Proton Binding of Halloysite Nanotubes at Varied Ionic Strength: A Potentiometric Titration and Electrophoretic Mobility Study

The development of stimuli-responsive drug nanocarriers is an increasingly important area in nanomedicine because efficient delivery of toxic drugs to targeted tissues minimizes side effects. The specific objective of this study was to synthesize and characterize a novel pH-responsive drug carrier based on halloysite nanotubes for the controlled release of the anticancer drug doxorubicin. Poly(N,N-dimethylaminoethyl methacrylate) brushes were grafted from the surface of halloysite nanotubes using the combination of mussel-inspired polydopamine surface modification and activators regenerated by electron transfer in atom transfer radical polymerization. The chemical structure and morphology of the modified halloysite nanotubes were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermal gravimetric analysis as well as scanning and transmission electron microscopies. Dynamic light scattering and zeta potential analysis were carried out to evaluate the pH-responsivity of the functionalized halloysite nanotubes. The results of the drug loading and release study of pristine and functionalized halloysite nanotubes showed that grafting of poly(N,N-dimethylaminoethyl methacrylate) brushes on the polydopamine-modified halloysite nanotubes surface leads to a drastic increase in doxorubicin loading capacity and a highly pH-sensitive release behaviour. Less than 10 % of the loaded doxorubicin was released from poly (N,N-dimethylaminoethyl methacrylate)-grafted halloysite nanotubes at pH 7.4 after 24 h; in contrast, at pH 5.5 there was a continuous release of doxorubicin totalling 13 % in the first 30 min, i.e. lower than for the pristine halloysite nanotubes (32 %), but reaching 48 % after 24 h. Poly (N,N-dimethylaminoethyl methacrylate)-grafted halloysite nanotubes can hence be considered as a potential candidate for delivering highly toxic drug molecules to the acidic target sites.

In this paper, new porous pH-responsive microspheres based on functionalized halloysite nanotubes (HNTs) with poly-lactic-co-glycolic acid (PLGA) are investigated as the phenytoin sodium (PHT-Na) carrier. For this purpose, the surfaces of HNTs were modified by a silane coupling agent, (3-aminopropyl)triethoxysilane (APTES) and then the desired microsphere was synthesized through PLGA coating on modified HNTs. Formation of these hybrid particles are confirmed using various characterization methods like Fourier transform infrared (FT-IR), transmission electron microscope (TEM), scanning electron microscope (SEM), EDX, zeta-potential, and X-ray diffraction. The results of the FT-IR spectrum show the presence of APTES, PHT-Na and PLGA peaks, which supported the modification of HNTs and drug capsulation. TEM images confirm the presence of APTES on HNTs, due to the increase in outer diameter. SEM images displayed that by grafting PLGA polymer to modified HNTs, the shape of nanotubes has changed from rod-like to microsphere. Hence, the prospering connection of APTES and PLGA on HNTs was emphasized by zeta-potential results. Moreover, the profile of drug release is recorded via HPLC. In vitro drug release tests show that both the presence of polymer chains around drug containers and the pH value of the release medium play an important role in controlled release. Eventually, the kinetics of drug release was evaluated based on Korsmeyer–Peppas kinetic model.

The superior physical and mechanical performances of halloysite nanotubes (HNTs) make them ideal fillers for rubber reinforcement. However, due to the abundant hydroxyl groups on their surface, they are highly polar and have poor compatibility with polymers, making it difficult for them to disperse uniformly in polymers, thus limiting their application in rubber composites to a certain extent. In this paper, noncovalently modified HNTs filled with a natural rubber/chloroprene rubber/chloro-isobutylene-isoprene rubber (NR/CR/CIIR) blend system, using polydopamine(PDA) as a surface modifier, was investigated. and a green and high-performance PDA/HNTs/NR/CR/CIIR composite was prepared by mechanical blending. Transmission electron microscopy (TEM) showed that the original HNTs had a hollow, multi-walled nanotube structure, and the outer surface of the PDA-modified halloysite nanotubes(PDA/HNTs)showed an obvious cladding layer. X-ray diffraction indicated that the modification experiments did not damage the halloysite crystal structure; the modifier only interacted on the outer surface of the HNTs. Zeta potential and thermogravimetric analysis (TGA) further indicated the presence of interfacial interactions between PDA and HNTs, suggesting that PDA was successfully grafted to the surface of HNTs. In addition, the effects of PDA/HNTs on the mechanical properties, vulcanization properties and Rubber Process Analyzer (RPA analysis) of the NR/CR/CIIR blend were also investigated. The dispersion of HNTs in the NR/CR/CIIR blend was enhanced by the adsorption of PDA on the HNTs surface by non-covalent bonds, and the comprehensive properties of the composites were improved.

Zinc sulfide (ZnS) nanoparticles are successfully deposited on the surface of natural halloysite nanotubes (HNTs) to produce ZnS /HNTs nanocomposites. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) analysis. The results indicate that ZnS nanoparticles are uniformly attached on the surface of HNTs with narrow particle size distribution center at ~10 nm, and are prevented from aggregation by HNTs and expose more active sites. ZnS /HNTs show excellent photocatalytic activity for the degradation of eosin B under UV light, better than pure ZnS and HNTs, indicating its potential application in the field of environmental protection. The mechanism for photocatalytic activity enhancement is also investigated.

Rubber composites based on halloysite nanotubes (HNT) have attracted tremendous attention during the last decades owing to the improved mechanical, dynamic mechanical, and thermal properties [1–3]. Before exploring the various aspects of HNT/rubber composite systems, we first briefly describe the HNT. It was first introduced by Berthier long back in the year of 1826 [4]. HNTs are a type of naturally occurring silicates with rolled nanotubular or spiral morphology, which have an analogous chemical structure of kaolinite [5]. They are naturally occurring and economically viable, and many countries such as China, France, Belgium, and New Zealand have deposits of these clay minerals. The major origin of HNT is a clay mineral obtained from a weathering product of granitic and rhyolitic volcanic rocks. The chemical composition of HNT is reported as Al2(OH)4Si2O5∙nH2O with n 1⁄4 0 or 2 for the anhydrous and the fully hydrated halloysite, respectively. The nanotubular structure was evolved from kaolinite by rolling up the layers under natural conditions. Most of the HNTs are multiwall or double-wall nanotubes [6]. Usually, the inner diameter of HNT is in the range of 50–80 nm and length of about 1,000 nm. Based on the state of hydration, HNTs are classified in two categories: hydrated HNTs with a crystalline structure of 10 A (d001) spacing and dehydrated ones with 7 A (d001) spacing [7]. HNTcontains two different types of hydroxyl groups, inner and outer hydroxyl groups, which are positioned in between the layers and on the surface, respectively. Attributed to the multilayer structure, most of the hydroxyl groups are inner groups, and only a few hydroxyl groups are located on the surface of HNT. The surface of HNT is mainly composed of outer O–Si–O (silanol) groups and O–Al–O (aluminol) groups situated inside the lumen. It is interesting to note that the density of surface hydroxyl groups in HNT is rather lower compared to other silicates clay minerals, for instance, kaolinite and montmorillonites (Fig. 1).

Halloysite nanotubes functionalization with phosphonic acids: Role of surface charge on molecule localization and reversibility

Co3O4 nanoparticles were successfully deposited on the surface of natural halloysite nanotubes (HNTs) to produce Co3O4/HNTs composites. The structure and morphology of the samples were characterized using X-ray diffraction, field-emission scanning electron microscope, transmission electron microscope and Fourier transform infrared. The results indicated that Co3O4 nanoparticles were uniformly attached on the surface of HNTs with narrow size distribution. Co3O4/HNTs exhibited an excellent photocatalytic efficiency for degradation of methyl blue under UV light, better than Co3O4 and HNTs mixture, HNTs and pure Co3O4. The mechanism of enhanced photocatalytic activity of Co3O4/HNTs was also proposed.

ABSTRACTFully renewable soy protein isolate (SPI)–based film with rigid strength and sufficient water resistance is difficult to attain. In this study, the mussel‐inspired surface chemistry of ɛ‐poly‐L‐lysine (ɛ‐PL)/dopamine was exploited for codeposition onto halloysite nanotubes (HNTs) to engineer a multinetwork of HNT/SPI bionanocomposite films via physicochemical bonds. A series of ɛ‐PL/dopamine aqueous solutions at different concentration ratios were employed. The ɛ‐PL with abundant cationic amine groups could prevent the overoxidation of dopamine on HNT surfaces, thus maintaining sufficient free catechol groups for highly active reactions that improve the biphase interfacial adhesion. Moreover, HNTs surface entangled by ɛ‐PL chains could be more compatible with peptides. This codeposition of ɛ‐PL/dopamine on HNT (DLHNT) surfaces was analyzed by X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X‐ray diffraction, and thermogravimetric analysis. Compared to the control SPI film, the tensile strength of the nanocomposite film (DLHNTs0.5/SPI) was increased from 5.9 MPa to 8.25 MPa, the Young's modulus was improved by 166.4%, and the moisture absorption was reduced to 56.1% (87.2% of the control). In summary, a facile and mild bioinspired surface chemistry of ɛ‐PL/dopamine codeposition onto HNT surfaces was performed to prepare SPI‐based nanocomposite films with improved interfacial adhesion and benign compatibility. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46197.

At present, petrochemical-based plastics are commonly used in many industries. Yet, environmental concerns over these materials have led to the search for biodegradable alternatives in several industries including packaging. In this regard, the renewable resource-based polymer, poly (lactic acid) (PLA), holds a greater demand due to its unique properties (such as biodegradability and biocompatibility). Despite the merits of PLA, limitations in mechanical, thermal and gas barrier properties have restricted the usage of PLA in a wider range of applications in the packaging industry. Therefore, it has become a necessity to reinforce PLA to overcome these drawbacks. The main objective of this research is to investigate the feasibility of halloysite nanotubes (HNTs) as a nanofiller to reinforce PLA. PLA/HNTs nanocomposite films were prepared using the solution casting method by varying the HNTs loading (from 2.5 – 10 (w/w %)). Evaluation of the physico-chemical properties of the films revealed that the mechanical and thermal properties of PLA films were enhanced with the addition of HNTs. The interfacial interaction was further investigated with Fourier transform infrared (FTIR) spectroscopy and the end-hydroxyl groups of PLA were found to chemically interact with outer surface siloxane groups of HNTs. Furthermore, a comparison study was conducted to investigate the influence of the nanocomposite processing methods, namely melt compounding and solution casting, on the thermo-mechanical properties of PLA/HNTs nanocomposite films. Moreover, the ductility of PLA/HNTs films was improved by incorporating a plasticiser, poly (ethylene) glycol (PEG), while the dispersion of HNTs in PLA at high filler loadings was further improved by modifying the outer surface of HNTs with a silane modifier, γ-aminopropyltriethoxysilane (APTES). In addition, after evaluating the effect of three different types of HNTs (which are structurally different) on the tensile properties, this study suggests the best type of HNTs that can be incorporated into the PLA matrix in order to obtain the optimum tensile properties. In this dissertation, an alternative finite element (FE) approach to accurately predict the elastic modulus of polymer/HNTs nanocomposites is proposed. A real-structure-based 3-D computational model with randomly oriented HNTs was developed and compared with the conventional, idealized modelling approach. The developed idealized model consists of nanotubes with fixed aspect ratio and the proposed alternative real-structure-based model takes the experimentally observed variations in HNTs sizes, impurities and aspect ratios into account. According to the parametric studies, a unit cell model with cylindrical reinforcements (representing HNTs) and at least 30 inclusions gave promising results, provided the model included actual information about HNT's size ranges and aspect ratios. Numerical studies were validated with experimental findings and the developed real-structure-based model gave more accurate results than idealized and analytical models. Furthermore, the reinforcing mechanism was carefully studied in terms of the stress distribution. As the final stage of this research, the PLA/HNTs nanocomposite films were further developed for better end user application by creating high performance, multifunctional, active packaging films. ZnO nanoparticles have been successfully investigated to remarkably enhance the antimicrobial properties of poly (lactic acid) (PLA) composite films for active packaging, where they can enhance the shelf life of goods, but the addition of ZnO into PLA decreases its thermo-mechanical properties. In this study, ZnO nanoparticles were deposited on the outer and inner surfaces of halloysite nanotubes (HNTs) using a novel solvothermal method and these ZnO deposited HNTs (ZnO-HNTs) were incorporated into the PLA matrix as a reinforcing filler. PLA nanocomposite films with ZnO had inferior mechanical properties compared to PLA films with ZnO-HNTs which showed significant improvements, with an increase in the tensile strength and modulus by 33% and 74%, respectively, with the addition of 5 (w/w %). Antimicrobial tests revealed that ZnO-HNTs can act as a promising antimicrobial agent against bacteria such as Escherichia coli and Staphylococcus aureus where the bacteria count reduced by more than 99%.

In this study, we demonstrate the use of poly (ethylene oxide) (PEO) for in-situ modification of the inner surface of halloysite nanotubes (HNTs) with water molecules as the hydrogen bond forming medium, as well as the nano-confinement of PEO molecular chains within the nanotube. Before testing, the Soxhlet experiment of PEO/HNTs powder is applied in order to remove the physical adsorption of PEO molecules onto the outmost surface of HNTs. The crystal temperature of PEO changes sharply from 36.9 °C of neat PEO to −25 °C of PEO in the PEO/HNTs powder and the decomposed temperature of PEO in the PEO/HNTs powder is about 13.1 °C higher than that of neat PEO, which is mainly owing to the nano-confinement effect of PEO within the HNTs with a diameter of about 10 nm. From thermo-gravimetric (TG) analysis, about 7.71 % by weight of PEO has been chemically bonded to HNTs. The hydrogen bonds among PEO, HNTs, and water molecules are evidenced by FTIR and XPS performances. Meanwhile, the binding energy of Al2p in the innermost surface of HNTs shifts from 74.7 eV in the neat HNTs to 74.5 eV in the PEO/HNTs powder, while that of Si2p on the outmost surface of HNTs keeps almost constant, indicating that the hydrogen bonds only exists inner the nanotube and PEO molecular chains have been trapped in nano-scale within HNTs, which is in accordance with the DSC and TG observation.

Benzoxazine monomer (named as B-aptes) was synthesized from 3-aminopropyltriethoxysilane (KH-550), bisphenol A (BPA), and paraformaldehyde. Subsequently, functionalized halloysite nanotubes were obtained by introducing B-aptes onto the surface of halloysite nanotubes (HNTs). Then, benzoxazine-modified halloysite nanotubes (B-HNTs) were used to combine with BPA epoxy resin to prepare the diglycidyl ether of bisphenol-A (DGEBA)/B-HNTs composites. The homogeneous dispersion state of modified HNTs in the cured composite matrix was observed by scanning electron microscopy. Differential scanning calorimetry was used to investigate polymerization behaviors of ternary composites. The results showed that the ternary composite possessed lower polymerization temperature compared with the neat DGEBA/benzoxazine. According to the results of thermogravimetric analysis, the thermal stability of DGEBA/benzoxazine copolymers was improved by the modified HNTs, the char yield increased with the increase of HNTs mass ratio. The results of mechanical tests and dynamic mechanical analysis displayed that the DGEBA/B-HNTs composites possessed promoted mechanical properties.

Halloysite nanotubes@carbon/manganese dioxide nanocomposites (HNTs@C/MnO2) with coaxial tubular structure were prepared by introducing manganese dioxide on the surface of carbon-coated halloysite nanotubes (HNTs@C). The HNTs@C nanocomposites were synthesized by hydrothermal carbonization of glucose on the surface of HNTs and further activated at high temperature to improve the degree of carbonization. The use of halloysite nanotubes can effectively induce the heterogeneous deposition of carbonaceous species on the surface of the halloysite nanotubes, and then uniformly generate MnO2 nanoflakes via the redox reaction between the carbon and potassium permanganate to construct the hybrid coaxial structure. The as-prepared hybrid materials were characterized by transmission electron microscope, Fourier transform infrared spectrum, X-ray diffraction, and thermogravimetric analysis. The specific capacitance of HNTs@C/MnO23 nanocomposites can reach 274 F g−1 at 1.25 mA cm−2 in 1.0 M Na2SO4 electrolyte after correcting the weight percent of electroactive materials. Furthermore, the special coaxial tubular structure and the manganese dioxide nanoflakes facilitated the ion diffusion between the electrode/electrolyte interfaces. The results indicate that the novel coaxial tubular hybrid nanocomposites can be a promising candidate as electrode material for supercapacitors.

Addition of natural inorganic nanotubes to epoxy acrylate resin can significantly improve their mechanical properties. Halloysite nanotubes are one kind of novel reinforcing and toughening materials for polymers. In the present work, halloysite nanotubes were compounded to epoxy acrylate resin to improve the wear resistance and toughness of the composites. To improve the embedding of halloysite nanotubes within the epoxy acrylate resin matrix, the halloysite nanotubes surface was grafted by poly (methyl methacrylate). The morphology of the grafted halloysite nanotubes particles demonstrates a core-shell structure with halloysite nanotubes as the core and the grafted polymer as the shell. Poly (methyl methacrylate) was chemically attached to the halloysite nanotubes particles as indicated by the Fourier transform infrared and X-ray photoelectron spectroscopy results. The poly (methyl methacrylate) grafted halloysite nanotubes (poly (methyl methacrylate)- g-halloysite nanotubes) then compounded with epoxy acrylate resin and composites are formed via ultraviolet-curing method. The hardness, flexibility, impact resistance and wear resistance of epoxy acrylate/poly (methyl methacrylate)- g-halloysite nanotubes composites were markedly improved in comparison with that of neat epoxy acrylate. The improvement was correlated to the well-dispersed halloysite nanotubes and the interfacial bonding between epoxy acrylate resin and poly (methyl methacrylate)- g-halloysite nanotubes. The well dispersion of halloysite nanotubes in the matrix is attributed to the modification on the surface of halloysite nanotubes.

Selective modification of the inner surface of halloysite nanotubes (HNTs) by the cycloaddition of azides and alkynes (click reaction) was successfully achieved. Fourier transform infrared spectroscopy and thermogravimetry confirmed that the modification involved only the HNT cavity. Morphological investigations evidenced that the functionalized nanotubes formed microfibers and clusters in the micrometer range. By means of the casting method, these nanomaterials were dispersed into biopolymeric matrixes (chitosan and hydroxypropyl cellulose) with the aim of obtaining nanocomposite films with tunable properties from the physicochemical viewpoint. For comparison purposes, we also characterized composite nanomaterials based on pristine halloysite. The mesoscopic structure of the nanocomposites was correlated with their tensile, thermal, and wettability properties, which were found to be strongly dependent on both the nature of the polymer and the HNT functionalization. The attained knowledge represents a bas...

Nanometer ferriferrous oxide (Fe3O4) with about 10–30 nm diameter were deposited on the inner and outer surfaces of natural halloysite nanotubes (HNTs) via the reduction co-precipitation process. The aggregation of Fe3O4 nanoparticles will be significantly reduced as they are distributed on the surfaces of HNTs with higher specific surface areas. Powder X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectrometer showed that Fe3O4/HTNs composites had a good thermal stability at 373 K and 523 K. Furthermore, Vibrating-sample magnetometer (VSM) confirmed the superparamagnetism of Fe3O4/HTNs composites could be preserved at different heat treatment of 373 K and 523 K.