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Topic Name: Study Finds Quantum Dots Nanoparticles Can Penetrate Skin Through Minor Abrasions

Category: Nanocharacterization

Research persons: Dr. Nancy Monteiro-Riviere, Leshuai Zhang

Location: College of Veterinary Medicine, North Carolina State University, United States

Details

Researchers at North Carolina State University have found that quantum dot nanoparticles can penetrate the skin if there is an abrasion, providing insight into potential workplace concerns for healthcare workers or individuals involved in the manufacturing of quantum dots or doing research on potential biomedical applications of the tiny nanoparticles.

While the study shows that quantum dots of different sizes, shapes and surface coatings do not penetrate rat skin unless there is an abrasion, it shows that even minor cuts or scratches could potentially allow these nanoparticles to penetrate deep into the viable dermal layer – or living part of the skin – and potentially reach the bloodstream.

Dr. Nancy Monteiro-Riviere, professor of investigative dermatology and toxicology at NC State's College of Veterinary Medicine, tested the ability of the quantum dots to penetrate rat skin at 8 and 24 hour intervals. The experiment evaluated rat skin in various stages of distress – including healthy skin, skin that had been stripped using adhesive tape and skin that had been abraded by a rough surface. The researchers also assessed whether flexing the skin affected the quantum dots' ability to penetrate into the dermal layer. Monteiro-Riviere co-authored the study with doctoral student Leshuai Zhang.

While the study indicates that acute – or short-term – dermal exposure to quantum dots does not pose a risk of penetration (unless there is an abrasion), Monteiro-Riviere notes "there is still uncertainty on long-term exposure." Monteiro-Riviere explains that the nanoparticles may be able to penetrate skin if there is prolonged, repeated exposure, but so far no studies have been conducted to date to examine that possibility. Quantum dots are fluorescent nanoparticles that may be used to improve biomedical imaging, drug delivery and diagnostic testing.

This finding is of importance to risk assessment for nanoscale materials because it indicates that skin barrier alterations – such as wounds, scrapes, or dermatitis conditions – could affect nanoparticle penetration and that skin is a potential route of exposure and should not be overlooked.

The study found that the quantum dots did not penetrate even after flexing the skin, and that the nanoparticles only penetrated deep into the dermal layer when the skin was abraded. Although quantum dots are incredibly small, they are significantly larger than the fullerenes – or buckyballs – that Monteiro-Riviere showed in a 2007 study in Nano Letters can deeply and rapidly penetrate healthy skin when there is repetitive flexing of the skin.

Additionally, Monteiro-Riviere's laboratory previously showed quantum dots of different size, shape and surface coatings could penetrate into pig skin. The anatomical complexity of skin and species differences should be taken into consideration when selecting an animal model to study nanoparticle absorption/penetration. Human skin studies are also being conducted, but "it is important to investigate species differences and to determine an appropriate animal model to study nanoparticle penetration," Monteiro-Riviere says. "Not everyone can obtain fresh human skin for research."

Nanoparticles are generally defined as being smaller than 100 nanometers (thousands of times thinner than a human hair), and are expected to have widespread uses in medicine, consumer products and industrial processes.

About Quantum Dot
A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules.

Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits. Some quantum dots are commercially available.

There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.

An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the quantum confined size of the nanocrystal is more significant at energies near the band gap. Thus quantum dots of the same material, but with different sizes, can emit light of different colors.

The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit bluer (higher energy) light. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the bandgap energy that determines the energy (and hence color) of the fluoresced light is inversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energy levels which are more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e. those closer to the red end of the spectrum. Recent articles in nanotechnology and other journals have begun to suggest that the shape of the quantum dot may well also be a factor in the coloration, but as yet not enough information has become available. Furthermore it was shown recently that the lifetime of fluorescence is detemined by the size. Larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer and thus these large dots show a larger lifetime.

As with any crystalline semiconductor, a quantum dot's electronic wave functions extend over the crystal lattice. Similar to a molecule, a quantum dot has both a quantized energy spectrum and a quantized density of electronic states near the band edge.

About Nanoparticle
The term nanoparticle is generally used to refer to a small particle with all three dimensions less than 100 nanometres . The term also includes subcategories such as nanopowders, nanoclusters and nanocrystals.

A nanocluster is an amorphous/semicrystalline nanostructure with at least one dimension being between 1-10nm and a narrow size distribution. This distinction is an extension of the term "cluster" which is used in inorganic/organometallic chemistry to indicate small molecular cages of fixed sizes.

A nanopowder is an agglomeration of noncrystalline nanostructural subunits with at least one dimension less than 100nm.

The term nanocrystal is not a generic term. The term is actually a registered trademark of Elan Pharma International (EPIL) used in relation to EPIL’s proprietary milling process and nanoparticulate drug formulations.

Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.

The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometre the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. The interesting and sometimes unexpected properties of nanoparticles are partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties.

Nanoparticles exhibit a number of special properties relative to bulk material. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper. The change in properties is not always desirable. Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid. Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution.

Nanoparticles have a very high surface area to volume ratio. This provides a tremendous driving force for diffusion, especially at elevated temperatures. Sintering can take place at lower temperatures, over shorter time scales than for larger particles. This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate complicates matters. The large surface area to volume ratio also reduces the incipient melting temperature of nanoparticles.

Moreover nanoparticles have been found to impart some extra properties to various day to day products. Like the presence of titanium dioxide nanoparticles impart what we call as the self-cleaning effect, and the size being nanorange, the particles cant be seen. Nano Zinc Oxide particles have been found to have superior UV blocking properties compared to its bulk substitute. This is one of the reasons why it is often used in the sunscreen lotions. Clay nanoparticles when incorporated into polymer matrices increase re-inforcement, leading to stronger plastics, verified by a higher glass transition temperature and other mechanical property tests. These nanoparticles are hard, and impart their properties to the polymer (plastic). Nanoparticles have also been attached to textile fibers in order to create smart and functional clothing.