Quantum Dots, The Future of Biomedicine

The word 'quantum' means a small quantity of energy proportional to its radiation frequency. In other, simpler words, it is the smallest unit of something, such as how a photon is a quantum of light. That is why a new microscopic technology is aptly called quantum dots (QDs). QDs are semiconductor crystals (materials that exhibit electrical conductivity) that have optical and electronic properties because of their small size, which is about 1 - 10 mm.

In recent years, researchers have found that QDs hold the key to further progression in the medical field, though they do have their faults as well.

Types of QDs

QDs are categorized according to elemental composition, properties, size, shape, and structure. This categorization is important for researchers, since they need to understand every aspect of behavior to build safe QDs.

Properties of QDs

There are three categories that QDs can be divided between based on properties - carbon QDs, functionalized QDs, and surface-modified fluorescent carbon QDs.

The first category, carbon QDs, are a group of dots that are viable alternatives for semiconductors due to their biocompatibility (ability to coexist with tissues without causing harm) and stability. Carbon QDs are also known to have high photoluminescence at low costs. Due to the high photoluminescence (emission of light), it is a perfect candidate to use for lymph node mapping and tumor diagnosis. Because these QDs emit light well, doctors can easily visualize parts of the body.

The second category is functionalized QDs (f-QDs). This specific type are efficient, safe, and effective for delivering bio-activities. These bio-activities can include medication (drugs) or gene delivery "vehicles."

Surface-modified fluorescent carbon QDs are a category of functionalized QDs and are known as acting ligand targeters (binds to receptors on cells). Because of this characteristic, this sub-category of QDs have potential to be used for enhanced, specific cellular targeting. Another property f-QDs have is high quantum yield, the efficiency of converting light energy into emitted light. This characteristic is similar to photoluminescence, as it helps QDs emit a strong light.

Size of QDs

QDs can range from 1 to 10mm, which is roughly equivalent to about 0.04 to 0.4 inches. Here, QDs are again separated into different categories.

Small QDs range from 1mm to 4 mm. Their small sizes determine its unique properties of better tissue penetration, adaptability to in-vivo techniques, and usefulness in imaging cellular structures. These dots have larger bandgaps (energy that determines electrical conductivity), therefore needing more energy to excite electrons.

Large QDs, on the other hand, range from 5mm to 10mm. These QDs are more efficient at attaching to ligands that target cancer, and antibodies, due to having smaller bandgaps and requiring less energy.

Therefore, the two different types of QDs (depending on size) have their own strengths and weaknesses.

Shape of QDs

Two common shape-based classifications are spherical QDs and nanorods. Spherical QDs have "symmetrical" properties. In other words, light is emitted in all directions and is useful for LEDs. Nanorods have asymmetrical properties and only emit light in a certain direction, and is useful for controlled light emission. Nanorods are well suited towards optoelectronic devices (LEDs, lasers, and barcode scanners are a few examples).

Structure of QDs

Figure 1. Classification of Quantum Dots (QDs) based on structure

1. Core-shell QDs

Core-shell QDs have a core material surrounded by a shell. The core has the unique properties of QDs while the shell works to improve stability and improve optical (visual) properties. Careful selection of shell materials can reduce sensitivity to outside materials and even more stability. Additionally, biomolecules (antibodies, etc.) can be added. For instance, folic acid (a vitamin), enhances capability to enter cancer cells.

2. Janus QDs

Janus QDs are special in the way they have two sides with different surface properties and this causes them to have asymmetric characteristics. These QDs are also able to perform self-assembly and are skilled at building complex structures. This is important because janus QDs can combine multiple functions, making them promising candidates for medical use.

3. Alloyed QDs

Alloyed QDs are also somewhat like janus QDs. They are made up of many different elements, allowing its properties to be fine-tuned and their composition easily modified, making it perfect for targeting a specific part of the body.

4. Doped QDs

Doped QDs involve the incorporation of impurities. Although it sounds ineffective, the use of impurities can positively alter properties. Doping can improve light absorption and change the way light is emitted, among other benefits. Many doped QDs are used in solar cells and electronic devices.

Surface Modifications

One drawback of QDs in biomedical research is their toxicity. QDs release lead and arsenic, which is significant cause of concern. They are also often not soluble (dissolvable in water), which poses a concern for how the body will get rid of QDs.

However, surface modifications are a solution. One way is to perform surface ligand exchange, which is exchanging toxic ligands with biocompatible ligands. Another way is to coat QDs in biocompatible materials, barring the release of toxins.

Surface ligand exchange

Encapsulation

In the image portraying the encapsulation of a PEG-PLA copolymer, the TOPO is added to the copolymer, causing the copolymer's hydrophilic ("water-loving") area to point out, blocking the inner, hydrophobic ("water-hating") area.

Conclusion

The many unique properties of QDs hold the future of the medical field, with its potential applications in imaging, disease diagnosis, and other biomedical technologies. They are also easy to manipulate - in both size, functionality, and targeting capabilities - and this makes them even more promising. With further research and knowledge, disease diagnoses will be easier and more accurate than ever.