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Researchers Identify a Method to Accurately Measure Tiny Nanoparticles

Most consumers may not be aware of the presence of nanoparticles, but these tiny molecules play an important role in modern life.

Scientists have long debated the most effective way to measure nanoparticles so that results can be shared across labs. NIST researchers have found that one approach—particle number concentrations—may be better than others for most applications. (Image credit: N. Hanacek/NIST

Nanoparticles fight microorganisms on bandages, prevent athlete’s foot fungus in socks, and serve as vital ingredients in sunscreen lotions. They make sure that the powdered sugar on doughnuts remains powdery and also improve the colors of popular candies. They are also used in sophisticated drugs that focus on certain types of cells in cancer therapies.

However, when chemists examine a sample, it is difficult to determine the quantities and sizes of these nanoparticles, which are usually 100,000 times smaller than the width of a piece of paper. Although many options are provided by technology to assess nanoparticles, experts are yet to reach an agreement on the most optimized method.

In a recent paper reported by the National Institute of Standards and Technology (NIST) and partnering institutions, scientists concluded that instead of measuring the average size of the nanoparticles, measuring the range of their sizes would be optimal for a majority of applications.

It seems like a simple choice. But it can have a big impact on the outcome of your assessment.

Elijah Petersen, Study Lead Author, NIST

The results of the study were recently reported in Environmental Science: Nano.

Precision is important, as with many measurement questions. Exposure to a specific amount of some nanoparticles can lead to unfavorable outcomes. In order to increase the efficacy of a drug, pharmaceutical researchers usually need precision. Moreover, environmental scientists have to know, for instance, the number of titanium, silver, or gold nanoparticles that can pose a risk to organisms in water or soil.

Unreliable measurements may lead to the use of more nanoparticles than required in a product which may also result in unwanted expenditures for manufacturers.

Nanoparticles may sound ultramodern, but they are neither novel nor based only on high-tech manufacturing processes. A nanoparticle is actually a submicroscopic particle measuring less than 100 nm on at least one of its dimensions. In fact, hundreds of thousands of these particles can be placed onto a pin’s head.

Nanoparticles appeal to researchers because a majority of materials act in a different way at the nanometer scale than they do at larger scales. These particles can also be made to do plenty of useful things.

Since the days of ancient Mesopotamia, nanoparticles have been used when ceramic artists decorated vases and other vessels using very tiny bits of metal. During the 4th century, Roman glass artisans used to ground metals into very small particles to alter the color of their wares under varied lighting. These methods were not recalled for a while but in the 1600s, they were eventually rediscovered by resourceful manufacturers for glassmaking.

Later, in the 1850s, scientist Michael Faraday thoroughly investigated ways to utilize different kinds of wash mixes to alter the gold particles’ performance.

In the mid-20th century, contemporary nanoparticle research developed rapidly owing to technological breakthroughs in optics. The fact that individual particles can be visualized and their behavior can be examined further extended the possibilities for experimentation. However, the biggest developments came after investigational nanotechnology took off in the 1990s. All of a sudden, the behavior of various substances, including individual gold particles, can be closely studied and exploited.

There were numerous findings on how tiny amounts of a substance would absorb light, reflect light, or alter in behavior, which led to the use of nanoparticles in many additional products.

Debates have since followed about their measurement. When it comes to evaluating the response of organisms or cells to nanoparticles, some teams prefer to measure particle number concentrations (also referred to as PNCs by researchers). For many researchers, PNCs are quite difficult because additional formulas have to be used when establishing the end measurement. Others prefer to determine surface or mass area concentrations.

Most often, PNCs are used to characterize metals in chemistry. However, the situation for nanoparticles is innately more complicated than it is for dissolved inorganic or organic substances. This is because nanoparticles are different from dissolved chemicals and are available in a wide range of sizes. At times, they adhere together when added to testing materials.

If you have a dissolved chemical, it’s always going to have the same molecular formula, by definition. Nanoparticles don’t just have a certain number of atoms, however. Some will be 9 nanometers, some will be 11, some might be 18, and some might be 3.

Elijah Petersen, Study Lead Author, NIST

The issue is that each of those nanoparticles may be fulfilling a critical role.

For some industrial applications, a basic estimate of particle number may be perfectly fine but therapeutic applications call for much more powerful measurement. For instance, in the case of cancer treatments, each particle, regardless of how big or small, could be delivering a required antidote. And just like any other kind of dosage, the dosage of nanoparticles should be precise to be both safe and effective.

According to Petersen, using the particle size range to determine the PNCs will often be the most helpful in the majority of cases. The size distribution notes the entire distribution of particle sizes instead of using an average or a mean. This way, formulas can be effectively utilized to find the number of particles present in a sample.

However, regardless of the type of method used, investigators must make note of this in their papers, for the purpose of comparability with other studies. “Don’t assume that different approaches will give you the same result,” he stated.

Petersen added that he and his coworkers were surprised to see how measurements are considerably  influenced by the amount of coatings on nanoparticles. He observed that some coatings can have a positive electrical charge, leading to clumping.

Petersen worked in association with scientists from federal laboratories in Switzerland, and also with researchers from 3M who have earlier made several nanoparticle measurements for applications in industrial settings. Swiss researchers, like those in much of the rest of Europe, are trying to learn more about determining nanoparticles since PNCs are needed in several regulatory situations. However, little information is available on which methods are best or more likely to produce the most accurate outcomes across a number of applications.

Until now we didn’t even know if we could find agreement among labs about particle number concentrations. They are complex. But now we are beginning to see it can be done.

Elijah Petersen, Study Lead Author, NIST


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