Science Things I'm Thinking About: When Gold Isn't Gold
Updated: May 3, 2020
On Gold Nanoparticles
The microscopic world is so fascinating. I have a materials science background-- if I learned only one thing from my classes it’s that when you "zoom-in" to the microscale, you will be able to experience some interesting properties that aren’t observable on the macroscale. One example that I like to use are the optical properties of gold nanoparticles compared to bulk gold. Visually bulk gold is, well, gold. That’s not surprising. It gets interesting when you look at smaller gold particles however. At the nanoscale, gold particles actually aren’t gold! In fact, gold nanoparticles can look red, orange, or even blue! You might have recognized this effect if you’ve worked with gold nanoparticles in research.
The color of gold nanoparticles is very dependent on particle diameter.
Image from 4nDOTS
Gold nanoparticles absorb and scatter light very efficiently because the conductive electrons on the metal surface undergo a collective oscillation when they are excited by light. This oscillation is known as a surface plasmon resonance (SPR). The electrons move back and forth across the gold surface, similar to the way waves of water move across the surface. When the electrons are moving at the same frequency as the light, the plasmon is ‘in resonance.’ In this resonance phase, electrons are able to absorb and scatter light, producing the colors you see with your eye.
The optical properties of spherical gold nanoparticles are highly dependent on the particle diameter. The colors of visible light that you see are complementary to the light that a material absorbs. Smaller gold nanoparticles absorb and resonate with purple, blue, green, and yellow wavelengths of light, so they look red. Larger gold nanoparticles absorb and resonate with green, yellow, and red wavelengths of light, so they look blue. Cool right?
On Quantum Dots in Biomedical Sciences
Research about quantum dots and their applications have really picked up over the years. When I first heard about quantum dots, I was like “whaaat, is this science fiction?” If you’ve heard of quantum dots before, it’s probably after going to Best Buy. The most commonly known use of quantum dots nowadays may be TV screens. Quantum dots are synthetic semiconductor crystals that can transport electrons. Quantum dots sound like the stuff of the future, (and they are) but the principles surrounding their great applicability in biomedical sciences is actually similar to the case of the optical properties of gold nanoparticles.
If semiconductor particles are made small enough, a quantum effect called quantum confinement comes into play. Quantum confinement limits the energies at which electrons and holes (the absence of an electron) can exist in the particles. This can get complicated really quickly, so feel free to delve into your own research. While the mathematical models and quantum mechanics behind quantum confinement is outside the scope of this article, what is important to know is that this movement of electrons into different energy levels is the key to understanding light emission. When quantum dots are illuminated by UV light, some of the electrons receive enough energy to break free from the atoms. This capability allows them to move around the nanoparticle, creating a conductance band in which electrons are free to move through a material and conduct electricity. When these electrons drop back into the outer orbit around the atom (the valence band), as illustrated in the following figure, they emit light. The gap between the valence band and the conductance band, which is present for all semiconductor materials, is called a band gap and is responsible for the fluorescent properties of quantum dots.
Smaller crystals typically have larger electronic band gaps, meaning that the energetic difference between energy states is greater. The color of that light depends on the energy difference between the conductance band and the valence band.
Many semiconductor substances can be used to produce quantum dots, such as cadmium selenide, cadmium sulfide, or indium arsenide. I said in the first paragraph that the applicability of quantum dots in biomedical sciences is similar to the case of gold nanoparticles. Then I talked about quantum confinement and conduction/valence bands. You might have been thinking “what is the connection here?” The answer is that similar to the example about gold nanoparticles, quantum dots are also extremely amenable to customization to absorb certain wavelengths of light. In fact, my rotation lab focuses on the application of quantum dots for neurological labeling and in vitro/ex vivo probing molecular and cellular processes as they are better than traditional organic dyes for fluorescence. There are several advantages to quantum dots over nanoparticles. Quantum dots are a subset of nanoparticles, wherein the quantum effects start to affect the optical properties. As this size is pretty small (around 10nm), quantum dots are less likely to aggregate within tissue and imaging samples. Quantum dots also have a narrower and more symmetrical emission peak than nanoparticles, allowing for increased color saturation and spectra multiplexing.
(A) Size dependent fluorescence spectra of quantum dots and (B) different relative particle diameters corresponding to the size (Girma et al. Journal of Materials Chemistry (2017))
Current methods of labeling are restricted by rapid photobleaching, non-specificity, and aggregation of fluorescent dyes. Quantum dots are tunable molecular probes that can approach challenges in imaging where current technology is limited. Another interesting biomedical use for quantum dots is in the development of synthesis platforms. Due to the fact that certain biological molecules are capable of molecular recognition and self-assembly, nanocrystals themselves could also become an important building block for self-assembled functional nanodevices. There are so many potential applications for quantum dots, and I’m so excited to see how this field blossoms in the coming years! The microscopic world truly is fascinating!