Biosynthesis of silver nanoparticles on yellow phosphorus slag and its application in organic coatings

: This work concentrated on the decoration of AgNPs to yellow phosphorus slag (YPS) using both chemical (NaBH 4 ) and biological ( Areca catechu nut and Jasminum subtriplinerve leaf extracts) reducing agents, as well as its use as antibacterial and enhancement additives for organic coatings based on acrylic emulsion resin. It is the ﬁ rst study about the decoration of AgNPs on the surface of YPS using bio-reduction agents ( A. catechu nut and J. subtriplinerve leaf extracts). The characteristics of YPS decorated by AgNPs (YPS@AgNPs) were determined using attenuated total re ﬂ ec-tance infrared spectroscopy, scanning electron microscopy, X-ray di ﬀ raction, ultraviolet-visible spectroscopy and dynamic light scattering methods. To quantify the amount of AgNPs in YPS@AgNPs, atomic absorption spectroscopy was used. The results of YPS@AgNPs fabrication con ﬁ rmed that the above green reduction agents had a higher e ﬃ ciency and were more suitable than the chemical reduction agent (NaBH 4 ).


Introduction
One of the most often studied nanoparticles among all others in recent years has been silver.The diameter of silver nanoparticles (AgNPs), which typically have 20-15,000 silver atoms, is less than 100 nm [1].Typical applications of AgNPs are in the pharmaceutical industry and agriculture sector [2], and they have been used as antimicrobial and anticancer agents [3].AgNPs have a high surface-to-volume ratio, which results in extraordinary antibacterial action even at low concentrations [1].It has been proposed that the antibacterial mechanisms used to kill Staphylococcus aureus and Escherichia coli involve bacteriolysis and the degradation of bacterial cell walls by Ag + ions.By controlling peptidoglycan (PGN) synthetic transglucosylase (TG) and transpeptidase (TP), as well as enhancing the activation of PGN autolysins of amidases, the Ag + ions promote bacterial lysis and destroy the PGN cell wall of S. aureus [4].Pseudomonas stutzeri is a nonfluorescent denitrifying bacterium that is widely distributed in the environment.People also have identified it as an opportunistic disease.Much work has been achieved in understanding the taxonomy of this varied taxonomic group, proving the clonality of its populations over the last 15 years [5].It was reported that AgNPs could both lead to oxidative damage to cells through a peroxidation reaction with cellular lipids and influence the denitrification and cytotoxicity of P. stutzeri [6].
Depending on the manufacturing process, metal nanoparticles can be made in a variety of sizes and forms, with chemical reduction being the most popular.They consist of butylhydroxyanisole (BHA), butylhydroxytoluene (BHT), NaBH 4 , etc.Because of the potential toxicity and health hazards of chemical reduction, synthetic chemical anti-oxidants should be replaced with natural anti-oxidants [7][8][9][10].Because of their relative safety, the utilization of naturally occurring anti-oxidantsprimarily phenolic compoundshas received a lot of interest.The redox characteristics of phenolic compounds, which allow them to function as reducing agents, hydrogen donors, singlet oxygen quenchers, or metal chelators, are primarily responsible for their anti-oxidant ability [11].Some studies have been carried out on more sustainable processes of synthesizing AgNPs by using natural extracts, such as Aesculus indica [12], Moringa oleifera [13], Cannabis sativa [14], Areca [15], and Jasminum subtriplinerve Blume [16].Areca nut extract has major ingredients of phenolic compounds such as flavonoids, tannins, and alkaloids [17].It has been used to produce metal nanoparticles.For instance, phytochemicals like amines, alcohols, and phenols present in the areca nut husk extract facilitate the reduction of Pd 2+ to Pd 0 .It is an eco-friendly synthesis with low cost, high reaction yield, and mild reaction conditions, and areca nut husk extract can be recycled [15].The J. subtriplinerve Blume (Oleaceae) contains phenolic compounds and is one of the herbal plants that has been used widely as a tea for weight loss or to stimulate milk glands.The primary components discovered in Oleaceae oxidation were linalool monoterpenes (14.8-44.2%),terpineol (15.5%), geraniol (19.4%), cis-linalool oxide (8.8%), and benzopyrone coumarin (48.9%).In other words, it is a great natural source of phenolic compounds.It is also investigated for anti-oxidant, cytotoxic, and antimicrobial activities [18].
To enhance the antibacterial ability of inorganic particles as well as improve the stability of the AgNPs, AgNPs could be decorated on the other nanoparticles, for example, TiO 2 @Ag [19], ZnO@Ag [20], SiO 2 @Ag [21], or Fe 3 O 4 @Ag [22].In particular, zeolite with a hollow structure is an ideal carrier for the AgNPs, reducing the agglomeration of AgNPs, thereby increasing the bactericidal ability of zeolite and the stability of AgNPs [23].When combined with zinc, the antibacterial synergistic effect of the Ag-Zn/zeolite increases the antibacterial ability of the hybrid material [24].The coating film contains 1 wt% of Ag-Zn/zeolite that can kill most (>99%) of E. coli and S. aureus bacteria after 24 h exposure [25].On the other hand, the Ag-Zn/zeolite is considered as an environmentally friendly antibacterial agent that is applied in various fields such as dentistry [26] and food preservation film [27].
Yellow phosphorus slag (YPS) is a solid byproduct formed during the manufacturing of yellow phosphorus in an electrical furnace using phosphate ore, silica, and coke [28].Tang Loong Industrial Zone (Lao Cai Province, Vietnam) YPS comprises 46.62% CaO, 38.85% SiO 2 , 3.31% Al 2 O 3 , and 1.4% P 2 O 5 .In other words, the YPS can be a great carrier for the AgNPs used as an antibacterial and enhancement additive for organic and inorganic materials.In addition, the compressive strength of the YPS is high, reaching 14-17 MPa, and can be used to create unburned material in the building field [29].Besides, the YPS can be used in various applications, such as the activation of an alkaline rice fertilizer [30], absorbent for chromium (VI) ion and methylene blue [31], cement substitute [32], production of calcium carbonate whisker [33], or recovery of rare earth metals [34].In addition, Vietnam is the only country in Southeast Asia with abundant apatite ore (basic material for yellow phosphorus production).Vietnam produces 6-8,000 tons of yellow phosphorus annually to meet the domestic demand and export.Only seven phosphorus companies in Tang Loong Industrial Zone (Lao Cai Province, Vietnam) have released more than 600,000 tons of slag annually, putting some factories in danger of shutting down owing to overload and a lack of slag storage [29].As a result, processing them optimally is a big problem.
In this work, AgNPs were decorated on YPS using NaBH 4 , Areca catechu nut extract, and J. subtriplinerve leaf extract as reducing agents.Polyethylene glycol (PEG400) was used as a co-surfactant for the preparation of the YPS@AgNP hybrid material.Subsequently, a comparison of the reducing ability of these agents was assessed based on the results of UV-Vis, ATR-IR, AAS, size distribution, zeta potential analysis, and antibacterial activity test.This hybrid material is also expected to be a great additive for polymer coating applications.

Materials
YPS was pretreated by a flotation system from the waste product of Lao Cai Yellow Phosphorus JSC (Tang Loong Industrial Zone, Lao Cai Province, Vietnam), as reported in the study of Pham et al. [29].The calcium silicate content in YPS particles was higher than 90%, and the BET surface area of YPS was 1.3145 m 2 •g −1 .AgNO 3 (>99.7%)and NaBH 4 (99%) were purchased from Aladdin Reagent Co., Ltd.(China).Ethanol 99.5% was provided by Duc Giang Chemical Co.(Vietnam).Polyethylene glycol (PEG400, molecular weight of 400) was purchased from Sigma Aldrich Co. for 120 min.Then, the mixture was filtered to obtain the A. catechu nut extract (abbreviated as AN extract).

Preparation of YPS@AgNPs using the AN extract
A mixture of YPS and AgNO 3 solution was prepared as above, in which 2 g of YPS was dispersed in 60 mL of distilled water.Then, 10 mL of the AN extract was dropped slowly to this mixture, and an additional 0.02 g of PEG400 was added before stirring for 4 h.Subsequently, the solid part was treated as mentioned above.The obtained product was named as YPS@Ag-ANE.were added into 50 mL of ethanol 99.5% before stirring on a magnetic stirrer at 60 o C for 120 min.Then, the mixture was filtered to obtain the J. subtriplinerve leaf extract (JS extract).

Preparation of YPS@AgNPs using the JS extract
A mixture of YPS and AgNO 3 was prepared as above, in which 2 g of YPS was dispersed in 60 mL of distilled water.
Then, 50 mL of the extract (JN extract diluted with distilled water at a ratio of 1:4 v/v) was dropped slowly into this mixture, and an additional 0.02 g of PEG400 was added before stirring for 4 h.Thereafter, the solid part was treated as mentioned above.The obtained product was named as YPS@Ag-JSE.

Attenuated total reflectance infrared (ATR-IR) spectroscopy
The ATR-IR spectra of YPS and YPS@AgNP samples were recorded using a Nicolet iS10 spectrometer (Thermo Scientific, USA) in the wavenumber range of 4,000-400 cm −1 , a resolution of 8 cm −1 , and a scan average of 16 times.

Scanning electron microscopy (SEM)
SEM of YPS and YPS@AgNPs samples was performed on a SEM-S-4800 device (Hitachi, Japan).The samples were coated with Pt to increase their conductivity.

X-ray diffraction (XRD) analysis
XRD patterns of the YPS and YPS@AgNPs samples were performed on a Siemens D5000 X-ray diffractometer (Germany) with 2θ from 10 to 80°and an X-ray CuKα incident beam with a wavelength of 0.154 nm.

Size distribution and zeta potential
The size distribution and zeta potential of the YPS and YPS@AgNP samples were obtained using a SZ-100Z2 nanoparticle analyzer (Horiba, Japan).The samples were weighed in the same amount and dispersed in distilled water by ultrasonication for 30 min before recording.The experiment was repeated three times.

Ultraviolet-visible (UV-Vis) spectroscopy
UV-Vis spectra of the YPS and YPS@AgNP samples dispersed in distilled water were recorded on a UV-Vis spectrometer (Libra S80, Biochrom, UK).UV-Vis diffuse reflectance spectra of powdery samples were recorded using a UV-2450 spectrometer (Shimadzu, Japan) in a wavelength range of 200-1,400 nm.

Atomic absorption spectroscopy (AAS)
The AAS spectra of the YPS@AgNP samples were recorded on an AASICE3500 spectrometer (Thermo Scientific, Germany).
The particles weighed 0.01 g and were dissolved in 1 mL concentrated HNO 3 solution before adding an additional 4 mL of distilled water.The samples were then diluted 50 times to evaluate the concentration of Ag(I) ions in the solution.

Water contact angle (WCA)
The WCA of AC-based coatings was carried out using an OCA 50 device (Dataphysics, Germany) at 20°C using an unchanged shape of 4 mL dose-distilled water droplet with a dropping rate of 2 mL•s −1 .The ambient phase was air.

Determination of the antibacterial activity of YPS decorated by AgNPs
The antibacterial activities of the YPS@AgNP samples were evaluated using the agar diffusion method.Bacterial strains (ATCC, Manassas, USA) included S. aureus (ATCC 6538), a Gram(+) bacterium, and E. coli (ATCC 8739), a Gram(−) bacterium.P. stutzeri B27, a Gram(−) marine bacterium was isolated from the marine region at Phu Quoc City, Kien Giang Province (Vietnam).Tryptic soy agar (TSA) was provided by Merck Co. (Germany).
The YPS@AgNP samples and YPS in powder form were dispersed in distilled water using ultrasonication to form suspensions.Next, 50 µL of the suspension was dropped into an agar well.The agar dishes were kept at 4°C for 4 h and then cultured at 37°C.After 24 h of testing, the inhibition zone of the sample was observed and measured.
Ampicillin was used as a positive control sample at a concentration of 10 µg•mL −1 .Reverse osmosis (RO) water was a negative control sample.The minimum inhibitory concentration (MIC) value of the YPS@AgNP sample was determined by diluting the mother suspension of 1 mg•mL −1 YPS@AgNPs to 10, 20, 40, 80, 160, and 320 times corresponding to 100, 50, 25, 12.5, 6.25, and 3.125 µg•mL −1 to obtain the lowest concentration that could inhibit the growth of bacteria in agar wells.

ATR-IR spectra of YPS@AgNP samples
Functional groups of YPS and YPS@AgNP samples were determined by ATR-IR spectroscopy.The IR spectra of the YPS and YPS@AgNP samples are shown in Figure 1.From the IR spectrum of the YPS sample, two main characteristic absorption peaks were observed: (1) the absorption peaks of M-O bonds, including the absorption bands around 598 cm −1 attributed to polyhedral (CaO n ), the presence of Al-O and Si-O bonds and pseudowollastonite α-CaSiO 3 at 712 cm −1 , the stretching vibration of Si-O groups at 923 cm −1 , indicating the glass-like earth silicon, and the deformation vibration of the bridge Si-O-Si and terminal O-Si-O groups at 592 cm −1 .(2) The sharp peaks around 1,428 cm −1 , representing the vibration of the CO 3 2− anion in calcite.These results can be supported by the reports of Zinesh [34] and Nguyen et al. [31].Consequently, it can be observed that the functional groups remain the same with the YPS sample.In other words, the process of decorating AgNPs did not modify the functional groups in the YPS.The appearance of new peaks at 3,252 cm −1 and at around 1,563-1,591 cm −1 indicated O-H linkage in the plant extract residues, as there were no signals of the -OH group in the YPS@Ag-Na sample.

Morphology of YPS@AgNP samples
The morphology of the YPS@AgNP sample was determined by SEM.The SEM images are shown in Figures 2 and 3.The YPS has a cubic shape with a smooth surface, while the YPS@AgNP samples have a rougher surface with a smaller cubic size (Figure 2).Interestingly, there are many spherical particles on the surface of YPS, indicating that AgNPs were successfully produced on the surface of YPS.
The SEM images of YPS@Ag samples indicate that AgNPs are attached to the surface of YPS and retain the initial morphology of the YPS.Obviously, the density of AgNPs clinging to the YPS surface depended on the nature of the reducing agents used.In detail, the number of AgNPs on the YPS surface decreased, respectively, when reducing agents were used in the following order: JSE, NaBH 4, and ANE.The AgNPs synthesized using JSE adhered evenly on the YPS surface.Besides, the SEM images of AgNPs produced using NaBH 4 as a reducing agent illustrate that the AgNPs have good adhesion and coating on the YPS surface, especially when JSE was used for reducing.However, the SEM image of YPS@Ag-ANE indicated that the AgNPs were not well-adhered to the YPS surface and tended to attach in arrays.Overall, it can be concluded that AgNPs were effectively synthesized using the reducing agents JSE and NaBH 4 , but ANE was quite ineffective compared to the two mentioned reducing agents above.
It can be seen that AgNPs were successfully synthesized in a large quantity using JSE (Figure 3 -YPS@Ag-JSE).The AgNPs have a spherical shape and a particle size in the range of 20-40 nm.Apart from the reductive reaction, the acid-base oxide reaction between organic substances in the JSE (tannins, oleanolic acid, betulinic acid, etc.) and CaO particles in the YPS may contribute to the uniform dispersion of AgNPs on the YPS surface [34][35][36][37].

Crystallographic structure
The structures of crystal YPS and YPS@Ag-JSE were identified by the XRD method, as displayed in Figure 4.
It can be observed from the XRD pattern of the YPS sample that it has calcium phosphate β and hatrurite crystals.The peak intensity of the calcium phosphate crystal in the XRD pattern of YPS is greater than that of the hatrurite crystal, which indicates that calcium phosphate crystals account for the majority of the YPS sample.The calcium phosphate β crystals were placed at the diffraction angles 2θ of 31° [31].
In the XRD analysis of the YPS@Ag-JSE sample, the calcium phosphate β crystals remained in the YPS@Ag-JSE XRD pattern but with much lower intensity.This can be explained by the formation of CaO/AgNPs that decreased the crystals of calcium phosphate [37].The diffraction peak characterized for AgNPs in the XRD pattern of the YPS@Ag-JSE samples was presented at diffraction angles 2θ: 38.117°, 44.301°, 66.447°, and 77.402° [38].Overall, the above XRD analysis results confirmed that AgNPs were successfully produced on the YPS surface.

Size and zeta potential
Table 1 and Figure 5 present the Z-average particle size (ZP), polydispersity index (PI), zeta potential, and size distribution of YPS@Ag samples, as determined by DLS methods.
YPS was settled at the bottom of the jar due to their enormous size, ca.micrometers (Figure 5a).Although tPEG400 has been used as a co-surfactant, it was difficult to disperse YPS@Ag-Na in distilled water (Figure 5a).In contrast, the YPS@Ag-JSE and YPS@Ag-ANE samples were dispersed better in water.This may be owing to the presence of plant extracts as stabilizer agents.
As shown in Figure 5a, AgNPs were separated partly from the YPS@AgNP solution using ultrasonication.It can be concluded from the ZP value (Table 1) that the JSE gave the best result as JSE can effectively reduce Ag(I) ions to both largest amount and smallest particle size in comparison with the other reduction (about 218.43 ± 18.42 nm). Figure 5b illustrates the size distribution of AgNPs using JSE as a reducing agent.The particle size of YPS@AgNPs ranged from 72.87 to 454.69 nm, with 171.25 nm mostly found at a frequency of 11.08%.This means the JSE is not only effective in reducing Ag(I) to AgNPs but also stabilizes YPS@Ag-JSE particles [39].
The obtained values bring insights into the electrostatic repulsion values between particles of YPS@AgNPs, which are −28.33,−32.3, −35.83, and −49.90 mV for the YPS@Ag-ANE, YPS@Ag-Na, YPS@Ag-JSE, and YPS samples that conventionally represent the least stable to the most stable one, respectively.The YPS sample exhibited the most   negative value of zeta potential, as the majority of YPS are CaO and SiO 2 , both of which have negative surface charges [40,41].Therefore, the existence of AgNPs decreased the negative charge of the YPS surface, leading to the lower negativity of the zeta potential of the YPS@AgNPs samples.In other words, AgNPs were successfully produced on the YPS surface.The AgNPs synthesized using the JSE reducing agent have the best particle stability in a suspension.Additionally, the YPS@Ag-JSE sample showed the best mobility in a suspension.

UV-Vis spectra
To get a better understanding on the formation of AgNPs on the YPS surface as reported by the above measurements, the UV-Vis method was used for finding the UV absorption peak at around 400 nm, characterized for surface plasmon resonance (SPR) of the AgNPs [5,7,8].Figure 6 demonstrates the UV-Vis spectra of YPS and YPS@AgNP samples.
From Figure 6, the UV-Vis spectrum of the YPS sample does not show a peak at around 400 cm −1 but, for the YPS@AgNPs samples, it is obvious that there was a peak at around 400 cm −1 .This indicates that AgNPs were successfully synthesized on the YPS.This result was similar to that reported in the study of Jorge et al. [5].YPS@Ag-JSE exhibited the highest absorbance of 0.8, and the absorbances of YPS@Ag-ANE and YPS@Ag-Na were about 0.5 and 0.55, respectively.These absorbance data satisfy the size and zeta potential results of the YPS@AgNP samples that used PEG400 as the surfactant for good dispersion of AgNPs in solution.As a result, the absorbance intensity of YPS@AgNPs using bio-reduction agents was higher than that of the YPS@AgNPs that used a chemical reduction agent.This may be because the plant extracts may play a role as stabilizers, as mentioned above.

Concentration of AgNPs in YPS samples and the yield of the reduction process
The concentrations of AgNPs in the YPS@Ag samples were determined using the AAS method and are presented in  ).The ANE had the lowest AgNP concentration in the YPS@ AgNP samples (64.92%).These yields could be attributed to different natures, structures, and compositions of the reducing agents as well as their active mechanism.The ANE contains flavonoids, tannins, and alkaloids, which are easily reduced by NaBH 4 .For example, flavonoids can be reduced to flavanols by using NaBH 4 as an anti-oxidant.However, in the case of coumarins (the main component of JSE -49%), they are stronger reductive agents than NaBH 4 , and the reducing process of coumarins using NaBH 4 is hard: harsh temperatures or using reagents such as Ni 2 B and NaBH 4 /MeOH [42].The obtained results can be strongly confirmed by SEM and UV-Vis analysis.The AAS analysis results emphasize the strength of the reduction, as JSEa bio-reduction, exhibited the strongest ability to reduce Ag(I) to Ag(0) because it gave the highest reduction yield (Table 2).Compared with the conventional method, using chemical reduction (NaBH 4 ), for instance, the biosynthesis provided much higher efficiency in reducing Ag(I) to the Ag metal.Additionally, in bio-reduction, JSE also caused ion reduction with good efficiency and was environmentally friendly as it decreased the use of chemical materials.Also, JSE is affordable and much cheaper than other chemical reducing agents.

UV-Vis diffuse reflectance spectroscopy
Figure 7 illustrates the ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) of the YPS and YPS@Ag samples.Despite having the capacity to absorb photons, calcite oxide's photon absorbance was reduced by the emergence of silicate oxide and phosphorus, making it impossible to observe bands in the UV-Vis DRS spectra [43].YPS can absorb poorly in visible and near-infrared light with an absorbance of 0.2-0.3.However, the presence of AgNPs and the black color of the powder caused a strong increase in the absorbance in the UV-Vis-Nir region of YPS@AgNP samples.The UV-Vis-Nir absorbance of the YPS@Ag-JSE and YPS@Ag-ANE was greatly elevated, especially, the absorbance of YPS@Ag-JSE was maintained at 1.2-1.3 in the wavelength range of 200-1,400 nm.The difference in the UV-Vis-Nir absorbance of the YPS@AgNP samples may be because the amount of AgNPs formed in the powder samples was affected by the different reducing agents.JSE has the highest reduction yield, as shown in Table 2; therefore, the absorbance in visible and near-infrared light of the YPS@Ag-JSE sample was the highest.Although the reduction yield of YPS@Ag-ANE was lower than that of YPS@Ag-Na, the visible and near-infrared light absorbance of YPS@Ag-ANE was higher because the ANE was absorbed partly on the YPS, as exhibited in Figure 5a.
The band gap energy (E g ) value of YPS@AgNP samples according to the Tauc plot was calculated using UV-Vis DRS data in order to have a better knowledge of the photocatalytic capability of YPS and YPS@AgNP samples.The equation for calculating E g of samples is where n = 2. From Eq. ( 1), the E g values of YPS, YPS@Ag-Na, YPS@Ag-JSE, and YPS@Ag-ANE were 3.3, 2.4, 1.1, and 1.2 eV, respectively (Figure 8).The E g values of the samples tend to decrease with the decoration of AgNPs along with the  reduction agent, especially bio-agents.In other words, the green reduction, together with AgNPs, have a significant role in the improvement of the photocatalytic ability of YPS because the energy requirement for the photon to jump to another level is low.Notably, from these E g values, it is evident that YPS@AgNPs are potential materials in dye photodegradation, as the lower the E g values, the better the photocatalytic process of the dye.Hence, it might solve several problems related to environmental pollution caused by dyes.Additionally, it could act as a painting-addictive for self-cleaning paints when exposed to sunlight.Therefore, this biosynthetic process of AgNPs can contribute to various fields of environment.In particular, YPS is a byproduct of the yellow phosphorus production.The reuse does not involve cost and also solves the environmental problem.JSE is easily found in our country, and the extraction of plants with ethanol involves low cost and is environmentally friendly.The decoration yield of AgNPs on YPS is quite high, reaching 94.45%.The experimental procedure is simple and does not need complex devices; therefore, this process can be expanded on a pilot or industrial scale.

Antibacterial ability of YPS@Ag samples
The negative control sample was sterilized in RO aqueous solution (50 µL•well −1 ), and the positive control sample was 10 µg•mL −1 ampicillin solution (50 µL•well −1 ).Inhibition zones of samples that were prepared with a concentration of 0.01 g• mL −1 are listed in Table 3.It can be concluded that the YPS@Ag-JSE sample exhibited the best antibacterial activity.
The YPS@Ag-JSE sample can strongly fight against E. coli (with inhibition zones of 6 mm) and S. aureus (with inhibition zones of 9 mm).With smaller inhibition zones, YPS@Ag-Na shows good results in killing E. coli and S. aureus.It can also be seen from Table 3 and Figure 9 that YPS@Ag-ANE cannot aggravate E. coli but can resist S. aureus.These results were confirmed by the above SEM, AAS, and UV-Vis results.The antibacterial ability of YPS@Ag-JSE when using JSE anti-oxidant as a reducing agent is the same as that reported in the study of Prabodh et al. [44].In conclusion, the YPS@Ag-JSE samples have high potential for resistance to bacteria.
The effect of YPS@Ag-JSE concentrations on the antibacterial activity against E. coli and S. aureus is demonstrated in Table 4.The YPS@Ag-JSE samples were diluted 10, 20, 40, 80, 160, and 320 times in sterilized RO aqueous solution to obtain different concentrations.The negative control sample was sterilized in RO aqueous solution (50 µL•well −1 ), and the positive control sample was 0.25 mg•mL −1 ampicillin solution (50 µL•well −1 ).
The YPS@Ag-JSE sample has an MIC for E. coli lower than 6.25 µg•mL −1 because, at a concentration diluted 160 times (equivalent to the concentration of 6.25 µg•mL −1 ), it still has antibacterial activity (Table 4).The undiluted YPS@Ag-JSE sample was resistant to S. aureus (the diameter of the ring was 3 mm), and when diluted 10 times (50 µg•mL −1 ), it could not inhibit the growth of S. aureus.The MIC of YPS@Ag-JSE for P. stutzeri B27 was determined to range from 20 to 40 diluted times (50 and 20 µg•mL −1 , respectively).Hence, the antimicrobial activity of YPS@Ag-JSE depended strongly on the sample's concentration with the change in the inhibition zones of bacteria similar to that using the YPS@Ag agents.In particular, the E. coli and S. aureus remained in the same inhibition zone after 24  and 48 h of testing.However, for the P. stutzeri B27, the inhibition zone tends to become larger, which caused the shorter diameter of the zone (decrease from 10 to 8 mm).This can be explained by the excess and careless washing of PEG400 in the samples.Although the PEG400 was reported as an antibacterial agent to kill E. coli and S. aureus [45], it was also a nutrient source for the P. stutzeri B27 growth in general [5].Therefore, after 48 h of testing, it can be seen to grow and shorten the zone diameter to P. stutzeri (Figure 10).
3.9 Use of YPS@AgNPs as an enhancement additive in emulsion acrylic coatings 3.9.1 IR spectra YPS@AgNPs with a good antibacterial activity could be used as an effective additive to improve the mechanical properties and antibacterial activity of the organic matrix.The emulsion acrylic (AC) resin (commercial name: R4152, China) coatings contained 10 wt% of the YPS@AgNP samples synthesized using different reducing agents that have been prepared and studied.The AC and AC/YPS coatings were used as control samples.The emulsion acrylic coatings were left at ambient temperature to dry naturally for 7 days for stability before evaluating their properties.IR spectra of emulsion acrylic (AC), AC/YPS, and AC/YPS@AgNP coatings are shown in Figure 11.The sharp band located at 1,727 cm −1 corresponded to the stretching vibration of the C═O linkage in the AC resin.The weak bands at 2,952 and 1,440 cm −1 were assigned to the stretching and bending vibrations of the C-H bond in the AC resin.The band at 1,142 cm −1 was attributed to the stretching vibration of the C-O linkage in the AC resin.The position of functional groups in the AC resin was not shifted, suggesting that the presence of YPS and YPS@AgNP particles did not influence the functional groups of the AC coatings.

Coating abrasion
The abrasion resistance or falling sand durability (which was determined by using a Falling sand abrasion tester [Erichsen] according to ASTM D968-15 standard) of emulsion acrylic coatings containing YPS@AgNPs are shown in Table 5.
As observed from Table 5, the AC coatings contained 10 wt% YPS@AgNPs, and the abrasion resistance of AC/YPS and AC/YPS@AgNP coatings is higher than that of AC, showing an enhancement in the abrasion resistance of YPS and YPS@AgNPs on AC.In particular, the AC/YPS@Ag-JSE coating has the best abrasion resistance.The one-way analysis of variance (ANOVA) test result for independent treatments (five tested coatings) presented an F-statistic of 6.154 and a p-value of 0.0092.The p-value is lower than 0.05, suggesting that the difference in the abrasion resistance of all coatings is significant.However, the result of the Tukey honestly significant difference (Turkey HSD) test for each pair of coatings showed insignificance (p > 0.05), except for the pair of AC/YPS@Ag-JSE coating versus AC coating (p < 0.01).This indicated that the YPS@Ag-JSE had enhanced ability in the mechanical properties of AC coating more than the YPS and other YPS@AgNPs.

Relative hardness
The relative hardness of AC coatings containing various reducing agents is also illustrated in Table 5.It can be seen that these AC coatings with YPS@AgNPs have higher relative hardness than the AC coating containing only the YPS particles or neat AC coating.This indicates that YPS@AgNPs have the ability to strengthen the elastic modulus of the emulsion acrylic coatings.In comparison, YPS@AgNPs were prepared using different reducing agents, and the YPS@Ag-JSE enhanced the relative hardness of AC coatings better than the YPS@Ag-Na and YPS@Ag-ANE.This may be due to the distribution of YPS@Ag-JSE in the water being better than that of YPS@Ag-Na or YPS@Ag-ANE; hence, it could disperse into the AC coating more regularly.Thus, the relative hardness of AC coating was improved.The F-statistic and p-value of the one-way ANOVA test for the relative hardness of five tested coatings are 17.7025 and 0.0002, respectively.The p-value <0.05 corresponded to a significant difference.
To strongly outline the potential of bio-reduction, Tukey HSD results were considered.From the data in Table 6, a significant difference in the treatment pairs was observed compared to the relative hardness of AC (E) with AC/ YPS@AgNPs (A, B, C); however, an insignificant difference was observed compared to the relative hardness of AC (E) vs AC/YPS (D).This result confirmed that YPS@AgNP particles play an important role in the improvement of the relative hardness of AC coatings, which was better than the YPS particles.However, when comparing the relative hardness of AC/YPS@AgNP coating with that of the AC/YPS coating, only YPS@Ag-JSE exhibited a better enhancement for AC coating.As compared with the relative hardness of AC/ YPS@Ag-ANE (A) and AC/YPS@Ag-Na (C), the relative hardness of AC/YPS@Ag-JSE (B) exhibited a significant difference.
YPS@Ag-JSE could significantly improve the mechanical properties of AC coatings.This may be due to the regular dispersion of YPS@Ag-JSE due to the small size as well as the residues of JSE and PEG400 co-surfactant could help fill defects in the AC coating, leading to the structure of the coating becoming tighter; thus, the ability to withstand external forces of the coating was enhanced.Moreover, under the impact of stress, the forces can transform from the AC resin to the YPS@Ag-JSE particles, helping to distribute evenly the applied force to the whole of the coating's surface.

Adhesion ability
The adhesion ability of AC coatings containing YPS@AgNPs, which was determined by Elcometer 510 Automatic Pull-Off Adhesion Gauge according to ASTM D4541, is also    5. It is clear that these AC coatings with YPS and YPS@AgNPs have good adhesion ability.Interestingly, the YPS itself can improve the adhesion force of the AC film, owing to the main component of the YPS being calcite oxide, which can increase the adhesive stress of the materials [46].The adhesion ability of the coatings was in the order AC < AC/YPS <AC/YPS@Ag-Na <AC/YPS@Ag-ANE <AC/YPS@Ag-JSE.The mobility of YPS@AgNPs can contribute to the explanation of this order, in which phenolic compounds in bio-reducing agents contain mobile hydrogen atoms in the -OH group that can interact with the AC coatings through the lone-pair electron in the oxygen atom of the AC.In other words, it can enhance the adhesion force of atoms in samples with the steel substrate.The F-statistic and p-value obtained from the one-way ANOVA test for the adhesion of coatings are 16.0253 and 0.0002, respectively.The p-value < 0.05 corresponded to a significant difference in the adhesion of the tested coatings.The Turkey HSD test results in a significant difference in the adhesion of AC/YPS and AC/YPS@AgNP coatings vs AC coating (p < 0.05), though the difference in the adhesion of AC/YPS coating vs AC/YPS@AgNPs coatings and that of AC/ YPS@AgNPs coating are insignificant.

Hydrophilic/hydrophobic properties of emulsion acrylic coatings
Another property of the AC/YPS and AC/YPS@AgNPs coatings that can be affected by YPS and YPS@AgNPs is the WCA, which evaluates the hydrophilic/hydrophobic properties of the AC, AC/YPS, and AC/YPS@AgNP coatings.The WCA of AC-based coatings, such as AC, AC/YPS@Ag-JSE, AC/ YPS@Ag-Na, AC/YPS@Ag-ANE, and AC/YPS, are presented in Figure 12.The WCA of the tested coatings are in the order: AC (43.90 ± 3.58°) < AC/YPS (56.78 ± 3.78°) < AC/ YPS@Ag-ANE (57.80 ± 0.72°) < AC/YPS@Ag-JSE (61.15 ± 4.29°) < AC/YPS@Ag-Na (63.72 ± 1.83°).The increased tendency in WCA of coatings containing YPS and YPS@AgNP particles is mainly due to the presence of metal oxides in YPS.Consequently, the dispersion of YPS and YPS@AgNPs particles in AC coatings could play the role of a barrier to prevent the permeation of water into the coatings.The difference in WCA of AC/YPS@AgNPS and AC/YPS is slight; therefore, it is an insignificant difference in the adhesion of these samples as aforementioned.
From the above results, it can be seen that YPS@AgNP particles are promising enhancement additives for organic coatings, especially when using 10 wt% of YPS@AgNP particles, the abrasion resistance, relative hardness, and adhesion of the AC coatings improved significantly.The evaluation of the antibacterial activity of the organic coatings containing YPS@AgNPs is in progress, and the results will be published in the future.

Conclusion
The use of "green chemistry" is not only to solve the environmental pollution problem due to recycling YPS (a potential carrier for AgNPs reduced by plant extracts [ANE, JSE]) but also is the green procedure to fabricate YPS@AgNP particles with high efficiency.More importantly, it gave a magnificent result against E. coli and S. aureus bacteria, especially the new strain of P. stutzeri B27.YPS@AgNP were prepared using the reduction agent of phenolic-rich plant extract, JSE, which plays a good reductive role in the AgNP-producing process with high efficiency and controls AgNP morphology.Using 10 wt% of YPS@AgNPs particles, the abrasion resistance, relative hardness, and adhesion of emulsion acrylic (AC) coatings were improved significantly.The results obtained from evaluating the antibacterial activity of AC coatings with YPS@AgNP particles indicated that the YPS@AgNP samples, especially YPS@Ag-JSE, are potential antibacterial agents with low cost and simple strategy for production.

Figure 11 :
Figure 11: IR spectra of the AC coating samples.

Table 1 :
Z-average size, PI, and zeta potential of YPS@Ag nanoparticle samples

Table 2 .
With the highest yield of 94.45%, the JSE yielded the largest quantity of AgNPs modified on the YPS surface.The second highest one was AgNPs, synthesized using NaBH 4 (81.19%

Table 3 :
Inhibition zones of YPS and YPS@AgNP samples

Table 4 :
MIC values of YPS@Ag-JSE samples

Table 5 :
Effect of YPS@AgNPs prepared with various reducing agents on the mechanical properties of emulsion acrylic coatings

Table 6 :
Tukey HSD results for relative hardness of treatment pair of ACbased coatings