Plasma sprayed rutile titania-nanosilver antibacterial coatings

Highlights

• TiO₂/Ag feedstock powders containing 1–10,000 ppm silver nanoparticles were double sintered and deposited by plasma spray.

• TiO₂/Ag coatings were composed of pure rutile phase and homogeneously-distributed metallic silver.

• TiO₂/Ag coatings with more than 10 ppm silver nanoparticles exhibited strong antibacterial activity against E. coli and S. aureus.

Abstract

Rutile titania (TiO₂) coatings have superior mechanical properties and excellent stability that make them preferential candidates for various applications. In order to prevent infection arising from bacteria, significant efforts have been focused on antibacterial TiO₂ coatings. In the study, titania-nanosilver (TiO₂/Ag) coatings with five different kinds of weight percentages of silver nanoparticles (AgNPs) were prepared by plasma spray. The feedstock powders, which had a composition of rutile TiO₂ powders containing 1–10,000 ppm AgNPs, were double sintered and deposited on stainless steel substrates with optimized spraying parameters. X-Ray diffraction and scanning electron microscopy were used to analysize the phase composition and surface morphology of TiO₂/Ag powders and coatings. Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were employed to examine the antibacterial activity of the as-prepared coatings by bacterial counting method. The results showed that silver existed homogeneously in the TiO₂/Ag coatings and no crystalline changed happened in the TiO₂ structure. The reduction ratios on the TiO₂/Ag coatings with 10 ppm AgNPs were as high as 94.8% and 95.6% for E. coli and S. aureus, respectively, and the TiO₂/Ag coatings with 100–1000 ppm AgNPs exhibited 100% bactericidal activity against E. coli and S. aureus, which indicated the TiO₂/Ag coatings with more than 10 ppm AgNPs had strong antibacterial activity. Moreover, the main factors influencing the antibacterial properties of TiO₂/Ag coatings were discussed with grain size and the content of silver as well as the microstructure of the coatings.

Introduction

Titania coatings are the materials highly appropriate to be employed as self-cleaning surfaces, wastewater treatment and disinfection, protective coatings and biomaterials because of their excellent stability, nontoxicity, superior mechanical properties, good biocompatibility and low cost [1], [2], [3]. The emergence of infectious diseases in general poses a serious threat to public health worldwide, especially with the emergence of antibiotic-resistant bacterial strains [4]. Therefore, increasing attentions has been focused on developing TiO₂ coatings with antibacterial properties in order to reduce the frequency of infections and diseases resulting from bacteria [5].

The well-known TiO₂ polymorphs are rutile, anatase and brookite. The structure of rutile (tetragonal, space group P4₂/mnm) is made of chains of TiO₆ octahedra that share a vertex along the c-axis, and rutile is the most thermodynamically stable phase of the three polymorphs [6]. Moreover, rutile is known as an anti-wear and anti-corrosion material with excellent mechanical properties [7], [8]. It has been reported that rutile layers could decrease the levels of bioreactivity and improve biocompatibility [9]. Therefore, titania coatings were tentatively prepared by using rutile powders.

There are many methods that have been used for the preparation of TiO₂ coatings, including sol–gel [10], chemical vapor deposition (CVD) [11], physical vapor deposition (PVD) [12], atmospheric plasma spraying (APS) [13] and suspension flame spraying [14]. Among these techniques, APS is a particularly appropriate technique because a large area of TiO₂ can be easily deposited without any special requirements, such as vacuum pressures or reactive atmospheres [6].

Extensive effort has been made to develop the antibacterial TiO₂ coatings. Heidenau et al. reported a study of the antibacterial as well as the biocompatible potential of different metal ions includes Ag⁺, Zn²⁺, Co²⁺, Al³⁺ and Hg²⁺ in TiO₂ coatings [15]. Zhao et al. fabricated antibacterial nano-structured titania coating incorporated with silver nanoparticles on titanium implants [16]. Recently, researchers have focused on the modification of TiO₂ coatings with different antibacterial agents such as Ag, Cu, Zn and Au [5], [17], [18], [19]. Among these materials, silver has long been known to exhibit strong bactericides and has attracted increasing attention because of other benefits such as a broad antibacterial spectrum including antibiotic-resistant bacteria, non-cytotoxicity at suitable doses, satisfactory stability, and smaller possibility to develop resistant strains [20], [21], [22]. So it is possible to fabricate antibacterial TiO₂ coatings by introducing silver nanoparticles into the rutile powders. But nanoscale Ag could enter human bodies, tissues or even cells, and becomes toxic to human cells at higher levels, Vik et al. pointed that the maximum silver concentration released in vitro should be no more than 10 ppm [23]. Therefore, it is significant to prepare TiO₂ coatings with excellent antibacterial property and suitable AgNPs doses. But there are limited studies that consider the effect of silver concentration on antibacterial property in the plasma sprayed TiO₂/Ag coatings.

In this work, titania coatings with different amounts of silver nanoparticles were deposited on stainless steel substrates by plasma spray with a optimized spraying condition. The TiO₂/Ag coatings, which were denoted as T1, T2, T3, T4 and T5, were prepared by using rutile TiO₂ powders containing 1 ppm, 10 ppm, 100 ppm, 1000 ppm and 10,000 ppm AgNPs, respectively. The feedstock powders were prepared by mechanically mixing TiO₂ powders sintered at 1300 °C and AgNPs powder, followed by sintering and crushing the mixtures to make silver nanoparticles homogeneously distribute on the surface of rutile powders. The objective of this work is to investigate the effects of composition on antibacterial property for a material with minimum amount of silver nanoparticles (AgNPs) as well as maintain the excellent antibacterial property.