Quantum Dots: Applications in Biotechnology
What Are Quantum Dots?
Quantum dots are nanoparticles approximately 2-10 nm in diameter that are made of semiconducting material such as cadmium selenide. They are unique in the sense that they are small enough for quantum confinement effects to dominate their behaviour. Dr Booth said that, “Quantum dots are nanocrystals of semiconducting material with dimensions smaller than the exciton Bohr radius.” (A measure of the separation between the electron and hole in an exciton, or excited-state electron-hole pair in the dot). At this small scale, the particle is confined to dimensions smaller than its exciton Bohr radius, meaning that its electrons are limited to discrete energy states, unlike in larger semiconductors where the states form continuous bands.
The energy levels in quantum dots can be calculated by treating them as a particle-in-a-box problem. This simplified approach helps explain why larger dots emit light at longer wavelengths (red) while smaller dots emit at shorter wavelengths (blue).
What Can Quantum Dots Be Used For?
The most widely exploited property of quantum dots is their ability to emit light at precise wavelengths depending on their size, due to the quantum confinement effect which makes their energy levels discrete. Through the process of photoluminescence, quantum dots emit light when they absorb energy. When electrons in the conduction band drop back down to the valence band, the released energy produces light. Because of this, quantum dots are used in many fields including imaging and light emitting diodes.
History of Quantum Dots
Quantum dots were first discovered by Alexey Ekimov in the Soviet Union in 1981 at the Vavilov State Optical Institute. Research continued on semi conductors until Moungi Bawendi developed a method to produce nearly perfect quantum dots. Due to this, in 2023 these three were awarded the Nobel Prize in Chemistry for “the discovery and synthesis of quantum dots”
How Are Quantum Dots Used in Biotechnology?
In his lecture, Dr Booth focuses on how quantum dots can replace organic dyes in biological systems. Matt says that, “compared with organic dyes, quantum dots are much more photostable; they can undergo many excitation and emission cycles without significant degradation.”
Because of their sensitivity, quantum dots can be used in biological imaging. Quantum dots can be bound to cellular components and other molecules, and emit light in specified wavelengths, allowing us to track and visualise biomolecules over extended periods with remarkable precision and detail. This is helpful in diagnostics when we need to detect low-abundance molecules, like early cancer biomarkers, or tracking cells in the body.
Future Applications
A paper from MIT written by McHugh et al, discusses a method of delivering vaccines through biocompatible (safe) quantum dots (McHugh et al, 2019). The vaccine needle is coated in dissolvable microneedles that contain quantum dots encapsulated in biocompatible microparticles. The small size of the capsules means that the fluorescent signal can be detected using light in the near-infared (NIR) range; this means they can be detected with a modified smartphone. These quantum dot ‘vaccinations’ have the potential to greatly accelerate our ability to mass-vaccinate populations. Removing the need for conventional records, and even electricity, and could be instrumental in vaccinating children in developing nations.
References
McHugh et al (2019): Biocompatible Near-Infrared Quantum Dots Delivered to the Skin by Microneedle Patches Record Vaccination
<https://pmc.ncbi.nlm.nih.gov/articles/PMC6946931/#:~:text=Here%2C%20we%20developed%20an%20approach>
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This article expertly discusses the applications of quantum dots in biotechnology, including their photostability and fluorescence, while also addressing early controversies over their discovery. Although most of the relevant information has been added, I believe the article would benefit from the addition of the maths used in calculating the energy levels and how it differs between regular semiconductors.