Quantum Dots and Their Applications in Biotechnology
Date: 5th November 2025 Speaker: Dr Matthew Booth
Researchers at MIT have developed an invisible quantum dot “tattoo” that stores vaccination records beneath the skin, readable by standard smartphones. This technology sounds like science fiction, yet it represents just one application of quantum dots—semiconductor nanocrystals whose remarkable properties are transforming biotechnology, displays, and medical imaging.
What Are Quantum Dots?
Quantum dots are semiconductor crystals measuring just 2-10 nanometers across—roughly one-thousandth the width of a human hair. At this scale, quantum mechanics governs their behavior, producing properties impossible in bulk materials. When illuminated, these nanocrystals emit brilliant, pure colors determined entirely by their size.
The physics underlying this phenomenon resembles a jack-in-the-box. Imagine compressing the box: when released, the jack springs out with greater energy than from a larger box. Similarly, confining an exciton—a bound electron-hole pair—within a shrinking crystal increases its energy. Smaller quantum dots emit bluer light; larger ones emit redder light. This size-dependent color tunability distinguishes quantum dots from traditional fluorescent dyes.
The technical explanation involves quantum confinement. In semiconductors, electrons and holes bind together forming excitons with characteristic radii—typically 1-20 nanometers. When crystal dimensions fall below this exciton Bohr radius, the particles become “squashed,” raising their energy levels. The band gap—the energy difference between electron states—increases as crystal size decreases, shifting emission toward shorter wavelengths.
Why Quantum Dots Beat Traditional Dyes
Both quantum dots and fluorescent dyes emit light in narrow spectral bands, producing vivid colors. However, quantum dots possess a critical advantage: while dyes absorb and emit at similar wavelengths, quantum dots absorb broadly across the spectrum but emit narrowly. This enables multiplexed imaging—illuminating different colored quantum dots with a single ultraviolet source.
For cellular imaging, this matters immensely. Scientists can label different cellular components with various quantum dot colors, then excite all simultaneously. Traditional dyes require separate excitation wavelengths for each color, creating cross-channel interference and complicating quantitative analysis. As Dr Matthew Booth explains in his lecture, “if you want to see how different parts of the cell interact, quantum dots enable this.”
Quantum dots also demonstrate superior photostability. Fluorescent dyes suffer photodegradation—repeated excitation triggers chemical reactions permanently destroying their fluorescence. Quantum dots withstand thousands of excitation cycles without degradation, maintaining detectability for five years or longer. This longevity makes them essential for applications like MIT’s vaccination tracking system, which would fail rapidly using conventional dyes.
Medical Applications
At nanometer scales, quantum dots can be functionalized—coated with targeting molecules such as antibodies. Cancer researchers exploit this by engineering quantum dots that bind selectively to tumor cells. When injected into the body, these nanocrystals accumulate in cancerous tissue while bypassing healthy cells. Under appropriate illumination, tumors glow distinctly, providing surgeons with real-time visualization of cancerous margins.
The “biological window”—wavelengths penetrating tissue most effectively—lies in the red to near-infrared range around 650-900 nanometers. Water absorption drops significantly here, allowing light to pass through centimeters of tissue. Quantum dots can be precisely tuned to emit at these optimal wavelengths, maximizing imaging depth. Traditional dyes offer limited wavelength options; quantum dots provide continuous tunability simply by adjusting crystal size during synthesis.
Quantum dots also enable therapeutic applications. Photothermal cancer therapy uses metal nanoparticles or certain quantum dots coated with tumor-targeting antibodies. Once accumulated in cancerous tissue, these nanocrystals absorb light and convert it to heat, destroying cancer cells while sparing surrounding healthy tissue. This approach promises fewer side effects than conventional radiotherapy.
The Nanotoxicity Question
Despite their promise, quantum dots raise important safety concerns. Nanotoxicity—the biological effects of nanoscale materials—remains poorly understood. Bulk material toxicity provides limited guidance; properties like size, shape, and surface chemistry profoundly influence nanomaterial behavior in living systems.
Early quantum dots contained cadmium or lead—highly toxic elements. While encapsulation strategies minimize exposure, concerns persist. Studies in primates reveal accumulation in organs like the spleen, though long-term consequences remain unclear. As Dr Matthew Booth emphasizes in his lecture, “for new types of nanomaterials, it is important to carefully assess nanotoxicity… there are many more considerations when thinking about toxicity at the nanoscale.”
Each application requires rigorous safety evaluation. The MIT vaccination tracking system undergoes scrutiny precisely because these quantum dots remain in human tissue for years. Regulators must balance potential benefits against incompletely understood risks.
Synthesis and Practical Considerations
Manufacturing quantum dots involves solvothermal synthesis—heating chemical precursors in solution until crystal nucleation occurs. Interestingly, controlling size proves easier at larger scales. Making small (blue-emitting) quantum dots requires stopping growth almost immediately after nucleation, presenting technical challenges. This explains why copper indium sulfide quantum dots appear predominantly yellow, orange, and red—smaller crystals dissolve back into solution before achieving stability.
The Bigger Picture
Quantum dots demonstrate how nanoscale engineering unlocks properties absent in bulk materials. From cancer diagnosis and therapy to next-generation displays and solar cells, these semiconductor nanocrystals exemplify convergence between physics, chemistry, and biology.
Understanding quantum confinement—how size determines electronic properties—enables precise control over emission wavelengths, offering capabilities unmatched by traditional materials. The broad absorption, narrow emission, photostability, biocompatibility, and tunability collectively position quantum dots as transformative tools for medicine and beyond.
Yet these advantages come with responsibilities. As nanotechnology advances, researchers must carefully evaluate long-term biological effects, ensuring therapeutic benefits outweigh potential risks. The invisible vaccination tattoo represents not just technological achievement but also the delicate balance between innovation and safety that defines modern biotechnology.
References:
- McHugh, K.J. et al. (2019). “Biocompatible near-infrared quantum dots delivered to the skin by microneedle patches record vaccination.” Science Translational Medicine, 11(523), eaay7162. Massachusetts Institute of Technology.
- Bawendi, M.G., Ekimov, A.I., and Efros, A.L. (2023). “For the discovery and synthesis of quantum dots.” Nobel Prize in Chemistry citation.
- Resch-Genger, U. et al. (2008). “Quantum dots versus organic dyes as fluorescent labels.” Nature Methods, 5(9), 763-775.