Fungal keratitis (FK) is an infectious form of keratitis primarily caused by filamentous fungi such as Aspergillus and Fusarium, as well as yeast-like fungi such as Candida (Spierer et al., 2015; Mishra et al., 2020). Currently, more than 100 fungal species have been identified as potential causes of FK (Donovan et al., 2022). The incidence of infectious keratitis is on the rise (Ung et al., 2019), with FK accounting for approximately 45% of the 1 million new corneal infections reported annually (Niu et al., 2020). It is estimated that there are 1 million cases of FK globally each year, and more than half of the patients with FK may lose their vision and suffer from monocular blindness (Brown et al., 2021). This condition is particularly prevalent in developing countries in Asia and Africa (Bisen et al., 2024), where it poses a significant public health challenge. Compared to bacterial keratitis (BK), the diagnosis of FK is often delayed, and treatment options are limited. As a result, clinical outcomes and prognoses for FK tend to be worse than those for BK (Prajna et al., 2012). Mild cases of FK can progress to severe corneal disease (Kern, 1990; Whitcher and Srinivasan, 1997; Garg and Rao, 1999). FK frequently manifests alongside corneal ulcers, a condition referred to as ulcerative fungal keratitis (UFK) (Anderson et al., 1959). UFK tends to be particularly serious (Whitcher and Srinivasan, 1997; Brown et al., 2022) and can result in permanent visual impairment or blindness.

The development of FK is closely associated with several factors, including prolonged contact lens wear, eye trauma, the overuse of immunosuppressants or broad-spectrum antibiotics, eye surgeries (such as corneal transplantation), and immunodeficiency diseases (including acquired immunodeficiency syndrome (AIDS), diabetes, and systemic immunodeficiency disorders) (Jin et al., 2022; Ler et al., 2022; Reginatto et al., 2023; Stapleton, 2023). FK results from the interaction between fungi and their host. Fungi can damage the human corneal epithelium, exposing the basement membrane, which allows the fungus to attach to the extracellular matrix of the corneal tissue. The fungi then produce enzymes that degrade the tissue, facilitating the spread of mycelia within the cornea. Fungal attachment enhances the transmembrane signaling mechanisms and alters the cytoskeletal structure of host cells, leading to the rearrangement of microtubules and microfilaments, ultimately resulting in apoptosis (Wu et al., 2016; Sitnova and Svetozarskiy, 2023). In response to fungal invasion, the host activates its defense mechanisms; pattern recognition receptors stimulate neutrophils to secrete various interleukins and promote the production of reactive oxygen species (ROS) (Abbondante et al., 2023). However, excessive ROS can lead to an overproduction of IL-1β in the body (Liu et al., 2023), causing eye tissue damage and promoting fungal growth, which further exacerbates FK. Figure 1 illustrates the risk factors, pathogenic fungi, and pathogenic mechanisms associated with FK.

Currently, the drug treatment for FK primarily focuses on the combination of antifungal and anti-inflammatory medications (Manzouri et al., 2001; Lakhani et al., 2019; Fisher et al., 2022). This includes local applications of natamycin (NATA), amphotericin B, fluconazole, and voriconazole, often combined with dexamethasone and non-steroidal anti-inflammatory drugs (Shen et al., 2011; Hou et al., 2023; Mishra et al., 2024). Amphotericin B is frequently used as a first-line treatment for FK (Ansari et al., 2013), while topical NATA eye drops are the preferred option for filamentous FK (Mahmoudi et al., 2018; Rohira et al., 2021). At present, there are no commercially available amphotericin B eye drops, and the amount of the drug that reaches the affected area is low (Diebold et al., 2007), necessitating frequent long-term administration. Similarly, NATA eye drops have a short residence time in the cornea (Gu et al., 2022) and require frequent application. Prolonged medication can exacerbate the underlying condition and/or lead to complications, posing challenges for patient compliance (Chandasana et al., 2014; Chhonker et al., 2015; Bhattacharya et al., 2020; Vitiello et al., 2023). Furthermore, antifungal drugs are susceptible to developing resistance, which can result in treatment failure (Wiederhold, 2017; Kaul et al., 2022). Although new antifungal agents such as posaconazole (Ferguson et al., 2022) and esaconazole (Cultrera et al., 2021) have been introduced, but their therapeutic effects remain limited. Study suggests that the use of traditional Chinese medicine in combination with antifungal drugs can enhance therapeutic efficacy and reduce drug side effects (Huang et al., 2022). In addition to the aforementioned medications, research on surgical treatments for FK, such as corneal transplantation (Thomas and Kaliamurthy, 2013), photodynamic therapy (Amescua et al., 2017; Hung et al., 2021) and sutureless corneal adhesion (Choi and Jeon, 2022), is also increasing. However, these methods all have certain limitations and drawbacks. There is an urgent need to explore innovative therapeutic approaches for FK.

Nanomaterials are particles with diameters ranging from 1 to 100 nm, characterized by high specific surface areas and adjustable physical and chemical properties. They have found widespread applications in various medical fields, including cancer treatment, drug delivery, diagnostic imaging, antibacterial therapy, burn treatment and wound healing (Aflori, 2021; Kakodkar et al., 2023). Metal nanoparticles and metal oxide nanoparticles exhibit broad-spectrum antimicrobial activity, and certain nanomaterials have demonstrated antibacterial effects against filamentous fungi such as Aspergillus, Coccidioides, and Mucor (Leon-Buitimea et al., 2021). Furthermore, nanomaterials can serve as carriers for drug transport. Drug molecules can bind to nanoparticles, resulting in the formation of coupled nanoparticle-drug complexes. Drug molecules can be loaded into nanoparticles by following methods: attachment to the surface, conjugation, entrapment, and encapsulation (Liu et al., 2012; Achouri et al., 2013). Solid lipids encapsulated by nanoparticles are referred to as lipid nanocarriers (Gu et al., 2022). A drug delivery system (DDS) composed of nanomaterials for FK can enhance the residence time of drugs in the cornea (Liu et al., 2012), improve bioavailability and stability, ensure continuous and controlled release, and provide targeted delivery to specific ocular tissues (Kakodkar et al., 2023). Compared to traditional treatment methods, nanopreparations can significantly enhance the quality of medical treatment, offering higher efficacy with fewer adverse reactions, thus demonstrating substantial clinical therapeutic potential.

This review utilizes bibliometrics to analyze the current status and progress in the field of nanomedicine therapy for FK in recent years. It begins by introducing the anatomical and physiological characteristics of the eye. We then discuss the advancements in nanomaterials for the treatment of FK from both direct and indirect therapeutic perspectives. Additionally, we explore the mechanisms and properties of various nanomaterials used in FK treatment, with a particular focus on the action of nanoparticles against biofilms and the DDS involved in FK therapies. Finally, we present prospects for future development in this field, aiming to investigate the clinical application value of nanomaterials in treating FK.

Bibliometrics is an emerging method of literature analysis that systematically evaluates publications within a specific field. By statistically analyzing the characteristics and trends of papers published in this area, bibliometrics examines relationships between years, countries and regions, and keywords (Lin et al., 2022). This approach aims to reveal research progress and predict future development trends, providing valuable insights for researchers and guiding investigations within the field (Lin et al., 2020). On August 18, 2024, we conducted a search in the Web of Science Core Collection (WoSCC) database and successfully obtained 105 publications. Our retrieval method utilized the following search string: (fungals OR fungal OR fungi) AND (keratic OR keratitis OR keratitides) AND (nanomedicinal OR nanomedicine OR nanomedicines OR nanostructure OR nanostructures OR nanomaterial OR nanomaterials OR nanopreparation OR nanopreparations OR nanoparticles OR nanoparticle). We focused on articles and reviews, manually screening to retain 100 relevant studies related to FK and nanomedicine. To avoid the influence of subjective factors on the screening results, the screening process was independently completed by three authors. If there were any discrepancies in the results, discussions were held until all three authors reached a consensus and obtained the same screening results. Complete records for each publication, along with cited references, were exported. Subsequently, we employed VOSviewer, CiteSpace 6.2. R4, and Microsoft Office Excel for data statistics and visual analysis. Our analysis included the annual number of published papers, keyword co-occurrence, keyword evolution over time, and country co-occurrence.

As nanomedicine has only emerged in the treatment of FK in recent years, the number of relevant articles is relatively small (Figure 2). There were only relevant articles from 2012, and merely 100 articles were retrieved by August 2024. Since 2018, this research field has received attention with an exploding growth in the number of studies. The field declined in popularity in 2023, but gained traction again in 2024. The most cited paper investigates a hybrid hydrogel-based contact lens, which comprises quaternized chitosan (HTCC), silver nanoparticles, and graphene oxide (GO). This contact lens can encapsulate voriconazole and may be a promising method for the rapid and effective treatment of fungal keratitis (Huang et al., 2016). The second most cited paper examines lecithin/chitosan nanoparticles that can encapsulate amphotericin B, thereby prolonging its residence time on the cornea and treating fungal keratitis (Chhonker et al., 2015). The third most cited paper explores NATA solid lipid nanoparticles (NATA-SLNs). These nanoparticles extend the drug release rate, enhance corneal permeability, increase antifungal activity, and exhibit no cytotoxic effects on corneal tissue (Khames et al., 2019).

We utilized VOSviewer to extract and analyze keywords, retaining a total of 46 keywords after screening. The visual analysis of keyword clustering is presented in Figure 3. Different colors in the figure represent distinct clusters, while larger node labels indicate more frequent appearances of the corresponding keywords. The top three keywords by frequency are “fungal keratitis,” “nanoparticles,” and “drug delivery,” respectively. We also examined the evolutionary trends of these keywords over time, with the results depicted in Figure 4.

Finally, we used CiteSpace to construct the cooperation network graph among countries/regions (Figure 5). China has published the most papers (39), followed by India (19) and Egypt (17). The largest connected component for a specific country/region consists of 30 nodes and 28 connections, with a map density of 0.0644. After calculating the betweenness centrality, the top three countries identified are China, India, and Malaysia, with both China and India exhibiting betweenness centrality values greater than 0.1. This indicates that these countries/regions play significant roles in research within this field. According to the statistical results presented, China emerges as the core country in this area of study.

The eyeball is composed of the sclera, cornea, and its internal contents. The primary structures are situated within the eye’s framework, with the optic nerve at the rear connecting to the brain (Figure 6). The anatomy of the eyeball consists of two main parts: the eyeball wall and its contents. The eyeball wall can be further divided into three layers: the fibrous layer, the vascular layer, and the retina. The internal contents of the eyeball include the lens, vitreous body, and aqueous humor (Baino and Kargozar, 2020; MercuȚ et al., 2020; Xu et al., 2022). The anterior portion of the eyeball wall is the cornea, which constitutes approximately one-fifth of the total structure. The cornea is a transparent refractive surface characterized by its convex shape and lack of blood vessels, although it is densely packed with nerve endings (Thomasy et al., 2014).

In microanatomy, the cornea is composed of five layers: the epithelial cell layer, anterior elastic layer, interstitial layer, posterior elastic layer, and endothelial cell layer (Figure 6). The epithelial cells of the cornea create tight junctions that help prevent foreign objects and drugs from penetrating. Additionally, a tear film coats the surface of the corneal epithelial layer, providing a barrier function that inhibits drug absorption. As a result, conventional ophthalmic eye drops face challenges in the effectively delivering of therapeutic agents to the eye.

Normal physiological activities in humans, such as tear production, blinking and eye tissue metabolism, can significantly influence the drug delivery of Enhanced Filamentous Growth Protein 1 (EFG1) (Patel et al., 2018). The human eyeball possesses both dynamic and static barriers to drug penetration. The static barriers include the corneal epithelial cells and stroma, while the dynamic barriers consist of conjunctival blood flow, lymphatic circulation, and tear flow. These barriers collectively hinder drugs from entering the anterior chamber of the eye (Pal et al., 2013). The anterior chamber contains aqueous humor, which is produced by the ciliary body and serves to nourish the cornea. Aqueous humor circulates from the posterior chamber into the anterior chamber through the pupil (Figure 6). In systemic drug administration (Goel et al., 2010), drugs are delivered to the cornea via this aqueous humor circulation. Under physiological conditions, the typical tear flow rate in humans is approximately 1.2 mL/min, with the tear film being replenished about every 5 min. However, during tear production, this flow rate can increase dramatically to 300 mL/min, resulting in the rapid washing away of drugs. Moreover, the tear film features a hydrophilic layer formed by mucin, which protects the eyeball from foreign objects but also presents a barrier to drug absorption (Mannermaa et al., 2006).

Due to the anatomical and physiological barriers present in the eyeball, it is challenging for traditional therapeutic agents for FK to remain in the eye and achieve effective absorption. Consequently, there is an urgent need to develop new drug formulations that can enhance the therapeutic efficacy for treating FK.

Traditional ophthalmic medication uses eye drops, but due to the special anatomical and physiological structure of the eye, it is difficult for the drug to stay for a long time. At the same time, FK often occurs in the form of chronic diseases and needs to be administered frequently (Nayak and Misra, 2018). Nanocarriers are used to ensure drug targeting and controlled drug release.

Nanocarriers have specific surface charges, which are conducive to their conjugation and retention at specific sites. This potential is measured as zeta potential (Patra et al., 2018). Zeta potential refers to the potential difference between the surface charge of the nano carrier and the opposite charge from the medium arranged around the particles. Zeta potential reflects the stability of the nano carrier. According to the principles of physics, if two particles have high zeta potential of the same charge, they will repel each other because of the repulsive force, preventing the aggregation of particles. The surface of the human cornea carries a negative charge. Therefore, cationic nanoparticles will be attracted to the cornea due to electrostatic forces, which enables the cationic nanoparticles to remain on the cornea (Tsai et al., 2018). This is the mechanism by which nano carriers deliver drugs to the anterior eye region.

Ophthalmic nanodrugs can achieve targeted delivery of drugs, thereby reducing the required drug concentration and reducing the occurrence of side effects. At the same time, nanomaterials have a certain adhesion effect. Nanoparticles adhere to the surface of fungi through the action of electric charge, cutting off some key enzyme pathways and damaging the integrity of the membrane. At the same time, some nano materials have certain antifungal effect, which can reduce the fungal resistance of drugs (Guo and He, 2024).

Ophthalmic nano materials can be divided into lipid based nano carriers, polymer nano carriers and inorganic nano carriers according to their materials (Guo and He, 2024). Lipid nanocarriers include liposomes, niosomes, solid lipid NPs, nanostructured lipid carriers and microemulsion. Polymer nanocarriers include dendrimers, micro micelles, hydrogel, chitosan nanoparticles. Inorganic nano carriers include carbon nanotubes, metal particles.

Nanomedicine can induce fungal cell death or reduce the expression of mycotoxins by directly disrupting the fungal cell structure, thereby achieving therapeutic effects in the treatment of FK. The related pathogenesis is illustrated in Figure 7. Silver nanoparticles exhibit significant antifungal activity and hold high therapeutic value for FK (Xu et al., 2013). Silver ions can interact with the cell wall and membrane, disrupt intracellular biomacromolecules, affect cellular respiration, generate ROS, interfere with nucleic acid synthesis, and thus inhibit the growth and reproduction of fungi (Shehabeldine et al., 2022; Piecuch et al., 2023). Studies have indicated that AgCu₂O-EDTA nanoparticles (AgCuE NPs) can treat FK caused by Candida albicans by destroying the cell wall and membrane of the fungus (Ye et al., 2022). This therapeutic effect may be related to the down-regulation of the Bcr1-related pathway, EFG1, and virulence-related genes, including High Osmolarity Glycerol Response 1 (HOG1) and Checkpoint Kinase 1 (CHK1), which are essential for maintaining the full virulence of Candida albicans, along with the up-regulation of oxidative stress-related genes (SOD5, Prx1). The former group is associated with the formation of the cell wall and biofilm of Candida albicans, as well as its virulence production and cell adhesion. In contrast, the latter group is linked to the production of ROS, which contribute to fungal cell death (Ye et al., 2022). Scientists have also developed contact lenses loaded with silver nanoparticles (AgNPs), which release these particles to help treat FK (Liu et al., 2018). Hydrogel materials impregnated with silver nanoparticles during the production of contact lenses demonstrate antibacterial properties, reducing the likelihood of microbial keratitis (Fazly Bazzaz et al., 2014). Additionally, silver microspheres (AgMPs) show effective treatment results for candidal keratitis and exhibit biosafety for corneal epithelial cells. Therefore, they can serve as candidate drugs for ocular surface drops aimed at treating FK (Shi et al., 2021).

In addition to silver ions, propolis-based nanofibrous patches (Ulag et al., 2021) and phomopsidione nanoparticle-coated contact lenses can also be utilized to treat corneal microbial keratitis (Bin Sahadan et al., 2019), particularly cases associated with contact lens use. Furthermore, Dectin-1 specific nanobodies have demonstrated a therapeutic effect on Aspergillus fumigatus keratitis in murine models, leading to reductions in the mRNA and protein expression levels of IL-1β and IL-6 in human corneal epithelial cells (HCEC) stimulated by infected corneas and Aspergillus fumigatus, thereby exerting an anti-inflammatory effect (Liu et al., 2022). Scientists have developed a nanosystem that co-integrates lyticase and gallium ions (MLPGa), which effectively degrades the extracellular polysaccharides found in the cell wall and biofilm of Candida albicans, resulting in antifungal activity (He et al., 2022). Moreover, an ultrasmall positively charged carbon dot can penetrate the corneal barrier, open tight junctions, reach the lesion site, and effectively eliminate fungal pathogens (Chen et al., 2024).

Despite the many advantages of nanodrugs in directly treating FK, they still have certain limitations. Nanomaterials have potential toxicity to the human body. For example, silver ions may cause toxic reactions by disrupting cholesterol in cell membranes. Moreover, the metabolism and excretion of some nanomaterials in the body are complex, and their accumulation in tissues may lead to long-term toxicity (Chetoni et al., 2003). Nanoparticles possess metabolic stability, and their metabolism and excretion from the body require the support of the liver and kidneys. Therefore, they may potentially cause damage to liver and kidney functions (Ravindran et al., 2018). Additionally, nanoparticles can affect the activity of enzymes (such as cytochrome P450), thereby influencing the normal metabolism of the body (Ravindran et al., 2018). Additionally, the development of drug resistance in fungal cells and the protective role of biofilms can reduce the effectiveness of nanodrug treatments. The high cost of producing some nanodrugs also limits their large-scale application (Gorantla et al., 2020). Although laboratory studies have shown that nanomaterials have significant therapeutic effects on fungal infections, further research is needed in the academic community to address the challenges in clinical applications, such as individual differences among patients, varying degrees of infection, and the synergistic effects with other treatment methods.

Nanomaterials can be used as drug carriers to increase the residence time of drugs in the cornea and form DDS to improve the treatment efficacy for FK. Additionally, nanomaterials can be utilized to create contact lenses that deliver therapeutic agents for the treatment of FK. Furthermore, nanomaterials can be combined with other treatment modalities, such as photodynamic therapy, to significantly enhance the clinical effectiveness of these approaches.

Due to the unique anatomical and physiological structure of the eye, conventional drugs often encounter challenges in reaching ocular tissues, resulting in a typically short retention time. This makes the treatment of FK particularly challenging. Nanomaterials have introduced innovative therapeutic strategies for FK, encompassing both direct antimicrobial effects and their application as carriers for indirect treatment. Research on direct treatment has primarily focused on silver nanoparticles, which exhibit significant antifungal properties and demonstrate ocular biocompatibility. In addition, zinc oxide nanoparticles possess a unique mechanism for treating FK, as they can activate autophagy pathways and downregulate inflammatory responses. Gold nanoparticles are used in photodynamic or photothermal therapy for FK, exerting direct effects in FK treatment through photothermal/photodynamic effects. However, due to the relatively enclosed structure of the eye, the complete degradation and clearance pathways of nanomaterials are still under investigation, and the long-term effects of these materials on the human body require careful monitoring. In addition to silver nanomaterials, other polymer-based nanomaterials and co-integrated nanosystems have also shown promising antimicrobial activity. Indirect treatment research has mainly concentrated on utilizing nanomaterials as DDS for ocular administration. These include nanoparticles, lipid-based nanocarriers, microemulsions, liposomes, polymeric micelles, cubosomes, nanocarrier-loaded gels, dendrimers, and vesicles. These systems can achieve targeted drug delivery by prolonging drug release, increasing drug retention time on the cornea, and enhancing therapeutic efficacy. Compared to oral administration or conventional topical eye drops, targeted delivery via nanocarriers is both safer and more effective.

Nanomedicine for FK offers many advantages, such as improving patient compliance, enhancing drug efficacy, and reducing side effects. The emergence of drug delivery systems based on nanocarriers is necessary. These systems will provide a paradigm shift for antifungal treatment by enhancing the solubility and stability of drugs, thereby offering sustained release at the site of infection. Meanwhile, these delivery systems will help reduce systemic exposure and toxicity by increasing therapeutic levels within the corneal tissue, thus improving patient compliance and clinical outcomes. Future research directions are expected to focus on refining these novel designs to make the systems more biocompatible and to enhance their stability. Moreover, there is a need for further exploration of the combination of antifungal agents with these new systems to combat multidrug-resistant fungal strains and reduce the likelihood of treatment failure. However, it is undeniable that the high production costs, challenges in storage and transportation, and certain safety concerns associated with some nanomaterials limit their clinical application. To address these issues, more innovative nanomaterials need to be developed, while existing materials should be improved through optimization of material composition, manufacturing processes, and structural design. It is foreseeable that in the future, more novel or improved nanomaterials will emerge, providing new therapeutic options for FK and advancing their clinical application in FK treatment.