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. 2019 Jul 1;4(7):11354-11363.
doi: 10.1021/acsomega.9b00646. eCollection 2019 Jul 31.

Sol-Gel-Deposited Ti-Doped ZnO: Toward Cell Fouling Transparent Conductive Oxides

Rehab Ramadan  1   2 , David Romera  3 , Rosalía Delgado Carrascón  1 , Miguel Cantero  1 , John-Jairo Aguilera-Correa  3 , Josefa P García Ruiz  1 , Jaime Esteban  3 , Miguel Manso Silván  1
Affiliations

Sol-Gel-Deposited Ti-Doped ZnO: Toward Cell Fouling Transparent Conductive Oxides

Rehab Ramadan et al. ACS Omega. .

Abstract

Ti-doped ZnO thin films were obtained with the aim of tailoring ZnO film bioadhesiveness and making the optoelectronic properties of ZnO materials transferable to biological environments. The films were prepared on silicon substrates by sol-gel spin-coating and subsequent annealing. A Ti-O segregation limits the ZnO crystallite growth and creates a buffer out-layer. Consequently, the Ti-doped ZnO presents slightly increased resistivity, which remains in the order of 10-3 Ω·cm. The strong biochemical interference of Zn2+ ions released from pure ZnO surfaces was evidenced by culturing Staphylococcus epidermidis with and without the Zn2+ coupling agent clioquinol. The Ti-doped ZnO surfaces showed a considerable increase of bacterial viability with respect to pure ZnO. Cell adhesion was assayed with human mesenchymal stem cells (hMSCs). Although hMSCs find difficulties to adhere to the pure ZnO surface, they progressively expand on the surface of ZnO when the Ti doping is increased. A preliminary microdevice has been built on the Si substrate with a ZnO film doped with 5% Ti. A one-dimensional micropattern with a zigzag structure shows the preference of hMSCs for adhesion on Ti-doped ZnO with respect to Si. The induced contrast of surface tension further induces a cell polarization effect on hMSCs. It is suggested that the presence of Ti-O covalent bonding on the doped surfaces provides a much more stable ground for bioadhesion. Such fouling behavior suggests an influence of Ti doping on film bioadhesiveness and sets the starting point for the selection of optimal materials for implantable optoelectronic devices.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(a) X-ray diffractograms for ZnO:Ti films with doping levels of 0.1, 0.5, 1, and 5% Ti with respect to Zn (from top to bottom) annealed at 400 °C (top set) and 600 °C (bottom set). Miller indices according to the ZnO wurtzite phase. FE-SEM perspective view micrographs and AFM 500 × 500 nm2 topography images (insets) of ZnO samples with 5% Ti after annealing at 400 (b) and 600 °C (c) (the z-scale in the AFM images is 25 nm).
Figure 2
Figure 2
Characteristic Zn 2p (a) and Ti 2p (b) core-level XPS spectra of 5% Ti-doped ZnO sample annealed at 600 °C. (c) Normalized [Ti]/([Ti] + [Zn]) ratio calculated from the survey spectra of the ZnO samples doped at 0.1, 0.5, 1, and 5% Ti after 400 and 600 °C annealing compared with the nominal expected value (lines indicate exclusively trends). (d) Variation of Ti at. % for increasing photoemission angle for samples doped at 5% at 400 and 600 °C.
Figure 3
Figure 3
Variation of WCA for the samples treated at 400 °C (squares) and 600 °C (circles) and the different Ti doping from 0 (pure ZnO) to 5% Ti.
Figure 4
Figure 4
(a) Nyquist diagrams and impedance analysis corresponding to pure and 5% doped ZnO films after annealing at 400 and 600 °C. (b) Tauc plots for ZnO, ZnO:Ti 1%, and ZnO:Ti 5% after annealing at 400 °C (left) and 600 °C (right).
Figure 5
Figure 5
(a) Area of adhesion (% coverage) of S. epidermidis on the surfaces of ZnO doped with 5% Ti (annealed at 600 °C) compared with controls for pure ZnO and TiO2. (b) Particle counting on the same surfaces. (c) Viability (% of dead adhered bacteria) on the same surfaces. (d) Comparison of the previous parameters on the ZnO surface in the presence of clioquinol. Fluorescence microscopy images of S. epidermidis adhered on the surface of ZnO without (e) and with (f) addition of clioquinol. ###: p-value < 0.0001 for Wilcoxon test compared to TiO2. *: p-value < 0.05 for Wilcoxon test compared to ZnO. **: p-value < 0.01 for Wilcoxon test compared to ZnO. ***: p-value < 0.001 for Wilcoxon test compared to ZnO.
Figure 6
Figure 6
hMSCs cultured on the surface of the ZnO:Ti films annealed at 400 (top) and 600 °C (bottom) and different doping levels (from left to right: ZnO, ZnO:Ti 0.1%, ZnO:Ti 5%, and TiO2). Green, actin cytoskeleton/red, α-catenin intermediate fibers/blue, DAPI nuclear staining.
Figure 7
Figure 7
Expansion area of hMSCs cultured on the surface of pure ZnO, ZnO:Ti 0.1 at. %, ZnO:Ti 5 at. %, and TiO2 after annealing at 400 and 600 °C.
Figure 8
Figure 8
(a) SEM image of a ZnO:Ti/Si zigzag 1D micropattern grown with 5% Ti and annealing at 600 °C. (b) hMSCs cultured on the surface of ZnO:Ti/Si zigzag 1D micropattern (green, actin cytoskeleton/red, magnified ZnO:Ti self-fluorescence/blue, DAPI nuclear staining). (c,d) EDS complementary maps of Zn and Si, respectively.
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