Plectranthus barbatus Leaf Extract-Mediated Synthesis of ZnS and Mg-Doped ZnS NPs: Structural, Optical, Morphological, and Antibacterial Studies

In the current study, the researchers have explored the infuence of doped Mg ions on the optical, morphological, and structural properties of zinc sulfde (ZnS) nanoparticles (NPs). Te green technique was employed to prepare pure and 2% and 5% Mg-doped ZnS NPs using the Plectranthus barbatus leaf extract as a capping agent. XRD, SEM, FTIR, and UV-visible were used in the investigation process. Te XRD results showed that all the synthesized materials have a cubic structure with space group F-43m. Te D av was nearly in the range of 2.02–2.20 nm. Te SEM images illustrated that NPs were agglomerated. Te UV-visible results showed that the optical bandgap increased as Mg 2+ ions increased, which was in the range of 3.81–4.42eV. Te absorption shoulder of the prepared NPs is blue-shifted with increasing dopant concentration. Te FTIR spectrum gives characteristic peaks for Zn-S bonds and asserts NPs’ formation. Te antibacterial check against E. coli and S. aureus bacterial strains revealed that pure and Mg-doped ZnS NPs have higher activity for both bacterial strains. Te results have shown that the prepared materials can be used for antibacterial activities and optoelectronic applications.


Introduction
Among the family of semiconductor compounds, II-VI groups hold tremendous technological importance in many felds of science and technology. For instance, ZnS, CdS, ZnSe, CdTe, etc., are of great importance for solar cells and optoelectronics because they possess exceptional structural, electronic, and optical properties [1,2]. Te unique properties and applications of zinc sulfde (ZnS) nanoparticles (NPs) have recently made them one of the most promising research materials. It is a semiconductor with large band gap energy (3.68 eV at 298 K), wide exciton binding energy, and cubic unit cell with lattice constant a � b � c � 5.406Å. Due to their ability to demonstrate new properties when compared to bulk ZnS, NPs have garnered a lot of attention [3][4][5][6][7][8]. As a result of their excellent optoelectronic properties, low cost, nontoxicity, and natural abundance, ZnS NPs have been utilized for photocatalysts, solar cells, light diodes, and bio-applications [9][10][11][12][13][14], among others. Generally, chemical approaches for synthesizing nanomaterials are faster and cheaper, and they provide a higher yield than physical approaches. Terefore, such chemical methods are more suitable for industrial scales.
Te literature review illustrates that ZnS NPs have been fabricated using diferent techniques involving sol-gel [15], coprecipitation [16,17], solvothermal [18], green synthesis [19], microwave [9], and hydrothermal techniques [20]. As it is well known, doping ZnS NPs with diferent metal atoms, especially alkaline earth metals, is an efective method of modifying the nanocrystals' optical properties and electrical properties [14,[21][22][23]. Furthermore, it is well known that NPs have a high surface-to-volume ratio, resulting in unstable properties when oxygen is chemisorbed at their surface in an ambient atmosphere leading to higher resistivity or uncontrolled luminescence emission. To overcome this obstacle, the surface bonds of the NPs can be adequately passivized with appropriate guest atoms, thereby promoting a stable state. Generally, choosing the suitable doping atom depends on the following criteria: (i) the ionic radius values of guest and host atoms should be similar and (ii) the thermal solubility limit should be as high as possible to maintain the high content of guest atoms [4,24,25]. Magnesium (Mg) ion is one possible doping atom, being a promising candidate to replace Zn sites in order to tune the optical properties of the host lattice.
In addition, Mg is a naturally abundant and low-toxic element. Moreover, due to its similar ionic radius to Zn, Mg ions are not expected to cause a signifcant lattice distortion of ZnS [26]. Indeed, doping ZnS NPs by various transition metals has been tried using diferent approaches [5,15,27,28]. In the middle of these is the green method. It is an efcient approach to preparing ZnS NPs due to its simplicity and scalability. Several valuable reports regarding ZnS's structural, morphological, and optical properties have been published. For example, a work by Ashokkumar and Boopathyraja [11] has revealed that E g gets increased with the increase in Mg dopant concentration. Te results suggest that these materials can be used in optoelectronic applications [11]. Ayim-Otu et al. [29] have synthesized pure and (3%) Cu-doped ZnS NPs by coprecipitation, showing improved optical bandgap after doping and proving their possible use in photovoltaics [16]. According to the literature [11,30,31], doping of ZnS by Mg can be performed using diferent chemical processes; however, to the best of our knowledge, no research studies on using the Plectranthus barbatus (P. barbatus) leaf extract as a medium for the biosynthesis of Mg-doped ZnS NPs have been reported.
Herein, we focused on synthesizing ZnS pure and doped by Mg using the P. barbatus leaf extract as a capping agent. P. barbatus (also known as Coleus forskohlii) is a medicinal herb that belongs to Lamiaceae family and has proven its value as an antimicrobial, antioxidant, and antiseizure drug [32,33]. Furthermore, it is used in herbal treatment of many diseases, including hypertension, congestive heart failure, eczema, colic, respiratory disorders, painful urination, insomnia, and convulsions [34]. Te P. barbatus leaf extract has exhibited antibacterial, antioxidant, and immunomodulatory activities and were cytotoxic to gastric adenocarcinoma cells. Te secondary metabolites of P. barbatus leaves have reported to include eugenol, α-pinene, and β-caryophyllene, which are toxic to larvae of Anopheles subpictus, Stegomyia albopicta, Culex tritaeniorhynchus [35], and A. aegypti fourth-instar larvae, but no insights into the mechanism of action have been reported [36]. A recent report has evaluated the phytochemical components in the P. barbatus leaves from both aqueous and organic extracts. Te main fndings revealed the presence of favonoids, cinnamic derivatives, steroids, and ellagic acid, with higher activity for the organic extract [37]. Plants of this species grow in many countries around the world, such as India, Ethiopia, Egypt, tropical East Africa, Yemen, and Brazil [32].
Te efect of doping on the morphology, optical, and structural properties of ZnS was investigated, and furthermore, the antibacterial activities against Gram-positive and Gram-negative bacteria were assessed.  ) and Escherichia coli (E. coli)) were kindly obtained as a gift from Al-Jarf Medical Lab (Dhamar City, Yemen). Mueller-Hinton agar (MHA) was acquired from Sigma-Aldrich (Darmstadt, Germany) and used as recommended. Distilled water (DW) was used wherever.

Plant Collection.
Fresh leaves of the target P. barbatus plant were collected from Dhamar valleys farms, Yemen. Tey were washed several times with tap water and DW. After that, the leaves were chopped into small pieces and crushed in a mortar with a pestle till they became dough [38].

2.3.
Preparation of the Extract. 13 g of the dough-like crushed leaves were suspended in 200 mL DW. Ten, the mixture was stirred for 90 min at 25°C, during which the color of the solution was changed from colorless to brown. After fltering, the resultant solution was immediately employed for production of the target pure and Mg-doped ZnS NPs [39].

Green Synthesis of ZnS NPs.
To synthesize ZnS NPs, equal moles (3.5 moles) of Zn(CH 3 OO) 2 ·2H 2 O and Na 2 S were separately prepared in 25 mL each of the P. barbatus leaf extract (termed solutions A and B, respectively). Solution B was then added, in a dropwise manner, to solution A, and the mixture was stirred using a magnetic stirrer for 1 h at 25℃. After that, the product was fltered, washed with ethanol and DW, and dried at 25℃ for 48 h. Finally, the obtained powder was further dried at 100°C for 1 h and then quashed into fne powder [39,40]. Te same procedure was repeated for doping ZnS with 2 and 5 mol% Mg. Te overall scheme for the biosynthesis of ZnS and Mg-doped ZnS NPs and their bioactivity are summarized in Figure 1.

Antibacterial Test.
Te antibacterial efect of the prepared materials was executed for S. aureus (Gram-positive) and E.coli (Gram-negative) bacteria by the disc difusion route [41,42]. Te concentrations of 70, 140, and 210 mg/mL of the synthesized NPs were taken for the antibacterial check. Te inhibition zones were measured after incubation at 36-37℃ for 24 h.

Characterization.
Te X-ray difraction (XRD) profles for the pure and doped ZnS were recorded using an XD-2 Xray difractometer (Beijing, China) with CuKα radiation of λ � 0.15418 nm. Fourier transform infrared (FTIR) spectra were recorded via the Nicolet iS10 FTIR spectrometer from Termo Scientifc (Madison, WI, USA) on the range of 650-4000 cm −1 . Te scanning electron micrograph (SEM) was imaged by JSM − 6360 LV (Tokyo, Japan). Electronic spectra were obtained using a Hitachi UV-vis U3900 spectrophotometer (Tokyo, Japan) coupled with Varian Cary-50 software and acquired in the range of 200-800 nm at room temperature. Te NPs were suspended in DW and immediately analyzed using a 1 cm cuvette.

Results and Discussion
3.1. XRD Analysis. XRD analysis was employed to defne the structural purity and crystallite size of the NPs. Te XRD patterns for pure, 2% Mg, and 5% Mg-doped ZnS NPs are ofered in Figure 2 and assured agreement PDF card no. 05-0566 [43]. Tus, three prominent peaks coinciding to (111), (220), and (311) planes of ZnS were observed, respectively, at 2θ of 28.75°, 48.01°, and 56.46°. Te results revealed wellresolved patterns of ZnS NPs. Te broadening in the diffraction peaks might be due to the size efect; therefore, the crystallite size is in the nanoregime. Te average crystallite size (D) of the NPs is estimated by Debye Scherrer's relation D � 0.9λ/β cos θ [4]. Bragg's formula was applied to estimated d-spacing nλ � 2d sin θ [13], while the lattice parameter (a) was calculated using the equation , where hkl is Miller's plans. Te peak position (2θ), lattice parameter, d-spacing, and D ave values for the highest intensity peak are collected in Table 1. From this table, it is found that the D value increases with the increase in the Mg doper concentration, i.e., from 2.021 nm for the undoped ZnS to 2.204 nm for 0.05 Mg-doped ZnS. Tese results elucidate the role of the plant extract in facilitating the NPs' production.
Furthermore, it was observed that the position of the difraction peak of (111) was slightly shifted to higher angles after doping, and it was proven that Mg is well superseded Zn in the ZnS matrix. Te lattice (a) parameter and d-spacing values were slightly decreased due to the coalition of smaller ionic radii of Mg into the position of larger ionic   radii Zn [11]. Te reduction in the parameter of lattice (a) and d-value lead peak position shifts into higher value. Te slight increase in D ave after Mg doping is due to the improvement of crystalline surface growth by the existence of Mg [11]. Tese results agree with the fact that has been reported by Ashokkumar and Boopathyraja, who prepared pure and Mg-doped zinc sulfde using the coprecipitation method [11].

SEM Analysis.
Te SEM analysis was performed to study the surface morphology of the synthesized NPs. Figures 3(a) and 3(c) illustrate the SEM images of pure ZnS and 5% Mgdoped ZnS NPs. It can be seen that the NPs were irregular in shapes and sizes and agglomerated into higher clusters. Furthermore, it could be seen that the pure sample is less lumpy than the doped one, while the extent of particles' agglomeration was higher after doping. Due to the low magnifcation of the images, counting of the particles and particle sizes is difcult; however, the insert images (Figures 3(b) and 3(d)) could visualize traced particle sizes.

UV-Visible Spectroscopic Analysis.
Te UV-vis absorption spectra of Mg ions doped ZnS NPs are listed in the range 200-900 nm as shown in Figure 4(a). Te pure and Mgdoped ZnS NPs revealed an absorbance shoulder at 321, 296, and 270 nm, respectively. In addition, it illustrates that the synthesized samples have excellent absorption at the short wavelength region. Also, it shows that the absorption shoulder of the prepared NPs shifts towards the shorter wavelengths as the concentration of the dopant increases. Tis can be explained by the strong sp-d exchange interaction between the zinc sulfde band electrons and localized d spins related to the magnesium ions. Te energy gap (E g ) for the synthesized NPs is computed using the relation E g � 1240/λ c [44], where λ c is the cut-of wavelength. Te estimated E g for the synthesized NPs was 3.86, 4.19, and 4.59 eV, respectively. Te Beer-Lambert formula was used to compute the absorption coefcient (α) [4] α � 2.303A/t, where A and t represent the absorbance and the thickness, respectively. Figure 4(b) displays α for all the samples (pure and Mgdoped ZnS) as a function of wavelength (λ). It illustrates that α decreases as the λ increases. Tis made Mg-doped ZnS NPs an excellent nominee material for optoelectronic instruments as a window layer. Figure 4(c) displays the transmission spectra (T s ) of ZnS (pure), 2% Mg-doped ZnS, and 5% doped ZnS NPs. By incorporating Mg ions into the Zn-S lattice, distortions and imperfections are created, thereby suppressing transparency. Tis result was in coincidence with literature [11]. Te extinction coefcient (k) as a function of λ for pure (ZnS) and Mg (0.02 and 0.05) doped ZnS NPs is shown in Figure 4. Te k was computed via the equation k � ∝ λ/4π [45]. Figure 4(c) shows that the extinction coefcient (k) at short wavelengths was increasing, and then as the wavelength increased, the k decreased up to 290 nm, and then a sudden increase was observed. Furthermore, as the wavelength increased, the k increased as well.
For a direct allowed transition, the E g value was estimated using Tauc's equation (44) where A is a constant, α is the absorption coefcient, hv is the incident photon energy, and n takes the values 1/2, 3/2, 2, and 3 depending on the material and the type of the optical transition whether it is direct or indirect by using n � 1/2. Te estimated E g is 3.81, 4.19, and 4.42 eV, respectively, as shown in Figure 5. Tese results are in a close agreement with previous studies reported by Mani et al. [19] and Ashokkumar and Boopathyraja [11].
Te E g values obtained by applying both absorption edge and Tauc's equation methods were nearly equal. Accordingly, the E g value of the NPs is larger than that of the bulk material. Te increase in the E g value of a semiconductor is due to the increase in the life-time of holes and electrons. Also, the observed E g is blue-shifted, which can be attributed to the quantum confnement efects. Indeed, the reproducibility of the particle sizes and optical and other properties of bio-based NPs are challenging. Terefore, the primary concern to using the bio-synthetic route is to optimize the critical protocols. In certain cease, the authors have reported a highly reproducible green method for the production of NPs as assessed by UV-visible spectroscopy [46]. However, this is not always the case, and extensive evaluation is needed.

FTIR Studies.
FTIR spectra of the prepared pure ZnS, 2%, and 5% Mg-doped ZnS are shown in Figure 6. In principle, FTIR analysis helps in identifying the functional groups existing in the synthesized NPs as well as the various adsorbing chemical species. As can be seen, the spectra of pure ZnS and Mg-doped ZnS are nearly the same with a slight diference in some peak positions and intensities assigned to the variation in the crystals' nanostructures [47]. According to the literature [6,47,48], the peaks at 1127, 1109, and 617 cm −1 are due to Zn-S vibrations. After doping ZnS with Mg, a change in its microstructure is expected. Te case could be followed through the change in the intensities of peaks at 1127 and 1109 cm −1 , thus supporting Mg inclusion into the main ZnS crystal [47]. Te broad bands in the range of 3050-3670 cm −1 are due to HO stretching of the adsorbed water molecules on the surface of the nanocrystals [47]. It may also suggest absorption contributed from OH of alcohol and phenol of extract biomolecules. Te strong bands at 1557, 1387, 1331, 1022, and 932 cm −1 could be assigned to the stretching bands of C�O, C�C, C-C, asymmetric C-O, and symmetric C-O which associated with the capping biomaterials, driving the production of ZnS and its doped Mg NPs [49].

Synthetic Mechanism.
Based on the phytocompounds reported in the literature [37,50], the reaction mechanism is depicted in Figure 7. Accordingly, favonoids, cinnamic derivatives, steroids, terpenoids, saponins, and essential oils were major constituents of the P. barbatus leaf [50]. Notably, diterpenoids, including forskolin [51], plectranthone, and plectrinone [52], are one of the most bioactive compounds in the P. barbatus plant [33]. Hence, to simplify the illustration of the suggested mechanism, the diterpene called forskolin was used to represent the stabilizing agents used to prepare the target NPs (pure and Mg-doped ZnS NPs). As can be seen, forskolin combines several functional groups that could contribute to capping and stabilizing ions for the next production of NPs. Hence, one forskolin molecule could coordinate as one, bi-, or multi-dentate with metal ions (Zn 2+ and Mg 2+ ). On the other hand, essential oils and Nanomaterials and Nanotechnology amino acids can stabilize the sulfde ions. Tus, by further hydrolysis of the coordinated ions, the end NPs can be obtained [53].

Antibacterial Studies.
Te antibacterial efect of the asprepared pure and (0.02, 0.05) Mg-doped ZnS NPs was investigated against S. aureus and E. coli strains to check their utility as potential materials for biological applications. Figures 8(a) and 8(b) show the zone of inhibition (ZOI), and Table 2 displays the measured ZOI for all the prepared compounds. Te results indicated higher antibacterial potency of pure ZnS NPs than the Mg-doped one, with lesser activity against Gram-negative than Gram-positive bacteria. Te diference can be explicated by the chemical composition and various structures of each cell surfaces, as the cell wall of Gram-positive and Gram-negative bacteria is different [54,55]. Te higher activity is due to the greater ability of prepared NPs to generate reactive oxygen species, which leads to oxidative stress and destruction of bacterial cells [11,56]. According to previous studies, several mechanisms could be proposed for the antibacterial resistance of NPs. Due to smaller sizes of NPs, they can easily adhere with the cell wall of the bacterium and can cause destruction, which will lead to the death of the cell. Te electrostatic interaction of the nanomaterials with the cell wall and photocatalytical light activation are also a common possible reason behind the antibacterial activity of nanomaterials [57,58]. Te antibacterial activity of the P. barbatus extract has been reported to be solvent-dependent, with the aqueous extract efect being lower than the organic one [37]. In the present study, the experiment was performed to evaluate the antibacterial activity of the synthesized inorganic NPs; however, the plant extract was not targeted and thus was not compared. It is worth mentioning that even though a trace of the plant phytocompounds can be detected, their efect would not be counted due to the traced concentration. On the other hand, the antibacterial activity of chemically synthesized ZnS NPs is low, as reported elsewhere [59], supporting the advantage of the biosynthesis method in producing NPs with a better performance against microbes.
According to Ashokkumar and Boopathyraja [11], the antibacterial activity of ZnS NPs prepared via a coprecipitation method was dopant-ratio dependent; however, the case was not straightforward, and instead, a peak of efcacy was found at 0.2Mg-doped ZnS among the tested (0.0-0.4)Mgdoped ZnS NPs. A similar trend was seen for ZnS-flled polyvinylpyrrolidone/Chitosan [60] against S. aureus and     both strains, exhibiting similar ZOI at similar concentrations [59,62,63]. Despite the synthesis method, the antibacterial activity of doped ZnS NPs varies depending on the dopant type and concentration. Hence, Ag-doping has enhanced the ZnS activity [64], Mg-doped ZnS has an optimal value [11], and Sn-doping showed the same or slightly lesser activity than pure ZnS [63]. Te latter one supports our fndings that the activity of pure ZnS is marginally higher activity than that of Mg-doped ones. Such diferences are brought about by the various sizes and shapes present in each doped ZnS product, which in turn rely on the synthesis method.

Conclusion
In this paper, pure and (2% and 5%) Mg-doped ZnS NPs were successfully prepared via an economical, easy, and environment-friendly route using the P. barbatus leaf extract. Te XRD pattern proved the cubic phase with crystallite sizes in the range of 2.02-2.204 nm. Te SEM revealed that the obtained NPs are agglomerated. Te UV-visible results have shown that the optical band gap was increased as Mg 2+ ions increased, which were in the range 3.81-4.42 eV. Te FTIR study denotes the existence of OH, C-C, and C�O functional groups which are the surface-active molecules that stabilize the ZnS NPs. Te antibacterial test demonstrated that the biosynthesized pure ZnS and Mg-doped ZnS NPs had remarkable inhibition of growth of both Gramnegative and Gram-positive bacteria.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest. Nanomaterials and Nanotechnology 9