Last October I attended the National Women in Physics Conference at Lincoln Nebraska. For an undergraduate women in Physics this is an amazing conference that offers a great opportunity to network with other young physicists and learn about the research going on around the country. While at the conference, I met many great people who got me connected with the Scientific American blogs and ScienceOnline. I also presented a poster of my research on the Design and Characterization of Thermally Responsive Nanoparticles. I was fortunate enough to win one of the top poster prizes at the conference and the chance to write about my work in blog form.

I work with thermally responsive nanoparticles. That may sound a bit scary but it’s not hard to break down those words and understand what I work with. These thermally responsive (responds to temperature change) particles are called Elastin-Like polypeptides (ELP). ELP’s are a single chain of amino acids that bond together (Glycine, Valine, and Proline).

I know what you are thinking… where is the physics in all this chemistry? If I am in physics why do I work with these particles and how do I even know what to do with them? When I started I had the same questions. Here is where the Chemical Engineers come in. I am in a joint program with the lab of Dr. Nolan Holland in the Chemical and Biomedical Engineering Dept. at CSU. Ali Ghoorchian is a graduate student in Dr. Holland’s lab and he makes the samples I work with. They have been studying this system for a very long time and know how to create and manipulate our samples.

So why should you care about these particles? The ELP system is a really amazing system to work with. The ELP chains have a transition temperature where they change from a random coil to a hydrophobic b-spiral. The b-spirals clump together at high temperature and it is very difficult to control the ELP chains. To fix this the Chemical Engineers added a “foldon” head group to three ELP chains. This foldon head group stabilizes the transition of the ELP to control the clumping. The chains with the head group are known as a “Trimer”. Above the transition temperature the ELP’s in each Trimer fold together and clump with other Trimers. The ELP tails are hydrophobic so by clumping with other trimers they avoid water. The head groups are hydrophilic and form an outer barrier around the tails. What forms from many of these trimers together are micelles.

Figure 1: The three armed star elastin-like polypeptide (GVGVP)40 Foldon before (a) and after transition (b). A micelle formed by many of these foldons and polypeptide after transition (c)1.

Micelles are the key to what we are studying. The micelles can be useful for various reasons. They can be used for a drug delivery system, biosensors or viscosity modifiers. How can this be? The fantastic thing about the ELP’s is that they are man-made and can be manipulated very easily. ELP’s have been extensively studied and the Chemical Engineers can change the amino acid sequence to manipulate transition temperature of the micelles. The micelles can be manipulated by salt, pH, temperature, light and solvent.

Since micelles are thermally reversible they are promising as a Drug Delivery System. At high temperatures the trimers group together to form the micelles but at lower temperature they will break up. One can do this over and over again and the ELP’s will act the same. This can be used for drug delivery because if one were to add drugs to the micelle when it is at a high temperature, send it to the infected part of the body, then lower the temperature, the micelle would break up and release the drugs. This is a very interesting and exciting idea for researchers to explore.

So far I have told you a lot of chemistry and engineering but I think you may have noticed I have neglected the physics side of my research. This is the most important part and this is where I come in handy for the Chemical Engineers. They want to know how these ELP’s change with different stimuli. At the moment we are focusing on salt concentration in the samples. We want to be able to characterize the size and shape of the micelles with increase in salt concentration.

To do this we use Light Scattering Spectroscopy. I work in the Light Scattering Lab of Dr. Kiril Streletzky in the Physics Dept. of CSU and we have this setup. What is light scattering spectroscopy? This is a system that involves two components: Dynamic and Static Light Scattering. Dynamic light scattering (DLS) uses Brownian motion to give a size for the particles in the samples. Brownian motion is the random movement of particles due to the bombardment by the solvent molecules that surround them. The larger the particle the slower the Brownian motion will be. DLS yields correlation functions and by fitting these functions we are given a diffusion coefficient for the particles which lets us know how a particle diffuses within a fluid. The diffusion coefficient relates directly to the hydrodynamic radius (Rh) of the particle using the Stokes-Einstein equation (for spherical particles). Using DLS gives us a very good idea of size of our particles if they are spherical.

Figure 2: (a) Light Scattering Spectroscopy setup. (b)Example of Intensity fluctuations over time.

We also want to know the shape of our particles as we change salt. This is where Static Light Scattering (SLS) comes in. SLS focuses on average intensity of the scattered light to give us the radius of gyration (Rg). Rg is calculated as the root mean square distance of the objects' parts from either its center of gravity or an axis. Rg measurements are independent of shape assumption; whereas, any Rh value assumes a sphere. By taking the ratio of radius of gyration over the hydrodynamic radius you can obtain a ratio that directly correlates to shape. A sphere will give a ratio of .77 while a random coil will give a ratio of 1.5-1.8. SLS can also yield molecular weight by looking at the intercept of a one over Intensity vs. angle SLS measurement.

So, we do all these measurements but what are the results? We found some interesting facts about the micelles. With increase in salt concentration we saw an increase in radius, molecular weight, and ellipsoidal shape change of our micelles. We saw that, looking at hydrodynamic radius, there was an increase in size through 3 different salt regimes. In the lower salt concentrations (0-15mM) the size was about 15nm. The middle salt concentration (15-30mM) had an increasing radius from 15-60nm. When we got to the high salt concentrations (30-60mM) there was another plateau of radius between 60-80nm.

Figure 3: (a) Salt concentration and size dependence for samples 67-78.

After gathering our size data we noticed an issue with shape. In the lower salt concentrations the shape seemed to be very spherical but as we increased salt concentration the micelles became more ellipsoidal. We also wanted to understand how the molecular weight changed as a function of our salt concentration. We noticed that the molecular weight followed the same pattern as the radius. This meant that not only was the size of our micelles getting larger but they were increasing in number of trimers.

Figure 4: The ratio of Rg/Rh for our salt concentrations show us that the micelles are hyperbranched spheres for most salt concentrations up to 50mM of salt.

 

Figure 5: Mw vs. salt concentration for various salt samples. We see that the molecular weight increase within the same regimes that the size increases.

These data will be very useful in understanding how to further manipulate the particles and help us find the right direction to continue our work. The data helps us find the best samples for further experimentation including mixing different types of foldons and using different size chains. We have learned quite a bit but there is still quite a lot of work to be done on this project.

Now, everything sounds like rose petals, but this project also offered its fair share of frustrations. The samples we worked with were extremely temperamental. One observation was that it is important to focus on the sensitivity of our sample to various external stimuli. The biggest factor we had to deal with was pH. pH of the sample would lower over night and we only have consistent data within a very small pH window (10.1-10.4). Because of this we measured each sample on the day it was made. Also, since we are dealing with such a high pH, the way we cleaned the cells also had to be changed. We began using chemicals to make sure all dirt was off the sides of the cell before we used it. After getting through the many trials we were able to form a system for consistent data.

I hope this has sparked you interest in this area of research. It is a very exciting field of study and there is still a lot of work to be done on this project.

I would like to thank my Advisors and the National Science Foundation for funding this adventure. I would also like to thank Kiyomi Deards and Bora Zivkovic for the opportunity to write for this blog.

If you would like to read more on this you can look up Ali Ghoorchian’s paper at http://pubs.acs.org/doi/abs/10.1021/ma100285v

References:

  1. A. Ghoorchian, J. T. Cole, and N. B. Holland, Macromolecules 43, 4340 (2010);
  2. K.A. Streletzky, J. McKenna, R. Mohieddine, J. Polym Sci B: Polym. Phys. 46, 771, (2008)
  3. “Instruction manual for BI-9000AT Digital Autocorrelator”. Brookhaven Instruments Corp. Holtsville NY. 1998
  4. "Dynamic Light Scattering: an Introduction in 30 Minutes." Malvern Instruments: Technical Note: Worcestershire UK. 1-8. Print.
  5. Weiner, Bruce B. “What is Particle Size?” Brookhaven Instruments, Holtsville NY. November 2010.