Quantum Dots Illuminate Life's Mechanisms

How do bodies become images, and images become bodies?

Bodies are opaque. Penetrating the boundary of their skin without dismembering them is one of the most surprising feats of contemporary science. Technology has given us many tools to extend our gaze inside of ourselves, disclosing the inner workings of our biological machinery. Some of these are based on electromagnetic radiation, others on magnetic fields or ultrasonic waves. Paradoxically, we need something invisible to make the darkness of our bodies visible.

Quantum dots are materials that exist at the boundary of invisibility and visibility. They are nanocrystals of only a few billionths of a meter in size. Much smaller than our cells, these particles can only be seen with the most advanced electron microscopes. Yet, quantum dots are light emitters. A solution containing quantum dots is fluorescent, producing extremely bright light of a very specific color.

Microscopic photographs of cells and tissues stained with fluorescent quantum dots. From [1].

Quantum dots are made of semiconducting materials, often containing rare-earth and heavy metals like cadmium and lead. These materials are excited when exposed to UV radiation and start emitting light in response. This is where the word “quantum” comes into play: the fluorescence of quantum dots is the result of the resonating vibration of the electrons on the surface of each nanocrystal. These surface electrons get entangled with the incoming radiation, similar to a spiderweb which starts vibrating when it catches a flying bug.

Since their discovery in the 1980s, quantum dots have been used to shed light — figuratively and literally — on the inner workings of our biological bodies. Because they are so small, quantum dots can be used as imaging probes by injecting them into living organisms — or even individual cells — and following their diffusion through the body. Their luminous emission can be tuned by changing the size and composition of the nanocrystals, producing a rainbow of fluorescent light.

Quantum dots are used to visualize veins (red), arteries (blue), and a tumor (green) in the brain of a rat. From [2].

In a process known as “bioconjugation”, the surfaces of quantum dots are often coated with biological molecules to stabilize them, aid their diffusion through living tissues, and target specific areas of the body. In a study from 2017, quantum dots were used to image the brain of a rat, distinguishing between healthy tissue and a brain tumor.

Because they are transmitted during cell division, quantum dots have also played a role in embryology, being used as “lineage tracers” capable of tracking embryo development from one generation of cells to the next. They have been injected into the embryos of zebrafish and of the frog Xenopus laevis to illuminate the developmental stages of these model organisms. The surfaces of quantum dots are spaces where light and matter, images and bodies become so deeply entangled that it becomes impossible to draw a line between them.

Cell division in an embryo of Xenupus laevis injected with quantum dots. From [3].

How does matter become an image, and how do images become matter? Lately, I’ve found myself thinking often about visualization technologies, especially those that we use to produce images of biological bodies. The histories of these technological gazes have often been controversial, channeling an epistemic ambition for absolute transparency that has often revealed itself as a political desire for control. Did you know that zebrafish owe their success as model organisms to the crystal-clear transparency of their flesh? What does this say about the way science conceptualizes the body?

An embryo of Xenopus laevis imaged with fluorescent quantum dots. From [3].

As feminist epistemology has taught us, the technologies we use to see in science are not neutral or innocent: they are also processes of materialization, manifesting the reality of the very bodies they are seeking to unveil. In this process of materializing bodies as images, and images as bodies, the materiality of the interfaces that allow us to see often becomes invisible. Approaching quantum dots in their physicochemical weirdness, with their vibrant and complex interfaces, can maybe help us grasp this forgotten materiality.

References

[1] Gao, X., Yang, L., Petros, JA. et al. In vivo molecular and cellular imaging with quantum dots. Curr Opin Biotechnol 16(1), 63-72 (2005).

[2] Bruns, O., Bischof, T., Harris, D. et al. Next-generation in vivo optical imaging with short-wave infrared quantum dots. Nat Biomed Eng 1, 0056 (2017).

[3] Benoit Dubertret et al., In Vivo Imaging of Quantum Dots Encapsulated in Phospholipid Micelles. Science 298, 1759-1762 (2002).