Measuring the Solubility of Ligated Gold Nanoparticles in
by Jeffrey Powell
Supervisor: Dr. Chris Sorensen
Thanks to Brandon Lohman
Who without I would not have been able to take a part in this research.
Summer Research Experience for Undergraduates 2009
Gold Nanoparticles (AuNP) 5 nm in diameter, ligated with various alkane thiols, were dissolved in various alkane solvents ranging from hexane to hexadecane and also three aromatics including toluene, para-xylene, and mesitylene. These solutions were centrifuged at 12000g acceleration for 5 minutes to 1 hour depending on the solvent. A two-phase system forms with dissolved monomers on top and large clusters on the bottom. The top layer of the liquid was removed and studied because it contained pure monomers and can tell us about the solubility of the nanoparticles in a saturated solution of monomers. A UV-Vis Spectrophotometer was used to find the absorbance of the solution. The darker the liquid meant more absorbance of light and therefore more monomers of gold nanoparticles in solution. The data from the UV-Vis was converted into moles Au atoms per liter. Each nanoparticle system with different ligands behaved differently in the various solvents. We attempted to accurately find the solubility of each solvent and plot the trend to determine the overall tendency of the AuNP with that particular ligand.
A nanometer is one billionth of a meter.
A carbon-carbon bond is about .15 nm
The DNA double helix diameter is 2 nm.
The smallest bacterium is 200 nm.
When comparing a nanometer to a meter, a marble is the same as that of the earth.
Nanotechnology is at the edge of quantum mechanics and condensed matter.
They can act and behave as a “supermolecule” as it is on a nanometer scale.
Gravity becomes less important and forces such as surface tension and Van der Waals become more important.
The dramatic increase in surface area to volume ratio alters the mechanical, thermal, and catalytic properties of the materials.
Gold is one example of being stable and inert in everyday quantities and sizes but becomes a potent chemical catalyst and highly reactive at nanoscales.
Nanotechnology is only now coming out of its infancy and we are at the tip of the iceberg. Any data or research on these nanoparticles will be helpful for the future.
The fact that these nanoparticles can be manipulated in size and chemical makeup makes it able to create structures with an incredible amount of new properties and applications.
A gold metal salt is reduced to slowly grow nanocrystals. The addition of a soap causes “pods”, or micelles, to be formed where the nanocrystals are grown. This is called the inverse micelle method. The growth to the final size involves diffusive interactions between the inverse micelle which contain only a few atoms. The slow growth determines the final size of the particles.
1) A gold salt like AuCl4- is dissolved into a solution with a solvent like toluene.
2) A surfactant is added to the solution to promote inverse micelle formation.
3) A stabilizing ligand is added to the solution and is present in the inverse micelle environment.
4) A reducing agent such as NaBH4 is added to the solution to reduce the dissolved gold ions into atoms.
5) Micellar diffusion is responsible for a slow growth rate of particles giving rise to nanocrystalline structures instead of disordered clusters.
6) The product of inverse micelle synthesis is digestively ripened.
For the nanoparticles to be useful for any systematic study or size-dependent application, they must be monodisperse in size distribution. Digestive ripening is a technique used in which polydisperse ligated gold nanoparticles are heated and refluxed anaerobically in the presence of excess ligand. The mechanism is poorly understood but involves the nanoparticles trading their constituent atoms or groups of atoms back and forth until and equilibrium size is reached. A driving force for this favored equilibrium size can be a consideration of the competition between the surface energies of the particles favoring large size and the interaction of the ligand with the metal surfaces favoring small sizes.
Electron Micrograph Pictures Post Digestive Ripening Pre Digestive Ripening
Transmission Electron Micrograph Pictures
Post Digestive Ripening
Pre Digestive Ripening
is a rough sketch of what a thiol ligated nanoparticle would
look like. The center sphere is the
gold nanoparticle while the floppy “spaghetti” is
the thiols attached to the surface of the gold
through covalent bonds. A typical dodecanethiolated
gold nanoparticle has the chemical formula of Au3850(C12SH)350.
This is a rough sketch of what a thiol ligated nanoparticle would look like. The center sphere is the gold nanoparticle while the floppy “spaghetti” is the thiols attached to the surface of the gold through covalent bonds. A typical dodecanethiolated gold nanoparticle has the chemical formula of Au3850(C12SH)350.
Our experiment was to test the solubility of these alkanethiolated gold nanoparticles in various solvents. We took dry AuNP and used alkane and aromatic solvents to dissolve the particles. Dry AuNP were added to 300 microliters of solvent and sonicated to dissolve all particles. This was done until no more gold would dissolve in the solvent and a precipitate was seen. This meant that there was an equilibrium of gold in solution and precipitated gold. These samples were then spun in a centrifuge at 12000g acceleration until a distinct two-phase system appeared. This high acceleration pushed all of the clusters to the bottom and left the monomers in solution. This supernatant of monomers in solution was removed carefully. The supernatant was then taken to the UV-Vis spectrophotometer where its absorption was measured between 400 nm and 600 nm. Below is a plot of the data gathered from the UV-Vis.
The peak that is seen is the plasmon peak. The plasmon is a quantum of plasma oscillation of the free electron gas at the surface of the gold particle. The absorbance at this peak was what was used to find the concentration of gold atoms in this solvent. Using Beer’s Law, the absorbance is converted into concentration in moles of gold atoms per liter solvent. This procedure was done using octanethiolated AuNP, decanethiolated AuNP, dodecanethiolated AuNP, and hexadecanethiolated AuNP. The results are shown below.
In these two plots, it is easy to see the trend among the alkane solvents. In the decanethiolated AuNP, the solubility increases from hexane to decane where it peaks. The solubility then drops in the higher chain alkanes. The same is seen in the dodecanethiolated AuNP except the peak concentration is seen around dodecane. The trend seems to suggest that the ligated AuNP follows a “like dissolves like” conclusion. The ligands on the gold sphere are attached by a sulfur atom. This sulfur atom is “busy” and occupied with bonding to the gold. The “tail” of carbon atoms left on the thiol is left to move around and “flop”. When dissolved in a solvent, an alkane similar in length to the ligand will be more likely to draw up into solution the entire AuNP. Dodecanethiolated AuNP will be most soluble in dodecane, as seen in the plot.
In the decanethiolated AuNP plot, there is a “hiccup” at nonane where the solubility decreases after octane instead of following the trend upwards toward decane. This was acknowledged during our experiment that something strange and as of now unexplainable. We are confident it is a true point because of our experimental method and techniques.
The “like dissolves like” trend is seen in the example of how a small amount of ethanol can be dissolved in water but an infinite amount of water can be dissolved in water. The “sameness” of the nanoparticle and the solvent leads to better solubility.
The two previous graphs of the octanethiolated AuNP and the hexadecanethiolated AuNP show a different story. They obviously do not follow the “like dissolves like” hypothesis as the octanethiolated AuNP does not have a peak at octane and the hexadecanethiolated AuNP does not have any solubility after dodecane. If the magnitude of the concentration is compared to that of the decanethiolated AuNP and dodecanethiolated AuNP the C8 and C16 particles are an order of magnitude less. The octanethiolated AuNP also have an obvious stair-stepping with an odd-even functionality. These two plots do not support the hypothesis of “like dissolves like” and some unknown forces are acting on these particles that dramatically affect their solubility in these solvents.
This is the first example of a suspension of particles acting as a thermally reversible solution. These nanoparticles are acting as molecules in solution and are capable of changing phases with no change in entropy. This is unique because of the sizes of these “supermolecules.” They are not just a colloid with particles suspended in liquid but actual “molecules” with physical properties directly affecting the entire solution.
Future work on this project would deal with finding why the intermolecular forces decide the solubility they way that they do. This would answer the question of why changing the ligand length can increase or decrease the solubility of the nanoparticles.
NSF NIRT CTS0609318
This work was partially funded under NSF grant number PHY-0851599.
Any opinions, findings, and conclusions or recommendations expressed in this material
are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.
Dr. Chris Sorensen
Kansas State Physics and Chemistry Departments
Kansas State Biochemistry Department and Dr. John Tomich
My name is Jeffrey Powell and I graduated May 2010 from Benedictine College in Atchison, Kansas. I am double majored in Physics and Chemistry with a minor in Mathematics. I am currently in physics graduate school at Kansas State University. This project was completed with the help of Brandon Lohman under the guidance of Dr. Sorensen during my 10 week summer REU internship with the Kansas State Physics Department. The project was started May 2009 and ended July 2009.