- New color-coding technique can be used as a quick visual marker to establish the size of nanoparticles.
- The colors are distinguishable enough that one can determine a size difference of at least 8 nanometers.
Color-coding is used in almost all fields, whether it’s electronic components where color-code indicates the value/ratings of resistors, inductors, capacitors, or a simple way to organize items such as folders and subway lines.
In electromagnetics, light scattering by tiny particles is one the basic concepts that has attracted a lot interest throughout modern history. Since resonant scattering features a strong local field and can be analyzed via spectroscopy, it has enabled numerous applications of nanoparticles.
Now, a team of scientists at the University of Michigan has applied color-coding to nanoparticles, enabling researchers to rapidly deduce their their sizes. It could have a variety of applications, including biological sensors, drug delivery system, lithography of machine chips, and advanced coatings.
In this study, researchers have used a special type of particles called silica nanoparticles, which have low refractive index. They have wavelengths ranging from 390 nm to 700 nm (wavelength of visible light falls under the same range), thus they don’t much interact with light. This makes them nearly invisible to the naked eye.
How Nanoparticles Shines In Different Colors?
Scientists figure out that they can make nanoparticles shine based on their size. All they need to do is add metal caps to individual nanoparticles: larger nanoparticles of 486 nm in diameter appear reddish hue, while smaller ones around 421 nm appear green (shown in the image). These colors are distinguishable enough that one can determine a size difference of at least 8 nm.
Scanning electron microscope image of metal-dressed silicon dioxide nanosphere | Courtesy of researchers
To make this possible, scientists first distinguished nanoparticles’ Mie resonances – sizes that scatter visible light weakly or strongly. More specifically, they used homogenous spheres to describe the scattering of an electromagnetic plane wave. Then, they improved the Mie resonance to make it visible in light.
The team coated a glass slide with a thin gold film, sprinkled silica nanoparticles on it, and again coated it with another thin gold film. This enhanced the capability of particles to trap light while making it visible when observed via a dark-field microscope.
At present, dynamic light scattering is used to size particles, where a sample is exposed to a laser pulse and size distribution is analyzed with a scanning electron microscope over a specific period of time.
This new technique, one the other hand, is more precise than traditional methods. A scanning electron microscope requires an expensive setup and is capable of handling only one particle at a time, whereas the new methodology enables sizing of multiple nanoparticles at once.
Bottom line: the metal dress produced by a 2-step deposition process significantly increases the Mie resonances of low-index nanoparticles. It improves the light confinement ability of low-index nanoparticles through enhanced boundary reflection.
Moreover, at the enhanced Mie resonance, the high electric field induces strong circulating displacement currents, and thus reasonable magnetic response. It can be used to sense minute alterations in refractive index or size.
Researchers can use this technique to verify the size of a nanoparticle, and see changes in color (due to alterations in refractive index) when nanoparticles carry payloads. This would help doctors ensure nanoparticles are correctly picking up molecules and delivering them to targets, simply by observing their color changes. This color signal can notify biological sensors if there is any alteration in state.
The size of nanoparticles as well as the quantity of each size alters the characteristics of advanced coatings, making a surface rougher or smoother, trap or reflect light, or absorb or repel water.
The method could also be applied to nanosphere lithography for sensors and machine chips, where particles are uniformly ordered in size to generate precise patterns for printing circuits.