**2.2 Properties of Nanodiamonds**

The properties of the nanomaterials depend on their size. In Nanodiamonds also, depending on the size the properties vary. When the size of the cluster changes, the discrete electronic energy levels become visible at the edges of the bands. As a result the band gap increases as a consequence of separation of frontier orbitals' energy which follows the quantum confinement effect. Quantum confinement will occur when the diameter of the Nanodiamond becomes trivial say 2–3 nm thereby structure variation possible (Bucky diamonds or fullerene structure). When the diameter of the cluster of Nanodiamond increases, the surface carbon atoms reduces and hence the characteristics of the Nanodiamonds improves. During the development of the dangling bonds on the outer layer of the Nanodiamond accords the stabilization of the material. The surface terminations have influence on the stabilization of carbon compounds drained the center of attention to several studies. For good functionalization and surface termination, the surface of the Nanodiamond can be enhanced with the organic functional groups. These groups can be easily identified by FTIR studies.

The surface functionalization and size studies make Nanodiamonds a potential material in drug delivery, biomolecule conjugation, uploading sorbent molecules, catalytic application and polymer matrix. At room temperature, the nitrogen vacancies in Nanodiamonds provide red emission spectrum and visible emission spectrum. Nanodiamonds also favors the fluorescent properties empowers the nanoelectrometry, nano-magnetometry, etc. Nanodiamonds can tune their magnetic and optical properties by adjusting the surface chemistry of the material.

Nanodiamonds have high hardness, thermal conductivity, biocompatibility, Young's modulus, chemical stability, high electrical resistivity, resilience to a dictatorial setting. Nanodiamonds are known for their exceptional mechanical and optical qualities, as well as their large surface areas and tunable surface topologies. They're also nontoxic, making them ideal for biological applications [4]. The most featured properties are fluorescence and biocompatibility which are depicted below.

### *2.2.1 Fluorescence*

Introducing a nitrogen vacancy in the lattice is known as NV center or nitrogen vacancy center which dominance the fluorescence properties in Nanodiamonds. A nitrogen vacancy is created by bombarding elevated particles and vacuum annealing between 600 and 800°C. During the irradiation vacancy will form at the centers and during annealing the vacancies will emigrated and confined by the nitrogen atoms. During this process two types of vacancy center forms namely negatively charged nitrogen vacancy and neutral nitrogen vacancy center. In addition, both vacancy centers will have distinct emission spectra. Among these, peculiarity falls into negatively charged nitrogen vacancy center as it has a spin S = 1 ground level results in spin polarization through optical pumping and controlled by electromagnetic resonance. The spin coherence time for this vacancy center is quite long. Nitrogen vacancy became highly capable for bioimaging, magnetic sensing, and fluorescence resonance high energy transfer.

Under high temperature and pressure synthesis, blazing photo luminescent Nanodiamonds can be produced in materials later squeeze down to nano sized particles. The concentration of NV imperfections developed from electron irradiation, does not depend upon the size of the Nanocrystals but they can decrease the size of the Nanodiamonds as the electrons might get pin down at the surface.

Recent studies picturizes the interest on imaging applications from nitrogen vacancy centers. In a bare Nanodiamond (~5 nm) developed from trinitrotoluene (TNT) and hexagon precursor, the intermittent luminescence is emanated from nitrogen vacancy center is reported [5]. Nanodiamonds which are considered larger (>20 nm) grown from TNT, graphite precursors show the presence of stable luminescence [6]. When fluorophores are linked or adsorbed with Nanodiamond fluorescent particles are formed. This fluorophore linked Nanodiamond can pass through different cell chambers (variable pH) without changing the cell feasibility [7]. When octadecylamine is covalently linked with carboxylic acid on a nanodiamond surface a bright blue fluorescent Nanodiamond is produced and reported [8].

### *2.2.2 Biocompatibility of Nanodiamonds*

Diamond is known as a non-toxic material. The toxicity of Nanodiamonds is investigated through in-vivo as well as in-vitro studies. Both studies scrutinize the characteristics of a cell feasibility, cell mechanism and behavior. Through in-vitro cytotoxicity, carbon nanomaterials like nanotubes (single as well as multi walled), carbon black the toxicity were studied. They also examined with Nanodiamonds. The studies concluded the carboxyl nanotubes are more toxic rather than Nanodiamonds as carboxyl Nanodiamonds shows relatively less toxicity [9].

In human beings, in-vivo toxicity is deeply developed with the help of animal models. By the detonation techniques Nanodiamonds are formed in a powder form (low density) which will be easily spread in the environment. The respiratory tracking is much more efficient in human beings to study the toxicity. To know more about the tracking of the toxicity intratracheal instillation can be selected. Nanodiamonds controlled by intratracheal instillation is examined by biomedical measurement is diffused into spleen, liver, heart and bones [10]. The interaction of nanomaterials introducing to living system is monitored through adsorption, distribution, metabolism and excretion process (ADME process). As nanomaterials are very small, it is arduous to spot the interaction through a microscopic method. One of the most promising technique for good reliability, high sensitivity, and easy detection is radionuclide tracer technique. Gallium, Indium, Copper are used for labeling carbon nanoparticles. Rhenium Nanodiamonds are appropriate Nanodiamonds used for experimental research.
