Enhanced visible-light photodegradation of fluoroquinolone-based antibiotics and E. coli growth inhibition using Ag-TiO2 nanoparticles

Abstract

Antibiotics in wastewater represent a growing and worrying menace for environmental and human health fostering the spread of antimicrobial resistance. Titanium dioxide (TiO₂) is a well-studied and well-performing photocatalyst for wastewater treatment. However, it presents drawbacks linked with the high energy needed for its activation and the fast electron-hole pair recombination. In this work, TiO₂ nanoparticles were decorated with Ag nanoparticles by a facile photochemical reduction method to obtain an increased photocatalytic response under visible light. Although similar materials have been reported, we advanced this field by performing a study of the photocatalytic mechanism for Ag-TiO₂ nanoparticles (Ag-TiO₂ NPs) under visible light taking in consideration also the rutile phase of the TiO₂ nanoparticles. Moreover, we examined the Ag-TiO₂ NPs photocatalytic performance against two antibiotics from the same family. The obtained Ag-TiO₂ NPs were fully characterised. The results showed that Ag NPs (average size: 23.9 ± 18.3 nm) were homogeneously dispersed on the TiO₂ surface and the photo-response of the Ag-TiO₂ NPs was greatly enhanced in the visible light region when compared to TiO₂ P25. Hence, the obtained Ag-TiO₂ NPs showed excellent photocatalytic degradation efficiency towards the two fluoroquinolone-based antibiotics ciprofloxacin (92%) and norfloxacin (94%) after 240 min of visible light irradiation, demonstrating a possible application of these particles in wastewater treatment. In addition, it was also proved that, after five Ag-TiO₂ NPs re-utilisations in consecutive ciprofloxacin photodegradation reactions, only a photocatalytic efficiency drop of 8% was observed. Scavengers experiments demonstrated that the photocatalytic mechanism of ciprofloxacin degradation in the presence of Ag-TiO₂ NPs is mainly driven by holes and ˙OH radicals, and that the rutile phase in the system plays a crucial role. Finally, Ag-TiO₂ NPs showed also antibacterial activity towards Escherichia coli (E. coli) opening the avenue for a possible use of this material in hospital wastewater treatment.

Scheme 1. Synthetic route for the preparation of Ag–TiO₂ NPs.

Fig. 1. Powder XRD patterns of TiO₂ P25 and Ag–TiO₂ NPs. The blue lines represent the diffraction peaks positions for the typical Ag XRD pattern (JCPDS no. 87-0717).

Fig. 2. (a) HRTEM image of Ag–TiO₂ NPs; (b) EDS map of Ag–TiO₂ NPs with Ag (red) and Ti (green) atomic distribution; (c) EDS map of Ag–TiO₂ NPs with Ag atomic distribution (in red).

Fig. 3. HRTEM images at low magnification (a) and high magnification (b) of Ag–TiO₂ NPs; (c) SAED (selected area electron diffraction) pattern of Ag–TiO₂ NPs.

Fig. 4. (a) UV-Vis absorption spectra of TiO₂ P25 and Ag–TiO₂ NPs with the inset of Tauc plot and (b) PL spectra of TiO₂ P25 and Ag–TiO₂ NPs.

Fig. 5. (a) Representative variations of the absorption spectrum of CIP (3 mg L⁻¹, pH = 3) under visible light irradiation in the presence of Ag–TiO₂ NPs (300 mg L⁻¹); (b) time-dependent variation of the concentration of CIP solution upon exposure to visible light in the presence of TiO₂ P25 (300 mg L⁻¹, black squares) and Ag–TiO₂ NPs (300 mg L⁻¹, red circles); reported values are the mean of 3 replicates.

Fig. 6. (a) Representative variations of the absorption spectrum of NFX (3 mg L⁻¹, pH = 3) under visible light irradiation by using Ag–TiO₂ NPs (300 mg L⁻¹); (b) time-dependent concentration of NFX solution upon exposure to visible light in the presence of TiO₂ P25 (300 mg L⁻¹, black squares) and Ag–TiO₂ NPs (300 mg L⁻¹, red circles); reported values are the mean of 3 replicates.

Fig. 7. Five photocatalytic degradation reactions of fresh CIP solution (3 mg L⁻¹, pH = 3) with reused (for up to 5 cycles) Ag–TiO₂ NPs (300 mg L⁻¹) under visible light irradiation.

Fig. 8. Visible-light photocatalytic degradation percentage of CIP (3 mg L⁻¹, pH = 3) in the presence of only Ag–TiO₂ NPs (300 mg L⁻¹, in black) or of both Ag–TiO₂ NPs (300 mg L⁻¹) and the scavengers AgNO₃ (in red), EDTA (in green) or isopropanol (in blue).

Scheme 2. Photocatalytic mechanism scheme of Ag–TiO₂ NPs under visible light.

Fig. 9. Modified streak method demonstrating the inhibitory bacteria growth potential of TiO₂ (a) and Ag–TiO₂ NPs (b) against E. coli: (a) E. coli in LB broth on the left part of the agar plate, TiO₂ P25 and E. coli in LB broth on the right part of the agar plate; (b) E. coli in LB broth on the left part of the agar plate, Ag–TiO₂ NPs and E. coli in LB broth on the right part of the agar plate.