Effect of Amino-, Methyl- and Epoxy-Silane Coupling as a Molecular Bridge for Formatting a Biomimetic Hydroxyapatite Coating on Titanium by Electrochemical Deposition

doi:10.1016/j.jmst.2016.07.012

Abstract

Abstract:

The objective of this study was to determine the role of functional groups of silane coupling on bioactive titanium (Ti) surface by electrochemical deposition, and calcium phosphate (CaP) coating, as well as bone cell adhesion and proliferation. Methyl group (—CH₃), amino group (—NH₂), and epoxy group (—glyph name—C(O)C) were introduced onto the bioactive Ti surface using self-assembled monolayers (SAMs) with different silane coupling agents as molecular bridges. The effect of the surface functional groups on the growth features of the CaP crystals was analyzed (including chemical compositions, element content, minerals morphology and crystal structure etc.). CH₃-terminated SAMs showed a hydrophobic surface and others were hydrophilic by contact angle measurement; NH₂-terminated SAMs showed a positive charge and others were negatively charged using zeta-potential measurement. Scanning electron microscopy results confirmed that flower-like structure coatings consisting of various pinpoint-like crystals were formatted by different functional groups of silane coupling, and the CaP coatings were multicrystalline consisting of hydroxyapatite (HA) and precursors. CaP coating of CH₃-terminated SAMs exhibited more excellent crystallization property as compared to coatings of —NH₂ and —C(O)C groups. In vitro MC3T3-E1 cells adhesion and proliferation were performed. The results showed that CaP coatings on silane coupling functionalized surfaces supported cell adhesion and proliferation. Thus, these functional groups of silane coupling on Ti can form homogeneous and oriented nano-CaP coatings and provide a more biocompatible surface for bone regeneration and biomedical applications.

Key words: Silane coupling, Molecular bridge, Calcium phosphate coatings, Titanium, Electrochemical deposition

Tan Guoxin,Ouyang Kongyou,Wang Hang,Zhou Lei,Wang Xiaolan,Liu Yan,Zhang Lan,Ning Chengyun. Effect of Amino-, Methyl- and Epoxy-Silane Coupling as a Molecular Bridge for Formatting a Biomimetic Hydroxyapatite Coating on Titanium by Electrochemical Deposition[J]. J. Mater. Sci. Technol., 2016, 32(9): 956-965.

Figures/Tables 13

Fig. 1. Schematic illustration of electro-nucleation, crystallization and growth for CaP coatings on different functionalized Ti surfaces (—CH3, —NH2 and —C(O)C groups) by ECD.

Fig. 2. ATR-FTIR absorbance spectra of pTi, CH3-terminated, NH2-terminated and —C(O)C-terminated Ti surface. O—H peak of pTi, C—H peak of Ti-OTS, —NH2 peak of Ti-APTES and epoxide of Ti-GPTMS were obviously observed. (ν (O—H): 3200-3500?cm-1, ν (C—H): 2929 and 2855?cm-1, ν (Si—O): 1056?cm-1, δ (—N—H): 1536?cm-1 and ν (—C(O)C—): 910?cm-1).

Fig. 3. Water and diiodomethane contact angles of different functionalized pTi surfaces. Ti-OTS (—CH3 group) presents hydrophobic surface and others (pTi, —NH2 and —C(O)C groups) present hydrophilic surface.

Table 1 Surface energy on different surfaces of titanium

Fig. 4. pH dependence of the zeta-potential (a) and zeta-potential at pH?=?6 (b) of different functionalized Ti surface. Ti-APTES (—NH2 group) surface covered with positive potential and others (pTi, —CH3 and —C(O)C groups) were negative potential at pH?=?6.

Fig. 5. SEM and EDS of CaP coatings on different surfaces with different functional groups ((a) pTi, (b) Ti-OTS (—CH3), (c) Ti-APTES (—NH2), and (d) Ti-GPTMS (—C(O)C)), and (e-h) were the magnified images of (a-d), respectively. pTi were covered with flower-like CaP precipitates clustered with submicro pinpoint-like crystals, whereas others exhibited diverse nano pinpoint-like crystals (Ti-OTS), nano squame-like crystals (Ti-APTES) and nano flake-like crystals (Ti-GPTMS).

Fig. 6. EDS element mapping of CaP coatings on different surface ((a) pTi, (b) Ti-OTS, (c) Ti-APTES, (d) Ti-GPTMS); Ca and P elements of coatings distributed on pTi were the most sparse, while Ti-APTES surface (—NH2 group) were more dense than other surfaces (—CH3 and —C(O)C group).

Table 2 XPS spectral components of different CaP coatings surface

Samples	Elemental composition (at. %)
Samples	C	O	N	Ca	P	Si
pTi	38.37	48.65	-	6.6	4.92	-
Ti-OTS	34.68	45.80	-	11.90	6.31	1.31
Ti-APTES	32.28	48.66	1.08	10.08	6.89	1.01
Ti-GPTMS	34.75	44.66	-	12.11	7.27	1.21

Fig. 7. High-resolution XPS full spectra CaP coatings of pTi (a), Ti-OTS (b), Ti-APTES (c) and Ti-GPTMS (d) were distinctly obtained. N1s (399.6?eV) of APTES and Si2p (about 102.2?eV) of silane coupling agents were clearly observed. Ca2s, Ca2p, P2s and P2p peaks markedly appeared on spectra.

Fig. 8. TEM images, SAED graphs and XRD patterns of CaP coatings on different Ti surface ((a) pTi, (b) Ti-OTS, (c) Ti-APTES, and (d) Ti-GPTMS). The CaP coatings were multicrystalline minerals that consisted of HA and precursors (DCPA, DCPD, and OCP).

Table 3 Crystallite sizes of CaP coatings on different surface calculated by Scherrer formula with the full width at half maximum (FWHM)

Samples	(002)		(211)		Average size/nm
Samples	FWHM	Crystal size/nm	FWHM	Crystal size/nm	Average size/nm
pTi	0.199	40.55	0.166	49.27	181.7
Ti-OTS	0.253	31.89	0.384	21.30	100.3
Ti-APTES	0.547	14.75	0.216	37.87	95.9
Ti-GPTMS	0.650	12.42	0.357	22.91	88.1

Fig. 9. Representative SEM images of MC3T3-E1 cells on the different CaP coating surface after culturing for 24?h ((a) pTi, (b) Ti-OTS, (c) Ti-APTES, (d) Ti-GPTMS). Cell density: 2?×?104?cells/mL.

Fig. 10. Viability/proliferation (MTT assay) of MC3T3-E1 cells on different CaP coatings surfaces after 1, 3, 5 and 7 days. Cell density: 2?×?104?cells/mL. The data are reported as the mean?±?standard deviation (n?=?4), *p?<?0.05, and **p?<?0.01 relative to controls (pTi). Error bars represent the SD of four independent samples.

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