A mini-review on dispersion and functionalization of boron nitride nanotubes
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Boron nitride nanotubes (BNNTs) are of intense scientific interest due to their unrivaled physicochemical characteristics, extraordinary thermal, electronic, mechanical, optical properties and prospective applications in various nanotechnologies. Therefore, they are particular candidates for the development of new materials potentially applied in abundant applications. However, obtaining homogenous composite materials requires a good dispersion of BNNTs, as well in solvents or in the matrix, which still has remained challenging. The preparation of effective dispersions of BNNTs presents a major impediment to the extension and utilization of BNNTs. BNNTs intrinsically tend to bundle and/or aggregate. The prevention of such behavior has been explored by testing various techniques to improve the dispersibility of BNNTs in a variety of solvents. There are mainly two approaches to obtain a good quality dispersion; chemical functionalization and physical interactions. The chemical functionalization technique has been found effective, but deteriorates the intrinsic properties of BNNTs through the introduction of defects on the wall. Physical blending approaches with the ultrasound and high-speed shearing have been proven capable of debundling BNNTs and stabilizing individual BNNTs while maintaining their integrity and intrinsic properties. Attention has been brought to the study of the dispersion of BNNTs in aqueous media and most attention is paid to molecular dynamics simulation techniques and other physical techniques, as well as to the use of various surfactants and polymers.
KeywordsBoron nitride nanotubes (BNNTs) Dispersion Polymer Surfactant Molecular dynamics (MD) simulation
In this section, the historical perspective, properties, and applications of BNNTs are discussed. Investigations regarding nanotubes, which were constituted of non-carbon materials, were carried out after discovering carbon nanotubes (CNTs) . Boron nitride is considered as a synthetic material, which consists of equivalent atoms of boron (B) and nitrogen (N) . According to the periodic table, boron and nitrogen are placed adjacent to carbon; also, they have identical atomic radius with carbon. Therefore, it is well-known that boron nitride (BN) and carbon have similar crystal structures . As carbon exists in two forms of graphite and diamond, it is possible that BN becomes synthesized in hexagonal and cubic forms . According to neat high-temperature stability of BN , it could utilize to fabricate ceramic parts , coatings , or lubricants . It should be noted that the hexagonal form of BN (“h-BN” or “α_BN”) is analogous to a hexagonal form of graphite because it crystallizes in a hexagonal layered structure; however, it is sometimes called “g-BN” derived from graphitic BN. It is found that BNNTs contain a layered plane structure; however, they were successfully synthesized in 1995 , and other 1D BN nano-materials, including nanowires and nano-fibers were produced after that . Considering layered 2D nanostructures, single-layered BN spherical structures (fullerenes) were produced in 1998; however, this process is similar to the production of graphene in 2005 . Also, insulating properties and good thermal conductivity were indicated by these materials . There are some characteristics considered for hexagonal BN including high-temperature solid lubricants , good thermal conductor , and good electrical insulator . Results show different low-specific gravities including stable in air, under vacuum, and in an inert atmosphere for temperatures up to 1000 °C, 1400 °C, and 2800 °C, respectively . Also, its hardness is considered to be identical to graphite; therefore, BN materials produced by hot pressing can be easily machined to reduce costs . Atar and Yola succeeded to develop a novel molecular imprinted sensor based on core–shell type nanoparticles incorporated BN nanosheets and used for cypermethrin detection in wastewater samples  and also for etoposide detection . Also, BN nanosheets was used to synthesize an electrochemical sensor for serotonin detection in urine samples , and also for diethylstilbestrol detection . Also, BN quantum dots were presented to prepare an imprinted biosensor for cardiac troponin-I detection in plasma samples . Also, Atar and Lutfi Yola showed that the BN nanosheets are able to determine of β-agonists in urine samples  and organophosphate pesticides in water samples .
According to the mentioned properties, BNNTs can be considered as complementary materials for CNTs; also, they are permitted to replace CNTs for applications that require chemical stability, high-temperature resistance, and electrical insulation. Significant characteristics of BNNTs lead them to be a favorable candidate to be used in many applications such as, chemical sensor , gas adsorbent , drug delivery , cancer therapy , enhanced catalysis , hydrogen storage , and so on [30, 31, 32]. Contrary to that of the CNT–polymer composites [33, 34, 35, 36], studies of BNNT–polymer composites are scarce because it is extremely difficult to obtain a highly pure BNNT phase with a sufficiently high yield to fabricate and test a composite material. In this review, recent developments concerning the dispersion and functionalization of BNNTs obtained by experimental and theoretical are presented.
Dispersion of nanotubes
Although BNNTs are associated with superior characteristics, they are recognized as structural analogs of CNTs . Thermal and mechanical conductivities of BNNTs are the same as their CNTs counterparts, but they have obvious differences compared to those of the CNT, including electrical insulation, or profound chemical and thermal stabilities [38, 39]. According to the van der Waals interaction existed between tubes, it could be found that pristine BNNTs are normally entangled and form a bundle during the experimental synthesis, therefore, they are rarely soluble in organic and aqueous solutions . Individual BNNTs and the bundle have to be separated in order to be utilized in practical applications. Although this process could be carried out by applying both covalent and non-covalent functionalization, the second one seems to be more suitable because it maintains the intrinsic properties of the BNNTs [41, 42]. In addition to the above-mentioned advances in the field of dispersion and functionalization of BNNT, several new approaches could be suggested to extend the possibility of different technical applications; however, there are other significant developed covalent and non-covalent approaches [41, 43]. In this regard, we will discuss the recent developments concerning the dispersion and functionalization of BNNTs obtained by experimenting in the “”Experimental evidence”, and to get more insights on the details, theoretical studies are presented in the “Theoretical investigations”.
Performing the process of covalent functionalization of BNNT is very difficult because they are highly resistant to harsh chemical conditions. However, it can be found by investigating recent studies that it is possible to carry out the process of covalent modification. It should be noted that the most prevalent chemical functionalization is respectively carried out using –OH on B atoms and –NH2 groups at the edges and defects . Considering non-covalent modifications, one can coat BNNTs with a polymeric material using weak interactions, including π–π, hydrophobic, and van der Waals forces . The BNNTs were also coated with PEGylated phospholipid 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))] conjugates (mPEG–DSPE)] . Zhi et al. investigated wrapping BNNTs with a π-conjugate conducting polymer-poly[m-phenylenevinylene-co-(2,5-dioctyloxy-p-phenylenevinylene)] (PmPV) . Using the cathodoluminescence and transmission electron microscopy, they showed that PmPV-wrapped BNNTs could be dispersed in organic solvents. This finding is also in good agreement with molecular dynamics (MD) simulation . Also, Velayudham et al. studied non-covalent functionalization of BNNT utilizing conjugated poly(p-phenylene ethynylene)s (PPEs) and polythiophene; however, the results of this investigation also showed that BNNTs could be dispersed in organic solvents . Recently Serizawa et al. have investigated the potential of different water-soluble polymers, including a poly(p-phenylene) derivative ((–)PPP), poly(xylidine tetrahydrothiophene) (PXT), poly(sodium styrene sulfonate) (PSS), poly(sodium vinyl sulfonate) (PVS), and poly(sodium acrylate) (PAA), for dispersing BNNTs in water. They found that (–) PPP has the highest potential for dispersing BNNTs among these polymers . These results indicated that polymers with an extension π-plane show stronger interactions with BNNTs than those with a smaller π-plane, suggesting that the π–π stacking interactions play a very important role in the functionalization of BNNTs with polymers.
Considering a dual surfactant/polydopamine (PD) process, Biggs et al. suggested a process for efficiently disentangled BNNTs. Results of high-resolution transmission electron microscopy (HR-TEM) showed that an individual BNNT is coated with a uniform PD nano-coating; therefore, it significantly enhances dispersion BNNT in aqueous solutions . Results indicated that there were PD-functionalized BNNTs, which were individually localized in the cytoplasm by endosomal escape; also, it was found that PD-BNNTs with up to 30 μg/mL concentrations were cytocompatible in osteoblasts cells after 72 h of the exposure. Also, stabilizing a BNNT solution during several months is carried out by dispersing multi-walled BNNTs using ammonium oleate surfactants; however, this process is based on the non-covalent functionalization of nanotube surfaces . Results of Fourier-transform infrared spectroscopy (FTIR) and photoluminescence (PL) analysis with synchrotron radiation source showed that intrinsic optical properties of BNNTs are preserved by BNNT aqueous solution. Using FTIR-ATR spectra and TEM images, Green et al. suggested that t-butanol plays a critical role in the stability of BNNSs dispersions and this assertion supported by MD simulations. Taken together, the simulation results and experimental findings showed that t-butanol has good miscibility with water and effective shielding of BNNS from water, providing colloidal stability . Moreover, different conjugated molecules, such as perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) , flavin mononucleotides (FMN) , peptide , poly(xylylene tetrahydrothiophenium chloride) (PXT), denatured DNA , long alkyl chains , dichlorocarbenes , epoxy , proteins , polyvinylpyrrolidone  are also explored for functionalization of BNNTs.
According to energetic, geometric, and electronic properties, covalent functionalization of a zigzag BNNT with acetylene, is investigated by DFT . It is recognized that a BNNT is considered as the most stable functionalized one in which acetylene is diffused into the tube wall; therefore, two heptagonal and two pentagonal rings are formed and release 1.54 eV energy. Moreover, C2H2 hydrogen atoms are substituted by different functional groups including –F, –CH2F, –CN, and –OCH3; however, this process has impacted on the geometric and electronic properties of BNNT that are accurately investigated. Results show that the range of reaction energies is − 1.03 to − 3.13 eV, so that their relative magnitude order is as follows: C2F2[(OCH3)2C2[C2H2[(CH2F)2C2[(CN)2C2, and implies on the increased functionalization energy, which is because of the enhancement of electron donating property of functional groups. Generally, little changes that occur in the electronic properties of tube lead to chemical modification of BNNT; however, it might be considered as an effective approach of purification of BNNTs. Also, Goel et al. studied the adsorption behavior of oxazole and isoxazole hetero-cycles over the (6,0) zigzag and (5,5) armchair BNNT by considering DFT . Although the existence of the adsorption on armchair BNNT surfaces is physical in nature, they showed that adsorption energies, frontier molecular orbital (FMO) analysis, and structural changes at the adsorption site imply on covalent adsorption of the zigzag BNNT surface. Performing the process of reoptimization of structures in the aqueous phase illustrates the role of solvent in improving adsorption properties over the BNNT surface. A considerable energy enhancement in the solubility of BNNTs after adsorption of heterocyclic rings is indicated by the solvation energy. Occurring substantial changes in BNNT systems’ electronic properties is carried out after these hetero-cycles’ attachment to the tube surface; however, DOS plots, natural bond orbital (NBO) analysis and the quantum molecular descriptors (QMD) are evidences of these changes. Moreover, different molecules, and [p-(1,1,3,3-tetramethylbutyl)phenyl ether (Triton X-100) , isoniazid , amine , methanol  are also explored for functionalization of BNNTs.
As discussed above, developing investigations regarding BNNTs is still limited due to the existence of BNNT samples that are utilized in widespread researches about their properties and applications. Most scientists carry out investigations in this field to find a good surfactant/polymer to disperse BNNTs. The continuity of this effort underlines its importance and highlights the fact that we have not been sufficiently successful. Although quite some knowledge has been gained in the process, only now it begins to coalesce into understanding. In our view, significant progress in the field can only be achieved through understanding. Applying computer simulations leads us to understand some things at the single tube–tube contact level; also, they can be verified by performing accurate experiments. There are guidelines about exfoliation, solvation, and stabilization of BNNTs in the solution provided by MD simulations. However, they suggest that it is necessary to apply dispersing agents that (1) can be strongly adsorbed on the nanotube surface, (2) present hydrophilic groups that will have a better operation if their rigidness is proved, and (3) cannot have significant movements on the nanotube surface. If dispersing agents become aggregated with a nanotube geometry-dependent structure, then obtaining the nanotube diameter and chirality sorting will be possible.
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