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

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 Zn 2+ ions released from pure ZnO surfaces was evidenced by culturing Staphylococcus epidermidis with and without the Zn 2+ 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.

Self-Organized In-Depth Gradients in Highly Ti-Doped ZnO Films: Thermal Versus MW Plasma Annealing

“…The two peaks in the diagram of the TZt300 film are in agreement with the (100) and (002) peak assignments for ZnO crystals in a wurzite structure [2]. In any case, the lattice constriction effects induced by the high concentration of Ti dopant are responsible for an inhibition of the crystallization of ZnO for a given thermal activation, as previously observed [19]. This decrease in the crystal structure by TiO 2 attributed to the higher activation energy required for the crystallization of TiO 2 [30].…”

“…An R vs. C map is presented in Figure 6b and shows the improvement in the electrical conduction for MwPA with respect to TA films as demonstrated by the decreasing resistance. We had previously shown that inclusion of Ti implies an increase in surface resistivity with respect to pure (TA) ZnO [19], so that a MwPA process can partially compensate for this drawback. This confirms the trend of reduced surface resistivity of MwPA samples for MwPA with respect to TA [26].…”

“…Additional examples can be found of deposition of B-doped (n-type) ZnO by metal organic chemical vapor deposition [10] or Zr-doped ZnO by atomic layer deposition (ALD) [11]. of monoethanolamine diluted with ethanol was mixed with the first solution for stabilization [19]. The mixture was stirred under reflux for 10 h at 60 • C. TiO 2 sol was prepared by dissolving 0.2 M of titanium isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 ≥98%, Sigma-Aldrich) in ethanol.…”

“…The mixture was stirred under reflux for 10 h at 60 • C. TiO 2 sol was prepared by dissolving 0.2 M of titanium isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 ≥98%, Sigma-Aldrich) in ethanol. We added 0.35 µl of HCl to the solution to induce an acid condensation catalysis [19]. Our study was performed with 5% Ti-doped ZnO solution (5% TZO) prepared by mixing aliquots of the previously described sols.…”

“…Significant efforts have been devoted to optimize Ti doping by different techniques and at different doping levels. Although most reports refer to magnetron sputtering systems [17,18], chemical alternatives also exist, with an attractive control of the Ti doping level in a wide range of compositions demonstrated by the sol-gel process [19]. This latter work highlights that, the condensation of Ti-O within ZnO sols takes preferentially place at the film surface for Ti doping in the 1%-10% range.…”