On Oct. 4, the UNCG Department of Chemistry welcomed Dr. Rigoberto Hernandez, a researcher at Johns Hopkins University, as he gave a presentation on his recent nanoparticle research. Nanoparticles that are created today will end up as waste in the environment tomorrow, and Hernandez and his group aim to uncover what unintended effects these nanoparticles might bring about.
A nanoparticle is a collection of hundreds of atoms that, despite not being chemically bonded together, are still stable enough to remain in conformation. And they’re small—10,000 times smaller than the width of a human hair. At such a small scale, known as the nanoscale, particles tend to react differently with their environment compared to materials at larger sizes, and this is what makes engineered nanoparticles advantageous in advanced technologies.
While some nanoparticles do occur naturally, others are specifically engineered to be used in consumer products such as eyeglasses and sunscreen, or in the medical field for things like drug delivery, gene therapy and DNA probes.
A nanoparticle can have a very complex composition, depending on what other chemicals or particles it has interacted with during its lifetime, and normal human defense mechanisms may not be able to protect against engineered nanoparticles that persist in the environment.
“Our fear is that when we employ [nanoparticle] solutions in the environment, that they could lead to new, unwelcome and unanticipated consequences,” said Hernandez. “So the motivation of our center is to figure out the rules of making these nanoparticles now that will ensure that as they’re transformed in the environment, they will have no unwelcome consequences.”
In an effort to do this, Hernandez generates computational models that predict how nanoparticles will behave when in contact with membranes and other parts of the cellular matrix. However, a computer powerful enough to calculate the dynamics of the hundreds of billions of atoms that exist simultaneously at the nanoscale simply does not exist.
To circumvent this problem, Hernandez and his group shifted the model from the nanoscale up to the mesoscale, or ‘middle scale,’ by averaging some of the particles together and representing how the resulting assemblies interact with each other. The result is a coarse-grained model, which is able to mimic the complex behavior of the particles by lumping them together into larger structures.
“As a human being, you’re used to doing that coarse-graining,” said Hernandez. “Because you don’t worry about every atom in your body—you don’t even worry about every organ in your body—when you ask how you’re going to move from one place to another. You are a coarse-grained particle.”
One of the features of coarse-grained models is that they operate on a different timescale than what actually occurs. Because the model contains fewer particles than the original system, there is less friction between particles and they are subsequently able to move faster. This means the time in the coarse-grained models is accelerated, and that the model is dynamically inconsistent.
This can sometimes be beneficial because it allows researchers to make quicker calculations while maintaining accurate structures, but is detrimental to those, like Hernandez, who wish to study the true rates of reactions.
While reflecting on what led him to where he is today, Hernandez admitted an odd phenomenon would occur in college in which girls walking in front of him would speed up and girls walking behind him would slow down.
He then showed a picture of himself as a young man, complete with long hair and an ill-fitting suit, and laughed.
“All I did was I shaved, cut off the mustache, and I figured out how to dress more like someone from the northeast; and that stopped happening,” said Hernandez.
Similarly, he and his research group figured out a way to ‘dress up’ their coarse-grained models to slow down the processes of the system and get the results they wanted.
“So it turns out that you can take that coarse-grained model, decorate it in just the right way, and suddenly you get the right [timescale],” said Hernandez. “There is a possibility of developing mesoscale equations of motion that are coarse-grained, that will be able to get not just structure but also dynamics. And that’s the great dream, I think, for the century.”