Review on advances in photocatalytic water disinfection utilizing graphene and graphene derivatives-based nanocomposites

Journal of Environmental Chemical Engineering 7 (2019) 103132

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Review on advances in photocatalytic water disinfection utilizing graphene and graphene derivatives-based nanocomposites

T

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Vishal Duttaa, Pardeep Singha,b, , Pooja Shandilyaa, Sheetal Sharmaa, Pankaj Raizadaa,b, Adesh K. Sainib,c, Vinod Kumar Guptad, Ahmad Hosseini-Bandegharaeie,f, Shipli Agarwald, ⁎⁎ Abolfazl Rahmani-Sanie, a

School of Chemistry, Faculty of Basic Sciences, Shoolini University, Solan, Himachal Pradesh, 173212, India Himalayan Centre for Excellence in Nanotechnology, Shoolini University. Solan, HP, 173229, India c School of Biological and Environmental Sciences, Faculty of Basic Sciences, Shoolini University, Solan, HP, 173229, India d Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia e Department of Environmental Health Engineering, Faculty of Health, Sabzevar University of Medical Sciences, Sabzevar, Iran f Department of Engineering, Kashmar Branch, Islamic Azad University, PO Box 161, Kashmar, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene based nanocomposite Enhanced heterogeneous photocatalysis Bacterial disinfection Mechanistic view Kinetic study

Graphene, the recently discovered allotrope of carbon, is a single atom layered aromatic carbon material which is tightly packed into a two-dimensional hexagonal crystal lattice. It is the thinnest material known to man and has a huge application in various fields of science and technology. However, despite having excellent characteristics, the use of pure graphene sheets has limited application. Graphene-based composites offer a considerable potential in the field of environmental remediation for the efficient removal of biological contaminants. Therefore, a comprehensive review of the antibacterial property of the graphene-based nanocomposites and advancements in this field is of significant value for the scientific community. In the present review, firstly, a concise overview about graphene, its exceptional chemical and physical features, and different fabrication and characterization techniques employed for graphene-based nanocomposites are discussed. Then, a comprehensive discussion was performed on the disinfectant property of binary, ternary, and complex metal oxide-graphene and graphene derivatives-based composites, along with the mechanistic models of disinfection. Furthermore, the future prospects and the remaining challenges in utilizing graphene nanocomposites in energy and environmental disciplines is discussed by giving a precise conclusion and a prospective outlook.

1. Introduction In the present world, water contamination and pollution is one of the most worrying and the biggest world’s problems which should be combated by formidable and practical solutions. As the water usage is not limited to household activities but also widely utilized in agricultural and industrial activities, the main cause for water pollution is the wide range of chemicals released by industries [1–4]. Organic synthetic dyes are mainly utilizing by textile industries and, because of their high stability, their ejection in water bodies leads to harmful effect on humans and animals. Several dyes such as methylene blue (MB), bromophenol blue (BPB), and phenolic compounds when ejected in higher quantity result in serious contamination of the water bodies. Methylene blue (MB) is a basic cationic dye used for dying wool, cotton,

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silk, etc. When MB is used wildly, it can cause serious diseases like hard breathing, vomiting, and mental disorders. In many industrial fields, BPB is utilized for textile, dermatological agent and also for veterinary medicines [5–10]. Out of all, phenolic compounds are of the environmental pollutants which are toxic for aquatic animals at very low concentration. Many industries such as olive oil mills, petroleum refineries, and petrochemical plants release waste products into water bodies which mainly consist of phenol and phenolic compounds. Therefore, to avoid health and environmental threats by phenolic derivatives, wastewater pollutants should be mitigated [10–12]. Many waterborne diseases, such as typhoid fever, cholera, amoebic, and other diarrheal diseases, which appalled millions of people across the world are transmitted through microorganisms like bacteria, protozoa, and viruses [13–15]. Thus, due to the noteworthiness of public health and

Corresponding author at: School of Chemistry, Faculty of Basic Sciences, Shoolini University, Solan, Himachal Pradesh, 173212, India. Corresponding author. E-mail addresses: [email protected] (P. Singh), [email protected] (A. Rahmani-Sani).

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https://doi.org/10.1016/j.jece.2019.103132 Received 26 October 2018; Received in revised form 9 April 2019; Accepted 29 April 2019 Available online 08 May 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

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because of environmental concern issues, refinement of water with effectual and low-cost techniques is very essential. Also using effective and inexpensive strategies to meet the need of pure and safe drinking water is always vital from the economical point of view. In emerging countries, the condition is worse since the problem is daunting and the economical water treatment techniques are not available. Existing techniques for the removal of the contaminants form the wastewater are adsorption, physical and biological treatments, ozone treatment, microbial activity, electrochemical methods, chemical oxidation, free chlorine, chloramines, chlorine dioxide, ozone, germicidal UV radiation, and advanced filtration process which have been used in water purification plans for many years. All of these techniques hold some serious drawbacks, such as being non-economical and failing in complete degradation, the some of them also results in the formation of perilous by-products and also display low efficiency [16–21]. However, chlorine and its derivatives can react with various natural carbon-based compounds in the water to form many cancer-causing disinfection byproducts (DBPs). Moreover, complete disinfection of microorganisms usually needs higher doses than normal, since some organism develops resistance to chlorine and its derivatives [22,23]. The ozonation also forms DBPs like carboxylic acid, aldehydes, ketones and bromates and it requires expensive and complicated technological equipment. Besides the above methods of disinfection, numerous antibacterial agents comprising enzymes, antibiotics, metal ions, and ammonium compounds have been broadly used to protect public health. But these materials are accompanying with many shortcomings like the need of post-treatment process, antibiotic resistance, relatively high expense, environmental contamination, leaching, and short effective life time. Therefore, there is an urgent need to re-evaluate disinfection methods and to consider innovative approaches to develop a new generation of antimicrobial agents which are reliable and have high ability to effectively kill pathogenic bacteria [24–28]. An attractive technology for treatment of waste water is photocatalytic disinfection, due to its ability in direct utilization of solar energy for achieving disinfection, as well as chemical detoxification. Solar photocatalysis has become very significant area of research since visible light is the main component of solar radiation [29,30]. In fact, during a typical photocatalytic process, the sunlight is directly used to carry out a variety of chemical reactions which can ultimately leads to water splitting, degradation of organic pollutants, and disinfection of water [31–34]. The photocatalytic reactions mainly depend upon photon (light energy) or the wavelength of light and catalyst. The necessary steps involved in semiconductor photocatalysis are discussed in Eqs. (1)–(20). When light energy falls on semiconductor surface, holes left in valence band (VB) of semiconductor can oxidize donor molecules and react with H2O molecules to generate OH radicals. On the other hand, conduction band (CB) electrons react with dissolved O2 species to form superoxide ions and these electrons encourage redox reactions. The holes and electrons undergo consecutive reduction and oxidation reactions with any species, which might be adsorbed on the semiconductor surface, to yield required products. Route of charge separation for reactive oxygen species (ROS) production:

Photocatalyst + hυ → e− (conduction band) + h+ (valence band)

E0′ = −0.33V

O2 + e− → O2⋅

O2⋅ + 2H+ + e− → H2 O2 E0 ′ = 0.89V

(4)

H2 O2 → O2 +

HO2−

2H+

+

+ H+ pK a = 11.7

2e−

→ H2 O2 E0 ′ = 0.28V

H2 O2 + e− → OH− + OH⋅ E0 ′ = 0.38V

OH⋅ → O2− + H+ pK a = 11.8

(9)

→ H2 O2

(10)

O2⋅ − + h+ → 1O2 E0′ = 0.65V

(11)

Interaction between radicals:

2HO2∙ → H2 O2 + O2

(12)

2HO2∙

(13)

+

O2∙ −

+ H2 O → H2 O2 + O2 + OH−

2HO2∙ → H2 O2

(14)

H2 O2 + hυ (UV ) → H2 O2 +

O2− ∙

→

2OH ∙

OH ∙

+ O2 +

(15)

OH−

3O2 → Photocatalyst intersystem crossing →

(16) 1

O2

(17)

Overall photocatalytic oxidation:

ROS, h+ + pollutants → oxidised products (H2 O, CO2)

(18)

O2 + pollutants + hυ → Photocatalyst → oxidized products ( H2 O, CO2) (19) Recombination of electron and hole pairs:

e − + h+ → Heat + Radiation less decay

(20)

Recently several natural and engineered nanomaterials have shown strong antimicrobial property, including chitosan, fullerene, aqueous fullerene nanomaterial (nC60), carbon nanotubes (CNTs) and inorganic nanoparticles like Cu, Ag, ZnO, TiO2, etc. [35–40]. Although these nanomaterials have found a wide application as photocatalyst but still they suffer various limitations like fast rate of electron-hole pair recombination, wide band gap which restricts the absorption only to the UV region, and other limitations regarding to recovery of nano-sized nanoparticles from treated water, and their reusability in many cycles. These limitations have restricted the direct use of nanoparticles for practical application in full-scale water treatment plants. The doping, along with coupling and supporting of two or more semiconductors are of the methods which are used to overcome this limitations. Supporting and coupling of semiconductor with an appropriate support or dopant causes well-dispersion of semiconductor nanoparticles which further inhibits from their aggregation, leaching, and recombination of electron-hole pairs. The interaction between two semiconductors will stimulate an internal electric field that increase probability of e−/h+ separation [41–51]. But still there are a few limitations in doping and coupling of two or more semiconductors and, therefore, some innovative approaches have been sought for synthesizing new antimicrobial agents with improved photocatalytic efficiency, like metal oxide-based graphene composites. The immobilization of catalyst nanoparticles (NPs) on graphene surface, as a two-dimensional support, has opened up new opportunities for synthesis and designing a new generation of catalysts. Graphene, which is the thinnest material in universe, has found vast applications in various fields of science technology such as physics, material science, chemistry, biology, and so on. However, despite having excellent characteristics, the use of pure graphene sheets is limited. For instance, although both graphene and graphene-related materials have been considered and studied as environment-friendly materials which have strong antibacterial activity, the benefits of using graphene-based nanocomposites and its hybrids for environmental application is more attractive. Various semiconductor photocatalysts such as TiO2, ZnO, etc. have also broadly been studied due to their various properties like chemical stability, low cost, non-toxic nature, etc. However, they suffer high recombination of electron-hole pairs, which limits their widespread application. The great interest towards graphene-based

(2) (3)

(8)

2HO2∙ 1O2

(1)

HO2⋅ → O2⋅ pK a = 4.8

H2 O + h+ → OH⋅ + H+ E0 ′ = 2.32V

(5) (6) (7) 2

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3. Bactericidal potential of graphene derivatives-based nanocomposites

nanomaterial in water disinfection is because of easy availability of bulk quantities, easy functionalization by chemical reaction, good water dispersion, highly biocompatibility, and not being robust oxidants which are expected to not produce hazardous disinfectant byproducts (DBPs) [52–56]. In the present review, after extensive mentioning of several preparation and characterization techniques employed for graphene-based composites, applicability of binary, ternary and complex graphenebased composites for water disinfection was comprehensively discussed. Furthermore, the mechanistic models of disinfection used for explanation of bactericidal activity of graphene-based composites and the applicable kinetic models were discussed. Furthermore, giving a precise conclusion and a prospective outlook, the outlooks and remained challenges for utilizing graphene nanocomposites in energy and environmental sciences were mentioned.

Amongst, the nano-sized materials, some carbon nanomaterials have emerged as promising materials with efficient antimicrobial action, such as carbon nanotubes (CNTs), graphite, fullerenes, graphene, and graphene based materials. Graphene, which is newly discovered nanomaterial possesses great charge carrier mobility (˜1 × 105 cm2 V−1 s−1) at room temperature [83,84], large surface area (˜ 2630 m2 g−1) [85], optical transparency [86], high mechanical stiffness (2.4 ± 0.4 TPa) [87], high thermal conductivity (˜2000 to 5000 W m K−1) [88], high electrical conductivity (108 A cm−2) [89] and exceptionally conjugated structure. Owing to its unique properties and biocompatibility, graphene emerges as an attractive nanomaterial which is exceptionally suitable for versatile applications in liquid crystal devices, sensors, solar cells, capacitors, batteries, wastewater treatments, and disinfection of microbes like bacteria [90–94]. In addition, out of the above-mentioned applications, investigation on solar light-driven killing of bacteria by using different graphene composites is an emerging field which has attracted considerable attention. The favorability of graphene for transportation of charge carriers in photocatalytic reactions is originated from the extraordinary properties which are ingrained in long-range π-conjugation offered by this material. Despite having many applications in the other fields, applications of graphene in biological studies are comparatively limited. Thus, in recent years, notable concern has been focused on the study of interactions between graphene derivatives and living cells. Recently, it has been investigated that both graphene oxide (GO) and reduced graphene oxide (rGO) exhibited strong antibacterial activity. Among GO and rGO, GO showed higher antibacterial activity due to the small size of its nanosheets. Since the stronger van der waal forces between rGO nanosheets facilitate aggregating of particles, the size of rGO is around nine times greater than the GO nanosheets. The bactericidal effect of GO and rGO has been prompted by sharp edge of graphene sheets which consequences in disruption of cell membrane and, therefore, leads to loss of membrane integrity and release of cellular content and ultimately cell death (Fig. 2a, b) [95–97]. Graphene-based nanomaterials can also oxidize bacterial lipids, DNA, proteins and other components of cells. Moreover, the antibacterial behavior of rGO and GO have been observed to be time and concentration dependent, so that increasing the incubation time and concentration of GO and rGO increases rate of bacterial cell death. At all examined concentrations and incubation intervals, the GO dispersion show much higher antibacterial activity. The bactericidal activity of rGO and GO are ascribed to its effect on both membrane and increasing the oxidative stress. Membrane stress is stimulated by direct interaction with graphene-based material. The sharp edge of graphene nanosheets might disrupt the cell membrane which leads to loss of bacterial membrane integrity and outflow of intracellular material. The antibacterial mechanism via oxidative stress has been often introduced as a fundamental mechanism for carbon based nanomaterials such as fullerene and CNTs [98,99]. Generally, oxidation by graphene-based materials is supposed to happen via two pathways: one is reactive oxygen species (ROS) and other is ROS-independent which is also happens in fullerene, in which graphene-based material oxidizes a vital cellular structure and, therefore, disrupt a specific microbial process or hinders its respiration. The oxidation via reactive oxidative species has been explained in Fig. 2c [97]. GSH oxidation is used to determine the possibility of ROS-independent oxidative stress mediated by dispersion of graphene derivatives. GSH is a tripeptide with thiol groups and an important antioxidant in bacteria which prevent damage to cellular components caused by ROS [100]. The higher level of GSH indicates a higher amount of oxidative stress in bacterial cells [101]. The GSH oxidation is an indirect confirmation of the ability of graphene-based materials to mediate ROS-independent oxidative stress in the bacterial cells.

2. Use of semiconductor metal oxides as active photocatalysts The use of photocatalyst is an efficient, economical and green technique for water purification. Recently, advancement in photocatalytic materials and nanotechnology has led to the production of innovative photocatalysts by which degradation of organic contaminants and bacteria can be achieved with high efficiency. As a result, many active photocatalysts in the visible light range have been fabricated, and their performance for refinement of water has been studied [57–61]. Among the engineered nanomaterials employed for decontamination of water, the most widely used ones are TiO2 [62–65], ZnO [66–69], and CdS [70–72] which have shown strong antimicrobial activities by generating reactive oxidation species (ROSs) capable of disinfecting microorganisms, like bacteria, virus, protozoa, and spores [73–78]. When light is irradiated on photocatalysts with a photo-energy greater than the band gap, electron of nano photocatalyst (NPs) are stimulated across the band gap to conduction band, which generates holes in the valence band [79] and electron and hole generated display oxidizing and reducing power, respectively as shown in Fig. 1. A reductive reaction can happen between the generated electron and molecular oxygen which lead to the production of superoxide anion (O2·-). On the other hand, an oxidative reaction between the generated hole and hydroxyl ions or water molecules leads to the production of hydroxyl radical (OH·), via an electron abstraction process [80]. Both hydroxyl radical and superoxide anion are very strong and nonselective oxidants which can destruct structure of all types of biomolecules such as lipids, carbohydrates, nucleic acids, amino acids, proteins, and DNA [81]. Therefore, these radicals are very effective for disinfection of microorganisms, like bacteria, since 96% of bacterial cell is comprised of macromolecules and 3% of bacterial cell include monomer like amino acids, nucleotides, sugars, etc. and the inorganic ions form only the remaining 1% [82].

Fig. 1. General mechanism of photocatalytic oxidation process (With permission from Elsevier, license number 4372030368370). 3

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Fig. 2. (a) Wrapping of E. coli by graphene sheets (b) Slicing of cell membrane by sharp edges of graphene (c) Mechanism depicting bacterial disinfection of cells by metal oxide/graphene composite (With permission from Royal Society of Chemistry).

superior mass electro-catalytic activity (two times) and a better capacity to tolerate poisoning concerns in methanol electro-oxidation [106]. The growth of metal oxide on the graphene and producing graphenemetal oxide nanocomposites are affordable by utilizing the hydrothermal or solvothermal method. Zou et al. used a seed directed hydrothermal approach reported to grow different metal oxide nanorods, like TiO2, ZnO, MnO2, CuO, ZrO2 on the surface of graphene to form metal oxide-graphene-metal oxide heterostructures which their photocatalytic efficiency was four times more than that of pristine metal oxide nanorods [107]. A single step hydrothermal route was used for fabrication of TiO2-graphene nanocomposites and the product exhibited good adsorption properties for dyes, wide visible light absorption range, and effective charge separation which significantly enhanced efficiency of the produced nanocomposite as compared to pristine TiO2 and TiO2-CNTs [108]. The crystal facets of TiO2 NPs in graphene nanocomposites synthesized by the hydrothermal method can also be easily controlled. Besides the vast use of TiO2, and Fe3O4 NPs with hollow interior and spherical geometry for production of graphenebased nanocomposites, the hydrothermal reaction can be utilized for the growth of NiO nanosheets, MoO3 nano belts, VO2 nanotubes, and Mn3O4 NPs on graphene surface [109]. Moreover, metal chalcogenidesgraphene nanocomposite is also synthesizable by this method. CdS quantum dots has been also grown on the graphene by Cao et al., using the solvothermal method and reacting Cd2+ ions and GO in DMSO as a solvent at 180 °C [110]. The decoration of CdS on graphene surface was found to prohibit from stacking of graphene sheets as well as from CdS quantum dot aggregation. The produced nanocomposite showed an ultrafast electron transfer in picosecond from QDs to graphene which is a beneficial character in optoelectronics materials. Similarly, the deposition of metal sulphides like SnS2 used in lithium-ion batteries can also be done by solvothermal method. ZnSe-graphene nanocomposite can also be synthesized by utilizing GO and [ZnSe](DETA)0.5 nano belts as precursors and employing one-pot hydrothermal method at 180 °C for 12 h [111]. The synthesized nanocomposite shows improved electrochemical and photocatalytic property for methyl orange degradation. Besides metal oxides and chalcogenides mentioned above, complex compounds like Li4Ti5O12, LiFePO4, and Bi2WO6 can also be used for synthesis of composites by hydrothermal and solvothermal method [112].

Furthermore, chitosan has been also utilized as an antibacterial agent which is useful because of its compatibility and biodegradability [102]. Chitosan is the second most abundant polymer which is a product of deacetylation of chitin and co-polymer of N-acetyl-D glucosamine and glucosamine linked by a glycosidic bond. Many theories explain that interaction between anionic groups on bacteria surface and polycationic chitosan leads to an increase in membrane permeability and thereby, facilitate the leakage of cellular proteins by disrupting the bacterial membrane [103]. 4. Synthesis methods of graphene nanocomposites In order to attain a excessive yield of fabrication, many efforts have already been made for developing a synthetic route for fabrication of graphene and graphene-based composites. Some of the examined routes are hydrothermal method and solvothermal method, chemical vapor deposition method, deposition via electrochemical and electrophoresis processes, chemical electrolysis deposition, deposition via physical contact and mixing, covalent reaction, non-covalent interaction and photochemical reactions. Each synthesis method has its own advantages and limitations and therefore, production of a specific nanocomposite can be done by employing a suitable method of preparation [103,104]. 4.1. Hydrothermal and solvothermal method This method is very versatile and is widely applied to prepare graphene nanocomposites and other different kinds of materials. The high pressure used in the hydrothermal and solvothermal method leads to close covalent contacts between the deposited material and graphene, and the synthesized composites through this method are advantageous for various applications. In this method graphene or graphene oxide is mixed with the precursor in solution and, then, the precursor becomes deposited on the graphene surface in an autoclave. The metal/alloy nanoparticles such as Ag, Pt, Ru, metal oxide (TiO2, SnO2, VO2, and Mn3O4), metal chalcogenides (ZnS, SnS, and MoS2), and complex compound (LiFePO4) have been grown on graphene surface with good control [104]. Shen et al. fabricated rGO-Ag NPs nanocomposite by hydrothermal method with AgNO3 and GO as a precursor and ascorbic acid as a reducing agent as described in Fig. 3. [105]. Similarly, Pt-Ru nanoparticles were also deposited on graphene surface and, compared to nanoparticle alone, the composite displayed 4

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Fig. 3. Schematic illustration of preparation of Ag-RGO via hydrothermal method (With permission from Elsevier, license number 4372031235339).

Electrophoresis is another convenient method to synthesize graphene nanocomposite [121]. Zhu et al. synthesized graphene-CNTs nanocomposite with tunable CNTs content with an applied voltage of 30 V for 30 min [122]. Graphene-NiOH nanocomposites were synthesized via electrophoresis of colloidal solution of Ni2+ ion and GO in which GO surface was charged positively by Ni2+ ion and deposited on the substrate under negative potential with immediate reduction of GO to rGO [123]. MnO2, Cu2O and ZrO2 deposition on the graphene coated textile surface can be performed by electrochemical method. To synthesize MnO2/graphene nanocomposite, graphene-coated textile surface is subjected to electrochemical deposition in a solution having Mn(NO3)2 and NaNO3. Metal has been deposited on graphene with the rate of ˜5 μg min−1 on applying a constant rate of 0.1 mA cm-2. Likewise, Cu2O and ZrO2–graphene nanocomposites can also be produced via a similar method [124].

4.2. Chemical vapor deposition (CVD) method Among the methods reported for producing graphene composite, the CVD of carbon atom on a metal substrate has attracted a large interest, because of low-cost production of graphene layer with high surface area [113]. CVD method is useful to grow both CNTs and metal oxide on graphene to form graphene heterostructure. The direct synthesis of graphene materials through this method is utilized to develop large area, particular and few-layered graphene sheets on the metal substrates such as Ru, Cu and Ni [114]. High degree of crystallinity and negligible bulk defects can be maintained while introducing a graphene layer onto the desired substrate via this method. Kim et al. used vapor phase epitaxy with higher vertical alignment to grow ZnO nano needles on graphene [115]. ZnO with denser nanorod array with graphene can be easily tuned by controlling CVD growth temperature. Zhu et al. prepared 3D CNTs-graphene material with surface area of 2 × 103 m2 g−1 [116]. 3D CNTs mediated graphene sandwich with surface area of 612 m2 g−1 which is much greater than that of graphene films without CNTs (202 m2 g−1) can also be easily synthesized by utilizing the CVD method.

4.4. In situ chemical electroless deposition In situ chemical electroless deposition method is a simple route for preparation of different graphene-based composites with metal, metal oxide and di-chalcogenide like Ag, Au, Pt, Cu, TiO2, Fe2O3, Mn3O4, SnO2 and CdS [125,126]. Specific nanorods, nanosheets and nanoparticle can be accumulated on graphene surface to fabricate nanocomposite by this method. Ag-graphene nanocomposite can be synthesized by simply heating AgNO3-GO solution at 75 °C or by in situ reduction of silver salt which can be done by glucose or NaBH4 in the presence of GO [127]. The copper ion were also reduced by KBH4 in the presence of GO to synthesize copper nanoparticles-graphene composite. Li et al. employed chemically reduction of a Pt-containing precursor, like H2PtCl6, and GO with a NaBH4 reducing agent to grow Pt nanoparticle on graphene and form graphene-Pt nanocomposite [128]. The growth of bimetallic NPs is also affordable on the graphene sheets by this method. Guo et al. synthesized bimetallic NPs-graphene nanocomposite which, compared to platinum black or commercially available Pt/C catalyst, showed a superior electrocatalytic activity towards oxidation of methanol [129]. Graphene-TiO2 nanocomposite has been synthesized via in situ hydrolysis of absorbed Ti(IV) butoxide (Ti[O (CH2)3CH3]4) on the reduced graphene surface.

4.3. Electrochemical and electrophoresis deposition method One important way to easily deposit metal, metal alloy, and a metal oxide on graphene surface is through electrochemical and electrophoresis deposition method [117]. By operating a potential electrochemical reduction induces the synthesis of metal nanoparticle from the solution onto graphene. Semiconductors like CdSe, ZnO, and metals such as Cu, Pt, Ni, and bimetallic NPs can be electrochemically deposited onto graphene surface. ZnO nanorods can be deposited on conductive rGO by this method [118]. The morphology of a ZnO nanoparticles has strongly affected by the conductivity of rGO. For example, low conductivity of rGO results in particle shaped structure and high conductivity of rGO results in hexagonal nanorods. To grow ZnO nanorods on rGO surface, a constant potential was used. Further metals like Cu, Pt, Ni, and even bimetallic NPs such as Au, Pd can also be deposited on the graphene sheets via electrochemical method [119]. The deposition of bimetallic Au, Pd NPs on graphene can be done by applying a constant potential at - 0.2 V in an aqueous phase containing HAuCl4, PdCl2, and 0.1 M KCl. Furthermore, polymers such as polyaniline and polypyrrole can also be electro-deposited onto graphene. To form graphene–polyaniline nanocomposite, aniline monomer can be mixed with GO in the presence of H2SO4 and then non-adsorbed part of aniline monomer can be eliminated by washing. Further, GOaniline can be electro-polymerised in 1 M H2SO4 electrolyte via electrochemical scanning between −1.3 to +1.0 V vs SCE [120].

4.5. Covalent reactions based methods The most convenient applied method to fabricate graphene nanocomposite is performed through a covalent interaction. In this method, graphene and GO are bonded covalently to the surface by amide bonding, atom transfer radical polymerization (ATRP), diazonium salt, and click chemistry [130]. Among these, amide bonding has been vastly 5

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5. Characterization techniques and antibacterial activity determination methods

employed for covalently synthesized graphene nanocomposite. The amine-terminated ionic liquid can be attached to graphene surface to produce a poly-dispersed nanocomposite which, due to the electrostatic interaction among ionic liquid units on surfaces of graphene can be well dispersed in aqueous solution, DMSO, and DMF for over three months. During covalently fabricating of graphene nanocomposite via diazonium salt GO is partially reduced by NaBH4 and then sulfonated with aryl diazonium salt, which inserted sulfonated groups on the graphene sheets with S/C ratio 1: 35. Further reduction of sulfonated partially reduced GO with hydrazine was carried to remove the residual oxygen functional group. Above prepared sulfonated graphene has an exceptional solubility of 2 mg mL−1 (at pH 3–10) [131]. Graphene nanocomposite can be covalently functionalized by growing polystyrene chain on the graphene surface via atom transfer radical polymerization (ATRP) with 80% grafting efficiency [132]. The obtained polystyrenegraphene nanocomposite showed improvement in thermal conductivity by the factor of 2.6 with 2 wt % graphene as compared to polystyrene, and improvement in tensile strength by 70% and Young’s modulus by 57% with only 0.9 wt % of graphene as compared to polystyrene.

The material characteristics related to antibacterial activity of graphene-based nanomaterial can be identified by various characterization methods such as Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Thermogravimetric analysis (TGA), Dynamic light scattering analysis (DLS), UV–vis diffuse reflectance spectra (DRS), Raman spectroscopy, cyclic voltammetry, Electrochemical impedance spectroscopy (EIS), Tauc plots, point of zero charge pH (PZC), etc. [141–143]. The XRD patterns of GO, graphene, and graphite, also tell us about the degree of oxidation. The higher interlayer spacing of the materials shows a higher oxidation degree. Generally, graphene has a sharp and strong peak at 2θ = 32 °, indicating a highly ordered structure [144]. The size distribution in the composite can be analyzed by SEM. The morphological characteristics of the composite can be evaluated by TEM technique, and it is also applicable for the demonstration of the homogeneity and dispersion of one compound over other. TGA can be applied to analyzing the thermal stability of the composite. Identifying the type of chemical bonds or functional groups present in the molecule can be performed by using Fourier transform infrared spectroscopy (FTIR) over the range of 4000−500 cm−1. The graphene composite in aqueous dispersion can be characterized by DLS, which shows the size differences between the different materials. The XTT method can be used to evaluate the generation of superoxide anion-induced reactive oxygen species (ROS). Raman spectroscopy is an effective and reliable method for determining crystal quality of carbon based materials [145]. In Raman spectroscopy, the properties used for carbon materials are D and G lines which are corresponding to sp3 and sp2 C stretching modes [146]. The D and G band of GO are seen at 1305 cm-1 and 1586 cm-1 respectively. On reduction of GO to rGO the D and G band shifted to 1282 cm-1 and 1566 cm-1, respectively [147]. Diffuse reflectance spectroscopy is used to measure band gap using equation K = (1-R)2/2R, i.e. Kubelka-Munk equation, where R is reflectance (%) and K is reflectance transformed. The Tauc plots are used to calculate direct and indirect transition bandgaps using Tauc’s equation, i.e. α = α° (hυ − Eg )n / hυ where α is absorption coefficient, α° and h are the constants, and Eg is the optical bandgap of material. Eg has been explained by deducing linear portion of plot of (αhυ)2 versus energy axis [148]. Cyclic voltammetry is a technique to study electrochemical reactions or to study electrochemically generated free radicals. This type of study helps in understanding the fact of solar energy conversion and catalysis [149]. Electrochemical impedance spectroscopy is used to reveal changes of interfacial properties of electrode after analyte interaction with their probing molecules restrained on surface of electrode. The Bode plots are mostly used in electrochemical works. In the circuit, impedance is calculated using equation Z = (Rs + R ct ) 1 + jωτ2 where τ1 and τ2 are Bode

4.6. Non-covalent reactions based methods Another commonly applied method to prepare graphene-based nanocomposite is through non-covalent interaction. This method is based on the non-covalent interaction between organic species and graphene. Graphene surface can be modified by a surfactant and a polymer, like 1pyrenebutyrate (PB−), PDI, 1-pyrenecarboxylic acid, sodium dodecyl benzene sulfonate (SDBS), dendronized perylene bisimides, etc. which interact with graphene surface by π-π stacking, electrostatic or hydrophobic interaction to enhance water solubility and stability of graphene [133]. An et al. functionalized graphene directly from graphite, without oxidation with 1-pyrenecarboxylic acid which intercalated and exfoliated graphite to multi-layer and single graphene sheets [134]. The π-stacking of pyrene ends in molecule and −COOH functional group at another tail of molecule help graphene sheets to be stabilized in the solution. Graphene can be non-covalently functionalized by polymers such as polystyrene, polyaniline, and conjugated triblock copolymer [135]. Stankovich et al. synthesized graphene-polystyrene nanocomposite by mixing solution of isocyanate treated GO with polystyrene followed by chemical reduction to achieve molecular level dispersion of graphene in polystyrene [136]. Further biomolecules such as nucleotide (both single and double-stranded DNA and RNA), peptide, and protein can be employed to form graphene nanocomposite via π-π and electrostatic interaction [137,138].

4.7. Photochemical reactions based methods

1 + jωτ 1

Photochemical reactions can be utilized to prepare graphene nanocomposite under sunlight. For example, TiO2 -graphene nanocomposite can be synthesized under UV light-induced photochemical reaction by mixing GO with TiO2 nanoparticles in ethanol. As TiO2 is UV active, therefore on UV irradiation GO was reduced by accepting excited electrons from TiO2 [139]. This strategy can also be applied to synthesize WO3- and BiVO4-graphene nanocomposites [140]. Like metal oxide, metal nanoparticles can also be deposited easily on graphene by radiation assisted chemical reaction. Using a two-step photochemical method, noble metals such as Au, Pt, and Ag, NPs can be well dispersed on graphene in dimethylformamide (DMF). In the first step GO is photo chemically reduced to graphene by phosphotungstate under UV irradiation. In the next step, injection of noble metal ion or compound precursor into the system leads to immediate formation of graphene-based nanocomposite [141].

characteristic time constants. The point of zero charge pH (PZC) is associated with surface charge density σ calculated from acid-base titration [150,151]. The PZC calculated from titration data by defining pH condition at which net surface proton charge is zero using equation δP = F (ΓH + − ΓOH −) where, ΓH + is surface concentration of H+, ΓOH − is surface concentration of OH- and F is faraday constant. Two methods have been explained in literature to calculate (ΓH + − ΓOH −) . In the first method, proton budget in titration system involved water hydrolysis and consumption at solid surface and in the second method, proton charge is equal to difference between amount of base and acid and number of protons and OH groups left in the solution [152].

6. Photocatalytic disinfection by graphene derivatives-based nanocomposites The graphene and its derivatives have been preferred for fabrication 6

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formation between graphene and TiO2. Rahimi et al. reported TiO2graphene nanocomposite sensitized by tetrakis (4-carboxyphenyl) porphyrin (TCPP) via solvothermal technique [159]. Rahimi et al. reported TiO2-graphene (TG) nanocomposite sensitized by tetrakis (4-carboxyphenyl) porphyrin (TCPP) via solvothermal method [159]. The disinfectant feature of the TGP composite was observed against E. coli bacteria under visible light illumination. The photo-antibacterial activity of TG nanocomposite was comparatively low compared to TG sensitized by porphyrin dye (TGP). The disinfection performance of 64% for TGP was observed within 440 min under visible light. Here, TCPP generates photo induced electrons and holes, thus acting as a sensitizer, whereas graphene inhibits the rate of recombination by accepting the electron. Thus higher photocurrent density will be generated which, in turn, is helpful in generating ROS (OH·, O2·−). These ROS are responsible for photo-damaging of bacterial membrane. In addition to TiO2 composite, some other semiconductor nanoparticles which have also been studied as antimicrobial agents with graphene and its derivatives are ZnO, Ag3PO4, Bi2MoO6, etc. Kavitha and co-workers synthesized ZnO/graphene composite via in situ thermal decomposition of zinc benzoate dihydrazinate compound on graphene surface [160]. The antibacterial action of composite was surveyed against E. coli, gram-negative bacteria under UV light irradiation. The antibacterial property of ZnO might be due to either photocatalytic production of H2O2 or by penetrating cell envelope, resulting in disruption of bacteria cell walls. Deng at al. synthesized graphene-CdS (G-CdS) nanocomposite through two-step solvothermal method [161]. The composite showed a better bacterial disinfection than pristine CdS nanoparticles under visible light radiation. The G-CdS nanocomposite destroys the antioxidant defence system, i.e. catalase (CAT) activity and superoxide dismutase (SOD) activity, and enhances oxidative stress by increasing lipid peroxidation and producing intercellular ROS which ultimately results in cell death. However, decreased rate of antibacterial performance of G-CdS in the presence of humic acid (HA) may be ascribed due to the prevention of physical contact between bacteria and the composite by HA and HA acting as an antioxidant which reacts with ROS and, thus, reduces the antibacterial efficiency of the nanocomposite. Ibanez et al. fabricated TiO2-RGO composite by photocatalytic reduction of GO under UV light irradiation in presence of methanol [162]. The disinfectant property of composite was studied against E. coli bacteria and F. solani spores under solar light illumination. TiO2RGO composite exhibited an enhanced disinfection rate for E.coli as compared to pure TiO2, whereas such enhanced effect was not observed for F. solani spores with both TiO2-RGO composite and TiO2. The complete inactivating of E. coli was obtained with 500 mg/L of TiO2RGO within 10 min of solar irradiation. The less susceptibility of F. solani spores was due to the rigid structure of spores wall made up of polymeric sugars, proteins and glycoproteins which provide high resistance against different stress-inducing factors [163,164]. The biocidal action of the composite may be attributed to the generation of oxygen under visible light irradiation [165]. ZnO semiconductor with a band gap of 3.2–3.4 eV is a novel material for various applications and can be deposited onto graphene and can be easily utilized for forming various nanostructured morphologies. Many investigations on fabrication of graphene decorated ZnO composites and their applications have been done till date [166–169]. However, this study is restricted to their antibacterial application. The antibacterial property of ZnO may be due to oxidative stress and release of Zn2+ ion, resulting in the damage of bacterial membrane. Haldorai et al. synthesized ZnO/rGO nanocomposite via thermal decomposition method [170]. The nanocomposite was characterized by various techniques like Raman, XRD, XPS, and TEM. The antibacterial activity of nanocomposite was examined against E. coli, B. cereus, and S. aureus by CFU method. The mechanism of antibacterial activity of this nanocomposite was attributed to the synergistic effect of both rGO and ZnO. The rGO inhibit bacterial growth by disruption of cell membrane,

of hybrid nanocomposites due to property of graphene in forming composite with the other nanomaterials. Graphene effectually separates an electron-hole pair by accepting photo-induced electrons from the conduction band of metal. Further, a chemically modified graphene (GO) with carboxyl and hydroxyl group with high water solubility and rGO-based composites can form hybrid materials with desirable properties. Various graphene, GO, and rGO based hybrid materials have been consecutively fabricated with several metals, non- metals, metal oxides, etc. [153]. The lateral dimension and morphology of GO sheets play an important role in deciding its efficiency as an antimicrobial agent. For instance, as compared to smaller GO sheets, larger GO sheets display superior antimicrobial activity due to considerable coverage of cell surface. This enveloping of bacterial cell by larger GO sheets (> 0.4 μm2) inhibit the cell proliferation by blocking all the available active site on the cell membrane, whereas small sized GO sheets (< 0.2 μ m2) may stick to the surface but are not able to envelop and isolate the whole cell, which leads to a significant reduction in the efficiency [154]. Furthermore, well-dispersed GO sheets show better antimicrobial activity as compared to aggregated GO, due to presence of higher surface area in well-dispersed GO sheets. Antibacterial activity of both GO and rGO towards many bacteria is high. The strong disinfectant property can be ascribed due to the factors such as size, aggregation extent, membrane stress, etc. The GO nanosheets with smaller size and less aggregation display increased toxicity as compared to rGO with bigger size and less degree of aggregation. Ahmed and Rodrigues evaluated the effect of GO on water borne microbial community [155]. They found that GO weaken metabolic action of microorganism. This causes reduction in oxygen utilization and eventually deduces the value of biological oxygen demand (BOD), which finally cause cell death. Apart from the size and dispersibility of GO sheets, usage of adequate dosages of graphene for antimicrobial activity has also become of high importance. The antibacterial activity of graphene sheets was found to be more superior when compared with antibiotic kanamycin. The application of graphene/MO composites as antibacterial agents still needs more exploration, in comparison to their photocatalytic and absorptive applications, and there is a tremendous need to understand their antimicrobial behavior. 6.1. Graphene derivative-based binary metal oxide nanocomposites A huge number of graphene mediated metal oxide nanocomposites as photocatalytic water disinfection agents has been exploited due to their excellent properties such as stability, high absorptivity, conductivity, biocompatibility, tunable optical behavior, low cost, excellent physicochemical properties, etc. The graphene and rGO composite hybrids with metals and metal oxides such as Ag, Fe, TiO2 have appeared as antimicrobial agents for water disinfection. Among several graphene derivatives, graphene nanosheet, as two dimensional materials, has found countless promising purposes. The graphene nanosheet has been prepared by the reduction of GO via the hydrothermal method and extensively investigated for various application [156, 157]. Eaton and co-workers investigated antibacterial activity of graphene nanosheets against both gram-negative (E. coli & S. typhimurium) and gram-positive (B. subtilis & E. faecalis) bacteria. Mostly, as compared to the gram-positive bacteria, graphene nanosheets are more toxic to gram-negative bacteria due to difference in type of cell walls, as gramnegative bacteria and gram-positive bacteria, respectively possess a thick (20–80 nm thick) and a thin (7–8 nm thick) peptidoglycan layer [157]. Also, the antibacterial activity of carbon based materials depends upon structure, size, and composition of individual materials. Cao et al. synthesized TiO2 decorated graphene composite (4.2 wt % of graphene) via direct redox reaction and reported a higher antibacterial performance for the composite than for bare TiO2 [158]. The disinfectant features of nanocomposite were studied against E. coli bacteria under visible light irradiation. The composite extended light absorption ranges to visible region which was attributed to Ti-C bond 7

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was able to inactivate 90% of E. coli within 27 min and, thus, showed higher a bactericidal action as compared to TiO2 nanoparticle/GO as it took 52 min for the same photoinactivation. Moreover, the percentage of graphene content in the composite can shift the photoactivity of the composite to the visible light region. The antibacterial activity of GOTiO2 NRCs composite is higher than those of TiO2 nanoparticles and graphene oxide- TiO2 nanoparticle composite (GO-TiO2 NPCs). The complete destruction of E. coli under solar irradiation within 2 h has been detected by the composite. Gao et al. prepared CdS/GO nanocomposite via two-phase (watertoluene) method [174]. The disinfection property of the composite was studied against both gram-negative bacteria (E. coli) and gram-positive (B. subtilis). The CdS/GO nanocomposite killed 100% of both E. coli and B. subtilis within 25 min exposure to visible light whereas, within the same duration, unsupported CdS nanoparticle only was able to kill 55% of bacteria species. The CdS/GO nanocomposite has shown a lower antibacterial performance against gram-negative bacteria as compared to gram-positive ones. This was attributed to the complex membrane structure of gram-negative bacteria, having an additional protective outer membrane which preserves the inner membrane from active radical species. The composites are photo-stable during the disinfection process [175]. The disinfection property of GO-CdS nanocomposite was originated from the production of OH· radical due to irradiation with visible light. ZnO has a wide band gap of 3.27 eV and only UV light can activate it. the photo-response range of ZnO can be extend to visible region by its hybridizing with GO to fabricate the composite. Wu et al. reported synthesis of GO-ZnO composite via hydrothermal method for disinfection of E. coli K-12, by which inactivation of the bacteria took only 10 min, under solar light irradiation [176]. The mechanism of bacteria disinfection was attributed to strong interaction between GO and ZnO nanoparticles. GO enhanced production of ROS such · by facilitated the charge separation and acted as photosensitizer. The functionalization of graphene can well stabilize and disperse the nanocrystal without introducing organic surfactant. Further, because of being less expensive and biocompatible nature of TiO2, it has emerged as an important material for bacteria disinfection. Also, the combination of two materials, for example graphene and TiO2, will enhance the antibacterial property of the product. A few work has been reported on the antibacterial applications of the composites. Akhavan et al. prepared multiwall carbon nanotubes ZnO nanocomposite (MWCNT-ZnO) via the solgel method [177]. The functionalized and un-functionalized MWCNTZnO nanocomposites were used to study the antibacterial activity against E. coli. The optimum value for MWCNT content was 10 wt % in both the functionalized and un-functionalized composites. The functionalized nanocomposite disinfected 100% of the bacteria within 10 min of UV–vis light irradiation as compared to un-functionalized one which disinfected 63% of bacteria. The higher activity of functionalized composite was due to the easier charge transfer through Zn-O-C carbonaceous bond formed between Zn atom of ZnO and oxygen atom of carboxylic functional group present on functionalized MWCNTs. Further, the bactericidal property of composite increased as the MWCNT content was increased. Akhavan et al. synthesized heterojunction arrays of TiO2/multi-wall carbon nanotube (MWCNT) via dip-coating sol-gel method and annealed the product at 400 °C [178]. The composites were utilized for disinfection of E. coli bacteria, under visible light radiation. The antibacterial activity of TiO2/MWCNTs annealed at 400 °C was higher than that of the TiO2/MWCNTs annealed at 100 °C. The higher activity of TiO2/MWCNTs annealed at 400 °C is due to formation of TieOeC and TieC bonds at heterojunctions. The formation of such type of carbonaceous bond led to the extension of absorption towards visible region. The TiO2/MWCNTs heterojunctions disinfected 99.8% of bacteria within 120 min of visible light illumination. In the heterojunction, electron and holes migrated towards different directions and hence reduced the rate of recombination which, in turn, increased the rate of

whereas ZnO inhibits bacterial growth by photocatalytic production of H2O2 or through penetrating into cell envelope. Nourmohammadi and co-workers fabricated GO decorated ZnO composite via drop-casting and electrophoretic deposition (EPD) process, and rGO sheets reduction was performed within the composite via UV-assisted photocatalytic [171]. The reduced composite was then exploited for disinfection of E. coli bacteria using visible light radiation. Two methods used to fuse Graphene oxide sheets with ZnO nanowires (NWs) are electrophoretic deposition (EPD) and drop-casting methods. ZnO nanowires alone disinfect 58% of bacteria, whereas GO/ZnO composite showed strong antibacterial activity. Especially, GO/ZnO synthesized with EPD process disinfected 99.5% of bacteria, when was irradiated under visible light for 1 h. The GO/ZnO produced by EPD procedure exhibited, respectively 1.9 and 6.2 folds higher antibacterial property than drop casting prepared GO/ZnO composite and ZnO NWs, using visible light irradiation. This might be due to this reality that positively charged GO sheets effectively penetrate into ZnO NWs and form a net-like spider structure. This may facilitate charge exchange, and also provide easier interaction of bacteria with the sharp tip of NWs which provide better charge transfer to bacteria and thus inducing physical disruption of bacterial membrane. The ability of GO/ZnO composite to work under the visible light is attributable to the fact that Fermi level of rGO with zero band gap is positioned below conduction band of ZnO, as shown in Fig. 4 [172]. This acted as a bridge for electrons to jump from VB of ZnO to VB of rGO with lower energy. Thus the composite synthesized via EPD route is a UV or visible light photoexciting composite with a lower rate of recombination and enhanced disinfection activity compared to ZnO nanowires and GO/ZnO fabricated via drop-casting procedure. Akhavan and co-workers fabricated GO-TiO2 via chemical exfoliation process followed by post-annealing of composite at 400 °C [173]. In comparison to bare TiO2, the GO-TiO2 composite showed 25% more E. coli disinfection, under solar light. It was observed that GO/TiO2 composite exhibited enhanced antibacterial activity by a factor of 1.1, 1.7, 3.7, 7.5 as compared to Ag-TiO2/Ag/a-TiO2, Ag nanorods, Ag-SiO2, and TiO2, respectively. The reduced graphene platelets acted as an excellent sensitizer of TiO2 mediated materials and improved their efficiency in the photocatalytic process under solar light. GO behaved as an electron reservoir which concurrently increased the quantum efficacy of photocatalysis by reducing electron-hole recombination. The one-dimensional nanorods of TiO2 combined with GO (GO-TiO2 NRCs) via two-phase hydrothermal method have also been used as a bactericidal agent under solar radiation for photocatalytic degradation of E. coli bacteria [174]. The two-phase water-toluene interface has been confirmed by TEM, XRD, and XPS. The TiO2 nanorods/GO composite

Fig. 4. Band structure of ZnO and Fermi energy level of graphene (With permission from Elsevier, license number 4372040293254). 8

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formation of OH· radicals. Thus, a higher photocatalytic efficiency of the heterojunction was observed. Moreover, during heterojunction formation between TiO2 and other semiconductors/metals/CNTs, a space charge layer was generated near the interface ranging from several tens to hundred nanometres to equalize Fermi levels. This generated space charge layer helped the separation of the photogenerated electron-hole pairs and it also reduced the rate of recombination. Chang et al. prepared magnetic GO/TiO2 (MGO/TiO2) composite via a simple synthesis method [179]. The antibacterial performance of MGO-TiO2 composite was investigated against E. coli bacteria. Depending upon the TiO2 content three kinds of composite materials were generated. MGO-TiO2-1, MGO-TiO2-2 and MGO-TiO2-3 in which the TiO2 content was respectively 4.79%, 6.02%, and 8.21%. The optimal value of Ti contents in MGO-TiO2 composite was 6.02% which offered the highest antibacterial activity. At the minimal concentration of 180 mg/L of the MGO-TiO2-2 composite, 30 min irradiation with solar light almost caused complete disinfection of E. coli. The MGO-TiO2-2 composite exhibited 5 fold enhanced antibacterial property as compared to pure TiO2. In addition, the pure magnetic iron nanoparticles also showed capability in disinfection of bacteria. Thus, the antibacterial property of the MGO-TiO2 composites is due to all the three components present in their structure, i.e. TiO2, GO, and magnetic nanoparticles. Although the presence of magnetic nanoparticles causes easy separation of the composite, the superior activity of composites was due to the efficient charge transfer from TiO2 to GO which suppress the rate of recombination of photo-generated electron-hole pair, and an additional antibacterial property originates from the presence of GO and magnetic nanoparticles and their combinational effects. Also, the studies showed that the bacterial disinfection rate increased with increase in TiO2 content up to a certain optimal value, since more TiO2 content generate more ·OH radicals. Further increase in TiO2 content decreases the antibacterial performance. This was explained as an increased in TiO2 content covered the active site of GO which, in turn, decreased the bacterial disinfection rate. Additionally, the researchers discussed the influence of inorganic ions on the antibacterial property of the composites, using visible light, and their observations showed that the addition of inorganic ions causes a decrease in the antibacterial efficiency. The inorganic ions like HCO3− and HPO42− reduced the antibacterial performance to a greater extent as compared to other ions like K+, Na+, Cl-, SO42−, and NO3−, because the former ions could react with ·OH radical which is a strong oxidant for disinfection of bacteria. There are countless approaches to assess the antibacterial activity of the nanocomposites such as, optical density method, colony forming unit method (CFU), minimum inhibition concentration method (MIC), disk diffusion method, micro dilution method and well diffusion method etc. Optical density method directly assesses the turbidity of bacterial culture using spectrophotometer. For this, 10 mL of sterile nutrient broth with known concentration of nanocomposites were homogeneously dispersed in 50 mL of test tube. This solution is then seeded with bacterial cells and optical density is determined at 600 nm before and after incubation [152]. CFU method is used to determine the number of viable bacterial cell which have the ability to multiply. In CFU method 106-107 CFU/mL bacterial culture with optimized concentration of nanocomposites were plated on agar plates followed by incubation at 37 °C for 24 h [175]. After 24 h CFU were calculated either manually or by using click counter and automated colony counter using image processing. The MIC is the lowest concentration at which the nanocomposites displays antimicrobial property. In this method 10 mL of saline water were inoculated with bacterial culture and diluted up to 108 CFU/mL. To this, different concentration of nanocomposites was added and the bacterial culture were then incubated for 4 h at 37 °C. After incubation 0.1 ml of each bacterial solution containing different concentration of nanocomposites were plated on sterile nutrient agar plate. Again the agar plates were incubated overnight at 37 °C followed by colony

counting to determine the MIC [180]. In disk diffusion method (KirbyBauer method) all the disks were autoclaved and the experiment were carried out in triplicate. Bacteria were grown overnight at 37 °C in nutrient broth, the bacterial suspension then diluted to approximately108 CFU/mL which then plated on nutrient agar plates. The equal sized nanoparticle/nanocomposites loaded paper disks were then placed gently on each petri dish. The plates then incubated and the zone of inhibition were measured [181]. Another method to assess the antibacterial activity is micro dilution method where MIC is determined. For this approximately 105 CFU/mL were inoculated in 96 well plates containing different concentration of nanocomposites. The final volume of each micro well is adjusted to 0.2 mL, the plates then incubated at 37 °C for 24 h [182]. 6.2. Graphene derivatives-based ternary metal oxide nanocomposites Zhang and his co-workers fabricated Bi2MoO6-RGO nanoplates via hydrothermal process [180]. The resultant composite exhibited an enhanced photocatalytic bacterial disinfection, compared to pristine Bi2MoO6, against E. coli K-12 bacteria under visible light radiation. The fabricated composite was characterized by FE-SEM, TEM, XRD, and EIS (electrochemical impedance spectroscopy). The photogenerated holes and the ROS like hydroxyl radical, hydrogen peroxide, superoxide anion radical, etc. were the principal responsible active species for destruction of bacteria via the photocatalytic process. The bacteria disinfection efficiency was mainly depended upon amount and recombination rate of generated electrons and holes. In Bi2MoO6-RGO composite, Bi2MoO6 possessed extraordinary charge carrier mobility which enables charge separation and, therefore, enhanced the bacteria disinfection performance of composite. Also, graphene showed excellent electrical conductivity which made it exceptionally good electron transporter, thus leading to suppressed charge recombination and increased overall bacteria disinfection efficiency. Liu et al. fabricated GO-TiO2-Ag as a multifunction composite via a facile two-phase process by using 0D silver (Ag) nanoparticles, 1D TiO2 nanorods, and 2D GO sheets The multifunction composite exhibited enhanced bacteria disinfection activity as compared to GO-Ag and GOTiO2 composites. Its antibacterial property was measured against E. coli bacteria under solar light radiation. It was observed that antibacterial activity of nanocomposite was related to the amount of Ag in the nanocomposite, so that the increase in the Ag content caused an increase in the rate of bacterial disinfection. The antibacterial abilities of GOTiO2, GO-Ag, GO-TiO2-Ag-1, GO-TiO2-Ag-2, GO-TiO2-Ag-3 were observed in the presence or absence of solar radiation. The Ag amount in GO-TiO2-Ag-1, GO-TiO2-Ag-2, and GO-TiO2-Ag-3 was 1.9, 4.68, and 9.72 μg/mL, respectively. Using visible light, GO-TiO2 nanorode, GOTiO2-Ag-1, and GO-TiO2-Ag-3 respectively showed 5.1, 7.15, and 8.24 log decrease in bacterial cell within 120 min with bacterial disinfection rate constant of 0.0411 min−1, 0.0596 min−1 and 0.0716 min−1, respectively, whereas for GO-TiO2-Ag-2 the complete killing of 8.24 log within 90 min of solar irradiation was observed with rate constant of 0.0985 min−1. For Go-Ag composite the rate of bacterial disinfection was not dependent on the solar irradiation because the nanocomposite cannot act as a photocatalyst. The above data clearly indicated that GOTiO2-Ag-2 composite exhibited the utmost photocatalytic antibacterial action, among the others, the reason behind which may be the decreased rate of recombination by Ag nanoparticles [181]. Gao and co-workers fabricated a multi-component sulfonated GOZnO-Ag (SGO-ZnO-Ag) composite via hydrothermal technique and a polyol-reduction procedure [182]. Under visible light illumination, SGO-ZnO-Ag composite disinfected 99% of E. coli within 20 min. SGOZnO-Ag composite showed enhanced antibacterial activity as compared to SGO-ZnO, ZnO-Ag and ZnO. The enhanced efficacy of composite was attributed to synergistic effect of ZnO nanorods, SGO sheets, and Ag nanoparticles. This effect copmprise of resonance energy transfer from Ag to ZnO, extension of life time of charge carriers by effective charge 9

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6.3. Graphene derivatives- based complex metal oxide nanocomposites

separation, increased light absorption, and effective scattering of light due to hierarchical “treelike” structure. In addition to TiO2, ZnO, CdS, etc., Bi2WO6 has also attracted much attention due to the narrow band gap (˜2.8 eV), chemical inertness, photostability and biocompatibility. Chen et al. prepared Bi2WO6/GO hybrids by cetyltrimethylammonium bromide (CTAM) assisted hydrothermal method which exhibited a high antibacterial performance [183]. CTAB is a cationic surfactant and stabilizer which couple GO with Bi2WO6 and is also helpful in the shape-controlled synthesis of nanostructures. The antimicrobial property of the fabricated composite was studied on the gram-positive bacillus bacteria and was assessed by the CFU method under sunlight. The optimal sample where no colony formation found under sunlight for 2 h has 2.0-BWO/15 mL GO hybrids. The prevention of bacterial cell growth was due to the photo-generated oxy-radicals which disrupt the bacterial cell. The rate of recombination of e− and h+ pair assessed by fluorescence spectra. The bigger the probability of recombination causes a higher fluorescence intensity. The BWO/GO hybrid showed low fluorescence intensity which confirms the inhibition of recombination rate. Yanag et al. prepared composite (P25/Ag3PO4/GO) via electrostatically focused assembly and ion-exchange technique [184]. The ternary composite proficiently harvested solar light and showed exceptional bactericidal behavior. The bacteria used to investigate the antibacterial action are P. aeruginosa, S. typhi, S. aureus and E. coli. The P25/Ag3PO4/GO composite acted as a broad spectra antibacterial agent by exhibiting bactericidal activity towards these pathogenic bacteria. The MIC (MBC) toward the studied pathogenic bacteria, i.e. E. coli, S. aureus, S. typhi and P. aeruginosa were respectively 100(100), 6.25(12.5), 6.25(6.25), 6.25(6.25), under solar light irradiation. The bactericidal activity of composite was attributed to synergistic effect of intrinsic bactericidal action of Ag+ ion and photo-induced bacterial disinfection by P25 and Ag3PO4 on GO sheets which have high surface area. There are a range of mechanisms via which antimicrobial agent interacted with bacterial cell. In P25/Ag3PO4/GO composite, antibacterial mechanism was investigated by radical capture experiment. Under visible light irradiation, e− moves from CB of silver orthophosphate (Ag3PO4) to GO surface and the h+ moves from VB of Ag3PO4 to P25 (Fig. 5) [184]. This separation of photogenerated e− and h+ decreased the rate of recombination and enhanced the photo-stability of the composite. In addition to direct oxidation of bacteria by photogenerated hole, the reaction of adsorbed oxygen on GO surface with electrons and its reduction to ROS, principally superoxide radicals (O2·−), efficiently degraded the bacteria.

Wang and his co-workers enfolded rGO and C3N4 (CN) sheets on cyclooctasulfur (α- S8) crystal for the fabrication of metal free heterojunction catalyst [185]. They fabricated two different structure by wrapping CN and rGO in various different orders. The prepared structures were CNRGOS8 and rGOCNS8, as shown in Fig. 6. Both structures showed antibacterial activity under visible light radiation in diverse conditions. In aerobic conditions, the CNRGOS8 composite exhibited greater bacteria disinfection than rGOCNS8 whereas, rGOCNS8 revealed enhanced bacteria disinfection than CNRGOS8 in anaerobic conditions. The electronic states of composite surface were analyzed by XPS, and morphology of CN sheets was examined by TEM technique (Fig. 7). Antibacterial activity of composites was examined against E. coli K-12 bacteria. Various mechanisms for bacterial disinfection were suggested under aerobic and anaerobic conditions. The oxidative bacteria disinfection in aerobic condition was confirmed by enzyme catalase (CAT) activity in bacterial cell which acted as a defensive tool over oxidative stress, by converting H2O2 to O2 and H2O. The reductive bacteria disinfection under anaerobic condition was confirmed by fatty acid distruction by coenzyme-A (CoA) which is responsible for electron transporting between bacterial cell and the outer layer of graphene. Therefore, rGOCNS8 displayed a higher efficiency for reductive bacteria disinfection than the CNRGOS8 composite. Yi and coworkers found that silver orthophosphate (Ag3PO4) can achieve almost 90% of quantum efficiency for O2 evolution from H2O, under solar light exposure [186]. Due to higher photo-oxidative abilities of Ag3PO4, it has been considered as a favorable choice for practical application as compared to AgX, TiO2, BiVO4, etc. Nonetheless, numerous limitations of Ag3PO4 limit its practical application for waste water purification, like instability through illumination and easy decomposition and photo-reduction to feebly active Ag which definitely prohibit from visible light absorption and diminish its photo efficiency. Further, Ag3PO4 possess uneven microstructure, which is not soluble in most of solvents. On the other part, GO is a favorable choice for being used in fabrication of a composite photocatalyst because of its solubility in solvents and negatively charged functional sites on surface. Thus, hybridizing Ag3PO4 with GO sheets elevated the absorption of visible light and improved photo-efficiency, as GO can confer facilitation of charge transfer and suppression of the recombination of the photogenerated electron-hole pair [187,188]. Yang et al. fabricated TiO2/Ag3PO4/GO heterojunction composite via combining ion exchange and hydrothermal technique [189]. The synthesized composite displayed exceptional bactericidal performance against S. aureus, S. typhi, E. coli, P. aeruginosa, B. pumilus, and B. subtilis. The TiO2/Ag3PO4/GO composite revealed better antibacterial effect than bare TiO2, Ag3PO4, and two-phase composite. Because of synergistic effect of Ag3PO4 and TiO2, the composite emerges as a promising material for water disinfection. Liu et al. fabricated GO with Ag3PO4 via ion exchange procedure of CH3COOH and Na2HPO4 in presence of GO sheets [190]. The composite exhibited a stronger absorbance and higher antibacterial activity in the visible light region, as compared to the Ag3PO4. The antibacterial property has been studied against E. coli as a pathogenic bacterium. Since the Ag-based materials are efficient biocides against different kind of fungi and bacteria, the possible disinfectant ability of the GO-Ag3PO4 was ascribed to its intrinsic antibacterial property and visible light-driven photocatalytic disinfection property. 7. Mechanism and kinetic modeling of photocatalytic disinfection Owing to low cytotoxicity, graphene, graphene oxide, and reduced graphene are always preferred over carbon nanotubes (CNTs) and fullerene as antibacterial materials. It is due to the fact that graphene and its derivatives can be made metal free easily by using modified Hummer’s methods. However, CNTs are produced by carbon containing

Fig. 5. Photocatalytic mechanism of P25/Ag3PO/GO heterojunction for the degradation of Rhodamine B and bacterial disinfection under visible light irradiation (With permission from Elsevier, license number 4372040293254). 10

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Fig. 6. Photocatalytic mechanism of (a) CNRGOS8 and (b) RGOCNS8 in aerobic condition, (c) CNRGOS8 and (d) RGOCNS8 in anaerobic condition for bacterial disinfection under visible light illumination (With permission from Elsevier, license number 4372350298670).

Fig. 7. TEM images of E. coli K-12 photocatalytically treated with the natural sphalerite under VL irradiation adding quadruple scavengers (KI, isopropanol, Fe(II) and TEMPOL). (A) 0 h, (B) 6 h, (C) 12 h, and (D) 30 h (With permission from American Chemical Society).

carboxylic acid, hydroxyl, ether, etc. The reduction of GO results in formation of reduced graphene resembling pristine graphene. GO with functionalized surface can cause cracking of bacterial cell walls faster as compared as graphene and rGO. The sp3 hybridized carbon atom on the surface of GO is responsible for their high degree of defects as compared to rGO where the percentage of sp3 hybridized carbon atom decreases significantly. Both graphene and rGO sheets tend to stack face to face causing agglomeration (Fig. 8). Among these carbonaceous material the higher bactericidal effect was displayed by GO as compared to rGO followed by graphene against E. coli [147]. Dallavalle et al. studied effect of size on antibacterial property and concluded that smaller sized graphene sheet ruptured the phospholipid layer of bacterial cell membrane easily as compared to large graphene sheet [192]. Further, the surface area of the nanomaterial is controlled by roughness, the

gases in the presence of metallic nano catalysts, and only 50 ppm of residual metallic impurities can cause significant cytotoxicity. The antibacterial activity of graphene-based materials is ascribed to their dispersibility, size, surface functional groups and oxidizing capacity which significantly affect the cell deposition chance. All the carbonaceous materials tends to increase the specific surface area of nanoparticles, inhibit leaching, lower the rate of recombination and, hence, increase photostability. The antibacterial property of nanomaterials is evaluated by its shape, size, stability, surface functionalization, hydrophilicity, and dispersibility [191]. The pristine graphene with sp2 carbon domain has limited or no oxygen functional groups. However, GO has oxygen functional groups on the basal planes and edges containing sp2/sp3 hybridized carbon atoms. Depending on the method of preparation, GO possesses oxygen functionalities such as 11

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Fig. 8. Different functional groups and stacking of sheets present on (a) Pristine graphene (b) Graphene oxide (c) Reduced graphene oxide (With permission from Royal Society of Chemistry).

cell. Beside these two enzymes, other enzymes such as peroxidase, glutathione, and carotenoid might also be responsible for preserving the bacteria against oxidative stress. These enzymes protect bacterial cell from oxidative stress, so that higher CAT and SOD level means a higher activity of bacterial cell against ROS. CAT enzyme catalyze the decomposition of H2O2 to H2O and O2, whereas SOD is a metallo-enzyme which catalyzes the O2·− dismutation into H2O2 and O2. These enzymes can convert ROS into less or non-harmful species [197]. During the process of photocatalytic disinfection, firstly, the enzyme level increases, which indicates the defensive mechanism of bacteria against ROS, and then decreases with time, leading to accelerating of the accumulation of ROSs in the bacterial cell which cause the loss of cells viability. The destruction of the defence system against oxidative stress results in proteins fragmentation, release of ions, and production of protein carbonyl derivatives by the cells [198]. The bacterial destruction through photocatalytic disinfection begins from cell envelope comprise of cell wall and cell membrane and then the destruction of intracellular components. Disruption of cell membrane results in discharge of membrane potential and leakage of K+ ion involved in the synthesis of protein and regulating of polysomic contents [199]. The diffusion method is employed for assessing antibacterial activity which is indicated by formation of a zone of inhibition. A zone of inhibition is the area on an agar plate where the growth of bacteria is usually inhibited by antibiotics. The largeness of area of the zone of inhibition is a measure for the effectiveness of the antibiotic. The biggest challenge in the present time is the absence of sufficient reliable models for describing the inactivation process, which are vital for achieving to the proper optimization of photocatalytic disinfection. Thus, to improve photocatalytic disinfection methods for efficient water purification, improved methodologies must be adopted for realization and determination of the influence of certain parameters such as the rate of inactivation, catalyst dosage, and light intensity. The photocatalytic disinfection mechanism is still under debate. Most of the mechanism proposed so far have declared the cell membrane destruction as an important stage in bacterial disinfection. The major part of bacterial cell structure is made up of organic molecules (96%) such as polysaccharide, protein, nucleic acid (DNA and RNA), lipid, and lipopolysaccharide. A 3% part includes monomers such as sugars, amino acids, nucleotides, etc., and the remaining 1% is inorganic ions. Thus photocatalyst can easily inactivate microorganism by oxidizing most of the organic material that it is made up of [200,201]. The general mechanism of photocatalytic disinfection can be understood in the belowdiscussed steps, and both electron and hole are responsible for the generation of oxidative species. Initially, light absorption leads to the generation of electron-hole pair as represented by Eq. (21).

increased surface roughness enhances the bacterial cell adhesion which in turn increased the membrane permeability. The high roughness and ridges on the surface can easily cause the cracking of bacterial cell wall [193]. Moreover, GO also exhibit a greater dispersibility owing to its hydrophilic functional group present on their surface, which is also one of the reason assigned to higher antibacterial activity of GO as compared to rGO. The antibacterial activity of graphene containing materials is ascribed to their dispersibility, size, and oxidizing ability which significantly affect the cell deposition chance. Vecitis et al. suggested a cytotoxicity mechanism for SWCNTs, which is composed of three step, and believed that the same antibacterial mechanism is applicable in the case of graphene-based materials [194]. The first step is bacterial deposition or adhesion onto the graphene-based material, i.e. the direct physical contact between the microorganism and the graphene-based material. In the second step, the interaction between the graphenebased material and the microorganism become intimate and membrane disruptive, and cause membrane stress. In the last step, a precise microbial process would be interrupted by troubling or oxidization of cellular structure or a vital component. The extend of dispersibility in a graphene-based material is related to kind of functional groups existing on surface, so that the incorporation of hydroxyl, carboxyl, and/or epoxy groups increases the dispersibility of graphene sheets. Besides pristine graphene and GO, graphene-based composites have also shown a great antibacterial activity. It has been proved that large graphene sheets (> 0.4 μm2) exhibit higher antimicrobial activity, because they can cover the surface of bacterial cell extensively which results in blocking all available active sites and preventing cell proliferation. Further, well-dispersed graphene oxide sheets show better antimicrobial activity than the aggregated ones, because the former provides a larger surface area to be exposed [195]. Graphene oxide weakens the microorganism metabolic activity which reduces the oxygen intake and decreases the amount of biological oxygen demand (BODs) and ultimately causes cell death. Further, it is believed that graphene and its derivatives involve superoxide anion-independent oxidative stress. Another possible mechanism of antibacterial activity of graphene is the charge transfer between graphene or GO sheet and the bacteria. Since the edges of GO are negatively charged, it can act as a good electron acceptor to influence the damage of cell membrane. The prokaryotic cells are more susceptible to graphene than the eukaryotic cells, since prokaryotic cells do not possess nuclear membrane DNA structure and can be easily damaged by electrons released from graphene [196]. The bacterial disinfection caused by oxidative stress can be indicated by two enzymes, intracellular antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD), present in the bacterial 12

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TiO2 + hv → e−cb (TiO2) + h+vb (TiO2) h+vb

determining factor on the rate of disinfection of bacteria. The phospholipid bilayer and LPS layer are composed of fatty acids which are more vulnerable to peroxidation and, thus, lipids peroxidation is of much probability. The polyunsaturated fatty acids (PUFA) are highly oxygen sensitive molecules in nature and are main target for oxidative radical attack. ROS can be either generated through the photocatalytic system or naturally occur in the biological system. These ROS are responsible for destructing important cells components like proteins, nucleic acids, lipids, etc. The lipids peroxidation can be assessed by the production of malondialdehyde (MDA). Increase in MDA concentration increase the rate of cell disinfection [208]. The intracellular target sites are in cytoplasm which include DNA, RNA, enzymes, ribosome, etc. Each of these substances is of importance for proper cell functioning, and all of them are prone to oxidative stress. On oxidation, DNA molecules undergo fragmentation which is responsible for the bacterial disinfection. Thus a number different mechanisms and models has been suggested for photocatalytic water disinfection, and some of them are discussed below. Some of the disinfection models are Chick, ChickWatson, delayed Chick-Watson, Hom, and Kinetic power law models. Chick's model is of the simplest formulation and has vast applicability where chemical disinfection agents are used, like ozone, chlorine, chloramines, hydrogen peroxide, etc. [207]. Chick's law expresses the rate of first-order reaction as:

(21)

e−cb

is the valence band hole and is the conduction band where electron. The generated hole is believed to react with H2O molecule or OH ions to produce reactive OH radicals as mentioned in Eqs. (22) and (23). These radicals behave as a primary oxidant in the photocatalysis system [202,203].

TiIV|H2 O+ h+vb → TiIV|OH⋅

(22)

TiIV|OH− + h+vb → TiIV|OH⋅

(23)

In a typical system, for instance, the photo-generated electron in the conduction band reduces the TiIV to Ti III which in turn reduces the adsorbed oxygen to ROS such as superoxide radical as illustrated in Eqs. (24) and (25).

TiIV + e−cb ↔ TiIII

(24)

TiIII + O2 ↔ TiIV + O⋅− 2

(25)

These ROS can further oxidize organic materials like polysaccharides, nucleic acids, lipids and proteins which constitute bacterial cell and, subsequently, leads to the cell death. There are two targeted sites which are affected during the photocatalytic inactivation: extracellular target sites and intracellular target sites. The extracellular target sites include the sites located on the membrane and wall of the cells which are made up of peptidoglycan, present in gram positive and gram negative bacteria. The extracellular target sites are peptidoglycan layer a cross-linked polysaccharide matrix that encloses the cell. It maintains the shape, rigidity and internal pressure of the cell. Nearly 90% of cell wall in gram-positive bacteria is made up of peptidoglycan layer, whereas in gram-negative it contributes only 10% to the cell wall as shown in Fig. 9 [204]. Peptidoglycan layer is very porous in nature and can allow particle of approximately 2 nm in size to cross through [205]. The layer is porous enough to permit the diffusion of oxidative species up to the inner membrane. However, it has been shown that the rate of disinfection is independent of thickness of peptidoglycan layer [206,207]. Instead, the density and complexity of the cell wall as a whole is a

(26)

r= −kN

But the disinfection of various microbes deviates from first-order kinetics [209]. The Chick- Watson model has combined concentrationtime (CT) product concept whit Chick's law. By taking c and n as constants, Chick- Watson model can be given as: ' N = e−k t No

(27)

'

where, k is pseudo-kinetic constant and can be given as:

k ' = − k[c]n

(28)

In delayed Chick-Watson model, a time lag parameter (tlag) is used to estimate an early lag phase in disinfection process [204].

Fig. 9. Pictorial representation of outer layers of a bacteria (a) Gram-positive (b) Gram-negative (With permission from Elsevier, license number 4373031151908). 13

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fort≤ tlag N ⎧1 = −k′(t−t ) lag ⎨e No for t> tlag ⎩

(TBA) assay, chromatography, and other suitable techniques. The loss of viability in the cells is the function of rate of lipid peroxidation and, therefore, higher disinfection is observed with the increase in the rate of lipid peroxidation [210].

(29)

The Hom model explains the effect of disinfectant on microorganism by representing a differential expression for time-concentration relationship. The equation can be given as:

dN = −kNtmcn dt

8. Conclusion and prospects Graphene-based composites have an enormous potential to be employed as antibacterial agents in the area of wastewater remediation. The exceptional chemical and physical features of graphene have led to conduction of unrivaled research works in advanced water disinfectant technologies. The application of graphene nanocomposites for the removal of pathogens provides greater sensitivity, lower cost, and effectiveness in water purification. Additionally, graphene acts as an antibacterial agent by damaging metabolic pathway of the bacterial cell and, also, by disruption of the bacterial membrane via its sharp edges. The discussed literature proposes that different synthesized binary, ternary, and complex metal oxide-based graphene and graphene derivatives-based nanocomposites exhibit an enhanced antibacterial property under solar irradiation. By the present time, metal oxide/graphene composites have appeared as capable candidates for wastewater disinfection and other applications. However, extensive progress in the field of photocatalytic disinfection by metal oxide/graphene composites has not been attained yet. Moreover, precise kinetic information is required for effective designing of photocatalytic disinfection system. Also, understanding the elementary reaction behind the disinfection process is important in development of a correct mechanistic model. Therefore, a comprehensive study about the mechanism of action and how kinetics of lipid peroxidation is related to death kinetics need to be explored. Moreover, the large-scale application of photocatalytic disinfectant agents of metal oxide/graphene nanocomposites has remained unaddressed. Thus, further development and proper optimization are required to enlarge the prospect of graphene nanocomposites as photocatalytic disinfectant agents. This review can be a contribution to understand mechanistic model of photocatalytic and disinfection properties of metal oxide/graphene nanocomposites for water mitigation.

(30)

The Hom model reduces to Chick's Law, following first-order kinetics, where reaction is zero-order w.r.t concentration and time. However, when m ≠ 0, n ≠ 0 and cn t = k ' , the following equation can be followed:

ln

N −Kk 'tm = No m

(31)

Another model is kinetic power law model in which rate of disinfection is not supposed to follow first-order kinetic w.r.t. microbial concentration and can be given as: (32)

r= − kN xcn

By integrating, Eq. (32) is reduced to the following expression which gives the survival ratio of the organism.

ln

N −1 = ln[1 + (x − 1) kc ntNox − 1] No x−1

(33)

The main limitations of the above-discussed models are: their empirical nature, and not being purely logical, because they are not formed on basic physical and chemical methods of photocatalytic disinfection. Moreover, despite of heterogeneous nature of photocatalytic processes, the models have been assembled on assuming that the process a homogenous chemical reaction. However, mechanistic models usually are chosen over empirical model even though they are clumsier from a computational point of view. But they are more reliable than the empirical model as they can be applied to complex situations that arise practically. The mechanistic models include lipid peroxidation mechanism, microbe-catalyst interaction model, and series-event model, some of which are discussed below. The lipid peroxidation mechanism, particularly in the case of PUFA, starts from extracellular target sites in the presence of hydroxyl radical generated in the extracellular environment. The oxidation proceeds via a free radical mechanism which includes three steps as given below: chain initiation Eq. (34), chain propagation Eq. (35) and chain termination Eq. (36). Ri

Ti|OH⋅ + LH⟶Ti|H2 O+ L⋅ Ko

L⋅ + O2 ⟶LOO⋅

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(34) (35)

Kp

LOO⋅ + LH⟶LOOH + L⋅ Kt

2 LOO⋅ ⟶products + O2

(36) (37)

In the initiation step, hydroxyl radicals present on the catalyst surface abstract hydrogen from the lipids (LH) and produce carbon free radical, L·, as indicated by the first step. The rate of oxygen consumption during lipid peroxidation can be given as:

−

d [O2 ] R = Kp [LH] ⎛ i ⎞ 1/2 dt ⎝ 2kt ⎠ ⎜

⎟

(38)

where [LH] is concentration of oxidizable intracellular lipids, kp is propagation rate constant, Ri is the initial rate, and 2kt is termination rate constant. The rate of reaction of lipid with hydroxyl radical is governed by the following equation:

Ri = ki [OH·][LH]

(39)

where ki is the initial rate constant of the reaction. The lipid peroxidation products, like MDA, can be assessed by thiobarbituric acid 14

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[34]

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