Green approach in gold , silver and selenium nanoparticles using co ff ee bean extract

Green fabrication of metal nanoparticles (NPs), using natural reducing and stabilizing agents existed in plants and their derivatives, due to their unique properties, has gained more attention. The present study focuses on the synthesis of gold (Au), silver (Ag) and selenium (Se) NPs using coffee bean extract under hydrothermal conditions (1.5 atm and 121°C, for 15min). Coffee bean extract obtained in 2 h processing using Clevenger apparatus and Fourier transform-infrared (FT-IR) spectroscopy indicated five highlighted peaks, namely, hydroxyl, amide, aromatic, alkane and ring groups. Dynamic light scattering analysis revealed that among three different NPs formed, fabricated Ag NPs had small particle size (153 nm) and high zeta potential value (16.8mV). However, synthesizedAuNPs hadminimumpolydispersity index (0.312). Results also indicated that fabricated Au, Se and Ag NPs had low antioxidant activity with values of 9.1, 8.9 and 8.7%, respectively. Morphological and antibacterial activity assessments, demonstrated that synthesized Ag, Au and Se NPs had spherical shape and high bactericidal activity against E. coli and S. aurous. Obtained results indicated that the synthesized NPs, can be utilized in various areas.


Introduction
According to the National Institute of Health of America, nanotechnology is defined as fabrication of organic and inorganic materials, devices and systems using various techniques in the scale of nanometer (Nadagouda and Varma 2008). Metal nanoparticles (NPs) have unique characteristics compared to their bulk form. In fact, metal and metal oxide NPs have obtained more interests from the scientists because of their use in numerous areas and fields such as medicine, electronic, food and agriculture, biotechnology and wastewater treatment (Mohammadlou et al. 2017; Keshavarzi et al. 2018). Among metal NPs, gold (Au), silver (Ag) and selenium (Se) NPs draw great attention due to their wide applications and benefits. Au NPs are utilized in chemical catalysts, gene expression, drug delivery, sensors and fuel cells (Yola et al. 2015; Elahi et al. 2018). Ag NPs have been used in various areas including food packaging, biolabeling, waste water treatment, electoral devices, texture, paint and ceramics due to their high antimicrobial activities against several microorganisms (Zhang et al. 2016). Due to its anticarcinogenic activity and muscle functioning, Se NPs are utilized in medicines and pharmaceutics (Khurana et al. 2019).
Generally, three types of synthesis methods are involved in forming inorganic NPS, namely, physical, chemical and green techniques. As compared to the numerous conventional methods of synthesis of NPs which are based on utilizing chemical components as reducing and stabilizing agents, green procedures use natural and biodegradable reducing and capping agents presented in biological systems, including microorganisms, enzymes and plants and their derivatives (Wu and Chen 2010; Eskandari-Nojehdehi et al. 2018). Because the green synthesis methods use less energy, without using toxic components and generating hazardous by-products, these fabrication techniques have gained more attention these days; therefore, this method could be used as an alternative of the common physicochemical NPs synthesis methods (Gour and Jain 2019; Salem and Fouda 2020).
Coffee is dark colored, bitter and slightly acidic from the genus Coffee which is native to tropical Africa (Páscoa et al. 2013;Rein et al. 2013). Several studies demonstrated that coffee plays protective role against colorectal cancer and Alzheimer's disease and possesses high antioxidant activity (Carman et al. 2014;Vignoli et al. 2014). Furthermore, coffee contains important minerals such as calcium, potassium, iron, phosphorous, nickel and magnesium. Furthermore, coffee beans contain polyphenols, carbohydrates, caffeine, coffeic acid, chlorogenic acid, diterpenes and trigenilline, which can be utilized as reducing and capping biomolecules in inorganic NPs (Beer 1988;Bułdak et al. 2018). Our present study focuses on (i) the potential application of coffee extract to form three different metal NPs, namely, Ag, Au and Se NPs; (ii) the assessment of physicochemical attributes of the formed three NPs and also highlight the suitability of the coffee extract in metal ion reduction and (iii) measure the antioxidant and bactericidal properties of the resultant different NPs. Plate count agar (PCA) was obtained from Oxoid (Oxoid Ltd, Hampshire, England).

Coffee bean extract preparation
Coffee beans were powdered by a miller (MX-GX1521; Panasonic, Tokyo, Japan) and 100 g of that was subjected into a Clevenger device for 2 h. Coffee bean extract with a pleasant odor was collected in a dark vial and kept at 4°C.   Dynamic light scattering technique (Zetasizer Nano ZS; Malvern Instruments, Worcestershire, UK) was utilized to measure and particle size and its distribution (PSD), zeta potential and its distribution and polydispersity index (PDI) of the produced NPs.

Physicochemical analysis
Antioxidant activity of the formed three different NPs was assessed with method described by Anzabi (2018) and equation (1): In the mentioned equation, I%, A control and A sample are percentage of inhibition and absorbance of control and samples, respectively, at wavelength of 517 nm.

Bactericidal activity
Bactericidal effect of the synthesized NPs was measured using the technique described by Bakht Dalir et al. (2020), based on well diffusion method and the diameter of the formed transparent zone around the holes.

Microstructure analysis
Shape and size of the produced Au, Ag and Se NPs, as their morphological attributes, were studied by transmission electron microscopy ([TEM]; CM120; Philips, Amsterdam, the Netherlands) with an acceleration voltage of 120 kV.

Statistical analysis
Characteristics of the fabricated three studied NPs were measured three times and the mean values of data were compared together utilizing Tukey's comparison test with p value < 0.05, using Minitab v.16 statistical package (Minitab Inc., PA, USA).

Specifications of coffee bean extract
FT-IR spectra of the provided coffee extract are presented in Figure 1. As can be observed in this figure, five predominated peaks were detected with wavenumbers 3454.16,

Formation of fabricated NPs
UV-Vis spectra for the synthesized Se, Ag and Au NPs are shown in Figure 2

Fabricated NPs characteristics
Characteristics of the formed Se, Ag and Au NPs using coffee bean extract are shown in Table 1. As can be seen in Table 1, among all three NPs formed, fabricated Ag NPs had small particle size (153 nm) and high zeta potential value (16.8 mV). However, synthesized Au NPs had minimum PDI (0.312). Figure 3(a-c) indicates PSD of the resultant Se, Ag and Au NPs. The presence of the broad and sharp peaks for all the NPs formed revealed that polydispersed Se, Ag, and Au NPs were fabricated using coffee bean extract (Figure 3). Achieved results were validated by high values of the PDI for three different metal NPs formed (Table 1)

Antioxidant and antibacterial activities of the fabricated three different NPs
Results indicated that fabricated Au, Se and Ag NPs had low antioxidant activity with values of 9.1, 8.9 and 8.7%, respectively. Statistical analysis indicated that antioxidant activity of the synthesized Au NPs was significantly (p < 0.05) higher than that of the fabricated Ag NPs.  Figure 4(a and b). In the plate containing PCA amended with S. aureus, the diameter of formed clear zones around the holes having Ag, Au and Se NPs was 13 ± 1, 10 ± 1 and 8 ± 1 mm, respectively (Figure 4a). However, in the plate containing PCA amended with E. coli, the diameter of the formed transparent zones for the holes having Ag, Au and Se NPs was 11 ± 1, 8 ± 1 and 7 ± 1 mm, respectively. It can be explained by the fact that the antibiotics have less bactericidal effects on gram-negative bacteria as compared to the gram-positive bacteria strains. In fact, the presence of lipopolysaccharide and protein layers on the cell wall of gram-negative bacteria do not allow them to access and digest the bacterial cell wall

Microstructure of the formed three different metal NPs
TEM images for the resultant Au, Ag and Se NPs are shown in Figure 5(a-c). These figures indicated that approximately spherical and polydispersed NPs were created with particle size of 300, 700 and 100 nm for the fabricated Au, Ag and Se NPs, respectively. Other research have fabricated spherical Se, Au and Ag NPs in hydrothermal manner. Fardsadegh and Jafarizadeh-Malmiri fabricated spherical and monodispersed Se NPs using Aloe vera leaf extract with a mean particle size of 50 nm (Fardsadegh and Jafarizadeh-Malmiri 2019). Eskandari-Nojehdehi et al. (2016) also green synthesized well-dispersed Au NPs with a mean particle size ranging 10-50 nm using mushroom extract. Ahmadi et al. (2019) also green synthesized spherical Ag NPs with a mean particle size ranging 10-25 nm, using Aloe vera leaf extract.

Conclusions
In the present study, green synthesis of three different metal NPs, namely, Au, Ag and Se NPs based on subcritical water and coffee bean extract, as a non-toxic solvent and natural reducing agent, was achieved. Simple, clean, and fast developed inorganic NP synthesis technique makes it feasible to commercially fabricate metal NPs in large scales. Furthermore, the resultant stable and spherical Au, Ag and Se NPs, with high antibacterial activity, using coffee bean extract indicated high-potential application of coffee bean extract in converting inorganic ions into their NPs. It seems that by optimizing the synthesis parameters, different metal NPs with more desirable attributes can be fabricated for applications in various areas, including drug delivery, food packaging and formulation and medicine.