# Immobilization of Detonation Nanodiamonds on Macroscopic Surfaces
## 3 Results  
Initially, de-agglomerated ND suspensions were obtained using bead milling of the commercialized NDs. The reduction of the ND agglomeration size is a compulsory process to deploy the full potential of these carbon-based nanoparticles. The high surface-to-volume ratio is necessary both for homogeneous coatings and for further efficient functionalization processes. In this work, in-house bead-milled 1 mg/mL aqueous ND suspensions were employed with a mean hydrodynamic diameter of 43 \(\pm\) 2 nm at pH 12. See Figure S3 for details regarding the zeta potential and mean agglomerate size of NDs as a function of the ionic strength of the dispersion medium. Figure S4 shows TEM images to confirm ND agglomerate size and individual particle size. The aim of the current study is to investigate the immobilization strategy of NDs as a coating material in combination with metallicimplant materials. ND coatings for further chemical vapor deposition (CVD) treatments are widely discussed [37; 38; 39]. These coatings are based on electrostatic interactions and van der Waals forces of the substrates and NDs. We demonstrate the immobilization of NDs using covalent bonds with higher adhesive properties compared to adhered NDs. The model for the ND immobilization using silanized surfaces is shown in Figure 1.  
The covalent immobilized NDs will be compared to physisorbed NDs. In order to achieve a homogenously dense and nanometre-sized ND coating of commercial pure (cp) grade 4 Ti, three different coating strategies were studied using topographic investigation methods. Figure 2 shows the SEM and AFM images, including the correlated sample profile. Suitable coating methods were identified, namely dip coating and electrophoretic deposition (EPD) for physisorbed NDs, as well as amide-boned NDs for the covalent immobilization pathway. The SEM images of the NDs coated onto Ti are shown in Figure 2a. The white arrows mark the ND agglomerates on the Ti surface, and serve as a guide to the eye. Due to the homogenously  
Figure 1: Covalent immobilization using (3-Aminopropyl)triethoxysilane and 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-Hydroysuccinimide (EDC/NHS) activation of carboxylated NDs. Modified from [40]. Figure 2: (**a**) Scanning electron microscopy (SEM) images and (**b**) atomic force microscopy (AFM) images, including the correlated profile measurements of dip-coated, electrophoretic deposition (EPD)-treated and covalently immobilized nanodiamonds (NDs) on native TiO\({}_{2}\), in comparison to the uncoated surface. applied electric field, the EPD-treated surfaces display a higher ND density compared to the dip-coated samples. In both cases, electrostatic and van der Waals interactions lead to the loose attachment of the NDs. The highest immobilization rate of NDs is achieved using the formation of amide bridges between silanized surfaces and carboxylated NDs. Figure 2b represents the correlated AFM images. Scratches and inhomogeneities in the AFM images are related to the polishing process and sample roughness. The covalent immobilization strategy requires carboxylated NDs and amine-terminated silanized surfaces. The agglomerates of carboxylated NDs are bigger, due to their higher number of functional carboxylic acid surface groups and intrinsic hydrogen bonding [41]. This explains the 40 nm height difference in the sample profile of amide-bonded NDs. Table 1 provides the information on surface roughness parameters for uncoated, dip-coated, EPD-treated, and amide-bonded NDs on Ti calculated from Figure 2b. Overall, the covalent, immobilized NDs provide the best results in terms of reproducibility, with homogeneous and almost total surface coverage. In order to qualify the attachment of the immobilized NDs, nanoindentation measurements were carried out on planar Si as reference material, to avoid the influence of the surface inhomogeneities of the employed Ti.