Periodontal disease is the inflammation and progressive destruction of the tooth supporting structures, often leading to tooth mobility and eventual loss and globally a major public health challenge. Complex microbial populations including Pseudomonas aeruginosa, Streptococcus pyogenes, Escherichia coli, and Bacillus cereus, causes the disease. B. cereus, S. pyogenes, P. aeruginosa, and E. coli are commonly found in higher proportions compared to healthy oral environments[1]. Following conventional scaling and root planing, antibiotics are often prescribed to minimise microbial recolonisation and lower bacterial load[2]. However, antibiotic-resistant species has become a significant concern. Therefore, novel, and efficient treatment modalities are needed for the effective management of periodontitis[3]. 

Nanotechnology deals with manipulating materials with dimensions typically below 100 nanometers, collectively referred to as nanoparticles[4]. They show distinctive physicochemical characteristics like increased surface area, enhanced reactivity, and quantum effects that contribute to their antimicrobial behaviour[5]. Nano-biotechnology is an interdisciplinary approach combining nanoscience and biological systems and has inspired the synthesis of innovative nanobiomaterials[6, 7]. Conventional nanoparticle production methods are costly, environmentally detrimental and involves toxic chemicals that can harm living systems. 

Green nanoparticle synthesis employs eco-friendly biological routes using naturally derived materials such as plant extracts, fungi, bacteria, and yeast[8]. Plant based synthesis is particularly helpful because plant extracts are abundant, globally accessible, safe to handle, and rich in metabolites with strong reducing potential, enabling low-cost and sustainable nanoparticle formation. Plant biomolecules serve as reducing and stabilising agents during synthesis, promoting nanoparticle stability and yield[9]. Green synthesis via biological pathways produces higher nanoparticle yields and is more environmentally benign[10]. Green synthesized metallic and metal oxide nanoparticles have shown promising biomedical applications in diagnostics, tissue engineering, wound care, immunotherapy, biosensing, regenerative medicine, and dentistry[11]. 

Compared to microbial routes, green synthesis is a simpler, scalable, and non-toxic process for generating biogenic nanoparticles[12]. These biogenic metallic nanoparticles have extremely small dimensions and a high surface-to-volume ratio, characteristics that arise naturally from biological reduction mechanisms[13]. 

Plant extracts possess diverse phytochemicals, such as terpenoids, amides, carboxylic acids, flavonoids, ketones, and ascorbic acid, for the conversion of metallic salts into nanoparticles[14]. These phytochemicals function as reducing, capping, and stabilising agents during the formation process[15]. The plant metabolites found in leaves, such as proteins, polysaccharides, vitamins, alkaloids, amino acids, polyphenols, tannins, saponins, flavonoids, and organic acids enables sustainable synthesis of nanoparticles[16].

The nanoscale metal particles (<100 nm), display novel characteristics compared to their bulk counterparts due to quantum size effects[17]. Their minute size enhances cellular interaction and antimicrobial activity, as they can penetrate microbial membranes, disrupt metabolic processes, and hinder bacterial proliferation. The high surface-to-volume ratio promotes the generation of reactive oxygen species (ROS). Furthermore, since metal-based nanoparticles target multiple cellular structures rather than specific receptors, bacteria face greater difficulty developing resistance[18]. 

Bacteria have long been exploited for their biotechnological potential in bioremediation, bioleaching, and genetic engineering. Bacterial strains, including Bacillus cereus, Bacillus amyloliquefaciens, Bacillus indicus, and Bacillus cecembensis, have the ability to reduce metal ions to nanoparticles under mild conditions[12, 19]. Their metabolic versatility allows them to serve as biofactories for eco-friendly nanoparticle production. 

Fungal systems are highly effective biological agents for nanoparticle biosynthesis owing to their enzymatic richness and robust tolerance to metal ions. They can generate monodispersed nanoparticles with well-defined morphologies through enzymatic reduction processes occurring within or on their cell walls. Commonly used fungi for metal and metal oxide nanoparticle production include Penicillium, Fusarium, Aspergillus and Trichoderma species[12, 20]. Metals such as silver, gold, titanium dioxide, and zinc oxide can be synthesized using fungal extracts, yielding particles suitable for biomedical applications. 

Microalgae are photosynthetic aquatic microorganisms that serve as natural producers of metallic nanoparticles such as ZnO, Au, and Ag[21] and detoxifies the metal ions by converting them into their less toxic nanoparticle forms. For instance, titanium nanoparticles have been synthesised from Phaeodactylum tricornutum, using its culture supernatant. These nanoparticles demonstrate antibacterial, cytotoxic, and biogenic effects, and can further be utilised in imaging, biosensing, drug delivery, cancer therapy, and immunological applications[22]. 

Yeasts, are single-celled eukaryotic organisms that exhibit a high surface-to-volume ratio and significant metal ion uptake capacity, making them ideal candidates for nanoparticle biosynthesis[23]. Silver-tolerant yeast strains, such as Saccharomyces cerevisiae, have successfully produced silver and gold nanoparticles through biologically mediated processes[12]. 

Plant-mediated synthesis is considered the most straightforward, cost-effective, and sustainable strategies for nanoparticle generation[24]. Phytochemicals in plant tissues catalyse the reduction of metal ions into their nano particulate forms. Early studies reported the formation of silver and gold nanoparticles from extracts of Aloe barbadensis, Avena sativa, Citrus limon, Azadirachta indica, Coriandrum sativum, and Brassica juncea. Plants such as Helianthus annuus, Hibiscus rosa-sinensis, and Camellia sinensis have been employed in synthesising ZnO nanoparticles[12]. The broad diversity of phytochemical constituents in plants supports efficient and stable nanoparticle formation across multiple metal systems. 

Green synthesised nanoparticles show enhanced stability, low toxicity, and promising biological activity[25, 26]. The redox potential of plant-derived compounds such as terpenoids, polyphenols, and flavonoids enables them to act as reducing and stabilising agents during synthesis[27]. Secondary metabolites, particularly polyphenols, play a pivotal role in the formation and stabilisation of nanoparticles[28]. Typically, higher plant extract volumes yield smaller nanoparticles. Additionally, light-induced electrostatic interactions are thought to contribute to nanoparticle aggregation control and antibacterial efficacy[29]. 

Based on the synthesis of green nanoparticles, they can be categorised as extracellular, intracellular, or phytochemical-driven. Plant-mediated synthesis produces higher yields due to the abundance of bioactive reducing agents in extracts[11]. They have broad applications as antibacterial, anticancer, and catalytic agents, as well as in wastewater treatment and degradation of hazardous compounds[30]. The antibacterial mechanism primarily involves the release of metal ions that interact with bacterial membranes, causing structural disruption, leakage, and eventual cell death[31]. 

In restorative dentistry, green nanoparticles such as silver, zinc oxide, titanium dioxide, and zirconium have been incorporated into cements, composites, and adhesive systems that improve the mechanical integrity and impart long-lasting antibacterial capabilities. These materials minimise the occurrence of secondary caries by continuously releasing ions to inhibit microbial growth. In preventive dentistry, these nanoparticles are used in mouth rinses, varnishes, and toothpastes, where they suppress biofilm accumulation and enhance enamel remineralisation. The pronounced antibacterial efficacy of green nanoparticles against common oral pathogens—including Streptococcus mutans, Enterococcus faecalis, and Porphyromonas gingivalis—makes them ideal for applications in intracanal medicaments, root canal irrigants, and periodontal gels in periodontology and endodontics. In implantology, nanoparticles synthesized from plant extracts such as garlic and ginger have shown excellent antibacterial and antibiofilm properties. These particles help prevent microbial colonisation on implant surfaces and promote osseointegration, demonstrating their potential for future use in dental implant coatings[32]. Recently, AgNP-coated sutures with nanoparticles of low silver concentration emerge as a compelling alternative in surgical practice, that exhibited a sustained and stable antibacterial effect for up to 7 days during healing post periodontal surgeries[33].

Overall, these applications underscore the broad potential of green nanoparticles in advancing environmentally responsible and biologically compatible dental materials and therapeutics. 

Gold nanoparticles (AuNPs) show biocompatibility, stability, and minimal toxicity, making them promising agents in dental and biomedical sciences. Certain plants, referred to as “hyperaccumulators,” possess the natural capacity to sequester metals from the environment, facilitating the biogenic synthesis of gold nanoparticles. Extracts from Azadirachta indica, Aloe vera, Avena sativa, Medicago sativa, Hibiscus sabdariffa, and Cymbopogon citratus have all been utilized for the eco-friendly extracellular synthesis of AuNPs. Owing to their distinctive optical and plasmonic properties, AuNPs are valuable for periodontal diagnostics, as bone-inducing agents in regenerative procedures, and as analgesic compounds in dental therapeutics[32]. 

Silver nanoparticles (AgNPs) exhibit powerful antimicrobial and anti-biofilm effects, primarily through mechanisms involving membrane disruption, increased permeability, and the intracellular release of silver ions. These ions generate reactive oxygen species (ROS) that damage vital biomolecules within bacterial cells, leading to death. Although chemical reduction methods using AgNO₃ are well established, biogenic synthesis via plant extracts provides a more sustainable, cost-effective, and safer alternative[34]. In dentistry, AgNPs have been successfully integrated into acrylic resins, adhesives, nanocomposites, and implant coatings, enhancing the antibacterial resistance of restorative and prosthetic materials. 

Iron-based nanoparticles exist as iron oxide, iron oxide hydroxide, and zero-valent iron[35]. Green synthesis routes using Citrus sinensis extracts have been shown to yield environmentally benign and cost-effective iron nanoparticles[36]. These iron oxide nanoparticles, such as magnetite and maghemite, demonstrate synergistic antimicrobial effects when combined with both natural and synthetic agents, thereby improving antibacterial efficacy[32]. 

Copper nanoparticles (CuNPs) are valued for their broad-spectrum antimicrobial activity and their ability to stabilise matrices in medical and industrial applications. While chemical and physical fabrication methods such as microemulsion, aerosol processes, and laser ablation are often expensive and energy-intensive, green synthesis offers a more sustainable route. The inclusion of ascorbic acid during synthesis enhances nanoparticle stability and minimises oxidation[37]. Plants such as Zingiber officinale, Azadirachta indica, Punica granatum, and Citrus medica have been used to derive CuNPs with strong bactericidal potential[38]. In dentistry, copper nanoparticles exhibit antibiofilm and cytocompatible properties, making them particularly effective as anti-peri-implant agents against Aggregatibacter actinomycetemcomitans[32]. 

Titanium dioxide (TiO₂) nanoparticles are most widely studied due to their chemical stability, photocatalytic behaviour, low cost, and potent oxidative capability. Synthesized through precipitation using titanium isopropoxide, TiO₂ nanoparticles display antimicrobial, antiparasitic, and photodynamic properties[39], and on exposure to ultrasonic or light-induced excitation, help generate reactive oxygen species (ROS), which destroys bacterial cells effectively. The interaction between plant-derived compounds and Ti precursors enhances the rate of synthesis while improving nanoparticle stability[40]. 

Zinc oxide (ZnO) nanoparticles is one of the safest and most widely used nanomaterials in dentistry, that is recognised by the FDA for their biocompatibility. Zinc plays a vital role as a cofactor in numerous enzymatic reactions and commonly incorporated into restorative materials, including ZnO-eugenol cement, ceramics, and amalgam. ZnO nanoparticles synthesized using phenolic and flavonoid-rich plant extracts exhibit strong antibacterial and antioxidant properties, making them useful in dental composites and sealers[41].

Bismuth (Bi), due to its low toxicity, non-carcinogenicity, and limited bioaccumulation potential is categorised as an environmentally benign or “green” element. Bi₂O₃ nanoparticles derived via green synthesis are cost-effective and straightforward to produce, often employing ethanol and distilled water as safe solvents. Compared with microbial routes, plant-based synthesis yields bismuth nanoparticles with superior biocompatibility and reduced hazard potential[42]. 

Cobalt oxide (Co₃O₄) nanoparticles have broad-spectrum antibacterial activity and potential for environmentally sustainable synthesis. Ecological production methods are preferred because they minimise chemical waste and enhance nanoparticle safety. Studies show that these particles exhibit strong antimicrobial effects against both Gram-positive and Gram negative bacteria, including Escherichia coli, Bacillus subtilis, B. licheniformis, and Klebsiella pneumoniae[32]. 

Although green synthesis methods are recognized for their environmental advantages, several practical limitations remain. One major drawback is the limited and seasonal availability of certain biological materials, as well as the requirement for controlled harvesting to maintain consistency. Moreover, batch variability can lead to inconsistencies in nanoparticle size, shape, and stability[43]. 

Large-scale production also remains a challenge, as some synthesis routes require prolonged reaction times and high energy inputs, partially contradicting the sustainable principles of green chemistry[40]. In addition, differences in phytochemical composition among plant species complicate standardisation and reproducibility. Variability in particle size distribution can affect the optical, mechanical, and biological performance of the nanoparticles, making industrial scale production difficult. Consequently, optimisation of synthesis parameters and characterization methods is essential to improve reproducibility and ensure consistent nanoparticle quality[19]. 

Nanotechnology has revolutionised biomedical science, introducing materials with unique size dependent properties that offer superior functional performance compared to conventional bulk materials. In both dentistry and medicine, nanomaterials have emerged as innovative antibacterial agents capable of overcoming microbial resistance and promoting tissue regeneration[44]. 

The use of green-synthesized nanoparticles, derived from natural plant and microbial extracts, represents a sustainable approach that aligns with environmental and safety considerations. Their cost-effectiveness, biocompatibility, and reduced toxicity make them promising alternatives to chemically synthesized nanoparticles. 

Soon, nanotechnology is expected to become deeply integrated across all dental specialties— from restorative and preventive applications to periodontology, prosthodontics, and implantology—providing enhanced mechanical, antimicrobial, and regenerative capabilities. Furthermore, nano-carrier systems derived from herbal extracts may overcome limitations traditionally associated with phytotherapy, such as poor solubility, instability, and low bioavailability[45]. 

Collectively, green nanotechnology offers a pathway toward sustainable, safe, and high performance materials that will continue to advance modern dentistry and biomedical science.