Emerging technologies for the removal of pesticides from contaminated soils and their reuse in agriculture.

Pesticides are becoming more prevalent in agriculture to protect crops and increase crop yields. However, nearly all pesticides used for this purpose reach non-target crops and remain as residues for extended periods. Contamination of soil by widespread pesticide use, as well as its toxicity to humans and other living organisms, is a global concern. This has prompted us to find solutions and develop alternative remediation technologies for sustainable management. This article reviews recent technological developments for remediating pesticides from contaminated soil, focusing on the following major points: (1) The application of various pesticide types and their properties, the sources of pesticides related to soil pollution, their transport and distribution, their fate, the impact on soil and human health, and the extrinsic and intrinsic factors that affect the remediation process are the main points of focus. (2) Sustainable pesticide degradation mechanisms and various emerging nano- and bioelectrochemical soil remediation technologies. (3) The feasible and long-term sustainable research and development approaches that are required for on-site pesticide removal from soils, as well as prospects for applying them directly in agricultural fields. In this critical analysis, we found that bioremediation technology has the potential for up to 90% pesticide removal from the soil. The complete removal of pesticides through a single biological treatment approach is still a challenging task; however, the combination of electrochemical oxidation and bioelectrochemical system approaches can achieve the complete removal of pesticides from soil. Further research is required to remove pesticides directly from soils in agricultural fields on a large-scale.

Eco-friendly extraction of antibacterial compounds from enriched olive pomace: a design-of-experiments approach to sustainability.

The increasing interest in utilizing olive pomace bioactive molecules to advance functional elements and produce antioxidant and antimicrobial additives underscores the need for eco-friendly extraction and purification methods. This study aims to develop an eco-friendly extraction method to evaluate the effect of extraction parameters on the recovery of bioactive molecules from enriched olive pomace. The effects were identified based on total phenolic and flavonoid contents and antioxidant activity, employing a design of experimental methodology. The positive and the negative simultaneous effects showed that among the tested enrichments, those incorporating Nigella Sativa, dates, and coffee demonstrated superior results in terms of the measured responses. Furthermore, chromatographic analysis unveiled the existence of intriguing compounds such as hydroxytyrosol, tyrosol, and squalene in distinct proportions. Beyond this, our study delved into the structural composition of the enriched pomace through FTIR analysis, providing valuable insights into the functional groups and chemical bonds present. Concurrently, antimicrobial assays demonstrated the potent inhibitory effects of these enriched extracts against various microorganisms, underscoring their potential applications in food preservation and safety. These findings highlight enriched olive pomace as a valuable reservoir of bioactive molecules for food products since they can enhance their anti-oxidative activity and contribute to a sustainable circular economy model for olive oil industries.

Novel rod-like [Cu(phen)2(OAc)]·PF6 complex for high-performance visible-light-driven photocatalytic degradation of hazardous organic dyes: DFT approach, Hirshfeld and fingerprint plot analysis.

A novel octahedral distorted coordination complex was formed from a copper transition metal with a bidentate ligand (1,10-Phenanthroline) and characterized by Ultraviolet-visible spectroscopy, Ultraviolet-visible diffuse reflectance spectroscopy, Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller, Field emission scanning electron microscopy, and Single-crystal X-ray diffraction. The Hirshfeld surface and fingerprint plot analyses were conducted to determine the interactions between atoms in the Cu(II) complex. DFT calculations showed that the central copper ion and its coordinated atoms have an octahedral geometry. The Molecular electrostatic potential (MEP) map indicated that the copper (II) complex is an electrophilic compound that can interact with negatively charged macromolecules. The HOMO-LUMO analysis demonstrated the π nature charge transfer from acetate to phenanthroline. The band gap of [Cu(phen)2(OAc)]·PF6 photocatalyst was estimated to be 2.88 eV, confirming that this complex is suitable for environmental remediation. The photocatalytic degradation of erythrosine, malachite green, methylene blue, and Eriochrome Black T as model organic pollutants using the prepared complex was investigated under visible light. The [Cu(phen)2(OAc)]·PF6 photocatalyst exhibited degradation 94.7, 90.1, 82.7, and 74.3 % of malachite green, methylene blue, erythrosine, and Eriochrome Black T, respectively, under visible illumination within 70 min. The results from the Langmuir-Hinshelwood kinetic analysis demonstrated that the Cu(II) complex has a higher efficiency for the degradation of cationic pollutants than the anionic ones. This was attributed to surface charge attraction between photocatalyst and cationic dyes promoting removal efficiency. The reusability test indicated that the photocatalyst could be utilized in seven consecutive photocatalytic degradation cycles with an insignificant decrease in efficiency.

Adsorption of perfluorooctanoic carboxylic and heptadecafluorooctane sulfonic acids via magnetic chitosan: isotherms and modeling.

This paper evaluates the adsorption mechanism of perfluorooctanoic carboxylic acid (PFCA) and heptadecafluorooctane sulfonic acid (HFOSA) on magnetic chitosan for the first time via a statistical physics modeling. Magnetic chitosan (MC-CoFe2O4) was produced from shrimp wastes and used in standard batch adsorption systems to remove PFCA and HFOSA. The experimental isotherms indicated that the maximum adsorption capacities ranged from 14 to 27.12 mg/g and from 19.16 to 45.12 mg/g for PFCA and HFOSA, respectively, where an exothermic behavior was observed for both compounds. The adsorption data were studied via an advanced model hypothesizing that a multilayer process occurred for these adsorption systems. This theoretical approach indicated that the total number of formed layers of PFCA and HFOSA adsorbates is about 3 (Nt = 2.83) at high temperatures (328 K) where a molecular aggregation process was noted during the adsorption. The maximum saturation-multilayer adsorption of PFCA and HFOSA on magnetic chitosan was 30.77 and 50.26 mg/g, respectively, and the corresponding adsorption mechanisms were successfully investigated. Two energies were responsible for the formed adsorbate layer directly on the surface and the vertical layers were computed and interpreted, reflecting that physical interactions were involved to bind these molecules on the adsorbent surface at different temperatures where the calculated adsorption energies ranged from 14 to 31 kJ/mol. Overall, this work provides theoretical insights to understand the adsorption mechanism of PFCA and HFOSA using the statistical physics modeling and its results can be used to improve the adsorbent performance for engineering applications.

Photocatalytic transfer of aqueous nitrogen into ammonia using nickel-titanium-layered double hydroxide.

The development of solar-driven transfer of atmospheric nitrogen into ammonia is one of the green and sustainable strategies in industrial ammonia production. Nickel-titanium-layered double hydroxide (NiTi-LDH) was synthesised using the soft-chemical process for atmospheric nitrogen fixation application under photocatalysis in an aqueous system. NiTi-LDH was investigated using advanced characterisation techniques and confirmed the potential oxygen vacancies and/or surface defects owing to better photocatalytic activity under the solar spectrum. It also exhibited a bandgap of 2.8 eV that revealed its promising visible-light catalytic activities. A maximum of 33.52 µmol L-1 aqueous NH3 was obtained by continuous nitrogen (99.9% purity) supply into the photoreactor under an LED light source. Atmospheric nitrogen supply (≈78%) yielded 14.67 µmol L-1 aqueous NH3 within 60 min but gradually reduced to 3.6 µmol L-1 at 330 min. Interestingly, in weak acidic pH, 20.90 µmol L-1 NH3 was produced compared to 11.51 µmol L-1 NH3 in basic pH. The application of NiTi-LDH for visible-light harvesting capability and photoreduction of atmospheric N2 into NH3 thereby opens a new horizon of eco-friendly NH3 production using natural sunlight as alternative driving energy.

Novel gC3N4/MgZnAl-MMO derived from LDH for solar-based photocatalytic ammonia production using atmospheric nitrogen.

The development of catalysis technologies for sustainable environmental applications, especially an alternative to ammonia (NH3) production under the Haber-Bosch process, has gained a lot of scope in recent days. The current work demonstrated a green synthesis of graphitic carbon nitride (gC3N4) containing magnesium-zinc-aluminium mixed metal oxides (MgZnAl-MMO) derived from layered double hydroxide (LDH) for visible light aided catalytic production of ammonia. Pyrolysis-hydrothermal techniques were adopted for the synthesis and fabrication of the gC3N4/MgZnAl-MMO catalytic composite. Characterization results of field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), UV-visible spectroscopy, photoluminescence (PL), etc. showed the desired properties and functionalities like semi-crystalline structure with rough surface morphology that enhance the sorption reactions. Catalytic composite gC3N4/MgZnAl-MMO showed a bandgap energy of 2.16 eV that is considerably shifted toward the visible range when compared to gC3N4 (2.39 eV) and MgZnAl-MMO (2.93 eV). The results were also well complied with XPS results obtained that promote solar-based photocatalysis. The gC3N4/MgZnAl-MMO assisted photocatalytic production of NH3 in an aqueous media proved to be acceptable by the production of a maximum 47.56 µmol/L NH3 under visible spectrum employing a light emitting diode (LED) source. The results showed that the advancement of catalyst for desired functionalities and NH3 production using LED simulating solar light-aided catalysis would be an alternative to the Haber-Bosch process and solar-based sustainable processes for NH3 production.

Combined conversion of lignocellulosic biomass into high-value products with ultrasonic cavitation and photocatalytic produced reactive oxygen species - A review.

The production of high-value products from lignocellulosic biomass is carried out through the selective scission of crosslinked CC/CO bonds. Nowadays, several techniques are applied to optimize biomass conversion into desired products with high yields. Photocatalytic technology has been proven to be a valuable tool for valorizing biomass at mild conditions. The photoproduced reactive oxygen species (ROSs) can initiate the scission of crosslinked bonds and form radical intermediates. However, the low mass transfer of the photocatalytic process could limit the production of a high yield of products. The incorporation of ultrasonic cavitation in the photocatalytic system provides an exceptional condition to boost the fragmentation and transformation of biomass into the desired products within a lesser reaction time. This review critically discusses the main factors governing the application of photocatalysis for biomass valorization and tricks to boost the selectivity for enhancing the yield of desired products. Synergistic effects obtained through the combination of sonolysis and photocatalysis were discussed in depth. Under ultrasonic vibration, hot spots could be produced on the surface of the photocatalysts, improving the mass transfer through the jet phenomenon. In addition, shock waves can assist the dissolution and mixing of biomass particles.

Photoelectrochemical properties and reactivity of supported titanium NTs for bacterial inactivation and organic pollutant removal.

In this work, we report on the effect of anodization time on the morphology, optical, and photocatalytic properties of TiO2 nanotubes (NTs) allowing bacterial inactivation and two organic pollutant degradation under low-intensity solar-simulated light. Scanning electron microscopy (SEM) showed that the length of the TiO2 NTs increased from 2.8 to 25.8 µm as anodization time was increased from 15 to 300 min at 60 V, respectively. The X-ray diffraction (XRD) patterns showed that all samples crystallize in the anatase phase after annealing at 400 °C for 3 h. Samples anodized for 30 and 60 min exhibit low diffuse reflection at 400 nm, which was attributed to the disorder-induced exciton scattering at the molecular level. The intensity of the photoluminescence (PL) spectra was found to increase as the length of the NTs increases up to a maximum anodization time of 300 min, revealing the contribution of bulk excitonic states. A maximum photoelectric conversion efficiency of 0.55% was obtained at a potential of - 0.5 V vs. Ag/AgCl for TiO2 NTs anodized for 60 min. The optimized NTs (anodized for 60 min) showed a photocatalytic bacterial inactivation of a magnitude of 6 log within 360 min and a degradation of indole and methylene blue (MB) under low-intensity solar-simulated light (50 mW/cm2). The stability of the prepared catalyst was tested over several cycles.

Advances in 2D MXenes-based materials for water purification and disinfection: Synthesis approaches and photocatalytic mechanistic pathways.

MXenes two-dimensional materials have recently excited researchers' curiosity for various industrial applications. MXenes are promising materials for environmental remediation technologies to sense and mitigate various intractable hazardous pollutants from the atmosphere due to their inherent mechanical and physicochemical properties, such as high surface area, increased hydrophilicity, high conductivity, changing band gaps, and robust electrochemistry. This review discusses the versatile applications of MXenes and MXene-based nanocomposites in various environmental remediation processes. A brief description of synthetic procedures of MXenes nanocomposites and their different properties are highlighted. Afterward, the photocatalytic abilities of MXene-based nanocomposites for degrading organic pollutants, removal of heavy metals, and inactivation of microorganisms are discussed. In addition, the role of MXenes anti-corrosion support in the lifetime of some semiconductors was addressed. Current challenges and future perspectives toward the application of MXene materials for environmental remediation and energy production are summarized for plausible real-world use.

Recent progress and challenges on the removal of per- and poly-fluoroalkyl substances (PFAS) from contaminated soil and water.

Currently, due to an increase in urbanization and industrialization around the world, a large volume of per- and poly-fluoroalkyl substances (PFAS) containing materials such as aqueous film-forming foam (AFFF), protective coatings, landfill leachates, and wastewater are produced. Most of the polluted wastewaters are left untreated and discharged into the environment, which causes high environmental risks, a threat to human beings, and hampered socioeconomic growth. Developing sustainable alternatives for removing PFAS from contaminated soil and water has attracted more attention from policymakers and scientists worldwide under various conditions. This paper reviews the recent emerging technologies for the degradation or sorption of PFAS to treat contaminated soil and water. It highlights the mechanisms involved in removing these persistent contaminants at a molecular level. Recent advances in developing nanostructured and advanced reduction remediation materials, challenges, and perspectives in the future are also discussed. Among the variety of nanomaterials, modified nano-sized iron oxides are the best sorbents materials due to their specific surface area and photogenerated holes and appear extremely promising in the remediation of PFAS from contaminated soil and water.

Preparation and applications of chitosan and cellulose composite materials.

Chitosan is a natural fiber, chemically cellulose-like biopolymer, which is processed from chitin. Its use as a natural polymer is getting more attention because it is non-toxic, renewable, and biocompatible. However, its poor mechanical and thermal strength, particle size, and surface area restrict its industrial use. Consequently, to improve these properties, cellulose and/or inorganic nanoparticles have been used. This review discusses the recent progress of chitosan and cellulose composite materials, their preparation, and their applications in different industrial sectors. It also discusses the modification of chitosan and cellulose composite materials to allow their use on a large scale. Finally, the recent development of chitosan composite materials for drug delivery, food packaging, protective coatings, and wastewater treatment are discussed. The challenges and perspectives for future research are also considered. This review suggests that chitosan and cellulose nano-composite are promising, low-cost products for environmental remediation involving a simple production process.

Emerging technologies for biofuel production: A critical review on recent progress, challenges and perspectives.

Due to increasing anthropogenic activities, especially industry and transport, the fossil fuel demand and consumption have increased proportionally, causing serious environmental issues. This attracted researchers and scientists to develop new alternative energy sources. Therefore, this review covers the biofuel production potential and challenges related to various feedstocks and advances in process technologies. It has been concluded that the biofuels such as biodiesel, ethanol, bio-oil, syngas, Fischer-Tropsch H2, and methane produced from crop plant residues, micro- and macroalgae and other biomass wastes using thermo-bio-chemical processes are an eco-friendly route for an energy source. Biofuels production and their uses in industries and transportation considerably minimize fossil fuel dependence. Literature analysis showed that biofuels generated from energy crops and microalgae could be the most efficient and attractive process. Recent progress in the field of biofuels using genetic engineering has larger perspectives in commercial-scale production. However, its large-scale production is still challenging; hence, to resolve this problem, it is essential to convert biomass in biofuels by developing novel technology to increase biofuel production to fulfil the current and future energy demand.

Investigation and modeling of odors release from membrane holes on daily overlay in a landfill and its impact on landfill odor control.

In the present work, we studied the NH3 and H2S odor fluxes between the exposed working area and the HDPE covering film holes of the daily overlay in an actual landfill site with a daily operating area of 1600 m2 in Hangzhou, China. We showed that the odors were released from the membrane pores and the average concentrations of NH3 and H2S release reached 109.6 ± 56.6 and 86.0 ± 31.1 mg/m2/s, respectively. These concentrations are 43.8 and 57.3 times the exposed working surface. Furthermore, mathematical modeling based on the total amount of odor release revealed that there was a linear positive correlation between the total odor amount and the landfill operation area. However, the maximum number of film holes allowed on the covering layer has nothing to do with the working area and exposed working time, which is mainly determined by the HDPE film width in terms of ensuring the deodorizing effect of the covering operation. If the HDPE film with a width of more than 4 m is used, the number of film holes allowed within 100 m is more than 8. Therefore, in order to reduce the odor, the appropriate film width should be selected according to the actual operating conditions such as the mechanical operation level at the time of welding, the design of the landfill site, and the operational norms. This study explores the effect of film hole quantity of the daily cover in the landfill on the odor release from the landfill, which can provide an important reference for the design, operation, and decision-making of the daily cover operation of the sanitary landfill.

Combining photocatalytic process and biological treatment for Reactive Green 12 degradation: optimization, mineralization, and phytotoxicity with seed germination.

In this study, we show that the combination of a photocatalytic process (as a pretreatment step) combined with the conventional biological treatment of wastewaters can improve the process and achieve satisfactory efficiency. In this context, Reactive Green 12 (RG-12) solutions were photocatalytically pretreated using TiO2-impregnated polyester as supported catalyst under UV light in batch reactor. Photocatalysis as pretreatment (during 4 and 8 h of irradiation) was combined with 7 days of aerobic biological treatment using activated sludge. As first assays, respiratory tests revealed that the removal of RG-12 was improved by 5.4% and 11.7% for the solutions that were irradiated for 4 and 8 h in the presence of TiO2, respectively. However, 34.5% and 19% of dye solution was discolored after 7 days of biological treatment for the pretreated solutions during 4 and 8 h of UV light exposure, respectively. The discoloration efficiency obtained by the combined processes achieved 59.6% and 74.9% for the samples under photocatalysis during 4 and 8 h, respectively. A significant decrease in chemical oxygen demand (COD) of about 74.9% was achieved after photocatalysis/biodegradation processes. In addition, a decrease in the phytotoxicity was obtained as followed by the germination index (GI) values of cress seeds that increased from 46.2 to 88.7% after 8 h of photocatalysis and then to 92.8% after further 7 days of biological treatment.

Emerging technologies for the recovery of rare earth elements (REEs) from the end-of-life electronic wastes: a review on progress, challenges, and perspectives.

The demand for rare earth elements (REEs) has significantly increased due to their indispensable uses in integrated circuits of modern technology. However, due to the extensive use of high-tech applications in our daily life and the depletion of their primary ores, REE's recovery from secondary sources is today needed. REEs have now attracted attention to policymakers and scientists to develop novel recovery technologies for materials' supply sustainability. This paper summarizes the recent progress for the recovery of REEs using various emerging technologies such as bioleaching, biosorption, cryo-milling, electrochemical processes and nanomaterials, siderophores, hydrometallurgy, pyrometallurgy, and supercritical CO2. The challenges facing this recovery are discussed comprehensively and some possible improvements are presented. This work also highlights the economic and engineering aspects of the recovery of REE from waste electrical and electronic equipment (WEEE). Finally, this review suggests that greener and low chemical consuming technologies, such as siderophores and electrochemical processes, are promising for the recovery of REEs present in small quantities. These technologies present also a potential for large-scale application.

Biological responses at the interface of Ti-doped diamond-like carbon surfaces for indoor environment application.

Diamond-like carbon (DLC) and titanium-doped DLC coatings were prepared by hybrid PECVD/direct current magnetron sputtering (DCMS). In this study, we show that the operating conditions of titanium-doped DLC coatings used for implants in surgical devices significantly modify their surface properties and consequently their interaction with cells. The coatings showed uniform distribution on the substrate and their biocompatibility was tested by way of rat calvaria osteoblasts. Doping DLC with Ti changed the roughness and wettability of the film interface. The autoclaving of the samples led to the surface oxidation and the formation of TiO2 on the top-most layers of Ti-doped DLC. This was quantitatively assessed by X-ray photoelectron spectroscopy (XPS) and revealed the presence of Ti3+ and Ti4+ species in redox reactions during their interactions with cells. By XPS analysis, the oxidative carbonaceous species C=O and O=C-C were detected during the bacterial inactivation. Reactive oxygen species (ROS) were identified on the sputtered samples and the ⦁OH radical was identified as the most important oxidative radical intermediate leading to bacterial disinfection. The position of the intra-gap of the oxidized C species is suggested within the TiO2 bandgap.

New Evidence for Ag-Sputtered Materials Inactivating Bacteria by Surface Contact without the Release of Ag Ions: End of a Long Controversy?

The study provides new evidence for Ag-coated polyester (PES) mediating Escherichia coli inactivation by way of genetically engineered E. coli (without porins, from now denoted porinless bacteria). This allows the quantification of the bactericidal kinetics induced by the Ag surface without the intervention of Ag ions. Bacterial inactivation mediated by Ag-PES was seen to be completed within 60 min. The samples were prepared by high-power impulse magnetron sputtering (HiPIMS) at different sputter powers. In anaerobic media, this process required 120 min. The amounts of ions (Ar+, Ag+, and Ag2+) generated during the deposition by direct current magnetron sputtering (DCMS) and HiPIMS were determined by mass spectrometry. The thickness of the Ag films sputtered on PES by DCMS (0.28 A) during 100 s was found to be 340 nm. Thicknesses of 250, 230, and 200 nm were found when sputtering with HiPIMS was tuned at 8, 17, and 30 A, respectively. By scanning transmission electron microscopy (STEM-HAADF), the atomic distribution of Ag and oxygen was detected. By X-ray photoelectron spectroscopy (XPS), a shift in the Ag oxidation state was observed within the bacterial inactivation period. This reveals redox catalysis within the time required for the total bacterial inactivation due to the interaction between the bacterial suspension and Ag-PES. Surface properties of the Ag-coated PES samples were additionally investigated by X-ray diffraction (XRD). The formation of Ag plasmon was detected by diffuse reflectance spectroscopy (DRS) and was a function of the applied sputtering energy. The indoor sunlight irradiation dose required to induce an accelerated bacterial inactivation was found to be 5-10 mW/cm2.

Deciphering the Mechanisms of Bacterial Inactivation on HiPIMS Sputtered CuxO-FeOx-PET Surfaces: From Light Absorption to Catalytic Bacterial Death.

The production of nontoxic, affordable, and efficient antibacterial surfaces is key to the well-being of our societies. In this aim, antibacterial thin films have been prepared using earth-abundant metals deposited using high-power impulse magnetron sputtering (HiPIMS). The sputtered FeOx, CuxO, and mixed CuxO-FeOx films exhibited fast bacterial inactivation properties under exposure to indoor light (340-720 nm) showing total bacterial inactivation within 180, 120, and 60 min, respectively. The photocatalytic mechanisms of these films were investigated, from the absorption of photons up to the bacteria's fate, by means of ultrafast transient spectroscopy, flow cytometry, and malondialdehyde (MDA) quantification justifying the cell wall disruption. The primary driving force leading to bacterial inactivation was found to be the oxidative stress at the interface between the sputtered thin films and the microorganism. This was justified by using engineered porinless bacteria disabling the possible ion diffusion leading to internal bacterial inactivation. Such stress is a direct consequence of the photogenerated electron-hole pairs at the interface of the sputtered layers. By diffuse reflectance spectroscopy, we found that both FeOx and CuxO present a band gap of ∼2.9 eV (>425 nm), while the mixed CuxO-FeOx thin film has a band gap below 2.3 eV (>540 nm). The structure and atomic composition of the films were characterized by energy-dispersive X-ray, X-ray photoelectron, and optical spectroscopy. While the composition and metal oxidation states are distinct in all three films, the difference in photocatalytic efficiency can, at first sight, be explained as the direct consequence of their absorbance and the unique interaction between Fe and Cu oxides in the composite film.

Femtosecond Spectroscopy of Au Hot-Electron Injection into TiO2: Evidence for Au/TiO2 Plasmon Photocatalysis by Bactericidal Au Ions and Related Phenomena.

In the present work, we provide evidence for visible light irradiation of the Au/TiO2 nanoparticles' surface plasmon resonance band (SPR) leading to electron injection from the Au nanoparticles to the conduction band of TiO2. The Au/TiO2 SPR band is shown to greatly enhance the light absorption of TiO2 in the visible region. Evidence is presented for the light absorption by the Au/TiO2 plasmon bands leading to the dissolution of Au nanoparticles. This dissolution occurs concomitantly with the injection of the hot electrons generated by the Au plasmon into the conduction band of TiO2. The electron injection from the Au nanoparticles into TiO2 was followed by femtosecond spectroscopy. The formation of Au ions was further confirmed by the spectral shift of the transient absorption spectra of Au/TiO2. The spectral changes of the SPR band of Au/TiO2 nanoparticles induced by visible light were detected by spectrophotometer, and the morphological transformation of Au/TiO2 was revealed by electron microscopy techniques as well. Subsequently, the fate of the Au ions was sorted out during the growth and biofilm formation for some selected Gram-negative bacteria. This study compares the bactericidal mechanism of Au ions and Ag ions, which were found to be substantially different depending on the selected cell used as a probe.