Green-Synthesized AgNPs for Bacterial Efficacy and Sustainable Development Goals

• INTRODUCTION

Materials in nano-confines (1–100 nm) have remarkable differences in parcels compared to bulk materials. These differences exist in the physical and structural parcels of particles, molecules, and bulk paraphernalia owing to differences in physicochemical parcels and face-to-volume rates¹. With advances in nanotechnology, numerous nanomaterials with unique properties have emerged, opening new avenues for research. Metal nanoparticles, including silver, platinum, and gold, are used for various purposes^2,3. AgNPs have been used extensively because of their unique physical and chemical characteristics, including electrical conductivity, thermal, optical, antimicrobial, and natural properties. AgNPs are widely used in biomedical applications, such as in crack dressings, antiseptic fabrics, creams, and sprays. AgNPs affect microorganisms through membrane disruption and enzymatic inhibition. Researchers have developed nanomaterials with distinct size-controlled physicochemical features and applications⁶. AgNPs are significant for their operations in healthcare, medicine, photothermal treatments, visual operations, catalytic processes, energy transfer, solar cells, optoelectronics, and environmental applications.⁷ Biological methods are safe, provident, doable, and ecologically sound, with downsides of time consumption and lower influence on size, shape, and liquid structure⁸. Among metallic rudiments, Ag is a noble metal that is central to antibacterial research. This review emphasizes the outlook, development, antibacterial functioning & mechanism, characterization methods, factors affecting bactericidal functioning, antimicrobial relevance, and future perspectives for green nanotechnology-based AgNPs in antibacterial therapeutic applications and infection control for SDG-3⁹.

• THEORETICAL FRAMEWORK

Silver nanoparticles (AgNPs) have attracted attention from researchers because of their defense against microorganisms and drug resistance against commonly used antibiotics¹⁰. The characteristics of AgNPs have made them applicable in biomedical, drug delivery, water treatment, and agrarian fields¹¹. AgNPs are used in ink bonds, electronic devices, and pastes because of their high conductivity¹². AgNPs have been synthesized using various physicochemical methods, such as chemical reduction, gamma shaft radiation¹³, microemulsion, electrochemical system¹⁴, laser ablation¹⁵, autoclave, microwave oven roasting, and photochemical reduction. These methods have effective yields but are limited by toxic chemicals, high costs, and energy requirements. Given these downsides, cost-effective and energy-efficient approaches for AgNP emulsions using microorganisms, plant extracts, and natural polymers as reducing and capping agents are emerging. The combination of nanotechnology and green chemistry will expand biologically and cytologically compatible metallic nanoparticles^16,17,18.

Phytochemicals in plant leaves, including flavonoids, alkaloids, ketones, carboxylic and ascorbic acids, tannins, amides, and phenols, drive the development of metal nanomaterials. These substances reduce metal salts to produce metal nanoparticles¹⁹ and exhibit antibacterial and antioxidant properties. Green synthesis of nanoparticles has lower toxicity and higher biocompatibility than chemical synthesis because of the longer nanomaterial half-life and increased efficiency²⁰. Although the green synthesis of silver nanoparticles has been extensively studied, particularly regarding the synthesis using bacteria and fungi, to our knowledge, there is a lack of detailed information on environmentally friendly synthesis using plant extracts or their antiviral, antifungal, antimicrobial, and anticancer qualities²¹.

Table 1: Silver nanoparticles, their characterization, and applications

S. No	Metal nanoparticles	Reducing agent or extract	Characterization	Particle characteristic	Applications	Author	References
1	AgNPs	Sambucus ebulus	FTIR, UV-Vis XRD HPLC	Size 18.6nm Shape- FCC	Antioxidant,& antibacterial activity	Karan T. et al. (2024)	[22]
2	AgNPs	Rubus discolor leave	UV- Vis, TEM FTIR EDX	Size-37 nm Shape- predominantly spherical	Antibacterial activity against Escherichia coli and Pseudomonas aeruginosa	Ghasemi S. et al. 2024)	[23]
3	AgNPs	Ocimum tenuiflorum and Azadirachta indica	UV- Vis XRD	Shape- FCC Size- 21 nm	S.aureus, E. coli, P.aeruginosa	Reen G. K.. et al. (2024)	[24]
4	AgNPs	Azadirachta indica, Syzygium aromaticum	UV-Vis,		Against Enterococcus	Chandran N. et al. (2024)	[25]
5	AgNPs	Terminalia bellirica	UV- Vis, FTIR, XRD, FESEM	Size- 44.5 nm Shape- FCC	Antimalarial activity	Singh S. et al. (2024)	[26]
6	AgNPs	Pimpinella anisum aqueous seed extract	UV- Vis FTIR XRD	Shape- spherical Size – 20.18 nm, 21.00 nm, 40.08 nm	Antibacterial against a strain of Escherichia coli	Barabadi H. et al.(2023)	[27]
7	AgNPs	Moringa oleigera leaf extract	UV- Vis, FTIR, TEM	Size – 24 -40 nm	antibacterial  Staphylococcus aureus ATCC6538 and pseudomonas aeruginosa ATCC9027	Shaaban T. M. et al (2023)	[28]
8	AgNPs	Equisetum diffusum extract	UV- Vis FTIR, DLS, SEM EDX	Size- 62.6  nm	Antibacterial L.monocytogenes and E. coli	Jabbar A. et al (2023)	[29]
9	AuNPs	Gelidiella acerosa	UV-Vis FTIR	Size- 5-20 nm Shape- Spherical	Antibiotic against  Staphylococcus aureus and, G. acerosa	Subbulakshmi A. et al (2023)	[30]
10	AgNPs	Green tea leaf extracts	UV- Vis., FTIR	Size- 30 – 150 nm Shape- quasi-spherical	High killing ability for bacteria (E. Coli)	Parvathalu K., et al (2023)	[31]
11	AgNPs	Eucalyptus camaldulensis and Terminalia arjuna extracts	UV- Vis, SPR, SEM , FTIR	Size- 23nm, 13 nm, 42 nm	Bacillus subtilis, Staphylococcus aureus, Pasteurella mutocida	Liaqat N. et al (2022)	[32]
12	AgNPs	Tridax procumbens plant (TNP) extracts	TEM, Zeta potential FTIR, HRLC- MS HPLC	Size- 23.17 nm Shape-  FCC	Against Anticancer Escherichia coli, Shigella spp., Aeromonas spp.	Pungle R. et al.(2022)	[33]
13	AgNPs	Conocarpus Lancifolius fruit extracts	UV- Vis, FTIR, XRD	Size- 22.5 nm, Shape – spherical	Anticancer and Antibacterial Streptococcus p. , S. Aureus.	Oves M.et al. (2022)	[34]
14	AgNPs	Syzygium cumini fruit extract	UV- Vis, FTIR, XRD, SEM	Size-  47 nm Shape- FCC	Inhibit free radical orientation diseases., (G +ve & G –ve) bacteria	Chakravarty A. et al. (2022)	[35]
15	AgNPs	Cassia tora seed extracts	UV- Vis, FTIR, SEM, XRD	Size- 55.80 nm, 58 nm, 61.06 nm, 63.26 nm, 64.80 nm	Antibacterial efficacy against S. aureus.,	Nawabjohn M. S. et al (2022)	[36]
16	AgNPs	East extract,  malt extract	UV- Vis, FTIR, TEM, XRD	Size- 13.2 nm Shape – spherical	Antibacterial & cancer activity against (MCP) Escherichia coli, Klebsiella pneumonia	Wypij M. et al.(2021)	[37]
17	AgNPs	Cucumber leaf and rice husk extract	—–	———–	Compared study between g-AgNPs & Chem. AgNPs Strong antibacterial activity Escherichia coli	Zhang H. et al (2021)	[38]
18	AgNPs	Areca catechu extract	UV- Vis, SEM, DLS, FTIR	Size – 25 nm Shape- spherical (round)	Against antibiotic-resistant bacteria	Choi S. J. et al (2021)	[39]
19	AgNPs	Berberis vulgaris B. nigra, Capsella bursa-pastoris, L. angustifolia  plant extract	UV- Vis, FTIR, TEM, XRD, PCCS	Size- 46.1 nm Shape- spherical, truncated octahedron	Antibacterial activity L.monocytogenes (gram +ve)	Salayova A. et al. (2021)	[40]
20.	AgNPs	Grapefruit peel extract	UV- Vis, FTIR	Size – 13.56 nm	Staphylococcus A., Enterococcus faecalis & S. aureus	Arsene M.M.J. et al (2021)	[41]
21	AgNPs	Onion (O), Tomato (T), Acacia catechu (C) alone, and Mixed COT extracts	UV- Vis, FTIR, TEM, XRD, DLS	Size –  25  nm Shape – FCC	Phytocatalitical Deradation of  methyl orange (MO), methyl red (MR), & congo red (CO)	Chand K. et al. (2020)	[42]
22	AgNPs	Brillantaisia patula, Crossopreryx febrifuga, and Senna siamea leaf extract	UV- Vis, FTIR, XRD, TEM	Size- 17 nm Shape- spherical	(G +ve and G – ve Bect.) Staphylococcus A., Escherichia coli & Pseudomonas A.	Kambale K. E. et al. (2020)	[43]
23	AgNPs	Ocimum Canum Sims leaf extract	XRD, SEM	Size – 15.72 nm Shape- spherical and rod shape	Escherichia coli	Tailor G. et al (2020)	[44]
24	AgNPs	flower extract of Aerva lanata	UV- Vis, FTIR, AFM, SEM, TEM	Size- Shape –	Bacterial activity against Klebsiella planticola	Kanniah P. et al (2020)	[45]
25	AgNPs	Gymnema sylvestre leaf extract	UV- Vis, FTIR, XRD, TEM,	Size – 20 nm & 30 nm Shape – FCC	Staphylococcus aureus and Escherichia coli	Rajkumar P. V. et al (2020)	[46]

The plant species used for the eco-friendly synthesis of AgNPs are listed in Table 1. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) are common methods for confirming nanoparticle formation and examining their shape, size, and surface area. The diameters of the reported particle sizes ranged from 5 to 100 nm, except for one. Their shapes included spherical, face-centered cubic (FCC), quasi-spherical, and occasionally truncated or rod-like forms. Nanoparticles exhibit biological and catalytic actions, such as antibacterial, antioxidant, anticancer, and photocatalytic effects, in addition to structural characterization. This indicates that Ag nanoparticles can be widely used in biomedical and environmental fields.

3. SYNTHESIS METHODS FOR NANOPARTICLES

3.1 Top-Down and Bottom-Up Methods: The synthesis of nanoparticles through biological systems is not only advantageous but also imperative for advancing sustainable technology. This method offers unparalleled benefits, including high-yield production, scalability, non-toxicity, and precise morphologies. It is crucial to pioneer novel methods for nanoparticle production, and green synthesis techniques are at the forefront of this innovation. These techniques are not only straightforward and safe but also environmentally friendly, making them indispensable in modern science⁴⁷. Saratale et al have meticulously reviewed various green nanoparticle synthesis techniques, highlighting their transformative applications in agriculture and biomedicine⁴⁸. The synthesis of green nanoparticles employs two distinct approaches: top-down and bottom-up approaches. The top-down approach involves mechanical methods to form larger nanoparticles, whereas the bottom-up approach, which is synonymous with green synthesis, utilizes acids to reduce the particle size. It is essential to integrate the bottom-up method with complex analysis when employing the top-down approach to maximize efficiency and effectiveness⁴⁹. Embracing these green synthesis methods is not just a choice but a necessity for a sustainable future.

Figure 1: Top-down and bottom-up synthesis⁴⁹.

• GREEN SYNTHESIS

Embracing the future of sustainable technology involves the environmentally friendly synthesis of AgNPs. By utilizing a solution of silver metal ions and an organic reducing agent, nanoparticle production can be revolutionized without the need for external stabilizing and capping agents. Remarkably, the natural components found in plants, such as flavonoids, alkaloids, ketones, aldehydes, amides, and ascorbic acid, serve as effective agents, making the process not only efficient but also eco-friendly. The synthesis of AgNPs is not just a possibility; it is a reality that harnesses the power of nature⁵⁰.

The cornerstone of this innovative synthesis is Ag+ ions, which are readily available in various water-soluble silver salts. Among these, the aqueous AgNO3 solution stands out, with an optimal Ag+ ion concentration range of 0.1–10 nm, most commonly at 1 nm. This precise concentration ensured the successful development of AgNPs, paving the way for advancements in nanotechnology. By adopting this method, researchers can lead the charge in creating a sustainable future, demonstrating that cutting-edge technology and environmental responsibility can go hand in hand⁵¹.

3.3. AgNP SYNTHESIS USING PLANT EXTRACT

The green synthesis of AgNPs using plant extracts is not just an advancement in nanotechnology; it is a revolutionary leap forward, offering a sustainable and eco-friendly alternative to traditional methods of synthesis. Harnessing the power of various plant extracts from leaves, flowers, bark, roots, and stems, especially from medicinal plants like Azadirachta indica⁵¹, Aloe vera⁵², Ocimum tenuiflorum⁵³, Emblica officinalis⁵⁴, Tinospora cordifolia⁵⁵, Cocos nucifera⁵⁶, and common spices such as piper nigrum⁵⁷, serves as a potent and natural solution. These plant components underwent a meticulous cleaning process, first with tap water and then with distilled water, to ensure their absolute purity. After drying, they were either ground into a fine powder or used directly to create extracts with varying pH values, while enhancing the stability of AgNPs, with protein metabolites and chlorophyll acting as effective agents⁵⁸.  The efficiency of this process is undeniable, as evidenced by the synthesis of spherical AgNPs averaging 20 nm in a mere 40 min using 60 ml of plant leaf extract and 10 ml of AgNO₃ at 60°C. Prathinha et al.⁵⁹ further substantiated this method by successfully synthesizing AgNPs using Azadirachta indica and Ocimum tenuiflorum green extracts. The striking visual transformation from green to dark brown is a definitive indicator of successful AgNP formation (Figure 2). Importantly, higher concentrations of plant extracts significantly accelerate the synthesis rates ²⁵, while factors such as pH, temperature, optical activity, and AgNO₃ concentration are crucial for optimizing production. The precise collection of AgNPs for characterization was ensured by centrifuging the silver nitrate and plant solutions²⁴. Neem and Ocimum tenuiflorum extracts have been extensively studied, with neem plants exhibiting extraordinary antibacterial and antifungal properties, offering promising applications in bacterial treatment, as illustrated in Figure 3. This innovative approach to AgNP synthesis champions environmental sustainability and holds immense potential for propelling medical and technological advancements^60,62.

Figure 2: Tulsi extract explanation setup during AgNP formation⁵⁹.

This groundbreaking approach not only underscores the remarkable versatility of plant materials but also highlights their pivotal role in advancing nanotechnology. The evidence is clear: plant-based synthesis of AgNPs is not just a possibility but a revolutionary step forward in the field⁶³.

4. ANTIMICROBIAL PROPERTIES OF THE PRODUCED AgNPs

4.1. Well Diffusion Assay

The antibacterial efficacy of AgNPs was rigorously evaluated using the agar well diffusion assay, a method that leaves no room for doubt. The results, measured by inhibition zones (mm) in 6 mm wells on Muller-Hinton agar, were nothing short of compelling. The particles, serially diluted 2-fold from 360 μL/mL to 11.25 μL/mL⁶⁴, demonstrated a formidable antibacterial effect. E. faecalis, a notorious Gram-positive cocci bacterium, leading cause of enterococcal infections in humans and a prevalent pathogen in hospitals, often found in the urinary tract, bloodstream, and surgical sites. The positive control, vancomycin, exhibited its well-documented antibacterial action against E. faecalis, with an impressive inhibition zone of 27.0 ± 0.0 mm. However, the challenge of vancomycin-resistant enterococci (VRE)⁶⁵, a global threat, remains unyielding, as no inhibition zone was observed for either VRE or E. faecalis strains. The AgNPs data, as illustrated in Figure 3, reveal a strain with sustained release, greater stability, tensile strength, nanoscale porosity, flexibility, and dominant antimicrobial efficacy against both gram-negative and gram-positive strains.

Figure 3: Antibacterial effects of AgNPs using A. catechu against E. faecalis and VRE. (a,b) Inhibition zone of AgNPs against E. faecalis; (c,d) inhibition zone of AgNPs against VRE⁶²

This highlights the potential of microemulsions in wound healing applications⁶⁶. P. aeruginosa, a gram-negative bacterium notorious for its green pigment pyocyanin, poses severe health risks, including meningitis, septicemia, post-transplant infections, persistent pneumonia, and urinary tract infections⁶⁷. The rise of multidrug-resistant Pseudomonas aeruginosa (MRPA) is a serious public health concern, escalating morbidity and mortality in hospitalized patients. Alarmingly, MRPA showed no inhibition zone for gentamicin, whereas P. aeruginosa exhibited a substantial inhibition zone of 25.0 ± 0.7 mm diameter. Notably, both bacteria displayed concentration-dependent inhibition zones, with P. aeruginosa treated with AgNPs achieving a slightly larger inhibition zone than MRPA. The results, as depicted in Figure 4 and the accompanying Table, underscore the potential of AgNPs in combating these formidable pathogens⁶⁵.

• CHARACTERIZATION:

The characterization of the synthesized AgNPs using techniques such as Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction (XRD), and scanning electron microscopy (SEM) is critical for revealing details of size, shape, and surface charge ^22,78. Dynamic light scattering (DLS) and zeta potential methods are essential for characterizing antibacterial agents, providing crucial insights into their application ^42,66,79. Atomic force microscopy (AFM) and high-performance liquid chromatography (HPLC) provide additional information for characterizing AgNPs. The transformation of Ag⁺ to Ag° in the aqueous phase is evidenced by a band in the silver colloid spectra at approximately 410 nm³³. As the silver concentration increased, the absorption band sharpened, and a red shift from 402 to 407 nm occurred (Figure 4) when the AgNO₃ concentration increased from 250 to 1000 mg/dm³, indicating larger silver aggregates and increased particle size. The TEM images confirmed the transformation of spherical silver particles into prismatic structures ⁶⁷. The size, morphology, and surface charge of nanocarriers are pivotal determinants of their physical stability and function. Smaller particles with larger surface areas influence drug release, whereas the surface charge affects the interactions between the biological environment and bioactive compounds ⁷⁰. The safety, behavior, and efficacy of biodistribution depend on these factors. A comprehensive description of AgNPs is essential to assess their functional elements.⁶⁸

Figure 4: Scanning electron microscopy images of S. aureus (control and AgNP-treated) and P. aeruginosa (control and AgNP-treated). Red arrows indicate bacterial cell distortions, and the red circle shows nanoparticle accumulation on cells.⁶²

Figure 5: (a) XRD pattern showing peaks at 2-theta angles. (b) The curve shows the absorption feature, indicating the energy band gap. (c) TEM image reveals the spherical morphology and size distribution⁶².

6. APPLICATIONS OF GREEN SYNTHESIZED SILVER NANOPARTICLES FOR HEALTH AND WELL-BEING, SDG 3

Green-synthesized AgNPs have gained attention owing to their unique properties and applications across fields. Their environmentally friendly synthesis using biological agents offers advantages over traditional chemical methods of synthesis. AgNPs exhibit antimicrobial properties against bacteria, fungi, and viruses, making them promising candidates for use in wound dressings, medical devices, and water purification^69,79. Green-synthesized AgNPs have applications in drug delivery, biosensing, and cancer therapy, enabling targeted drug delivery and cancer cell apoptosis⁷⁰.

SDG 12 Catalysis: AgNPs are efficient catalysts in chemical reactions, including organic synthesis and environmental remediation. Their surface area and electronic properties enhance their catalytic activity^71,78.

SDG 6, Environmental Remediation: Green-synthesized AgNPs remove pollutants from water and soil by adsorbing heavy metals and organic contaminants^72,73,76,77.

Textile Industry: AgNPs incorporated into textiles provide antimicrobial properties, odor resistance, and UV protection, improving fabric hygiene and durability^74,75.

7. FUTURE PROSPECTS OF GREEN SYNTHESIZED SILVER NANO-PARTICLES

Green-synthesized AgNPs represent a groundbreaking advancement with transformative potential in multiple industries, including personalized medicine, sustainable agriculture, and environmental remediation. These nanoparticles are not just an option; they are a necessity for achieving SDG-3 by promoting the good health and well-being of the population. In personalized medicine and tissue engineering, AgNPs offer unparalleled opportunities for innovation and improvements. As sustainable alternatives to chemical pesticides, they promise to revolutionize agriculture and ensure food security while protecting the planet. Moreover, their application in air purification systems and smart textiles for healthcare monitoring is expected to redefine industry standards. However, to fully harness these benefits, it is imperative to establish standardized protocols for their synthesis, application, and disposal. This will ensure safety and minimize environmental impacts. Furthermore, rigorous research on the long-term effects of AgNPs on human health and the environment is essential to determine safe usage levels. The time to act is now; embracing AgNPs is not just a choice but a strategic imperative for a sustainable future.

• CONCLUSION:

Characterization of green-synthesized AgNPs is imperative to unlock their potential in biomedical applications. Techniques such as SEM, TEM, UV-visible spectroscopy, FTIR, XRD, dynamic light scattering (DLS), and zeta potential measurements provide insights into the size, shape, morphology, and surface charge of these nanoparticles. The spectral shifts and morphological transformations from spherical to prismatic structures indicate changes in particle size and aggregation, which influence their functional properties, including drug release behavior and biodistribution. Advanced techniques such as atomic force microscopy (AFM), high-performance liquid chromatography (HPLC), and zeta potential measurements for antibacterial characterization highlight the role of surface charge in AgNP-biological interactions. Smaller particle sizes and optimal surface charges enhance AgNP efficacy by improving their interactions with bioactive compounds and refining their distribution. Therefore, a comprehensive characterization of AgNPs is essential for optimizing their application in antimicrobial treatments and ensuring their stability in biomedical contexts.

• ACKNOWLEDGMENT

I express my deep sense of gratitude to my principal, Professor S. P. Singh. St. John’s College, Dr. Bhimrao Ambedkar University, Agra, for their encouragement and constant help during the study period.

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