Effects of acid treatment on the physico-chemical and pore characteristics of halloysite

Ligand-Directed Shape Reconfiguration in Inorganic Materials

Polymer elastomers with reversible shape-changing capability have led to significant development of artificial muscles, functional devices, and soft robots. By contrast, reversible shape transformation of inorganic nanoparticles is notoriously challenging due to their relatively rigid lattice structure. Here, the authors demonstrate the synthesis of shape-changing nanoparticles via an asymmetrical surface functionalization process. Various ligands are investigated, revealing the essential role of steric hindrance from the functional groups. By controlling the unbalanced structural hindrance on the surface, the as-prepared clay nanoparticles can transform their shape in a fast, facile, and reversible manner. In addition, such flexible morphology-controlled mechanism provides a platform for developing self-propelled shape-shifting nanocollectors. Owing to the ion-exchanging capability of clay, these self-propelled nanoswimmers (NS) are able to autonomously adsorb rare earth elements with ultralow concentration, indicating the feasibility of using naturally occurring materials for self-powered nanomachine.

Morphology and Selected Properties of Modified Potato Thermoplastic Starch

Potato thermoplastic starch (TPS) containing 1 wt.% of pure halloysite (HNT), glycerol-modified halloysite (G-HNT) or polyester plasticizer-modified halloysite (PP-HNT) was prepared by melt-extrusion. Halloysites were characterized by FTIR, SEM, TGA, and DSC. Interactions between TPS and halloysites were studied by FTIR, SEM, and DMTA. The Vicat softening temperature, tensile, and flexural properties were also determined. FTIR proved the interactions between halloysite and the organic compound as well as between starch, plasticizers and halloysites. Pure HNT had the best thermal stability, but PP-HNT showed better thermal stability than G-HNT. The addition of HNT and G-HNT improved the TPS’s thermal stability, as evidenced by significantly higher T5%. Modified TPS showed higher a Vicat softening point, suggesting better hot water resistance. Halloysite improved TPS stiffness due to higher storage modulus. However, TPS/PP-HNT had the lowest stiffness, and TPS/HNT the highest. Halloysite increased Tα and lowered Tβ due to its simultaneous reinforcing and plasticizing effect. TPS/HNT showed an additional β-relaxation peak, suggesting the formation of a new crystalline phase. The mechanical properties of TPS were also improved in the presence of both pure and modified halloysites.

Green method of conversion of geraniol to value-added products in the presence of selected minerals

The study presents the results of research on the process of geraniol (GA) transformation in the presence of natural minerals: montmorillonite, mironekuton, halloysite and also in the presence of halloysite modified with 0.1 M water solution of H2SO4. To obtain information on the structure of the used catalysts, instrumental studies were performed (SEM, XRD, FT-IR, XRF, BET). The second part of the research consisted in examining the influence of individual parameters (temperature, catalyst content, and reaction time) on the course of GA transformation process. The syntheses were carried out without the application of solvent and under atmospheric pressure. To determine the most beneficial process conditions, two functions were selected: GA conversion and selectivity of GA. The proposed method of GA transformation on such minerals: montmorillonite, mironekuton, halloysite, has not been described in the literature so far.

Efficient Rhodamine B Dye Removal from Water by Acid- and Organo-Modified Halloysites

The halloysite has been subjected to modification through ultrasound (HU), sulfuric acid (HU-SA), and oligocyclopentadiene resin (HU-OCPD). The modified materials were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry (TG), and N2 adsorption-desorption isotherms, and tested as low-cost adsorbents for removal of Rhodamine B dye (RhB) from aqueous solutions. Batch experiments were conducted to study the effect of different operational parameters such as adsorbent dose, solution pH, and contact time. It was observed that the adsorption was strongly pH-dependent and that solution pH at 2.0 had the greatest removal efficiency for the dye. The experimental data were modeled using several isotherm and kinetic models such as Freundlich, Langmuir, Temkin as well as pseudo-first-order, pseudo-second-order, and intraparticle diffusion. It was found that the equilibrium adsorption data can be fitted well using the Freundlich isotherm model and the adsorption kinetics follows a pseudo-second-order model. The adsorption capacity of HU, HU-SA, and HU-OCPD was found to be 8.37, 13.1, and 17.8 mg/g, respectively. The results revealed that surface modification of halloysite via acid activation and polymer loading results in a significant increase in the removal of RhB from aqueous solution. This study has shown potential on organo-halloysite for organic dye adsorption from water.

Immobilization of Trametes versicolor laccase on chitosan/halloysite as a biocatalyst in the Remazol Red RR dye

In this study, the laccase obtained from Trametes versicolor was immobilized onto the chitosan(CTS)/halloysite (HNT) beads. In the immobilization step, the effects of chitosan (1-3% w/v), halloysite (0-2% w/v), glutaraldehyde (0.5-1.5% v/v) and enzyme concentrations (1-3%) on loading and immobilization efficiency were investigated. SEM, FT-IR, XRD, TGA and XPS analyses were performed to examine the structure of beads. In addition, the effects of parameters such as pH (4-10), temperature (25-55 °C), storage life on the activity of free and immobilized laccase were also investigated. The activities of free and immobilized laccase preserved 23% and 56% of its initial activity at the end of 59 days of storage. The effects of mediators such as 2.2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS), 1-Hydroxybenzotriazole hydrate (HBT), 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) and violuric acid (VLA) on the dye removal efficiency were investigated. Reusability of the CTS/HNT/Lac in the presence of HBT and VLA mediators, which enable the highest dye removal, was tested. After 15 cycles, 42% and 54% dye removal were achieved with the CTS/HNT/Lac in the medium containing HBT and VLA, and 42% and 49% of the activity is preserved, respectively. This study showed that CTS/HNT/Lac can be used repeatedly for Remazol Red RR dye removal.

Selectively Etched Halloysite Nanotubes as Performance Booster of Epoxidized Natural Rubber Composites

Halloysite Nanotubes (HNT) are chemically similar to clay, which makes them incompatible with non-polar rubbers such as natural rubber (NR). Modification of NR into a polar rubber is of interest. In this work, Epoxidized Natural Rubber (ENR) was prepared in order to obtain a composite that could assure filler-matrix compatibility. However, the performance of this composite was still not satisfactory, so an alternative to the basic HNT filler was pursued. The surface area of HNT was further increased by etching with acid; the specific surface increased with treatment time. The FTIR spectra confirmed selective etching on the Al-OH surface of HNT with reduction in peak intensity in the regions 3750-3600 cm^-1 and 825-725 cm^-1, indicating decrease in Al-OH structures. The use of acid-treated HNT improved modulus, tensile strength, and tear strength of the filled composites. This was attributed to the filler-matrix interactions of acid-treated HNT with ENR. Further evidence was found from the Payne effect being reduced to 44.2% through acid treatment of the filler. As for the strain-induced crystallization (SIC) in the composites, the stress-strain curves correlated well with the degree of crystallinity observed from synchrotron wide-angle X-ray scattering.

Development of a Sodium Alginate-Based Active Package with Controlled Release of Cinnamaldehyde Loaded on Halloysite Nanotubes

The worsening environment and the demand for safer food have accelerated the development of new food packaging materials. The objective of this research is to prepare antimicrobial food packaging film with controlled release by loading cinnamaldehyde (CIN) on etched halloysite nanotubes (T-HNTs) and adding it to sodium alginate (SA) matrix. The effects of T-HNTs-CIN on the physical functional properties and antibacterial activity of the film were systematically evaluated, and the release of CIN in the film was also quantified. Transmission electron microscopy and nitrogen adsorption experiments showed that the halloysite nanotubes had been etched and CIN was successfully loaded into the T-HNTs. The addition of T-HNTs-CIN significantly improved the water vapor barrier properties and tensile strength of the film. Similarly, the presence of T-HNTs-CIN in the film greatly reduced the negative effects of ultraviolet rays. The release experiment showed that the diffusion time of CIN in SA/T-HNTs-CIN film to fatty food simulation solution was delayed 144 h compared with that of SA/CIN film. Herein, the antibacterial experiment also confirmed the controlled release effect of T-HNTs on CIN. In conclusion, SA/T-HNTs-CIN film might have broad application prospects in fatty food packaging.

Preclinical developments of natural-occurring halloysite clay nanotubes in cancer therapeutics

The natural world holds useful resources that can be exploited to design effective therapeutic approaches. Ready-to-use tubular nanoclays, such as halloysite clay nanotubes (HNTs), are widely available, cost-effective, and sustainable submicron crystalline materials that have been showing great potential towards chronic multifactorial and malignant diseases, standing out as a promising anticancer nanotherapeutic strategy. Currently, several preclinical studies have reported the application of HNTs in cancer research, diagnosis, monitoring, and therapeutics. This groundbreaking review highlights the preclinical knowledge hitherto collected concerning the application of HNTs towards cancer therapy. Despite their reproducibility issues, HNTs were used as nanoarchitectonic platforms for the delivery of conventional chemotherapeutic, natural-occurring, biopharmaceutical, and phototherapeutic anticancer agents in a wide range of in vitro and in vivo solid cancer models. Overall, in different types of cancer mice models, the intratumoral and intravenous administration of HNTs-based nanoplatforms induced tumor growth inhibition without causing significant toxic effects. Such evidence raises a relevant question: does the therapeutic benefit of the parenteral administration of HNTs in cancer outweigh their potential toxicological risk? To answer this question further long-term absorption-distribution-metabolism-excretion studies in healthy and cancer animal models need to be performed. In cancer therapeutics, HNTs are envisaged as promising platforms for cancer multi-agent therapy, enabling the combination of different therapeutic modalities. Furthermore, HNTs might constitute suitable nanotheranostic platforms. Nevertheless, to confirm the potential and safety of the application of HNTs as nanodelivery systems for cancer therapy, it is necessary to perform in-depth in vivo pharmacokinetics and pharmacodynamic studies to further the translation to clinical trials.

Influence of selective acid-etching on functionality of halloysite-chitosan nanocontainers for sustained drug release

The functionality of halloysite (Hal) nanotubes as drug carriers can be improved by lumen enlargement and polymer modification. This study investigates the influence of selective acid etching on Hal functionalization with cationic biopolymer chitosan. Hal was subjected to lumen etching under mild conditions, loaded under vacuum with nonsteroidal antiinflammatory drug aceclofenac, and incubated in an acidic solution of chitosan. The functionality of pristine and etched Hal before and upon polymer functionalization was assessed by ζ-potential measurements, structural characterization (FT-IR, DSC and XRPD analysis), cell viability assay, drug loading and drug release studies. Acid etching increased specific surface area, pore volume and pore size of Hal, decreased ζ-potential and facilitated binding of the cationic polymer. XRPD and DSC analysis revealed crystalline structure of etched Hal. Successful chitosan binding and drug entrapment were further confirmed by FT-IR and DSC studies. XRPD showed surface polymer binding. DSC and FT-IR analyses confirmed the presence of the entrapped drug in its crystalline form. Drug loading was increased for ≈81% by selective lumen etching. Slight decrease of drug content occurred during chitosan functionalization due to aceclofenac diffusion in the polymer solution. The drug release was more sustained from etched Hal nanocomposites (up to ≈87% for 12 h) than from pristine Hal (up to ≈97% for 12 h) due to more intensive chitosan binding. High human fibroblast survival rates upon exposure to pristine and etched Hal before and after chitosan functionalization (>90% in the concentration of 1000 μg/mL) confirmed that both lumen etching under mild conditions and polymer functionalization had no significant effect on cytocompatibility. Based on these findings, selective lumen etching in combination with polycation modification appears to be a promising approach for improvement of Hal nanotubes functionality by increasing payload, polymer binding capacity, and sustained release properties with no significant effect on their cytocompatibility.

Surface modified novel magnetically tuned halloysite functionalized sulfonic acid: synthesis, characterization and catalytic activity

The present work demonstrates the synthesis of magnetically tuned halloysite solid acid, the physiochemical properties of which are thoroughly studied using different characterization techniques and has been successfully used for the synthesis of bisamides and 4H-pyrans.

Synthesis and Electrochemical Performance of Graphene @ Halloysite Nanotubes/Sulfur Composites Cathode Materials for Lithium-Sulfur Batteries

Natural halloysite nanotubes (HNTs) and reduced graphene oxide (RGO) were introduced into the S cathode material to form HNTs/S and RGO@HNTs/S composite electrode to improve the electrochemical performance of Li-S batteries. The effect of acid etching temperature on the morphology and pore structure of HNTs was explored and the morphological characteristics and electrochemical performance of composite electrodes formed by HNTs that after treatment with different acid etching temperatures and RGO were compared. The result shows that the cycling stability and the utilization rate of active substances of the Li-S battery were greatly improved because the pore structure and surface polarity functional groups of HNTs and the introduction of RGO provide a conductive network for insulating sulfur particles. The RGO@HNTs treated by acid treatment at 80 °C (RGO@HNTs-80/S) composite electrode at 0.1 C has an initial capacity of 1134 mAh g-1, the discharge capacity after 50 cycles retains 20.1% higher than the normal S electrode and maintains a specific discharge capacity of 556 mAh g-1 at 1 C. Therefore, RGO and HNTs can effectively improve the initial discharge specific capacity, cycle performance and rate performance of Li-S batteries.

Past, Present and Future Perspectives on Halloysite Clay Minerals

Halloysite nanotubes (HNTs), clay minerals belonging to the kaolin groups, are emerging nanomaterials which have attracted the attention of the scientific community due to their interesting features, such as low-cost, availability and biocompatibility. In addition, their large surface area and tubular structure have led to HNTs' application in different industrial purposes. This review reports a comprehensive overview of the historical background of HNT utilization in the last 20 years. In particular it will focus on the functionalization of the surfaces, both supramolecular and covalent, following applications in several fields, including biomedicine, environmental science and catalysis.

Cation Doping Approach for Nanotubular Hydrosilicates Curvature Control and Related Applications

The past two decades have been marked by an increased interest in the synthesis and the properties of geoinspired hydrosilicate nanoscrolls and nanotubes. The present review considers three main representatives of this group: halloysite, imogolite and chrysotile. These hydrosilicates have the ability of spontaneous curling (scrolling) due to a number of crystal structure features, including the size and chemical composition differences between the sheets, (or the void in the gibbsite sheet and SiO2 tetrahedron, in the case of imogolite). Mineral nanoscrolls and nanotubes consist of the most abundant elements, like magnesium, aluminium and silicon, accompanied by uncontrollable amounts of impurities (other elements and phases), which hinder their high technology applications. The development of a synthetic approach makes it possible to not only to overcome the purity issues, but also to enhance the chemical composition of the nanotubular particles by controllable cation doping. The first part of the review covers some principles of the cation doping approach and proposes joint criteria for the semiquantitative prediction of morphological changes that occur. The second part focuses on some doping-related properties and applications, such as morphological control, uptake and release, magnetic and mechanical properties, and catalysis.

Tailoring magnetic resonance imaging relaxivities in macroporous Prussian blue cubes

In order to unravel the relationship between zeta potential values and r₂/r₁ ratios for contrast agents in MRI application, a series of macroporous Prussian blue cubes were successfully synthesized by HCl etching and used as model samples for relaxivity investigation. It was found that their r₂/r₁ ratios firstly decreased and then increased with the increasing HCl concentration, while the variation trend for zeta potential is quite the opposite. By employing Gauss fitting and eliminating the HCl concentration in the resultant equations, a relationship between zeta potential values and r₂/r₁ ratios, i.e. ζ = 229 × (563 -r₂/r₁)^0.012- 267, was finally obtained. This result showed that magnetic resonance imaging relaxivities (viz. r₂/r₁) could be tailored through altering zeta potential values (surface charges) of the contrast agent.

Mesoporous halloysite nanotubes modified by CuFe2O4 spinel ferrite nanoparticles and study of its application as a novel and efficient heterogeneous catalyst in the synthesis of pyrazolopyridine derivatives

In this study, mesoporous halloysite nanotubes (HNTs) were modified by CuFe₂O₄ nanoparticles for the first time. The morphology, porosity and chemistry of the CuFe₂O₄@HNTs nanocomposite were fully characterized by Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM) image, transmission electron microscope (TEM) images, energy-dispersive X-ray (EDX), X-ray diffraction (XRD) pattern, Brunauer-Emmett-Teller (BET) adsorption-desorption isotherm, thermogravimetric (TG) and vibrating sample magnetometer (VSM) curve analyses. The results confirmed that CuFe₂O₄ with tetragonal structure, uniform distribution, and less agglomeration was located at HNTs. CuFe₂O₄@HNTs nanocomposite special features were high thermal stability, crystalline structure, and respectable magnetic property. SEM and TEM results showed the nanotube structure and confirmed the stability of basic tube in the synthetic process. Also, inner diameters of tubes were increased in calcination temperature at 500 °C. A good magnetic property of CuFe₂O₄@HNTs led to use it as a heterogeneous catalyst in the synthesis of pyrazolopyridine derivatives. High efficiency, green media, mild reaction conditions and easily recovery of the nanocatalyst are some advantages of this protocol.

Functionalization of halloysite nanotubes by enlargement and layer-by-layer assembly for controlled release of the fungicide iodopropynyl butylcarbamate

Iodopropynyl butylcarbamate (IPBC) is currently one of the most important fungicides widely used for industrial coatings and bamboo treatment. In this work, a controlled release composite with IPBC for inhibition of mold and stain fungi was prepared using enlarged halloysite nanotubes (HNTs) with layer-by-layer (LbL) assembly of polyelectrolyte multilayers. The acid-treated HNTs retained their tubular structure with increased internal diameter, and IPBC loading efficiency was therefore increased to 24.4%, approximately three times the amount of raw HNTs (8.4%). In vitro drug release assay showed that IPBC could be released from HNTs in a sustainable manner with a total release amount of 33.8% over a period of 35 days. The release rate of IPBC could be further controlled by adjusting the number of LbL layers on the tubes and the released amount of IPBC could be limited to less than 10% in 100 days. An inhibition zone test indicated the as-prepared nanocomposites exhibited significant anti-fungal performance against three mold fungi (Aspergillus niger, Trichoderma viride, and Penicillium citrinum) and one stain fungus (Botryodiplodia theobromae). The results support the potential use of HNTs for a prolonged service life of bamboo products.

Controlled release of iodopropynyl butylcarbamate from functionalized halloysite nanotubes was realized by coating with LbL polyelectrolyte multilayers, with potential for protection of bamboo materials with a prolonged lifetime.

Preparation and characterization of porous starch reinforced with halloysite nanotube by solvent exchange method

The aim of this study was to improvement of adsorption capacity of porous starch (PS) by incorporation of halloysite nanotube (HNT). PS/HNT carrier was synthesized through the solvent exchange method. Various fabrication approaches for PS were introduced and different ethanol ratio (40, 60, 80, 100%), HNT concentration (0, 0.1, 0.2, 0.3, 0.4 and 0.5 mg/mL) and drying method (oven, freeze dryer and microwave) were applied. The results indicated that high ratio of ethanol created larger pores and also the best results were obtained from freeze-dried samples (p < 0.05). Incorporation of HNT could further improve the adsorption capacity. However, the best oil and water adsorption capacity were related to PS alone. The nitrogen adsorption-desorption measurement and scanning electron microscopy elucidated the porous structure of samples. The experimental data were fitted successfully by the Langmuir model rather than Freundlich isotherm. Also PS/HNT had higher apparent density and more negative zeta potential rather than PS. However, the DSC results showed the similar thermal patterns for PS and PS/HNT.

Halloysite nanotubes in analytical sciences and in drug delivery: A review

Halloysite (HNT) is a natural inorganic mineral that has many applications in manufacturing. This review (with 192 references) covers (a) the chemical properties of halloysites, (b) the effects of alkali and acid etching on the loading capacity and the release behavior of halloysites, (c) the use of halloysite nanotubes in analytical sciences and drug delivery, and (d) recent trends in the preparation of magnetic HNTs. Synthetic methods such as co-precipitation, thermal decomposition, and solvothermal method are discussed, with emphasis on optimal magnetization. In the analytical field, recent advancements are summarized in terms of applications of HNT-nanocomposites for extraction and detection of heavy metal ions, dyes, organic pollutants, and biomolecules. The review also covers methods for synthesizing molecularly imprinted polymer-modified HNTs and magnetic HNTs. With respect to drug delivery, the toxicity, techniques for drug loading and the various classes of drug-halloysite nanocomposites are discussed. This review gives a general insight on the utilization of HNT in analytical determination and drug delivery systems which may be useful for researchers to generate new ideas. Graphical abstract Schematic presentation of the structure of halloysite nanotubes, selected examples of modifications and functionalization, and represetative field of applications.

Preparation of hollow iron/halloysite nanocomposites with enhanced electromagnetic performances

Nanostructures loaded on halloysite nanotubes (HNTs) have attracted global interest, because the nanotubular HNTs could extend the range of their potential applications. In this study, we fabricated a novel nanocomposite with hollow iron nanoparticles loaded on the surface of HNTs. The structure of the iron nanoparticles can be adjusted by ageing time. Owing to the increased remnant magnetization and coercivity values, the nanocomposites loaded with hollow iron nanoparticles showed better electromagnetic performance than that with solid iron nanoparticles. This study opens a new pathway to fabricate halloysite nanotubular nanocomposites that may gain applications in the catalytic degradation of organic pollutants and electromagnetic wave absorption.

Impact of Sulfuric Acid Treatment of Halloysite on Physico-Chemic Property Modification

Halloysite (HNT) is treated with sulfuric acid and the physico-chemical properties of its morphology, surface activity, physical and chemical properties have been investigated when HNT is exposed to sulfuric acid with treatment periods of 1 h (H1), 3 h (H3), 8 h (H8), and 21 h (H21). The significance of this and similar work lies in the importance of using HNT as a functional material in nanocomposites. The chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR). The spectrum demonstrates that the hydroxyl groups were active for grafting modification using sulfuric acid, promoting a promising potential use for halloysite in ceramic applications as filler for novel clay-polymer nanocomposites. From the X-ray diffraction (XRD) spectrum, it can be seen that the sulfuric acid breaks down the HNT crystal structure and alters it into amorphous silica. In addition, the FESEM images reveal that the sulfuric acid treatment dissolves the AlO₆ octahedral layers and induces the disintegration of SiO₄ tetrahedral layers, resulting in porous nanorods. The Bruncher-Emmett-Teller (BET) surface area and total pore volume of HNTs showed an increase. The reaction of the acid with both the outer and inner surfaces of the nanotubes causes the AlO₆ octahedral layers to dissolve, which leads to the breakdown and collapse of the tetrahedral layers of SiO₄. The multi-fold results presented in this paper serve as a guide for further HNT functional treatment for producing new and advanced nanocomposites.

Amine-Impregnated Mesoporous Silica Nanotube as an Emerging Nanocomposite for CO2 Capture

Pristine halloysite nanotubes (HNTs) were pretreated to produce mesoporous silica nanotubes (MSiNTs), which was further impregnated with polyethenimine (PEI) to prepare an emerging nanocomposite MSiNTs/PEI (MP) for CO2 capture. Thermogravimetric analysis (TGA) was employed to analyze the influences of PEI loading amount and adsorption temperature on CO2 adsorption capacity of the nanocomposite. The Brunauer-Emmett-Teller (BET) surface area (SBET) of MSiNTs was six times higher than that of HNTs, and the corresponding pore volume was more than two times higher than that of HNTs. The well dispersion of PEI within the nanotubes of MSiNTs benefits more CO2 gas adsorption, and the adsorption capacity of the nanocomposite could reach 2.75 mmol/g at 85 °C for 2 h. The CO2 adsorption on the nanocomposite was demonstrated to occur via a two-stage process: initially, a sharp linear weight increase at the beginning, and then a relatively slow adsorption step. The adsorption capacity could reach as high as 70% within 2 min. Also, the nanocomposite exhibited good stability on CO2 adsorption/desorption performance, indicating that the as-prepared emerging nanocomposite show an interesting application potential in the field of CO2 capture.

Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds

Halloysite is an alumosilicate tubular clay with a diameter of 50 nm, an inner lumen of 15 nm and a length of 600-900 nm. It is a natural biocompatible nanomaterial available in thousands of tons at low price, which makes it a good candidate for nanoarchitectural composites. The inner lumen of halloysite may be adjusted by etching to 20-30% of the tube volume and loading with functional agents (antioxidants, anticorrosion agents, flame-retardant agents, drugs, or proteins) allowing for formulations with sustained release tuned by the tube end-stoppers for hours and days. Clogging the tube ends in polymeric composites allows further extension of the release time. Thus, antioxidant-loaded halloysite doped into rubber enhances anti-aging properties for at least 12 months. The addition of 3-5 wt% of halloysite increases the strength of polymeric materials, and the possibility of the tube's orientation promises a gradient of properties. Halloysite nanotubes are a promising mesoporous media for catalytic nanoparticles that may be seeded on the tube surface or synthesized exclusively in the lumens, providing enhanced catalytic properties, especially at high temperatures. In vitro and in vivo studies on biological cells and worms indicate the safety of halloysite, and tests for efficient adsorption of mycotoxins in animals' stomachs are also carried out.

Does halloysite behave like an inert carrier for doxorubicin?

Thermogravimetric analysis-mass spectrometry revealed more details of the interaction between doxorubicin and halloysite surface.

Effective activation of halloysite nanotubes by piranha solution for amine modification via silane coupling chemistry

Piranha solution is shown to be an effective agent in activating the surface of HNTs for silanization modification.

Opening and blocking the inner-pores of halloysite

Aggregation and dispersion behaviours of halloysite nanotubes (HNTs) were influenced by pH. The meso- and micro-porosity of halloysite were determined by N2 gas physisorption. The blocking and opening of halloysite nanotube pores with acid, neutral, and base treatment were detected in a micro- and mesoporous matrix of tunable porosity. The inner pores of HNTs were opened with base treatment, but were closed with acid or neutral treatment.

Enlargement of halloysite clay nanotube lumen by selective etching of aluminum oxide

Halloysite clay tubes have 50 nm diameter and chemically different inner and outer walls (inner surface of aluminum oxide and outer surface of silica). Due to this different chemistry, the selective etching of alumina from inside the tube was realized, while preserving their external diameter (lumen diameter changed from 15 to 25 nm). This increases 2-3 times the tube lumen capacity for loading and further sustained release of active chemical agents such as metals, corrosion inhibitors, and drugs. In particular, halloysite loading efficiency for the benzotriazole increased 4 times by selective etching of 60% alumina within the tubes' lumens. Specific surface area of the tubes increased over 6 times, from 40 to 250 m(2)/g, upon acid treatment.