Why ionic liquids coated ZnO nanocomposites emerging as environmental remediates: Enhanced photo-oxidation of 4-nitroaniline and encouraged antibacterial behavior

https://doi.org/10.1016/j.molliq.2020.114107Get rights and content

Highlights

  • Benzimidzole based Ionic Liquid (ILs) coated ZnO NPs emerging as photocatalyst for the oxidation of nitroaniline.

  • The influence of ionic liquids on the ZnO crystalline phase, and also on the morphology

  • XPS peaks exhibit a photoemission from the benzimidazole nitrogen coated over ZnO NPs.

  • Encouraging antibacterial properties from ZnO@ILs and their illustration

Abstract

In this paper, benzimidazole based ionic liquid (IL) was prepared, and then coated on ZnO to yield ZnO@IL which was then characterized systematically by different analytical methods such as X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS) before exploring as a photo-catalyst for the oxidation of 4-nitroaniline (4-NA). In XRD, for ZnO@IL, the plane [100] has been shifted to higher angle at 64° in due to the influence of IL on the ZnO surface. The intensity and shape of the XPS peaks (appearing between 404 and 398 eV) originated from the IL's imidazole nitrogens indicate the existence of a sizable photoemission due to the interaction of NHCs with the electrophilic surface of ZnO. The catalytic oxidation of 4-NA (0.1 M) by ZnO@IL under UV light reveals a considerable degradation, which follows a first order kinetics. The formation of intermediates was analyzed by HPLC-MS, and proposed a possible mechanism for the oxidation. At low pH = 4, a greater oxidation rate (k = 2.01 × 10−4 mM.s−1) was obtained than that observed in higher pH of 10 (k = 0.26 × 10−4 mM.s−1). Additionally, ZnO@IL shows an antibacterial behavior exhibiting a significant microbial inhibition about 45%, and observed a considerable decrease in the cell growth if ZnO@IL presents in the culture medium.

Introduction

Research on ionic liquids (ILs) has been recently attracted because of interesting physico-chemical properties such as (i) a very low vapor pressure due to a low volatility, (ii) high thermal stability (physical, and chemical inertness), (iii) non-flammability character, and (iv) high ionic conductivity [[1], [2], [3], [4]]. ILs, organic molten salts, which are being considered to be an effective green solvents, alternative to traditional organic solvents [[5], [6], [7]]. They can be used as electrolytes [[8], [9], [10]], chemical solvents [[11], [12], [13]], and catalysts [14,15], chemical adsorption [16], membrane separation [17,18], membrane modification [19], gel [[20], [21], [22], [23]], surfactant [24], organic synthesis/polymerization reactions [25], or epoxy-/oxidation reactions [26,27]. In particular, the use of ILs as electrolytes in electrochemical batteries at high temperature has been gotten a great attention as it prevents significantly the crystallization of ions due to their low melting point, and the widening of the range of electrochemical potential. Many studies have been recently focused on the use of hydrophobic associated hydrogel electrolyte in supercapacitors [28], and also the fabrication of new electrodes based on nanocomposite/graphene quantum dots/ionic liquid/multiwall carbon nanotubes/polyaniline for electrochemical interest [29]. Importantly, in the electrochemical applications, new electrodes based on the ionic liquids have been fabricated, and used as electrochemical sensors for the detection of different organic moieties; it has been shown that there is a significant improvement in the electrochemical sensing capabilities due to the synergetic effect generated between ionic liquid with metal oxide nanoparticles. Moreover, as it is known that high surface area and good electrical conductivity of nanoparticles would convalesce the current density of the electrode. So, the combination of nanoparticles with ionic liquid which has an ionic nature to have a good adsorption character in order to enhance the oxidation current, can improve the sensing ability such as high selectivity and sensitivity due to the generation of good electrochemical windows. Therefore, several new electrodes were fabricated based on the ILs, and employed electrochemically in different applications; for example, the detection of benserazide and levodopa by using newly designed electrode (CPE/nMBZBr/NiO-NPs) containing carbon paste with NiO NPs having n-methyl-3-butylimidazolium bromide [30]; the sensing of epinine by carbon paste electrode modified with CuO NPs mixed with n-hexyl-3-methylimidazolium hexafluorophosphate (CPE/CuO-NPs/HMIPF6) in serum and urine samples [31], and the recognition of vanillin by carbon paste electrode (CPE) modified with NiFe₂O₄ NPs binded with 1-hexyl-3-methylimidazolium chloride [32] have been reported. Additionally, the determination of tert-butylhydroxyanisole in the presence of kojic acid by CPE/MgO-NPs-/M3BIBr (carbon paste electrode with MgO NPs and n-methyl-3-butylimidazolium bromide) [33] were performed. Furthermore, the sensing of carmoisine in the presence of tartrazine in dried fruit and soft drink samples was carried out by using another electrode system (CPE/1-M-3BIBr/NiO/CNTs) having carbon paste mixture of NiO/CNTs containing 1-methyl-3-butylimidazolium bromide [34], and also N-hydroxysuccinimide by PtNPs/POM/2D-hBN (electrode designed with carbon paste with platinum nanoparticles/polyoxometalate/two-dimensional hexagonal boron nitride nanosheets/1-hexyl-3-methylimidazolium chloride) [35] have been reported.

In addition, it was shown the elimination of heavy metal ions efficiently using ILs [36,37] because of their ionic character, and in the same way, it was found the removal of VOCs [38,39] by ILs without emission of greenhouse gases [[40], [41], [42]]. Nevertheless, there are also some disadvantages in the usage of ILs from the environmental points of view because of their high stability, strong ionic character, non-biodegradable nature; eventually, these factors limit their usages in the industrial applications [43,44]. So, the researchers are now exploring a way to minimize their hazard nature, and to discover new type of ILs possessing the remarkable properties; thereupon, it has constantly gained an attention to focus on the choice of suitable cation and anion in the ILs. It has been illustrated that the properties of the ILs can be tuned effectively by altering (i) the nature of cation/anions with improving the characteristic of acidity/basicity, (ii) viscosity, and (iii) (im) miscibility with other solvents. After considering above points, several ILs have been designed by contemplating different combinations of cations, anions as well as altering carbon chains length [6]. Owing to the structural composition of the ILs including the nature of cation, and its alkyl chain length attached to aromatic ring can easily influence the toxicity [[45], [46], [47]]. It means that the toxicity of ILs is closely related to the length of the carbon chains (i.e. the toxicity is being increased if the carbon chain attached to ring is lengthy) [2,48]. As it can be seen that the toxic nature of 1-alkyl-3-methyl imidazole bromide ILs [Cnmim]Br (n = 2,4,6,10,12) was reported to be increased if alkyl chain length is augmented from [C2mim]Br to [C10mim]Br [49], despite of the fact that the types of cations and anions that could also influence the toxicity of ILs [50].

In literature, imidazolium-based ionic liquids (ILs) are significantly studied [2,[51], [52], [53]]; namely, ILs of imidazolium, pyridinium and pyrrolidinium cations and simple anions (trifluoromethanesulfonate, hexafluorophosphate, tetrafluoroborate and bis(trifluoromethane-sulphonyl)imide), and showed the extraction of chemical compounds by using ILs as non-conventional solvents, where the size of cation and anion has been played a crucial role in determining the efficiency. Exemplarily, the extraction of pro-anthocyanidins from grape seeds in aqueous phase [54], and the dissolution of cellulose in ionic liquid/water mixtures [55] were employed successfully. This manifests that the greater surface area of larger cations or anions could increase the adsorption character of ILs through van der Waals forces. This is the reason why inexpensive ILs based on amino acids/amides are being contemplated for the catalytic applications [56]. Thus, amphibian-inspired amino acid related ILs which have high water permeability were used to treat pigment wastes; in the same way, an acid functionalized IL was considered as eco-friendly catalyst in the following reactions such as polyoxymethylene dimethyl ethers [57], and also in dehydrochlorination [58], besides the use in sensing of cyanide ions [59]. Furthermore, the conducive nature of carbon dots (CPDs) on the surface of ILs such as PbBiO2Br material has been proved by the existence of hydrogen bond, and showed interesting properties because of coulomb force that persists between them [39]. Thus, magnetite/graphene oxide nanocomposite (rGO–Fe3O4) was efficiently employed as an adsorbent for the removal of phenazopyridine residues from water samples [60]. Although metal oxides such as TiO2 and ZnO have been considered as active photocatalysts for the oxidation of different contaminants [[61], [62], [63], [64], [65]], the mixture of these semiconductors with ILs to deem as photocatalyst is limited even though there are so many works related to the hybrid nature of different metal oxides that exhibits unique properties. Particularly, the importance of ZnO is growing rapidly in electrochemical and photochemical applications due to its friendly nature in both biochemically and environmentally besides to its low cost and large availability. Thus, ionic liquid based ZnO NPs were successfully employed as electrochemical biosensors for the detection of epinephrine by employing carbon paste with ZnO NPs /IL (IL = 1,3-dipropylimidazolium bromide) [66]. In the same way, with the modified electrode containing ZnO/CNTs/IL (IL = 1-methyl-3-butylimidazolium bromide as binder), it was employed to recognize ascorbic acid in food samples [67]. These studies illustrate how the combination of ZnO with ionic liquids impacts greatly in improving the detection ability with high selectivity/sensitivity, and giving a fast response for the recognition of the desired compound in the real samples. Interestingly, the following two breast anticancer drugs such as doxorubicin and dasatinib have been detected cleverly by using ZnO-NPs/BMTFB/CPE (BMTFB1 = -butyl- 3-methylimidazolium tetrafluoroborate) as amplified electrosensor [68]. Yet, the employment of ILs adhered with ZnO NPs in the photocatalytic and antibacterial studies are highly limited in literature although those combinations would exhibit unique properties because of strong synergetic effect that being generated between the nanoparticles and ILs through electrostatic coulomb force. This strategy can enhance their applicability; so, the fabrication of IL based catalyst is being explored as photo-catalysts for the oxidation of contaminants. For the purpose, benzimidazole based ionic liquid (IL) was prepared and coated on ZnO to result as IL@ZnO, which was then characterized completely by different analytical methods before employing for the catalytic oxidation of 4-nitroaniline (4-NA). The formation of intermediates was studied by HPLC-MS and proposed a possible mechanism for the oxidation. Finally, the antibacterial properties of ZnO@IL were studied and analyzed in detail.

Section snippets

Synthesis of benzimidazolium diacid (IL)

This benzimidazole based ionic liquid was prepared as reported elsewhere [69]. The mixture of benzimidazole (118 mg, 1.0 mmol) and chloroacetic acid (276 mg, 2.0 mmol) was dissolved in dry acetonitrile, and the pH of the solution was adjusted to 8 by using sodium hydroxide. The resulting mixture was refluxed for 5 h, and then the solution was cooled to room temperature. After completion of the reaction, the pH of the solution was adjusted to 2.0–3.0 to result a precipitate of white solids (see

XRD characterization

The XRD diffraction studies were performed for IL, ZnO and ZnO@IL samples, and the data collected were analyzed by using Match Software. The results show that ZnO presents in a hexagonal crystalline system having spatial group of P63mc (186). The parameters are: a = 3.25 Å c = 5.21 Å, density (d) = 5.67 g/cm3. The crystalline pattern of ZnO is clearly coincided with that of wurtzite as its characteristic peaks appeared at are: 32.5 [100], 34.3 [002], 36.3 [101], 47.5 [102], 56.7 [110], 63.5

Conclusions

Benzimidazole based ionic liquid (IL) coated ZnO@IL was prepared and characterized by different analytical methods. In XRD, the plane [100] of ZnO has been shifted higher angle at 64° in for ZnO@IL apparently from the influence of the IL on the ZnO surface. This observation is consistent with the intensity and shape of the XPS peaks (404–398 eV) which has been originated from imidazole nitrogens (cationic) of the IL as a result, there is a sizable photoemission because of the interaction of

CRediT authorship contribution statement

Liliana Margarita García Rojas: Characterization of intermediates in the oxidation analysis; Carlos Alberto Huerta-Aguilar: Oxidation kinetics and mechanism; Eduardo Daniel Tecuapa-Flores: Antibacterial studies; Daniela Soledad Huerta-José: XRD and SEM analyses; Pandiyan Thangarasu: Analysis of all results, and writing/revision of manuscript; Jagpreet Singh Sidhu: Preparation and characterization of benzimdazole based ionic liquid, and its coating on ZnO NPs; Narinder Singh: Designing the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge Dirección General Asuntos del Personal Académico, (DGAPA, UNAM Project No. 222419 and CONACYT No. 266150) for support of this work. The authors are also grateful to USAI (Unidad de Servicios de Apoyo a la Investigación, Facultad de Química, UNAM) for analytical services and DGSCA-UNAM for computational facilities.

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