Nano-Diamond Hybrid Materials for Structural Biomedical Application

Among the different allotropic forms of Carbon, graphite is the more thermodynamically stable at ambient temperatures and pressures, while diamond, in these conditions, may exist only in its metastable state. In fact, due to the high-energy barrier that separated the graphitic sp2 and diamond sp3 configurations (Fig. 1A and B), high temperatures and pressures in presence of catalysts are needed to transform graphite in diamond.

Nevertheless, a third parameter (surface area) becomes crucial at the nanoscale level and it become relevant in the definition of the system equilibrium energy levels: At this nano-dimensions, the Gibbs free energy becomes dependent on the contribution of the surface energy, leading to changes in the thermodynamic equilibrium phase diagram (Barnard et al., 2003; 2007; Viecelli et al., 2001). Tetrahedral hydrocarbons in the form of nano-diamonds of 3 nm have been demonstrated by atomistic models to be more stable than poly-aromatics graphite (Fig. 1C).

In addition, a more complex morphological structure is generated at the nanodiamond interface; Barnard and Sternberg (2007) reported that cuboctahedral clusters presented a transition from Sp3 to Sp2 carbons at the surfaces of aggregations of 1.0-3.0 nm (Aversa et al., 2016 a-o, 2017 a-e; Mirsayar et al., 2017).

On this morphological transition at the interface, it has been recently demonstrated by Xiao et al. (2014) that reversible nanodiamond-graphitic carbon onion like phase transformation can occur even at room temperature and pressure leading to the formation of diamond cores with graphitic shells (bucky-diamond) (Fig. 1C) (Barnard and Sternberg, 2007).

These findings allowed us to understand that the nanodiamond surfaces can be then easily modified through the chemistry of graphitic carbon in many different chemical methods, such are the Diels–Alder cycloaddition reactions between conjugated diene and dienophile, to form functionalised cyclohexene systems (Jarre et al., 2011).

This new class of materials based on Carbon Sp2 and Sp3 nanocrystalline structures is very attractive for future nanotechnological development in biomedical structural applications. Nanocrystalline particles, which are often named detonation nanodiamond and characterized by sizes of 3-6 nm, are produced by detonation of carbon explosive materials (Danilenko, 2004; Greiner et al., 1988; Ozawa et al., 2007; Chang et al., 2008).

Detonation nanodiamond have been initially utilized in applications such as galvanic coatings, polishing systems, polymer nano-composities, lubricants. New niche applications, however, are recently developing; magnetic recording, adsorbents, diamond ceramics production, coatings in field emission devices, catalyzes of heterogeneous catalysts and in fuel cells as proton-conducting nanocomposite membranes. Preliminary investigation demonstrated that detonation nanodiamonds are non-toxic and biocompatible, making them very attractive for bio-medical applications considering its easy controllable rich surface chemistry.

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However, it has been reported that detonation nanodiamonds may be characterized by different levels of purity and by the presence of several undesired functional groups/elements at the diamond particles surface, while high surface chemical purity and uniformity surfaces are needed for biomedical applications (Lai and Barnard, 2011a; 2011b). A simple purification method utilizes oxidation procedures. Depending on the type of procedure, the detonation powder of different levels of purities and specific surface characteristics can be obtained. The fraction of the Carbon that is not present as diamond can be purified up to 95% by weight by oxidation at high temperatures in air/Ozone atmosphere (Osswald et al., 2006; Shenderova et al., 2011).

Oxidation, while removing undesired processing functional compunds at nanodiamond surfaces, forms oxygen-containing groups, such are anhydrides and carboxylic acids (Shenderova et al., 2011).

The simple air/ozone purification, then, produces carboxylated nano-diamond with highly reactive and hydrophilic surface OH terminations appropriate in biomedical applications (Krueger et al., 2008; Kruger et al., 2006).

Diamond and glassy carbon has been recognized in literature, however, the toxicity of nano-diamonds remains a real concern (Schrand et al., 2009). In vitro and in vivo studies are still needed to evaluate characteristics such as in vivo mechanical and physiological behaviours (Zhang et al., 2011; Schrand et al., 2009a; 2009b; Yuan et al., 2010; Mohan et al., 2010) as well as cell viability or undesired gene modification activity.

Previous investigations of our group have shown that high level of biocompatibility and bioactivity has been observed for nano-composite materials made combining amorphous silica nanoparticles of about 7 nm.