Synthesis of Schiff Base Encapsulated ZnS Nanoparticles: Characterization and Antibacterial Screening

In this study, Schiff bases containing salicylaldehyde moiety (namely salicylaldehyde 2-methyl-3-thiosemicarbazone and salicylaldehyde triazole) were synthesized using the conventional refluxing method. The Schiff bases were utilized in the encapsulation of ZnS nanoparticles using the co-precipitation method. The nanoparticles were characterized using FTIR, UV-visible absorption spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis. X-ray diffraction analyses suggest that the Schiff base encapsulated ZnS particles form the cubic crystal phase of ZnS, with the average crystallite sizes being approximately between 56 and 60 nm. The interaction between the Schiff bases and ZnS was also evaluated by photoluminescence spectroscopy. The antibacterial activities of the Schiff base encapsulated ZnS nanoparticles were screened against four different gram-positive and gram-negative bacterial strains (i.e. Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, and Staphylococcus aureus) using the agar diffusion method. The antibacterial activities of the nanoparticles were compared with those of their respective Schiff bases. Although in the current study the Schiff base encapsulated ZnS nanoparticles were found to be inactive against those bacteria, they could be applicable as multifunctional materials for fluorescence probes, photocatalysts, and other biological applications.

Antibacterial activity; Co-precipitation; Encapsulation; Schiff base; ZnS nanoparticles

        Semiconductor materials of groups II–IV are commonly used due to their attractive electronic and optical properties (Keskin et al., 2019). These types of semiconductor materials are used in a wide range of applications due to their broad absorption range, tunable bandgap, spectral purity, and photochemical stability (Mansur et al., 1999; Carrillo-  Carrión et al., 2009; Mansur, 2010; Wei et al., 2016).

An interesting feature of semiconductor materials is that they can be prepared in a few nanometer-sized crystals, which have physical and chemical properties that are different from those of crystals with ‘bulky’ structure.  Most of the physical and chemical properties are controlled by the particle size (Attanayake et al., 2020). However, by bringing the size down to the nanometer scale, a large number of atoms on the particle surface are then less coordinated, so making the particles thermodynamically unstable. The less coordinated atoms readily chelate with ligands or surfactants (Yang et al., 2014). Among semiconductor nanocrystals, ZnS nanoparticles (NPs) are of interest due to their fascinating crystalline structures. It is well known that ZnS NPs may exist in cubic phase (zincblende) at room temperature with a bandgap of 3.68 eV, which converts to hexagonal phase (wurtzite) at higher temperatures with a bandgap of 3.77 eV (Tiwari and Dhoble., 2016). Based on the wide bandgap, ZnS NPs have the potential to be utilized in a wide range of applications in photonics, electronics, solar cells, and LEDs (Niu et al., 2014), and drug development (Ajibade et al., 2020). The latter, in particular, also relies on chelating ligands or capping agents.

Because the chelating agents are situated on the particle surface and control the chemical reactivity of the ZnS NPs, one can design the chelating ligands with functional groups that can be used to coordinate with the atoms on the surface to stabilize the ZnS NPs as well as be used for specific applications. The interesting chelating ligands are Schiff bases, which are referred to as the organic compounds containing an imine or azomethine moiety (?CH=N?). These compounds are synthesized based upon the condensation of primary amine and carbonyl compounds, such as an aldehyde or a ketone (Golcu et al., 2005; Da Silva et al., 2011; Patil et al., 2016; Bhat and Wagay, 2017), involving the replacement of the carbonyl group (C=O) of an aldehyde or ketone with an imine or azomethine moiety followed by elimination of a water molecule (Sinha et al., 2008; Md Yusof et al., 2015; Umofia et al., 2016). Schiff bases are very general and useful ligands that readily form stable complexes with most of the transition metals and hence play an important role in the development of coordination chemistry (Abdel Aziz et al., 2012). It is very interesting to note that Schiff bases exhibit strong bioactivities, such as antiviral (Sriram et al., 2006), antifungal (Zishen et al., 1993), antimalarial (Rathelot et al., 1995), antibacterial (Karthikeyan et al., 2006), antitumor (Kowol et al., 2009), and anticancer (Zishen et al., 1993; Shi et al., 2007) activities. Due to their stability, Schiff bases have a broad scope of organic and medicinal chemistry applications (Qin et al., 2013). They contain strong donor sites due to imine nitrogen atoms, and therefore, the azomethine structure has the ability to exhibit biological activities (Safari and Gandomi-Ravandi, 2014). Schiff base-related compounds, such as triapine, have potential for medicinal applications (Rejmund et al., 2018). In particular, Schiff bases containing salicylaldehyde moiety have been known to have anticancer activities (Qin et al., 2013).

It is also interesting to note that due to their simple chemical reactions, Schiff bases can be synthesized by different methods, including conventional reflux and eco-friendly or green syntheses (Ali et al., 2020). Various green synthesis methods have been introduced, including synthesis that uses water as solvent, the grinding method, microwave irradiation, and the sonication method. Green synthesis in general has been found to minimize the reaction time, but results in high efficiency. 

In light of the above background, in this study two Schiff bases containing salicylaldehyde moiety were synthesized using the conventional method, where salicylaldehyde and thiosemicarbazide or 4-amino-4H-1,2,4-triazole were heated under reflux in ethanolic solution. The compounds were isolated and purified with the recrystallization method, giving a 25–50 percentage yield. These two Schiff bases were then used for the first time to encapsulate ZnS NPs, employing the co-precipitation method, which is a rapid, simple, energy efficient, and low temperature process (Rane et al., 2018). The chemical structure of the Schiff bases and their encapsulated ZnS NPs were confirmed by the spectroscopic method. The antibacterial activities of the synthesized Schiff bases and their encapsulated ZnS NPs were screened against four different species of bacterial strains (namely Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus), and they were compared with the standard antibiotic, streptomycin. The main objective of this study is to investigate the effect of the encapsulation of the two Schiff bases on the antibacterial activity of ZnS NPs against the four bacterial strains, and the novelty is the usage of the two Schiff bases as capping agents.

    Two new Schiff bases of salicylaldehyde derivatives (namely ZnS-Sal2me3TSC and ZnS-SalTriazole) have been successfully synthesized using the conventional refluxing method. Their chemical structures have been confirmed by spectroscopic methods. The two Schiff bases were utilized as stabilizing agents to encapsulate ZnS NPs using the co-precipitation method. The chemical, electronic, composition, and crystal structures of ZnS-Sal2me3TSC and ZnS-SalTriazole NPs were characterized using FTIR, UV-Vis, EDX, and XRD analyses. The antibacterial studies suggested that the Schiff base ligands, as well as the ZnS-Sal2me3TSC and ZnS-SalTriazole NPs, are inactive and are most likely due to the higher minimum inhibition concentration beyond the experimental range in this current study. Overall, the results suggest the possibility of exploring ZnS NPs further in the future for their potential bioapplications, investigating Schiff bases and other bioactive ligands.

    The authors would like to extend their deepest gratitude to the Chemical Sciences, Environmental and Life Sciences, Applied Physics, Geosciences of the Faculty of Science, Centre for Advanced Material and Energy Sciences (CAMES), Universiti Brunei Darussalam, and Faculty of Resource Science and Technology, Universiti Malaysia Sarawak. MHSAH is especially grateful to the Chemical Sciences for FIC Grant No. UBD/RSCH/1.4/FICBF(b)/2020/024, and YWS is grateful for grant from Universiti Brunei Darussalam (Research Grant No. UBD/RSCH/1.4/FICBF(b)/2018/019).  

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