Green biomimetic synthesis of Ag–TiO2 nanocomposite using Origanum majorana leaf extract under sonication and their biological activities

Background Studies of plant extract-mediated synthesis of nanoparticles is extensively explored and studied in recent time due to eco-friendly, cost-effectiveness and minimal use of toxic chemicals for synthesis. In this study, the synthesis of Ag–TiO2 nanocomposites (NCs) was carried out using Origanum majorana leaf extract under ultrasound irradiation. Origanum majorana leaf extract plays an important role as reducing and capping agent in synthesis of Ag–TiO2 nanocomposites (NCs). The antimicrobial activities of synthesised Ag–TiO2 NCs have been studied against Gram-positive and Gram-negative bacteria. In addition to this, the antioxidant activity of green Ag–TiO2 NCs was also evaluated on the basis of free radical scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid (ABTS), and hydrogen peroxide free radicals. Results Green-synthesised Ag–TiO2 NCs were successfully characterised on the basis of UV–Vis spectrophotometer, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction analysis (XRD), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS) and transmission electron microscopy (TEM). The results revealed the spherical shape of nanocomposite with an average size 25–50 nm. The synthesised Ag–TiO2 NCs have showed significant antimicrobial activity against Escherichia coli, Bacillus subtilis and Aspergillus niger in comparison to TiO2 nanoparticles (NPs). The antioxidant evaluation of biomimetic synthesised Ag–TiO2 NCs also exhibited strong activity than TiO2 NPs and comparable to standard. Conclusion Green-synthesized Ag–TiO2 NCs provide a promising approach that can satisfy the requirement of large-scale industrial production bearing the advantage of low cost, eco-friendly and reproducible.


Background
Nanotechnology is an emerging as a rapidly growing field with its applications in science and technology and nanostructures are important tool in different areas of research (Manke et al. 2013;Hussain et al. 2016;Ostovan et al 2017). Metal and metal oxide nanoparticles (NPs) have received considerable research attention due to their exceptional electrical, optical, magnetic, catalytic and pharmacological properties. Traditional methods for the synthesis of metal and metal oxide NPs include reducing and stabilising chemical agents that are expensive and have an adverse effect on the environment (Guo et al. 2015). In response, researchers are now looking for alternative "green synthesis" approaches in an effort to reduce or eliminate harmful chemicals during the production of NPs (Singh et al. 2018). Green synthesis of metal and metal oxides using plant extract has been extensively studied in recent time in eco-friendly manner with using minimum amount of hazardous chemicals (Zheng et al. 2015;Yulizar et al. 2018, Yulizar and Apriandanu 2019Pirtarighat et al. 2019).
Among various metal oxides, titanium dioxide (TiO 2 NPs) have demonstrated as a most valuable material in various fields due to its unique surface chemistry, high chemical stability, non-toxicity and clean photocatalytic nature with great morphologies that have significant impact on both academia and industries especially in biomedicine field (Wu 2011;Daghrir et al. 2013;Shi et al. 2013). TiO 2 NPs are environmentally harmonious material having remarkable biological activities including antibacterial (Visai et al. 2011), antioxidant (Sethy et al. 2020), anti-parasitic (Durairaj et al. 2014) and anticancer activities (Trouiller et al. 2009). The two inherent properties of such as large bandgap and the fast recombination of electron-hole pairs are make the applicability of TiO 2 limited (Chai et al. 2017;Monfort et al. 2017).
Recent developments in the synthesis of nanomaterials with metal nanoparticles (Ag, Au, Fe, Cu, Ru, and Pd) deposited on metallic oxide surfaces have gained considerable curiosity in nanotechnology and material science because of their significant applications in diverse fields such as biomedical, catalysis, biosensing, information storage, solar cells, optical and many more Gawande et al. 2016;Zheng et al. 2016;Zhang et al. 2018;. Silver and gold nanoparticles have been increasingly used due to their powerful optical, electrical and microbial properties in various areas of research as biological sensors, catalysis, drug delivery vehicles, and antimicrobial agents as well as having low cytotoxic effects on mammalian cells (Mahmoudi and Serpooshan 2012;Crites et al. 2013;Padmos et al. 2015).
Moreover, investigations reveals that Ag-doping regulates the cytotoxicity of TiO 2 NPs by selectively kill human cancer cells while sparing normal cells (Ahamed et al. 2017). Recently, it was explored that without any light illumination, Ag-TiO 2 produces a typical ROS (reactive oxygen species) potential in killing microbial communities (Lin et al. 2011). Besides this, generation of ROS also causes damage to protein, lipids and rupture the DNA cells which confirms Ag-TiO 2 NCs as potential antibacterial agent (Jin et al. 2011). Thus, Ag-TiO 2 NCs are a suitable candidate due to its simple and inexpensive synthesis, high availability, low-toxicity with unique optical-physio and biological properties.
Synthesis of Ag-TiO 2 NCs has been reported by various methods such as sol-gel process (Zhao and Chen 2011), chemical vapour deposition (Lee et al. 2014), thermal dissociation (Saravanan et al. 2018) and electrochemical oxidation (Avciata et al. 2016). Most of these physio-chemical methods not only violates green principles but also suffers irreversible agglomeration and poor dispersibility of nanoparticles causing weak biological activities. Hence, to fulfil these limitations, nowadays the focus of researchers have shifted towards the plant extract-mediated synthesis of diverse nanomaterials but little attention has been paid on the synthesis of Ag-TiO 2 NCs using plant extracts. Recently, green synthesis of Ag-TiO 2 NCs using different plant extracts has been reported such as Citrus lemon (Liang et al. 2012), Acacia nilotica (Rao et al. 2019), Nephelium lappaceum (Kumar et al. 2016) and Euphorbia heterophylla (Atarod et al. 2016) for improved photocatalytic and biological activities. However, these synthesis favours green methodology, but in most of cases, synthesis of NCs completed in two or more steps and conventionally requires much time for reaction process. In this regard, the use of ultrasound as a non-conventional energy source is an effective alternative eco-compatible approach in materials synthesis (Xu et al. 2013). Ultrasounds in nanomaterial synthesis reduced the time and some reports proved the formation of lower particles size due to its cavitational effect (Neppolian et al. 2008;Jordens et al. 2016). During cavitation, bubbles are formed and subsequently collapse to form non-aggregated nanoparticles in short reaction time with high yields.
Keeping in view the diverse applications and limited literature on plant-mediated synthesis of Ag-TiO 2 NCs, in the present study, we have reported a green Origanum majorana leaf extract-mediated synthesis of Ag-TiO 2 NCs under sonication for the first time. The leaves of medicinally important plant Origanum majorana (lamiaceae) contain flavonoids, phenolic terpenoids, phenolic glycosides and oxygenated monoterpene (Goel and Vasudeva 2015), and these biomolecules act as the reducing and stabilising agents in the nanomaterial synthesis Mohammadian et al. 2018;Nezhad et al. 2020).

Characterisation methods and instruments
Green-synthesised Ag-TiO 2 NCs were characterised using UV-Vis, FTIR, XRD, SEM-EDS and TEM analysis. Perkin-Elmer LAMBDA 750 UV-Vis NIR spectrophotometer was used taking quartz cuvette with de-ionised water as a reference. FTIR spectra were recorded on a Perkin-Elmer spectrum version 10.4.00 and using a spectral range of 4000-400 cm −1 with KBr pellets. XRD measurements were carried out on X-ray diffractometer (Panalytical X Pert Pro) equipped with a CuKα radiation (λ = 1.54060 Å) operated at a voltage of 45 kV and current 40 mA. Surface morphology and shape of the NCs were estimated by SEM [Nova Nano FE-SEM 450 (FEI)]. The FE-SEM is coupled to EDS detector for identification of elements present in nanoparticles and to analyse its chemical composition. Transmission electron microscope was recorded using a Tecnai G2 20 (FEI) S-Twin 200 kV. The ultrasoundassisted reactions were carried out using an ultrasonic processor probe system (Qsonica700) operating at 20 kHz, 700 W with 12 mm tip. The operating conditions were a 30-s pulse on and 30-s pulse off time with an amplitude of 50% for 10 min.

Preparation of Origanum majorana leaf extract
The collected leaves of Origanum majorana were washed thoroughly under running tap water to remove the associated dust particles and dried. For preparation of leaf extract, about 20 g of dried leaves of Origanum majorana was powdered and then 100-ml deionized water was added and heated upto 80 °C for 10 min. The mixture was further allowed to cool down to room temperature, then the mixture was filtered using Whatman no.1 filter paper. The residue was removed and filtrate was used for the synthesis of NCs.

Synthesis of Ag-TiO 2 NCs using Origanum majorana leaf extract
For green synthesis of Ag-TiO 2 NCs, precursor solution 0.1 M of titanium (IV) isopropoxide (TTIP) with 15-ml deionised water was placed in a cylindrical glass vessel and irradiate for 10 min under sonication. After that, 1 mM AgNO 3 solution with 25-ml leaf extract of Origanum majorana was added dropwise with continuous sonication. After 10 min, the colour of the solution changed to grey due to the excitation of surface plasmon resonance (SPR) which indicated the formation of Ag-TiO 2 NCs. The resultant NCs solution was then centrifuged at 6000 rpm for 20 min to complete the precipitation process of Ag-TiO 2 NCs. The obtained precipitate was then washed three times with water to remove by-products and then dried at 80 °C in the oven for overnight followed by annealing at 400 °C for 2 h.

Antimicrobial activities
The antimicrobial activity of synthesised Ag-TiO 2 NCs was studied by standard Agar Well diffusion method as reported previously (Nguyen et al. 2019). The antibacterial activity was carried out against both Gram-positive (Bacillus subtilis, Staphylococcus aureus) and Gramnegative (Escherichia coli, Pseudomonas aeruginosa) pathogenic microorganisms and antifungal activity was evaluated against two selected fungi Aspergillus niger and Aspergillus solani. Each strain was swabbed uniformly onto the sterile nutrient agar Petri plates using cotton swabs. Wells of 8 mm diameter were then punched in the inoculated plates using a sterile plastic rod. Using a micropipette, four concentrations (25, 50, 75 and 100 µg/ ml) of synthesised Ag-TiO 2 NCs were loaded to the labelled wells, respectively. After incubation at 37 °C for 1 day, the diameter of inhibition zone was measured in millimetres (mm) using standard scale to determine the antimicrobial activity. The results of antimicrobial activity of synthesised Ag-TiO 2 NCs were also compared with previously synthesized green TiO 2 NPs using Origanum majorana leaf extract (Bhardwaj et al. 2019) to estimate the efficiency of Ag loading in NCs. The experiment also included reference standard.

Antioxidant activity
Antioxidant activity of the green synthesised Ag-TiO 2 NCs and TiO 2 NPs were determined and compared with standard on the basis of DPPH, ABTS and hydrogen peroxide-scavenging assays.

DPPH free radical-scavenging assay
The antioxidant activity of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were investigated on basis of DPPH as described by earlier method (Miliauskas et al. 2004). Various concentrations of synthesised Ag-TiO 2 NCs and TiO 2 NPs were prepared and added to 1 ml DPPH solution (0.1 mM DPPH in methanol) in the test tubes labelled accordingly. The reaction mixture was shaken and then incubated for 30 min in dark place at room temperature. The absorbance was recorded spectrophotometrically at 517 nm. BHT was used as the reference standard antioxidant compound.

ABTS radical-scavenging assay
ABTS free radical-scavenging activity of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were analysed according to the reported method described earlier with moderate modifications ). The stock solution of ABTS radical cation (ABTS +. ) was prepared by reacting 7 mM of ABTS stock solution with 2.45 mM potassium persulfate (K 2 S 2 O 8 ). After incubation at room temperature for overnight, the absorbance was recorded at 734 nm. Synthesised Ag-TiO 2 NCs and TiO 2 NPs were separately added with ABTS at different concentrations and again incubated for 15 min in the dark place. ABTS reagent without sample was used as control solution.

Hydrogen peroxide-scavenging assay
The hydrogen peroxide-scavenging potential of greensynthesised Ag-TiO 2 NCs and TiO 2 NPs were determined according to reported method (Bhakya et al. 2016). A hydrogen peroxide solution was prepared in phosphate buffer at pH 7.4. The different concentrations of Ag-TiO 2 NCs and TiO 2 NPs with ascorbic acid (reference) were taken in test tubes and mixed with 50 μL of 5 mM hydrogen peroxide solution. Afterwards, the mixture was incubated for 10 min at room temperature and absorbance was measured spectrophotometrically at 230 nm against a blank solution containing phosphate buffer without hydrogen peroxide. The percentage of H 2 O 2 -scavenging activity was calculated using a control (blank) and sample (nanocomposites treated) absorbance.

UV-Vis spectrophotometer
UV-Vis spectrum of Ag-TiO 2 NCs is presented in Fig. 1. The absorption band observed at 380 nm is due to TiO 2 (Tang et al. 2012) and absorption at around 570 nm is due to localised surface plasmon resonance (LSPRs) of deposit Ag NPs on the surface of NCs (Zheng et al. 2011;Ramchiary et al. 2014).

FTIR analysis
FTIR spectrum of Ag-TiO 2 NCs (Fig. 2) shows characteristics peaks of OH groups corresponding to water molecules absorbed on the surface of the nanoparticles. Broad peaks at 3432 cm −1 and small peaks at 1624 cm −1 can be attributed to the stretching and bending vibration modes of water molecule, respectively (Llano et al. 2014). The strong absorption band at 400-600 cm −1 is attributed to Ti-O vibration band. Sharp peak around 550-760 cm −1 found in FTIR spectrum can be attributed to Ti-O-Ti bonding (Fleaca et al. 2015).

XRD
X-ray diffraction measurements are used for phase investigation and crystallinity of the nanomaterials.  (311) in lattice planes of face-centred cubic (FCC) structure approving the formation of Ag NPs (Yuan et al. 2010). The XRD pattern of Ag-TiO 2 confirms the formation of dual phases including the anatase phase of TiO 2 and the FCC lattice of Ag and indicates Ag particles did not enter into the crystal lattice of TiO 2 and deposits only on its surface.

SEM and EDS analysis
The detailed morphology, particle size and shape of biomorphic Ag-TiO 2 NCs was investigated by SEM and TEM analysis which showed the distribution of spherical shaped Ag particles on the surface of TiO 2 which is not uniform. The average size of Ag-TiO 2 NCs is 25-50 nm. The elemental composition of the Ag-TiO 2 NCs was determined by EDS analysis and presence of titanium (Ti), silver (Ag) and oxygen (O) was confirmed ( Fig. 4a-d).

Plausible mechanism for the formation of Ag-TiO 2 NCs
The plausible mechanism for formation of Ag-TiO 2 NCs is presented in Fig. 5. TTIP hydrolyse to Ti(OH) 4 in aqueous media and Ti(OH) 4 is usually not stable and hence, it would go through the condensation process to produce amorphous hydrous oxide precipitates (TiO 2 xH 2 O) as stated in the following equations (Mahshid et al. 2007): The presence of various hydroxyl groups in leaf extract of Origanum majorana were responsible for the  Consequently, reducing phytochemicals bind and capped the Ag + ion to form the stable nanoparticles as presented in Fig. 5. The organic molecules which played the main role for formation of Ag-TiO 2 NCs are carnosol, ursolic acid, carsonic acid, lithospermic acid and biomolecules present in the leaf extract of Origanum majorana (Bina et al. 2017).
Finally, formed Ag-TiO 2 NCs were subjected to the calcination process at 500 °C to remove the water molecules.

Antimicrobial activities of green-synthesised Ag-TiO 2 NCs
The antibacterial assay of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were evaluated against pathogens of both Gram-positive Staphylococcus aureus and Bacillus subtilis and Gram-negative Escherichia coli and Pseudomonas aeruginosa bacteria. Figure 6 shows the zone of inhibition was observed on bacteria due to the effect of synthesised Ag-TiO 2 NCs at four different concentrations compared with TiO 2 NPs. The maximum zone of inhibition was noticed in E. coli (22 mm) followed by B. subtilis (20 mm) at 100 µg/ml concentration of Ag-TiO 2 NCs. The lowest inhibition zone was observed with P. aeruginosa (2 mm) at 25 µg/ml concentration of Ag-TiO 2 NCs. By doing experiments on different concentration of (3) AgNO 3 NCs, we found that zone of inhibition increases with the increasing concentration of Ag-TiO 2 NCs (Table 1). This dose-dependent inhibition might be due to the denaturation of bacterial cell wall as NPs binds to cell membrane and pierced inside the bacteria followed by depletion of intracellular ATP (Adenosine triphosphate) (Mamonova et al. 2015). Synthesised Ag-TiO 2 NCs exhibit good antibacterial activity with zone of inhibition against Escherichia coli and Bacillus subtilis pathogens. Ag-TiO 2 NCs shows more zone of inhibition as compared to TiO 2 NPs. In case of Ag-TiO 2 NCs, release of silver ions from NCs enhances its power to bind with bacterial enzymes which are responsible for inactivating the bio-cells by penetrating the cell walls leading to damage of the bacteria (Gupta et al. 2017).
Similarly, antifungal activity of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were studied against two selected fungi Aspergillus niger and Aspergillus flavus illustrated in Fig. 7. Significant antifungal activity of synthesised Ag-TiO 2 NCs was exhibited against A. niger while low activity was observed against A. flavus. From the minimum inhibitory concentration (MIC) values, it is clearly seen that considerably low amount of green Ag-TiO 2 NCs (25 µg/ml) was able to disrupt the fungal cell membrane and lead to its death (Table 2). Similarly as in antibacterial assay, antifungal activity of synthesised Ag-TiO 2 NCs shows more zone of inhibition as compared to TiO 2 NPs. The smaller particle size achieved under sonication also contributed the higher antifungal activity of synthesised Ag-TiO 2 NCs as smaller particle size have large surface to volume ratio due to which more number of drugs molecules get adsorbed on this surface that are expected to be work as a potent agent in disrupting the cell walls (Jalal et al. 2018). The results indicate that synthesised Ag-TiO 2 NCs are a potent antimicrobial agent carrying more capacity to kill the microbes compared to TiO 2 NPs.
However, from a very long time, Origanum majorana is a famous herb used in traditional medicines and its Fig. 4 a SEM image of Ag-TiO 2 NCs with magnification at 500 nm; b SEM image of Ag-TiO 2 NCs with magnification at 1 µm; c TEM image of Ag-TiO 2 NCs; d EDS spectrum of green-synthesised Ag-TiO 2 NCs remarkable biological activities added advantage along with Ag NPs and TiO 2 NPs own potent biological properties that would greatly promote green synthesis of Ag-TiO 2 NCs. Using this popular herb and from previous studies, it was also observed that biosynthesised NPs had showed higher antimicrobial activity than pure chemically synthesized NPs (Santhoshkumar et al. 2014).

Antioxidant activity of Ag-TiO 2 NCs
An antioxidant compound works in neutralising the free radicals to stop the oxidation process. The antioxidant assay of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were investigated against DPPH at different concentrations (Fig. 8). Significantly, high radical-scavenging activity was observed in Ag-TiO 2 NCs as compared to TiO 2 NPs suggesting the presence of silver particles enhances NC antioxidant nature by efficiently separating electron-hole pairs (Hatano et al. 1989). Capped nanocomposites were found to be potent free radical scavenger comparable to standard BHT. The results showed that DPPH free radical scavenging is inhibited by Ag-TiO 2 NCs in a dosedependent manner, i.e. with continuous increment in NC concentration, the scavenging activity was also increased.
The antioxidant assay at different concentrations of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were investigated against ABTS (Fig. 9). Leaves of Origanum majorana contain a high content of bioactive compounds (polyphenols and essential fatty acids oils) that have ability for potential scavenging owing to their hydroxyl groups (Kabeer et al. 2017). As similar to DPPH scavenging, ABTS was also found dose-dependent activity. Green-synthesised Ag-TiO 2 NCs showed strong activity than TiO 2 NPs via inhibiting ABTS radical and comparable to ascorbic acid (standard reference antioxidant) at higher concentrations.  Green-synthesized Ag-TiO 2 NCs were capped by the phytoconstituents found in Origanum majorana leaf extract that could scavenge a variety of free radicals. The phenolic content present in leaf extract can easily donate electron to hydrogen peroxide and thereby neutralising it into water (Lateef et al. 2017). The hydrogen peroxide-scavenging assay at different concentrations of green-synthesised Ag-TiO 2 NCs and TiO 2 NPs were investigated (Fig. 10). Results showed that Ag-TiO 2 NCs exhibited good-scavenging potential as compare to TiO 2 NPs. The capped Ag-TiO 2 NCs showed close scavenging activity to standard ascorbic acid only at high concentration (100 µg/ml) and indicated that H 2 O 2 activity is also a dose-dependent activity.
Phytoconstituents present in leaf extract of Origanum majorana are popular for their antioxidant nature along with Ag and TiO 2 own antioxidant property which are responsible for further enhancing its scavenging activity and made it more active and potent. From above results, it can be suggested that eco-friendly green Ag-TiO 2 NCs synthesized using Origanum majorana leaf extract under ultrasound irradiation could be a promising candidate for antioxidant drugs more than TiO 2 NPs and can be a best substitute of chemically synthetic ones.

Conclusion
In this study, Ag-TiO 2 NCs were synthesised via a simple, cost-effective, eco-friendly approach using leaf extract of Origanum majorana as a bio-reductant under ultrasound irradiation for the first time. The aqueous leaf extract containing phytoconstituents was used for reduction and stabilisation of NCs. The synthesised Ag-TiO 2 NCs were characterized by UV-Vis, FTIR, XRD, SEM-EDS and TEM analysis. Antimicrobial and antioxidant activities of Ag-TiO 2 NCs were performed and found with  dose-dependent variation. Green-synthesised Ag-TiO 2 NCs showed excellent biological activities compared to TiO 2 NPs and close to standard. There are many advantages of applying the present sonochemical route for synthesis such as low temperature, short duration of time, and well dispersibility of doped metal on the surface providing smaller size particles with high yields. All data analysed during this study are included in this article.