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Synthesis of boron nitride nanotubes and their applications

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Boron nitride nanotubes (BNNTs) have been increasingly investigated for use in a wide range of applications due to their unique physicochemical properties including high hydrophobicity, heat and electrical insulation, resistance to oxidation, and hydrogen storage capacity. They are also valued for their possible medical and biomedical applications including drug delivery, use in biomaterials, and neutron capture therapy. In this review, BNNT synthesis methods and the surface modification strategies are first discussed, and then their toxicity and application studies are summarized. Finally, a perspective for the future use of these novel materials is discussed.
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84
Synthesis of boron nitride nanotubes and their applications
Saban Kalay, Zehra Yilmaz, Ozlem Sen, Melis Emanet, Emine Kazanc
and Mustafa Çulha*
Review Open Access
Address:
Department of Genetics and Bioengineering, Yeditepe University,
Atasehir, 34755 Istanbul, Turkey
Email:
Mustafa Çulha* - mculha@yeditepe.edu.tr
* Corresponding author
Keywords:
boron nitride nanotubes; chemical modifications; medical applications;
synthesis methods; toxicity
Beilstein J. Nanotechnol. 2015, 6, 84–102.
doi:10.3762/bjnano.6.9
Received: 13 May 2014
Accepted: 04 December 2014
Published: 08 January 2015
This article is part of the Thematic Series "Atomic scale interface design
and characterisation: Experimental aspects and methods".
Guest Editor: C. Bittencourt
© 2015 Kalay et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Boron nitride nanotubes (BNNTs) have been increasingly investigated for use in a wide range of applications due to their unique
physicochemical properties including high hydrophobicity, heat and electrical insulation, resistance to oxidation, and hydrogen
storage capacity. They are also valued for their possible medical and biomedical applications including drug delivery, use in bioma-
terials, and neutron capture therapy. In this review, BNNT synthesis methods and the surface modification strategies are first
discussed, and then their toxicity and application studies are summarized. Finally, a perspective for the future use of these novel
materials is discussed.
84
Review
Introduction
Boron nitride nanotubes (BNNTs) are known as structural
analogs of carbon nanotubes (CNTs) but with superior prop-
erties [1-3]. Although they have structural similarities, they
significantly differ in their chemical and physical properties. In
contrast to CNTs, their electrical properties are not dependent
on their chirality and diameter since they have a large band gap
of about 5.5 eV. BNNTs also have excellent radiation shielding
properties when compared to CNTs [4]. Since the BNNTs are
composed of B and N atoms, their electronic structures are
expected to be rather different from that of CNTs. The charge
distribution is asymmetric in B–N bonds in BNNTs as
compared to the C–C bonds in CNTs [5]. The electron density
of B is attracted to the N atoms due to its higher electronegativ-
ity. Thus, the B–N bonds have a partially ionic character, which
causes a gap between the valence and conduction bands. There-
fore, the B–N bonds behave as a wide band gap semiconductor.
Some relevant properties of BNNTs are as follows: high hydro-
phobicity, resistance to oxidation and heat, high hydrogen
storage capacity and radiation absorption. Their electrical insu-
lation is indeed very high, despite a high thermal conductivity
[6]. Due to these properties, they can be used in a wide range of
applications. BNNTs can resist oxidation in air up to 1000 °C
while CNTs are resistant only up to 500 °C under the same
conditions [7]. This makes BNNTs useful additives to increase
Beilstein J. Nanotechnol. 2015, 6, 84–102.
85
stability against the oxidation of surfaces [4]. Due to their
highly hydrophobic character, BNNTs were also used to
prepare super hydrophobic surfaces [8,9]. A hydrophobic
surface was prepared by the synthesis of BNNTs on the surface
of a stainless steel substrate where the contact angle was found
to be more than 170° [8]. The origin of this super hydrophobic-
ity was attributed to the surface morphology and adsorption
capacity of BNNTs for airborne molecules [9].
BNNTs were also used to prepare composite materials to
enhance their physical properties. Bansal et al. fabricated a glass
composite by adding 4 wt % BNNTs and measured the strength
and fracture toughness as 90% and 35%, respectively, which
were greater than that of the constituents [10]. BNNTs also
have significant hydrogen storage capacity, which was
measured as 0.85 wt % – two times larger than that of the
commercial CNTs [11].
The use of BNNTs in medical and biomedical applications has
also been increasingly investigated [12-14]. Their hydrophobic-
ity and toxicity concerns are the two factors that may limit their
use in such applications. Due to their high hydrophobicity,
BNNTs can only be used in biological applications after nonco-
valent [7] or covalent [15,16] modifications to increase their
water dispersibility. Thus, they have been modified with several
surface modifiers such as PEGylated phospholipids [17], and
molecules of biological origin including DNA [18], proteins
[13], and flavin mononucleotides (FMN) [19].
The synthesis of BNNTs was first reported in 1995 [20] by
Chopra, based on an arc discharge method. Following the first
report, several methods including arc discharge [20-22], chem-
ical vapor deposition (CVD) [23-26], substitution reactions [27-
29], ball milling [30-35], laser ablation [36-38], and low
temperature methods [39-41] were reported. The CVD and ball
milling methods are currently the two most widely used
methods for the synthesis of BNNTs.
In this review, the most important BNNT synthesis methods are
summarized first, then in vitro and in vivo studies of their toxi-
city are addressed. Finally, the investigations utilizing BNNTs
in applications such as drug delivery, biomaterials preparation,
biosensors, hydrogen storage, and neutron capture therapy are
summarized by giving examples from the literature.
BNNT synthesis methods
There are several reports on the synthesis of BNNTs. The type
of boron precursor, catalyst, temperature, mode of heat and
duration are the key parameters in the synthesis procedure.
Depending on these conditions, the length and size of the
BNNTs will vary. In this section, a summary of the synthesis
methods and the nature of the generated BNNTs are discussed.
The precursors used in the synthesis of the BNNTs, formation
mechanisms; application areas and physical properties (such as
diameters and length) are summarized in Table 1.
Arc discharge
BNNTs were first synthesized by an arc discharge method
resulting in a 1–3 nm inner diameter and a length of 200 nm
[20]. An arc discharge was generated between a hexagonal BN
(h-BN)-filled tungsten rod as an anode and a cooled copper
electrode as cathode. The dark gray BNNTs were collected
from the surface of the copper cathode. Later, hafnium diboride
(HfB2) electrodes [21], and conductive boron substances such
as YB6 [22] were used to obtain BNNTs using this arc dis-
charge method.
Substitution reaction
Due to the structural similarity between CNTs and BNNTs,
BNNTs can be obtained from CNTs via substitution reactions.
BNNTs have been synthesized in high yields from CNTs and
B2O3 under a N2(g) atmosphere at 1773 K [27]. A represen-
tative substitution reaction is given below.
Another substitution reaction was performed by interacting
aligned carbon–nitrogen nanotubes (CNxNTs) or CNTs and
B2O3 under NH3 atmosphere at 1260 °C for 30 min to synthe-
size BxCyNz/NTs [28]. The BxCyNz/NTs were obtained from
CNxNTs with a higher yield than that of the BxCyNz/NTs
obtained from CNTs. Finally, single-walled carbon nanotubes
(SWCNTs) were used to obtain multi-walled boron nitride
nanotubes (MWBNNTs) by mixing with B2O3 (as the B
precursor) and MoO3 (as the catalyst) under N2(g) atmosphere at
1500 °C for 30 min [29]. Although this method can be used to
produce BNNTs, the outcome is not always pure BNNTs but
rather some B- and N-doped CNTs result in addition [48].
Chemical vapor deposition
Chemical vapor deposition (CVD) is a well-known, econom-
ical method that is widely used for CNT and BNNT synthesis
since it generates high yield products and requires simple
experimental procedures. Lourie et al. performed the synthesis
of BNNTs from borazine (B3N3H6) based on the reaction
provided below [49].
Although Co, Ni, NiB and Ni2B were found to be successful
catalysts for the synthesis, NiB and Ni2B were the most
efficient precursors to obtain the highest BNNT yield. Later,
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Table 1: Reaction conditions, growth mechanisms and applications of BNNTs reported in literature.a
Precursor T [°C];
t [h] Substrate Method Growth
mechanism
Physical
properties Modification Application Ref.
B, h-BN, NH3
<1100;
2iron
deposits
alumina
ball milling
(20 h), CVD base-growth
40–100 nm
diam.,
bamboo-like
– – [42]
1200;
2
40–100 nm
diam.,
cylindrical
shape
B:FeO:MgO
(2:1:1), NH3
1200;
0.5
Si/SiO2mechanic.
mixed CVD
base-growth
30 nm diam.,
random
direction,
closed tip ends
– – [43]1300;
0.5 tip-growth
60 nm diam.,
random
direction,
closed tip ends
1400;
0.5 mixed-growth
10 nm diam.,
flower-like,
closed tip ends
B:FeO:MgO
(1:1:1), NH3
1300;
0.5
tip-/base-
growth
100–500 nm
diam., closed
tip ends
B:FeO:MgO
(4:1:1), NH3
1300;
0.5
tip-/base-
growth
50–150 nm
diam., closed
tip ends
B2O3, CaB6,
Mg, NH3
1150;
6 CVD base-growth 150 nm diam.,
>10 µm length – – [23]
h-BN, N21250–1300;
10
ball milling
(100 h),
CVD
30–60 nm
diam.,
cylindirical
shape, 500 nm
length
covalent with
NH4HCO3
reinforced
material for
Al-matrix
composite
[44]
B, FeO, MgO 1100–1700;
1ball milling,
CVD
metal
catalytic
growth
50–80 nm
diam., up to
10 µm length,
straight
nanowires
noncoval.
polyaniline/
Pt/GOX
amper. glucose
biosensor [45]
B, iron particle,
N2
1100;
15 Si/SiO2ball milling
(50 h), CVD
metal
catalytic
growth
50–200 nm
diam., up to
1 mm length,
bamboo-like
insulators for
electromechanical
systems
[30]
MWCNT,
H3BO3, NH3
1300;
3 substitution – 40–50 nm
diam.
noncoval.
trioctylam.,
tributylam.,
triphenyphos.
gel
nanocomposite [46]
B, Co(NO3)2,
N2, H2
1100;
0.5–3
stainless
steel
ball milling,
CVD – bamboo-like superhydrophobic
surface [8]
B, N21200;
16
ball milling
(150 h),
CVD
20–50 nm
diam.
cylindrical,
cylindrical
capped by iron,
bamboo-like
– – [32]
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Table 1: Reaction conditions, growth mechanisms and applications of BNNTs reported in literature.a (continued)
KBH4, NH4CI,
N2
1200–1300;
5–10 CVD –
10–30 nm
diam., up to
5 µm length,
bamboo-like
– – [24]
B, Fe2O3, NH31200–1300;
2.25 CVD –
64–136 nm
diam.,
bamboo-like
– – [25]
MWCNT,
H3BO3, NH3
1080;
6 substitution –
10–100 nm
diam., 10 µm
length
coval. PVA
and HP-MEC
imp. mechanical
performance of
polymer
[47]
ammon.
borane,
ferrocn., N2
1450;
1
graphite
crucible
(graphite
paper
inner line)
CVD
(large diam.
catalyst)
300 nm diam.,
10 µm length,
bamboo-like
– – [26]
vapor–liquid–
solid (small
diam.
catalyst)
15–200 nm
diam., 100 µm
length,
cylindrical
shape
B, Fe2O3, NH3
600;
1 CVD –
20–60 nm
diam.
hydrogen storage [11]
B,
Fe3+-MCM-41,
NH3
2.5–4 nm diam.
YB6, N2/Ar – arc
discharge mixed-growth
4–10 nm diam.,
4–6 µm length,
closed or open
tip
– – [22]
aPVA: polyvinyl alcohol, HP-MEC: hydroxypropyl methylcellulose.
Ma et al. demonstrated that BNNTs could be synthesized
without any metal catalyst using melamine diborate
(C3N6H62H3BO3) [50]. A comprehensive study also showed
that B3N3H6 and decaborane could be used as precursors [51].
In a recent study, a large scale, high-yield BNNT synthesis
method was demonstrated based on CVD using boron and metal
oxides to produce so-called BOCVD methods [43,52-55]. The
chemical mechanism of BOCVD [43] is shown below.
This mechanism is called vapor–liquid–solid (VLS). The high
yield of BNNTs was observed as a white powder in the inner
wall of the aluminum boat and on the substrate. Note that for
the BOCVD mechanism to occur, variation in the types of cata-
lysts, boron compounds and nitrogen-containing gases should
be used.
The first successful synthesis of patterned BNNTs was
performed by catalytic CVD [56]. To produce pure and verti-
cally aligned BNNTs, a Si substrate was coated with Al2O3 of
30 nm thickness, then MgO, Ni, or Fe catalysts was deposited
on the surface of the Al2O3 by pulsed laser deposition. This
substrate was placed into a quartz tube with one end closed. The
quartz tube was placed in a tube furnace for the growth of high
yield BNNTs at 1100–1200 °C based on the growth vapor trap-
ping (GVT) mechanism as seen in the SEM image in Figure 1.
The latest BOCVD method was studied by Nithya et al. [57].
They claim that large-scale production of BNNTs can be
obtained using a mixture of B/V2O5/Fe2O3 and B/V2O5/Ni2O3
as precursors. In this experiment, the diameter and length of
BNNTs was controlled and various BN nanostructures were
obtained [57].
Recently, our group synthesized BNNTs from a boron ore, cole-
manite (Ca6B6O115H2O), for the first time by means of CVD
[58]. The reaction parameters such as type of catalyst, cole-
manite/catalyst ratio, reaction temperature and duration were
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Figure 1: SEM images of BNNTs grown based on a CVD method. (a) Experimental setup, (b) stretching of dense BNNTs from the sample surface,
(c) high magnification SEM image of BNNTs, (d) SEM images of slightly compressed BNNTs on a Si substrate, and (e) cross-sectional view of verti-
cally aligned BNNTs. Figure adapted with permission from [56], copyright 2010 American Chemical Society.
optimized. ZnO, Al2O3, Fe3O4 and Fe2O3 catalysts were
investigated with respect to their differences in performance. It
was found that only Fe2O3 was effective as a catalyst. Figure 2
shows the SEM images of the results of the BNNT synthesis
under several experimental conditions. The synthesized
MWBNNTs were in the range of 10–30 nm in diameter, with a
5 nm wall thicknesses and 0.34 nm between walls. This simple
method can be used to synthesize pure MWBNNTs on a large
scale. The mechanism of BNNT formation was from base-
growth, which included the conversion of Fe2O3 into metallic
iron, the formation of an initial complex between metallic iron
and BN, and the growth of a BN core into BNNTs on the
surface of the metallic iron when the surface was supersatu-
rated with B and N atoms.
In addition to the base-growth mechanism, BNNTs were also
formed using the tip-growth, mixed-growth, and metal-
catalytic-growth mechanisms (Table 1). During the annealing
step in the metal-catalytic-growth mechanism (same as for base-
growth), B atoms diffuse into the catalyst particle while the N2
is decomposed to N atoms on the surface of the catalyst. The
precursors precipitate layer-by-layer to form the BNNTs [30].
In the tip-growth mechanism, the catalyst is located on the tip of
growing BNNTs [59]. Thus, the BNNTs are generally formed
as bamboo-like structures.
The base- and tip-growth mechanisms of the BNNTs were
revealed by TEM showing that the tube diameter and catalysts
ratio were the important factors for the selection of growth
mechanisms [43]. A ratio of 8:1 colemanite:catalyst caused the
formation of BNNTs with a large diameter, thickness and a
zigzag structure (Figure 2e) [59]. Under the similar experi-
mental conditions with high catalyst content, the formation of
thick BNNTs was reported [43].
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Figure 2: SEM images of the BNNTs products at the different reaction time and colemanite/catalyst ratios (w/w) after CVD application. The respec-
tive reaction time and colemanite/catalyst ratio (w/w) were (a) 30 min and 12:1, (b) 60 min and 12:1, (c) 120 min and 12:1, (d) 120 min and 32:1, and
(e) 120 min and 8:1. (f) Boat surface after removal of the BNNTs, at 120 min and with a ratio of 12:1.
Ball milling method
Ball milling is commonly used in many studies to obtain a high
yield of BNNTs [30]. The main objective of the ball milling
method is to increase the surface area to bring the catalyst,
boron and nitrogen precursors into contact as much as possible
[32]. Although the impurities originating from the steel surface
may interfere with the reaction, the synthesis of high-yield
BNNTs was claimed [31]. The ball milling method allows
transfers a high amount of mechanical energy to the boron
powder, which results in an increased surface area and
increased number of contact points among the catalyst, boron
and nitrogen precursors, resulting in improved yield and prod-
uct quality [31]. The structural changes in the boron com-
pounds in the reaction mixture during ball milling were
observed using X-ray diffraction [33]. It was also reported that
there was no chemical reaction between NH3 and boron powder
during the ball milling process [33]. In the process, the boron
precursor is ground either with or without catalyst. However,
there is no report comparing both approaches. Li et al. reported
that only amorphous boron was ball milled under a NH3 atmos-
phere (300 kPa) for 150 h, at 155 rpm [60]. The milled boron
and Fe(NO3)3 were sonicated in ethanol for 30 min followed by
an annealing step under an 85% N2 and 15% H2 gas mixture.
Bamboo-like, 40–80 nm diameter BNNTs were synthesized.
Lim et al. reported that amorphous boron powder and NiBx
were ball milled in the presence of NH3 atmosphere for 48 h at
180 rpm and annealed in the presence of N2 and H2 gases [34].
Bamboo-like BNNTs of 20–40 nm diameter and >250 nm
length were obtained. Recently, Li et al. reported that the ball
milling of amorphous boron powder and Fe(NO3)39H2O is a
more effective precursor for a high-yield BNNT synthesis [35].
It was possible to obtain relatively long BNNTs, up to 1 mm in
Beilstein J. Nanotechnol. 2015, 6, 84–102.
90
length, using ball milling annealing technique [30]. These long
BNNTs were produced over the duration of a 50 h annealing
step in the presence of N2 gas at 1100 °C.
Laser ablation method
The synthesis of single- or double-walled BNNTs can general-
ly be achieved using laser ablation [36-38]. It was reported that
the only way to synthesize single-walled BNNTs (SWBNNTs)
was by the laser vaporization method [37]. Golberg et al.
synthesized pure BNNTs with 3–8 walls from BN under high
N2 pressure [36]. A single crystalline, 1–20 μm thick BN sub-
strate and a CO2 laser with a spot size of 80 µm diameter and
240 W power were used to synthesize pure BNNTs at tempera-
tures higher than 5000 K. Naumov et al. synthesized
MWBNNTs using a continuous wave CO2 laser at 500 W
power based on a BN substrate [61] and Lee et al. obtained
gram quantities of SWBNNTs using continuous wave CO2
laser [62].
BNNT synthesis methods at low temperatures
BNNT synthesis at low temperature is of great interest since it
reduces the synthesis cost. However, the temperature has an im-
portant effect on the formation mechanism of the BNNTs
[42,43]. It has been reported that the BNNT formation mecha-
nism and tube diameter can be varied depending on the
temperature or substrate/catalyst ratio [42,43]. Bae et al.
obtained exclusively bamboo-liked BNNTs when processed at
temperatures below 1100 °C and cylindrical-shaped BNNTs
at 1200 °C from the ball-milled boron and FeCI24H2O
precursors [42].
At low temperature, Xu et al. produced 60–350 nm diameter
MWBNNTs at 450–600 °C with 50% yield [39]. Wang et al.
synthesized BNNTs by means of microwave plasma at a
temperature lower than 520 °C [40]. In this technique, a
6–100 nm pore size, aluminum oxide template was used along
with microwave plasma. The BNNTs were grown on the
surface of this template in the presence of B2H6/Ar and NH3/N2
at 104 Pa pressure at 520 °C. The diameter of the synthesized
BNNTs was the same as the pore size diameter of the aluminum
oxide template. The BNNTs were synthesized in a stainless
steel autoclave at 380 °C from amorphous boron, NaN3, and
CH3CN for 14 h. The obtained product was washed with
ethanol, dried and a 5% BNNTs yield was calculated [41].
Modifications
Although the BNNTs have several unique properties, they are
highly hydrophobic and difficult to use when an aqueous media
is involved. Significant effort has been dedicated to increase the
dispersibility of the BNNTs in aqueous media to extend their
applicability to a variety of fields including medicine and
biomedical applications. Two approaches to alter the surface
properties of BNNTs are commonly employed: one is through
covalent attachment of a molecule or molecular structure, and
the other involves the physical adsorption of a molecular struc-
ture or a polymer onto BNNT surfaces. The chemical modifica-
tion can be achieved through the –OH groups on the B atom and
the –NH2 groups at the edges or defects of the BNNTs. There
are several polymeric structures that can be physical adsorbed
onto the BNNTs to generate coated nanotubes or composites. In
this section, examples of these two routes are addressed.
Chemical modifications
Due to their high resistance to harsh chemical conditions,
BNNTs are consequently difficult materials for covalent func-
tionalization (similar to CNTs). However, recent studies
demonstrate that covalent modification is possible. The most
commonly preferred chemical functionalization is through the
–OH on B atoms and –NH2 groups at the edges and defects, as
shown in Figure 3 [15,44,63-65].
In one study, BNNTs were functionalized with amine groups by
ammonia plasma irradiation [63]. It was predicted that NH2
radicals produced by NH3 plasma attached to B atom at the
defects and edges whereas H radicals attached to the N atoms
of BNNTs. It was shown the amine-functionalized BNNTs (AF-
BNNTs) were dispersible in chloroform. To investigate further
chemical functionalization, the AF-BNNTs were coated with
3-bromopropanoyl chloride (BPC) via amide groups of acid
chloride of BPC molecules and the amine groups of BNNTs.
The study concluded that formation of amine groups on BNNT
surfaces was helpful to increase the stability of molecules and
polymers on the BNNTs surfaces [63].
3-aminopropyltriethoxysilane (APTES) is a widely-employed,
aminosilane used in many applications. Ciofani et al. used
APTES as an agent for silica coating to functionalize BNNTs
[15]. For cellular uptake studies, a fluorescent dye, Oregon
Green 488 carboxylic acid, succinimidyl ester was covalently
bound to the functionalized BNNTs. The NIH/3T3 fibroblast
cells were treated with this fluorescent dye labeled with the
functionalized BNNTs. The study found that the labeled
BNNTs were localized in the cytosol (Figure 4) [15].
A method to functionalize N–H at the defects on BNNTs with
the –COCl groups of stearoyl chloride was successfully imple-
mented [66]. This functionalization increased the dispersibility
of the BNNTs in many organic solvents including chloroform,
ethanol, acetone, toluene, N,N-dimethylacetamide, N,N-
dimethylformamide, and tetrahydrofuran. BNNTs functional-
ized with proteins may increase their potential applications in
the field of nanomedicine. The covalent grafting of BNNTs
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Figure 3: Summary of chemical modification routes of BNNTs.
Figure 4: Low (a) and high (b) magnification confocal images of fluorescently labeled, functionalized BNNTs, where red, green, and blue are the
cytoskeletal actin, functionalized BNNT, and nuclei, respectively. Figure adapted with permission from [15], copyright 2012 Elsevier.
with human transferrin, linked through a carbamide bond,
was reported [67]. The transferrin–BNNTs were tested on
primary human umbilical vein endothelial cells (HUVECs)
to investigate their cellular uptake. It was concluded that the
functionalization of the BNNTs via a targeting protein could
generate smart and selective nanocarriers to be used in
nanomedicine [67].
Physical modifications
For these types of modifications, weak interactions such as ππ,
hydrophobic, and van der Waals forces are utilized to coat the
BNNTs with mostly a polymeric material. Poly[m-phenylene-
vinylene-co-(2,5-dioctoxy-p-phenylenevinylene)] (PmPV) was
used to cover BNNTs via ππ interactions [7]. The prepared
structure was more soluble in chloroform and ethanol than
water. It was claimed that this composition had potential use in
optical devices because of its luminescence properties and good
dispersibility in organic solvents [7].
The BNNTs were also coated with PEGylated phospholipid
1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-
[methoxy(poly(ethylene glycol))] conjugates (mPEG–DSPE)]
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Figure 5: TEM images of ferritin molecules immobilized onto BNNT
surfaces (a), EDS spectrum of BNNTs with immobilized ferritin mole-
cules (b), ferritin molecules on the surface and inside of a BNNT (c).
Figure adapted with permission from [13], copyright 2005 American
Chemical Society.
[17]. This polymer was selected due to its water solubility and
biocompatibility. The mPEG–DSPE/BNNTs suspension was
expected to be stable in water because fatty acids from DSPE
should noncovalently interact with BNNTs and the hydrophilic
mPEG could aid in the dispersion of the BNNTs in water.
Indeed, the mPEG–DSPE/BNNTs were highly dispersed in
water and slightly so in ethanol, acetone, methanol and chloro-
form. Furthermore, it was noted that sonication for long time
periods (hours) could help to better disperse the BNNTs [17].
FMN is derived from vitamin B2 and is a well-known phos-
phorylated biomolecule. The interaction of vitamin B2 with
BNNTs resulted in a highly fluorescent FMN–BNNT complex
under daylight and UV light irridation [19]. Furthermore, the
fluorescence from this complex was thermally stable and
pH-dependent. It was suggested that FMN–BNNT nanohybrids
could be used for biomedical imaging.
The adsorption of ferritin onto BNNTs was also reported. It was
found that there was a natural affinity of this protein to the
BNNT surfaces [13]. Figure 5 shows TEM images of ferritin
immobilized onto the BNNTs. As seen, the ferritin not only can
be adsorbed onto the BNNT surface but can also penetrate into
the BNNTs. In addition, 1-pyrenebutyric acid N-hydroxysuccin-
imide ester (PAHE) was used to obtain more efficient
immobilization. Because the PAHE has aromatic pyrenyl
groups in its structure, a strong ππ interaction between the
BNNT surface and PAHE is expected. When PAHE was used, a
denser ferritin immobilization was observed [13].
A computational investigation concerning the interactions of
BNNTs with tryptophan (Trp), aspartic acid (Asp), and argi-
nine (Arg) was also carried out [68]. It was found that the polar
Asp and Arg interacted with the BNNT surface through the
charge transfer and electrostatic interactions while Trp, a neutral
amino acid, had no interaction with the surface of the BNNTs.
This study provides a deeper understanding into the nature of
the interactions of amino acids (and perhaps similar molecules)
with surface of the BNNTs [68].
The interaction of a peptide, HWSAWWIRSNQS, with BNNTs
was studied using AFM [69]. It was found that the
peptide–BNNT structure had an excellent dispersibility in water
since the BNNTs were covered by this peptide. The study
revealed that the presence of Trp (W), which has a benzene ring
in the peptide sequence, exhibited a strong ππ interaction with
the BNNT surfaces [69].
Dendrimers prepared from synthetic carbohydrate ligands were
used to coat the BNNTs to mimic the cell surface receptors. The
[G-2] dendrimers possessing R-mannose moieties ([G-2] Man)
[70] were used to coat the BNNTs. Although uncoated BNNTs
were precipitated very quickly, the [G-2] Man-coated BNNTs
formed a stable suspension in water for weeks. The carbohy-
drates provide specific molecular recognition sites on cell
membranes. The [G-2] Man-coated BNNTs were incubated
with the R-mannose-specific receptor Canavalia ensiformis
agglutinin (Con A). To observe the fluorescence of this com-
plex, they were conjugated with fluorescein isothiocyanate
(FITC). Chinese hamster ovary (CHO) cells were treated with
this complex. It was found that the coated BNNTs had specific
molecular recognition capability [70].
Glycol chitosan (GC) is widely used due to its biocompatibility
and good solubility over a broad pH range [71]. The BNNTs
were coated with GC during a 12 h sonication process. The
TEM results indicated that the GC–BNNTs had two different
configurations: bamboo-like shaped and noncontinuous walled.
HUVECs were treated with the GC–BNNTs and the cellular
uptake of the GC–BNNTs was observed. However, the uptake
mechanism remains unclear and it might be worthwhile to
further investigate it [71].
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BNNT-grafted, poly(glycidyl methacrylate) and polystyrene
brushes were prepared via atom transfer radical polymerization
[72]. The resulting nanocomposite material was characterized
using FTIR, TGA, SEM and TEM. The TEM images clearly
show the formation of polymer grafts on the BNNT surface.
Toxicity of BNNTs
The potential adverse effect of nanomaterials on living systems
is a growing concern. Although many engineered nanomate-
rials (ENMs) are already in use in several applications, there is
no clear consensus regarding their possible impacts on living
systems and the environment. The main reason behind this
uncertainty is the lack of significant data on the subject. In addi-
tion, the diversity of nanomaterials and parameters further adds
to the uncertainty for the proper assessment of the safety of
these novel materials. Similar to many other ENMs, there are
several issues with the assessment of the possible toxic effects
of BNNTs. In early studies, there was no clear consensus
regarding their cytotoxicity. In some reports it was found that
BNNTs were toxic [73], and in others, not [74]. Naturally, first,
in vitro studies were undertaken to assess the toxicity of the
BNNTs. Similar to CNTs, one of the major problems in toxi-
city assessment of BNNTs is their low dispersibility in aqueous
media, due to their high hydrophobicity. In order to increase the
dispersion, either a surfactant or hydrophilic polymer is used to
alter the surface properties. However, this process adds further
uncertainties to the assessment since another material is intro-
duced into the system. For example, polyethylenimine (PEI) is a
cytocompatible polymer and principally used for DNA transfec-
tion and cell permeabilization. The BNNTs were coated with
PEI for dispersion in aqueous media for biological applications
[12]. The effect of the PEI-coated BNNTs with respect to
viability, metabolism and cell proliferation of human neuroblas-
toma cell line (SH-SY5Y) was investigated. The PEI-coated
BNNTs exposed cells analyzed at different time intervals. The
viability, metabolic activity and proliferation of the cells were
analyzed with MTT and trypan blue assays. The results showed
that the PEI-coated BNNTs did not affect the viability of
neuroblastoma cells up to 5 µg/mL [12].
Lahiri et al. studied the behavior of osteoblast cells in a scaf-
fold constructed from the BNNT-embedded polylactide-poly-
caprolactone (PLC–BNNT) in orthopedic implants [75]. Using
real-time PCR methods, they studied the RunX2 gene expres-
sion profile, which is a transcription factor responsible for
enhancing the cell proliferation. The results of the experiments
showed that the PLC–BNNTs increased the RunX2 gene expres-
sion of the osteoblast cells up to sevenfold. It was concluded
that the positive effect of the BNNTs embedded into the scaf-
fold on the cell proliferation was due to the natural affinity of
proteins to the hydrophobic BNNTs [75].
Ciofani et al. investigated the cytotoxicity of GC–BNNTs [74].
The production of reactive oxygen species (ROS), DNA content
in cell lysates, and apoptosis of cells were assessed using
SH-SY5Y cells. The cells were exposed to GC–BNNTs up to
100 µg/mL. They found that the GC–BNNT-dependent toxic
concentration was lower than the 50 µg/mL. On the other hand,
100 µg/mL of GC–BNNTs significantly decreased the cell
viability. It was also found that the ROS production was not
significant [74].
The hemolytic and cytotoxic effects of pure BNNTs on the
malignant U87 (wild type p53), T98 (mutant p53) glioblastoma,
MCF-7 adenocarcinoma mammary gland cells and normal
MRC-5 fibroblast lung cells were investigated [76]. The hemo-
lytic activity of the pure BNNTs was investigated by UV–vis
spectroscopy and the results showed no significant hemolysis of
cells that were exposed to the BNNTs. The metabolic activity of
the BNNT-exposed cells using an MTT assay was studied and
found that the BNNTs were significantly toxic at 200 µg/mL.
The biocompatibility tests indicated that the pure BNNTs were
good candidates at nontoxic concentrations for pharmacolog-
ical applications [76].
The cell lines A549, RAW264.7, 3T3-L1 and HEK293 were
exposed to BNNTs. The authors studied the cells with MTT and
fluorometric microculture cytotoxicity assay (FMCA). As an
indirect cytotoxicity measurement technique, FMCA assesses
the esterase activity of cells. In general, the results indicated
that the BNNTs were cytotoxic for the studied cell types even at
low concentrations. In addition, the authors evaluated the toxi-
city of BNNTs according to the cell type and endocytosis ability
of the cells. In particular, RAW 264.7 had high endocytosis
capability as compared to A549 and 3T3-L1 cell types, and
HEK293 showed low endocytosis capability. The results
showed that the BNNTs slightly affected the growth and meta-
bolic activity of HEK293 cells [77]. On the other hand, the
effect of the BNNTs on A549 and 3T3-L1 cell types was worse
than that of the HEK293 cells. The BNNTs were highly toxic
for the RAW 264.7 cell type. From this study, one can be
conclude that the toxicity of BNNTs is related to the cell type
and the ability to perform endocytosis [77].
The morphology and viability of the GC–BNNTs-exposed
HUVECs cells were also investigated [71]. The cells were incu-
bated at increasing concentrations of GC–BNNTs for 48 and
72 h. The cell morphology and cell viability by amido black
assay and trypan blue were studied. The total protein content
and E1/1 protein expression profile were determined. It was
found that a 50 µg/mL concentration of GC–BNNTs added to
the cell medium caused a decreased proliferation. On the other
hand, a 100 µg/mL concentration of GC–BNNTs showed a
Beilstein J. Nanotechnol. 2015, 6, 84–102.
94
Table 2: Toxicity behavior of the BNNTs on cultured cells and animals.
Physical coating In vitro / In vivo Type of assay Result Ref.
PEI SH-SY5Y Trypan Blue, MTT Nontoxic at 5 µg/mL [12]
glycol chitosan SH-SY5Y MTT, WST-1, Apo. kit, Image-IT
Green ROS kit
Low toxicity <100 µg/mL [74]
PLC Osteoblast cells (RunX2) Real-time PCR Increased cell growth [75]
nonfunctionalized U87, T98, MCF-7, MRC-5 MTT Low toxicity <200 µg/mL [76]
nonfunctionalized A549, RAW264.7, 3T3-L1, HEK293 MTT, FMCA Related to cell type [77]
glycol chitosan HUVECs Amido Black assay, Trypan Blue Nontoxic <50 µg/mL [71]
glucosamine, PEG,
chitosan
MRC-5 MTT Nontoxic <50 µg/mL [78]
PLL C2C12 Trypan Blue, MTT, LIVE/DEAD,
annexin V-FITC
Low toxicity <10 µg/mL [79]
PLL hOB MTT Low toxicity <10 µg/mL [80]
glycol chitosan Rabbit Blood tests Nontoxic [81,82]
modestly reduced proliferation at hours 48 and 72. Similar to
the results reported by Ciofani et al., the GC–BNNTs were
nontoxic at low concentrations [71].
The toxicity of glucosamine (GA)-, poly(ethylene glycol)1000
(PEG1000)- and chitosan (CH)-coated BNNTs using MRC-5
cells was studied [78]. The study found that the BNNT–CH and
BNNT–PEG were nontoxic in the range from 0.1 to 50.0 µg/mL
but they were significantly toxic at 100 µg/mL. It was con-
cluded that the GA–BNNT, PEG–BNNT and CH–BNNT were
nontoxic at low concentrations but further evaluation on more
cell types was suggested for their reliable use in biomedical
applications [78]. The GA–BNNTs were found to be nontoxic
to MRC-5 cells, but the previous studies claimed that
glucosamine prevented the cellular uptake of glucose by pancre-
atic beta cells and caused cell death [73].
Poly-L-lysine (PLL) is known for its cytocompatibility due to
the presence of amino groups and its use as a good dispersion
agent [79]. The BNNTs were first coated with PLL and then
with quantum dots (QDs) to observe the cellular uptake of
PLL–BNNTs in C2C12 mouse myoblast cells. To observe the
energy dependence of the uptake mechanism of the
PLL–BNNTs, sodium azide was used to block ATP. They con-
cluded that the PLL–BNNTs accumulation occurred in the cell
membrane with energy dependent pathways. At a concentration
of up to 10 µg/mL, the PLL–BNNTs exhibited no evidence of
apoptosis, necrosis and membrane permeabilization [79].
Danti et al. investigated the cellular uptake of the BNNTs in the
primary human osteoblasts (hOBs) under the exposure of low
frequency ultrasound (US) [80]. The BNNTs were wrapped
with PLL for stabilization in water. Furthermore, they found
that the PLL–BNNTs were localized in the cytoplasm in the
vesicles. Although the experimental system was complex, and
further studies were necessary to understand the molecular
mechanism, an obvious result was the increasing osteoblastic
maturation. The findings of this study indicated that the BNNTs
had potential use as nanotransducers for cellular therapies [80].
Ciofani et al. first reported a pilot investigation on the in vivo
toxicity of BNNTs and they applied a single intravenous dose of
BNNTs at 1 mg/mL. The changes in blood, kidneys and liver
parameters were observed [81]. In the following study, 5 and
10 mg/kg of the BNNTs were intravenously injected into
rabbits [82]. After the injection, the white blood cells, red blood
cells and hematocrit levels were analyzed to investigate the
possible adverse effects of the BNNTs. The authors did not see
any adverse toxicological effect of BNNTs in blood [82].
Some of the important studies from the literature are summa-
rized above and some of the toxicity assessment attempts are
provided in Table 2. As seen, a number of reports claim that the
BNNTs are nontoxic. Since the BNNTs are highly hydrophobic,
it is difficult to perform toxicity assays for these materials.
Therefore, a surface modification approach is generally
performed to increase their dispersibility in aqueous media
(Table 2). However, this may mask the real toxicity of the
BNNTs since mostly the surface coating comes in contact with
the cells. Based on the reports to date, their toxicity depends on
the concentration, cell type and surface modifications, as is the
case for all nanomaterials. A possible reason for the disagree-
ment among the reports could arise from the synthesis proce-
dure of the BNNTs, since the chemicals for the synthesis vary
from synthesis to synthesis. Therefore, a careful purification
step is vital for the further use of the BNNTs after the synthesis.
Finally, although all in vitro studies provide very valuable data
for the toxicity assessment, evaluation of this novel material
Beilstein J. Nanotechnol. 2015, 6, 84–102.
95
under in vivo conditions is critically important. At the moment,
a lack of in vivo data is one reason a solid conclusion about the
toxicity of the BNNTs cannot be drawn.
Drug delivery
The use of nanomaterials in drug delivery has also been investi-
gated in recent years. Although the BNNTs can potentially be
used for drug delivery, their behavior in biological environment
is not very well understood at the moment. However, there are a
few early reports and a review is included here.
A multifunctional, BNNT-based nanocarrier was synthesized
using an Fe catalyst to render them magnetic and coated with
PEI whereby QDs were attached to the PEI-coated BNNTs [83].
The cellular uptake of the BNNTs was tracked by QDs which
revealed that the cellular uptake of the BNNTs depends upon
the distance from the external magnetic field. A supercon-
ducting quantum interference device (SQUID) magnetometer
analysis was carried out and revealed that the magnetic prop-
erties of the BNNTs were related to the Fe catalysts. Consid-
ering the magnetic properties and the ability to bind molecules
on a large surface area of the BNNTs, it was concluded that the
constructed structure was ideal for drug delivery purposes [83].
The core–shell structures of BNNTs with europium-doped,
sodium gadolinium fluoride (NaGdF4:Eu) were fabricated by
using urea to demonstrate the chemotherapy efficiency of the
BNNT–NaGdF4:Eu composites in the presence and absence of
a magnetic field [84]. The BNNT–NaGdF4:Eu composites
simultaneously show fluorescent and magnetic properties. Thus,
imaging and targeting of the composites can be more easily
achieved. Human LNCaP prostate cancer cells were treated with
the BNNT–NaGdF4:Eu composites in the presence and absence
of a magnetic field and higher cell-associated uptake was found
in the presence of a magnetic field. Then, the composites were
loaded with doxorubicin (dox) to investigate the viability of
LNCaP prostate cancer cells in the magnetic field. It was found
that dox-loaded BNNT–NaGdF4:Eu composites had higher
toxicity in the presence of a magnetic field due to increased
cellular uptake of the composites and thus increased doxoru-
bicin delivery. It can be said that the BNNT–NaGdF4:Eu
composites increase the chemotherapy efficiency by the use of
an external magnetic field [84].
The BNNT–mesoporous silica (BNNT–MS) hybrids were
reported for their use in drug delivery [85]. In this study, a
negatively and positively charged BNNT–mesoporous silica,
BNNT–MS and BNNT–MS–NH2, respectively, were produced.
These structures were loaded with dox. The loading efficiency
was found to be pH dependent and both negatively and positive-
ly charged structures had the same dox loading capacity. The
BNNT–MS–NH2 had higher uptake potential in LNCaP
prostate cancer cells due to its charge. Thus, it had a higher
toxicity towards LNCaP prostate cancer cells. It was concluded
that the prepared structures have potential in cancer therapy
[85].
To investigate in vivo biodistribution of the BNNTs, they were
functionalized with GC then radiolabeled with 99mTc [86].
After 30 min of injection into mice, the BNNTs were in the
systemic circulation and accumulated in the liver, spleen and
intestinal tissues. After 4 h, radiolabeled BNNTs were found in
the bladder. They were eliminated from body by renal excre-
tion and the BNNTs conjugated with chitosan were degraded
through enzymatic degradation. After 24 h, there was a reduc-
tion of radiation in the related organs (Figure 6). These
promising results showed that the BNNTs have potential to
carry new drugs or radioisotopes [86].
Electroporation is used for increasing the cell permeability for
introduction of molecules into cells. However, it requires a high
voltage, which is one of the problems for drug delivery. The ap-
plicability of BNNTs in an electroporation process was investi-
gated [87]. The BNNTs were stabilized in phosphate buffered
solution (PBS) with the help of PLL. Before applying electropo-
ration to human neuroblastoma SH-SY5Y cells, the cells were
incubated in the BNNT suspension, and then the electropora-
tion was performed. The results demonstrated that a low elec-
tric field was adequate for electroporation. The BNNTs acted as
mediators for electroporation as they interacted with the cell
membrane. These experimental findings indicated that the
BNNTs are promising tools for drug and gene delivery using
electroporation [87].
A theoretical study revealed that platinum-based anticancer
drugs preferentially interacted with Al-doped BNNTs as
compared to pristine, zigzag and armchair BNNTs [88].
Cisplatin (cis-Pt) and nedaplatin (neda-Pt) molecules were used
as platinum-based anticancer drugs. An aluminum (Al) atom is
substituted for a boron atom. The Al atom induced a protrusion
out of the plane of the BNNT and a distortion occurred at the
doping site to relieve the stress. Density functional theory was
performed to observe the absorption of cis-Pt and neda-Pt on
pristine and Al-doped BNNTs. The results indicated that the
chlorine atom of cis-Pt and the oxygen atom of the carbonyl
group of neda-Pt interacted with the Al-doped BNNTs [88].
Biomaterial applications
The addition of BNNTs into a polymeric matrix could increase
the physical strength, degradation rate and durability of the final
product. Therefore, their use in polymeric biomaterials was
investigated.
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96
Figure 6: Scintigraphic image of radiolabeled, glycol chitosan BNNTs after (a) 30 min, (b) 1 h, and (c) 4 h after injection. Figure adapted with permis-
sion from [86], copyright 2012 Elsevier.
In one study, the BNNTs were used in polylactide-polycapro-
lactone (PLC) copolymer as additives to improve the properties
of the polymer as an orthopedic implant [75]. With the addition
of BNNTs, a 1370% increase in the mechanical strength of the
polymer was observed. The reason for such an improvement
was attributed to the formation of BNNT bridges among the
polymeric structures. Figure 7 shows such structures as marked
in red. When the osteoblast cell viability was evaluated with
respect to the BNNT–PLC composite, and compared only to
PLC, an increased cell viability was observed. In addition, it
was found that the RunX2 gene, which is the major regulator for
the differentiation of the osteoblasts cells, increased the expres-
sion level with the addition of the BNNTs into the polymer.
These results indicated that BNNTs plays an important role to
improve mechanical properties of a scaffold and up regulated
the gene expression for increased cell viability for orthopedic
applications [75].
Hydroxypatite (HA) is an important material used in ortho-
pedic implant applications [89]. 4 wt % of BNNTs were added
into HA to improve its mechanical properties. It was found that
compared to HA alone, an increase in elasticity of up to 120%,
a 129% increase in hardness, and an 86% increase in fracture
toughness were possible. The BNNT–HA composite also
showed a 75% increase in the wear resistance. It was noted that
the addition of BNNTs to HA did not have any adverse effect
on osteoblasts cell proliferation and viability [89].
Sensing applications
One interesting application area of BNNTs is the field of
sensors. Although there are not many reports regarding their use
in sensors, a few available reports are included here as exam-
ples. The unique properties of the BNNTs can be combined
with the properties of other nanomaterials to construct novel
sensor devices for humidity, carbon dioxide detection, and clin-
ical diagnostics.
A highly sensitive humidity sensor using BNNTs and silver
nanoparticles (AgNPs) for the rapid detection of humidity was
fabricated [90]. Figure 8 shows the humidity sensor system. The
adsorption–desorption tests showed that the AgNP–BNNT ma-
terial had a potentially fast response/recovery time of 100/15 s
for the detection of relative humidity at room temperature [90].
The use of Ni-encapsulated BNNTs in optomagnetic-based
sensors with respect to their magnetic and optical band gap
properties was evaluated [91]. Two intense blue emission peaks
at ~480 nm and ~365 nm were observed upon encapsulation of
BNNTs with Ni. The time-resolved photoluminescence spec-
troscopy (TRPL) provided a photoluminescence spectrum with
a bi-exponential decay of 280 ps. It was suggested that the
Ni-encapsulated BNNTs could be used in clinical diagnostics
and bioimaging applications due to their TRPL properties [91].
Hydrogen storage in BNNTs
Hydrogen is considered to be an exceptional energy source ma-
terial since it produces clean energy in high yield. Although
there are several techniques that can generate an abundant
amount of hydrogen, its storage and transportation is an
obstacle for its widespread use. In this respect, BNNTs were
also investigated for their hydrogen adsorption capacity. Molec-
ular dynamics simulations indicated that the collision and
Beilstein J. Nanotechnol. 2015, 6, 84–102.
97
Figure 7: Preparation process of PLC (left) and PLC–BNNTs (right) and (a,b) SEM images of a PLC–BNNT composite exhibiting improved mechan-
ical properties due to BNNTs bridges (red). Figure adapted with permission from [75], copyright 2010 Acta Biomaterialia.
Figure 8: Schematic representation of a humidity sensor test system with a single BNNT and a single BNNT–AgNPs. (a) SEM image (left) and EDS
spectrum (right) and (b) TEM and HRTEM image of the BNNTs, (c) and (e) the SEM images with a single BNNT and single AgNP–BNNT, (d) and
(f) the higher magnification SEM images in (c) and (e) marked with red square (f) AFM (upper) and TEM (lower) images. Figure adapted with permis-
sion from [90], copyright 2013 Elsevier.
Beilstein J. Nanotechnol. 2015, 6, 84–102.
98
adsorption behavior of hydrogen molecules in single-walled
BNNTs varies according to hydrogen incident energy. Addi-
tionally, the theoretical investigations showed that the B–N
bond length (1.46 Å) of the BNNTs was larger than the C–C
bond length of CNTs (1.42 Å). Therefore, the penetration of
hydrogen molecules through the BNNT walls is faster than that
through the CNT walls [92]. On the other hand, the B–N
heteropolar bonding structure of the BNNTs induced an extra
dipole moment on the hydrogen molecules contrary to the
CNTs [34]. In addition, there are experimental reports
supporting the simulation studies [11]. For the BNNTs, which
were synthesized with Fe3+/MCM-41 (mobile composition of
matter) complex catalysts, the percentage of the adsorbed
hydrogen molecules was two times larger than for the commer-
cial CNTs. The adsorption capacity of hydrogen molecules by
BNNTs was measured as 0.85 wt % by the Intelligent Gravi-
metric Analyser at room temperature [11].
Margulis at al. theoretically investigated the preferable adsorp-
tion sites on the BNNT surface by using the semi-empirical
AM1 method [93]. The computational models showed that the
hydrogen atoms favorably bonded to the nitrogen atoms as
compared to boron atoms due to the higher electronegativity of
the nitrogen atoms. Because of the high electronegativity of the
nitrogen atoms, the hydrogen atoms can come into closer
contact than the boron atoms can, but with the same energy.
The simulations indicated that the diameter was an important
property of the BNNTs to increase the amount of stored
hydrogen molecules [93].
The binding positions of the hydrogen molecules on the surface
of the SWBNNTs were also investigated [94]. It was claimed
that the hydrogen molecules had a capacity to bind to the
SWBNNTs as perpendicular, longitudinal and transversal posi-
tions in an ab initio theoretical study. The hydrogen molecules
approached the SWBNNTs in perpendicular position to the
surface, which slightly polarized and raised the binding energy
of the molecules. On the other hand, the hydrogen atoms inter-
acted with the nitrogen atoms more than the boron atoms [94].
The number of walls and the diameter of each wall affect the
hydrogen molecule storing capacity of the BNNT. The
hydrogen physisorption capacity of the SWBNNTs and
MWBNNTs was theoretically investigated by grand canonical
Monte Carlo theoretical studies [95]. The simulations showed
that the triple-walled (TW) BNNTs provided the necessary
limited spaces for hydrogen atoms between the tube walls. The
double-walled (DW) BNNTs were predicted to store more
hydrogen atoms due to the large space within the nanotubes as
compared to the TWBNNTs with the condition that the
inner tube diameter was sufficiently large in the DWBNNTs. As
a conclusion, the number of BNNT walls was found to be
very important as well as the wall diameter, which could be
chosen to inhibit the hydrogen–hydrogen repulsion in the inner
space of the BNNTs for improved hydrogen atom storage
capability [95].
The hydrogen molecule storage capacity for BNNTs of several
morphologies were also investigated experimentally [96]. The
studies showed that the number of absorbed hydrogen mole-
cules increased with the hydrogen pressure. The flower-type,
straight-walled and bamboo-type BNNTs were evaluated for
their hydrogen adsorption capacity. It was found that the
adsorption of the flower-type was 2.5 wt % at about 100 bar
hydrogen pressure. The straight-walled BNNTs exhibited an
increased hydrogen storage capacity up to 2.7 wt % and the
bamboo-type BNNTs had the highest hydrogen uptake capacity
of 3.0 wt % [96].
The BNNTs were synthesized using annealing and ball milling
methods and the hydrogen storage on the BNNTs was investi-
gated experimentally by pressure–composition isotherms (PCI)
and temperature-programmed desorption (TPD) methods [34].
The results showed that the hydrogen uptake capacity of puri-
fied BNNTs was 2.2 wt %. The temperature effect on the
hydrogen storage capacity of the BNNTs was also investigated
in the study. The BNNTs were exposed to the hydrogen atoms
at 180 and 250 °C and with resulting hydrogen absorption in the
range of 1.6–1.2 wt % [34].
The theoretical investigation of the hydrogen storage capacity
of BNNTs showed that the nature of the electronic structure of
boron and nitrogen atoms, as well as the diameter and dimen-
sions of the BNNT walls have an impact on their hydrogen
adsorption capacity. Although there are limited numbers of
reports, the studies show that BNNTs are possibly valuable ma-
terials for hydrogen storage. It is important to note that further
experimental investigations addressing the parameters such as
temperature, gas pressure, and purity of the BNNTs should be
conducted.
BNNTs for neutron capture therapy
The neutron absorption capacity of BNNTs was also reported
[97]. Ciofani at al. investigated the use of BNNTs as contrast
agents for neutron capture therapy, which could be an innova-
tive approach for treatment of several aggressive cancers such
as cerebral glioblastoma multiform. The main purpose of the
therapy was to target the tumor cells by 10B atoms. Accord-
ingly, BNNTs were used as carriers of boron atoms. Figure 9
shows how the BNNTs were functionalized with PLL, a fluo-
rescent probe (quantum dots) and folic acids. PLLs were
wrapped around the BNNTs to induce a hydrophilic property. In
Beilstein J. Nanotechnol. 2015, 6, 84–102.
99
addition, the BNNTs were coated with folic acids for selective
interaction with the tumor cells. The malignant glioblastoma
cells were exposed to functionalized BNNTs under in vitro
conditions. It was observed that the PLL–F–BNNTs were effec-
tively taken up by the malignant glioblastoma cells. This study
suggests that the use of BNNTs should be further investigated
for neutron capture therapy [97].
Figure 9: Schematic representation of a poly-L-lysine-, fluorescent
probe- and folate-modified BNNT. Figure adapted with permission from
the authors [97].
Gadolinium-doped BNNTs were fabricated as an effective
contrast agent in clinical applications of BNNTs [98]. Due to
the high magnetic moment property of gadolinium, Gd-doped
BNNTs can be applied as an MRI contrast agent. The high in
vitro biocompatibility property of the Gd-doped BNNTs and the
labeling of cell populations due to the Gd and B content, make
these structures a novel negative contrast agent [98].
Conclusion
It is clear that there is an increasing trend for the application of
BNNTs in several fields from medicine to sensors. Although
their synthesis is rather straightforward, it is still not possible to
produce large quantities with high purity and uniformity. Since
their physicochemical properties are independent of chirality, a
simple synthesis method can be suitable, in contrast to CNTs.
However, the choice of boron and nitrogen precursor catalysts
can be important factors for their possible medical and biomed-
ical applications. The other important point is the cost of the
preparation of BNNTs. In this sense, there is an effort to reduce
the synthesis temperature. However, to date, it seems that a
reasonably high temperature of around 1000 °C is currently
required for synthesis. The mechanism of BNNT formation is
mostly defined by the synthesis conditions, which should be
better understood for the control of the experimental parame-
ters. Another problem limiting their applicability is their low
dispersibility in aqueous media due to their high hydrophobic-
ity. In order to increase their hydrophilicity, either covalent
modifications or physical adsorption of a molecule or polymer
is typically performed. Covalent modification can be achieved
through –OH groups on B atoms and –NH2 groups converted
from N atoms at the edges and at defects.
The toxicity issues appear partially resolved in recent reports
after conflicting early reports. However, there is a lack of in
vivo data for the complete understanding of their toxicity. Most
of the toxicity studies reported to date involve a polymer or a
molecule attached to the BNNT surfaces to increase the disper-
sion in aqueous media, which might give misleading results
since toxicity is mostly defined by the coating material. In addi-
tion, there is not much information about the fate of these ma-
terials in biological systems and the environment, such as their
degradation after their release.
Acknowledgements
We acknowledge the support from the Scientific and Techno-
logical Research Council of Turkey (TUBITAK) (Project No.:
112M480) and Yeditepe University. We also acknowledge
support of the COST action MP0901 NanoTP (Designing novel
materials for nanodevices: From Theory to Practice).
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... Full analysis of synthesis methods is beyond the scope of this review. However, readers seeking a more detailed review of synthesis methods should look to Kim et al. [31], Ahmad et al. [56], Xu et al. [71], Yusuf et al. [72], Kalay et al. [73], and Tiano et al. [74]. ...
... Even now, the full extent of BNNT utility and applications is beyond the scope of this review, which cannot cover all the applications of BNNTs. Readers looking for a more detailed review of BNNT applications should consult these recommended citations [31,71,73,110,111,123,124]. ...
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Boron nitride nanotubes (BNNTs) will be one of the most important materials of this century. Recent synthetic advances have made BNNTs viable candidates for advanced multifunctional materials. Like carbon nanotubes and graphene, BNNTs and h-BN have extraordinary physical properties. Unlike CNTs, BNNTs have a wideband gap; they are piezoelectric, have neutron radiation shielding capability, and can withstand degradation up to 1000 °C. BNNTs could be the next big leap for nanocomposite advanced applications; however, significant scientific challenges must be addressed. Predominantly, large-scale synthesis techniques are immature. Production products require careful characterization, analysis, and purification. Impurities such as boron, amorphous BN, and h-BN lead to difficulty studying chemical modification and translation of BNNT properties. This review synthesizes relevant literature and state-of-the-art techniques regarding purification methods of BNNTs, classified as physical, chemical, and multi-step techniques and their applications. The review also discusses BNNT synthesis methods and future research directions. Graphical abstract
... In addition, the reduction in side effects and improvement in efficiency can be achieved by using nanostructures in drug delivery systems [13]. Among different nanostructures, boron nitride and carbon nanomaterials have applications in biomedicine as biosensors and drug delivery systems [14][15][16][17][18][19][20]. In recent years, g-C 3 N nanostructures have attracted considerable attention in the field of biomedicine based on their unique features, such as chemical stability which turn them into a suitable candidate for applications in biomedical science [21,22]. ...
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