Phytofabrication of bioinspired zinc oxide nanocrystals for biomedical application

Abstract

In the present study, we investigated a novel green route for synthesis of zinc oxide nanoparticles (ZnO NPs) using the extract of young cones of Pinus densiflora as a reducing agent. Standard characterization studies were carried out to confirm the obtained product using UV–Vis spectra, SEM–EDS, FTIR, and XRD. TEM images showed that various shapes of ZnO NPs were synthesized, including hexagonal (wurtzite), triangular, spherical, and oval-shaped particles, with average sizes between 10 and 100 nm. The synthesized ZnO NPs blended with the young pine cone extract have very good activity against bacterial and fungal pathogens, similar to that of commercial ZnO NPs.

Keywords::

• antimicrobial activity
• characterization
• nanoparticles
• phytofabrication
• pinus densiflora
• zinc oxide

Introduction

Green nanotechnology is an environmentally friendly alternative for synthesizing biocompatible nanoparticles. In the emerging field of nanobiotechnology, various biological materials have been used to synthesize nanoparticles (Prasad 2014). Among various sources, plant material holds a prime place due to fast production, low production cost, and definitive particle shapes; these attributes make plant material a feasible alternative to traditional physical and chemical methods (Yuvakkumar et al. 2014). Among various metal nanoparticles, zinc oxide nanoparticles (ZnO NPs) exhibit unique characteristics. Conventionally, ZnO NPs are synthesized by sol-gel processing, homogeneous precipitation, organometallic synthesis, spray pyrolysis, thermal evaporation, microwave methods, mechanical milling, and mechanochemical synthesis (Song and Kim 2009, Yuvakkumar et al. 2014). None of the methods of ZnO NP synthesis mentioned above are environmentally friendly. Due to multiple attractive characteristics including high surface area-to-volume ratio, high ultraviolet absorption, and long lifespan, ZnO NPs have been widely used in several applications, exploiting their semiconductor (Sangeetha et al. 2011), piezoelectric (Fujihara et al. 2011), and pyroelectric properties (Vayssieres et al. 2001). As a result, they have been used in many industrial applications including transparent electronics (Vanathi et al. 2014), ultraviolet (UV) light emitters (Akiyama et al. 1998), and space applications (Singh et al. 2011).

Biomaterial synthesis of metallic nanoparticles with well-controlled, well-defined size and shape has been attained through the use of plants and/or their extracts in eco-friendly approaches (Bar et al. 2009, Gnanasangeetha and Sarala 2013, Rajiv et al. 2013, Vanathi et al. 2014). Apart from plant materials, some enzymes (Prasad and Jha 2009), bacteria (Jayaseelan et al. 2012), Bhargavaea indica DC1 strain (Singh 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 NPs. However, obtaining nanoparticles through bacteria and other sources leads to several difficulties in terms of production, cultivation, maintenance, and cost effectiveness. To overcome these disadvantages, we used extract from the young cones of Pinus densiflora for. multicaulis Uyeki, which are cheap and readily available, to synthesize ZnO NPs. The principal compounds like fatty acids, resin aldehydes, resin hydrocarbons, resin alcohols, sterols, sterol esters, and triglycerides present in young pine cones act as natural ligation agents to obtain ZnO NPs (Kilic et al. 2010). All the above reports of synthesis of nanoparticles have a wide range of novelty regarding the conditions, size, shape, and stability of synthesis. Therefore, rapid synthesis of nanoparticles has to be explored by considering the diversity of plants and their parts, because only a little progress has been made on green synthesis of nanoparticles using plants and the extracts of their various parts.

The objectives of this study were to obtain ZnO NPs through a green route and to characterize the product obtained. In addition, the antibacterial and antifungal activity has been examined towards Gram positive bacteria and fungi.

Materials and methods

Chemicals and media

Zinc nitrate (Zn(NO₃)₂·6H₂O) (99.9%), acquired from DaeJung Chemicals, Seoul, South Korea, was used for the synthesis of ZnO NPs. Commercial zinc oxide nano powder (99.5% < 100 nm) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Mueller-Hinton agar, brain-heart infusion broth (BHI), Luria-Bertani broth (LB), and potato dextrose agar were purchased from MB Cell, Seoul, South Korea, for the antibacterial study. All chemicals were used without any modification. Nanopure water (conductivity = 18 μΩ/m, TOC < 3 ppb, Barnstead, Waltham, MA, USA) was used in all experiments.

Biological materials

The antimicrobial activity of biofabricated ZnO NPs and bulk metal zinc were evaluated for antibacterial activity against Brevibacterium linens (KACC-14346), Propionibacterium acnes (KACC-11946), Bacillus cereus (KACC-10001), and Staphylococcus epidermidis (KACC-13234), and for antifungal activity for Aspergillus, Candida sp., and Cryptococcus sp. were obtained from KACC (Korean Agricultural Culture Collection). The microbial cultures were maintained in a nutrient agar, potato dextrose agar medium, and stored at 4°C for further use.

Preparation of pine cone extract and fabrication of ZnO NPs

Young pine cones (50 g) were washed thoroughly with distilled water, cut into small pieces, and boiled in 200 ml of sterile Nanopure water and 50 ml of ethanol for a period of 2 h in a heating mantle at 100°C to obtain the extract. One hundred milliliters of 0.1 M zinc nitrate [(Zn(NO₃)₂·6H₂O); Sigma-Aldrich, St. Louis, MO, USA] were added dropwise using a peristaltic pump (1 drop/5 sec) into 20 ml of the hot young pine cone extract under continuous stirring at 80°C for 12 h. The ZnO NPs were synthesized through reduction of zinc nitrate by the principal phytochemicals present in young pine cone extract, which are essential for the ligation of transition metal ions to form metal oxide nanocrystals (Yuvakkumar et al. 2014, Vanathi et al. 2014). During the reaction, a brown-colored precipitate was obtained as an initial confirmation. The precipitate was obtained after evaporation at 100–150°C for 10–12 h, washed several times with ethanol, and calcined at 400°C for 1 h in a furnace. Finally, a white powder was obtained and used in subsequent experiments.

Characterization of ZnO NPs

The ZnO NPs formed by the young pine cone extract were scanned after color change with a UV-1800 UV–Vis spectrophotometer (Shimadzu, Kyoto, Japan) in the wavelength range of 200–800 nm using a dual beam operated at 1 nm resolution. Transmission electron microscopy (TEM; H-7650, Hitachi, Ltd., Tokyo, Japan,) was used to examine the surface morphology and size of the ZnO NPs. 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 NPs 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 NPs coated onto leather and fabric. The prepared samples were subjected to qualitative elemental analysis using energy dispersive spectroscopy (EDS; JED 2300, JEOL).

Antibacterial and antifungal activity

According to Gunalan et al. (2012), growth standardization of microbial cultures was followed. For the analysis of antimicrobial assay, fresh bacterial and fungal colonies were inoculated into 100 ml of nutrient broth and potato dextrose agar medium, respectively. Bacterial growth was monitored at every 4-h interval under a UV–visible spectrophotometer, till the optical density of 0.8–0.1 at 660 nm was reached.

In vitro antimicrobial activity of the phytofabricated and commercial ZnO NPs was determined using the agar well diffusion assay (Doménech and Prieto 1986, Perez et al. 1990, Nagajyothi et al. 2013). Approximately 20 ml of sterile molten and cooled media, Mueller-Hinton Agar and potato dextrose agar, were poured in sterilized petri dishes. The plates were left overnight at room temperature to check for contamination. Subsequently, inoculation of bacterial and fungal culture on the top of the agar medium and wells (5 mm) was done in selected areas on different plates with a sterilized stainless steel cork borer. About 0.1 ml of solution of different concentrations (2, 4, 6, 8 mM for both bacteria and fungi) of different zinc ions and prepared combinations of ZnO NPs were added in the wells. The green synthesized ZnO NPs blended with pine cone extract combination were prepared by adding 0.25 mL of young pine cone extract in all the concentrations. The plates were incubated at 37°C and 25°C for bacteria and fungi, respectively. The plates were examined for evidence of zones of inhibition, which appear as a clear area around the wells (Gunalan et al. 2012). The diameter of such zones was measured using a meter ruler, and the mean value for each organism was recorded and expressed in millimeters.

Determination of microbial growth

To examine the microbial growth rate and behavior in the presence of bulk fabricated ZnO NPs, fabricated ZnO NPs blended with extract, and commercial ZnO NPs, various microorganisms were grown in liquid medium supplemented with varying concentrations (4, 8 and 16 mM) of nanoparticle colloidal suspensions. To avoid potential interference during optical measurements of the growing cultures caused by the light-scattering properties of the nanoparticles, the same liquid medium without microorganisms, but containing the same concentration of nanoparticles cultured under the same conditions, was used as blank controls. All the fresh cultures were inoculated into the respective growth medium and then the flasks were put on a rotary shaker (180 rpm) at 37°C. Following inoculation, the OD of the cultures was serially monitored at every 3-h interval for up to 18–24 h for bacteria, and for fungi, assessment was carried out every 7 h for up to 49–77 h by using a UV–visible spectrophotometer at 660 nm (Doménech and Prieto 1986, Gunalan et al. 2012).

Results and discussion

Physical characterization of ZnO NPs

Figure 1 shows the UV–Vis absorption spectra of the ZnO NP peak at 375 nm, which confirms that ZnO NPs were formed. The UV absorbance at 378 nm was in accordance with previous reports (Akiyama et al. 1998, Vanathi et al. 2014), and the band gap of the ZnO NPs was calculated according to the method proposed by Vanathi et al. 2014. The results of the EDS analysis of the green synthesized ZnO NPs are shown in Figure 2a. These results demonstrate strong signals in zinc and oxygen, further confirming the formation of ZnO NPs. Our EDS results are in good agreement with earlier reports (Hong et al. 2009, El Ghoul et al. 2012, Rajiv et al. 2013, Nagajyothi et al. 2013, Yuvakkumar et al. 2014, Vanathi et al. 2014). SEM imaging of the morphology of the NPs revealed non-uniform distribution in the form of a needle-like structure, as shown in the inset in Figure 2a, which is very similar to the results of earlier studies (Yuvakkumar et al. 2014). An FTIR spectrum (Figure 2b) confirmed the structure of young pine cone extract, with bands at 3400, 2810, 1610, 1120, 800, and 500 cm^{− 1} (Velmurugan et al. 2013). The significant absorption peaks in the FTIR spectra (Figure 2b) of the prepared ZnO NPs at 3420, 1630, and 1110 cm^{− 1} could be assigned to O–H stretching, the H–O–H bending vibration mode to the adsorption of moisture, and the Zn–O stretching vibration, respectively. The bands around 1040–3500 cm^{− 1} corresponded to the symmetric C–O vibration associated with C–O–SO₃. Further, the spectrum showed bands at 440 cm^{− 1}, corresponding to metal–oxygen (M–O) (Doménech and Prieto 1986, Yuvakkumar et al. 2014). The XRD peaks at 2θ = 32.10°, 34.62°, 36.37°, 47.24°, 56.70°, 64.20°, 67.82°, 68.68°, 69.18°, 73.82°, and 77.92° were assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (200), (1 1 2), (201), (004), and (2 0 2) crystal planes of the ZnO NPs, respectively (Figure 3a). The diffraction peaks are spherical and hexagonal wurtzite structure phases of zinc oxide, which was confirmed using data from JCPDS card No. 89-7102. The distinct diffraction peaks give evidence of a well-crystallized structure of phytosynthesized ZnO NPs. Further, the strong and narrow peak shape denotes that the product was crystalline in nature. The mean particle sizes of the nanoparticles were estimated from the full width at half maximum (FWHM) using Scherrer's equation (Azizi et al. 2014).

Figure 1. UV–Vis spectra of the young pine cone extract, zinc nitrate, and reaction mixture, where the inset shows the whole cone in tree and the separated cone material.

Figure 2. (a) SEM–EDS and (b) FTIR analysis of ZnO NPs.

Figure 3. (a) HR–TEM image, (b) XRD, (c) fringe pattern, and (d) SAED pattern analysis of ZnO NPs.

d = 0.89λ/ß cos θ

Here d, λ(, θ(, and β(indicate the mean particle size, the X-ray wavelength (1.5406 Å), Bragg diffraction angle corresponding to the (101) plane, and FWHM of the (101) plane, respectively. TEM images indicated that the ZnO NPs had an average size between 10 and 100 nm and included hexagonal (wurtzite), triangular, spherical, and oval-shaped NPs, while some were non-uniformly shaped (Figure 3b). The obtained image is in accordance with the results of an earlier study (Sivaraj et al. 2014, Vanathi et al. 2014). Supplementary Figure 1 to be found online at http://informahealthcare.com/doi/abs/10.3109/21691401.2015.1058811 shows the TEM images with different ZnO NP morphologies, and shapes are indicated by the arrows.

Antimicrobial assay of bulk metal and nanoparticles

The antimicrobial activity of bulk metal zinc, phytofabricated, and commercial ZnO NPs of different concentrations (2, 4, 6, and 8 mM) were studied toward various bacterial and fungal pathogens by the agar well diffusion methods, and the results are represented in Table I. As described by Gunalan et al. 2012, the inhibition zone clearly indicates that the mechanism of the biocidal action of ZnO NPs involves disruption of the membrane, with a high rate of generation of surface oxygen species, and finally leads to the death of pathogens. Interestingly, the inhibition zones were in various sizes according to the type of pathogens, materials, and the concentration used (bulk, fabricated ZnO NPs, fabricated ZnO NPs with extract, and commercial ZnO NPs). As it was shown in the study of (Rizwan et al. 2010, Gunalan et al. 2012), it has been found that, by increasing the concentration of ZnO NPs in wells, the growth inhibition also increased consistently because of proper diffusion of nanoparticles in the agar medium. Both nano and bulk ZnO NPs showed antimicrobial activity against selected pathogens but maximum activity (2.8 mm) was observed in P. acnes and B. cereus followed by B. linens and S. epidermidis (2.6 mm), for commercial nanoparticles. Earlier Velmurugan et al. 2015a and 2015b, has elaborated the mechanism of antibacterial action against various human pathogens by silver nanoparticles. The possible mechanism behind the inhibition of bacteria by nanoparticles, is as follows: (1) the nanoparticle will anchor to the bacterial cell wall and subsequently penetrate it, thereby causing structural changes in the cell membrane, like the permeability of the cell membrane and death of the cell, (2) Formation of free radicals by the nanoparticles may be considered to be another mechanism by which the cells die, (3) ability to damage the cell membrane and make it porous, which can ultimately lead to cell death, (4) interact with the thiol groups of many vital enzymes and inactivate the cells, (5) generation of reactive oxygen species, which are produced possibly through the inhibition of a respiratory enzyme by silver ions and attack the cell itself, and (6) soft acid in nanoparticles, and there is a natural tendency of an acid to react with a base, in this case, a soft acid to react with a soft base. Lowest inhibition zones were observed in fabricated ZnO NPs with extract (Table I). Among fungal pathogens, maximum activity was noticed for Aspergillus sp. > Cryptococcus sp. > Candida sp. for commercial ZnO NPs (Table I). Gunalan et al. (2012) and Doménech and Prieto (1986), have described that the release of Zn²⁺ ions is responsible for the antibacterial activity. In our study, pine cone extract-mediated ZnO nanoparticles blended with extract showed a less similar significant zone inhibition when compared to commercial ZnO nanoparticles. However, low enhancement of the antimicrobial activity was recorded in the cases of bulk ZnO/green synthesized ZnO NPs alone and at lower concentration (2, 4 and 6 mM), but medium inhibition was noticed at higher concentrations.

Table I. Representation of well method of inhibition of bulk, fabricated ZnO NPs, fabricated ZnO NPs with extract, and commercial ZnO NPs against various human bacterial and fungal pathogens.

Bacterial strains ZoI (mm)
Materials	Bulk metal Zn^+ ion				Fabricated ZnO NPs				Fabricated ZnO NPs + extract				Commercial ZnO NPs
Concentration (mM)	2	4	6	8	2	4	6	8	2	4	6	8	2	4	6	8
B. linens	–	–	0.3 ± 4.12	0.7 ± 4.11	–	0.2 ± 2.22	0.9 ± 1.18	1.9 ± 12.4	0.6 ± 3.34	0.9 ± 7.72	1.4 ± 7.16	2.0 ± 5.18	0.5 ± 2.48	0.9 ± 3.25	1.8 ± 4.25	2.6 ± 6.66
P. acnes	–	0.3 ± 3.13	0.5 ± 8.19	0.5 ± 6.17	–	0.3 ± 4.28	0.8 ± 2.14	1.5 ± 16.12	0.4 ± 5.62	1.5 ± 6.82	2.0 ± 4.12	2.5 ± 4.18	0.4 ± 2.51	1.5 ± 4.28	2.2 ± 9.14	2.8 ± 8.97
B. cereus	–	0.2 ± 4.11	0.3 ± 4.12	0.7 ± 4.11	–	0.3 ± 4.29	0.6 ± 0.18	1.8 ± 12.42	0.8 ± 2.89	1.4 ± 3.72	1.8 ± 4.12	2.2 ± 4.18	0.2 ± 1.18	1.4 ± 2.31	1.9 ± 8.26	2.8 ± 5.29
S. epidermidis	–	0.2 ± 4.11	0.3 ± 4.12	0.7 ± 4.11	–	0.4 ± 2.26	0.8 ± 0.18	1.5 ± 14.44	0.6 ± 3.05	1.2 ± 3.32	1.9 ± 4.12	2.4 ± 4.18	0.5 ± 2.20	1.4 ± 3.68	1.6 ± 2.14	2.6 ± 5.24
Fungal strains ZoI (mm)
Aspergillus sp.	–	0.1 ± 2.85	0.3 ± 4.12	0.5 ± 1.18	–	0.1 ± 3.28	0.6 ± 2.18	1.0 ± 16.12	0.2 ± 5.42	1.0 ± 6.82	1.2 ± 9.24	1.9 ± 5.22	0.4 ± 2.54	1.8 ± 8.26	2.1 ± 9.35	2.8 ± 6.28
Candida sp.	–	0.1 ± 1.82	0.3 ± 2.12	0.3 ± 4.18	–	0.2 ± 3.22	0.6 ± 219	1.2 ± 12.42	0.2 ± 4.02	0.9 ± 4.22	1.2 ± 6.32	1.8 ± 8.21	0.6 ± 2.22	1.4 ± 4.22	1.8 ± 8.22	2.2 ± 7.61
Cryptococcus sp.	–	–	0.3 ± 4.12	0.5 ± 4.18	–	0.1 ± 4.32	0.4 ± 1.17	1.0 ± 14.44	0.2 ± 4.02	0.8 ± 6.64	1.2 ± 3.32	1.4 ± 9.22	0.5 ± 5.32	1.2 ± 6.64	1.8 ± 3.34	2.6 ± 4.42

Determination of microbial growth

Figures. 4 and 5 shows the effect of bulk, green synthesized, and a combination of green synthesized, extract, and commercial ZnO nanoparticles on the growth of bacterial and fungal pathogens. Time-dependent changes in microbial growth were monitored by measuring OD at 660 nm for bacterial growth determination. The antimicrobial activity is probably derived, through the electrostatic attraction between the negatively charged cell membrane of the microorganism and the positively charged nanoparticles (Dragieva et al. 1999, Hamouda et al. 2001, Dibrov et al. 2002, Gunalan et al. 2012), interaction of metal ions, including zinc, with microbes, and orientation of ZnO NPs. Both the green combination with extract and the commercial ZnO treatments exhibited significant inhibitory effect on the growth of pathogens during 24 and 72 h of incubation. The optical density of the medium was investigated as the number of microbes after contact with the nanoparticles. The growth inhibition of the pathogens by both treatments, recorded as a function of time, suggested significant differences in antibacterial and antifungal activity of the nanoparticles (Figures. 4b, d and Figure 5b, d) for bacteria and fungi, respectively. (Figures 4a, c and 5a, c) show maximum growth in zinc ion and the green synthesized ZnO NPs alone for bacteria and fungi. The concentration of 16 mM leading to the continuous inhibition of growth of the tested organisms during 24 and 77 h cultivation has been used. The measurement of OD was carried out at 660 nm, to avoid strong absorption due to the ZnO NPs in the region of 380–450 nm and from bacterial cellular components such as nucleic acids (A260), proteins (A280), and molecules present in the medium (Gunalan et al. 2012). The OD at 660 nm is due to the scattering of light by the bacterial and fungal cells. Increasing concentration of ZnO NPs decreases the growth of microbes, and the concentration at which growth stopped altogether was higher in green ZnO NPs than commercially available nanoparticles. Hence, we suggest that the commercial nanoparticles that were prepared by using several chemicals which might be bound on the surface of the particles will play an additional role in the inhibitory action. Also, commercial ZnO NPs are of uniform shape and size, this may also be another possibility for the enhanced inhibitory action against cells.

Figure 4. Growth curves of various bacterial strains treated with bulk (a), fabricated ZnO NPs (b), fabricated ZnO NPs with extract (c), and commercial ZnO nanoparticles (d).

Figure 5. Growth curves of various fungal strains treated with bulk (a), fabricated ZnO NPs (b), fabricated ZnO NPs with extract (c), and commercial ZnO NPs (d).

Conclusions

In this manuscript, we have reported a green synthetic approach employing simple, inexpensive, and abundant eco-friendly material (young pine cones) for the synthesis of ZnO NPs without using any toxic chemicals. The results from TEM images and XRD demonstrated that the particles were on the nanoscale with maximum similar shapes and sizes. In addition, we have demonstrated the enhanced bioactivity of green synthesized ZnO nanoparticles blended with the young pine cone extract, by studying the antimicrobial activity of suspensions with various other formulations using a standard microbial method. The growth inhibition was solely higher in biologically synthesized ZnO NPs than chemical ZnO NPs and other common antimicrobials. The enhanced bioactivity of smaller particles is attributed to the higher surface area-to-volume ratio. The young pine cone-mediated synthesized ZnO NPs have immense potential, and are expected to have a notable impact on manifold uses in pharmaceutical, textile, biomedical, and cosmetic industries.

Supplementary material available online

Supplementary Figure 1 to be found online at http://informahealthcare.com/doi/abs/10.3109/21691401.2015.1058811.

Acknowledgments

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 National Research Foundation (Korean Ministry of Education, Science and Technology, Award NRF-2011-35B-D00020).

Declaration of interest

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