first
fabricated non-graphene-containing structures—the electrodes and
gates—on plastic sheets. Separately, they grew large graphene sheets on
metal, then peeled them and transferred them to the plastic. Finally,
they topped the sheet with a waterproof layer. The devices work after
being soaked in water, and were flexible enough to be folded.
In 2015 researchers devised a digital switch by perforating a
graphene sheet with boron-nitride nanotubes that exhibited a switching
ratio of 105 at a turn-on voltage of 0.5 V. Density functional theory suggested that the behavior came from the mismatch of the density of states.
Single atom
In 2008, a one atom thick, 10 atoms wide transistor was made of graphene.
In 2022, researchers built a 0.34 nanometer (on state) single
atom graphene transistor, smaller than a related device that used carbon
nanotubes instead of graphene. The graphene formed the gate. Silicon
dioxide was used as the base. The graphene sheet was formed via chemical vapor deposition, laid on top of the SiO 2. A sheet of aluminum oxide was laid atop the graphene. The Al 2O x and SiO 2 sandwiching the graphene act as insulators. They then etched into the sandwiched materials, cutting away the graphene and Al 2O x to create a step that exposed the edge of the graphene. They then added layers of hafnium oxide and molybdenum disulfide
(another 2D material) to the top, side, and bottom of the step.
Electrodes were then added to the top and bottom as source and drain.
They call this construction a "sidewall transistor". The on/off ratio
reached 1.02×105 and subthreshold swing values were 117 mV dec–1.
Trilayer
An electric field can change trilayer graphene's crystal structure,
transforming its behavior from metal-like into semiconductor-like. A
sharp metal scanning tunneling microscopy
tip was able to move the domain border between the upper and lower
graphene configurations. One side of the material behaves as a metal,
while the other side behaves as a semiconductor. Trilayer graphene can
be stacked in either Bernal or rhombohedral
configurations, which can exist in a single flake. The two domains are
separated by a precise boundary at which the middle layer is strained to
accommodate the transition from one stacking pattern to the other.
Silicon transistors are either p-type or n-type, whereas graphene
can operate as both. This lowers costs and is more versatile. The
technique provides the basis for a field-effect transistor.
In trilayer graphene, the two stacking configurations exhibit
different electronic properties. The region between them consists of a
localized strain soliton where the carbon atoms of one graphene layer shift by the carbon–carbon bond
distance. The free-energy difference between the two stacking
configurations scales quadratically with electric field, favoring
rhombohedral stacking as the electric field increases.
This ability to control the stacking order opens the way to new devices that combine structural and electrical properties.
Transparent conducting electrodes
Graphene's high electrical conductivity and high optical transparency
make it a candidate for transparent conducting electrodes, required for
such applications as touchscreens, liquid crystal displays, inorganic photovoltaics cells, organic photovoltaic cells, and organic light-emitting diodes. In particular, graphene's mechanical strength and flexibility are advantageous compared to indium tin oxide, which is brittle. Graphene films may be deposited from solution over large areas.
Large-area, continuous, transparent and highly conducting
few-layered graphene films were produced by chemical vapor deposition
and used as anodes for application in photovoltaic
devices. A power conversion efficiency (PCE) up to 1.7% was
demonstrated, which is 55.2% of the PCE of a control device based on
indium tin oxide. However, the main disadvantage brought by the
fabrication method will be the poor substrate bondings that will
eventually lead to poor cyclic stability and cause high resistivity to
the electrodes.
Organic light-emitting diodes
(OLEDs) with graphene anodes have been demonstrated. The device was
formed by solution-processed graphene on a quartz substrate. The
electronic and optical performance of graphene-based devices are similar
to devices made with indium tin oxide. In 2017 OLED electrodes were produced by CVD on a copper substrate.
In 2014 a prototype graphene-based flexible display was demonstrated.
In 2016 researchers demonstrated a display that used
interferometry modulation to control colors, dubbed a "graphene balloon
device" made of silicon containing 10 μm circular cavities covered by
two graphene sheets. The degree of curvature of the sheets above each
cavity defines the color emitted. The device exploits the phenomena
known as Newton's rings
created by interference between light waves bouncing off the bottom of
the cavity and the (transparent) material. Increasing the distance
between the silicon and the membrane increased the wavelength of the
light. The approach is used in colored e-reader displays and
smartwatches, such as the Qualcomm Toq. They use silicon materials instead of graphene. Graphene reduces power requirements.
Frequency multiplier
In 2009, researchers built experimental graphene frequency multipliers that take an incoming signal of a certain frequency and output a signal at a multiple of that frequency.
Optoelectronics
Graphene strongly interacts with photons, with the potential for direct band-gap creation. This is promising for optoelectronic and nanophotonic devices. Light interaction arises due to the Van Hove singularity.
Graphene displays different time scales in response to photon
interaction, ranging from femtoseconds (ultra-fast) to picoseconds.
Potential uses include transparent films, touch screens and light
emitters or as a plasmonic device that confines light and alters
wavelengths.
Hall effect sensors
Due to extremely high electron mobility, graphene may be used for production of highly sensitive Hall effect sensors. Potential application of such sensors is connected with DC current transformers for special applications. New record high sensitive Hall sensors are reported in April 2015.
These sensors are two times better than existing Si based sensors.
Quantum dots
Graphene quantum dots (GQDs) keep all dimensions less than 10 nm. Their size and edge crystallography govern their electrical, magnetic, optical, and chemical properties. GQDs can be produced via graphite nanotomy or via bottom-up, solution-based routes (Diels-Alder, cyclotrimerization and/or cyclodehydrogenation reactions). GQDs with controlled structure can be incorporated into applications in electronics, optoelectronics and electromagnetics. Quantum confinement can be created by changing the width of graphene nanoribbons (GNRs) at selected points along the ribbon. It is studied as a catalyst for fuel cells.
Organic electronics
A semiconducting polymer (poly(3-hexylthiophene) placed on top of single-layer graphene vertically conducts electric
charge better than on a thin layer of silicon. A 50 nm thick polymer
film conducted charge about 50 times better than a 10 nm thick film,
potentially because the former consists of a mosaic of variably-oriented
crystallites forms a continuous pathway of interconnected crystals. In a
thin film or on silicon, plate-like crystallites are oriented parallel to the graphene layer. Uses include solar cells.
Spintronics
Large-area graphene created by chemical vapor deposition (CVD) and layered on a SiO2 substrate, can preserve electron spin over an extended period and communicate it. Spintronics
varies electron spin rather than current flow. The spin signal is
preserved in graphene channels that are up to 16 micrometers long over a
nanosecond. Pure spin transport and precession extended over 16 μm
channel lengths with a spin lifetime of 1.2 ns and a spin diffusion
length of ≈6 μm at room temperature.
Spintronics is used in disk drives for data storage and in magnetic random-access memory.
Electronic spin is generally short-lived and fragile, but the
spin-based information in current devices needs to travel only a few
nanometers. However, in processors, the information must cross several
tens of micrometers with aligned spins. Graphene is the only known
candidate for such behavior.
Conductive ink
In 2012 Vorbeck Materials started shipping the Siren anti-theft packaging device, which uses their graphene-based Vor-Ink circuitry to replace the metal antenna and external wiring to an RFID chip. This was the world's first commercially available product based on graphene.
Light processing
Optical modulator
When the Fermi level
of graphene is tuned, its optical absorption can be changed. In 2011,
researchers reported the first graphene-based optical modulator.
Operating at 1.2 GHz without a temperature controller, this modulator has a broad bandwidth (from 1.3 to 1.6 μm) and small footprint (~25 μm2).
A Mach-Zehnder modulator based on a hybrid graphene-silicon
waveguide has been demonstrated recently, which can process signals
nearly chirp-free. An extinction up to 34.7 dB and a minimum chirp parameter of -0.006 are obtained. Its insertion loss is roughly -1.37 dB.
Ultraviolet lens
A hyperlens
is a real-time super-resolution lens that can transform evanescent
waves into propagating waves and thus break the diffraction limit. In
2016 a hyperlens based on dielectric layered graphene and h-boron nitride
(h-BN) can surpass metal designs. Based on its anisotropic properties,
flat and cylindrical hyperlenses were numerically verified with layered
graphene at 1200 THz and layered h-BN at 1400 THz, respectively. In 2016 a 1-nm thick graphene microlens that can image objects the size
of a single bacterium. The lens was created by spraying a sheet of
graphene oxide solution, then molding the lens using a laser beam. It
can resolve objects as small as 200 nanometers, and see into the near
infrared. It breaks the diffraction limit and achieve a focal length
less than half the wavelength of light. Possible applications include
thermal imaging for mobile phones, endoscopes, nanosatellites and photonic chips in supercomputers and superfast broadband distribution.
Infrared light detection
Graphene reacts to the infrared spectrum at room temperature, albeit
with sensitivity 100 to 1000 times too low for practical applications.
However, two graphene layers separated by an insulator allowed an
electric field produced by holes left by photo-freed electrons in one
layer to affect a current running through the other layer. The process
produces little heat, making it suitable for use in night-vision optics.
The sandwich is thin enough to be integrated in handheld devices,
eyeglass-mounted computers and even contact lenses.
Photodetector
A graphene/n-type silicon heterojunction has been demonstrated to
exhibit strong rectifying behavior and high photoresponsivity. By
introducing a thin interfacial oxide layer, the dark current of
graphene/n-Si heterojunction has been reduced by two orders of magnitude
at zero bias. At room temperature, the graphene/n-Si photodetector with
interfacial oxide exhibits a specific detectivity up to 5.77 × 1013 cm Hz1/2 W2
at the peak wavelength of 890 nm in vacuum. In addition, the improved
graphene/n-Si heterojunction photodetectors possess high responsivity of
0.73 A W−1 and high photo-to-dark current ratio of ≈107.
These results demonstrate that graphene/Si heterojunction with
interfacial oxide is promising for the development of high detectivity
photodetectors. Recently, a graphene/si Schottky photodetector with record-fast
response speed (< 25 ns) from wavelength 350 nm to 1100 nm are
presented. The photodetectors exhibit excellent long-term stability even stored in
air for more than 2 years. These results not only advance the
development of high-performance photodetectors based on the graphene/Si
Schottky junction, but also have important implications for
mass-production of graphene-based photodetector array devices for
cost-effective environmental monitoring, medical images, free-space
communications, photoelectric smart-tracking, and integration with CMOS
circuits for emerging interest-of-things applications, etc.
Energy
Generation
Ethanol distillation
Graphene oxide membranes allow water vapor to pass through, but are impermeable to other liquids and gases. This phenomenon has been used for further distilling of vodka
to higher alcohol concentrations, in a room-temperature laboratory,
without the application of heat or vacuum as used in traditional distillation methods. Further development and commercialization of such membranes could revolutionize the economics of biofuel production and the alcoholic beverage industry.
Solar cells
Graphene has been used on different substrates such as Si, CdS and
CdSe to produce Schottky junction solar cells. Through the properties of
graphene, such as graphene's work function, solar cell efficiency can
be optimized. An advantage of graphene electrodes is the ability to
produce inexpensive Schottky junction solar cells.
Charge conductor
Graphene solar cells use graphene's unique combination of high electrical conductivity and optical transparency. This material absorbs only 2.6% of green light and 2.3% of red light. Graphene can be assembled into a film electrode with low roughness.
These films must be made thicker than one atomic layer to obtain useful
sheet resistances. This added resistance can be offset by incorporating
conductive filler materials, such as a silica matrix. Reduced conductivity can be offset by attaching large aromatic molecules such as pyrene-1-sulfonic
acid sodium salt (PyS) and the disodium salt of
3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid (PDI).
These molecules, under high temperatures, facilitate better
π-conjugation of the graphene basal plane.
Light collector
Using graphene as a photoactive material requires its bandgap to be
1.4–1.9 eV. In 2010, single cell efficiencies of nanostructured
graphene-based PVs of over 12% were achieved. According to P.
Mukhopadhyay and R. K. Gupta organic photovoltaics
could be "devices in which semiconducting graphene is used as the
photoactive material and metallic graphene is used as the conductive
electrodes".
In 2008, chemical vapor deposition produced graphene sheets by depositing a graphene film made from methane gas on a nickel plate. A protective layer of thermoplastic
is laid over the graphene layer and the nickel underneath is then
dissolved in an acid bath. The final step is to attach the
plastic-coated graphene to a flexible polymer
sheet, which can then be incorporated into a PV cell. Graphene/polymer
sheets range in size up to 150 square centimeters and can be used to
create dense arrays.
Silicon generates only one current-driving electron for each
photon it absorbs, while graphene can produce multiple electrons. Solar
cells made with graphene could offer 60% conversion efficiency.
Fuel cells
Appropriately perforated graphene (and hexagonal boron nitride hBN) can allow protons
to pass through it, offering the potential for using graphene
monolayers as a barrier that blocks hydrogen atoms but not
protons/ionized hydrogen (hydrogen atoms with their electrons stripped
off). They could even be used to extract hydrogen gas out of the
atmosphere that could power electric generators with ambient air.
The membranes are more effective at elevated temperatures and when covered with catalytic nanoparticles such as platinum.
Graphene could solve a major problem for fuel cells: fuel crossover that reduces efficiency and durability.
In methanol fuel cells, graphene used as a barrier layer in the
membrane area, has reduced fuel cross over with negligible proton
resistance, improving the performance.
At room temperature, proton conductivity with monolayer hBN,
outperforms graphene, with resistivity to proton flow of about 10 Ω cm2 and a low activation energy of about 0.3 electronvolts. At higher temperatures, graphene outperforms with resistivity estimated to fall below 10−3 Ω cm2 above 250 degrees Celsius.
In another project, protons easily pass through slightly imperfect graphene membranes on fused silica in water. The membrane was exposed to cycles of high and low pH. Protons
transferred reversibly from the aqueous phase through the graphene to
the other side where they undergo acid–base chemistry with silica
hydroxyl groups. Computer simulations indicated energy barriers of
0.61–0.75 eV for hydroxyl-terminated atomic defects that participate in a
Grotthuss-type relay, while pyrylium-like ether terminations did not. Recently, Paul and co-workers at IISER Bhopal demonstrated solid state
proton conduction for oxygen functionalized few-layer graphene (8.7 × 10−3 S/cm) with a low activation barrier (0.25 eV).
Thermoelectrics
Adding 0.6% graphene to a mixture of lanthanum and partly reduced strontium titanium oxide produces a strong Seebeck
at temperatures ranging from room temperature to 750 °C (compared to
500–750 without graphene). The material converts 5% of the heat into
electricity (compared to 1% for strontium titanium oxide.)
Condenser coating
In 2015 a graphene coating on steam condensers quadrupled
condensation efficiency, increasing overall plant efficiency by 2–3
percent.
In February 2013 researchers announced a novel technique to produce graphene supercapacitors based on the DVD burner reduction approach.
In 2014 a supercapacitor was announced that was claimed to achieve energy density comparable to current lithium-ion batteries.
In 2015 the technique was adapted to produce stacked, 3-D supercapacitors.
Laser-induced graphene was produced on both sides of a polymer sheet.
The sections were then stacked, separated by solid electrolytes, making
multiple microsupercapacitors. The stacked configuration substantially
increased the energy density of the result. In testing, the researchers
charged and discharged the devices for thousands of cycles with almost
no loss of capacitance. The resulting devices were mechanically flexible, surviving 8,000
bending cycles. This makes them potentially suitable for rolling in a
cylindrical configuration. Solid-state polymeric electrolyte-based
devices exhibit areal capacitance of >9 mF/cm2 at a current density
of 0.02 mA/cm2, over twice that of conventional aqueous electrolytes.
Also in 2015 another project announced a microsupercapacitor that
is small enough to fit in wearable or implantable devices. Just
one-fifth the thickness of a sheet of paper, it is capable of holding
more than twice as much charge as a comparable thin-film lithium
battery. The design employed laser-scribed graphene, or LSG with manganese dioxide.
They can be fabricated without extreme temperatures or expensive "dry
rooms". Their capacity is six times that of commercially available
supercapacitors. The device reached volumetric capacitance of over 1,100 F/cm3. This corresponds to a specific capacitance of the constituent MnO2 of 1,145 F/g, close to the theoretical maximum of 1,380 F/g. Energy density varies between 22 and 42 Wh/L depending on device configuration.
In May 2015 a boric acid-infused,
laser-induced graphene supercapacitor tripled its areal energy density
and increased its volumetric energy density 5-10 fold. The new devices
proved stable over 12,000 charge-discharge cycles, retaining 90 percent
of their capacitance. In stress tests, they survived 8,000 bending
cycles.
Stable lithium ion cycling was demonstrated in bi- and few layer graphene films grown on nickelsubstrates, while single layer graphene films have been demonstrated as a
protective layer against corrosion in battery components such as the
battery case. This creates possibilities for flexible electrodes for microscale
Li-ion batteries, where the anode acts as the active material and the
current collector.
In 2014 researchers built a lithium-ion battery made of graphene and silicon, claiming took only 15 minutes to charge. In 2014, graphene with controlled topological defects was demonstrated
to adsorb more ions, resulting in high-efficiency batteries.
In 2015 argon-ion
based plasma processing was used to bombard graphene samples with argon
ions. That knocked out some carbon atoms and increased the capacitance
of the materials three-fold. These "armchair" and "zigzag" defects
reflect the configurations of the carbon atoms that surround the holes.
In 2016, Huawei announced graphene-assisted lithium-ion batteries with greater heat tolerance and twice the life span of traditional Lithium-Ion batteries, the component with the shortest life span in mobile phones.
Electrode
In 2010, researchers first reported creating a graphene-silicon
heterojunction solar cell, where graphene served as a transparent
electrode and introduced a built-in electric field near the interface
between the graphene and n-type silicon to help collect charge carriers. In 2012 researchers reported efficiency of 8.6% for a prototype
consisting of a silicon wafer coated with trifluoromethanesulfonyl-amide
(TFSA) doped graphene. Doping increased efficiency to 9.6% in 2013. In 2015 researchers reported efficiency of 15.6% by choosing the optimal oxide thickness on the silicon. This combination of carbon materials with traditional silicon
semiconductors to fabricate solar cells has been a promising field of
carbon science.
In 2013, another team reported 15.6% percent by combining titanium oxide and graphene as a charge collector and perovskite
as a sunlight absorber. The device is manufacturable at temperatures
under 150 °C (302 °F) using solution-based deposition. This lowers
production costs and offers the potential using flexible plastics.
In 2015, researchers developed a prototype cell that used
semitransparent perovskite with graphene electrodes. The design allowed
light to be absorbed from both sides. It offered efficiency of around 12
percent with estimated production costs of less than $0.06/watt. The
graphene was coated with PEDOT:PSS conductive polymer (polythiophene)
polystyrene sulfonate). Multilayering graphene via CVD created
transparent electrodes reducing sheet resistance. Performance was
further improved by increasing contact between the top electrodes and
the hole transport layer.
Copper wire
has long been used for power transmission for its high conductivity,
ductility, and low costs. However, traditional wire fails to meet the
transmission requirements of many new technologies. Thermally dependent resistivity in mesoscopic copper wire limits efficiency and current carrying capacity in small-scale electronics. Additionally, copper wire exhibits internal failure by electromigration
at high current density, limiting miniaturization of wire. Copper's
high weight and low temperature oxidation also limit its applications in
high-power transmission. Increasing demand for high ampacity transmission in electronics and
electric vehicle applications necessitate improvements in conductor
technology.
Graphene-copper composite conductors are a promising alternative to standard conductors in high-power applications.
In 2013, researchers demonstrated a one-hundred-fold increase in current carrying capacity with carbon nanotube-copper
composite wires when compared to traditional copper wire. These
composite wires exhibited a temperature coefficient of resistivity an
order of magnitude smaller than copper wires, an important feature for
high load applications.
Additionally, in 2021, researchers demonstrated a 4.5 times increase
in the current density breakdown limit of copper wire with an axially
continuous graphene shell. The copper wire was coated by a continuous
graphene sheet through chemical vapor deposition. The coated wire exhibited reduced oxidation of the wire during joule heating, increased heat dissipation (224% higher), and increased conductivity (41% higher).
Sensors
Biosensors
Graphene does not oxidize in air or in biological fluids, making it an attractive material for use as a biosensor. A graphene circuit can be configured as a field effect biosensor by
applying biological capture molecules and blocking layers to the
graphene, then controlling the voltage difference between the graphene
and the liquid that includes the biological test sample. Of the various
types of graphene sensors that can be made, biosensors were the first to
be available for sale.
Pressure sensors
The electronic properties of graphene/h-BN heterostructures can be
modulated by changing the interlayer distances via applying external
pressure, leading to potential realization of atomic thin pressure
sensors. In 2011 researchers proposed an in-plane pressure sensor
consisting of graphene sandwiched between hexagonal boron nitride and a
tunneling pressure sensor consisting of h-BN sandwiched by graphene. The current varies by 3 orders of magnitude as pressure increases from 0 to 5 nN/nm2.
This structure is insensitive to the number of wrapping h-BN layers,
simplifying process control. Because h-BN and graphene are inert to high
temperature, the device could support ultra-thin pressure sensors for
application under extreme conditions.
In 2016 researchers demonstrated a biocompatible pressure sensor
made from mixing graphene flakes with cross-linked polysilicone (found
in silly putty).
NEMS
Nanoelectromechanical systems
(NEMS) can be designed and characterized by understanding the
interaction and coupling between the mechanical, electrical, and the van
der Waals energy domains. Quantum mechanical limit governed by
Heisenberg uncertainty relation decides the ultimate precision of
nanomechanical systems. Quantum squeezing can improve the precision by
reducing quantum fluctuations in one desired amplitude of the two
quadrature amplitudes. Traditional NEMS hardly achieve quantum squeezing
due to their thickness limits. A scheme to obtain squeezed quantum
states through typical experimental graphene NEMS structures taking
advantages of its atomic scale thickness has been proposed.
Molecular absorption
Theoretically graphene makes an excellent sensor due to its 2D
structure. The fact that its entire volume is exposed to its surrounding
environment makes it very efficient to detect adsorbed
molecules. However, similar to carbon nanotubes, graphene has no
dangling bonds on its surface. Gaseous molecules cannot be readily
adsorbed onto graphene surfaces, so intrinsically graphene is
insensitive. The sensitivity of graphene chemical gas sensors can be dramatically
enhanced by functionalization, for example, coating the film with a thin
layer of certain polymers. The thin polymer layer acts like a
concentrator that absorbs gaseous molecules. The molecule absorption
introduces a local change in electrical resistance
of graphene sensors. While this effect occurs in other materials,
graphene is superior due to its high electrical conductivity (even when
few carriers are present) and low noise, which makes this change in
resistance detectable.
Piezoelectric effect
Density functional theory simulations predict that depositing certain adatoms on graphene can render it piezoelectrically
responsive to an electric field applied in the out-of-plane direction.
This type of locally engineered piezoelectricity is similar in magnitude
to that of bulk piezoelectric materials and makes graphene a candidate
for control and sensing in nanoscale devices.
Body motion
Promoted by the demand for wearable devices, graphene has been proved
to be a promising material for potential applications in flexible and
highly sensitive strain sensors. An environment-friendly and
cost-effective method to fabricate large-area ultrathin graphene films
is proposed for highly sensitive flexible strain sensor. The assembled
graphene films are derived rapidly at the liquid/air interface by
Marangoni effect and the area can be scaled up. These graphene-based
strain sensors exhibit extremely high sensitivity with gauge factor of
1037 at 2% strain, which represents the highest value for graphene
platelets at this small deformation so far.
Rubber bands infused with graphene ("G-bands") can be used as
inexpensive body sensors. The bands remain pliable and can be used as a
sensor to measure breathing, heart rate, or movement. Lightweight sensor
suits for vulnerable patients could make it possible to remotely
monitor subtle movement. These sensors display 10×104-fold
increases in resistance and work at strains exceeding 800%. Gauge
factors of up to 35 were observed. Such sensors can function at
vibration frequencies of at least 160 Hz. At 60 Hz, strains of at least 6% at strain rates exceeding 6000%/s can be monitored.
Magnetic
In 2015 researchers announced a graphene-based magnetic sensor 100
times more sensitive than an equivalent device based on silicon (7,000
volts per amp-tesla). The sensor substrate was hexagonal boron nitride. The sensors were based on the Hall effect, in which a magnetic field induces a Lorentz force
on moving electric charge carriers, leading to deflection and a
measurable Hall voltage. In the worst case graphene roughly matched a
best case silicon design. In the best case graphene required lower
source current and power requirements.
Environmental
Contaminant removal
Graphene oxide is non-toxic and biodegradable. Its surface is covered
with epoxy, hydroxyl, and carboxyl groups that interact with cations
and anions. It is soluble in water and forms stable colloid suspensions in other liquids because it is amphiphilic (able to mix with water or oil). Dispersed in liquids it shows excellent sorption capacities. It can remove copper, cobalt, cadmium, arsenate, and organic solvents.
Research suggests that graphene filters could outperform other techniques of desalination by a significant margin.
In 2021, researchers found that a reusable graphene foam
could efficiently filter uranium (and possibly other heavy metals such
as lead, mercury and cadmium) from water at the rate of 4 grams of
uranium/gram of graphene.
Permeation barrier
Instead of allowing the permeation, blocking is also necessary. Gas
permeation barriers are important for almost all applications ranging
from food, pharmaceutical, medical, inorganic and organic electronic
devices, etc. packaging. It extends the life of the product and allows
keeping the total thickness of devices small. Being atomically thin,
defectless graphene is impermeable to all gases. In particular,
ultra-thin moisture permeation barrier layers based on graphene are
shown to be important for organic-FETs and OLEDs. Graphene barrier applications in biological sciences are under study.
Other
Art preservation
In 2021, researchers reported that a graphene veil reversibly applied via chemical vapor deposition was able to preserve the colors in art objects (70%).
Aviation
In 2016, researchers developed a prototype de-icing system that
incorporated unzipped carbon nanotube graphene nanoribbons in an epoxy/graphene
composite. In laboratory tests, the leading edge of a helicopter rotor
blade was coated with the composite, covered by a protective metal
sleeve. Applying an electrical current heated the composite to over
200 °F (93 °C), melting a 1 cm (0.4 in)-thick ice layer with ambient
temperatures of a -4 °F (-20 °C).
Catalyst
In 2014, researchers at the University of Western Australia discovered nano sized fragments of graphene can speed up the rate of chemical reactions. In 2015, researchers announced an atomic scale catalyst made of
graphene doped with nitrogen and augmented with small amounts of cobalt
whose onset voltage was comparable to platinum catalysts. In 2016 iron-nitrogen complexes embedded in graphene were reported as
another form of catalyst. The new material was claimed to approach the
efficiency of platinum catalysts. The approach eliminated the need for
less efficient iron nanoparticles.
Coolant additive
Graphene's high thermal conductivity suggests that it could be used
as an additive in coolants. Preliminary research work showed that 5%
graphene by volume can enhance the thermal conductivity of a base fluid
by 86%. Another application due to graphene's enhanced thermal conductivity was found in PCR.
Lubricant
Scientists discovered using graphene as a lubricant works better than traditionally used graphite.
A one atom thick layer of graphene in between a steel ball and steel
disc lasted for 6,500 cycles. Conventional lubricants lasted 1,000
cycles.
Nanoantennas
A graphene-based plasmonic nano-antenna (GPN) can operate efficiently
at millimeter radio wavelengths. The wavelength of surface plasmonpolaritons
for a given frequency is several hundred times smaller than the
wavelength of freely propagating electromagnetic waves of the same
frequency. These speed and size differences enable efficient
graphene-based antennas to be far smaller than conventional
alternatives. The latter operate at frequencies 100–1000 times larger
than GPNs, producing 0.01–0.001 as many photons.
An electromagnetic (EM) wave directed vertically onto a graphene
surface excites the graphene into oscillations that interact with those
in the dielectric on which the graphene is mounted, thereby forming surface plasmon polaritons
(SPP). When the antenna becomes resonant (an integral number of SPP
wavelengths fit into the physical dimensions of the graphene), the
SPP/EM coupling increases greatly, efficiently transferring energy
between the two.
A phased array antenna 100 μm
in diameter could produce 300 GHz beams only a few degrees in diameter,
instead of the 180 degree radiation from a conventional metal antenna
of that size. Potential uses include smart dust, low-power terabit wireless networks and photonics.
A nanoscale gold rod antenna captured and transformed EM energy
into graphene plasmons, analogous to a radio antenna converting radio
waves into electromagnetic waves in a metal cable. The plasmon wave
fronts can be directly controlled by adjusting antenna geometry. The
waves were focused (by curving the antenna) and refracted (by a
prism-shaped graphene bilayer because the conductivity in the
two-atom-thick prism is larger than in the surrounding one-atom-thick
layer.)
The plasmonic metal-graphene nanoantenna was composed by
inserting a few nanometers of oxide between a dipole gold nanorod and
the monolayer graphene. The used oxide layer here can reduce the quantum tunneling effect
between graphene and metal antenna. With tuning the chemical potential
of the graphene layer through field effect transistor architecture, the
in-phase and out-phase mode coupling between graphene plasmonics and
metal plasmonics is realized. The tunable properties of the plasmonic metal-graphene nanoantenna can
be switched on and off via modifying the electrostatic gate-voltage on
graphene.
Plasmonics and metamaterials
Graphene accommodates a plasmonic surface mode, observed recently via near field infrared optical microscopy techniques and infrared spectroscopy Potential applications are in the terahertz to mid-infrared frequencies, such as terahertz and midinfrared light modulators, passive terahertz filters, mid-infrared photodetectors and biosensors.
Radio wave absorption
Stacked graphene layers on a quartz substrate increased the
absorption of millimeter (radio) waves by 90 per cent over 125–165 GHz
bandwidth, extensible to microwave and low-terahertz frequencies, while
remaining transparent to visible light. For example, graphene could be
used as a coating for buildings or windows to block radio waves.
Absorption is a result of mutually coupled Fabry–Perot resonators represented by each graphene-quartz substrate. A repeated transfer-and-etch process was used to control surface resistivity.
Redox
Graphene oxide
can be reversibly reduced and oxidized via electrical stimulus.
Controlled reduction and oxidation in two-terminal devices containing
multilayer graphene oxide films are shown to result in switching between
partly reduced graphene oxide and graphene, a process that modifies
electronic and optical properties. Oxidation and reduction are related
to resistive switching.
Reference material
Graphene's properties suggest it as a reference material for characterizing electroconductive and transparent materials. One layer of graphene absorbs 2.3% of red light.
Researchers demonstrated a graphene-oxide-based aerogel
that could reduce noise by up to 16 decibels. The aerogel weighed 2.1
kilograms per cubic metre (0.13 lb/cu ft). A conventional polyester urethane
sound absorber might weigh 32 kilograms per cubic metre (2.0 lb/cu ft).
One possible application is to reduce sound levels in airplane cabins.
Sound transducers
Graphene's light weight provides relatively good frequency response, suggesting uses in electrostatic audio speakers and microphones. In 2015 an ultrasonic microphone and speaker were demonstrated that
could operate at frequencies from 20 Hz–500 kHz. The speaker operated at
a claimed 99% efficiency with a flat frequency response across the
audible range. One application was as a radio replacement for
long-distance communications, given sound's ability to penetrate steel
and water, unlike radio waves.
Structural material
Graphene's strength, stiffness and lightness suggested it for use with carbon fiber.
Graphene has been used as a reinforcing agent to improve the mechanical
properties of biodegradable polymeric nanocomposites for engineering
bone tissue.
It has also been used as a strengthening agent in concrete.
Thermal management
In 2011, researchers reported that a three-dimensional, vertically
aligned, functionalized multilayer graphene architecture can be an
approach for graphene-based thermal interfacial materials (TIMs) with superior thermal conductivity and ultra-low interfacial thermal resistance between graphene and metal.
Graphene-metal composites can be used in thermal interface materials.
Adding a layer of graphene to each side of a copper film
increased the metal's heat-conducting properties up to 24%. This
suggests the possibility of using them for semiconductor interconnects
in computer chips. The improvement is the result of changes in copper's
nano- and microstructure, not from graphene's independent action as an
added heat conducting channel. High temperature chemical vapor
deposition stimulates grain size growth in copper films. The larger
grain sizes improve heat conduction. The heat conduction improvement was
more pronounced in thinner copper films, which is useful as copper
interconnects shrink.
Attaching graphene functionalized with silane molecules increases its thermal conductivity (κ) by 15–56% with respect to the number density
of molecules. This is because of enhanced in-plane heat conduction
resulting from the simultaneous increase of thermal resistance between
the graphene and the substrate, which limited cross-plane phonon scattering. Heat spreading ability doubled.
However, mismatches at the boundary between horizontally adjacent crystals reduces heat transfer by a factor of 10.
