Effect of hydrothermal treatment of titanium in high concentration of AgNO₃ solution on surface morphology and roughness

Introduction

Titanium (Ti) has widely been used clinically for dental implants due to their excellent mechanical properties, biocompatibility and osteoconductivity.^1,2 Development of Ti implant for dental application is still challenging. Dental implant not only requires osseointegration capability but also antibacterial property.³ Titanium demonstrated satisfactory osseointegration clinically. However, its antibacterial capability is lacking.

Silver coating has emerged as an alternative to prepare antibacterial titanium surface.⁴ Silver is considered a promising element to prevent and combat implant-related infection. Its main advantage is that it would not induce bacterial resistance.⁵ Silver is coated onto Ti surface often in the form of particles by immersion in AgNO₃ solution mixed with a reduction agent.^6,7 This results in silver particulates being deposited on the Ti surface often in nanoscale, thus called silver nanoparticles (AgNPs). The use of reduction agents is problematic because they are often toxic chemicals such as Sodium borohydride_, ammonium formate and hydrazine.^8,9 A recent study has been conducted to coat Ti surface with silver under hydrothermal without the need for reduction agents.¹⁰

Direct Ag coating onto Ti surface using hydrothermal has not been clearly described especially in high concentration of AgNO₃ solutions and without the addition of toxic reduction chemicals. Therefore, this study aimed to coat the Ti surface with Ag particles using hydrothermal using solely AgNO₃ solution. Surface morphology including the distribution of the Ag coating and the change in surface roughness were then evaluated.

Methods

Results and discussion

Figures 1 and 2 show the photographs and SEM images of Ti surface before and after hydrothermal in AgNO₃ solutions. Bright particles were observed on Ti surface hydrothermally treated in 0.01 mol/L and 0.1 mol/L AgNO₃. An area showed more concentrated particles, which may indicate the agglomeration. At higher concentration of AgNO₃ (0.1 mol/L), that concentrated area was larger (Figure 2). The elemental analysis from the surface using energy dispersive X-ray analysis (EDX) indicated that those particles are Ag (Figures 3-5). Many methods have been developed to coat Ag into Ti surface both in the form of particles or ions.^4,11

Figure 1. Photo of untreated Ti, Ag coated Ti from 0.01 mol/L AgNO₃, and Ag coated Ti from 0.1 mol/L AgNO₃.

Figure 2. Scanning electron microscope images of untreated Ti (A and B), Ag coated Ti from 0.01 mol/L AgNO₃ (C and D), and Ag coated Ti from 0.1 mol/L AgNO₃ (E and F).¹⁴

Figure 3. Scanning electron microscope image (A) and energy dispersive X-ray analysis element mapping images of Ag coated Ti from 0.01 mol/L AgNO₃ (B).¹⁴

Figure 4. Scanning electrom microscope image (A) and energy dispersive X-ray analysis element mapping images of Ag coated Ti from 0.1 mol/L AgNO₃ (B).¹⁴

Figure 5. Element composition of untreated Ti (A), Ag coated Ti from 0.01 mol/L AgNO₃ (B), and Ag coated Ti from 0.1 mol/L AgNO₃ (C) obtained from energy dispersive X-ray analysis.¹⁴

The formation of silver particles on Ti surface under hydrothermal from AgNO₃ solution was still unclear in this study. Several different mechanisms may be responsible for how Ag could be deposited on Ti surface from AgNO₃ under hydrothermal treatment. One possible way is through thermal decomposition.¹² The deposition of Ag particles from only AgNO₃ solution up to 75 μmol/L under hydrothermal conditions was recently reported.¹⁰ However, the exact mechanism on how Ag particles could be deposited on Ti surface was not clearly described. AgNO₃ was also reported to transform into Ag nanoparticles under hydrothermal condition at 121^oC.¹³ Hydroxyl ions exist on Ti oxide layer may also play a role on the Ag particles growth on its surface.⁶ It is known that Ti surface naturally forms thin oxide layer which contain Ti-OH groups on its outmost part. Figure 2 has confirmed the formation of Ag particles on Ti surface both in 0.01 mol/L and 0.1 mol/L AgNO₃ solutions. The Ag particles were also observed in the solution after hydrothermal (solution turned darkish color). The Ag particles seem to be non-homogenously distributed on Ti surface. As explained above, there is an area which contain thicker aggregated Ag particles masking the Ti surface. The concentrated Ag area was found to be larger in 0.1 mol/L AgNO₃ than that in 0.01 mol/L AgNO₃. These findings suggest that at high concentration of AgNO₃, the Ag coating tends to be aggregated and thicker, thus the use of a lower concentration might be preferred.

The next question is that whether the Ag coating deposited on Ti surface is in the metallic or oxide forms. One way to find this is using the EDX elemental mapping to the aggregated Ag coating. Figures 3 and 4 show the elemental mapping of Ag coated Ti prepared from 0.01 mol/L and 0.1 mol/L AgNO₃ solutions.¹⁴ In Figure 3, a thick Ag coating area (white dash line) shows a very strong purple color compared to the area in which less Ag coating was observed (white arrows). In Ti and oxygen (O) element mapping (green color and yellow color respectively), the Ag coating area was darker compared to the rest. The O elemental mapping provided very important data about the deposited Ag coating. The darker area (white dash line; Figure 3) in O elemental mapping indicated that that area was composed mostly of metallic Ag. Ag oxide also existed since the bright yellow color was also observed sporadically inside the white dash line (Figure 3; O Kα1). A similar trend was shown in Figure 4 where the Ag aggregate coating is larger. The thick Ag coating was most likely composed mainly from metallic Ag and smaller portion of Ag₂O

Surface treatment often changes surface roughness.¹⁵ The change in surface roughness might alter the biological performance of Ti implant. Therefore, it is necessary to evaluate whether the current method of Ag coating changed the surface roughness of Ti surface. Surface roughness parameters roughness average (Ra), maximum profile peak height (Rp), maximum profile valley depth (Rv), and mean roughness depth (Rz) were measured from all sample surfaces. Surface roughness texture of the sample surfaces were shown¹⁵ in Figure 6.¹⁶ The surface texture of all Ti samples before and after Ag coating were comparable. This data suggests that no significant changes were observed on Ti surface after Ag coating (Table 1). Comparison of SEM images between Ag coating and untreated Ti surfaces (Figure 2) also support surface texture data. The Ti substrate in which Ag coating deposited was found to be comparable (Figure 2A and B).

Figure 6. Representative of surface roughness texture of untreated Ti (A), Ag coated Ti from 0.01 mol/L AgNO₃ (B), and Ag coated Ti from 0.1 mol/L AgNO₃ (C) generated from roughness tester.¹⁶

Table 1 shows the quantitative values of surface parameter obtained from the roughness tester. The surface Ra and Rv values of all samples were relatively similar which indicating its comparable average surface height and deepest valley of the substrate. A slight increase of Rp and Rz values were recorded in Table 1. Surface roughness Rp shows the maximum peak height which come from the Ag coating aggregate and was larger when a solution of 0.1 mol/L AgNO₃ was used. The slight increase of Rp was followed by slight increase of Rz. Taken together, all surface roughness parameter demonstrated comparable values between untreated Ti (UnTi) and Ag coated Ti samples. This result suggests that hydrothermal treatment of Ti in 0.1 mol/L (Ag-Ti 0.1) and 0.01 mol/L AgNO₃ (Ag-Ti 0.01) did not cause a notable change in the surface roughness.

Conclusion

This study reported the effect of hydrothermal treatment of Ti surface in AgNO₃ solution on the surface morphology and roughness. After hydrothermal treatment, an Ag coating was observed in all treated Ti surfaces. EDX mapping suggests that Ag coating composed of mainly metallic Ag and in smaller quantities, Ag oxide. Using a higher AgNO₃ solution concentration resulted in more Ag aggregates that mask Ti surface, creating a non-homogenous coating. Surface roughness of treated Ti surface did not change significantly when coated. Nevertheless, a slight increase of Rp and Rz was observed; this might be due to Ag coating aggregates.

Data availability

Author contributions

Conceptualization and methodology, S.; validation, S.; investigation, R.J.H.P.; resources, S.; writing—original draft preparation, S.; writing—review and editing, S., C.F.T. and A.I.P.; visualization, S.; supervision, S. and A.I.P.; funding acquisition, S. and A.I.P.