3.1. Surface Characterization
The chemical structure of both samples, anodized titanium Ti-bare and coated Ti/CaPO4, were characterized by X-ray diffraction techniques. As represented in Fig. 1a, the XRD chart for anodized Ti surface. The characteristic peaks of titanium were observed at 2θ = 39.8. 52.3 and 71.1o for attributed to miller indices (100),(200), and (211) respectively [38, 39]. Accordingly, TiO2 exists in crystal structure and point group of hexagonal and 6/mm, respectively. Furthermore, the anatase titania (TiO2) phase formed via anodization was observed in Fig. 1a, which has three peaks at 2θ = 22.9, 38.2, 76.3 for corresponding miller indices of (101), (004), and (211), respectively (JCPDS card no. 21-1272). The point group and crystal structure are recommended to be P4₂/mnm and tetragonal, respectively. As represented in Fig. 1b, the calcium triphosphate was characterized by the peaks at 2θ =, 26.02, 31.6, and 32.3 o for corresponding miller indices (002), (211), and (300) [40, 41]. Whereas the space group and crystal structure are recommended to be R3̅m and Trigonal, respectively. Furthermore, the crystal structure of calcium phosphate consisted of two inequivalent Ca+2 points. The Ca+2 is bounded to six equivalent O−2 atoms in the first site. Thus, the Ca-O bond lengths are equal to 2.46 Å. On the other hand, the second site of Ca+2 atoms is bounded to 10 coordinate geometry to 10 oxide atoms (O−2). Hence, the Ca-O bond is ranging 2.25–2.72 Å. At the same time, the P-O is represented in short and long bond lengths equal to 1.54 and 1.56 Å, respectively[42,43].
The bond stretching within the molecule of the prepared CaPO4 was studied by the FT-IR spectroscopy method to determine the functional groups. As illustrated in Fig. 2. The peaks in the range of 3427 cm− 1 and 1639.1 m− 1 refer to O-H stretching vibration modes of adsorbed water traces[44]. In addition, the peaks at 1034 and 563.3 cm− 1 correspond to the orthophosphate group vibration modes[45]. The surface morphology of the CaPO4, which will deposit on the anodized Ti surface, was characterized by SEM, as represented in Fig. 3, at two different magnifications. As represented in Fig. 3., the nanosized CaPO4 particles were confirmed, and it was found to have a nano-flake shape and measured to be 60 ~ 80 nm. This figure illustrates more cavities on the surface, which increases the surface roughness, which is important to raise the biocompatibility of the Ti surface in saliva solution via increasing the corrosion resistance, as illustrated in the following sections. As we know, ACP is essential for the formation of mineralized bone and is used for bone substitutes [46].
In the following sections, the electrochemical behavior of the anodized Ti/CaPO4 samples in compared with Ti-bare metal was investigated through the electrochemical impedance studies to estimate the corrosion resistance of these samples towards the simulated physiological solutions which containing the concentrations of salts and ions like; Cl, Na, P that play a responsible role in changing on the stability of these surfaces. We will discuss in detail the interpretation of the EIS measurements for the coated Ti/CaPO4 sample with or without the effect of essential oils (EO) in comparison with the Ti-bare sample. During the fitting process, the EIS data were fitted using ANOVA software, and the fitted circuit is illustrated in Fig. 4. The fitting model consists of solution resistance (Rs) connected in series with (Rc//C), where Rc is a charge transfer resistance, and C is a capacitor element. The presence of a constant phase element regards the nationally formed anatase layer (TiO2), as reported in the XRD part. Furthermore, the constant phase element of EIS is mathematically like the capacitor component. Where the following equation can employ the impedance of CPE [44]:
Z = (1/Yo)/ (Jω)α (1)
Where the proportional factor (Yo) is the CPE constant, the angular frequency is (ω) (in radians/sec), the imaginary number j2 = -1, and n is the CPE exponent ranges from zero to one.
Given the EIS Nyquist spectra of the present samples after 3 hrs. of immersion in simulated saliva solution containing EO, was represented in Fig. 5. We noticed that the EO (L1 or L2 or L3) plays a great role in raising the corrosion resistance of the anodized Ti/CaPO4. These results support our hypothesis that while the CaPO4 on anodized Ti acts as a bioactive coat, a synergistic effect emerges between the EO CaPO4, acting as a superior for the stability of Ti sample in the oral environment. In the following sections, we will be giving more evidence to confirm the synergistic relation between the CaPO4 on the surface and the EO in the solution to prove its positive impact on the corrosion resistance of Ti in simulated solutions. For this reason, we will discuss two categories of EIS measurements. The first one focuses on the study of the electrochemical behavior of Ti/CaPO4 in comparison to Ti-bare sample without the addition of EO in saliva solution for two periods, a short time of 240 min and a long time of 336 hrs. In the second category, we study the effect of the addition of EO in the solution for the same two periods.
Figure 6(a-b) shows the Nyquist plot of the Ti-bare and Ti/CaPO4 surface through 240min. As represented in Fig. 6a, the Nyquist plot of Ti-bare in saliva solution shows the increase of resistance by time of soaking. As appeared in Figs. 6b, the value of the impedance increases in the resistive component, which indicates that the CaPO4 coat enhances the corrosion resistance by improving the surface compatibility [47].
A comparison between the charge transfer resistance, calculated over different time intervals for Ti-bare and Ti/CaPO4 electrodes, was represented in (Fig. 6c). Where the Rct values for Ti-bare are less than Ti/CaPO4 values, this explains the role of CaPO4 in enhancing the corrosion resistance between the coating and Ti surface in saliva solution. Additionally, the fitting parameters of Ti-bare and modified Ti/CaPO4 by using the fitting circuit appeared in Fig. 4, are represented in Table 1. From this Table, we can deduce the many important features as the resistance of Ti-bare redouble from 424 ohm to 844 ohm after 240 min (Table.1a), while for Ti/CaPO4 it 4 times multiplied during the same time, where the resistance increases from 460.08 to 1875.3 ohm (Table.1b). Also, the high-value Rs values of Ti-bare than Ti/CaPO4 samples refers to the role of CaPO4 to decrease the Rs values during the time of immersion, which means that the good migration of the ions to the surface. The small capacitance value C points to the low capability of the surface to store charge. This result supports the good corrosion resistance of the surfaces where the capacitance is inversely proportional to the resistance [46].
Table 1
a: Representation of the fitting parameters of Ti-bare electrode for 240 mins in simulated saliva solution.
Immersion time (min)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
Zero
|
25.3
|
424
|
0.0019465
|
0.0011911
|
15
|
22.98
|
479
|
0.0022877
|
0.0013932
|
30
|
21.85
|
541
|
0.0017624
|
0.001487
|
60
|
21.23
|
645
|
0.001549
|
0.0015886
|
120
|
13.65
|
721
|
0.0022474
|
0.0014037
|
180
|
31.21
|
801
|
0.0016422
|
0.0017883
|
240
|
29.51
|
844
|
0.0015989
|
0.0017496
|
Table 1
b: Representation of Ti/CaPO4 electrode fitting parameters for 240 mins in simulated saliva solution.
Immersion time (min)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
Zero
|
20.31
|
460.08
|
0.0087754
|
0.00066173
|
15
|
18.75
|
534.9
|
0.0087735
|
0.00098218
|
30
|
18.34
|
878.4
|
0.0087121
|
0.00070918
|
60
|
23.96
|
1329.3
|
0.0081457
|
0.00106471
|
120
|
18.56
|
1250.1
|
0.0066522
|
0.00108867
|
180
|
18.21
|
1577.4
|
0.0083068
|
0.00104972
|
240
|
17.6
|
1675.3
|
0.0064027
|
0.00099656
|
The effect of increasing the time of immersion from 240 min to 336 hours or 14 days was studied in this section. As seen in Fig. 7(a, b), the Nyquist plot of the Ti-bare and Ti/CaPO4 after 14 days of soaking in the saliva solution (without EO). The linear Nyquist plot indicates a non-charge transfer process, whereas the process is mainly diffusion [46]. Another time, the increase of the corrosion resistance of the Ti/CaPO4 sample reflects the positive impact of the CaPO4 layer, where the coating layer promotes surface durability. In Fig. 7c, a comparison between the charge transfer resistance over different time intervals for Ti-bare and Ti/CaPO4 electrodes was studied, and the results indicated that the charge transfer resistance reached a steady state after 14 days of soaking in the saliva solution. The resulting EIS fitting parameters based on the same fitting circuit (Fig. 4) were reported in the Table. 2. Where the resistance increases from 2831 to 3985 ohm for Ti-bare and from 4624 to 6890 ohm for Ti/CaPO4 during 336 hrs.
Table 2
a: Representation of the fitting parameters of Ti – bare electrode for 336 hrs. in simulated saliva solution.
Immersion time (hrs)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
24
|
13.1
|
2831
|
0.0010499
|
0.0037738
|
48
|
27.2
|
2921
|
0.0010345
|
0.0036871
|
72
|
12.2
|
3013
|
0.0010318
|
0.0034524
|
96
|
7.3
|
3214
|
0.0010199
|
0.0032382
|
192
|
17.6
|
3487
|
0.0007478
|
0.0031025
|
240
|
11.9
|
3548
|
0.0005098
|
0.0020324
|
264
|
26.4
|
3672
|
0.0004855
|
0.0020198
|
336
|
28.8
|
3985
|
0.0003057
|
0.0020054
|
Table 2
b: Representation of Ti /CaPO4 electrode fitting parameters for 336 hrs. in simulated saliva solution.
Immersion time (hrs)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
24
|
13.7
|
4624
|
0.0013603
|
0.0023142
|
48
|
6.9331
|
4752
|
0.0017779
|
0.0010329
|
72
|
13.713
|
4823
|
0.0009519
|
0.0014964
|
96
|
18.326
|
4919
|
0.0010911
|
0.0013919
|
192
|
14.229
|
5052
|
0.0010391
|
0.0014489
|
240
|
13.241
|
5137.5
|
0.0011269
|
0.0018313
|
264
|
18.21
|
6033.2
|
0.0007816
|
0.0014469
|
336
|
11.45
|
6890.2
|
0.0008409
|
0.0013728
|
In this section, and for further enhancement of the corrosion resistance of the coated Ti/CaPO4 surfaces. Three EO were used for medical applications, named Cumin, Thyme, and Coriander oils (L1, L2, and L3). They injected into the operating solution. The effect of EO was studied over two periods, 240 min and 336 hrs., to compare the result of the same electrodes in the presence of the green extract. As represented in Fig. 8, the comparison between the effect of concentration of three EO on the charge transfer resistance Rct of Ti/CaPO4 appeared. Whereas different oil concentrations were used, i.e., 0.25% up to 2% (Wt/Wt). Rct of Ti/CaPO4 was observed to increase by 264.4, 88.2, and 437.5% for L1, L2, and L3, respectively, at 2% of EO concentration.
As illustrated in Fig. 9(a-c), the Nyquist plot of modified Ti/CaPO4 electrode in saliva solution in the presence of 2.0% of different essential oils (L1, L2, and L3) for different time intervals up to 4 hrs. of soaking. However, the oil started diffusion with time, and the oil adsorption on the electrode surface enhanced the corrosion resistance over time. Hence, the diffusion of the saliva solution to the inner layers of the Ti sheet promotes the layer of the anodized Ti form, which is important for the corrosion resistance process. The progress of charge transfer resistance for each oil was followed as represented in the Fig. 9d. By fitting the EIS result based on the fitting circuit (Fig. 4) for the Nyquist data, the Rct of the electrodes increased by 85, 27, and 33% for L1, L2, and L3, respectively. The change in the charge transfer value reflects the different abilities of an oil to be adsorbed on the Ti surface. Whereas the oil ingredients are different in chemical structures. The fitting parameters extracted from Nyquist plots are represented in Table.3
Table 3
a: Representation of the fitting parameters of Ti/CaPO4 electrode for 240 mins in simulated saliva solution in the presence of oil L1.
Immersion time (min)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
Zero
|
20.31
|
1278
|
0.00010635
|
0.00094051
|
15
|
32.95
|
1378
|
0.00012117
|
0.00080105
|
30
|
80.82
|
2928
|
0.00010347
|
0.00080572
|
60
|
28.32
|
1955
|
0.00099075
|
0.00088006
|
120
|
51.84
|
2256
|
0.00010146
|
0.00082697
|
180
|
79.44
|
2826
|
0.0001137
|
0.00078212
|
240
|
65.39
|
3234
|
0.00010993
|
0.00082703
|
Table 3
b: Representation of the fitting parameters of Ti/CaPO4 electrode for 240 mins in simulated saliva solution in the presence of oil L2.
Immersion time (min)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
Zero
|
8.1
|
1363.5
|
0.00072305
|
0.0009921
|
15
|
21.3
|
1571.8
|
0.00088464
|
0.00096715
|
30
|
26.5
|
1531
|
0.00097682
|
0.00090402
|
60
|
34.74
|
1634
|
0.00095467
|
0.00093186
|
120
|
21.1
|
1695
|
0.00070791
|
0.00100214
|
180
|
31.4
|
1713
|
0.00083436
|
0.00094872
|
240
|
27.1
|
1734
|
0.00079157
|
0.0009859
|
Table 3
c: Representation of the fitting parameters of Ti/CaPO4 electrode for 240 mins in simulated saliva solution in the presence of oil L3.
Immersion time (min)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
(F)
|
Yo
|
Zero
|
17.45
|
4140
|
0.00048172
|
0.00074208
|
15
|
38.16
|
4370
|
0.00057622
|
0.00066165
|
30
|
34.51
|
4621
|
0.00057219
|
0.00073426
|
60
|
34.12
|
4751
|
0.00060768
|
0.00067721
|
120
|
42.14
|
4982
|
0.00063483
|
0.00073281
|
180
|
31.41
|
5421
|
0.00060124
|
0.00073165
|
240
|
34.72
|
5520
|
0.00059045
|
0.00073124
|
The modified electrode Ti/CaPO4 was investigated in saliva solution and injected oils for 14 days (see Fig. 10(a-c)). After the 336 hrs. of soaking of Ti/CaPO4 in saliva, the electrode resistance started to reach the steady state where the charge transfer resistance was slightly changed. In the last 7 days, the resistance changed by 1.9, 7, and 2.8% for L1, L2, and L3, respectively. The anticorrosion activity of the EO was in the order of L3 > L1 > L2. The steric hindrance of the active component in the essential oils plays an important role in corrosion inhibition. For L3 oil, the most common ingredient, linalool, is long-chain alcohol, which has a smaller molecular volume than the bulky Thymol group (L2). The bulky group around the oxygen atom in thymol decreases the ability of thymol to be adsorbed on the titanium surface and diffusion through the CaPO4 layer. As shown in Table 4, the EIS-relevant data were reported for different essential oils corresponding to the different time intervals. The progress of charge transfer resistance for each oil was shown in Fig. 10d, where the promising effect of L3 & L1 appeared and reached the high value after 72 hrs. to be stable at 336 hrs. Although the value of the corrosion resistance is nearly the same for both L3 & L1 at the beginning of soaking (8166.5 & 8405 ohm, respectively), it doubles more than twice in the case of the L3 (20421ohm), compared to one and half times in the case of L1 (14523 ohm) after 336 hrs. at the ending soaking. While the L2 oil increases the corrosion resistance with low value in comparison to L1& L3, the progress of the resistance tripled through the time of soaking from 2680 to 7525 ohm.
Table 4
a: Representation of the fitting parameters of Ti /CaPO4 electrode for 336 hrs. in simulated saliva solution in the presence of oil L1.
Immersion time (h)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
F
|
Yo
|
24
|
20.31
|
8405
|
0.00014939
|
0.0012252
|
72
|
10.12
|
8810
|
0.00012538
|
0.001392
|
120
|
18.34
|
12954
|
0.00015158
|
0.0013833
|
336
|
23.96
|
14523
|
0.00013158
|
0.0010647
|
Table 4
b: Representation of the fitting parameters of Ti/CaPO4 electrode for 336 hrs. in simulated saliva solution in the presence of oil L2.
Immersion time (h)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
F
|
Yo
|
24
|
10.45
|
2680.83
|
0.00026199
|
0.00098412
|
72
|
51.54
|
3618.72
|
0.00022871
|
0.0008737
|
120
|
40.52
|
5647.54
|
0.00024685
|
0.00033963
|
336
|
35.55
|
7525.19
|
0.00013158
|
0.0010647
|
Table 4
c: Representation of the fitting parameters of Ti/CaPO4 electrode for 336 hours in simulated saliva solution in the presence of oil L3.
Immersion time (h)
|
Rs
|
Rct
|
C
|
w
|
|
Ω cm2
|
Ω cm2
|
F
|
Yo
|
24
|
18.56
|
8166.5
|
0.00009636
|
0.0010891
|
72
|
19.92
|
8696.5
|
0.00010278
|
0.0010288
|
120
|
37.75
|
17718
|
0.00009298
|
0.0010661
|
336
|
35.55
|
20421
|
0.00009821
|
0.0010414
|