摘要：

Synthesizing nanoparticles with the less environmentally malignant approach using plant extract is of great interest; this is because most of the chemical approaches can be very costly, toxic, and time-consuming. Herein, we report the use of Acacia senegal leaf extracts to synthesize silver nanoparticles (AgNPs) using an environmentally greener approach. Silver ions were reduced using the bioactive components of the plant extracts with observable colour change from faint colourless to a brownish solution as indication of AgNP formation. The structural properties of the as-synthesized AgNPs were characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and UV-Vis absorption spectrum. Antimicrobial assessment of the as-synthesized AgNPs was explored on some strains of gram-positive and gram-negative bacteria. The obtained results indicate that the as-synthesized AgNPs are pure crystallite of cubic phase of AgNPs, fairly dispersed with a size range of 10–19 nm. The AgNPs were found to be small in size and exhibit significant antibacterial activities, suggesting that the as-synthesized AgNPs could be used in the pharmaceutical and food industries as bactericidal agents. 1. Introduction The biosynthetic approach in the use of naturally occurring reducing agents such as plant extracts, biomass, and biological molecules has emerged as a simple alternative method to complex chemical method of nanoparticle synthesis. The use of silver particles in the medicinal field can be traced back to more than ten decades ago when silver was first used in medicine before the discovery of antibiotics [1, 2]. Silver nanoparticles are known to have unique properties that make them ideal for various biological and biomedical applications such as in the treatment and prevention of certain diseases, for therapeutic and diagnostic use including biomolecular detection [3–7], and in industry [8]. This is mainly a result of the antimicrobial, antibacterial, antifungal, antiviral, and anti-inflammatory capabilities of the AgNPs [4, 6, 9, 10]. Silver is additionally known to possess high thermal and electrical conductivity thus resulting in its good optical reflectivity as well as various biological and catalytic abilities [7, 11, 12], which has, in turn, resulted in its high demand [13, 14]. Although the processes involved in nanoparticle synthesis result in particles possessing different anticipated characteristics, the chemical and physical methods which include UV irradiation, lithography, ultrasonic fields, and photochemical reduction processes for the production of nanoparticles have their own pitfalls in that they are costly, labour-intensive, and toxic to both organisms and the environment [5, 15–18]. Hence, "green" or biogenic synthesis of nanoparticles is now preferred over physicochemical methods because it not only results in more eco-friendly, cost-effective, contamination-free, and nontoxic sustainable nanoparticles but also allows for higher yield of products with better defined characteristics [12, 19–23]. It has been well documented that living plants and bioactive compounds from their extracts such as polyphenols, phenolic acids or proteins, sugars, terpenoids, and alkaloids have the ability to reduce metal ions by acting as electron shuttles and can therefore be used in the bioreduction of harmful metal ions in the synthesis of nanoparticles [17, 20, 22, 24, 25]. Numerous plant parts have been shown to be effective in the reduction of Au and Ag ions for the formation of gold and silver nanoparticles; these include and not limited to lemon grass leaf extracts (Cymbopogon flexuosus), neem (Azadirachta indica), and tamarind (Tamarindus indica) and fruit extract of amla (Emblica officinalis), as well as the biomass of wheat (Triticum aestivum) and oats (Avena sativa) [26]. Through green synthesis, nanoparticles are synthesized within a short time frame in a single-step bottom-up approach where the use of toxic chemicals, high pressure, energy, or temperature

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