Eco-friendly approach towards green synthesis of zinc oxide nanocrystals and its potential applications

Abstract

In the present study, we investigated a novel green route for synthesis of zinc oxide (ZnO) nanocrystals using Prunus × yedoensis Matsumura leaf extract as a reducing agent without using any surfactant or external energy. Standard characterization studies were carried out to confirm the obtained product using UV–Vis spectra, SEM–EDS, FTIR, TEM, and XRD. In addition, the synthesized ZnO nanocrystals were coated onto fabric and leather samples to study their bacteriostatic effect against odor-causing bacteria Brevibacterium linens and Staphylococcus epidermidis. Zinc oxide nanocrystal-coated fabric and leather showed good activity against both bacteria.

Keywords::

• antibacterial activity
• application
• eco-friendly
• prunus × yedoensis
• synthesis
• zinc oxide

Introduction

The green synthesis of metallic nanoparticles has attracted tremendous attention in recent years due to lower cost and greater eco-friendliness. Among various metal nanoparticles, zinc oxide nanocrystals (ZnO) exhibit unique characteristics such as semiconducting (Sangeetha et al. 2011), piezoelectric (Fujihara et al. 2001), and pyroelectric properties (Vayssieres et al. 2001), for transparent electronics (Vanathi et al. 2014), spin electronics (Wang 2004), and coating (Waham et al. 2010), as ultraviolet (UV) light emitters (Akiyama et al. 1998), chemical sensors (Mochinaga et al. 1998), and catalysts (Elseviers and Verelst 1999), and in space applications (Singh et al. 2011). Several chemical and physical methods are employed to obtain ZnO nanocrystals, but these are generally not environmentally friendly and lead to health concerns. Biological synthesis of metallic nanoparticles with well-controlled and defined size and shape has been attained through the use of plant extracts in eco-friendly approaches with environmental concerns (Bar et al. 2009, Gnanasangeetha and Sarala Thambavani 2013, Rajiv et al. 2013, Vanathi et al. 2014, Yuvakkumar et al. 2014). Apart from plant material, some enzymes (Prasad and Jha 2009), bacteria (Jayaseelan et al. 2012; Wang et al. 2015)), seaweeds (e.g., Sargassum muticum) (Azizi et al. 2014), and gelatins (Darroudi et al. 2014) have been used as green routes for the synthesis of ZnO nanocrystals. However, obtaining nanoparticles through bacteria and other sources leads to several difficulties in terms of production, cultivation, maintenance, and cost effectiveness. Thus, to overcome these limitations, economical and widely available leaf extract from the Yoshino cherry (Prunus × yedoensis Matsumura; Japanese: Somei-Yoshino) was used for the synthesis of ZnO nanocrystals. Brevibacterium linens and Staphylococcus epidermidis are responsible for body odor as well as odor on feet, shoes and/or socks, which is mediated through the breakdown of amino acids present in sweat (Ara et al. 2006, Kanlayavattanakul and Lourith 2011). Nazeruddin et al. (2014) summarized a table in his article, in which nanoparticles that have been synthesized by various biological routes by various researchers were discussed well.

As one of the objectives of this study, a green chemistry approach is adopted for the synthesis of ZnO nanocrystals using Prunus × yedoensis leaf extract, which is an eco-friendly route, due to its vital applications against odor-causing bacteria. The synthesized ZnO nanocrystals have been subsequently characterized by various instrumental techniques. The antibacterial activity of ZnO nanocrystal-coated cotton fabrics and tanned leather has been examined toward B. linens and S. epidermidis, Gram-positive bacteria.

Materials and methods

Leaf extract preparation and synthesis of ZnO NPs

Mature Prunus × yedoensis leaves (50 g) were collected in and around Chonbuk National University, Iksan, South Korea and washed thoroughly with distilled water, cut into small pieces, and boiled in 200 ml of sterile Nanopure water (conductivity = 18 μΩ/m, TOC < 3 ppb, Barnstead, Waltham, MA, USA) for 30 min to obtain the extract, followed by filtration (Whatman No. 42) and storage in a refrigerator for future use. One hundred milliliters of 0.1 M zinc nitrate [(Zn(NO₃)₂.6H₂O)-, Sigma-Aldrich, St. Louis, MO, USA] was added dropwise using a burette into 20 ml of the hot leaf extract under continuous stirring at 80°C for 8 h. The ZnO nanocrystals were synthesized through reduction of zinc nitrate by the phytochemical polyphenols present in the leaf extract, which are essential for the ligation of transition metal ions to form metal oxide nanocrystals (Vanathi et al 2014, Yuvakkumar et al. 2014). During the reaction, a yellow-colored precipitate was obtained as an initial conformation. The precipitate was washed several times with ethanol, followed by calcination at 450°C for 1 h in a furnace (AJEON Heating Industrial Co., LTD, Gyeonggi-do, South Korea). Finally, a white powder was obtained and used in subsequent experiments.

Characterization of ZnO NPs

The obtained colloidal ZnO nanocrystals were scanned after color change with a UV–Vis spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) in the wavelength range of 200–800 nm using a dual beam operated at 1 nm resolution. Biological transmission electron microscopy (Bio-TEM, H-7650, Hitachi, Ltd., Tokyo, Japan,) was used to examine the surface morphology and size of the ZnO nanocrystals. For TEM measurements, a drop of solution containing the particle was deposited on a copper grid covered with amorphous carbon. After allowing the film to stand for 2 min, the extract solution was removed by means of blotting paper and the grid allowed to dry before the measurement. X-ray powder diffraction of the samples was obtained using a Rigaku X-ray diffractometer (XRD, Rigaku, Japan). Fourier transform infrared (FTIR) spectra of the ZnO nanocrystals were obtained using a Perkin-Elmer FTIR spectrophotometer (Norwalk, CT, USA) in the diffuse reflectance mode at a resolution of 4 particles cm^{− 1} in KBr pellets. Scanning electron microscopy (SEM) was used to examine the binding nature of the ZnO nanocrystals coated onto leather and fabric. Energy dispersive spectroscopic (EDS) (JED 2300, JEOL) analysis was carried out on the prepared samples for qualitative elemental analysis.

Treatment of fabric and leather with ZnO nanocrystals and their antibacterial activity

ZnO nanocrystals were applied to fabric and leather by employing a pad–dry–cure method (Thitilertdecha et al. 2010, Yuvakkumar et al. 2014). The antibacterial activities of the ZnO nanocrystals on treated and untreated fabric and leather were evaluated against odor-causing bacteria B. linens (KACC-14346) and S. epidermidis (KACC-13234) procured from the Korean Agriculture Culture Collection, Suwon, South Korea (KACC). The cultures were maintained on nutrient agar and the antibacterial activity was determined using the method developed by Velmurugan et al. (2014). The antibacterial activity was evaluated by measuring the zone of inhibition against the test organisms. Finally, we measured diameters (mm) of zone of inhibition of the control strain and test with a ruler and caliper readings.

Results and discussion

UV–visible spectroscopy of ZnO nanocrystals

In the present investigation, the synthesis of ZnO nanocrystals was carried out employing a novel, environmentally benign synthetic strategy using Prunus × yedoensis leaf extract. ZnO nanocrystals demonstrated a broad absorption peak at 378 nm, as shown in Figure 1. The UV absorbance at 378 nm was in accordance with previous reports (Nawaz et al. 2011, Vanathi et al. 2014) and the band gap of the ZnO nanocrystals was calculated according to the method proposed by Vanathi et al. (2014). It is reported in the literature that typical ZnO nanocrystals show the characteristic surface plasmon resonance (SPR) at the wavelength in the range of 350–400 nm. The SPR absorbance is sensitive to the nature, size and shape of particles present in the solution and it also depends upon their inner particle distance and the surrounding media.

Figure 1. UV–Vis spectra of the leaf extract, zinc nitrate, and reaction mixture, where the inset shows the whole tree and leaf material.

SEM–EDS spectra and the image of ZnO nanocrystals

The energy-dispersive X-ray spectroscopy (EDS) analysis results of the green synthesized ZnO nanocrystals are shown in Figure 2a, and the inset shows the SEM image of ZnO nanocrystals. The results demonstrate the strong signals in zinc and oxygen, further confirming the formation of ZnO nanocrystals. Our EDS results are in good accordance with earlier reports (El Ghoul et al. 2012, Rajiv et al. 2013, Vanathi et al. 2014, Yuvakkumar et al. 2014). The morphology of the nanocrystals revealed non-uniform distribution in the form of a needle-like structure, as shown in the inset SEM image in Figure 2a, which is very similar to the results of earlier studies (Yuvakkumar et al. 2014).

Figure 2. Results of analysis of ZnO nanocrystals by (a) SEM–EDS (b) FTIR.

FTIR spectra and XRD graph showing ZnO nanocrystals

The significant absorption peaks in the FTIR spectra (Figure 2b) of the prepared ZnO nanocrystals at 3420, 1630, and 1110 cm^{− 1} could be assigned to O–H stretching, the H–O–H bending vibration mode due to the adsorption of moisture, and Zn–O stretching vibration, respectively (Hong et al. 2009, Yuvakkumar et al. 2014). The XRD peaks at 2θ = 32.10°, 34.62°, 36.37°, 47.24°, 56.70°, 64.20°, 67.82° and 69.18° were assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), and (2 0 1) crystal planes of the ZnO nanocrystals, respectively (Figure 3a). The diffraction peaks could be referenced as the spherical and hexagonal wurtzite structure phase of zinc oxide, which was evaluated using data from JCPDS card No. 89-7102. The strong and narrow peak denotes that the product was crystalline in nature.

Figure 3. Results of analysis of ZnO nanocrystals by (a) TEM (b) XRD.

TEM image of ZnO nanocrystals and particle size distribution

TEM indicated that the ZnO nanocrystals were spherical or oval while some were non-uniformly shaped and distributed in the prepared sample (Figure 3b), and the inset shows the particle size distribution graph. The obtained image is in accordance with the results of an earlier study (Vanathi et al. 2014, Sivaraj et al. 2014). It was observed that the nanoparticles formed were of different sizes, and particle size was found to be 10.80 nm, 26.12 nm, 41.14 nm and 16.48 and the mean size was about 20.06 nm, which lies in the nano range. The histogram of particle size distribution in the inset Figure 3b indicates that 10% of the particles have a size of around 40 nm with a standard deviation of 22.12% in size, while the remaining particles are around 60 nm with a standard deviation of 14.12%.

SEM–EDS and SEM image studies of ZnO nanocrystal-treated and -untreated fabrics

The morphological changes of cotton and leather samples before and after treatment with ZnO nanocrystals were clearly observed from the SEM images. Figures 4a and 5a show the EDS spectra of ZnO nanocrystal-coated fabric and leather samples. Figures 4b and 5b show the morphology of ZnO nanocrystal-treated cotton fabric and leather samples. SEM images clearly show the imposition of ZnO nanocrystals onto the surface of the fabric and leather with good distribution. SEM images clearly show the significant difference between the untreated and treated cotton fibers and leather due to the imposition of ZnO nanocrystals onto the surface of the fibers and leather. Both the micrographs clearly show that the ZnO nanocrystals were well distributed onto the fabric and leather.

Figure 4. (a) EDS spectra (b) SEM image of ZnO nanocrystal-coated fabric.

Figure 5. (a) EDS spectra (b) SEM image of ZnO nanocrystal-coated leather.

Antibacterial studies

To test the ability of the prepared ZnO nanocrystals to inhibit bacterial (B. linens and S. epidermidis) growth, testing was performed using a disc diffusion method, using the standard antibiotic Penicillin-G (10 mcg) as a control. A zone of inhibition (ZoI) appeared around the ZnO nanocrystal-coated fabric and leather due to the bacteriostatic effect of the product. Table I shows the antibacterial activity of the ZnO nanocrystals and standard antibiotic-coated fabric and leather. The greatest inhibitory effect was observed against B. linens with a ZoI of a diameter of 26 mm followed by inhibition against S. epidermidis with a ZoI of a diameter of 14 mm on the fabric. The ZnO NP-coated leather showed ZoI diameters of 14 and 12 mm against B. linens and S. epidermidis, respectively. Penicillin-G exhibited ZoI diameters of 52 and 38 mm against B. linens and S. epidermidis on the fabric samples and 44 and 30 mm on the leather samples, respectively.

Table I. Zone of inhibition (mm) of control, ZnO NPs, standard antibiotic-coated fabric and leather against odor-causing bacteria.

Materials	Organisms	ZoI (mm)
Control fabric	B. linens	−
	S. epidermidis	−
Control Leather	B. linens	−
	S. epidermidis	−
ZnO NP-coated fabric	B. linens	26 ± 4
	S. epidermidis	24 ± 2
ZnO NP-coated leather	B. linens	14 ± 6
	S. epidermidis	12 ± 4
Penicillin G (10 mcg)-coated fabric	B. linens	52 ± 2
	S. epidermidis	38 ± 2
Penicillin G (10 mcg)-coated leather	B. linens	44 ± 2
	S. epidermidis	30 ± 2

Possible mechanism of ZnO nanocrystal formation using Prunus × yedoensis leaf extract

The possible mechanism of ZnO nanocrystal formation using Prunus × yedoensis leaf extract is shown in Scheme 1 [Note* This scheme is obtained from Yuvakkumar et al. (2014)]. The ester oxygen atom and the phenolic hydroxyl groups of polyphenols form a p-track conjugation effect when the hydroxyl groups bind with the metal to form metal–phenolate complex (Zinc–ellagate complex) by the chelating effect. These complexes undergo direct decomposition at 450(C and this leads to formation of ZnO nanostructures. Moreover, the Prunus × yedoensis leaf extract has significant potential due to its polyphenols and has an easy electron-losing capacity which results in the formation of green zinc–ellagate complex formation. In our synthesis, aromatic hydroxyl groups present in Prunus × yedoensis leaf extract ligate with zinc ions to form the zinc–ellagate complex at pH 5–7. The pH value varies depending upon the metal ions (Wang et al. 2009, Yuvakkumar et al. 2014). The Prunus × yedoensis leaf extract used has a rich source of active ingredients such as polyphenols, alkaloids, flavonoids, and vitamins. These active compounds are highly responsible for the reduction and capping of ZnO nanocrystals. In addition, these compounds could perform functions like involving their antibiotic, antioxidant, radical scavenging, and antiviral effects etc., (Yuvakkumar et al. 2014). These natural plant phytochemicals have favorable effects for the synthesis of ZnO nanocrystals and are also anticarcinogenic and antioxidant in nature (Przewloka and Shearer 2002, Wang et al. 2009, Yuvakkumar et al. 2014).

Scheme 1. Possible mechanism of ZnO nanocrystal formation (Yuvakkumar et al. [2014]).

Conclusion

A green synthetic approach employing simple, inexpensive and abundant eco-friendly material for the synthesis of ZnO nanocrystals would have a major impact in nanoparticle research. The antibacterial results clearly demonstrate that the ZnO nanocrystal-coated fabric and leather samples have the ability to inhibit odor-causing bacteria. The future studies may include the use of synthesized nanoparticles along with routinely used antibiotics to see their synergistic effects. It confirms that ZnO nanocrystals are capable of rendering high antibacterial efficiency and hence have great potential in the preparation of drugs against bacteria. Therefore, green-synthesized ZnO nanocrystal-coated socks and shoes have immense potential for use by athletes and the general public.

Declaration of interest

The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.

Acknowledgement

This research work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the government (MEST) (No. 2011-0020202). This research was also supported by the Korean Ministry of Education, Science and Technology, Award NRF-2011-35B-D00020.