In our lab, the research is all about gold nanoparticles, which are small particles made up of gold atoms on the nano scale – which overall just means they’re super tiny globs of gold. How tiny? Well, if you think of the size of an golden ant, and then think of something about a million times smaller, you’re in the right size range. You may also wonder why we chose to work with gold nanoparticles. Gold nanoparticles have proven to be very interesting because they’re relatively biocompatible and have seen a lot of activity in terms of applications to biomedical imaging, gene therapies, and even skin care. Gold also has a high electron density which leads to a unique plasmon resonance – or oscillation of electrons around the particles – which gives the particles really cool optical properties and extremely vivid colors. Most of the particles we’re working with this summer are spheres which have a ruby red color. Which definitely makes you rethink the color you’ve been calling gold, doesn’t it?
The Golden Goat
This summer we have three members in our lab who are each more or less working on separate projects. The two main projects that our lab is working on this involve polymer wrapping and cytotoxicity. If you’re interested in what has been done in our lab before, check out the posts from last summer and the summer before.
Meet the Lab Members:
Celina Harris is a rising senior chemistry major who first began doing nanoparticle work in the fall of her sophomore year in a Chemistry 290 – this is her first summer staying and doing research with the chemistry department. Celina is working on the quantification of polymer wrapping of gold nanoparticles. When Celina isn’t in lab, she works with the literary magazine on campus, is involved with Students Against Sexual Assault (SASA), and is a TA for general chemistry labs. This fall, she’ll be studying abroad in France and applying for graduate school.
Rich Gawel is a rising junior biochemistry and molecular biology major who is also just beginning his work this summer. He is taking over the nanotoxicity project – which has been traditionally done using tadpoles as the model organism. This year he’s working with snails and the delivery of antidepressants using gold nanoparticle carriers. In addition to BMB, Rich is working for a minor in Political Science. Rich is also a TA for general chemistry lab and serves on the Honor Commission.
Fontaine McFeaters is a rising junior chemistry major who is just beginning her work this summer. Fontaine is currently working with the new isothermal titration calorimeter to measure the thermodynamic properties of the binding interactions between nanoparticles and polyelectrolytes. Outside of the lab, Fontaine is a member of the women’s golf team, a TA for general chemistry lab, and an avid Servo sitter. This fall, she will be studying abroad in England and adventuring across Europe.
Nanoparticles have a lot of potential for biological applications – look at Rich’s project for example. To make these particles more biocompatible, it’s necessary to modify their surface chemistry. This basically means, we need to change what is on the outside of the particle so that when it comes in contact with a biological entity, it doesn’t do any significant damage. This surface chemistry can be modified by coating the particles. Electrostatic coatings (commonly called layer by layer coatings because it can be used to coat a lot of polymer layers) utilizes the electrostatic interactions between oppositely charged species to hold them together. For my project specifically, I’m aiming to quantify the polymer wrapping around a single nanoparticle using an electrostatic coating of polymer.
I work with cetyltrimethylammonium bromide (CTAB) stabilized nanoparticles for my project. CTAB is a positively charged, organic, detergent molecule which forms a bilayer structure on the surface of the particles during the synthesis process. I coat the particles with the negatively charged polymer polystyrene sulfonate (PSS) – which is a standard polymer used for polyelectrolyte coating on flat surfaces. PSS is really handy for what we’re doing because it has one sulfur atom for each monomer unit in the polymer and sulfur is detectable using our instrumentation. We verify this coating by measuring zeta potential – which is a measure of surface charge density of the particles. Since the coating is electrostatic in nature, the charge potential should flip from positive (CTAB layer) to negative (PSS layer) and when this is observed and the negative charge is maintained, I’m able to say the particles have been coated and the coating is maintained through the rinsing process. The rinsing process is used to remove excess PSS from the particles by running a NaCl (salt) solution passed the particles five times and then running ultrapure water passed the particles three times. Particles are characterized throughout this process by measuring them using UV-Vis to ensure that their optical properties aren’t significantly changing and that the particles aren’t aggregating.
I determine the amount of gold, sulfur, and sodium in my samples by using inductively coupled plasma – optical emission spectroscopy (ICP-OES). ICP-OES is a technique where the sample is passed through an extremely hot plasma flame which breaks all the chemical bonds and push all the atoms into higher energy states. When the atoms relax back to their ground states, they release elemental specific light and the machine measures the intensity of this light. This intensity signal can be compared to a standard series of solutions for the elements I’m interested in and then the machine is able to work out the exact concentration in the nanoparticle solutions. From all of this, I can figure out how many polymers are on each nanoparticle by computing the ratio of sulfur and gold in each sample. After some math using atomic masses, density values, and the average volume of a nanoparticle, each of these ratios can be transformed into the final PSS molecules per particle measurement.
During the spring semester, we were able to get some rough estimates and right now I’m actually working on reproducing those results so we can be more certain in the validity of these numbers. Our goal is to have this number definitely figured out in a week or two actually. We’re also getting ready to send some samples up to Penn State so they can run X-ray Photoelectron Spectroscopy (XPS) on the particles and we can get a fuller understanding of nitrogen and bromine content in the samples (both features of the CTAB surfactant layer) which can’t be measured using ICP. After that, I’ll change a variable from this experiment and see how it affects the coating of polymers onto nanoparticles. We’re considering looking at how other polymers similar to PSS coat nanoparticles or looking into the effects of salt concentration or salt species on PSS coating of nanoparticles.
Most of my research consists of working with the Isothermal Titration Calorimeter, or ITC for short. We just received the ITC in the Spring, so very few experiments with this instrument have been done. The ITC is just a fancier way of doing titrations at a much smaller scale. Isothermal means the instrument keeps the reaction cells at a constant temperature while the titration occurs. The ITC consists of two cells, a reference cell and a sample cell. The ITC uses a small syringe to automatically inject the sample cell, filled with one reagent, with another reagent over a period of time. The ITC specifically measures how much heat is absorbed or released from the reaction as the injections occur. With the data collected, we are able to determine the binding constants, entropy changes, and a few other important thermodynamic properties.
The main goal of my research is to better understand the binding interactions between polyelectrolytes and gold nanoparticles using the isothermal titration calorimeter. As Celina mentioned, gold nanoparticles have many potential biological implications such as cancer therapeutics and targeted drug delivery. With my research, I am looking at both CTAB and citrate-stabilized gold nanoparticles. A polymer is chosen based on whether CTAB or citrate is used. For the citrate gold nanoparticles, I’ve been looking at coating the gold nano with polyallylamine hydrochloride (PAH). By altering the concentrations of both the gold nanoparticles and the PAH, we are hoping to use the ITC to determine what happens when PAH binds to the surface of the particles.
Another part of my research is looking at the polymer, polyanetholesulfonic acid (PAS). This is the first time we have worked with this polyelectrolyte, but we are interested in PAS due to its similarities to polystyrene sulfonate (PSS). PAS has not been studied as much as other polymers such as PAH or PSS, but has been known to have biological and medical relevance. PAS differs from PSS in its structure, where the sulfur-containing group is in a different position on the molecule, and a methyl and methoxy group are also present. Since little information was found on coating gold nanoparticles with PAS, we are curious as to what PAS has to offer to the world of nanoparticle research. After coating the particles with PAS, we have to make sure the polymer is actually on the surface of the particles. In order to tell if the polymer is bound to the surface, we have a few different techniques to characterize the particles and compare them to the particles before the polymer coating. The three main techniques we use include zeta potential, dynamic light scattering (DLS), and Ultraviolet-Visible Spectroscopy (UV-Vis). These techniques ultimately tell us that the polyelectrolyte is bound to the surface of the particle by looking at the hydrodynamic radius, charge of the particle, and absorbance of light. Since we can’t exactly see what happens at the surface of nanoparticles while coating them with polyelectrolytes, these tests are very useful! After characterizing the nanoparticles with the PAS coating, we will be able to analyze the similarities and differences to PSS coated nanoparticles. Once we are able to better understand the surface-level interactions between nanoparticles and polyelectrolytes, we will someday be one step closer to using gold nanoparticles with other molecules such as drugs or DNA for further applications.
Despite their incredibly small size, nanoparticles have already been identified to serve in a diverse range of applications, with further uses continually being investigated. One of the most heavily researched applications of nanoparticles is their potential use in pharmaceuticals. A significant amount of work is currently being done examining their use in medical imaging, targeted therapies, and as drug carriers. While new applications of nanoparticles are constantly being investigated, little work has been done thus far examining the unanticipated effects that these nanoparticles may have. Because of their widespread use in industry and continually increasing use, it is essential to determine to what extent particles released in nature interact with both humans and other organisms.
Dr. Thompson has been collaborating with Dr. Fong for the past several years investigating the toxicity of nanoparticles on aquatic organisms. This summer I will be taking over for Kevin, who assisted with the nanotoxicity project in the past. Currently we are investigating the potential of gold nanoparticles as a carrier for antidepressants and the effect this may have on organisms. Dr. Fong has extensively investigated the impact of several antidepressants on snail mobility. This summer, our work involves linking the antidepressant fluoxetine to gold nanoparticles and then treating both freshwater and marine snails to see if the nanoparticle carrier alters the effect of the drug on these snails.
While it is possible to attach larger molecules to the surface of a nanoparticle individually via a specific linkage, the physics of the curved surface of a spherical nanoparticle often makes it difficult. For this reason, we are employing a polymer which we can attach to both the drug molecules and to the surface of a nanoparticle. Polyacrylamide-co-n-hydroxysuccinimide (PAN) is a prepolymer consisting of acrylamide and n-acryloxysuccinimide repeats. This particular combination is very useful because it contains a special type of ester (n-hydroxysuccinimide or NHS ester) along the polymer backbone which are highly reactive to amine groups. Therefore, through a substitution reaction, we can replace these esters with molecules that have amine groups. Fluoxetine has an amine, allowing us to be able to link it to the polymer. Because thiol groups associate with the nanoparticle surface, we will be using a linker molecule (3-methylthiopropylamine or MTP) which has a thiol ether on one side of a carbon chain and an amine on the other end, allowing us to couple the drug containing polymer to the nanoparticle. Therefore, we have fluoxetine carrying nanoparticle, ready to expose to our snails.
Currently, we are performing a series of pre-experimental tests, specifically assessing the kinetics of the amine containing molecules to ensure they will sufficiently bind to the polymer. Because of the nature of the biological tests with snails, it is extremely important that we are able to control how much fluoxetine binds to the nanoparticle to most accurately replicate the concentrations previously shown to impact the snails. Once we can sufficiently quantify fluoxetine binding and manipulate it accordingly, we will be able to bind the drug containing polymer to the nanoparticles, prepare the necessary environmental concentrations, and then treat the snails.
My interest in this project deals with its highly interdisciplinary nature. At this point, I want the opportunity to explore a variety of different topics, techniques, and ideas. Despite knowing some of the diversity associated with this project, I never expected that I would be performing atmospherically controlled organic syntheses using anhydrous reagents and radicals. Dr. Thompson, who has devoted his career to investigating surface chemistry and modifying nanoparticle and planar surfaces, often jokes that he is becoming an organic chemist. In addition to the organic reactions, we are also employing a variety surface chemistry techniques and analytical techniques, not to mention an entire set of biological assays.