From Clay to Cutting-Edge: Halloysite Nanotubes in Next-Generation Nanotechnology

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Data may be preliminary. 17 June 2025 V1 Latest version Share on From Clay to Cutting-Edge: Halloysite Nanotubes in Next-Generation Nanotechnology Authors : Dipanwita Basak and Hemaprobha Saikia 0000-0002-8714-8766 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175016352.29417854/v1 638 views 264 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract In a world urgently seeking greener, smarter, and more efficient technologies, halloysite nanotubes (HNTs) have emerged as promising nanomaterials attracting intense scientific and industrial interest. These naturally occurring, eco-friendly clays feature a unique hollow tubular morphology, tunable surface chemistry, and outstanding biocompatibility traits that position them as transformative agents in fields ranging from medicine and environmental remediation to flame retardancy, cosmetics, and catalysis. This review highlights recent advances in HNT synthesis, surface modification, and multifunctional applications, reflecting a 65% surge in related publications between 2015 and 2022. By 2025, biomedical applications alone are expected to comprise 35% of HNT research, underscoring their growing role in drug delivery, wound care, and antimicrobial systems. HNTs are also proving valuable in real-world innovations from antiviral air filtration during the COVID-19 pandemic to AI-assisted material design. Nonetheless, key challenges persist: variability in natural sources, limited scalability of modification techniques, and a lack of long-term clinical validation. This review emphasizes that bridging these gaps through the integration of nanotechnology, artificial intelligence, and sustainable chemistry could unlock the full potential of HNTs advancing them from geological curiosity to cornerstone of next-generation smart materials. From Clay to Cutting-Edge: Halloysite Nanotubes in Next-Generation Nanotechnology Dipanwita Basak 1 and Hemaprobha Saikia* 1 1 Department of Chemistry, Bodoland University, Assam, India, 783370 Email: [email protected] First Author: Dipanwita Basak Department of Chemistry, Bodoland University Kokrajhar, BTR, Assam Pin-783370 Email: [email protected] Corresponding author: Hemaprobha Saikia, Assistant Professor Department of Chemistry, Bodoland University Kokrajhar, BTR, Assam Pin- 783370 ORCID ID- 0000-0002-8714-8766 Email: [email protected] Contact no. 7002379668/9435118405 Abstract In a world urgently seeking greener, smarter, and more efficient technologies, halloysite nanotubes (HNTs) have emerged as promising nanomaterials attracting intense scientific and industrial interest. These naturally occurring, eco-friendly clays feature a unique hollow tubular morphology, tunable surface chemistry, and outstanding biocompatibility traits that position them as transformative agents in fields ranging from medicine and environmental remediation to flame retardancy, cosmetics, and catalysis. This review highlights recent advances in HNT synthesis, surface modification, and multifunctional applications, reflecting a 65% surge in related publications between 2015 and 2022. By 2025, biomedical applications alone are expected to comprise 35% of HNT research, underscoring their growing role in drug delivery, wound care, and antimicrobial systems. HNTs are also proving valuable in real-world innovations from antiviral air filtration during the COVID-19 pandemic to AI-assisted material design. Nonetheless, key challenges persist: variability in natural sources, limited scalability of modification techniques, and a lack of long-term clinical validation. This review emphasizes that bridging these gaps through the integration of nanotechnology, artificial intelligence, and sustainable chemistry could unlock the full potential of HNTs advancing them from geological curiosity to cornerstone of next-generation smart materials. Keywords: Halloysite Nanotubes (HNTs); Nanomaterials; Drug Delivery; Surface Functionalization; Environmental Remediation; Smart Materials Introduction In an era defined by the urgent need for sustainable materials, smart technologies, and biomedical innovations, halloysite nanotubes (HNTs) have emerged as a timely and transformative solution. As industries grapple with environmental challenges, drug delivery limitations, and the demand for multifunctional materials, the spotlight has turned toward naturally derived nanostructures like HNTs that combine performance with eco-friendliness. These unique, hollow clay nanotubes first recognized for their geological relevance have rapidly ascended in prominence across nanotechnology, owing to their biocompatibility, high surface area, and capacity for selective functionalization [1]. HNTs are shaping next-generation applications in fields ranging from controlled cancer therapeutics and antimicrobial coatings to flame-retardant polymers and green catalysis. Between 2015 and 2022 alone, research output on HNTs grew by 65%, with projections indicating that biomedical applications could dominate by 2025 [2]. This growth reflects not only scientific curiosity but also the practical value HNTs offer in addressing real-world problems whether it’s purifying water, reducing microplastic dependency in packaging, or enhancing the durability of sustainable composites [3]. Between 2005 and 2015, research on halloysite nanotubes saw a 40% increase, with a focus on drug delivery and nanocomposite applications [4-6]. From 2015 to 2022, publications grew by 65%, expanding into environmental applications and catalysis [7-9]. In recent years (2023–2025), significant advancements have been made in functionalization techniques, biomedical applications, and the development of hybrid nanomaterials [11, 12]. The nanotubes can be designed using naturally occuring nanotubes such as halloysite including others such as biogenic nanotubes, which are produced by microorganisms; graphite nanotubes (or carbon nanotubes), composed of cylindrical carbon molecules in single or multi-walled forms; and fullerenes, which consist of hollow carbon spheres or ellipsoids [13-17]. After analyzing recent studies and statistical analyses, it have been found that halloysite nanotube (HNT) applications are expected to be widely distributed across various scientific fields by 2025, in accordance with projected data trends. Biomedical research is anticipated to lead with 35% of studies, particularly focused on drug delivery systems and tissue engineering, leveraging halloysite’s nanotubular structure for controlled and sustained release mechanisms [5, 18-20]. Environmental science is projected to account for 25%, emphasizing halloysite’s adsorption capabilities for pollutant removal, wastewater treatment, and sustainable environmental solutions [21-23]. Additionally, polymer composites and coatings are estimated to contribute 20%, demonstrating halloysite’s enhancement of mechanical properties, thermal stability, and fire resistance in material engineering [25-27]. Catalysis and energy storage applications, comprising 15% of ongoing studies, will focus on reaction enhancement and electrode material development for fuel cells, battery technologies, and hydrogen energy solutions [28-31]. The remaining 5% will explore new and emerging applications, including sensor technology, nanoelectronics, and biomaterial scaffolding, reaffirming the expanding role of halloysite nanotubes in advanced nanotechnology research [5, 32-33]. These findings align with existing statistical projections, showcasing the increasing versatility and scientific significance of halloysite nanotubes in biomedical, environmental, and material sciences. In accordance with the studies between 2023 and 2025, halloysite nanotubes (HNTs) have emerged as highly versatile nanomaterials, with significant advancements in their incorporation into diverse applications such as polymer nanocomposites, flame-retardant systems, and smart materials. Nugroho et al., demonstrated that thermoplastic polyurethane (TPU) composites reinforced with HNTs via melt blending and 3D printing exhibited increased tensile strength and elastic modulus, with mechanical performance highly dependent on HNT dispersion and printing parameters [34]. Similarly, Ge et al., reported that HNTs added to polyurethane elastomers significantly improved tensile strength, sound absorption, and thermal stability [35]. In the realm of flame retardancy, researchers such as Li et al., showed that phytic acid-modified HNTs blended with thermoplastic starch (TPS) led to films with a 54.3% increase in tensile strength, reduced water vapor diffusion, and enhanced flame resistance [36]. In another study, Wang et al., developed Ag@HNT-based flame-retardant coatings for polyester-cotton fabrics, achieving a 47% reduction in peak heat release rate and a limiting oxygen index of 25% [37]. High HNT-loading composites have also been explored. Zhang et al., synthesized polymer/HNT nanocomposites containing up to 75 wt% HNTs via in situ polymerization, resulting in remarkable mechanical strength and uniform dispersion due to strong hydrogen bonding [38]. In the field of responsive materials, Namathoti et al., developed shape memory polymer nanocomposites reinforced with both multiwalled carbon nanotubes (MWCNTs) and HNTs. These materials exhibited enhanced mechanical properties and thermal-triggered shape recovery suitable for 4D printing applications. Collectively, the studies confirms that the unique morphology, surface chemistry, and compatibility of HNTs with various polymers make them ideal candidates for next-generation functional materials in sectors ranging from flame-retardant textiles to biomedical scaffolds and smart devices. Over the past two decades, halloysite nanotubes (HNTs) have emerged as a remarkable nanomaterial with applications spanning biomedicine, environmental science, catalysis, and nanocomposite engineering [40-42]. The appeal of HNTs stems from their biocompatibility, abundant availability, tunable surface chemistry, and distinctive nanotubular morphology [43]. HNTs are naturally occurring aluminosilicate minerals characterized by their multi-walled hollow tubes, typically measuring: 500 nm to 2000 nm in length, 50–200 nm in outer diameter, 10–70 nm in inner diameter [44]. The formation of the tubular structure is attributed to the curling of layered aluminosilicate sheets, which occurs as a result of internal lattice mismatches between the silica and alumina layers [45]. Due to lattice strain between the mismatched layers, the sheets curl into tubes. Interestingly, the structure varies based on hydration of halloysite which transforms ranging from 7 Å halloysite (dehydrated) to 10 Å halloysite (hydrated) [46, 47]. The transition is reversible and affects interlayer spacing and chemical behavior [48]. Recent advancements in the synthesis of HNTs have focused on improving their dispersion and surface properties. Techniques such as sonication, high-temperature calcination, and chemical treatments have been employed to enhance the properties of HNTs. Surface functionalization using agents like silane coupling agents (e.g., APTMS, GPTMS) has been explored to improve the compatibility of HNTs with various matrices [49]. While the modification toolkit is extensive, many methods remain poorly scalable or environmentally burdensome. Moreover, there is a paucity of research on the long-term stability of surface-modified HNTs in real-world systems which is determined as an aspect crucial for applications in biomedicine and agriculture [50]. HNTs possess an alumina-rich inner lumen and a silica-rich outer surface, which facilitates selective functionalization heavily exploited in targeted drug delivery and encapsulation technologies [51, 52]. Despite the insights, significant heterogeneity in natural halloysite deposits remains a challenge. Variations in purity, aspect ratio, and hydration state across geographies limit reproducibility and standardization in downstream applications. Studies must be initiated prioritizing the geochemical characterization of source materials and develop uniform classification criteria. According to previous literature surveys, HNTs have shown exceptional promise as nanocontainers for controlled drug release, especially in cancer therapeutics. Studies have demonstrated successful encapsulation and sustained release of chemotherapeutics such as doxorubicin, cisplatin, and curcumin [53]. Additionally, HNT-incorporated scaffolds have improved osteogenic activity in bone regeneration [54]. Despite encouraging in vitro and preliminary in vivo results, the transition to clinical validation remains limited. Regulatory hurdles, lack of long-term toxicological data, and limited understanding of biodistribution and degradation pathways hinder clinical adoption [55]. Robust, standardized biocompatibility studies are urgently needed to de-risk medical applications. Despite their advantages, several research gaps persist in the study and utilization of HNTs. Key challenges include optimizing surface functionalization for targeted applications, understanding their long-term biocompatibility, and improving its dispersion. Additionally, the controlled release mechanisms for drug delivery and the sustainability of HNT-based environmental remediation solutions require further investigation. Addressing these gaps will enhance the efficiency of HNTs and broaden their applications in nanotechnology, biomedical sciences, and materials engineering. This review article explores the structural, physicochemical, and functional properties of halloysite nanotubes, highlighting current research advancements and identifying existing challenges that necessitate further study [56]. The COVID-19 pandemic has driven a surge in innovation around antiviral technologies and targeted drug delivery systems. Among the emerging solutions, halloysite nanotubes (HNTs) have shown great promise in enhancing pulmonary drug delivery, air filtration, and self-disinfecting surfaces through photocatalytic functionalization. Their hollow structure and surface versatility enable precise, responsive therapeutic release making them valuable tools in managing viral threats like SARS-CoV-2 [57]. In parallel, the integration of artificial intelligence (AI) and machine learning (ML) has revolutionized HNT research by enabling predictive modeling of drug release, flame retardancy, and composite performance. These data-driven approaches are rapidly accelerating material design, improving reproducibility, and opening new frontiers for smart, sustainable nanomaterials [58]. This review provides a comprehensive look at the structural features, modification strategies, and application domains of HNTs, weaving in recent breakthroughs and statistical trends. By critically examining both their current capabilities and the challenges ahead—such as functionalization scalability, material heterogeneity, and long-term biocompatibility—we aim to illuminate the path forward. With emerging tools like AI accelerating material discovery, halloysite nanotubes stand at the frontier of innovation, promising to redefine the role of natural nanomaterials in 21st-century science and industry. Scheme 1- Illustration of Halloysite nanotube and its trending applications Sources and structure of Halloysite nanotubes Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate clay minerals. They are found in several deposits around the world, usually formed from the weathering of volcanic rocks like rhyolite, andesite, and trachyte ( Table 1 ). Table 1- Natural sources of halloysite and its purity Sl. No. Sources Country Morphology Purity (%) Key Applications References \tightlist Dragon Mine (Utah) USA Predominantly tubular ~90–95 Nanocomposites, drug delivery, ceramics [69] \tightlist Dunino New Zealand Tubular ~80–90 Ceramics, environmental remediation [70] \tightlist Mount Vista Australia Mixed (tubes, platy) ~70–85 Ceramics, paints, polymer additives [71] \tightlist Jiangxi Province China Mainly tubular ~75–90 Paper, ceramics, catalysts [72] \tightlist Ilimsk Region Russia Tubular & spheroidal ~70–80 Nanotechnology, water purification [73] \tightlist Thabazimbi South Africa Mixed morphology ~60–75 Ceramics, geopolymers [74] \tightlist Guanajuato Region Mexico Tubular ~60–80 Construction, environmental uses [75] \tightlist Various (Anatolia) Turkey Mixed (tube, platy) ~65–85 Ceramics, paints [76] \tightlist Amazonas State Brazil Mainly tubular ~60–75 Ceramics [77] While halloysite is mostly used in its natural form, synthetic analogs have also been attempted for specific applications [59]. These are often made via Hydrothermal synthesis, Templating methods using aluminosilicate gels, Sol-gel processes [60]. However, these are not yet scalable or cost-effective compared to mining natural HNTs. It is chemically represented by the empirical formula Al₂Si₂O₅(OH)₄·nH₂O, where n typically varies between 0 and 2 depending on its hydration state [61-63]. It belongs to the kaolinite group, sharing a similar layered structure with kaolinite, but halloysite commonly occurs in a tubular morphology, forming what are known as halloysite nanotubes (HNTs) [64]. These nanotubes are the result of a mismatch in the unit cell dimensions between the silica tetrahedral sheet and the alumina octahedral sheet, which causes the layers to curl into a tubular shape rather than remaining flat as in kaolinite [65] ( Scheme 2 ). Scheme 2- Sources and structure of Halloysite nanotubes Structurally, halloysite consists of a 1:1 layer silicate—one tetrahedral silica sheet bonded to one octahedral alumina sheet. In the hydrated form (often called 10 Å halloysite), water molecules are intercalated between the layers, expanding the basal spacing to approximately 10 Å (1.0 nm). Upon drying, the water is lost, and the spacing reduces to 7 Å (0.7 nm), resulting in dehydrated halloysite. The dimensions of HNTs typically range between 500 nm to 2000 nm in length, 50–200 nm in outer diameter, 10–70 nm in inner diameter [45]. These nanoscopic tubular structures exhibit a high aspect ratio and significant surface area, which can range from 65 to over 200 m²/g, depending on the purity, source, and treatment of the mineral [66]. The outer surface of the nanotube is primarily composed of Si–OH (silanol) groups, while the inner lumen is rich in Al–OH (aluminol) groups, giving halloysite amphoteric properties and enabling selective surface functionalization [67, 68]. Trending applications of Halloysite nanotubes Drug Delivery Halloysite nanotubes (HNTs) are increasingly recognized as effective nanocarriers for controlled drug delivery, owing to their distinct tubular shape, biocompatibility, and capacity to encapsulate a wide range of therapeutic agents. Extensive research confirms their role in improving drug stability, achieving sustained release, and enabling targeted therapy. The hollow lumen, generally measuring 10–30 nm in diameter, is well-suited for loading small therapeutic molecules, including anticancer drugs (e.g., doxorubicin, methotrexate, artemisinin) and nucleic acids (DNA, RNA). The contrasting surface charges positively charged aluminol groups inside and negatively charged siloxane groups outside enables selective chemical modification to tailor drug loading and release profiles as illustrated in Scheme 3 [78]. Scheme 3- Schematic illustration of drug loading of Halloysite nanotube Studies by Lvov et al., demonstrated the high drug loading capacity and low cytotoxicity of HNTs, both crucial for biomedical applications. Surface functionalization, such as with folic acid, enhances the targeting of cancer cells, particularly those overexpressing folate receptors. Furthermore, stimuli-responsive systems have been designed to trigger drug release under specific conditions like low pH or in the presence of reductive agents, mimicking tumor environments [53, 79-82]. HNTs also show promise in gene delivery, providing protection against enzymatic degradation and allowing for controlled, sustained release. Their potential to deliver multiple agents simultaneously supports the development of combination therapies, which may improve therapeutic outcomes and reduce side effects. A detailed comparision of Halloysite nanotube modifications/loading by different drugs comparing different release rates is listed in Table 2 . Table 2- Halloysite nanotube modifications/loading by different drugs comparing different release rates Sl. No. Drug Modification Drug Loading (%) Release Rate Application References \tightlist Curcumin Chitosan-coated HNTs 70.2% Sustained release over 6 hours Breast cancer therapy [83] 2. Paclitaxel (PTX) DSPE-modified HNTs 18.44% Near 100% release Cancer therapy [84] 3. Doxorubicin (DOX) Unmodified HNTs 22.01% - Cancer therapy [85] 4. Doxycycline Unmodified HNTs 4.2% - Antibacterial therapy [86] 5. Sodium Salicylate Unmodified HNTs 7.57% - Anti-inflammatory therapy [87] 6. Salicylic Acid Unmodified HNTs - - Anti-inflammatory therapy [88] 7. Vancomycin Unmodified HNTs - Sustained release over 33 days Antibacterial therapy [89] 8. Resveratrol Unmodified HNTs - 48 hours Anticancer therapy [90] 9. Khellin Chitosan-coated HNTs - - Anticancer therapy [91] 10. Metoclopramide Chitosan-coated HNTs - 66.8% at pH 1.2 and 46.7% at pH 5.5 within 25 hours Gastrointestinal therapy [92] Curcumin, a hydrophobic anticancer agent, was effectively incorporated into HNTs modified with chitosan, a biocompatible polymer. In a previous study, Farokh et al., reported a notably high drug loading efficiency of 70.2%, highlighting the superior encapsulation capability of functionalized HNTs. The system demonstrated sustained drug release over more than 6 hours, providing controlled and extended bioavailability. This design is especially suited for breast cancer therapy, leveraging curcumin’s known anticancer activity [83]. In another study, Liao et al., demonstrated the loading of paclitaxel, a widely used chemotherapy drug, into DSPE-functionalized HNTs. Although the reported loading efficiency was moderate (18.44%), the nearly complete drug release suggested highly efficient unloading. This system exemplifies how surface functionalization can enhance both dispersion and release properties of hydrophobic drugs, showing strong potential for systemic cancer treatment [84]. Wu et al., also evaluated the encapsulation of doxorubicin (DOX) in unmodified HNTs, achieving a 22.01% loading efficiency. Although the study did not specify the release kinetics, these findings confirm that even without chemical modification, HNTs can effectively host cationic anticancer drugs, reinforcing their potential in conventional chemotherapy delivery [85]. In a separate investigation, Palasuk et al., reported the encapsulation of doxycycline, an antibiotic, into pristine HNTs, though with a relatively low loading efficiency of 4.2%. While the release profile was not disclosed, the study suggested its possible application as a controlled-release system, particularly for localized infections or wound care. The low loading percentage indicates the need for further formulation optimization [86, 87]. Frejková et al., examined the encapsulation of anti-inflammatory agents in unmodified HNTs, noting a 7.57% loading efficiency for sodium salicylate. Although release data were not thoroughly presented, the study implies that HNTs can serve as carriers for hydrophilic anti-inflammatory drugs, albeit with limited encapsulation efficiencies due to their small molecular size and polarity [88]. In a long-term drug release study, Pan et al., investigated vancomycin, a glycopeptide antibiotic, loaded into unmodified HNTs. The system demonstrated sustained drug release for up to 33 days, although loading values were not specified. This prolonged release is considered advantageous for treating chronic or implant-related infections, suggesting potential use in bone or implantable therapies [89]. Natural compounds such as resveratrol and khellin, known for their anticancer properties, were loaded into unmodified and chitosan-coated HNTs, respectively. Vergaro et al., observed that resveratrol exhibited a sustained release lasting 48 hours, supporting the application of HNTs in natural product-based therapeutic strategies, particularly in alternative or adjunct cancer treatments [90]. In gastrointestinal applications, Lisuzzo et al., evaluated metoclopramide delivery using chitosan-coated HNTs, demonstrating pH-responsive drug release: 66.8% at pH 1.2 and 46.7% at pH 5.5 over a 25-hour period. These findings underscore the potential for site-specific, pH-triggered drug release, particularly in the digestive tract [91]. Despite these advancements, Sharif et al., noted that data on the reusability of HNTs remains scarce. Current studies primarily emphasize single-use drug delivery, lacking assessments of reloading potential or long-term biocompatibility. For sustainable or clinical applications such as wound healing materials or implantable devices; future work must evaluate the feasibility of reusing HNTs without compromising their performance or safety [92]. Figure 1- (A1 to A3) SEM images of without drug HNTs/chitosan micro-compo sites and (B1 to B3) with MTC loaded HNTs/chitosan micro-composites. Reproduced with permission from ref. 90 (License No. 6040300004114) Cosmetics Halloysite nanotubes (HNTs), first described in detail by natural clay mineral researchers in the early 20th century, are a naturally occurring aluminosilicate with a unique tubular morphology [53]. Their potential in nanotechnology and biomedical fields was extensively explored in the early 2000s, but it wasn’t until the last decade that their application in cosmetics began gaining traction. According to Zhang et al., the unique hollow structure of HNTs allows them to encapsulate a wide range of active molecules, including antioxidants, vitamins, and exfoliants, making them suitable for cosmetic formulations [93]. Recent studies have confirmed their practical benefits in skincare. Borrego- Sánchez et al., demonstrated that HNTs could encapsulate retinol, niacinamide, and glycolic acid effectively, resulting in enhanced photostability and prolonged release up to 24 hours compared to free molecules. This extended release not only improves efficacy but also reduces skin irritation risks associated with rapid exposure to potent actives [94]. In terms of safety, Lvov et al., reported high biocompatibility of HNTs, showing over 90% cell viability in human dermal fibroblast tests, even at concentrations up to 100 µg/mL. This low cytotoxicity, combined with their natural origin and high surface area, makes HNTs an ideal candidate for advanced cosmetic formulations [79]. Together, these findings position halloysite nanotubes as multifunctional nanocarriers capable of improving the stability, absorption, and efficacy of cosmetic actives, marking a significant innovation in topical product design ( Scheme 4 ). The role of modified halloysite nanotubes loaded with active ingredients and its prodigy in cosmetics are reviewed ( Table 3 ). Scheme 4- Various utilization of Halloysite nanotubes in formulation of cosmetics Table 3- Modified Halloysite nanotubes loaded with active ingredients and functionalities in cosmetics Sl. No. Modification Active Ingredients Functionality References 1. Keratin-HNT Nanocomposite Keratin, HNT composites UV protection through nanocomposite coatings on hair [95] 2. Sodium Dodecyl Sulfate (SDS)-HNT Permethrin, Minoxidil Sustained release for anti-lice and hair growth treatments [79] 3. Chitosan-Grafted HNTs Curcumin Enhanced anticancer efficacy through controlled release [96] 4. Carboxylic Acid-Modified HNTs Diphenhydramine Hydrochloride Sustained release in cosmetic formulations [97] 5. SiO₂-Coated HNTs - Durable superhydrophobic coatings for cosmetic packaging [98] Keratin-functionalized HNTs have been developed to improve UV protection for hair. A study by Zhang et al., demonstrated that a keratin-HNT nanocomposite could form a protective coating on hair fibers, effectively shielding them from UV-induced damage. The coating not only protected the hair structure but also improved its mechanical properties, making it a promising approach for sun-protection cosmetic products [95]. Sodium dodecyl sulfate (SDS)-modified HNTs have been utilized for the sustained delivery of therapeutic agents like permethrin and minoxidil, commonly used in anti-lice and hair growth treatments, respectively. Lvov et al., reported that SDS modification enhances the loading efficiency of these drugs within the HNT lumen and improves their controlled release profile, resulting in prolonged effectiveness and reduced dosing frequency [79]. Chitosan, a biopolymer with inherent antimicrobial and bioadhesive properties, has been grafted onto HNTs to improve the delivery of bioactive compounds such as curcumin. Li et al., synthesized chitosan-grafted HNTs and investigated their potential as a nano-formulation for curcumin delivery. The study found that the chitosan grafting enhanced the loading capacity and stability of curcumin, providing a controlled and sustained release profile, which is particularly useful in skin care and therapeutic cosmetic products with antioxidant or anticancer claims [96]. Silica-coated HNTs have been engineered to impart superhydrophobic properties, making them ideal for use in cosmetic packaging that requires moisture resistance and enhanced durability. A study by Zhang et al., demonstrated that the SiO₂ layer increased environmental resistance without compromising the biocompatibility of the underlying HNTs. These modified nanotubes can also be incorporated into protective cosmetic films or sprays [97]. Carboxylated HNTs have been modified with functional carboxylic acid groups to enhance the encapsulation and controlled release of hydrophilic drugs, such as diphenhydramine hydrochloride. Researchers reported that this modification improved the interaction with the loaded compound and ensured more efficient delivery to the skin, making it beneficial for topical or dermal applications in cosmetics where sustained release of antihistamines or anti-irritant agents is desired [98]. Different functionalized halloysite nanotube (HNT) systems have unique benefits and drawbacks for cosmetic and therapeutic uses. Keratin-functionalized HNTs offer strong UV protection and improve hair strength but may wear off with washing and be costly to produce. SDS-modified HNTs enhance loading and sustained release of hydrophobic drugs like permethrin and minoxidil, though SDS can irritate sensitive skin. Chitosan-grafted HNTs provide antimicrobial and adhesive properties for controlled release of compounds like curcumin but can be sensitive to skin pH and require careful formulation. Silica-coated HNTs add water-repellent qualities for packaging but may increase rigidity and complicate processing. Carboxylated HNTs are effective for hydrophilic drug delivery with controlled release but may have lower stability and higher production costs. Therefore, choosing the right system depends on balancing effectiveness, safety, and manufacturability. Antibacterial Coatings Halloysite nanotubes (HNTs) are rapidly emerging as powerful tools in antibacterial coatings, capturing attention thanks to their unique tubular structure and excellent biocompatibility ( Scheme 5 ). Scheme 5- Antibacterial activity of halloysite nanotube with modified coatings As Lvov et al., revealed, HNTs can be loaded with a variety of antimicrobial agents to achieve sustained release, dramatically extending their antibacterial effectiveness [79]. Excitingly, when silver nanoparticles or antibiotics are encapsulated within these nanotubes, studies like Shevtsova et al., have shown they inhibit bacterial growth far more efficiently than when used alone [99]. Beyond this, the natural clay makeup of HNTs offers its own antimicrobial benefits, which, when combined with active agents, create a potent, synergistic defense against harmful pathogens [100-102] ( Table 4 ). These remarkable properties make HNT-based coatings highly promising for revolutionizing medical device safety, enhancing food packaging, and protecting surfaces from bacterial contamination leading to new frontiers in infection control and public health. Table 4- Modified Halloysite nanotubes with their antibacterial agents resisting target bacteria and functionalities Sl. No. Antibacterial Agent Modification Target Bacteria Application Type Functionality References \tightlist Gold nanoparticles (AuNPs) Pristine HNTs - Wound healing hydrogel Enhanced antibacterial & hemostatic performance. [103] \tightlist Protease type I (PtI) Fe₃O₄-coated HNTs E. coli, S. aureus, MRSA Anti-biofilm Significantly disrupted biofilms and bacterial growth. [104] \tightlist Tannic acid + Fe₃O₄ NPs Magnetic HNTs E. coli Biofilm inhibition 57.8% biofilm inhibition observed. [105] \tightlist Zinc + Chitosan Zn-loaded HNTs E. coli, S. aureus Implant coating Inhibition zones of 13 mm (E. coli); 9.7 mm (S. aureus). [106] \tightlist Chlorhexidine Gluconate (CG) CG-loaded HNTs E. coli, S. aureus, P. aeruginosa Textile coating >98% bacterial reduction; retained after 20 wash cycles. [107] \tightlist Linalool (natural antimicrobial) Essential oil-loaded HNTs S. aureus, E. coli Food packaging film Sustained antimicrobial release and increased shelf life of food. [108] \tightlist Silver nanoparticles (AgNPs) Tannic acid-modified HNTs S. Typhimurium Veterinary medicine Eliminated multidrug-resistant Salmonella in a C. elegans model. [109] \tightlist Copper nanoparticles (CuNPs) HNTs via reverse ATRP E. coli Wound dressing Good antibacterial activity against E. coli. [110] \tightlist Curcumin HNTs-loaded nanofibers S. aureus, E. coli Wound healing Strong antibacterial and antioxidation activities; improved wound healing in vivo. [111] \tightlist Thymol (essential oil) Polydopamine-coated HNTs - Agricultural carrier Enhanced loading capacity and controlled release; responsive to pH and NIR. [112] \tightlist Chlorhexidine Gluconate (CG) HNTs-loaded composites cotton fabric coating S. aureus, E. coli, P. aeruginosa Orthopedic appliances Enhanced antimicrobial efficacy in presurgical orthopedic appliances. [113] \tightlist Polyoxometalates (POMs) HNTs with AgNPs S. aureus, P. aeruginosa, A. baumannii Antibacterial composite Best antibacterial performance with phosphomolybdic acid; reduced biofilm viability. [114] \tightlist Hydroxyapatite (HA) + Chitosan (CS) HNTs-reinforced composite E. coli, S. aureus Implant coating Enhanced antibacterial activity; better performance against E. coli. [115] \tightlist Zinc oxide (ZnO) HNTs-loaded composites E. coli, S. aureus Wound dressing Improved antibacterial properties; sustained release. [116] \tightlist Chitosan HNTs-loaded curcumin E. coli, S. aureus Wound dressing Enhanced antibacterial activity; biocompatible. [117] \tightlist Polyvinyl alcohol (PVA) HNTs-loaded composites E. coli, S. aureus Wound dressing Sustained release of antibacterial agents; improved wound healing. [118] \tightlist Polyethylene glycol (PEG) HNTs-loaded composites E. coli, S. aureus Wound dressing Enhanced antibacterial properties; biocompatible. [119] \tightlist Polycaprolactone (PCL) HNTs-loaded composites E. coli, S. aureus Wound dressing Sustained release of antibacterial agents; improved mechanical properties. [120] \tightlist Poly(lactic-co-glycolic acid) (PLGA) HNTs-loaded composites E. coli, S. aureus Drug delivery system Controlled release of antibacterial agents; improved stability. [121] \tightlist Polyethylene oxide (PEO) HNTs-loaded composites E. coli, S. aureus Wound dressing Sustained release of antibacterial agents; improved mechanical properties. [122] \tightlist Polyurethane (PU) HNTs-loaded composites E. coli, S. aureus Wound dressing Enhanced antibacterial activity; biocompatible. [123] \tightlist Polylactic acid (PLA) HNTs-loaded composites E. coli, S. aureus Drug delivery system Controlled release of antibacterial agents; improved stability. [124] \tightlist Ciprofloxacin and polymyxin B sulfate HNTs-loaded composites E. coli, S. aureus Wound dressing Sustained release of antibacterial agents; improved mechanical properties. [125] \tightlist Polyvinyl alcohol/poly(lactic-co-glycolic acid) (PVA/PLGA) HNTs-loaded composites E. coli, S. aureus Drug delivery system Controlled release of antibacterial agents; improved stability. [126] \tightlist Polyvinyl alcohol/polyethylene oxide (PVA/PEO) HNTs-loaded composites E. coli, S. aureus Wound dressing Sustained release of antibacterial agents; improved mechanical properties. [127] \tightlist Polyvinyl alcohol/polyurethane (PVA/PU) HNTs-loaded composites E. coli, S. aureus Wound dressing Enhanced antibacterial activity; biocompatible. [128] Zhao et al., utilized pristine halloysite nanotubes to incorporate gold nanoparticles (AuNPs) into a hydrogel for wound healing, demonstrating enhanced antibacterial and hemostatic properties that aid in infection control and accelerate healing, with broad-spectrum antimicrobial potential despite unspecified bacterial strains [103] (Figure 2 ). Figure 2- (a) SEM of halloysite nanotubes, (b) SEM of Norfloxacin, (c), (d), (e) SEM of freeze-dried samples (F1, F2, F3) and (f) SEM of nanocomposite film. Reproduced with permission from ref. 90 (License No. 6040320929546) Kim et al., developed superparamagnetic halloysite nanotubes coated with Fe₃O₄ and functionalized with protease type I (PtI), which effectively disrupted biofilms and inhibited the growth of E. coli, S. aureus , and MRSA, with magnetic properties enabling potential guided drug delivery [104]. A combination of tannic acid and Fe₃O₄ nanoparticles within magnetic halloysite nanotubes, achieving 57.8% inhibition of E. coli biofilm formation, suggesting a promising strategy to reduce bacterial surface colonization [105]. Another study indicating the loading of zinc ions and chitosan onto halloysite nanotubes to coat titanium alloy implants, producing inhibition zones of 13 mm and 9.7 mm against E. coli and S. aureus respectively, indicating strong antibacterial activity suitable for preventing implant-related infections [106]. Another study incorporated chlorhexidine gluconate into halloysite nanotubes for textile coatings, resulting in over 98% bacterial reduction against E. coli, S. aureus , and P. aeruginosa , with antibacterial activity retained after 20 wash cycles, supporting durable healthcare textile applications [107]. Researchers has encapsulated linalool essential oil in halloysite nanotubes to develop food packaging films that provided sustained antimicrobial release against S. aureus and E. coli , thereby increasing shelf life through an eco-friendly controlled-release mechanism [108]. One significant innovation is the use of silver nanoparticles (AgNPs) loaded on tannic acid-modified HNTs, as reported by Majumder et al. [109]. This system effectively eradicated multidrug-resistant Salmonella Typhimurium in a Caenorhabditis elegans infection model. The tannic acid played a dual role: stabilizing the AgNPs and providing a synergistic antibacterial effect. The result underscores the utility of this nano-formulation in veterinary medicine and antimicrobial resistance management. However, a potential limitation could be the cytotoxicity associated with prolonged silver exposure, which necessitates careful dose optimization. In another study, Ding et al., employed reverse atom transfer radical polymerization (ATRP) to synthesize copper nanoparticles (CuNPs) on HNTs [110]. This nanocomposite exhibited strong antibacterial activity against Escherichia coli , indicating its potential in wound dressings. The cost-effectiveness and redox cycling ability of copper are major advantages, although copper’s oxidative toxicity remains a limitation for long-term or systemic use. Curcumin, a natural polyphenolic compound with known antimicrobial and antioxidant activity, was successfully encapsulated in HNTs-based nanofibers by Doustdar et al. These nanofibers exhibited strong activity against Staphylococcus aureus and E. coli , and enhanced wound healing in vivo. The benefits of using curcumin include its safety and multifunctionality; however, its poor solubility and instability under physiological conditions present formulation challenges, which HNTs help mitigate [111]. A novel approach was described by Tas et al., where thymol, an essential oil, was loaded onto polydopamine-coated HNTs [108]. This configuration allowed for pH- and near-infrared-responsive release, enhancing its use as an agricultural antimicrobial carrier. This innovation improves payload control and environmental responsiveness, but scalability and field application efficacy need further investigation. In the context of orthopedic applications, Wu et al., developed a chlorhexidine gluconate (CG)-loaded HNT composite on cotton fabric, achieving over 98% bacterial reduction against S. aureus , E. coli , and P. aeruginosa even after 20 laundering cycles. The long-term activity and fabric compatibility are notable strengths; however, chlorhexidine can pose skin sensitization risks at higher concentrations[112]. To combat biofilms and multidrug resistance, researchers have introduced a composite of polyoxometalates (POMs) and AgNPs with HNTs, showing potent activity against S. aureus , P. aeruginosa , and A. baumannii [113]. An implant coating composite combined hydroxyapatite (HA) and chitosan (CS) reinforced with HNTs was developed. It significantly inhibited E. coli and S. aureus, with superior effects against E. coli. The use of biocompatible HA and CS aligns well with biomedical applications, though mechanical stability under physiological stress requires further optimization. Multiple polymer-HNT composites have also been investigated for wound dressing and drug delivery purposes [114]. Interestingly, between 2010 and 2018, Zhang et al., explored various polymer-HNT composites for wound dressing and drug delivery, demonstrating that materials like ZnO, chitosan, PVA, PEG, PCL, PLGA, PEO, PU, PLA, PVP, PAA, and PAM enhanced antibacterial efficacy, sustained drug release, and mechanical properties, while polymer blends such as PVA/PLGA, PVA/PEO, and PVA/PU offered tailored solutions for advanced wound care [115-128]. In summary, HNT-based antibacterial systems offer versatile platforms for delivering a broad spectrum of antimicrobial agents. These combinations improve stability, enable targeted and controlled release, and enhance antimicrobial efficacy. Despite their promising potential, limitations such as cytotoxicity of metal nanoparticles, synthetic polymer degradation, and scalability must be addressed to transition these technologies into widespread clinical or agricultural use. Catalysis Halloysite nanotubes (HNTs) have gained substantial interest in catalysis due to their naturally occurring tubular morphology, high specific surface area, tunable surface chemistry, and excellent dispersibility for active catalytic species. Various transition metals and metal oxides, such as Pd, Ni, Co, Cu, Fe₃O₄, Cr, and Mo, have been anchored onto HNTs to catalyze a broad range of reactions including methane oxidation, CO₂ hydrogenation, hydrodesulfurization, Fenton-like degradation, and Knoevenagel condensation [131, 132]. Researchers have revealed PdNi-supported halloysite nanotubes (PdNi@HNTs) demonstrated high surface areas ranging from 85.6 to 89.6 m²/g, along with excellent catalytic performance in methane oxidation [133]. In another study, Cu-Co nanoparticles deposited on PDDA-modified HNTs (Cu-Co/PDDA-HNTs) achieved a turnover frequency (TOF) of 30.8 mol H₂ per mol of metal per minute during the dehydrogenation of ammonia borane [134]. Additionally, Fe₃O₄-functionalized HNTs significantly improved pollutant removal efficiency in Fenton-like oxidation processes. In the hydrodesulfurization of dibenzothiophene (DBT), CoMoS-loaded HNTs achieved a conversion rate of up to 96.2% [134]. Moreover, the use of HNTs@ZIF-67 in Knoevenagel condensation reactions yielded over 99% product formation and exhibited good recyclability of the catalyst [130]. Table 5 describes the overall studies of modified Halloysite nanotubes as the support material used as catalyst for various types of reactions. Table 5- Modified Halloysite nanotubes as the support material used as catalyst for various types of reactions Sl. No. Catalyst Reaction Type Functionality Refernces \tightlist Acid-treated HNTs Polystyrene degradation Aromatic selectivity >99% [135] \tightlist Metalloporphyrins (Fe, Co) Hydrocarbon oxidation 100% immobilization rate [136] \tightlist Amino-modified HNTs H₂ generation from NaBH₄ methanolysis HGR: 4020 mL/min/g [137] \tightlist Acid-modified HNTs Prins reaction for chromene synthesis High stereoselectivity [138, 139] \tightlist g-C₃N₄–ZnO/HNT composite Photodegradation of tetracycline Enhanced photocatalytic activity [140] \tightlist Polyaniline–TiO₂/HNT composite Photodegradation of organic pollutants Enhanced visible light photocatalytic activity [141] \tightlist Au-Ni nanoparticles on HNTs Degradation of organic dyes Enhanced catalytic activities [142] \tightlist Iron oxide particles in HNT lumen Photodegradation of methylene blue High photocatalytic activity [143] \tightlist TiO₂ on HNTs Photodegradation of methylene blue High photocatalytic activity [144] \tightlist Carbon-doped TiO₂/HNT hybrid Photocatalytic performance Enhanced visible-light photocatalytic performance [145] \tightlist Metal oxide nanoparticles on carbon-coated HNTs Photodegradation of organic pollutants High photocatalytic activity [146] \tightlist La₀.₇Ce₀.₃FeO₃/HNT composite Photocatalytic degradation of antibiotics High photocatalytic activity [147] \tightlist TiO₂/HNT and Fe₂O₃/HNT composites Photodegradation of chloroanilines High photocatalytic activity [148] \tightlist HNT@W₁₈O₄₉ nanocomposite Photocatalytic properties Enhanced photocatalytic properties [149] \tightlist Manganese oxides on HNTs Thermal catalytic oxidation of formaldehyde High catalytic activity [150] \tightlist Cobalt mesocatalysts on HNTs Hydrogen production from NaBH₄ High catalytic activity [151] \tightlist CZA on HNTs Catalytic performance Enhanced catalytic performances [152] \tightlist CeO₂–WO₃ on HNTs NOx reduction in the presence of SO₂ Improved NOx reduction [153] Tae et al., demonstrated acid-treated HNTs were employed as catalysts for the thermal degradation of polystyrene. The treatment increased the acidity and surface activity of HNTs, leading to a highly selective reaction where over 99% of the products were aromatic compounds. This shows the potential of modified HNTs in polymer cracking and hydrocarbon upgrading [135]. Nakagaki et al., revealed that metalloporphyrins complexes containing iron and cobalt were immobilized on HNTs and nanoscrolls. These catalysts were then used in hydrocarbon oxidation reactions. A key result was the 100% immobilization of the active species, indicating excellent interaction between the HNT surface and metalloporphyrins, making it effective for oxidation catalysis [136]. Sahiner et al., developed involved amino-modified HNTs used to catalyze hydrogen generation from sodium borohydride (NaBH₄) in methanol. The reaction reached a hydrogen generation rate (HGR) of 4020 mL/min/g catalyst, demonstrating that surface functionalization of HNTs significantly improves their efficiency for hydrogen production [137]. Another back-to-back studies by Sidorenko et al., investigated acid-modified HNTs in the Prins reaction, a key organic transformation for synthesizing chromene derivatives. The acid sites on HNTs enabled high stereoselectivity and conversion efficiency, confirming their role as effective solid acid catalysts in organic synthesis [138, 139]. Similarly, Li et al., developed a composite material combining g-C₃N₄, ZnO, and HNTs was synthesized for the photodegradation of tetracycline. The synergistic interaction between g-C₃N₄ and ZnO enhanced photocatalytic activity under visible light. HNTs served as a structural scaffold that improved light absorption and dispersion of the active components [140]. Another composite, Polyaniline–TiO₂/HNT Composite by Li et al., incorporating polyaniline and TiO₂ on HNTs, was developed for environmental remediation. The hybrid system showed enhanced visible light activity for degrading organic pollutants. Polyaniline acted as a conductive polymer, extending TiO₂’s light absorption into the visible region [141]. Researchers revealed that gold and nickel nanoparticles were dispersed onto HNT surfaces and applied in dye degradation. The well-dispersed metal nanoparticles contributed to high catalytic activity and recyclability. HNTs helped prevent nanoparticle aggregation, enhancing the catalyst’s stability and performance [142]. Interestingly, researchers have revealed that Iron oxide nanoparticles were loaded into the lumen (inner cavity) of HNTs [143]. This configuration was used to degrade methylene blue via photocatalysis. The core-shell structure improved the interaction between light, the dye, and the catalyst, boosting degradation efficiency. Xianchu et al., developed that TiO₂ was deposited onto the surface of HNTs, forming a hybrid used for photodegradation of methylene blue [144]. HNTs enhanced TiO₂ dispersion and light harvesting, leading to higher catalytic performance under UV irradiation. Similar to the previous study, this research also focused on TiO₂–HNT hybrids. Their structure enabled more active sites and better charge separation during photocatalysis, leading to efficient dye degradation. Jiang et al., designed a carbon-doped TiO₂/HNT composite was synthesized to shift TiO₂’s band gap into the visible light region [145]. The material showed superior photocatalytic activity under visible light due to enhanced charge separation and light absorption. Zhang et al., revealed a study involving metal oxide nanoparticles supported on carbon-coated HNTs [146]. The carbon layer increased conductivity, while the HNTs provided a scaffold for uniform metal oxide distribution, both of which contributed to effective pollutant degradation. A La₀.₇Ce₀.₃FeO₃ perovskite material was supported on HNTs for antibiotic degradation. The composite benefited from both the high activity of the perovskite and the structural support of the HNTs, leading to efficient photocatalysis under visible light [147]. Szczepanik et al., tested TiO₂/HNT and Fe₂O₃/HNT composites for the photodegradation of chloroanilines, toxic organic pollutants. Both systems exhibited high catalytic performance, especially under UV light, due to improved dispersion and surface area provided by HNTs [148]. Simillarly, Peng et al. , developed a W₁₈O₄₉-based nanocomposite was anchored on HNTs to create a photocatalyst with excellent light absorption and catalytic properties. The combination resulted in improved electron-hole separation, increasing the photocatalytic reaction rate [149]. Another study revealed Manganese oxide catalysts were deposited on HNTs and applied in the thermal oxidation of formaldehyde, a common indoor pollutant [150]. The composite showed high activity and stability, suggesting potential use in air purification systems. Vinokurov et al., found the cobalt-based mesocatalysts were supported on HNTs to catalyze hydrogen production from sodium borohydride [151]. The catalyst was highly active and reusable, with HNTs helping to maintain the dispersion and prevent sintering of the cobalt particles. Copper-Zinc-Aluminum (CZA) catalysts were supported on HNTs for use in methanol steam reforming [148]. The presence of HNTs improved thermal stability and catalytic dispersion, enhancing overall activity and selectivity. Kang et al. , revealed that CeO₂–WO₃ was coated on HNTs to create a catalyst for selective catalytic reduction (SCR) of NOx in the presence of SO₂ [152]. The composite exhibited high NOx conversion and sulfur tolerance, making it promising for industrial emission control. In conclusion, Halloysite nanotube (HNT)-based catalysts have demonstrated remarkable versatility across diverse catalytic applications, from hydrogen generation (e.g., 4020 mL/min/g with amino-modified HNTs) to highly selective organic transformations (>99% aromatic selectivity in polystyrene degradation) and efficient photocatalysis (enhanced visible-light activity in g-C₃N₄–ZnO/HNT and carbon-doped TiO₂/HNT composites). While modified HNTs consistently outperformed unmodified ones such as acid-modified HNTs yielding high stereoselectivity in Prins reactions and metal nanoparticle-HNT hybrids achieving superior dye degradation their inherent limitations in conductivity and scalability highlight the need for further optimization. Compared to other supports, HNTs offer a low-cost, environmentally friendly platform with tunable functionality, positioning them as a promising candidate for sustainable catalytic systems. Adsorption Over the past twenty years, halloysite nanotubes (HNTs) have attracted growing interest as effective adsorbents for removing a wide range of contaminants from water and air [153]. Their inherent properties such as high surface area, thermal stability, and customizable surface chemistry make them well-suited for adsorbing pollutants including heavy metals (like lead and mercury), organic dyes (such as methylene blue and methyl violet), pharmaceutical residues (including ciprofloxacin and naproxen), and various gases [154-157]. To improve their adsorption efficiency, researchers have applied modification methods such as acid treatment, incorporation of metal oxides, and polymer grafting, which enhance surface charge, porosity, and the number of active binding sites [158]. Studies have shown that the adsorption performance of HNTs is often comparable to, or even better than, that of traditional materials like activated carbon and silica [159]. For example, acid-treated HNTs have demonstrated more than a sevenfold increase in gas adsorption capacity, and manganese oxide-coated HNTs have achieved lead ion removal efficiencies up to 59.9 mg/g [160]. These findings underscore the potential of HNTs as cost-effective, environmentally friendly solutions for treating wastewater, capturing gases, and removing pharmaceutical pollutants as listed in Table 6 . This study provides an overview of key studies on the adsorption behavior of HNTs, outlines various surface modification strategies to boost their performance, and explores current challenges and future prospects in their practical application. Table 6- Modified Halloysite nanotubes as the adsorbent for target contaminants and their parameters Sl. No. Adsorbent Target Contaminant Maximum Adsorption Capacity (mg/g) pH Range Kinetic Model Isotherm Model Thermodynamic Insights Regeneration Cycles References \tightlist Natural HNTs Neutral Red (cationic dye) 65.45 at 318 K 2–12 Pseudo-second-order Langmuir & Freundlich Spontaneous & endothermic 3 cycles [161] \tightlist Chitosan-HNTs composite Cr(VI) 72.22 2 Pseudo-second-order Langmuir Spontaneous & endothermic - [162] \tightlist HNTs/PPy Cr(VI) 149.25 2–11 Pseudo-second-order Langmuir Spontaneous & endothermic 3 cycles [163] \tightlist HNTs/HDTMA Cr(VI) 6.611 3–10 Pseudo-second-order Langmuir Spontaneous & endothermic - [164] \tightlist HNTs/KH-792 Cr(VI) 37.25 2–9 Pseudo-second-order Langmuir Spontaneous & endothermic - [165] \tightlist HNTs/CH₃COONa Cu(II) 52.3 6 Pseudo-second-order Langmuir Spontaneous & endothermic - [166] \tightlist HNTs/amino alcohols Pb(II), Cd(II), Zn(II), Cu(II) Not specified 2–6.5 Pseudo-second-order Langmuir Spontaneous & endothermic - [167] \tightlist HNTs/PPy Cr(VI) 149.25 2–11 Pseudo-second-order Langmuir Spontaneous & endothermic 3 cycles [168] Luo et al., explored the adsorption efficiency of pristine halloysite nanotubes (HNTs) for the removal of the cationic dye Neutral Red [161]. The study reported a notable adsorption capacity of 65.45 mg/g at 318 K across a broad pH spectrum (2–12). The adsorption mechanism adhered to a pseudo-second-order kinetic model and conformed to both Langmuir and Freundlich isotherms, indicating a combination of monolayer and heterogeneous surface adsorption. Thermodynamic evaluations confirmed the process to be spontaneous and endothermic. Furthermore, the material demonstrated consistent performance over three regeneration cycles. In a more recent study, Edebali et al., developed a composite adsorbent by integrating chitosan with HNTs for the effective removal of Cr(VI) ions [162]. The hybrid material exhibited a maximum adsorption capacity of 72.22 mg/g at pH 2. Kinetic modeling followed pseudo-second-order behavior, while equilibrium data best fit the Langmuir isotherm, suggesting homogeneous monolayer adsorption. The process was thermodynamically spontaneous and endothermic, reflecting strong interactions between Cr(VI) and the composite surface. Mishra et al., significantly enhanced Cr(VI) adsorption by modifying HNTs with polypyrrole (PPy). The resulting HNTs/PPy nanocomposite achieved the highest capacity among surveyed materials at 149.25 mg/g within a pH range of 2–11. The adsorption conformed to pseudo-second-order kinetics and the Langmuir isotherm, with thermodynamic data again indicating spontaneous and endothermic behavior. Notably, the adsorbent retained its efficiency over three reuse cycles, emphasizing its potential for practical applications [163]. Zhang et al., modified HNTs using hexadecyltrimethylammonium bromide (HDTMA) and evaluated its performance in Cr(VI) adsorption. The modified nanotubes exhibited a comparatively lower capacity of 6.611 mg/g over a pH range of 3–10. Despite the reduced capacity, adsorption followed pseudo-second-order kinetics and aligned with the Langmuir isotherm, with thermodynamic results verifying a spontaneous and endothermic uptake process [164]. In a parallel investigation, the same authors utilized KH-792 as a silane modifier for HNTs, resulting in a higher Cr(VI) adsorption capacity of 37.25 mg/g within the pH range of 2–9 [165]. Similar adsorption kinetics and isotherm behavior were observed, with thermodynamic analysis again confirming spontaneous and endothermic characteristics. Khelifa et al., investigated copper ion removal using HNTs modified with sodium acetate (CH₃COONa). The adsorbent achieved an adsorption capacity of 52.3 mg/g at pH 6. The adsorption process followed pseudo-second-order kinetics and matched the Langmuir isotherm model. Thermodynamic studies revealed spontaneous and endothermic adsorption, although the study did not report on regeneration performance [166]. Matusik et al., focused on the removal of multiple heavy metals such as Pb(II), Cd(II), Zn(II), and Cu(II) using HNTs functionalized with amino alcohols. While specific adsorption capacities were not provided, the material demonstrated effective adsorption within a pH range of 2–6.5. The kinetic and isotherm analyses revealed pseudo-second-order and Langmuir model compliance, respectively, with thermodynamic studies supporting a spontaneous and endothermic process [167]. Repetition of results from Mishra and Maity , Zhang et al. , and others further supports the consistency in the performance of HNT-based materials across different functionalization strategies and target contaminants. These findings consistently demonstrate the effectiveness of modified HNTs in pollutant removal, with adsorption behavior typically governed by pseudo-second-order kinetics, Langmuir-type isotherms, and favorable thermodynamic profiles [168]. Variations in adsorption capacity across studies underscore the influence of surface functionalization, contaminant type, and operational conditions on adsorption efficiency. This comparative assessment underscores the versatility and promise of HNT-based adsorbents for environmental remediation, offering a tunable platform for targeted pollutant removal from aqueous systems. Flame Retardant Halloysite nanotubes (HNTs), naturally occurring aluminosilicate clay minerals with a unique tubular morphology, have recently garnered significant attention as promising flame retardant additives in polymer composites [162]. Their inherent characteristics including high aspect ratio, thermal stability, and environmentally benign nature make them excellent candidates for enhancing fire resistance while maintaining or improving mechanical properties [163]. Over the past decade, numerous studies have demonstrated the effectiveness of HNTs in various polymer matrices such as epoxy, polypropylene, polyurethane, and biopolymers, often achieving significant improvements in key fire safety parameters. For instance, the incorporation of HNTs into epoxy resins has been shown to increase the limiting oxygen index (LOI) up to 33% and achieve UL-94 V-0 ratings, while reducing heat release rates (HRR) by up to 40–50% [164]. These enhancements are primarily attributed to mechanisms such as char layer formation, gas-phase flame inhibition, and barrier effects that slow down heat and mass transfer during combustion. Modification of HNTs with organophosphorus compounds, silane coupling agents, or metal oxides has further amplified their flame retardant performance by improving dispersion, interfacial bonding, and synergistic flame inhibition effects [165] ( Table 7 ). Additionally, HNT-based flame retardants have shown compatibility across a broad pH range and various polymer processing conditions, highlighting their versatility. Parameters such as thermal stability, tensile strength, smoke suppression, and regeneration capability have also been positively influenced, making HNTs a multifunctional additive [166]. Given their cost-effectiveness, renewability, and minimal environmental impact compared to conventional halogenated flame retardants, halloysite nanotubes are emerging as a sustainable alternative in flame retardant technology for advanced polymeric materials. Table 7- Modified Halloysite nanotubes as flame retardant material with different polymer matrix Sl. No. Polymer Matrix HNT Modification Flame Retardant Mechanism Performance Functionality References \tightlist Epoxy Resin (EP) DOPO (Organophosphorus) Gas-phase phosphorus release; char formation LOI 30.3%, UL-94 V-0, HRR ↓ from 1221 to 758 kW/m² Improved thermal stability [167] \tightlist Bamboo/PLA Composite Chitosan/Tannic Acid complex Char layer formation; reduced flammable gas release Improved flame retardancy and mechanical strength Bio-based modification [168] \tightlist Polyamide 6 (PA6) None Physical barrier; enhanced crystallinity LOI 31.7%; improved thermal stability Unmodified HNTs [169] \tightlist Polycarbonate (PC) Silane-treated HNTs Barrier effect; char formation Increased UL-94 rating Improved HNT-matrix compatibility [170] \tightlist Silicone Rubber Foam Silane coupling agent Protective char layer; reduced heat release Significant flame retardancy increase Improved mechanical properties [170] \tightlist Polyester-Cotton Fabric Silver-loaded HNTs Protective char; smoke and toxic gas suppression Self-extinguishing in 9 s; 21% smoke reduction Antimicrobial added benefits [171] \tightlist Epoxy Resin Amino-functionalized HNTs Char formation; thermal barrier LOI increased by 20%; UL-94 V-1 rating Enhanced interface bonding [172] \tightlist Polypropylene (PP) Organophosphorus + HNTs Synergistic gas-phase and condensed-phase mechanisms HRR reduced by 45%; increased LOI Synergistic flame retardant effect [173] \tightlist Polyvinyl Chloride (PVC) Acid-treated HNTs Barrier effect; char layer formation UL-94 V-1 rating; reduced peak heat release Acid treatment improves dispersion [174] \tightlist Polystyrene (PS) Silane-modified HNTs Char formation; thermal insulation LOI 28%; improved mechanical strength Enhanced dispersion [175] \tightlist Polyethylene (PE) Phosphorus-functionalized HNTs Gas-phase flame inhibition; condensed phase char LOI increased by 25%; lower heat release Phosphorus improves flame retardancy [176] \tightlist Epoxy Unmodified HNTs Physical barrier; thermal stability LOI increase by 12% Baseline HNT performance [177] \tightlist Polyurethane (PU) Functionalized HNTs Char layer; thermal stability 15% reduction in peak heat release rate Enhanced mechanical properties [178] \tightlist PLA Clay-coated HNTs Barrier and char effects LOI 27%, UL-94 V-0 rating Biodegradable composite [179] \tightlist PET Phosphorus-doped HNTs Gas-phase inhibition; char formation LOI increase by 30% Effective at low loading [180] \tightlist EVA Unmodified HNTs Thermal barrier Improved flame retardancy and tensile strength Baseline study [181] \tightlist PP Organosilane-modified HNTs Enhanced char and thermal barrier UL-94 V-1 rating Better dispersion and interface [182] \tightlist Nylon-6 DOPO-modified HNTs Gas-phase and condensed phase mechanisms LOI 33%, V-0 rating Synergistic flame retardancy [183] \tightlist Epoxy Silane-functionalized HNTs Char layer; improved thermal stability Increased LOI and reduced heat release Better polymer interaction [184] \tightlist PS styrene-maleic anhydride HNTs Physical barrier effect Moderate LOI increase Baseline reference [185] \tightlist Epoxy Amino-silane modified HNTs Enhanced char formation UL-94 V-0 rating Improved mechanical strength [186] Numerous studies have explored the use of halloysite nanotubes (HNTs) as flame retardants across a wide variety of polymer matrices, demonstrating their effectiveness in enhancing fire resistance through diverse modification strategies and mechanisms. Ruan et al., reported that DOPO-modified HNTs incorporated into epoxy resin significantly improved flame retardancy by releasing phosphorus in the gas phase and promoting char formation, achieving a limiting oxygen index (LOI) of 30.3% and a UL-94 V-0 rating, while reducing heat release rate (HRR) substantially [167]. Jin et al., introduced a bio-based modification using a chitosan/tannic acid complex to bamboo/PLA composites, yielding better flame retardancy and mechanical strength by reducing flammable gas emissions and facilitating char formation [168]. Unmodified HNTs have also been shown to improve flame retardancy in polymers such as polyamide 6 (PA6) where a physical barrier effect and enhanced crystallinity contributed to an LOI of 31.7% [169]. Other functionalization approaches include silane coupling agents, as demonstrated by Pang et al., in silicone rubber foam, which improved flame retardancy through protective char layers and heat release reduction while enhancing mechanical properties [170]. Qi et al., innovatively used silver-loaded HNTs on polyester-cotton fabrics, achieving self-extinguishing behavior within 9 seconds and reducing smoke production by 21%, adding antimicrobial benefits alongside flame retardancy [171]. Amino-functionalized HNTs in epoxy resin by Zhong et al., promoted stronger interfacial bonding and increased LOI by 20%. Synergistic flame retardant effects were evident in polypropylene (PP) composites containing organophosphorus-modified HNTs (Chen et al.,), which lowered HRR by 45% and increased LOI through combined gas-phase and condensed-phase mechanisms [172, 173]. Acid-treated HNTs in polyvinyl chloride (PVC) matrices by Singh et al., enhanced dispersion and barrier effects, leading to UL-94 V-1 ratings and reduced peak heat release rates [175]. Similarly, silane-modified HNTs incorporated into polystyrene (PS) improved char formation and mechanical strength ( Li et al., ), while phosphorus-functionalized HNTs in polyethylene (PE) effectively increased LOI by 25% through gas-phase flame inhibition and condensed-phase char formation [176, 177]. Unmodified HNTs served as baseline references for epoxy and EVA composites, confirming their role as physical barriers that improve thermal stability and tensile strength [177, 181]. Other modifications, such as clay coatings described by Yang et al., and Beryl et al., and silane treatments consistently enhanced char layer formation, thermal stability, and flame retardant performance across diverse polymer matrices [178-179]. Phosphorus-doped HNTs in PET Li et al., exhibited remarkable flame retardancy at low loadings, while organosilane-modified HNTs in polypropylene and polyamide-6 demonstrated synergistic effects combining gas-phase and condensed-phase flame retardant mechanisms [180-184]. Early studies such as those laid foundational understanding by showing that organophosphorus-HNTs and unmodified HNTs, respectively, provide improved LOI and mechanical properties through physical barriers and char formation [185. 187]. Across these studies, it is evident that HNTs, whether unmodified or functionalized with phosphorus, silane, or other agents, contribute to effective flame retardancy primarily via char layer formation, barrier effects, and in some cases gas-phase flame inhibition. This multi-faceted mechanism enhances polymer thermal stability and reduces flammability, with many studies also reporting improvements in mechanical properties, which is critical for practical applications. The wide range of polymer matrices and modification techniques underscores the versatility of HNTs as flame retardant additives in polymer composites. Halloysite nanotubes in SARS-CoV-2 The global spread of SARS-CoV-2 (COVID-19) has prompted a surge in research focused on advanced nanomaterials for antiviral strategies. Among these, halloysite nanotubes (HNTs) have garnered considerable interest due to their unique hollow tubular structure, biocompatibility, abundance, and ease of surface functionalization. HNTs offer a versatile platform for various biomedical and environmental applications, including drug delivery, antimicrobial coatings, and filtration systems [188-193]. Their lumen can encapsulate active molecules, while their surface can be modified with functional groups or nanoparticles, making them suitable for targeted and sustained release applications. In the context of COVID-19, several studies between 2020 and 2025 have investigated the role of HNTs in enhancing therapeutic efficacy, surface disinfection, and aerosol filtration ( Table 8 ). Table 8- Modified halloysite nanotubes utilized in SARS-CoV-2 treatment Sl. No. Modification Functionality Performance References \tightlist HNTs as vehicles for drug delivery Drug delivery across blood-brain barrier Demonstrated potential for targeted delivery through the blood-brain barrier [194] \tightlist PEGylated green halloysite/spinel ferrite nanocomposites for pH-sensitive delivery of dexamethasone Pulmonary drug delivery Enhanced drug stability and controlled release in acidic pulmonary environments [195] \tightlist TiO₂ nanotubes for production of free radicals that eliminate viruses Surface sterilization Effective inactivating SARS-CoV-2 with minimal cytotoxicity [196] \tightlist Polydopamine-coated HNTs in electrospun nanofibers Air filtration Improved filtration efficiency and antimicrobial properties [197] Saleh et al., investigated the feasibility of using HNTs for transporting antiviral or neuroprotective drugs across the blood-brain barrier (BBB), which is critical in cases where SARS-CoV-2 causes neurological complications. The study simulated BBB transport using in vitro models and demonstrated that surface-modified HNTs could effectively cross the barrier and release their cargo in the brain. This research opened possibilities for treating neuroinflammatory conditions induced by COVID-19 [194]. Wang and colleagues incorporated polydopamine-functionalized HNTs into electrospun nanofibers to enhance air filtration membranes. The resulting filters exhibited high mechanical strength, improved capture efficiency of viral aerosols, and intrinsic antimicrobial activity. This innovation addressed the need for multifunctional face masks and HVAC filters during the pandemic, combining physical filtration with antimicrobial protection [195]. HNTs were coated with titanium dioxide (TiO₂), a photocatalytic agent, to develop self-disinfecting surfaces capable of inactivating SARS-CoV-2. Upon light activation, TiO₂ generates reactive oxygen species (ROS) that can degrade viral proteins and nucleic acids. The study demonstrated that these materials could reduce viral activity on surfaces within minutes under UV or visible light exposure, offering a non-toxic and reusable disinfection solution [196]. In this study, HNTs were functionalized with zinc ferrite (ZnFe₂O₄) and polyethylene glycol (PEG) to create a biocompatible, pH-sensitive drug delivery system for pulmonary administration of dexamethasone, an anti-inflammatory drug used in severe COVID-19 cases. The composite nanoparticles exhibited controlled release in acidic environments (mimicking infected lung tissues) and showed enhanced cellular uptake with minimal cytotoxicity. This approach enabled targeted drug delivery to inflamed lung tissues, reducing systemic side effects [197]. AI and Machine Learning in the Study and Application of Halloysite Nanotubes The integration of Artificial Intelligence (AI) and Machine Learning (ML) into the research and development of halloysite nanotubes (HNTs) has emerged as a transformative approach to accelerate material innovation, optimize composite performance, and predict functional behavior across diverse applications. Traditionally, the development of HNT-based materials has relied heavily on experimental trial-and-error, which can be both time-consuming and resource-intensive. ML algorithms such as random forests, support vector machines (SVMs), and deep neural networks are now being used to predict key material properties—such as mechanical strength, thermal stability, and flame retardancy—based on input features like nanotube morphology, surface functionalization, and polymer matrix characteristics [198]. For instance, in flame-retardant polymer composites, ML models have been trained on datasets comprising various HNT modifications (e.g., silane, phosphorus, amino) and their respective loadings to predict limiting oxygen index (LOI) and peak heat release rate (PHRR), thereby guiding optimal formulation strategies without exhaustive testing [199]. Moreover, AI-assisted image analysis tools are increasingly used to interpret scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, allowing automated, high-throughput quantification of HNT dispersion and orientation within a matrix. This significantly enhances reproducibility and eliminates subjective errors in nanostructure characterization [200]. In biomedical applications, where HNTs are used as drug delivery vehicles, ML techniques have shown promise in modeling and predicting drug loading efficiency, release kinetics, and biocompatibility based on physicochemical parameters such as surface area, lumen size, and zeta potential [201]. These predictive models are particularly useful for screening various drug-HNT combinations and for designing stimuli-responsive delivery systems with tailored release profiles. As computational power and data availability continue to expand, AI is expected to play an even more central role in the materials informatics of halloysite-based systems. This includes the inverse design of multifunctional nanocomposites, the development of eco-friendly flame retardants, and the creation of bioactive materials for regenerative medicine. Overall, AI and ML are shifting the paradigm from traditional materials discovery toward a data-driven, predictive approach that enhances efficiency, precision, and sustainability in the use of halloysite nanotubes. Conclusion and future perspectives Halloysite nanotubes (HNTs) have swiftly moved from geological curiosities to cutting-edge components in next-generation nanomaterials. Their unique tubular morphology, biocompatibility, and chemical versatility make them exceptional candidates for a wide range of applications—from precision drug delivery systems and flame-retardant composites to smart antibacterial coatings and catalytic platforms. This review highlights how HNTs, once mainly studied for geological purposes, are now pivotal in biomedical engineering, environmental remediation, and smart material development. Yet, the story of HNTs is just beginning. Despite impressive advances, challenges remain scaling up functionalization methods, standardizing natural sources, and validating long-term safety in biological systems. Excitingly, the integration of artificial intelligence and machine learning into HNT research is opening new frontiers in predictive material design and performance optimization. As global demand grows for sustainable, multifunctional materials, HNTs stand poised to transform entire industries. Whether as nanocontainers for targeted therapies or as catalysts for greener chemistry, their potential is vast and largely untapped. With interdisciplinary collaboration and a focus on eco-friendly scalability, halloysite nanotubes could become the foundation of tomorrow’s most innovative technologies. The continuous growth in research reflects an evolving interest in functionalization techniques and commercial scalability, making halloysite nanotubes a key material for future technological advancements. 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Predictive modeling of drug release kinetics from HNTs using machine learning. Journal of Controlled Release, 355, pp.231 Information & Authors Information Version history V1 Version 1 17 June 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Authors Affiliations Dipanwita Basak Bodoland University View all articles by this author Hemaprobha Saikia 0000-0002-8714-8766 [email protected] Bodoland University View all articles by this author Metrics & Citations Metrics Article Usage 638 views 264 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Dipanwita Basak, Hemaprobha Saikia. From Clay to Cutting-Edge: Halloysite Nanotubes in Next-Generation Nanotechnology. Authorea . 17 June 2025. DOI: https://doi.org/10.22541/au.175016352.29417854/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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