Light induced antibacterial activity and photocatalytic properties of Ag/Ag₃PO₄ -based material of marine origin

Highlights

• Ag₃PO₄-containing material was successfully prepared from cod fish bones.

• Ag₃PO₄ antibacterial activity was tested under light irradiation for the first time.

• Light irradiation enhanced the antibacterial properties of the material.

• Photocatalytic activity was also tested, under both UV and white light.

• Under white light the material had better performance that commercial Degussa P25.

Abstract

Fish bones were converted into materials consisting of silver phosphate (Ag₃PO₄), β-calcium phosphate (β-Ca₃(PO₄)₂, β-TCP) and hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp), as well as of metallic silver (Ag⁰), with a simple treatment in solution and calcination (650 or 1000 °C). The antibacterial activity of the material was measured in the dark and under UV and white light irradiation; this is the first time that an Ag₃PO₄-based material was tested under these conditions. Results showed light-enhanced antibacterial properties toward Gram-positive and Gram-negative strains (Methicillin-resistant Staphylococcus aureus – MRSA, Escherichia coli, Pseudomonas aeruginosa), with inactivation rates of up to 99.999% under UV light, and 99% for E. coli under white light (artificial indoor lighting). The photocatalytic activity was also tested, and the degradation of methylene blue dye was observed under both UV and white light. Even if the MB degradation was to a smaller extent under white light, it was approximately twice that of the commercial photocatalyst P25. This work demonstrates the valorisation of a food industry by-product such as fish bones to form a potentially valuable material, with important applications in self sterilizing surfaces and environmental remediation.

Introduction

Microbial contamination due to human activities is a growing concern, as increasingly numbers of potentially toxic, and often drug-resistant, microorganisms are found to be present in the environment. Species such as Methicillin resistant Staphylococcus aureus (MRSA), for instance, were detected in both hospital and municipal wastewaters [1], [2]. Several Escherichia coli (E. coli) strains, including problematic multi-drug resistant ones, were also found in water streams strains; this is normally considered as an indicator of fecal contamination [3]. Other microorganisms such as Pseudomonas aeruginosa and Salmonella were detected in river and coastal waters as well [4], [5]. These data clearly indicate the scale of the problem.

Pollution from organic contaminants is also an increasing problem; species such as dyes, for instance, are often found in industrial wastewaters. Because of their toxicity and detrimental effects on aqueous ecosystems, they pose a serious threat to the environment [6], [7].

Human activities also generate ever increasing quantities of by-products and waste. Data reported by Eurostat, for instance, show that just in the EU area the amount of waste generated in the year 2012 was greater than 2500 million tons [8]. Moreover, a further increase in this value is predicted for the future [9].

Heterogeneous photocatalysis is one of the most promising ways to simultaneously reduce both microbial contamination and organic pollution in wastewaters. In the photocatalytic process, a semiconductor (the photocatalyst) modifies the rate of a reaction, via the action of light of a suitable wavelength [10], [11]. When the photocatalyst is irradiated with photons having energy higher than, or equal to, its energy band gap (E_g), an electron (e^–) is able to migrate from the valence band to the conduction band, leaving a hole (h⁺) behind. That photo-generated couple (e^––h⁺) is able to reduce and/or oxidize a pollutant adsorbed on the photocatalyst surface [12]. The first photocatalytic material developed was titanium dioxide TiO₂ [13]. Photocatalytic materials can also have antibacterial activity; in fact the reactive oxygen species (ROS) can react with the cell walls of the bacterial strain, having a lethal effect [14].

Since the discovery of the photocatalytic effect, many other materials have been studied. Silver orthophosphate (Ag₃PO₄) was recently reported as a photocatalytic material activated under visible light (λ > 420 nm) [15]. Successive investigations were performed on this material to improve its photocatalytic efficiency; for instance, Ag₃PO₄ nanoparticles and nanospheres showed very good photocatalytic properties [16], [17]. The control of the shape of the crystal structure during the synthesis process also led to a material with good activity [18].

Multi-phasic silver phosphate-based materials were also studied; combinations with other silver compounds, for instance, received particular attention. Indeed, an improvement in the photocatalytic activity (PCA) was observed for Ag₃PO₄/Ag composite, where metallic Ag was in the form of nanoparticles [19]; in one case a shift of the activity toward the UV region was observed [20]. Ag₃PO₄ was also combined with other phosphate materials – silver phosphate nanoparticles, for instance, were deposited on a hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) support. This led to better photocatalytic performance, as well as activity in both visible and UV regions [21].

The antibacterial properties of Ag₃PO₄, on the other hand, have been studied much less. Literature reports Ag₃PO₄ used as an additive in HAp-based materials, and good antibacterial activity was observed in these cases [22], [23]. Ag₃PO₄ nanocrystals were also tested, they showed activity toward Gram-negative strains [24]. Silver phosphate bactericidal properties, however, were never tested under light irradiation. Considering its photoactive nature, it is likely that the intrinsic antibacterial activity of Ag₃PO₄ will be enhanced by the presence of light with an appropriate wavelength.

In this work we report the synthesis of Ag₃PO₄-based material from a natural source, namely fish cod bones (Gadus morhua). In Portugal about 60,000 t per year of cod fish are consumed; this leads to the production of huge quantities of bones as by-product, which could be reused and/or valorized.

Previous work carried out by the authors with cod fish bones showed that their main component is hydroxyapatite; it was also demonstrated that the composition of the material could be changed by a simple treatment of the bones in an appropriate solution [25]. In this case, to introduce silver into the hydroxyapatite structure, the bones were treated in a silver nitrate solution. The obtained material was characterized with several techniques (X-ray diffraction, IR and UV spectroscopy, SEM); moreover, functional properties such as photocatalytic and antibacterial activity were also tested.

This was the first time that a silver phosphate-based material was obtained from a natural source; furthermore, for the first time, the antibacterial properties of a Ag₃PO₄-based material were tested under light irradiation.