# Micro-Raman Spectroscopy of Dental Implants Subjected to Different Surface Treatments
## 1 Introduction
Plasmas can be categorized as either thermal (hot) plasmas or cold (low-pressure) plasmas (CPs or LPVs) [38]. CP treatment allows surface activation while operating at low ambient temperatures, with an enhancement of the surface energy and an osteoblastic spread [39; 40; 41]. CP-treated titanium implant surfaces showed a significant reduction in surface-adsorbed C content and more uniform and higher osteogenic responses in vivo [42; 43]. Raman Spectroscopy (RS) is a non-invasive and non-destructive method widely used to investigate the crystalline degree and quality of material [44; 45], and in the case of TiO\({}_{2}\), RS can identify its naturally occurring crystalline phases [20; 46]. TiO\({}_{2}\) anatase phase is tetragonal, with two chemical formula per unit cell (space group D\({}^{19}\)4h(\(I\)4l)/_and_). Among its six Raman active modes (Table 1), the most intense is the E\({}_{\text{g}}\) mode, peaked at 144 cm\({}^{-1}\), whose linewidth is also correlated to the average grain size or crystalline disorder. The rutile structure is tetragonal, with two TiO\({}_{2}\) units per cell, and space group D\({}^{14}\)4h(\(P\)4l)/_mm_) has four Raman active modes (Table 1) [46; 47; 48]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Active Modes** & **Anatase (cm\({}^{-1}\))** & **Ratile (cm\({}^{-1}\))** \\ \hline A\({}_{1\text{g}}\) & 519 & 612 \\ B\({}_{1\text{g}}\) & 399, 519 & 143 \\ B\({}_{2\text{g}}\) & - & 826 \\ E\({}_{\text{g}}\) & 144, 197, 639 & 447 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Raman active modes for anatase and rutile. For the brookite structure, the lower symmetry and larger unit cell with respect to anatase and rutile cells result in a larger number of Raman active modes with a rich Raman spectrum. The reference Raman spectra for the three crystal cages of TiO\({}_{2}\) are reported in Figure 1a. Recently, the TiO\({}_{2}\) phases of the root surface of commercially available titanium dental implants, subjected to various surface treatments, were investigated by RS [18; 36]. It has been proposed that the rapid formation of the oxide layer during manufacturing leads to an amorphous TiO\({}_{2}\) layer on implant surfaces [19; 36], but information on the formation of TiO\({}_{2}\) phases during manufacturing stages is extremely limited. Figure 1: Raman spectra at room temperature for different systems: (**a**) anatase, rutile and brookite synthetic powders; (**b**) samples N1-N5 and the background signal for N5 sample; (**c**) CP-treated samples N1A-N5A. In panels (**d**) and (**e**) a comparison between untreated and CP-treated samples is reported respectively for N4 and N5 samples. The arrows in Figure 1e indicate the TiO2 Raman bands while the asterisks indicate the main ripple structures. Therefore, although the presence of TiO\({}_{2}\) has been identified as responsible for osseointegration, little is known about the quality of this TiO\({}_{2}\) or the relationship between the crystallographic conformation of TiO\({}_{2}\) and the surface treatment carried out. Since the crystalline conformation of TiO\({}_{2}\) could have interesting biological effects, in this study we tried to evaluate its presence following different commonly performed surface treatments. The aim of this article is to evaluate the presence and transformation of TiO\({}_{2}\) at the implant surface at different manufacturing stages by means of RS. The sampled thicknesses are strongly reduced by the highly inhomogeneous surfaces of the samples, and we estimate scattering volume heights of the order of tens of nanometers, markedly reduced with respect to the typical focus depth in transparent materials.