Antimicrobial activity of printed composite TiO2/SiO2 and TiO2/SiO2/Au thin films under UVA-LED and natural solar radiation
Graphical abstract
Introduction
Photocatalytic water treatment resulting in degradation of organic pollutants and pathogen’s elimination has gained extensive attention in last decades as a promising option for the development of environmentally friendly technologies [[1], [2], [3]]. The most widely used photocatalyst is titanium dioxide in form of nanoparticles. The advantages of titanium dioxide include its chemical non-toxicity, relatively low cost and high photocatalytic activity [[4], [5], [6]]. Despite all advantages, this method has not found many practical applications in water treatment due to difficulties arising when nanoparticles should be separated from treated water.
Modification of TiO2 with noble metals, such as silver and platinum, leads not only to the extension of photocatalytic activity towards visible light but also to the increase of TiO2 antimicrobial activity [7]. Thus, TiO2 modified with Au were reported to be more efficient for inactivation of various bacteria than TiO2 under radiation [[8], [9], [10]]. Interestingly, antimicrobial effects of TiO2 modified with Au in darkness were also recently reported [11,12]. However, little is known regarding antimicrobial properties of TiO2 modified with Au when prepared using inkjet printing (material printing) technique. Inkjet printing of sol-gel materials is novel approach and only few studies were reported so far on photocatalytic properties of printed semiconductors such as TiO2 [13,14]. It is worth making a point that material printing of photocatalyst can be highly beneficial for scaling up.
To date majority of studies on photocatalytic water disinfection are conducted with pure cultures of bacteria added to distilled or synthetic water [15]. Only few works deal with photocatalytic inactivation of consortia of bacteria naturally present in real water [[16], [17], [18]] or synthetic water prepared in accordance with recommendations of WHO [19].
It is worth making a point that most commonly used source of radiation for photocatalytic water disinfection is different types of lamps [15], while the use of solar light is less common.
As far as we know, there are no studies on solar inactivation of natural fecal bacteria consortia in drinking water using printed TiO2/SiO2 and TiO2/SiO2 modified with gold nanoparticles. Hence, in this study TiO2/SiO2 and TiO2/SiO2 modified with gold nanoparticles in form of bipyramid were prepared by material printing technique. Moreover, feasibility of printed films for solar disinfection of drinking water contaminated with natural consortia of fecal bacteria for production of potable water was studied. Effect of film thickness, source of radiation (solar light and UVA-LEDs) as well as water matrix on antimicrobial properties of printed films was investigated.
Section snippets
Preparation of TiO2/SiO2/Au thin films
Hybrid silica sol was prepared according to procedure reported in earlier works [[20], [21], [22], [23]]. Gold bipyramid-like nanoparticles were prepared following steps described below. 1200 μl of HAuCl4 (25 mM in water) was mixed with 40 ml CTAB (47 mM in water). After that 1050 μl of aqueous silver nitrate (5 mM) was added, followed by 700 μl of 8-hydroxyquinoline (0.4 M in ethanol). Addition of silver ions was of high importance for controlled growth of Au bipyramids [24]. It was observed
Characterization of printed TiO2/SiO2 and TiO2/SiO2/Au thin films
SEM was applied to study and compare morphology of composite TiO2/SiO2 and TiO2/SiO2/Au thin films. As shown in Fig. 1 all prepared coatings were porous with homogeneous microstructure. Morphology of prepared coatings was similar to those reported in preceding works [20,23]. The thickness of thin films was measured by profilometer.
The crystal phase of printed TiO2/SiO2 and TiO2/SiO2/Au films was detected by means of X-ray diffraction patterns. Both anatase and rutile phase were present in
Conclusions
This study focused on photocatalytic disinfection of drinking water with fecal contamination using printed TiO2/SiO2 and TiO2/SiO2 thin films modified with bipyramid-like gold nanoparticles under UVA-LED and natural solar radiation. Photocatalytic antimicrobial activity of printed thin films was investigated against E. coli, Total coliforms and Enterococci. Key findings of this work are listed below.
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Antimicrobial activity of printed coatings improved with increase of thin film thickness (under
Acknowledgements
This Special Issue is dedicated to honor the retirement of Dr. John Kiwi at the Swiss Federal Institute of Technology (Lausanne), a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern. Authors would like to express gratitude to Professors Eduardo Blanco and Almudena Aguinaco Martin for help with band gap measurements.
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