Ah, a day at the beach. You find the perfect spot near the water’s edge, spread out your towel, slather on some sunscreen, and settle in with a cool drink. You spot a sparkling white boat bobbing in the distance, full of relaxed passengers tanning in the sun.
While this scene is full of idyllic summer imagery, is it also full of lurking danger? I’m not talking about sharks, but rather something much smaller: both the sunscreen you put on and the paint on the boat contain nanoparticles that provide important benefits but can harm both you and the environment around you.
Nanoparticles are spheres made from a wide range of materials and are tiny in size. A nanoparticle is a million times smaller than the grains of sand clinging to your towel, making it about the same size as a single molecule and imperceptible to the naked eye.
In sunscreen, a chemical called zinc oxide offers protection against UV rays. This chemical can exist either as nanoparticles or as larger, non-nano particles. Have you ever seen a lifeguard in a movie with a white stripe down their nose? They used sunscreen that contains the larger zinc oxide particles, which are opaque. Zinc oxide nanoparticles, on the other hand, are transparent and leave no visible residue when applied to the skin, the advantages of which are pretty clear (pun intended). On the boat, silver nanoparticles in the paint ward off bacteria, preventing the boat from rusting. Silver is anti-bacterial regardless of its size, but only silver nanoparticles are small enough to blend in with the paint.
Thanks to their shrunken size, nanoparticles of a certain material either give that material new properties (like making zinc oxide transparent) or allow its existing properties to be used in a new context (like making silver compatible with paint). Commercial nanoparticles have harnessed and revealed the capabilities of countless materials for applications ranging from batteries to coolant liquids to television displays. But despite the prevalence of nanoparticles in consumer products, we have only recently begun probing the harm they could cause to humans and other organisms. Could nanoparticles be bad for you? What about the sea creatures that become exposed to them when they leach into the water? Scientists are trying to answer these questions by studying the danger, or toxicity, of nanoparticles.
Unfortunately, the very thing that gives nanoparticles so many desirable properties—their miniscule size—is the same thing that can cause them to be toxic. As an object gets smaller, its surface area increases relative to its volume. For example, think of donut holes and how they taste better than donuts. Because donut holes are smaller, you get more of the yummy donut coating (sprinkles, icing, powdered sugar, etc.) with each bite. Nanoparticles are like donut holes, in that they have a very large surface area compared to their size. And just like how donut holes can become coated with sprinkles or powdered sugar, a nanoparticle that finds itself inside of a human or a fish can become coated with proteins.
Proteins are the components of our bodies that form structures such as muscle, carry out chemical reactions, and perform many other important roles. When proteins cover the surface of the nanoparticle, both the protein and the particle can undergo certain changes that may be harmful to the organism.
For the protein, this is a change in shape. Proteins adopt different shapes that dictate their role in the body. The proteins that make up muscle are long and flexible to allow our muscles to contract. The proteins that carry out chemical reactions have crevices that specific molecules fit into so they can be converted into something else. A protein’s shape is crucial to its function. So, if a protein changes shape, it loses its function.
Scientists have confirmed that certain protein-nanoparticle interactions can alter the shape of the protein, impairing its function and interfering with the important processes this function serves. While the nanoparticles in sunscreen do not penetrate intact skin, they—as well as nanoparticles in all other commercial products—could enter the body through a cut, accidental ingestion, and even inhalation if used in an aerosol. Scientific studies have shown that proteins that store oxygen in muscle cells and transport cholesterol can lose their shape and function if they come into contact with different nanoparticles, increasing the risk of cardiovascular disease and other serious health problems.
The interaction between the nanoparticle and the protein can also create changes in the particle that affect its toxicity down the line. When the proteins in an organism’s body coat the surface of the nanoparticle, they form what is called a “protein corona.” While nanoparticles can’t get inside of a cell, proteins, like the ones in this corona, can. The corona essentially disguises the nanoparticle as a protein so that the cell will let it in, just like a party crasher dressing up like someone on the list. Upon entering the cell, the nanoparticle gains access to all the proteins that are inside the cell. It then has the opportunity to interrupt the critical functions of these proteins, which include replicating DNA to make new cells and generating life-sustaining energy sources.
The protein corona also influences the way in which the particles interact with one another. Nanoparticles typically carry an electrical charge at their surface. Like magnets, similar charges repel each other, preventing nanoparticles of the same type from clumping together. But, the protein corona can obscure a nanoparticle’s surface charge, preventing this repulsion and enabling multiple particles to form clumps. Clumping slows down the nanoparticles, affecting their mobility and the locations within the body at which they settle. For example, researchers discovered that inhaled nanoparticles deposit at different parts of the lung depending on how many were clumped together. Nanoparticles’ fate within the body will in turn determine which other proteins they may encounter and impair later on.
Finally, the protein corona can impact a nanoparticle’s toxicity by physically changing the particle itself. When a protein binds to a nanoparticle’s surface, it can trigger a chemical reaction that causes the particle to dissolve. Unfortunately, this can make the nanoparticle even more toxic.For example, the silver nanoparticles in the boat’s paint can dissolve when they become coated by a certain protein found inside an organism’s body. Scientific studies have revealed that dissolved silver can be toxic toward rainbow trout and zebrafish, even at low amounts.
What does all this mean for us and for our oceans? We’re not quite sure yet. It’s important to note that no study conducted in a laboratory can perfectly replicate the conditions of an organism’s body. Additionally, these toxic effects all hinge on initial exposure to nanoparticles, which may or may not actually happen. So, while the answer isn’t entirely straightforward, the potential for nanoparticle toxicity toward humans and marine life is undeniable.
But… we need nanoparticles, right? So we don’t have rusty boats and white lifeguard noses? This is true. Luckily, nanoparticles come in all different types of materials, shapes, sizes and surface charges, and these things can greatly affect the way they behave. For example, the cholesterol-transporting protein only binds to nanoparticles that carry a certain surface charge. Similarly, the protein interaction that causes the silver nanoparticles to dissolve depends on the size of the particle. Thanks to the development of new screening tools that can analyze thousands of nanoparticles at once, scientists can test nanoparticles of all designs for toxicity. This makes it possible to identify a specific nanoparticle that maintains its useful properties but does not hurt people or the environment.
Thanks to the work of foresighted scientists, we now know that nanoparticles can be toxic to living organisms and we have the opportunity to modify commercial particles to minimize their potential for harm. Whether or not manufacturers seize that opportunity is another matter. So, for now, sit back and enjoy the sights and sounds of the ocean. But keep your eyes peeled for danger.
Sarah Anderson is a PhD candidate in the chemistry department at Northwestern University.