Selective modification of inner surface of halloysite nanotubes: a review

1 Introduction

The breadth of research into halloysite nanotubes (HNTs) has expanded greatly over the past several years [1], [2]. The HNTs (Al₂Si₂O₅(OH)₄·nH₂O) are natural hollow like aluminosilicate clay, which spread in many continents including Asia, North America, Europe, Oceania, and South America. The formation of the multilayer may be owing to the hydration effect of neighboring silica and alumina layers (shown in Figure 1A) [3], [4]. Generally, the multilayer tubule walls consist of 15~20 aluminosilicate layers with a layer spacing of ca. 10 Å or 7 Å for the hydrated or dehydrated HNTs, respectively [5], [6]. The external diameter of the HNTs usually ranges from 40 to 70 nm, and the length is within the scope from 500 to 1000 nm (shown in Figure 1B). Duarte et al. [7] presented a cross-section view of an ideal single-walled halloysite. The outer surface of the HNTs consists of siloxane groups (Si–O–Si), whereas the gibbsite octahedral array groups (Al–OH) overspread on the inner surface [7], [8].

Figure 1:

The structure of HNTs: (A) the curly morphology of HNTs; (B) the micromorphology of HNTs measured by TEM, which was conducted by our group.

Owing to plenty of attractive virtues, including the high ratio of length to diameter, nanoscale hollow shape, lower density of hydroxyl groups on the outer surface, etc., the HNTs have been applied to numerous promising domains. The desirable biocompatibility and nontoxicity make the HNTs better candidates for bio-medical materials compared with other tubular nanomaterials, such as carbon nanotubes and TiO₂ nanotubes [9], [10], [11], [12], [13], [14]. In Price’ review, they point out that the HNTs can be used to load metal and plastic anticorrosion, which can be utilized for biocide protection [8]. It has been reported that dozens of classical drugs can be loaded into the cavities of the HNTs to achieve sustained, pH-/thermo-responsive and targeted release systems [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Lvov et al. pointed out that the relatively large inner diameter and the electropositive characteristic enable the HNTs to be DNA carriers. Afterward, some other biomacromolecules, especially for enzymes, were also reported to be loaded into the HNT lumen. The hydroxy groups make the HNT nano-catalysts in catalyzing esterification reactions, and the catalytic activity of metal nanoparticles can be significantly enhanced by the immobilization of the HNTs [25], [26], [27], [28], [29], [30], [31]. Moreover, the HNTs have been considered as attractive candidates for a number of diverse applications, ranging from biosensors [32], [33], hydrogen storage materials [34], adsorbing agents [35], [36], [37], to flame-resistant materials [38], [39].

In order to expand the application fields or their corresponding performances, the development of chemical modification methods toward the HNTs has gathered increasing concerns. Until now, the silane coupling agent remains the main strategy in the modification of the HNTs [40], [41]. However, the utilization of organosilane agents usually results in the modification of both inner and outer surfaces. With the strong demand to precisely design HNT-based materials, the selective covalent and supramolecular modification of the halloysite external surface has been achieved by Amorati, Riela, Lazzara, Massaro, Cavallaro, and others [42], [43], [44], [45]. Compared to the outer surface modifications, which are largely investigated, the modification of the alumina inner surface is much less present in the literature. Selective modification for the inner surface also gave excellent prospects for the hybrid HNT-based materials, which, however, remains a difficult task. Therefore, the main aim of this review is to summarize the selective modification methods upon the inner surface of the HNTs in order to provide some guiding principles on developing selectively modified HNTs. Moreover, some special changes about the ζ potential, morphologies, and some other physicochemical properties should be observed after halloysite lumen modification, which can serve as the confirmation of successful lumen modification and will also be briefly introduced in this review.

2 Alkyl phosphate

Alkyl phosphates are reported to be capable of self-assembling on some metallic oxide surfaces in aqueous solutions [46]. It has also been reported that organophosphorus molecules usually exhibit high affinity toward metal oxide surfaces. Michel et al. [47] used a photolithography method to create patterns consisting of TiO₂ and silicon dioxide (SiO₂). The XPS results show that alkane phosphates can only self-assemble on TiO₂, whereas the they cannot self-assemble on SiO₂. Lvov et al. [48] present that there is hope that aluminosilicate clays will exhibit a similar selectivity response to TiO₂. The HNTs were treated with octadecylphosphonic acid in a weak acid environment (shown in Figure 2). It was evidenced by XPS, NMR, and FTIR that the Al−O−P bonds were formed on the inner alumina surface of the HNTs. No octadeylphosphonic acid bonding was detected on the outer surface of the HNTs, which demonstrated that the inner surface of the HNTs can be selectively modified by alkyl phosphates. In return, an inorganic micelle-like construction was created, which contains a hydrophobic core and a hydrophilic shell. Moreover, the bifunctionalization of halloysite can be achieved by treating the outermost silica surface through silanization after the selective reaction of alkyl phosphate on the inner surface.

Figure 2:

Selective modification of the inner surface by octadecylphosphonic acid proposed by Lvov et al. [48].

Sahnoune et al. [49] employed this method to graft polystyrene derivatives onto the inner surface of the HNTs (shown in Figure 3). In their study, a styrene/(methacryloyloxy)methyl phosphonic acid (P(S-co-MAPC₁(OH)₂)) copolymer was synthetized by typical radical polymerization. The prepared P(S-co-MAPC₁(OH)₂ was dissolved in toluene and, then, heated at solvent reflux to react with the inner surface of the HNTs. An amphiphilic property of the modified HNTs was achieved.

Figure 3:

Preparation of amphiphilic HNTs by coupling P(S-co-MAPC₁(OH)₂ onto the inner surface of the HNTs [49].

On the other hand, it has been reported that aryl phosphates can also be used in the modification of HNTs. In Tang’s study, phenylphosphonic acid was employed to intercalate and unfold the sheet of the HNTs, leading to a significant improvement in basal spacing. A longer treatment time can transfer the nanotubes to nano-platelets [50]. However, no literature investigates whether aryl phosphates also show a selective modification capacity upon the inner surface of the HNTs.

3 Dopamine derivatives

Dopamine is a significant neurotransmitter in the brain, which can be released by nerve cells to send signals to other nerve cells [51], [52], [53]. As a catechol derivative, dopamine and its derivatives were reported to be utilized in the modification of metallic oxide surfaces for biomedical applications [54], [55], [56]. For example, Wang et al. [57] reported that a dopamine derivative bearing catechol units can be incorporated into magnetic Fe₃O₄ nanoparticles via a Schiff base bond.

Takahara et al. [58] pointed out that the binding of catechol groups to the outer surface is noncovalent, where the binding energy is calculated as 14 kcal/mol. They demonstrated that dopa, a dopamine derivative bearing catechol units, can also be coupled to the inner surface but not to the outer surface of the HNTs (shown in Figure 4). The surface modification of the HNTs was achieved by mixing the HNTs with a dopamine derivative in THF:H₂O (4:1) and stirring at room temperature for 2 days. It should be noted that Takahara et al. [58] proposed a model experiment to verify the selective adsorption nature of dopamine. In this experiment, alumina and silicon nanoparticles were treated with dopamine derivative, respectively, under the same condition with the modification reaction of the HNTs. The dopamine derivative can only bond to the alumina nanoparticles but not to the silicon nanoparticles. These results are also appropriate to the inner and outer surfaces of the HNTs. Furthermore, the atom transfer radical polymerization (ATRP) was then introduced in the dopa-modified HNTs to give PMMA-filled HNTs.

Figure 4:

Dopamine derivative for selective modification of the inner surface of HNTs [58].

Lin et al. [59] used this method to prepare sulfonated HNTs. It significantly improved the compatibility and interfacial adhesion and gave the sulfonated HNT-based membrane materials void-free and uniform morphology.

4 Ionic liquid

An ionic liquid (IL) is an ionic salt whose fusing points is less than some arbitrary temperature, resulting in a liquid state [60], [61], [62]. ILs exhibit many potential applications, especially in the field of electric battery. They are powerful solvents with good electrical conductivity electrically, which are also known as liquid electrolytes.

Dedzo et al. [63] used 1-(2-hydroxyethyl)-3-methylimidazolium, an imidazole-based IL, to functionalize the tubes’ inner surfaces through the condensation reaction between the aluminol and alcohol to yield stable Al−O−C bonds, evidenced by the ¹³C solid-state NMR (shown in Figure 5). The introduction of ILs in the inner surface provides a greater electric density, ensuring a better accumulation of the nanoparticle precursors at the inner surface of the HNTs. In this way, palladium nanoparticle (PdNP) precursors selectively deposited inside the lumen of the HNTs. In contrast, the unmodified HNTs displayed nanoparticles both inside and outside the tubes. The PdNP-loaded HNTs showed excellent catalytic activity for the reduction of 4-nitrophenol and can be recycled up to three times without any significant reduction of the catalytic activities. The modification strategy developed in Dedzo’s study could be applied to a broad range of metal nanoparticles by selecting the chemical nature of the modifiers.

Figure 5:

Selective modification of the inner surface by an ionic liquid (1-(2-hydroxyethyl)-3-methylimidazolium) [63].

5 Arylboronic acid

Arylboronic acid belongs to the class of organoboranes, which contains an aryl-substituted boric acid and at least a carbon-boron (B–C) bond. Early studies of alcohol-affinitive molecules revealed that arylboronic acid can rapidly react with diols via dehydration condensation; the reaction between arylboronic acid and pinacol is one of the most typical cases in organic chemistry. Yildirim et al. [64] used the reaction between vicinal diols and boronic acids to enlarge the interlamellar spacing of graphene oxide, where the gas adsorption capacity was dramatically increased.

Our group demonstrated that the arlyboronic acid can also be covalently linked to the inner surface but not to the outer surface of the HNTs [65]. In this way, 1-pyrenylboronic acid (PBA) was coupled onto the inner surface of the HNTs, which functionalized the modified HNTs with fluorescence properties (shown in Figure 6). Moreover, the established Al–O–B linkage gave a highly specific and sensitive H₂O₂ sensitivity to PBA-modified HNTs, which made them desirable fluorescence probes for the detection of hyperoxide.

Figure 6:

Selective modification of the inner surface by 1-pyrenylboronic acid [65].

6 Electrostatic lumen coating

There is an attractive character for the HNTs that they possess a positively charged inner lumen (ζ=+24 mV) and a negatively charged silicondioxide outer surface (ζ=−35 mV) [1], [2]. Therefore, the selective modification can be achieved by utilizing the difference in electric potentials. Some electronegative molecules, e.g. negatively charged proteins and anionic surfactants, may be immobilized onto the inner surface of the HNTs [66].

In Lvov’s study, negatively charged proteins were selectively immobilized onto the lumen surface of the HNTs after typical vacuum cycles. The immobilized negatively charged proteins showed enhanced thermal stability and temporal biocatalytic abilities compared to free enzymes in solution [67]. In this way, Yan et al. [68] immobilized lipase, a negatively charged protein, onto the lumen surface of the HNTs, and then, the resulting enzyme-nanotube complex was mixed with chitosan to give an enzymatic membrane, which is capable of hydrolyzing lipids without the loss of enzymes.

Cavallaro et al. [69], [70], [71] demonstrated that an anionic surfactant can be an alternative to the use of the selective modification of the alumina inner surface on the HNTs. In their study, the inner surface of the HNTs can be selectively modified by sodium dodecanoate (NaL), decyltrimethylammonium bromide (DeTAB), and perfluorinated anionic surfactants through magnetic stirring. The use of anionic surfactant modification has been considered as a good strategy for oil and gas entrapment as well as a way to stabilize the HNT dispersion in water or controlling its partition between an aqueous and an oil phase [42], [69], [70]. The inner cavity modification with fluorinated anionic surfactant was evidenced to be a good strategy to generate flame-retardant additives [71]. Moreover, after halloysite lumen modification, its ζ potential has to increase making its stable aqueous colloids [35], [69], [72]. It happens because any internal tube modification “kills” internal positive charges, and a total ζ potential of the tubes is an algebraic sum of external minuses and inner pluses. This may be a simple confirmation of successful lumen modification.

7 Etching

It is well accepted that HNTs possess better biocompatibility than other nanotubular materials, especially for carbon nanotubes. HNTs may have a much broader application in the bio-medical fields when getting a larger cavity volume. Abdullayev et al. demonstrated a feasible process to enlarge the lumen diameter of the HNTs by etching the alumina sheets on the inner surface of the HNTs. In their study, sulfuric acid was employed to selectively dissolve the alumina sheets on the inner surface of the HNTs, whereas the exterior surface was preserved, which enhances two~ to -threefold the loading capacity as well as the surface area of the tubes [73].

PVA is a water-soluble polymer, which has the idealized formula [CH₂CH(OH)]_n [74], [75]. Because of its excellent film forming, emulsifying, adhesive, and nontoxic properties, PVA has been widely used in the field of bio-medicine, gas storage, etc. [76], [77], [78]. Our group utilized a unidirectional freezing method upon PVA/HNTs aqueous dispersions to achieve an ordered LC mesophase with parallel-banded structures in the formed PVA/HNTs aerogel [79]. The HNTs showed a desirable dispersibility in PVA matrix, resulting in serious PVA/HNTs composite materials in recent studies [80].

Cheng et al. [81] utilized PVA to develop a selective modification method upon the inner surface of the HNTs. In their work, the HNTs were initially calcined at 550°C and then added into the PVA aqueous solution. After drying, the PVA-treated HNTs were rinsed under sonication aiming to remove those PVA residues on the outer surface of the HNTs, evidenced by XPS patterns. Interestingly, the PVA-modified HNTs were etched to denote carbon nanotubes. The as-mentioned method provided a low-cost, versatile, and facile approach to obtain carbon nanotubes.

8 Click chemistry

The classic click reaction is a copper-catalyzed reaction of an azide(−N₃) with an alkyne (−C≡H) to form a triazole ring [82], [83]. Owing to the desirable specificity and high efficiency, click reaction has been widely used to prepare organic-inorganic hybrid materials [84], [85]. In a previous study, we functionalized the azide groups onto the surface of the HNTs [86]. Polyfluorenes with terminal alkyne groups were grafted onto the azide-modified HNTs via a copper-catalyzed click reaction, where the grafting degree was significantly enhanced. However, the dispersibility of the HNTs was found to be worse mainly due to the fact that the treatment of organosilanes was for aluminosilicate surfaces usually resulting in nonselective modification.

Lazzara et al. [87] demonstrated a selective modification of the cavity in the HNTs by recourse to the click reaction (shown in Figure 7). To functionalize the internal surface of the HNTs, they facilitated two compounds with terminal azide or alkyne groups to be encapsulated into the HNT lumens, respectively. Two kinds of modified HNTs were catalyzed by Cu²⁺ to process the click reaction between −N₃ and −C≡H terminations to generate the connected HNTs and then to give the HNT-based microfibers with a significant enhancement in mechanical properties. The FTIR and TGA results indicated that the modification occurred only in the cavity.

Figure 7:

Selective modification of the inner surface of HNTs by click reaction [87].

9 Poly(ethylene oxide) (PEO)

Previously, it has been demonstrated that the selective modification of the inner surface of the HNTs by covalent bonds can be feasibly achieved, while the selective modification of the inner surface may also be achieved through hydrogen bonds.

In Liu’s study, the water natively living in the lumen of the HNTs was used as the medium to form the hydrogen bond between the HNTs and the PEO by extracting the PEO/HNT nanocomposite with a Soxhlet extractor. The hydrogen bonding interactions were evaluated, which confirmed that the PEO only interacts with the inner surface within the lumen of the HNTs [88].

10 Conclusion

We focus on the development of selective modification of the inner surface of the HNTs, including the employment of alkyl phosphate, dopamine derivative, IL, arylboronic acid, PVA, PEO, and click chemistry, which will be able to provide multiple choices to prepare the selectively modified HNT-based organic-inorganic hybrid materials. These advances may expand the application fields, as well as enhance the performance of the HNTs. Otherwise, the methods discussed in this paper may also be capable of providing some guiding lights on the selective modification of other kaolin-based inorganic materials, such as imogolite nanotubes. Furthermore, a further understanding of how structure and composition are precisely affected by the selective modifications, or the detailed applicability of these methods, cannot be overemphasized.