Graphene Oxide/Graphene Toxicity

As with all new materials, toxicity is a huge element that restricts or allows them to be used in various fields. Graphene oxide is no exception, as various studies have been conducted on the substance in attempt to decipher how its interaction with both animal and bacterial cells. As these studies uncover more information, it may seal graphene oxide’s fate in one of the largest markets: healthcare and pharmaceuticals. Read about breakthrough studies here.

 

The cell on the left was treated with GO, while the cell on the right acted as a control. Image property of Y. Change et al.

The cell on the left was treated with GO, while the cell on the right acted as a control. Image property of Y. Change et al.

Discussion: The Chemistry of Reducing Graphene Oxide

Perhaps the most important question in graphene oxide chemistry involves how it can be reduced into a material that is both electrically conductive as well as chemically, thermally, and mechanically stable. As discussed in the last rGO article, reduction involves removing the oxygen functional groups away from the planar carbon structure. Sounds easy enough, but there have been many issues through methods of reduction involving both structural failures as well as incomplete reduction.

First of all, when observing a molecule of graphene oxide, we need to look at the bond angles of the molecule. In rGO, there are two kinds of carbon molecules: sp2 and sp3 (in a simpler sense, sp2 carbons bind to three other atoms, sharing a double bond with one of them, while a sp3 carbon binds to a four other atoms).

 

methane and ethene diagrams

Examples of sp3 (methane) and sp2 (ethene) molecules.

These two are simple examples of sp2 and sp3 bonded carbons. In the methane (sp3) molecule, the optimal angle between all four C-H bonds are at 109.5 degrees. In the ethane, we see that the optimal angle is 120 degrees between the C-H bonds. Now that we understand the different angles on the different molecules, we can apply this to our model of graphene oxide before reduction.

graphene oxide example

The sp2 carbons are highlighted in blue, while the sp3 carbons are highlighted in red.

As we expected, we see both sp2 and sp3 carbons in this model. However, we also observe that all the carbon atoms, whether they are sp3 or sp2, are within the hexagonal structure: giving them bond angles of 120 degrees. However, as we learned above, the sp3 carbons would rather have bonds of 109.5 degrees, which results in bond angle strain.

To relieve this strain, the sp3 carbons would either like to shift themselves out of the planar structure of the graphene oxide itself, or to gain electrons (aka reduction) to make a double bond with a neighboring carbon atom, making themselves sp2 and therefore happier with a 120 degree angle. Of course, the first option does not happen due to the huge amount of force it would take to break the planar structure of the graphene oxide. However, the second option does happen, resulting in the following:

diagram of reduced graphene oxide

Our final product after reduction: all carbons are sp2!

Now we see that after the oxygen functional groups left and a double bond between the two sp3 carbons converted them to sp2 carbons, all the carbons are now locked in 120 degree angles, which reduces all angle strain. This explains why graphene oxide is relatively easy to reduce into rGO, while rGO itself is more stable.

The next few posts will be talking about how to reduce graphene oxide through the three main methods: chemically, thermally, and photochemically. All these methods have their advantages and disadvantages, which often result in some surprising results.

Definitions: What is Reduced Graphene Oxide?

Reduced graphene oxide, or rGO, has perhaps been one of the most discussed topics in graphene communities ever since its creation from graphene oxide. However, it has come to my attention that many people are confused about what the term entails, as well as how rGO is related to graphene oxide or graphene. I hope to educate and clear up these terms in this post!

Graphene oxide solution on the left shows polar dispersability , while reduced graphene oxide is unable to disperse in water. The container on the furthest side shows a much smaller amount of reduced graphene oxide, which is shown to still not disperse in water.

Graphene oxide solution on the left shows polar dispersability, while reduced graphene oxide is unable to disperse in water. The container on the furthest side shows a much smaller amount of reduced graphene oxide, which is still not able to disperse in water.

In order to understand rGO, it is important to first understand what the definition of graphene oxide is. In the simplest sense, reduced graphene oxide is the reduced form of graphene oxide. Obvious, but what exactly does this reduction entail? Reduction is the term for when electrons are added to an atom, or in this case, a network of atoms.

In graphene oxide, there are many protruding oxygen groups along the side of the flake as well as its planar surface, which consists of carbon atoms bound in hexagonal rings. Reducing it results in the addition of electrons to the actual carbon atoms, which results in two major chemical changes:

  1. The carbon atoms regain their double bonds, resulting in restoration of their conjugation as well as electrical conductivity in the flake as a whole.
  2. The oxygen functional groups leave, resulting in the loss of hydrophilicity and therefore loss of the flake to disperse in polar solvents.
A simple reduction, showing the leaving of functional groups alongside the restoration of double bonds.

A simple reduction, showing the leaving of functional groups alongside the restoration of double bonds.

As seen in the above figure, the addition of a chemical reagent (in this case, hydrogen gas over a catalyst of palladium), and the oxygen groups have been removed for the example molecule while it has regained a double bond. To put this in context, this is the reaction that happens with graphene oxide is reduced into rGO. There are many methods to do this, including chemically, thermally, and photochemically.

However, no one method of reducing graphene oxide to rGO is perfect. This is due to the fact that reducing graphene oxide often results in structural defects that make them unable to be used in electronics requiring a high level of specificity, such as semiconductors. Also, graphene oxide can only theoretically be totally reduced: there will always be oxygen groups that remain on the flake, thereby reducing the electrical conductivity of the rGO.

The rGO continues to loose mass as it is heated, indicating remaining functional groups. This image is the property of Cui et al.

This thermogravimetric analysis of graphene oxide and rGO demonstrate that the graphene oxide and rGO both continue to lose weight as the temperature increases [1]. This is indicative of further reduction through thermal means, as the oxygen functional groups leaving result in a loss of weight. At this point, you may be wondering why we can’t just heat up the GO to a super high temperature in order to reduce it completely? The reason we are unable to do this is due to the fact that the actual carbon structure will begin to break down, thereby causing the GO flakes to disintegrate.

rGO will also never be equivalent to pristine graphene, due to the fact that rGO films are formed with multilayered atoms, while graphene consists of a single, one atom-thick layer of carbon atoms.  Also, rGO films do not result in the formation of carbon-carbon bonds, so many small flakes of rGO cannot be formed into a large sheet of graphene. Graphene also contains no oxygen functional groups at all, while rGO often has leftover oxygen groups, no matter how reduced it is.

At this point, we can conclude that rGO is a flake of graphene oxide that has been treated so that it loses its oxygen functional groups for a restoration of its double bond conjugation and electrical conductivity. Interested in learning more about the potential applications of rGO? Stay tuned, as this will be discussed in future blog posts!

Resources:

1. DOI: 10.1039/C1CC15569E

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Discussion: The History of Graphene Oxide

Brodie's graphene oxide data

Original data from the first ever paper on graphene oxide, by Brodie.

Introduction

The history of graphene oxide (GO), like many other novel materials, begins with its unintended discovery. Though there is a gap of about 150 years between its creation and its renewed interest, graphene oxide is perhaps one of the most unique substances ever researched. Starting from its first synthesis in in the lab to its modern day capability for practical application ranging from radioactive waste cleanup to quantum dots, the story of this wondrous material begins in a single lab.

B.C. Brodie was a professor at the University of Oxford in 1859 who was attempting to find the molecular weight of graphite. Having noticed that graphite had very different properties than other forms of carbon such as diamond or charcoal, Brodie had hypothesized that graphite was a unique element and therefore decided that the best method was to oxidize the graphite and then to perform elemental analysis, since he already knew the molecular weights of hydrogen and oxygen: graphite would be the missing piece of his puzzle.

Discovery

However, Brodie knew that graphite was unreactive with strong oxidizers, as even submerging it in boiling sulfuric acid would result in no reaction, eliciting his usage of extremely strong chemicals. Therefore, after using a mixture of potassium chlorate, fuming nitric acid, and heating, Brodie observed that the dull grey graphite had gained weight and taken on a light yellow color. Upon weighing the substance, Brodie also noticed that the substance had gained weight from the original graphite used: an indication of oxidization.

At this point, Brodie set off to analyze his product and quickly noticed that it was divided into seemingly smaller flakes, which he deemed to be crystals. Though his observation through a light microscope, he made an observation that would be significant even today: noticing that “the crystals are so extremely thin in a direction perpendicular to the paper … that it is impossible to obtain any reflection” (251). Though he was observing a thin, dried film rather than a suspension, this is perhaps one of the first times a near-atom thick substance was observed by a human being.

Image

Brodie reduces graphene oxide through thermal means.

Impressions

Having no luck from optical observation, Brodie resorted to combusting it in order to discover what elements it contained. Luckily, he managed to determine it contained only carbon, oxygen, and hydrogen in the forms of carbonic acid and water, with a carbon:oxygen ratio of 11:6. Interestingly enough, it did not contain a detectable amount of nitrogen or chlorine, which was found in the original reagents. Upon ignition, Brodie also observed that the graphene oxide would “explode” into a black powder: the first ever reduction of graphene oxide ever performed [1].

Though Brodie’s conclusion that graphite was a unique element was eventually proven to be wrong, his lengthy observations and studies into the synthesis of graphene oxide offer us valuable insight in its physical and chemical properties. However, there was still one major issue: Brodie’s synthesis method involved chemicals that were extremely dangerous and prone to explosion, which would only be exacerbated by his addition of heat. It was only a century later, in 1957, when W. Hummers and R. Offeman would develop a safer method of oxidation.

Image

Legacy

Hummers and Offeman also used a method that involved extremely strong oxidizers: potassium permanganate, sodium nitrate, and sulfuric acid. However, their method used fewer reagents and had no water: allowing for both a speedier and safer synthesis, as well as producing a product with a slightly higher ratio of oxidation at 2.1 carbon atoms with every 1 oxygen atoms. Modern scientists still use a modified version of Hummer’s method, showing how Brodie, Hummers, and Offemans’ studies have ultimately been the root of thousands of studies of graphene oxide across a multitude of industries [2].

Today, graphene oxide has the potential to open thousands of doors of human potential. As we continue to research and streamline the process to create graphene oxide, we hope that our research will allow us to bring various graphene oxide applications into reality.

References:

  1. On the Atomic Weight of Graphite

B. C. Brodie

Philosophical Transactions of the Royal Society of London, Vol. 149, (1859), pp. 249-259

Published by: The Royal Society

Article Stable URL: http://www.jstor.org/stable/108699

  1. DOI: 10.1021/ja01539a017

Authors note:

There actually were only two notable papers during the history of graphene oxide; this is probably because:

  • Graphene oxide wasn’t created to be used, it was accidently created when Brodie tried to provide graphite was an element
  • The technology to utilize graphene oxide has only been recently discovered (last decade)
  • Graphene oxide mainly gained attention due to the graphene hype, which only started in the last few years as well

Definitions: What is Graphene Oxide, anyways?

In the simplest sense, graphene oxide refers to single-atom layers of carbon (otherwise known as graphene) with functional groups containing oxygen attached to both the sides and area of the flake. Since the addition of the oxygen groups result in the breakage of the double bonds holding the carbon atoms together, the material loses its electrical conductivity. However, these oxygen groups cause the flake as a whole to become polar, and therefore able to disperse in solvents such as water. Since water is a nonvolatile and nonreactive solvent, this allows graphene oxide solutions to be easily separated and dried into films, allowing for even distribution of graphene oxide on virtually all surfaces [1].

Image

A simplified diagram of graphene oxide.

As seen in Figure 1 above, this proposed structure of graphene oxide is very simple, consisting of planar carbons with oxygen functional groups (in this case, alcohols) protruding from the plane. The functional groups have the ability to bind to other groups on molecules due to an effect known as hydrogen bonding. Though not the strongest bond, this effect allows graphene oxide flakes to disperse in water easily, as well as cling to various other molecules with similar functional groups: the most important being biological molecules such as DNA and amino acids.

However, one of the main issues with understand what graphene oxide is comes with the mystery of its actual synthesis mechanism. Ours and others research has shown us so far that it forms when graphite is under the presence of strong oxidants, but no one currently knows the exact mechanism. Therefore, we are unable to visualize the exact structure of the flakes. It is also worth mentioning that various oxidation methods yield graphene oxide with different properties, hinting that there is no one “graphene oxide”: instead, graphene oxide is an umbrella term for all types of graphene flakes that contain oxygen groups, rather than a definition of a single type of unique molecule.

The uses of graphene oxide depend on these functional groups, whose reactivity allows them to be used in two ways: functionalization and in situ. Since these uses range multiple industries, methods, and fields, they can too numerous to be discussed in a simple blog post. However, I will post more information in future posts.

I hope this article helped you understand the basic structure and physical properties of graphene oxide. If you have any questions, criticisms, or queries, we encourage you to contact us at hypermark.go@gmail.com or leave a reply!

References

  1. DOI: 10.1039/B917103G

Want to learn more about Hypermark GO? Visit our website at http://hypermarkgo.webs.com/ .

Introduction: The Beginning of the Graphene Oxide Age

In the last few years, interest in the single-atom thick substance known as graphene has multiplied due to its potential applications in virtually all industries. However, I believe a close cousin of graphene has been overlooked: graphene oxide, a substance that has just as many potential applications that has been shown to be easier to synthesize and manipulate.

Consisting of one-atom thick layers of carbon atoms with protruding oxygen groups, graphene oxide’s potential uses are increased a thousandfold by one simple physical change from graphene: its ability to disperse in water. This allows graphene oxide to form suspensions which have uses ranging from tagging DNA to being reduced into films capable of being used in supercapacitators [1],[2].

I am starting this blog not only to educate and inform, but also to inspire. I invite you to join me on this journey to better understand the creation, usages, and implications of graphene oxide. This blog will include reviews on recently published research, discussions on its applications, as well as updates on my personal research at Hypermark GO.

This blog is written for both the professional and hobbyist, and I hope you enjoy reading it as much as I enjoy writing it.

Resources:

1. DOI: 10.1021/la1037926.

2. DOI10.1039/C1JM00007A

About our company: Hypermark GO is a research oriented company that is focused on improving the synthesis of graphene oxide, thereby allowing for creation of the highest quality GO at the lowest price. We hope that we will someday aid in the development of products based on GO. For more information, please visit our website at http://hypermarkgo.webs.com/.