AuNPs display remarkable diversity in their size and shape (Figure 1), ranging from spherical, rod-shaped, triangular, and beyond. This variability stems from the various synthetic methods tested, such as seed-mediated growth, chemical reduction, or templated synthesis (Jana et al., 2001). Importantly, the size and shape of AuNPs profoundly influence their optical, electronic, and catalytic properties. For instance, smaller nanoparticles tend to exhibit a more intense and tunable surface plasmon resonance (SPR) absorption peak in the visible to near-infrared (NIR) range, making them particularly valuable for biomedical imaging and photothermal therapy applications (Dreaden et al., 2012). AuNP size and shape are crucial for their antimicrobial activity (Shamaila et al., 2016; Osonga et al., 2020). Smaller AuNPs are generally more effective due to a larger surface area to volume ratio, leading to stronger membrane interactions and easier cell entry, ultimately disrupting bacterial membranes and causing cell death (Shamaila et al., 2016; Osonga et al., 2020). The shape also matters; different shapes (e.g., spherical, rod, star) interact differently with bacteria, with star-shaped AuNPs potentially piercing cell walls more effectively (Shamaila et al., 2016; Osonga et al., 2020). Shape also influences the mechanism of action, sometimes promoting ROS generation or disrupting bacterial processes (Osonga et al., 2020). Therefore, controlling AuNP size and shape is essential for optimizing their antimicrobial properties in nanomedicine.

The optical properties of AuNPs are primarily governed by SPR, a phenomenon arising from the interaction of incident light with the free electrons at the metal surface. This collective electron oscillation results in a characteristic absorption and scattering peak, typically in the visible and near-infrared regions (Giljohann and Mirkin, 2009). The SPR properties, and consequently the color of colloidal AuNP solutions, can be precisely tuned by controlling factors such as size, shape, composition, and the surrounding medium, enabling tailored optical responses and facilitating qualitative analysis (Murphy et al., 2005). Optical properties are employed in cancer research, with both photothermal and photodynamic therapies demonstrating promising outcomes in cancer treatment using AuNPs (Eker et al., 2024).

Surface functionalization plays a pivotal role in modulating the physicochemical properties of AuNPs to suit specific applications. Functional ligands, such as thiolates, amines, or polymers, can be attached to the nanoparticle surface to enhance stability, solubility, and biocompatibility (Khan et al., 2019). Furthermore, surface modification enables the introduction of functional groups or biomolecules for targeted binding, facilitating applications in drug delivery, biosensing, and biocatalysis. The surface chemistry of AuNPs is crucial in determining their interaction with bacteria, with factors such as surface charge and potential being important (Saed et al., 2024). The surface chemistry of AuNPs can be tailored through specific synthesis methods and molecular binding, involving polymers, ligands, or biomolecules. These modifications significantly influence the nanoparticles’ electrostatic interactions with bacteria (Saed et al., 2024). Additionally, the surface chemistry is essential for the effective functionalization of AuNPs with antibiotics, potentially enhancing their antibacterial properties (Wang et al., 2014).

While gold is renowned for its chemical inertness, AuNPs can undergo surface oxidation under certain conditions, leading to changes in their properties and reactivity. Factors such as pH, temperature, and the presence of oxidizing agents can influence the extent of nanoparticle oxidation (Dreaden et al., 2012). Understanding the mechanisms and kinetics of surface oxidation is crucial to mitigating undesired alterations in AuNPs’ properties, thereby ensuring their stability and performance in various applications. Previous studies have shown that storing purified AuNP suspensions in the dark at 4°C can extend their stability for up to 20 days (Balasubramanian et al., 2010). Additionally, AuNPs used in earlier research maintain their atomic form without undergoing oxidation, even after being aerosolized in air or exposed to temperatures as high as 500°C (Balasubramanian et al., 2010). Due to their stability, biocompatibility, and ease of manipulation, AuNPs are considered a promising material for antibacterial applications.

AuNPs exhibit remarkable catalytic activity due to their high surface area-to-volume ratio, as well as their unique electronic and geometric structures. This catalytic ability spans a wide range of reactions, including oxidation, reduction, and hydrogenation (Khan et al., 2019). Notably, AuNPs are efficient catalysts in environmentally significant processes such as pollutant degradation, hydrogen production, and carbon dioxide reduction (Ghorbani-Vaghei et al., 2021). The tunability of AuNPs’ catalytic activity, achieved through adjustments in size, shape, and surface modification, holds promise for enhancing reaction selectivity and efficiency in various catalysis-driven industries. The catalytic activity of AuNPs, particularly their ability to generate ROS through redox reactions, plays a crucial role in their antimicrobial properties by inducing oxidative stress and disrupting bacterial cell membranes (Saed et al., 2024).

AuNPs are widely recognized for their inherent biocompatibility, which is attributed to their inert nature and low cytotoxicity. These properties make them highly suitable for various biomedical applications, including drug delivery, imaging, and therapy (Wang et al., 2014). Functionalization with biomolecules such as antibodies, peptides, or nucleic acids enables targeted delivery and enhanced imaging in biological systems, improving the precision of diagnostic and therapeutic approaches (Tang et al., 2020). Additionally, AuNPs can be readily internalized by cells and interact with biomolecules, further highlighting their potential for advancing healthcare technologies. The synthesis method plays a crucial role in tailoring nanoparticle properties for specific applications, as precise control over size, shape, and morphology directly influences their optical, electronic, catalytic, and biocompatibility characteristics (Niżnik et al., 2024). Advances in synthesis techniques have significantly expanded the versatility of AuNPs, fostering new applications across nanomedicine, diagnostics, and catalysis.

The chemical reduction method is one of the most widely used techniques for synthesizing AuNPs due to its simplicity, cost-effectiveness, and ability to control nanoparticle size and shape (Haiss et al., 2007; Sardar et al., 2009). In this approach, a gold precursor, typically chloroauric acid (HAuCl ₄ ), is reduced by a chemical reducing agent in the presence of a stabilizing agent to prevent aggregation. Common reducing agents include sodium borohydride (NaBH ₄ ), sodium citrate, and ascorbic acid, each influencing the reduction kinetics and final nanoparticle properties. The choice of reducing agent, along with reaction parameters such as temperature, pH, and reactant concentrations, plays a crucial role in governing nanoparticle nucleation and growth, thereby controlling the size and morphology of the resultant AuNPs (Darwish et al., 2022). By fine-tuning these parameters, researchers can synthesize AuNPs with diverse morphologies, including spherical, rod-shaped, triangular, and more complex structures, which are essential for various biomedical and catalytic applications (Niżnik et al., 2024).

Another method for synthesis is the seed-mediated growth method. This method is a widely utilized technique for achieving precise control over the size and shape of AuNPs by using pre-formed nanoparticles as seeds for further growth (Grzelczak et al., 2020). Initially, small, typically spherical AuNPs are synthesized as seeds via chemical reduction methods. These seeds are then introduced into a growth solution containing a gold precursor, a reducing agent, and specific additives such as surfactants or capping agents, which help regulate particle stability and morphology (Xia et al., 2009). By carefully adjusting reaction parameters—including the concentration of reactants, temperature, and reaction time—the final size and shape of the nanoparticles can be finely tuned. This method enables the synthesis of AuNPs with well-defined morphologies, including spheres, rods, cubes, and more intricate structures (Jana et al., 2001; Xia et al., 2009).

Additionally, template-assisted synthesis is a versatile technique that employs structured templates or molds to precisely control the size and shape of AuNPs (Gangwar et al., 2024). Templates can be inorganic, such as silica or anodic aluminum oxide (AAO), or organic, including polymers and biomolecules, which guide the nanoparticle growth process. The gold precursor is deposited onto the template surface, followed by chemical or electrochemical reduction, resulting in nanoparticles that conform to the template’s morphology (Gangwar et al., 2024). After synthesis, the template is typically removed via chemical dissolution or thermal treatment, leaving behind nanoparticles with well-defined sizes and shapes (Sperling and Parak, 2010; Chanana et al., 2016).

Another method is electrochemical methods, which provide a highly versatile and controllable approach for synthesizing AuNPs with well-defined size and morphology. In these methods, gold ions (Au³⁺ or AuCl₄ ⁻) are reduced electrochemically at the electrode surface in an aqueous solution containing appropriate electrolytes and stabilizers (De Rooij, 2003). By precisely adjusting key parameters such as the applied potential, current density, electrolyte composition, and electrode geometry, researchers can fine-tune the nucleation and growth kinetics of AuNPs, thus controlling their size, shape, and distribution (Ramachandran et al., 2024). Additionally, the use of surfactants or capping agents helps regulate particle stability and prevent aggregation, further enhancing the reproducibility and uniformity of AuNP synthesis (De Rooij, 2003).

AuNPs exhibit antimicrobial activity primarily through direct physical interactions with microbial cells, leading to membrane disruption, oxidative stress, and intracellular disturbances (Akintelu, et al., 2021; Timoszyk and Grochowalska, 2022). Their high surface area-to-volume ratio enhances adsorption onto microbial cell membranes, destabilizing lipid bilayers and increasing membrane permeability (Timoszyk and Grochowalska, 2022). This disruption compromises membrane integrity, resulting in leakage of cytoplasmic contents and loss of cellular homeostasis, ultimately leading to bacterial cell death (Timoszyk and Grochowalska, 2022).

AuNPs contribute to oxidative stress within microbial cells by inducing the generation of reactive oxygen species (ROS), including superoxide radicals, hydroxyl radicals, and singlet oxygen (Fu et al., 2014). These ROS cause oxidative damage to essential biomolecules such as lipids, proteins, and nucleic acids, leading to cellular dysfunction and loss of viability (Juan et al., 2021). In bacteria, ROS disrupt membrane integrity and interfere with enzymatic pathways, while in eukaryotic microbes, they may impair mitochondrial function (Juan et al., 2021). The overwhelming oxidative stress ultimately compromises microbial survival and enhances the antimicrobial efficacy of AuNPs (Fu et al., 2014).

AuNPs disrupt essential microbial cellular processes, including DNA replication, transcription, and protein synthesis, thereby inhibiting growth and proliferation (Akintelu et al., 2021; Mikhailova, 2021). By binding to nucleic acids and proteins, AuNPs can induce conformational changes, disrupting enzymatic activities and ribosomal functions, which hampers microbial metabolism and impedes cellular replication (Akintelu et al., 2021). These interactions interfere with critical cellular machinery such as DNA polymerases and ribosomal subunits, preventing efficient protein synthesis and DNA replication. This disruption of vital cellular processes enhances the antimicrobial efficacy of AuNPs against a broad range of pathogens, including bacteria, fungi, and viruses (Wang et al., 2017; Mikhailova, 2021).

The conjugation of AuNPs with pathogen-specific antibodies enables selective binding to bacterial or fungal cells. Additionally, the combination of antibody targeting with gold nanoparticle photothermal therapy allows for an immunologically mediated destruction of bacteria. For example, S. aureus and its biofilms can be effectively eliminated through this dual mechanism (Figure 3) (Kirui et al., 2019).

Aptamers, which are short single-stranded DNA or RNA molecules with high specificity for bacterial surface markers, have been utilized to functionalize AuNPs for pathogen detection. This approach enables the precise identification of bacterial contaminants. For instance, Zhang et al. developed a dual-recognition system that combines vancomycin and aptamers to simultaneously detect Escherichia coli and Staphylococcus aureus (Zhang et al., 2018). In this system, vancomycin is incorporated into Fe₃O₄@Au nanoparticles, serving as a broad-spectrum bacterial capture agent that effectively concentrates the target microorganisms. Aptamer-functionalized AuNPs, along with two distinct types of surface-enhanced Raman scattering (SERS) tags, are then employed for highly sensitive and specific quantitative analysis. This innovative platform achieves detection limits as low as 20 cells/mL for S. aureus and 50 cells/mL for E. coli, demonstrating its potential for rapid and accurate bacterial identification.

Surface modification of AuNPs with ligands such as folic acid, transferrin, or peptides facilitates selective uptake by microbial cells. For instance, studies have demonstrated that functionalizing AuNPs with antimicrobial peptides can facilitate targeted delivery to bacterial cells, enhancing antimicrobial efficacy. For instance, Casciaro et al. demonstrated that the covalent attachment of Esc(1-21) to soluble AuNPs, forming AuNPs@Esc(1-21) through a poly(ethylene glycol) linker, enhanced the peptide’s activity by approximately 15 times against both motile and sessile forms of Pseudomonas aeruginosa, without showing toxicity to human keratinocytes (Casciaro et al., 2017). Additionally, AuNPs@Esc(1-21) exhibited significantly improved resistance to proteolytic degradation and was able to disrupt the bacterial membrane at very low concentrations (5 nM) (Figure 4) (Casciaro et al., 2017).

Ensuring the biocompatibility and safety of AuNPs for clinical use remains a primary challenge. Despite their promising antimicrobial properties, rigorous safety assessments are necessary to mitigate potential adverse effects, such as cytotoxicity and immunogenicity, before their widespread clinical application (Kus-Liśkiewicz et al., 2021). These assessments are crucial for determining appropriate dosages and identifying any long-term risks associated with their use (Xuan et al., 2023).

Reproducibility in nanoparticle research is critically dependent on standardization. Inconsistent synthesis methods often lead to batch variability, affecting both therapeutic performance and safety outcomes. Therefore, establishing internationally accepted guidelines for nanoparticle fabrication, purification, and physicochemical characterization is essential to advance regulatory approval and clinical implementation (Amina and Guo, 2020; Tehrani et al., 2023).

Tailoring AuNP formulations and delivery methods to specific clinical applications is a significant challenge. Optimizing parameters such as particle size, surface charge, and surface functionalization is critical for enhancing antimicrobial efficacy while minimizing off-target effects and improving bioavailability (Sibuyi et al., 2021). Effective formulations that enable targeted delivery can improve therapeutic outcomes and reduce potential toxicity to healthy tissues (Elumalai et al., 2024).

Navigating regulatory hurdles and obtaining approval from agencies such as the FDA (Food and Drug Administration) for clinical use represents a substantial challenge. Demonstrating the safety, efficacy, and quality of AuNP-based antimicrobial products through preclinical studies and clinical trials is essential. This process requires considerable time, resources, and investment but is necessary to bring these advanced therapies to market (Allan et al., 2021; Ramos et al., 2022).

Extensive in vivo investigations are needed to assess the safety, pharmacokinetics, and efficacy of AuNPs in combating MDR infections. Preclinical studies using animal models can provide valuable insights into the biodistribution, tissue penetration, and therapeutic potential of AuNP-based antimicrobial agents (Xia et al., 2019; Lama et al., 2020). Furthermore, nanotoxicological studies must explore oxidative stress induction, mitochondrial disruption, and potential genotoxicity. Future research should aim at identifying biomarkers of nanoparticle exposure and developing predictive models of toxicity (Kumar et al., 2017; Chenthamara et al., 2019; Xia et al., 2019; Li et al., 2020; Lama et al., 2020; Kus-Liśkiewicz et al., 2021).

Successful translation of AuNP-based antimicrobial technologies depends on robust collaboration among microbiologists, materials scientists, clinicians, regulatory authorities, and industry partners. Such cooperation will enhance the design of comprehensive evaluation protocols, streamline regulatory approval, and facilitate knowledge exchange to accelerate clinical translation (Malik et al., 2023; Fosse et al., 2023).

The development of personalized and targeted therapeutic strategies using AuNPs holds significant promise for addressing antimicrobial resistance and improving treatment outcomes. Tailoring AuNP formulations and delivery methods to target specific pathogens or infection sites while minimizing systemic toxicity can enhance therapeutic efficacy and reduce the emergence of resistance (Makabenta et al., 2021; Kamalavarshini et al., 2023).

Integrating AuNP-based antimicrobial therapies with existing treatments, such as antibiotics or antifungals, provides synergistic benefits and helps address the limitations of conventional therapies (Ahmed et al., 2016; Chintalacharuvu et al., 2021; Salehi et al., 2021; Abdallah and Ali, 2022; Priya et al., 2023; Azmy et al., 2024). Additionally, combining AuNPs with other therapeutic approaches, such as photothermal therapy or immunotherapy, can enhance their antimicrobial effectiveness and expand their therapeutic potential (Songca, 2023; Kumar and Lim, 2023). Further research is needed to explore the dual benefits of AuNPs when combined with other treatments, aiming to minimize side effects and improve their overall therapeutic efficacy.

Developing AuNP-based antimicrobial products suitable for bedside testing, such as diagnostic assays or wound dressings, can facilitate rapid and decentralized management of infectious diseases (Pang et al., 2023). Portable and user-friendly AuNP-based technologies capable of detecting and treating microbial infections can transform clinical practice, particularly in resource-limited settings (Nath, et al., 2020; Gradisteanu Pircalabioru et al., 2024). Further advancements in AuNP-based antimicrobial therapies could facilitate their translation into point-of-care applications, enabling rapid and effective treatment of infections in clinical and field settings.

Recent progress in the field has emphasized the development of efficient and scalable synthesis techniques for gold nanoparticles (AuNPs), with precise control over their size and shape. Eco-friendly “green” synthesis methods—using plant extracts, microorganisms, or natural biomolecules—are increasingly favored for their sustainability and potential to yield AuNPs with superior antimicrobial activity (Niżnik et al., 2024). In parallel, advanced methods such as microwave-assisted and electrochemical synthesis offer fast, reproducible, and fine-tuned control over nanoparticle characteristics (Adeola et al., 2023). Moving forward, research should prioritize the creation of synthesis strategies that are not only environmentally sustainable and cost-effective but also capable of producing AuNPs with improved stability and enhanced antimicrobial efficacy.

Surface functionalization of AuNPs plays a crucial role in improving their stability, biocompatibility, and targeting ability. Coating AuNPs with antimicrobial peptides, antibiotics, or polymers enhances their specificity towards MDR pathogens while minimizing off-target effects (Zhang et al., 2018; Kirui et al., 2019). Moreover, the conjugation of targeting ligands onto AuNPs enables selective interaction with pathogen-specific receptors, facilitating targeted drug delivery and enhanced therapeutic efficacy (Kamat and Kumari, 2023). Therefore, future research is essential to improve the functionalization of AuNPs.

Elucidating the underlying mechanisms of AuNP-mediated antimicrobial activity is essential for optimizing their efficacy and minimizing potential resistance development. Studies suggest that AuNPs exert antimicrobial effects through multiple mechanisms including membrane damage, ROS generation, and disruption of cellular functions (Lee and Lee, 2018; Sadeghi et al., 2024). However, the exact mechanism of action must be thoroughly investigated to optimize their clinical application.

In addition to their antimicrobial properties, gold nanoparticles (AuNPs) hold significant promise across a broad range of biomedical applications, including diagnostics, imaging, and targeted drug delivery. When functionalized appropriately, AuNPs can be utilized as versatile platforms for developing rapid and highly sensitive diagnostic tools to identify multidrug-resistant pathogens and infectious diseases such as COVID-19 (Yang et al., 2022; Li F. et al., 2023; Li Y. et al., 2023; Khalifa and Al Ramahi, 2024). Their distinctive optical characteristics also support advanced imaging modalities like photoacoustic imaging and surface-enhanced Raman scattering, enabling accurate localization and real-time monitoring of infections. Moreover, AuNP-based nanocarriers can facilitate targeted delivery of therapeutic agents directly to infected sites, thereby minimizing systemic toxicity and enhancing treatment outcomes (Kumar and Chawla, 2021; Wahnou et al., 2023). Continued research is warranted to further investigate their utility in other medical domains, such as cancer therapy, regenerative medicine, biosensing, and immune modulation, ultimately broadening their clinical relevance.