Nanodiamond Particles

The DND used in this work was represented by nanodiamond particles (4 nm in size, as determined from the pristine DND diffraction pattern peak width analysis. Fig. 1 b) partially aggregated into larger formations, including globules with the average diameter of -300 nm (Fig. la). Diamond nanoparticles within the aggregates are bound together with sp carbon [3] whose presence results in the broad peak at 20 = 22° in the diffraction pattern of the DND used. 

The research on nanodiamond has by now developed into a multifaceted and very active field with the main interest focusing on the physical and chemical properties. Numerous applications have already been realized, and further dynamic development can be envisaged once it becomes possible to examine and modify individual nanodiamond particles. 

Further defects include, for example, the directed doping with boron, nitrogen or nickel. These confer certain electronic or optical properties to the nanodiamond particles (Section 6.2.3). Experimental as well as theoretical results show that only few elements Hke boron, nitrogen, silicon, oxygen, or phosphorus can be incorpo-... 

Depending on the method of their preparation, the individual nanodiamond particles do not exist as isolated crystallites, but they form tightly bound agglomerates. Apart from unordered sp - and sp -hybridized carbon, they may also include other impurities. The latter may originate either from synthesis or purification, for example, finely dispersed material from the reactor walls may contaminate the sample (Section 5.3). This is especially true for material produced by the detonation or shock wave method, whereas hydrogen-terminated diamond nanoparticles do not show this effect. 

Figure 5.28 Current-voltage characteristic of field emission from a nanodiamond particle on a silicon substrate doped with nitrogen. The turn-on field intensity (attainment of a 0.1 iA current) is 3.2Vpm", and at 5Vpm" a current density of about 95 mAcm" is attained ( Elsevier 2000).

Particularities like unpaired electrons at unsaturated bonding sites play a role for the electronic properties too, of course. However, a spin density of only 10 -10 spins per gram is determined from respective measurements (Section 5.4.1.5), which corresponds to a one-digit number of spins per nanodiamond particle. It results from a strong tendency toward saturation by the formation of re-bonds. In doing so, surface states rather graphitic in character are formed that cause, among other effects, also an electric conductivity (see below). 

For various samples of nanodiamond, conductivity measurements have also been made. The degree of graphitization plays a major role here for the magnitude of resistance. On suitably purified nanodiamond particles, there are initially just small, incoherent ti-systems, and the conjugation is anything but pronounced. 

In comparison to bulk diamond, nanodiamond particles are distinctly more reactive. This may be explained by the larger number of defects and by a markedly enlarged surface. Both effects increase the number of potential sites for the attack of a reagent, thus facihtating chemical modifications of nanodiamond particles. These include not only a functionalization of the surface, but also a conversion into other forms of carbon as discussed in Section 5.5.3. Due to the defective structure and to the presence of small graphitic domains on the particle surface, these transformations as well proceed much easier here than with macroscopic diamond particles. 

As discussed in Section 5.2.2, nanodiamond particles produced by detonation or shock wave synthesis exhibit a primary surface functionalization right from the preparation. It comprises a multitude of different groups, so it should be possible to make use of these functional groups. To ensure a reproducible quality of the secondary products obtained, however, a homogenization of the primary functionalities is required. There are several strategies to achieve this (Figure 5.29). 

From the hydrogenation or fluorination of a diamond material, a very hydro-phobic surface results that may then enter into an exchange with rather nonpolar compounds. A connection via Jt-stadting, however, plays just a minor role because graphitic fragments are only found in small domains on the particle surface. In the case of thermally graphitized nanodiamond particles, on the other hand, the conditions largely resemble those observed for multiwalled nanotubes. The interaction of the 7t-electrons with the polymer molecule causes a stable noncovalent incorporation into the composite. 

A possible reason for this enhancement of material properties may be the higher degree of crosslinking observed in the polymers mixed with nanodiamond. Hence an incorporation of suitably functionalized nanodiamond particles should lead to even more favorable properties and to a more homogeneous distribution of... 

It is especially nitrogen defects that may be detected this way, that is, by fluorescence microscopy (Figure 5.46). Defective nanodiamond particles with surface functionalization may also be employed as fluorescence label in in vivo experiments, so a system complementary to the metal chalcogenide quantum dots usually applied is available here. The nanodiamond adducts in these processes are characterized by their small particle size, stable fluorescence, and (at least according to current knowledge) by their nontoxicity. 

Nanodiamond consists of particles showing a size in the range of nanometers. A distinction has to be made between materials containing very small particles (d 4nm) on the one hand and larger nanodiamond particles on the other. Very small particles tend toward agglomerate formation, which is favored by the presence of graphitic carbon as well as by the functional groups situated on the particle surface. The preparation can be achieved in different ways (Box 5.1)... 

Schrand AM, Hens SAC, Shenderova OA. Nanodiamond particles properties and perspectives for bioappUcations. Crit Rev Solid State Mater Sci 2009 34 18-74. 

For nanodiamonds, there are relatively less studies on their toxicity compared to the aforementioned carbon nanostructures. However, results from the limited studies consistently suggest that nanodiamonds are bioinert and biocompatible to a variety of cell types. A number of recent studies all showed low toxicity of nanodiamonds to osteoblasts, fibroblasts, human kidney cells, neuroblastoma, macrophage, keratino-cyte, and PC-12 cells, as well as little production of ROS [46,47]. Direct transmission electron microscopy (TEM) evidence also showed that nanodiamond particles were internalized by neuroblastoma cells but did not cause apparent toxicological effect to cell mitochondrial functions [47]. 

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