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Posted: Jan 08, 2007
Diamond nanoparticles appear not to be cytotoxic
(Nanowerk Spotlight) Finely divided carbon particles, including charcoal, lampblack, and diamond particles, have been used for ornamental and official tattoos since ancient times. The importance of carbon nanomaterials in biological applications has been recently recognized.
Owing to their low chemical reactivity and unique physical properties, nanodiamonds could be useful in a variety of biological applications such as carriers for drugs, genes, or proteins; novel imaging techniques; coatings for implantable materials; and biosensors and biomedical nanorobots. Therefore, it is essential to ascertain the possible hazards of nanodiamonds to humans and other biological systems.
Researchers now have, for the first time, assessed the cytotoxicity of nanodiamonds ranging in size from 2 to 10 nm. Assays of cell viability such as mitochondrial function (MTT) and luminescent ATP production showed that nanodiamonds were not toxic to a variety of cell types. Furthermore, nanodiamonds did not produce significant reactive oxygen species. Cells can grow on nanodiamond-coated substrates without morphological changes compared to controls.
These results suggest that nanodiamonds could be ideal for many biological applications in a diverse range of cell types.
"Although the scientific community has been so far primarily focused on the potential biological applications of fullerenes and carbon nanotubes, other carbon nanomaterials, especially nanodiamonds, are beginning to emerge as alternative candidates for similar and many other applications" Professor Liming Dai explains to Nanowerk. "Both carbon nanotubes and nanodiamonds can be similarly modified for nanocomposite and biological applications. It is envisaged that nanodiamonds may prove to be an even better drug carrier, imaging probe, or implant coating in biological systems compared to currently used nanomaterials due to its optical transparency, chemical inertness, high specific area, and hardness."
It has been recognized that a bulk material with good biocompatibility may not be as well-tolerated by the body once it is in a fine particulate or nanometer-sized form. Although diamond thin films produced by chemical vapor deposition (CVD) are generally regarded as biologically inert, noninflammatory, and biocompatible, are diamond nanoparticles also well tolerated by cells?
A remarkable observation that detonation of oxygen-deficient explosives in an inert medium produces ultra-fine diamond particles having diameters of 4–5 nm was made four decades ago, but this novel form of diamond has never been isolated in pure form until a few years ago.
"The availability of the newly produced detonation nanodiamonds in aqueous dispersed forms facilitates their possible use in nanomedicine (e.g., medical nanorobots made of nanodiamonds) and biorelated studies" says Dai. "However, the environmental impact of nanodiamonds, especially on humans and other biological systems, has not been properly studied."
Recent research by Dai, Wright Brothers Institute Endowed Chair in Nanomaterials at Dayton University, and colleagues from the Air Force Research Laboratory and the NanoCarbon Research Institute in Chiba/Japan under the NEDO International Joint Research Program, has brightened the prospects for using nanodiamonds as drug carriers, implant coatings, nanorobots and other medical applications that take advantage of diamond nanoparticles’ attractive properties.
Interaction of neuroblastoma cells with nanodiamonds (ND). (A) TEM image showing internalized nanodiamonds after exposure to 25 µg/mL ND-COOH for 24 h. (B) Higher magnification of lower agglomerate in (A). (C) SEM image of cell growth on a control collagen substrate after 6 h. (D) SEM image of cells grown on a ND-COOH coated collagen substrate after 96 h. Scale bars are (A) 500 nm, (B) 100 nm, and (C), (D) 10 µm.
The researchers demonstrated that 2-10 nm nanodiamonds, with and without surface modification by acid or base, are biocompatible with a variety of cells of different origins, including neuroblastoma, macrophage, keratinocyte, and PC-12 cells. Several methods for assessing toxicity were used to rigorously test the cytotoxicity of the nanodiamonds (2-10 nm) using carbon black (20-30 nm) and cadmium oxide (100-1000 nm) as negative and positive controls, respectively.
"Although the cell types used may have different mechanisms of internalization of the nanodiamonds and the long-term effect of the internalized nanodiamonds on the cells needs to be further investigated, the resultant retention of mitochondria membrane along with low levels of reactive oxygen species (ROS) suggests that once inside the cell the nanodiamonds remain nonreactive" says Dai.
ROS are naturally generated byproducts of cellular redox/enzymatic reactions such as mitochondrial respiration, phagocytosis, and metabolism. However, they can also unregulate generation, leading to a condition known as oxidative stress, which can cause numerous pathological conditions. Increases in intracellular ROS (oxidative stress) represent a potentially toxic insult which, if not neutralized by antioxidant defenses (e.g., glutathione and antioxidant enzymes) could lead to membrane dysfunction, protein degradation, and DNA damage.
In conjunction with the toxicity testing of nanodiamonds, cells were grown on nanodiamond-coated substrates to examine their interactions and sustained viability over time, which provided further assurance for the utility of nanodiamonds as biologically compatible materials.
"Clearly, a promising potential for future research and development exists in this area." says Dai.