Some of the main types of nanoengineered nanoparticles reported as toxic are (among others) fullerenes, carbon nanotubes (CNTs), inorganic nanoparticles, organic nanoparticles and quantum dots (QDs).
Following up the approach initiated in my article "The Roots of Nano-Fear Decoded", and continuing with the approach taken in my previous article - "Indicators of Nanotoxicity" - I will now address another nanotoxicity topic: some of the main types of nanoengineered nanoparticles reported as nanotoxic. Later on, I will attempt to point out ways and propose solutions in order fight and prevent nanotoxicity.
Some of the main types of nanoengineered nanoparticles reported as toxic are (among others):
- Carbon nanotubes (CNTs);
- Inorganic nanoparticles;
- Organic nanoparticles;
- Quantum dots (QDs).
Fullerenes are nanoparticles in the form of spherical cages containing 28 to 100+ carbon atoms.
The most studied fullerene contains 60 carbon atoms. This particular type of fullerene is a hollow sphere and its atomic arrangement consists of pentagons and hexagons of carbon interconnected.
The fullerenes show the capability of being subjected to extreme pressures and regain its original shape when pressure is no longer applied - something equal or identical to resilience, I would say.
When the fullerenes are not modified, these molecules do not react with each other.
However, when fullerenes are engineered, some specific carbon atoms may be replaced by other atoms. Consequently, the nanoengineered fullerenes show the capacity of establishing bonds with other molecule. The physical result obtained is a material hard and elastic at the same time.
Carbon nanotubes (CTNs) are hollow cylinders consisting of a crystalline form of carbon molecules arranged into a hexagonal network of carbon atoms. CNTs can have diameters around 0.7 nm.
Regarding the CNTs length, this can vary between a few nanometres to several millimetres. Each end of a CTN can be opened or closed by a fullerene half-molecule.
Regarding the complexity of the CNTs wall, there are two main groups of CTNs. These CNTs can have:
- A wall consisting of a single layer: Single-Walled Carbon NanoTubes (SWCNTs);
- A wall consisting of several layers: Multi-Walled Carbon NanoTubes (MWCNTs). MWCNTs can also be perceived as a coaxial arrangement of carbon cylinders of increasing diameters having a common axis.
It is considered that a CNT having the dimensions 20 nm width x 2.000 nm length may have 100 x more surface area than a spherical nanoparticle with a diameter of 20 nm. Under specific conditions, this can interpreted as an indicator of high toxicity of the CNTs.
On the other hand, CNTs show a great tendency to agglomerate. This fact may be a cause for a significant reduction of the total area of CNTs capable of interacting with Human cells or their components.
Inorganic nanoparticles are frequently insoluble or show a poor solubility. Inorganic nanoparticles can be composed of pure metals or various inorganic alloys.
Among the inorganic nanoparticles, nano-titanium dioxide (nano-TiO2) is one of the most studied due to their nanotoxic effects.
Organic nanoparticles are also frequently insoluble or show a poor solubility. Organic nanoparticles can be composed of various organic substances. These various organic substances are commonly insoluble polymers. Different organic radicals can be grafted to these insoluble polymers.However, organic nanoparticles can also be materials involving entrapment, encapsulation or surface adsorption of a bioactive substance.
Quantum dots (QDs) are nanoengineered nanoparticles.
The main properties of QDs are:
- High photostability;
- Strong light absorbance;
- Excitation by single-wavelength;
- Size-tunable emission.
QDs are usually composed of a core of a semiconductor material enclosed in a shell of another semiconductor material with a larger spectral band-gap.
A typical QD has a diameter ranging from 2 to 8 nm.
QDs are extremely diversified. Therefore, the toxicity of QDs requires to be carefully characterized individually for each type of QD.