The production of Carbon 60 crystals is a moderately complex sequence of steps involving soot production, fullerene extraction, C60 isolation and purification. The following is a representative model.
The first step is the formation of carbon soot. This is obtained by heating high purity carbon to a temperature where the atoms will evaporate and then re-condense onto chamber surfaces. Temperatures in excess of 1000 °C are needed and this is done in a low pressure helium atmosphere. The required temperature can be obtained via high current resistance heating, arc evaporation, pyrolysis, RF plasma or laser ablation. Figure 1 illustrates two of the methods.
The soot that is produced is a mixture of amorphous carbon and fullerenes. Only about 5 to 15 percent of the overall mix is fullerene molecules. The resultant fullerene mix will be about 70% C60 + 30% C70. Less than 1% will be larger fullerenes and possibly nano-tubes.
The second step involves the extraction of fullerenes from the soot by using a toluene solvent. If the soot material is mixed with toluene, C60 and larger fullerenes will go into solution and the amorphous component will settle to the bottom of the mixing vessel. As described, the technique is somewhat slow and the yield is less than optimal. The process can be improved up by using a Soxhlet extractor. The fullerene containing solution can then be filtered out.
Next in sequence is the process of C60 extraction. This is done using a laboratory technique called chromatography. Basically, the fullerene mixture (which may be further diluted) forms a mobile phase. This mobile phase flows through an adsorbent material referred to as the stationary phase. Because they have different properties, components in the mobile phase pass through the stationary phase at different speeds. Carbon 60 will pass through first and then the heavier fullerenes will come later. This process may be repeated one or more times to increase the fullerene purity. The solvent is then evaporated off to leave solid fullerene crystals.
The final step is that of C60 purification and is essential if the C60 crystals are to be used for biomedical purposes. The process involves baking the C60 crystals in a heated vacuum oven for a period of time, long enough to partially sublimate the material and vaporize the residual toluene. This must be done under tightly controlled conditions and can take 24 hours or more.
Figure 1. Schematic showing fullerene synthesis by (a) Huffman-Krätschmer method and (b) combustion method.
When procuring C60, a purity percentage is specified. It is important to understand that this relates to the fullerene content and not to other elements. A purity of 99.0% means there is 99.0 % C60, 1.0 % C70 and possibly trace amounts of higher fullerenes. Other impurities would need to be specified separately and are not usually provided by C60 vendors.
An angstrom (À) is a unit of measure equal to 0.1 nm (nano-meter).
|C-C length||1.45 À|
|C=C length||1.39 À|
|outer diameter||10.18 À|
|mean diameter||7.1 À|
|inner diameter||3.48 À|
|# pentagon sides||12|
|# hexagon sides||20|
Table 1. Carbon 60 molecule properties.
|lattice constant||14.17 À|
|molecular weight||720 g/mol|
|low T||-20 ºC|
|sublimation T||527 ºC|
|tetrahedral void diameter||2.24 À|
|octahedral void diameter||4.14 À|
Table 2. Carbon 60 crystal properties.
Tables 1 and 2 provide some of the more relevant properties for C60 molecules and the crystal structure observed at room temperature. More comprehensive information can be found here and here.
At room temperature, C60 exists in a face centered cubic (fcc) crystal structure as illustrated in figure 2 (C). In a fcc structure C60 molecules can be found at each corner and on each face center. Molecules are bound to each other by weak Van der Walls interactions.
Figure 2. (a) Carbon bond lengths, (b) carbon 60 and (C) crystal structure.
Figure 3. Fullerene biomedical applications.
Based on a keyword search of medical research papers the distribution in figure 3 was created. It illustrates the diverse range of fullerene applicability to biological systems. It also shows that a lot of work has been undertaken to investigate possible toxicity effects. Toxicity is discussed in more detail here.
A carbon 60 molecule (figure 4A) consists of 60 carbon atoms joined to form a hollow sphere. The surface consists of 20 hexagons and 12 pentagons. Carbon atoms are connected by either single or double bonds. If a hexagonal side is adjacent to another hexagonal side a double bond exists in between. Hexagonal to pentagonal edges contain single bonds.
The properties of carbon 60 molecules can be modified by chemical functionalization. This basically involves breaking open one or more of the double bonds and joining with other molecules. C60 can be functionalized to exhibit affinity to certain nucleic acids, proteins or cell receptors. Polar or ionic groups can be joined to make C60 water soluble.
Figure 4 illustrates a basic C60 molecule and some functionalized C60 molecules. The addition of carboxylic or hydroxyl groups significantly improves the solubility of C60 in water. Anywhere from 2 to 40 groups can be added. References ref1 and ref2 both contain tables listing many of the C60 derivative structures, the application and research papers.
Figure 4. Carbon 60, carboxyfullerene and fullerenol molecules (left to right).
To make C60 more lipophilic (fat soluble) the C60 molecule/particle may be incorporated into a liposome or a micelle (Figure 5). Both liposomes and micelles are vesicular structures composed of amphiphilic molecules that have hydrophilic (water loving) heads and lipophilic (lipid loving) tails. A micelle has one layer and a liposome has more than one layer. These structures are able to maintain membrane structural integrity with low concentrations of C60.
Figure 5. Illustration of a two layer C60 liposome and a micelle. C60 molecules would position within the (orange) lipophilic tails.
A solution is a homogeneous mixture of 2 or more substances. Of interest to us is the dissolution of C60 (solute) into a vegetable oil (solvent). Carbon 60 is in the form of a fine crystal powder when first introduced into the solvent. When mixed, these crystals slowly dissolve and bond with the triglyceride fatty acids. These triglycerides may then form liposome spheres. Once C60 bonds to the triglyceride fatty acids it cannot be separated out by filtration.
The solubility limit for olive oil is about 0.8 mg per liter. Other vegetable oils have about the same value or less. To obtain this level of saturation the mixture needs to be stirred for up to 2 weeks. Mixing can also be accomplished by ultrasound or increasing the temperature while mixing. These methods are discouraged by many as they may oxidize the oil and make it age faster. Using ultrasound can make polyunsaturated oil pro-oxidant (please refer to the Toxicity page for discussion on this).
If the amount of C60 added to the oil exceeds the solubility limit then the remaining crystals will stop dissolving. The amount and size distribution of the remaining crystals will of course depend on the amount of C60 added in the starting mixture. Larger crystal particles will slowly settle on the bottom of the flask under the force of gravity. The smallest of particles will not move at all and literally becoming suspended due to frictional forces imposed by the viscosity of the oil.
If a C60/oil mix ratio in excess of the solubility limit is used, then the residual crystals should be removed by centrifuge and/or by using a vacuum-based filtration system (0.22 um filter paper). The kidneys will need to process these crystals if not removed. Some users, on social media channels, have reported kidney pain when consuming unfiltered mixtures.
In the Baati research paper, they used a ratio of 5 grams of C60 per liter of olive oil during mixing. This is 4.2 grams more than the 0.8 g/L solubility limit. They used C60 in excess to ensure that they obtained the 0.8 g/L limit. Only a few of the C60/mixture vendors use this higher ratio; most use a 0.8 g/L ratio.
It should also be noted that it is unknown if the fine particles that remain in suspension (with size less than 0.22 um after filtration) are biologically active when consumed. These slightly larger (non-solution) nano particles may actually be beneficial. Some experimentation and analysis would be required to resolve this unknown.
Biomedical applications of functionalized fullerene-based nanomaterials
Fullerene - biomedical engineers get to revisit an old friend
Liposome Formulation of Fullerene-Based Molecular Diagnostic and Therapeutic Agents
Structural phase transitions of C60 under high-pressure and high-temperature
Carbon Materials: Chemistry and Physics
The design of a fullerene generator
Wikipedia Liposome Micelle