Green Synthesis of Highly Concentrated and Stable Colloidal Dispersion of Core-Shell Silver Nanoparticles (AgNPs-Shell) and their Antimicrobial and Ultra-high Catalytic Properties

The versatile one-pot green synthesis of a highly concentrated and stable colloidal dispersion of AgNPs was carried out using the self-assembled tannic acid without using any other hazardous chemicals. Tannic acid (Plant-based polyphenol) was used as a reducing and stabilizing agent for silver nitrate in a mild alkaline condition. The synthesized AgNPs were characterized for their concentration, capping, size distribution, and shape. The experimental results confirmed the successful synthesis of nearly spherical and highly concentrated (2281 ppm) AgNPs, capped with polytannic acid (AgNPs-PTA). The average particle size of AgNPs-PTA was found 9.90 ± 1.60 nm. The colloidal dispersion of synthesized nanoparticles was observed stable for more than 15 months in the ambient environment (25 oC, 65 % relative humidity). The synthesized AgNPs-PTA showed an effective antimicrobial activity against Staphylococcus Aureus Escherichia coli. Ag-PTA also exhibited enhanced catalytic properties. It reduces 4nitrophenol into 4-aminophenol in the presence of NaBH4 with a normalized rate constant (Knor = K/m) of 615.04 mL·s-1·mg-1. Furthermore, AgNPs-PTA were stable for more than 15 months under ambient conditions. The unique core-shell structure and ease of synthesis render the synthesized nanoparticles superior to others, with potential for large-scale applications, especially in the field of catalysis and biomedical.


Introduction
Metallic nanoparticles have attracted the attention of scientists and researchers and have found applications in the emerging fields of nanoscience and biomedical technology [1,2]. Among many metallic nanoparticles, silver nanoparticles (AgNPs) are well known for their wide range use for optical, catalytic, electrical, and biomedical applications [3][4][5]. Generally, AgNPs are prepared by the chemical [6], and physical [7] synthesis methods. The physical and chemical methods for the synthesis of AgNPs involve the use of toxic, costly, and environmentally hazardous materials. Most recently, the green synthesis methods [5] are introduced that allow the use of eco-friendly, cost-effective and nontoxic preparation of AgNPs. Plant-mediated green synthesis of AgNPs has several advantages over the physical and chemical synthesis methods as it is facile, cost-effective, easy to scale up, and eco-friendly [8].
Furthermore, the green synthesis technique results in higher stable dispersions of nanoparticles without using high energy input, high pressure, high temperature, and toxic chemicals [9].
Tannic acid (TA) contains catechol and galloyl groups, which are well known for their metal chelation and material surface binding properties and play a vital role in the green synthesis of nanoparticles at room temperature by acting as both, reducing and capping agent [10,11]. In alkaline conditions, TA polymerizes and forms a capping layer of poly tannic acid [12]. These two properties of TA make it an ideal candidate for the green synthesis of metallic nanoparticles. In the past, several attempts were reported to fabricate tannic acid-mediated nanoparticles and their potential applications in the field of surface-enhanced Raman scattering [13], catalysis [14], treatment of organic pollutants [15], visual detection of ions [16], toxic gas sensing [17], and biomedical [18].
Due to the multi-drug resistant bacteria, it is vital to find alternative means to kill them [19,20] The solution was cooled down to room temperature and the freshly prepared nanoparticles were collected by centrifugal separation (1100 RPM for 15 minutes) and were washed three times using DI water. Figure 1 shows a schematic of the steps involved in this novel method of Ag NPs preparation. were placed overnight in the vacuum oven for drying at 50 ºC.

Characterization
The successful synthesis of Ag NPs-PTA was confirmed by measuring the ultraviolet-visible (UV-Vis) spectrum using

Antimicrobial Tests
The Antimicrobial activity of synthesized nanoparticles was analyzed against Gram-negative, Escherichia coli (E. coli, ATCC1129) and Gram-positive, Staphylococcus aureus (S. aureus, ATCC 6538). The AATC147-2004 standard zone of inhibition test was adopted to determine the Antimicrobial performance of synthesized Ag NPs-PTA [26].
The Luria-Bertani (LB) agar solution (25 ml) was added to the agar plates and were placed into a refrigerator for 15 minutes. 50 µl of fresh suspension of bacterial strains (E. coli and S. aureus, 10 5 -10 6 CFU per ml) was transferred to LB agar plates. Bacterial colonies were spread gently in the plate surface with the help of a sterilized glass rod. Round shape ACF samples (1.5×1.5 cm) containing 0.5 mg, 1 mg, 2 mg, and 4 mg concentrations of synthesized Ag NPs-PTA were placed in the LB agar plates along with the reference (control) ACF sample (without Ag NPs-PTA). LB agar plates having ACF samples were placed into the incubator at 37 ºC for 24 hours. The zones of inhibition around the samples were measured to determine the antimicrobial activity of Ag NPs-PTA.

Catalytic reduction of 4-NP
The catalytic reduction of the 4-NP into 4-AP in the presence of excess NaBH4 was carried out to analyze the catalytic activity Ag NPs-PTA. Briefly, 2 mL of fresh DI water, 1 mL of freshly prepared NaBH4 3

Results and Discussion
UV-Vis absorption spectrophotometer was used to confirm the successful synthesis of Ag NPs-PTA. Figure 2 represents the characteristic UV-Vis spectrum of the synthesized Ag NPs-PTA nanoparticle dispersion. The concentrated sample was diluted 300 times for UV-Vis analysis. The results, figure 2, shows a sharp absorption at 440 nm, which is a typical surface plasmon resonance absorbance band for Ag NPs-PTA [12].   To analyze the long-term stability of the Ag NPs-PTA, the sample was aged for 15 months in ambient conditions.

Antimicrobial Response Analysis
The ACF sheets were drop coated with Ag NPs-PTA dispersion to analyze the antimicrobial response of the synthesized Ag NPs-PTA. In general, nanoparticles easily release from the ACF sheet and they can effectively kill the microbes [35]. Figure 6 represents the SEM images of the ACF sheets drop coated with (b & c) and without (a) AgNPs-PTA dispersion. The results, in figure 6, show that the Ag NPs-PTA are not aggregated and are homogeneously distributed on the ACF sheet surface which is beneficial for antimicrobial applications.

Catalytic Reduction of 4-NP
In addition to antimicrobial properties, synthesized Ag NPs-PTA nanocomposites also exhibited enhanced catalytic properties. The reduction of 4-NP to 4-AP was analyzed in the presence of excess NaBH4. This model reduction reaction was monitored by UV-Vis spectroscopy at different intervals of time. As shown in Fig. 8, after the addition of Ag NPs-PTA nanocomposites, the characteristic absorption peak of 4-NP at 400 nm continues to decrease with time, while a new peak at 300 nm, attributed to 4-aminophenol (4-AP), increases slowly and simultaneously. The characteristic yellow color of the 4-NP disappeared completely after the completion of the reaction. It is important to note that, in the absence of synthesized nanocatalyst, characteristics yellow color and absorption peak of 4-NP (λ =400 nm) does not change even after 24 hours of incubation.  Table 3 that the catalytic activity of synthesized Ag NPs-PTA nanocatalyst for the reduction of organic pollutant 4-NP was superior when compared to previously reported silver-based catalysts.
aureus, and E. coli microbes. The synthesized Ag NPs-PTA nanocatalyst also displayed an enhanced catalytic performance for the reduction 4-NP to 4-AP with rate constant of (Knor) 63.71 mL·s -1 ·mg -1 . This study may offer a unique opportunity for the fabrication of multifunctional metal-PTA nanocomposites, which will have many unique futures uses like catalytic, metal detection, and biomedical applications.