**1. Introduction**

Antibiotics have become emerging pollutants due to their widespread use and persistence in the environment [1–3]. The origin is very varied, although they come mainly from medical treatments, agricultural, livestock, and industrial production [4–7]. The presence of antibiotics in the natural environment presents a serious health risk, since they can lead to the development of antibiotic-resistant bacteria and, in general, promote the destabilization of the natural environment [8,9]. Fluoroquinolone-based antibiotics, particularly ciprofloxacin and levofloxacin, are widely used in human and veterinary medicine [1–3]. They are used to fight bacterial infections like pneumonia, kidney or prostate infections, and even skin infections. In fact, this family of antibiotics is by far the most widely used for medical and veterinary treatments. Due to the chemical structure, after consumption by humans or animals, only between 15 and 20% is metabolized, so the non-metabolized antibiotic is eliminated into the environment through urine or feces [10]. Once released into the natural environment, these antibiotics are highly recalcitrant to degradation, which is why they have been detected in very worrying quantities in wastewater, surface water, ground water, and even in drinking water [11]. In general, some antibiotics can be eliminated from water bodies by different techniques such as adsorption, nanofiltration, coagulation, electrolysis,

**Citation:** Machín, A.; Fontánez, K.; Duconge, J.; Cotto, M.C.; Petrescu, F.I.; Morant, C.; Márquez, F. Photocatalytic Degradation of Fluoroquinolone Antibiotics in Solution by Au@ZnO-rGO-gC3N<sup>4</sup> Composites. *Catalysts* **2022**, *12*, 166. https://doi.org/10.3390/ catal12020166

Academic Editor: Roberto Fiorenza

Received: 8 January 2022 Accepted: 22 January 2022 Published: 28 January 2022

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or even biodegradation [12,13]. These decontamination techniques have been implemented in wastewater processing plants, especially in developed countries, although these methods are very expensive, require large spaces and are not very efficient. In fact, already treated waters continue to show high levels of these pollutants [14]. As an alternative procedure to those already mentioned above, it is worth highlighting the chemical transformation through advanced oxidation processes (AOPs), and especially those involving semiconductor photocatalysts [15]. These processes are based on the catalytic photodegradation of organic pollutants in aqueous solution, being much more efficient than conventional methods, as well as being friendly with the environment [15]. The AOPs produce highly oxidized species such as radicals and other reactive species, which degrade, through a chain reaction, organic pollutants in solution allowing complete mineralization [16,17].

Among the most widely used semiconductor materials, it is worth mentioning titanium oxide (TiO2) and zinc oxide (ZnO), due to their low cost, non-toxicity, large specific surface area of some of their forms, and high catalytic activity [18,19]. The main limitation of both semiconductors is the wide bandgap (ca. 3.2 eV in TiO<sup>2</sup> anatase and 3.0 eV in TiO<sup>2</sup> rutile, and 3.37 eV in ZnO wurtzite), which requires irradiation with UV light for the photoactivation process [20]. UV radiation from the solar spectrum is limited to about 5% of the total radiation reaching the earth's surface, so these catalysts are clearly inefficient [21]. However, alternatives such as doping these semiconductors with metallic nanoparticles and other additives deposited on the surface have made it possible to reduce bandgap and substantially promote electron-hole separation, avoiding rapid recombination processes [22]. The incorporation of Au nanoparticles on the surface of semiconductors enables the appearance of surface plasmon resonance (SPR), greatly improving the storage capacity and charge separation which increases the photocatalytic activity of the heterostructure [23,24]. Nevertheless, there are many other factors that decisively affect the photocatalytic behavior of the material, especially morphology, particle size, crystallinity and specific area, controlled during the synthesis procedure [25]. Graphene and especially graphene oxide (GO) provides scaffolds to anchor other components, due to their two-dimensional structure and large surface area [26]. There are many examples of nanohybrids prepared with GO with extraordinary optical, electrical, and thermal properties that have a direct effect on the catalytic activity of the material [27,28]. On the other hand, the incorporation of GO to ZnO has been shown to increase the activity, reducing the photocorrosion of the semiconductor, facilitating the separation of charges and the inhibition of the electron-hole recombination processes [29]. Additionally, the two-dimensional structure of GO greatly improves the interaction with organic pollutants, accelerating the subsequent photocatalytic degradation [30]. Graphitic carbon nitride (gC3N4) [31], is an allotropic material of carbon nitride, rich in nitrogen, and with truly extraordinary properties that allow its use in different applications such as catalysis, photodegradation of organic pollutants, CO<sup>2</sup> fixation, catalysis and even in energy storage systems [32–34]. Among the most relevant properties, it is worth highlighting its great thermal and chemical stability, its two-dimensional structure capable of facilitating interaction with other materials, and its simple synthesis method [34]. The purpose of doping with noble metal nanoparticles and the manufacture of complex hybrid composites has been to develop more efficient catalytic heterostructures, capable of showing reduced bandgap, altered electronic properties that allow efficient generation of hole-electron pairs, and inhibition of recombination processes [35]. Over the last few years, complex systems have been developed, with improved and increasingly outstanding properties, which have been applied to many processes, and especially to the degradation of highly persistent organic pollutants. In this sense, it is worth highlighting the catalysts based on metallic nanoparticles, dispersed on semiconductors such as TiO2, ZnO, Fe2O3, ZnS, or CdS [36]. Other catalysts, based on heterostructures formed by rare earth metal oxides, combined with graphitic carbon nitride have also been designed, showing good catalytic behavior [37].

Taking into account all the previous research, the objective of this work has been to obtain catalysts, based on the incorporation of gold nanoparticles on the surface of heterostructures formed by ZnO nanoparticles, to which reduced GO (rGO) and graphitic carbon nitride have been incorporated. These catalysts were used for the photodegradation of ciprofloxacin (CFX) and levofloxacin (LFX) in aqueous solution. The structural properties and morphology of the most active catalyst has been investigated using different spectroscopic and analytical techniques, such as XRD, XPS, Raman, DRS, SEM, TEM, and BET analysis. Finally, based on the results obtained, a possible photodegradation mechanism of the two antibiotics studied is proposed.
