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Templated-assisted one-dimensional silica nanotubes: synthesis and
applications
Xiaofei Yang,
*
Hua Tang, Kesheng Cao, Haojie Song, Weichen Sheng and Qiong Wu
Received 24th December 2010, Accepted 28th January 2011
DOI: 10.1039/c0jm04516k
Silica (SiO
2
) is one of the most frequently used inorganic materials. This review covers the research
progress in the synthesis of one-dimensional silica nanotubes as well as the newest aspects of silica
nanotubes in applications where their structural attributes are exploited. The synthetic methods for
well-defined silica nanotubes and a variety of specific silica nanotubes including hollow silica
nanotubes, mesoporous silica nanotubes, chiral or helical silica nanotubes are summarized. One-
dimensional tubular silica nanomaterials display structures that differ from those of other kinds of
nanostructured silica materials and provide unique features such as very uniform diameter, open at
both ends. In addition, sol–gel process and silane chemistry offer the reliable and robust surface
modification or functionalization of silica nanotubes. Attractively, end functionalization of silica
nanotubes may be able to control drug release, resulting in their wide applications in controlled drug
and gene delivery; also their distinctive inner and outer surfaces can be differentially functionalized
making silica nanotubes ideal multifunctional nanostructure candidates for biomedical applications in
various areas such as biosensing, bioseparation and biocatalysis.
1. Introduction
Interest in one-dimensional (1D) nanomaterials received
a major boost with the discovery of carbon nanotubes by Iijima
in 1991.
1
Over the past few years, 1D nanostructures made from
materials other than carbon such as nanowires, nanotubes, and
nanorods have also become the focus of intensive research due
to their unique properties and applications in condensed-matter
physics, information technology and nanoscale devices.
2–8
It is
proposed that 1D nanostructures provide a good system to
investigate the dependence of electrical and thermal transport or
School of Materials Science and Engineering, Jiangsu University,
Zhenjiang, 212013, P. R. China. E-mail: xyang@ujs.edu.cn; Fax: +86-
511-88791947; Tel: +86-511-88780191
Xiaofei Yang
Xiaofei Yang was born in Hubei,
China, in 1980. He received
a BSc. degree in Chemistry from
Hubei Normal University in
2001 and a MSc. degree in
Physical Chemistry from
Shanghai Normal University in
2004. He continued to pursue his
PhD at the University of Leeds,
UK from 2005 supported by the
Overseas Research Students
Awards Scheme (ORSAS) and
obtained his PhD degree in
materials chemistry in 2009. He
joined the School of Materials
Science and Engineering,
Jiangsu University, China in 2009. His research interests include
nanostructured materials, molecular self-assembly, biological and
biomedical applications of nanomaterials.
Hua Tang
Hua Tang received his Bache-
lor’s degree in Mineral Science
and Engineering from Wuhan
Institute of Technology in 2001.
He joined Prof. Jiaguo Yu’s
group and then worked as
a graduate student in 2002,
obtained his Master’s degree in
Materials Science (2005) and
PhD degree in Materials
Physics and Chemistry (2008)
from Wuhan University of
Technology. He has been
working in the School of Mate-
rials Science and Engineering,
Jiangsu University since 2008.
His research interests are focused on the synthesis of inorganic
nanostructures and their applications.
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mechanical properties on dimensionality and size reduction
(quantum confinement). Although considerable efforts have
been directed at the large-scale synthesis of 1D nanomaterials,
and various strategies and techniques have been developed to
break the symmetry either physically or chemically, the advance
of 1D nanostructures has been slow due to the difficulties asso-
ciated with the synthesis and fabrication of these nanostructures
with well-controlled dimensions, morphology, phase purity and
chemical composition.
Silica is popular for several reasons: a great variety of possible
structures (flexibility of coordinated Si), a precise control of the
hydrolysis-condensation reactions (due to a lower reactivity),
thermal stability of the obtained amorphous networks, and
applications in many fields such as separation, adsorption,
catalysis, optics, electronics, drug delivery and chemical sensing.
In addition, a great number of structures found in nature pre-
senting complex architectures are silica-based. Silica nano-
materials, compared with carbon nanomaterials, are cheaper,
easier to separate from one another after they are made, and
more importantly, conveniently produced in large quantities and
easily chemically functionalized.
Inorganic nanotubes are attracting a great deal of attention
due to their fundamental significance and potential applications
in various areas. Among them, silica nanotubes are of special
interest because of their hydrophilic nature, easy colloidal
suspension formation, and surface functionalization accessibility
for both inner and outer walls. These modified silica nanotubes
and nanotube membrane have shown potential applications for
bioseparation and biocatalysis. In addition, the study of the
physical and chemical nature of molecules or ions confined
within silica nanotubes is of great current interest.
In this review, we summarize the templating approaches for
the synthesis of one-dimensional silica nanotubes with different
kinds of morphologies such as hollow silica nanotubes, helical or
chiral silica nanotubes, mesoporous silica nanotubes. Further-
more, applications of silica nanotubes are reviewed with an
emphasis on the areas including biomedical science, separation
science and molecular recognition.
Kesheng Cao
Kesheng Cao is currently a PhD
candidate in the School of
Materials Science and Engi-
neering, Jiangsu University. His
research interests are mainly
focused on the controlled fabri-
cation of inorganic nano-
materials and nanocomposites,
hydrothermal synthesis of novel
nanostructured materials and
applications of one-dimensional
nanostructures.
Weichen Sheng
Weichen Sheng received his BSc
degree in Polymer Engineering,
Zhejiang University in 2000, and
obtained his PhD degree from
State Key Laboratory of
Chemical Engineering, Zhejiang
University in 2005. He then
joined the School of Materials
Science and Engineering,
Jiangsu University in 2006. He
has wide-ranging interests in the
synthesis and characterization
of porous materials, and the
development of polymer-based
optoelectronic functional mate-
rials.
Haojie Song
Haojie Song received his BSc
degree in Chemistry from Henan
Normal University in 2003 and
obtained his PhD in Physical
Chemistry from Lanzhou Insti-
tute of Chemical Physics,
Chinese Academy of Sciences in
2008. He joined the faculty of
Jiangsu University in 2008 and
his research interests are in the
synthesis and properties of oxide
nanomaterials, metal-doped
composite materials, and their
applications in nanomedicine,
catalysis, gas sensors and energy
storage.
Qiong Wu
Qiong Wu received his Master’s
degree from Jilin Normal
University in 2005 and is
currently pursuing his PhD in
the School of Materials Science,
Jiangsu University. His current
research interests include the
synthesis of functional materials
and hybrids, ceramic materials
and composites with high
performance.
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2. Template-based synthesis of silica nanotubes
Silica nanotubes were first discovered by Nakamura and co-
workers
9
in 1995 and have since lead to an enormous amount of
research into their synthetic methods, fundamental properties and
functionality for a wide variety of applications. Early investigations
and efforts mainly focused on the development of novel synthetic
strategies and the understanding of the formation mechanism of
resulting silica nanotubes. For instance, Fumiuki and co-workers
10
reported the preparation of hollow silica nanotubes in the presence
of tartaric acid and ammonia where tartrate ammonium crystals
formed was proposed as the template for the generation of silica
materials. Adachi and co-workers
11–13
reported the sol–gel method
for fabricating single silica nanotubes by the surfactant-mediated
template mechanism and the method for controlling the geometry
of the tubes. They presented the first use of self-assembled micellar
aggregates formed by laurylamine hydrochloride in aqueous
conditions as templates for the formation of silica nanotubes. The
nanotubes are hundreds of nanometres long with diameters of 5–8
nm and of consistent quality. The group have shown that the length
of the nanotubes can be increased by introducing a quantity of
trimethyl silane to the synthesis mixture during the condensation of
the gel. It was revealed that the diameter of the nanotubes can be
easily controlled by using a different length of alkyl chain of
surfactant. Trimethyl silane reacts with surface OH groups on the
silica nanotubes and effectively coats them in a hydrocarbon layer
blocking any intertube polymerisation. Moreover, a novel self-
assembly method for fabricating size tunable silica nanotube using
a reverse-microemulsion sol–gel technique was reported by Jang’s
group
14
and Banerjee’s group.
15
The main advantages of the
method are that the diameter, wall thickness and morphology of
silica nanotubes were tuned by using different kinds of apolar
solvents
14
and the amount of water.
15
Although silica nanotubes have been synthesized by different
synthetic methods including the sol–gel method, hydrothermal,
microemulsion, they have something in common in the use of
template as a structure-directing agent, followed by the gentle
removal of the template by the post-treatment such as extraction,
chemical reaction or calcination. The key for a successful
template-directed synthesis is to select a template which ensures
the formation of a desired nanostructure and can be easily
removed without damaging it, which means the template should
be thermally and chemically stable with respect to the specific
reaction conditions. However the template should be chemically
reactive if post-treatment is absolutely necessary.
Templating is one of the most frequently used methods of
synthesizing materials with structural units ranging from nano-
metres to micrometres.
16–18
Template-based methods essentially
involve the replication of one structure into another under
structural inversion. In the context of materials chemistry,
a template is a structure-directing agent. Template methods
initially yield template-nanomaterial hydrids. The detached
nanomaterials are then obtained by selective removal of the
template. Templates can be classified into three categories, inor-
ganic templates, organic templates and biomolecular templates.
2.1 Inorganic templates
There are two main types of inorganic templates applied for
producing nanomaterials, namely, ordered porous inorganic
membranes and inorganic nanomaterials. A templated-directed
approach to preparing nanomaterials has been pioneered by
Martin and co-workers in the early 1990s.
19,20
The synthesis of
controlled and uniform-sized nanomaterials benefits from the use
of fabricated membrane templates with cylindrical pores which
are monodisperse in terms of the diameter and length. Depen-
ding on the properties of the material and the chemistry of the
pore wall, nanomaterials may be obtained by the removal of
template and may be solid (nanorods, nanowires) or hollow
(nanotubes). The most important type of template used by this
group has been porous alumina membranes from which they
have made a variety of tubular nanomaterials including silica
nanotubes,
21–23
carbon nanotubes
24–26
and TiO
2
nanotubes.
27
In
addition, the template-based synthetic strategy has been
developed to make gold nanotubes,
28–30
nano test tubes,
31–33
DNA nanotubes,
34–37
and protein nanotubes.
36,37
2.1.1 Inorganic anodic aluminum oxide (AAO) as the
template. Silica nanotubes have been synthesized typically within
the pores of porous alumina membrane templates using the sol–
gel coating technique. Alumina templates can be dissolved to
liberate single silica nanotubes. These silica nanotubes prepared
at low temperature have porous walls and are fragile. Once the
templates are removed, the silica nanotubes will bundle up and
become less oriented. Yu and co-workers
38,39
expanded the above
strategy and developed a mechanical capping method of silica
nanotubes for encapsulation of functional molecules by using an
alumina microbead hammering treatment without chemical
linkers or chemical reactions (Fig. 1). It was suggested that not
only inorganic metals Au and Ag, but also organic polymers can
serve as the caps for silica nanotubes.
The template synthesis of silica nanotubes using inorganic
anodic aluminum oxide (AAO) has been advanced by He and co-
workers
40–42
where silica nanotubes with well-defined cylindrical
pores were fabricated further with a surface sol–gel procedure to
produce shape-coded silica nanotubes as a new dispersible
microarray system. The results showed each shape-coded silica
nanotube has several segments with different reflectance values
after the fabrication by multistep anodization template synthesis,
and it was proposed that the difference in optical reflectance
between the segmented parts of individual silica nanotube made
it convenient and effective to identify each nanotube.
Fig. 1 Schematic diagram for the preparation of Au-cap using alumina
microbead hammering treatment; Ag and polymers were also used as
capping materials according to the same procedure. Reprinted with
permission from ref. 38, copyright 2009, American Chemical Society.
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2.1.2 Inorganic nanomaterials as templates. Moreover,
employing inorganic nanomaterials as hard templates is also
a reasonable way to prepare novel tubular silica nanomaterials.
Fabrication of silica nanotubes using multi-walled carbon nano-
tubes (MWCNTs) as the template was investigated by different
groups. Kim and co-workers
43
reported the synthesis and fabri-
cation of silica nanotubes by first preparing silica coated
MWCNT composites via a surface oxidation of MWCNTs using
KMnO
4
in the presence of a phase transfer catalyst, followed by
grafting of 2-aminoethyl 3-aminopropyl trimethoxysilane
(AEAPS). It was shown that the amine groups in grafted AEAPS
on MWCNTs could activate the silica shell formation by acid–
base interaction. The synthesized silica initially formed a uniform
layer on MWCNTs with a controllable thickness, then the silica
nanotubes were obtained with an average layer thickness of 13 nm
without distortion of its original shape after calcined the
composite at 800 degrees to completely remove the inner
MWCNTs. Yang and co-workers
44
also presented a facile method
to fabricate silica nanotubes with acid-oxidized multiwalled
carbon nanotubes (MWCNTs) by the sol–gel polymerization of
tetraethoxysilane (TEOS) at room temperature, followed by
a similar procedure of oxidizing the MWCNTs templates at high
temperature to produce hollow silica nanotubes. The thickness
and morphology of the silica shell were controlled by rationally
adjusting the concentration of TEOS, and by introducing cationic
surfactant as a structure-directing agent. The use of single wall
carbon nanotubes (SWCNTs) as templates was reported by Lin
and co-workers
45
to synthesize silica nanotubes with diameters
ranging from 5–23 nm where silica coated SWCNTs were first
obtained through the reaction between chlorosilane and acid-
treated SWCNTs, followed by the calcination of as-prepared
hybrids at 900
C in air to release free silica nanotubes.
The report of Wang and co-workers
46
showed that wildtype and
genetically engineered flagella (inner diameter of similar to 2 nm
and outer diameter of close to 14 nm) detached from the surface of
specific bacterial cells could also be used to modify silica nano-
tubes, resulting in the formation of double-layered silica/flagella
nanotubes. It was shown that the flagella templates inside the
silica/flagella nanotubes could be removed as previously described
to obtain by calcining the hybrid nanotubes at high temperature
(550
C). However, further calcination of the silica nanotubes at
a higher temperature (800
C) resulted in the damage of original
silica nanotubes and the generation of a periodic nanohole array
along the silica fibers with a center-to-center nanohole spacing of
similar to 79 nm. It was also found that flagella displaying
different peptides resulted in different morphologies of silica
nanotubes. It was concluded that the monodisperse diameter and
genetically tunable surface chemistry of the flagella could be
employed for the fabrication of silica nanotubes with uniform
diameter and controllable morphologies.
Tuan and co-workers
47
reported the synthesis of silica nano-
tubes by adding copper sulphide (CuS) nanocrystals and
monophenylsilane (MPS) to supercritical toluene at the desired
temperature and pressure. Amorphous silica nanotubes were
found to form in the presence of oxygen when small amounts of
water were introduced into the system. Approximately 5% of the
silica nanotubes were observed to be helically coiled like those in
Fig. 2. However, only crystalline silicon nanowires were
produced in the absence of water and oxygen, it was suggested
that the seed material, in fact, was Cu metal which was converted
from CuS during the reaction. In addition, when CuS nano-
crystals were replaced by Au nanocrystals, the majority of the
obtained samples consisted of solid amorphous silica nanofibers
with only few silica nanotubes. It was proposed that the differ-
ence in silica morphology produced by Au and CuS seeds results
from qualitative differences between the metal–silica interface
morphology for gold and copper and the fact that Cu is easier to
oxidize than Au, resulting in the oxidation enhancement of MPS
and the formation of silica.
Three-dimensional (3D) interconnected silica nanotubes were
synthesized by Zhu and co-workers (Fig. 3) using hyperbranched
PbSe nanowires as templates via simple coating and etching
steps.
48
The obtained silica nanotubes with a thick enough shell
retain the structural characteristic of the original hyperbranched
PbSe nanowires where they are either parallel or perpendicular to
each other. These promising hyperbranched silica nanotubes
afford interesting and potential application in constructing new
3D nanofluidic devices.
Fig. 2 TEM images of (a) silicon nanowires, (c) silica nanotubes and (d) helical silica nanotubes; (b) SEM images of amorphous silica nanofibers.
Reprinted with permission from ref. 47, copyright 2008, American Chemical Society.
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Many nanomaterials including silicon nanowires,
49
ZnO
nanowires,
50
Gd(OH)
3
nanorods
51
and gold nanorods
52
have also
been used as templates for the generation of nanostructured
materials.
2.2 Organic templates
The use of organic compounds as templates for the generation of
inorganic nanomaterials has received increasing attention over
the last decade.
53–57
Compared with inorganic templates, organic
molecules as templates have advantages of structural variety,
easy removal, good solubility and dispersion in solution. The
heat treatment, which is often employed for the removal of
organic template, can simultaneously lead to the formation of
harder, more stable inorganic nanomaterials.
The first example of using organic templates for the generation
of well-ordered materials was reported by Dickey and co-workers
in 1949.
58
Since then molecular templating has been used to
synthesize a broad range of organic and inorganic materials. The
template-directed process involves the specific nucleation and
growth of inorganic phases on the surface of functionalized
organic structures. Oriented nucleation can occur if there is a high
level of molecular recognition at the template-matrix interface.
A general organic template approach involves three main
steps: (1) self-assembly of organic molecules into nanostructures
(template); (2) the interactions between the template and the
embedding matrix and (3) the removal of the template. The
combination of molecular self-assembly and the template-based
approach enables the generation of a variety of well-defined
nanostructures which can be inorganic, organic–inorganic or
organic–metal hybrids (Fig. 4).
2.2.1 Organic templated hollow silica nanotubes. A variety of
organic templates have been successfully used for the sol–gel
synthesis of hollow silica nanotubes such as surfactants,
11–14
acids,
60,61
block copolymer
62
and gel systems.
63–66
Most recently,
Jung and Shinkai
67
reviewed the fabrication of silica nanotubes by
using self-assembled gels and their applications in environmental
and biological fields. In particular, the use of self-assembled
organogels as templates that have been developed by different
groups for the sol–gel transcription to create silica nanotubes was
summarized. It is suggested that the close association of template
and inorganic precursors is vital for successful sol–gel templating
of silica nanotubes. In general, two classes of interactions need to
be taken into consideration, noncovalent interactions and cova-
lent interactions. When dissolved in solution, non-covalently
bonded templates can template small inorganic groups via elec-
trostatic, van der Waals and hydrogen-bonding interactions to
form silica nanotubes with tailorable shapes and sizes; whereas
covalent bonding of the organic ligand to the inorganic frame-
work forces close association of template and framework, and
limits the independent organization of the organic and inorganic
moieties. It is proposed that, in order to obtain high quality silica
nanotubes, the template requires only a small excess of positive
charge on its surface. Thus the template used in the reaction
should be cationic or non-ionic compounds.
Lei and co-workers investigated the sol–gel synthesis of
uniform silica nanotubes in high yield using an amino acid
derivative surfactant N-dodecanoyl-
L-histidine (DHis).
68
Typi-
cally, cationic quaternary ammonium surfactant dodecyl-
triethylammonium bromide (DTEAB) was added to a DHis
aqueous solution to obtain desired aggregates, which can be used
as versatile template for opposite charged precursors in the
preparation of silica nanotubes. It was proposed that the
formation of silica nanotubes was proved to be synergistically
self-catalyzed hydrolysis and condensation of silica precursors
depositing on the self-assembled template surface driven by
Coulomb interactions between the two oppositely charged
building blocks (Fig. 5a). Silica nanotubes with tunable sizes in
diameter and uniform wall thickness could be obtained in high
yield by optimizing the sol–gel process (Fig. 5b–5g). The paper
provided a general route for the construction of well-defined one-
dimensional nanostructures in aqueous media.
2.2.2 Organic templated specific silica nanotubes. In the past
few years, silica nanotubes have been the subject of increasing
attention since the structural and morphological diversity of
silica nanotubes have inspired the scientific community to
Fig. 3 (a–b) High resolution SEM images of SiO
2
tubes with open end;
(c) A TEM image of silica nanotube with open end; (d) TEM images of
nanotubes network with an 80 nm-thick silica shell. Reprinted with
permission from ref. 48, copyright 2009, Wiley-VCH Verlag GmbH &
Co. KGaA.
Fig. 4 Construction of hybrid nanotubes via templating approaches.
Reprinted with permission from ref. 59, copyright 2005, American
Chemical Society.
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explore their potential wider applications in various areas. The
development of synthetic methods enables researchers to design
specific organic templates or modify existing templates, and to
control the self-assembly as well as sol–gel transcription to create
desired silica nanotubes.
Chirality is a critical property of amino acids that form
proteins, the building blocks of life, and of DNA, which encodes
the genetic traits passed from one generation to the next. If an
object and its mirror image are nonsuperimposable, then they
have the property of chirality. The property is found in nearly
every biological molecule and in many synthetic bioactive mole-
cules such as pharmaceuticals. Control of chirality and synthetic
strategies for enantiomerically pure products are of great impor-
tance. Also control of chirality is an attractive goal due to its wide
applications in the area of separation science, asymmetric catal-
ysis, and analytical technology. Chiral nanomaterials have been
produced using both chiral templates and achiral templates.
69–73
The use of chiral templates is the most common approach as the
chirality of the template may transfer simply to the final nano-
materials in the sol–gel process, resulting in the formation of chiral
nanomaterials. Chiral compounds, which can be used as
templates, are either prepared from available chiral starting
materials, or generated during an asymmetric catalytic reaction.
In general, two kinds of chiral compounds, chiral surfactants
74–76
and chiral gelators,
18,64
have been used as templates for the
formation of specific tubular silica nanostructures such as meso-
porous silica nanotubes,
77
helical or chiral silica nanotubes.
78,79
Most recently, Che and co-workers
73,80,81
discovered a novel
templating route for preparing well-ordered silica nanomaterials
based on the self-assembly of surfactants and precursors in the
presence of aminosilane or quaternary aminosilane as a costruc-
ture-directing agent (CSDA). For instance, they first synthesized
silica nanotubes with radially oriented mesoporous channels by
using self-assembled chiral N-acylamino acid surfactant formed
upon partial neutralization followed by the addition of CSDA,
N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride
(TMAPS) and silica precursor TEOS.
74
As shown in Fig. 6, the
tube diameter and wall thickness can be controlled by a simple
adjustment of the molar composition. Furthermore, Che and co-
workers
75
presented a new route for synthesizing chiral meso-
porous silica nanotubes by using achiral surfactant sodium
dodecyl sulfate (SDS) as a template in the presence of chiral
molecule (R)-(+)- or (S )-()-2-amino-3-phenyl-1-propanol ((R)-
(+)-APP or (S)-( )-APP). The effects of molar ratios of chiral
dopant APP/SDS on the formation of tube shape and handed-
ness were investigated in detail with the structural trans-
formation with reaction time.
Another class of compounds which have been used as
templates for the preparation of chiral or helical silica nanotubes
is chiral gelators.
78,79
Shinkai and co-workers have developed
a general sol–gel strategy using different kinds of gelators such as
sugar-appended porphyrin,
64
crown-appended cholesterol,
82
and
sugar-appended azonaphthol derivatives
66
to prepare silica
nanotubes. A series of silica nanotubes with specific structures
have been developed including helical silica nanotubes,
64,83
double silica nanotubes,
82
and double-helical silica nano-
tubes.
65,84
They also proposed a mechanism
83
for the formation
of the helical silica structure obtained from the organogel system.
Oligomeric silica species are absorbed onto the surface of the
organogel template by hydrogen-bonding or electrostatic
Fig. 5 (a) Schematic representation of synergetic growth mechanism of silica nanotubes; (b–d) SEM and (e–g) HRTEM images of silica nanotubes after
calcination synthesized in DHis/DTEAB ¼ 8/2, Ct ¼ 10 mM system, incubated at different temperature. Reprinted with permission from ref. 68,
copyright 2010, American Chemical Society.
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interactions and the polymerization further proceeds along these
fibrils. This propagation mode eventually yields silica with
hollow cavities after removal of the gelators by calcination.
Wan and co-workers
85
reported the preparation of helical
mesoporous silica nanotubes using a variety of chiral cationic
gelators which were synthesized from simpler chiral starting
materials. The use of cationic gelators resulted in the formation
of long-range order of the nanostructured silica, in which the
diameters of the nanotubes are similar to the length of gelators. It
is obvious that the transcription process is sensitive to synthetic
conditions such as pH, temperature, concentration, catalyst and
also the ratios of starting materials, which directly influence the
self-assembly of chiral gelators and absorption of silica oligo-
mers, resulting in difficult control of chirality of silica nano-
structures. For successful sol–gel transcription, all the factors or
parameters which may affect the generation of specific silica
nanomaterials should be investigated systematically.
An interesting example of the design and preparation of
organic–inorganic hybrid silica nanotubes was described by Ji
and co-workers.
86
In a first step, self-assembly was combined
with templating approaches to produce hybrid two-layered
silica–lipid nanotubes and consequently, hybrid nanotubes con-
sisting of concentric lipid–silica–lipid by concomitant self-
assembly of glycolipids on both the inner and outer surfaces of
silica nanotubes. Subsequent sol–gel polymerization using lipid–
silica–lipid hybrid nanotubes resulted in the formation of more
complex hybrid nanotubes with a five-layered structure (Fig. 7).
2.3 Biological and biomolecular templates
In nature, there are a number of biological systems that display
morphologically complex architectures potentially suitable for
templating. The aim of biotemplating is either to replicate the
morphological characteristics and the functionality of
Fig. 6 SEM (a1–a3) and TEM (b1–b3, c1–c3) images of mesoporous silica nanotubes prepared with different reaction conditions; (d) formation
mechanism of mesoporous silica nanotubes. Reprinted with permission from ref. 74, copyright 2008, Wiley-VCH Verlag GmbH & Co. KGaA.
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a biological species or to use a biological structure to guide the
generation of inorganic materials. Indeed, a large variety of
biological species have been used as templates and the majority
of the biological structures that have been used for replication
show nanostructured features.
70,87–90
Since nature is a great
source of inspiration for the formation of well-defined, struc-
tured silica, it seems only logical to use biomolecules or bio-
organic templates taken from nature to fabricate silica materials.
Biomolecules such as proteins, peptides and other naturally
occurring biomimetic or synthetic self-assembling systems (as
analogues of naturally occurring species or polymers containing
unnatural amino acids) can be used as organic templates for
silica deposition. The use of such templates allows for synthesis
of nanostructured materials over a wide pH range and through
careful choice or design of the template can provide a unique way
of understanding the processes involved in silica biomineralisa-
tion.
Indeed several research groups have employed natural
templates for such a purpose. Since the presence of positive
charges in the template facilitates the silica transcription process,
natural proteins that carry positive charges were investigated.
Members of the family of mosaic viruses were regarded as an
attractive class of biotemplates due to their high stability in non-
standard pH conditions and temperatures.
The well-known tobacco mosaic virus (TMV) is an extremely
stabile virion, consisting of lots of identical protein subunits,
which self-assemble around a single strand of RNA to form
a hollow protein tube with a length of 300 nm, an outer diameter
of 18 nm, and an inner diameter of 4 nm. The outer and inner
surfaces of the tubes are covered in charged amino acid residues,
which make the TMV an excellent candidate for transcription
into silica materials. Tobacco mosaic virus can be used to
template silica nanotubes using a pH < 3 and TEOS as a silica
source, the nanotubes are believed to form due to the interaction
of the positively charged protein surface of the virus and the silica
anion.
91,92
The actual inner pore width (11 nm) is narrower than
the width of the tobacco mosaic template (18 nm) indicating that
the template has become compressed during the mineralization
process.
It is well-known that the DNA molecule itself is negatively
charged and will therefore be difficult to template. To use DNA
as a template there are two problems that need to be overcome.
Firstly, sol–gel polycondensation is carried out in neutral or
alkaline media so that silica species are anionic. However DNA
itself is an anionic polymer, so one must transform the anionic to
a cationic species; secondly, the precursor TEOS is soluble in
organic solvents, whereas DNA itself is soluble virtually only in
water, hence some appropriate solvent system which can dissolve
both components had to be found. The first successful example
showing that DNA can be used as a template for sol–gel tran-
scription of silica nanotubes was reported by Namuta and co-
workers.
93
They designed and introduced a modified amine into
the system to form an intermolecular ion pair with the negatively
charged phosphate groups of DNA leaving the amine group free
and the DNA ion pair complex positively charged (Fig. 8). By
utilizing this strategy it was found that DNA can act as a cationic
template for sol–gel transcription and the resultant silica nano-
tubes transcribe the higher-order structures of DNA very
precisely.
Besides TMV and DNA, other biomolecules or organic
amphiphilic molecules such as peptides, lipids could be tran-
scribed to silica nanotubes using a sol–gel transcription protocol.
For instance, Ji and co-workers
94
demonstrated the diameter and
dimension control of silica nanostructures in different ethanol/
water mixtures. It was shown that the peptidic lipid first self-
assembled in water to from nanotube structures nanostructures,
followed by sol–gel transcription of self-assembled lipid nano-
tubes as templates into silica nanotubes. It was suggested that the
resultant self-assembled nanostructures depended heavily on the
composition of a mixture of ethanol and water, the morphology
of silica materials changed from tubular structures to spherical
ones with the increase of ethanol volume fraction. Meegan and
co-workers
95
reported the sol–gel hydrolysis and condensation of
tetraethoxysilane in the presence of designed self-assembled b-
sheet peptide fibril templates. Hollow silica nanotubes could be
obtained after the removal of the template by extraction. The
diameter and length of silica nanotubes are primarily determined
by the self-assembled fibril template. The effects of synthesis
conditions such as pH value, different peptides and catalysts on
the formation of silica nanotubes have been investigated and the
structures and morphologies of resultant silica nanotubes were
characterized by various techniques. Moreover, double-walled
silica nanotubes were also preparation by Brunner
96
via
a biomimetic synthesis process using a peptide as a template.
Fig. 7 Design and formation of hybrid nanotubes (left); electron micrographs of nanotubes (right). Reprinted with permission from ref. 86, copyright
2005, Royal Society of Chemistry.
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2.4 Dual templates
Dual-template assisted synthesis of silica nanotubes was
demonstrated by two research groups. Liao and co-workers
97
synthesized hollow silica nanotubes via a sol–gel process using
poly(ethylene glycol) functionalized multi-walled carbon nano-
tubes (MWNT-PEG) as the dual templates while the Song
group
98
produced silica nanotubes with mesoporous walls of
30 nm thickness and various internal morphologies using hard/
soft dual templates; cetyltrimethylammonium bromide was
chosen as the soft template and carbon nanotubes as well as
carbon nanofibers were used as the hard template.
3. Applications of silica nanotubes
Recently, multifunctional silica nanotubes have made, and will
continue to make, important contributions to biological sciences
such as drug delivery, imaging and screening, targeting and cell
sorting due to their structural benefits such as distinctive inner
and outer surfaces. Unlike 1D silica nanowires and nanorods, 1D
silica nanotubes have a hollow structure, which allows the
modification of their inner surface and filling with specific
biomolecules. The tube structure may act as a physical shield for
the inserted biomolecules and provide advantages for biomole-
cule delivery. Many efforts have been made to demonstrate the
potential for newly emerging applications in biological systems,
including gene delivery, molecular separations, biosensing, bio-
separation and biocatalysis. However, there are two barriers that
need to be overcome in applications of silica nanotubes as
vehicles or carriers for biomolecules. First, controlling the
structure of the open end of silica nanotubes is crucial to control
uptake and release rate for the development of effective drug/
gene delivery system and to fabricate multifunctional silica
nanotubes containing desired functional molecules or nano-
particles inside of nanotubes.
33,38
It is believed that capping silica
nanotubes would be the easiest approach to control the open
end’s geometry of silica nanotubes. In addition, the ability of
silica nanotubes to capture or fill, transport and release mole-
cules in a controlled manner is a challenge. The use of nano ‘‘test
tubes’’ where one end is open and the other is permanently sealed
was chosen as an elegant solution for controlled release of the
nanotube contents, the open end could be reversibly capped to
hold the contents until the release is triggered.
In this part, we mainly focus on the advances in the use of silica
nanotubes for biomedical applications. Most current research
has focused on spherical silica nanoparticles and their applica-
tion because they are easier to make; however, cylindrical silica
nanotubes offer many advantages over spherical nanoparticles.
One advantage is that the template for the synthesis of silica
nanotubes is tunable, which means the pore diameter and
template thickness can be controlled by reaction conditions,
resulting in larger payload capacities for silica nanotubes.
Another advantage is that templated silica nanotubes can be
differentially functionalized on their inner and outer surfaces
using simple silane chemistry with commercially available
reagents. Then the modification on the inner and outer surfaces
and application of silica nanotubes for use in various areas
including gene/drug delivery, controlled drug release, bio-
separation and catalysis, molecular imprinting and hydrogen
absorption is discussed.
3.1 Gene/drug delivery and controlled release
In order to demonstrate how measurements on silica nanotubes
can be used to monitor wetting and diffusion in the hydrophobic
nanotube interior under various conditions, Okamoto and co-
workers
99
investigated the use of template-synthesized nanotubes
as model systems for studying adsorption, wetting and diffusion
in nanoscale containers by using fluorescence microscopy tech-
niques, which provided a nondestructive method for verifying the
chemical modification of the silica nanotube interior surfaces
that was a task not readily achieved by conventional surface
methods. Since wetting processes will play a critical role in the
transport of solute molecules in and out of silica nanotubes,
further investigation on the wettability of the hydrophobic
interiors of individual silica nanotubes was carried out to
compare wetting on patterned surfaces with individual features
and to directly test theories of wetting and capillarity.
The first example of silica tubular-structured materials as
biomolecule carriers was reported by Chen and co-workers
100
(Fig. 9). The fluorescent nature of the nanotubes allowed visu-
alizing their localization in living cells. Nanotubes filled with the
gene encoding green fluorescence protein (GFP) were found to
enter monkey kidney cells and these cells exhibited GFP
expression. These results demonstrated a novel application of
nanotubes in biomolecule delivery, because the cargo molecules
carried by the nanotubes will not be limited to DNA in the
Fig. 8 Schematic representation of the transformation of the DNA
surface. Reprinted with permission from ref. 93, copyright 2004, Wiley-
VCH Verlag GmbH & Co. KGaA.
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future. Instead, RNA, proteins, and other biomolecules may also
be loaded into the nanotubes. In addition, the size of the nano-
tubes can be adjusted as required according to the size and the
amount of cargo molecules, and a smaller nanotube vehicle
might increase the transfection/delivery efficiency.
Chen’s group
101,102
investigated the synthesis and applications
of mesoporous silica nanotubes (MSNTs) and amine-function-
alized MSNTs (NH
2
-MSNTs). The resulting silica nanotubes
were not only utilized as a support for the immobilization for
glucose oxidase, they were also functionalized with blue fluo-
rescent CdS quantum dots to store ibuprofen. It was observed
that the enzymatic activity of the immobilized glucose oxidase
first increased with the increasing coverage of the silica nanotube
surface by glucose oxidase, followed by the decrease in the
enzymatic activity caused by the overlap and aggregation of
glucose oxidase molecules at high surface coverage.
102
It was
shown from the comparative study of the capacity of different
silica nanotubes that the drug-loading amount of ibuprofen in
CdS-NH
2
-MSNTs (CdS-incorporated NH
2
-MSNTs) could
reach up to 740 mg g
1
silica, which was similar to that in as-
prepared MSNTs (762 mg g
1
silica) and NH
2
-MSNTs (775 mg
g
1
silica). Drug release studies in simulated body fluid revealed
that the loaded ibuprofen released from amine-functionalized
systems at a significantly lower release rate as compared to that
from amine-free systems, and also indicated the incorporation of
CdS quantum dots had nearly no effect on the ibuprofen release
process. Further study on the ibuprofen release from CdS-NH
2
-
MSNTs in other media showed that CdS-NH
2
-MSNTs are pH-
and ion-sensitive drug carriers, which should facilitate controlled
drug delivery and disease therapy.
101
Chen and co-workers
103
also illustrated the application of silica
nanotubes as enzyme immobilization carriers. The immobiliza-
tion was carried out by the adsorption of lysozyme molecules
from aqueous solution onto the hydrophilic silica nanotube
surface. A study of the zeta potentials of silica with and without
the immobilized lysozyme showed that there was an increase in
the isoelectric point with the increase in the loading amount of
lysozyme. FTIR spectra indicated that protein secondary struc-
ture was maintained well in the immobilized molecules. It was
observed that enzymatic activities first increased and then
decreased with increasing surface coverage of silica nanotubes by
lysozyme, which was totally consistent of the immobilization of
glucose oxidase, suggesting the overlap and aggregation of
lysozyme molecules reduced enzymatic activities of adsorbed
lysozyme molecules at high surface coverage.
Besides the above controlled drug delivery, Chen’s group
104
also advanced the pH-controlled drug delivery systems via the
layer by layer self-assembly technique where two kinds of inor-
ganic/organic hybrid composites based on mesoporous silica
nanotubes (MSNTs) and pH-responsive polyelectrolytes were
embedded. One system was developed on the basis of loading
poly(allylamine hydrochloride) and sodium poly(styrenesulfo-
nate) onto MSNTs and releasing the positively charged drug
doxorubicin. The other system was designed by alternately
coating sodium alginate and chitosan onto amine-functionalized
MSNTs, which were used as vehicles for the loading and release
of the negatively charged model drug sodium fluorescein.
Controlled release of the drug molecules from these delivery
systems was then achieved by changing the pH value of the
release medium. It was shown from the results of in vitro cell
cytotoxicity assays that the cell killing efficacy of the loaded
doxorubicin against human fibrosarcoma (HT-1080) and human
breast adenocarcinoma (MCF-7) cells was pH dependent Thus,
these hybrid composites could be potentially applicable as pH-
controlled drug delivery systems. Further investigations of silica
nanotubes as supports for immobilization of penicillin G acylase
enzyme,
105
as well as silver nanoparticles and their antibacterial
properties was also carried out by this group.
106,107
Bhattacharyya’s group
108,109
also demonstrated the utility of
silica nanotubes both as an ultrasound-triggered controlled drug
delivery system and a biosensor. Confinement of biomolecules
inside silica nanotubes such as hemoglobin and ibuprofen was
investigated in detail. It was found that, in the case of controlled
drug delivery triggered by ultrasound, the drug yield as function
of time was heavily dependent on the ultrasound impulse
protocol.
109
Higher drug yields could be obtained when impulses
of shorter duration and shorter time intervals between successive
impulses were employed.
108
The influence of confinement on the
ligand binding activity of hemoglobin was studied systematically
and the results suggested that the immobilization of hemoglobin
inside silica nanotubes didn’t result in any distortion of the native
protein structure and function.
3.2 Bioseparation, biointeraction and catalysis
The fabrication of oriented, robust silica nanotube arrays is also
of interest for their potential use in nanoscale fluidic biosepara-
tion, sensing, and catalysis. Teo and co-workers
110
developed
a well-controlled process to translate vertical silicon nanowire
arrays into silica nanotube arrays through a thermal oxidation-
etching approach. The obtained nanotubes retained the orien-
tation of original silicon nanowire arrays. High-temperature
oxidation (800–1000
C) produces relatively thick and rigid walls
that are made of condensed silica. This method could be useful
for fabrication of single nanotube sensors and nanofluidic
systems.
Martin’s group
22
reported the use of differentially function-
alized silica nanotubes as smart nanophase extractors to remove
Fig. 9 Schematic illustration of fluorescent silica nanotube preparation and its gene delivery; (left) TEM and fluorescent images of fluorescent silica
nanotubes (right). Reprinted with permission from ref. 100, copyright 2005, Wiley-VCH Verlag GmbH & Co. KGaA.
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lipophilic molecule, hydrophobic 7,8-benzoquinoline (BQ) from
aqueous solution. Furthermore, antibody-functionalized silica
nanotubes were also employed in the extraction of one enan-
tiomer of a racemic pair. The immobilization of the enzyme
glucose oxidase (GOD) on both the inside and outside surfaces of
silica nanotubes was investigated via the aldehyde silane route.
In consideration of the importance of functional silica nano-
tubes, Son and co-workers
111
combined the attractive tubular
nanostructure with magnetic property and prepared magnetic
silica nanotubes (MNTs) toward biomedical applications such as
magnetic-field-assisted bioseparation, biointeraction, and drug
delivery. It was shown from the chemical extraction and sepa-
ration experiment that MNTs can be used for the extraction,
separation, release, and analysis of trace amounts of the
extremely hydrophobic toxic chemicals, such as polychlorinated
biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)
in water. Also MNTs could be used to facilitate and enhance the
biointeractions between the outer surfaces of MNTs and
a specific target surface because of their magnetic properties. It
was suggested that the efficiency of antigen-antibody interactions
could be controlled by means of an external magnetic field.
Model drug molecules including 5-fluorouracil (5-FU), 4-nitro-
phenol and ibuprofen were loaded into the pores of MNTs that
were functionalized with 3-aminopropyltriethoxysilane (APTES)
in order to study the effect of the charged hydrogen-bonding
interaction between the drug and inner pore surfaces on loading
and release. The results showed that the strongest interaction
happened in the carboxylic group of ibuprofen with the amine
group inside MNT and ibuprofen released with a slow rate. The
immobilization of mimetic enzyme Hemin into magnetic silica
nanotubes (MNTs) was achieved by Xing and co-workers
112
where the surface functionalized MNTs act as a promising
platform as biocatalyst carriers and the incorporated Hemin
exhibited excellent catalytic activity and reusability.
Zhou and co-workers
113
expanded applications of silica
nanotubes with other functional materials and demonstrated
potential biomedical applications of multifunctional water-
dispersible CdSe-Fe
3
O
4
-SiO
2
nanocomposites. The resulting
specific hydrophilic nanocomposites was found to be able to
enter into the interiors of cells without causing damage, sug-
gesting their capability not only as a fluorescent probe but also as
potential drug/gene delivery systems, which is of great impor-
tance to the design and development of novel multifunctional
carriers in nanobiotechnology.
Furthermore, a variety of nanomaterials including CdSe
nanorods,
114
Ge nanorods,
115
In
2
S
3
nanorods,
116
Bi
2
S
3
nano-
rods,
117
silicon nitride nanotubes,
118
and titania nanotubes
89
have
been filled into silica nanotubes to prepare composite functional
nanotubes for applications in various areas such as biosensing,
chemical sensors, photocatalysis, selective recognition and
separation. The design of silica nanotube-based nanofiltration
systems was developed by El-Safty and co-workers
119
where
mesoporous silica nanotubes hybrid membranes were found to
be suitable for separation of biomolecules and were used as
highly efficient ultrafine filtration systems for noble metal
nanoparticles and semiconductor nanocrystals.
3.3 Molecular imprinting and recognition
Most recently, Xie and co-workers
120
illustrated that molecule
imprinting at the walls of highly uniform silica nanotubes was
applicable for the recognition of 2,4,6-trinitrotoluene (TNT). It
was revealed that TNT templates were efficiently imprinted into
the matrix of silica through the strong acid–base pairing inter-
action between TNT and 3-aminopropyltriethoxysilane
(APTES). The imprinted silica nanotubes could potentially be
exploited in detecting the highly explosive and environmentally
deleterious TNT and its derivatives (Fig. 10).
In addition, Kim and co-workers
121
reported the use of
thymidine-functionalized silica nanotubes for selective recogni-
tion and separation of oligoadenosines. It was shown that
thymidine receptor residues were covalently attached to the inner
side wall of silica nanotubes (SNTs). Then the adsorption
capacities of thymidine-functionalized silica nanotubes (T-SNTs)
Fig. 10 (Left) SEM and TEM images of TNT-imprinted silica nanotubes; (upper, right) molecular imprinting mechanism of TNT in silica; formation
mechanism of TNT-imprinted silica nanotubes. Reprinted with permission from ref. 120, copyright 2010, American Chemical Society.
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with nucleic acids and oligonucleic acids were evaluated. T-SNTs
exhibited excellent selectivity both in recognition and separation
of adenosine and adenosine-based oligonucleotides, because of
selective bindings of adenosine and oligoadenosine to the inner
surface of T-SNTs by formation of intermolecular hydrogen
bonds. However, guanosine, cytosine and oligoguanosine
derivatives were found not interactive to T-SNTs. 95% of
oligoadenosine was selectively separated by T-SNTs from
a mixture of equal amounts of oligocytosine and oligoguano-
sine.
121
The findings implied that T-SNTs could be used as
a selective receptor and separator for the extraction and sepa-
ration of adenosine and oligoadenosine derivatives in aqueous
solution.
Despite the unique advantages of silica nanotubes in the above
biomedical applications, investigation of their interactions with
biological systems still remains at a very early stage and has
a long way to go. In order to effectively develop these systems for
application, it is necessary to systematically reveal the structural
and functional properties that influence biocompatibility and
mechanisms of cellular and interactions. On the basis of previous
research, Lee and co-workers described the cytotoxicity and
cellular uptakes of silica nanotubes with two different sizes and
surface charges in primary (nonmalignant) and cancer cell
models.
122
The effect of varying size and surface functionaliza-
tion of silica nanotubes on cytotoxicity was evaluated against
two different human cell models and it was suggested that the
silica nanotubes showed a similar trend for concentration
dependent cytotoxic effects, however the effect of size and charge
seemed to be more important. It was proposed that positively
charged silica nanotubes were more toxic than bare silica nano-
tubes even at a lower concentration of 0.5 mgml
1
, and in
addition to charge, the extent of interaction of silica nanotubes
with cells and toxicity might be determined by the size of silica
nanotubes. Shorter sized silica nanotubes were significantly more
toxic to the cells than their longer counterparts possibly due to
their increased cellular interactions, which is similar to the find-
ings on silica nanoparticles and carbon nanomaterials. The
possible mechanism of cellular uptake of silica nanotubes was
also proposed that silica nanotubes were internalized in cells at
least in part by endocytosis.
3.4 Hydrogen absorption and storage
Hydrogen has received much attention as an alternative energy
source for the replacement of fossil fuels in fuel-cell vehicles and
portable electronic over the past few years. Hydrogen storage
materials that may be able to store large amounts of hydrogen at
ambient temperature and relatively low pressures with a smaller
volume and lower weight and faster kinetics for recharging have
been a hot topic and been studied intensively. However, this area
has been dominated by carbon-based nanomaterials due to their
reported high-storage capacities, the design and applications of
silica-based hydrogen storage materials, in particular involving
silica nanotubes, at room temperatures has been rarely reported
so far.
It is well-known that a silica nanotube has a unique structure
with an open end at one side or both two sides, the inner and
outer surfaces of the silica nanotube could be functionalized
separately with different functional groups by silane chemistry. It
can also be prepared in diameters ranging from micrometre scale
to nanoscale with a mesoporous wall structure, as well as the
incorporation of a variety of dopants including fluorescent
quantum dots, magnetic or catalytic nanoparticles into the silica
matrix in a good manner, making it an excellent candidate for
a hydrogen storage medium. Many efforts have been made on
the design and applications of novel hybrid nanostructures based
on silica nanotubes and on the investigation of their hydrogen
adsorption and storage properties.
Jung and co-workers
123
exploited the application of palladium
(Pd)-doped doubled-walled silica nanotubes where Pd nano-
particles were loaded to act as the primary receptor for atomic
hydrogen. The results showed that the hydrogen adsorption
capacities of Pd-doped silica nanotubes was much higher than
that of silica nanotubes without Pd nanoparticles, suggesting the
doping of Pd nanoparticles into silica nanotubes is of great
importance to hydrogen absorption and uptake. It was also
demonstrated that nanostructures of hydrogen-storage mate-
rials, silica nanotubes, also influenced the hydrogen absorption
capacities heavily. The surface area, the pore size and the pore
volume of the silica nanotube play a very important role in the
uptake of high amounts of hydrogen, furthermore, a moderate
amount of Pd nanoparticles as well as open ends at both sides of
silica nanotubes were suggested.
Besides Pd nanoparticles, Lee and co-workers
124
described the
incorporation of aligned lithium (Li) into silica matrix to prepare
Li-dispersed silica nanotubes by a sol–gel template method, and
investigated their hydrogen absorption characteristics. XRD
results showed that the obtained silica nanotubes and as-
prepared Li-dispersed silica nanotubes had an amorphous silica
structure. It was shown from SEM and TEM images that silica
nanotubes and Li-dispersed silica nanotubes had an open end in
one side and a partially closed end in the other side. The
hydrogen uptake of different silica nanotubes suggested the
hydrogen absorption capacities of Li-dispersed silica nanotubes
were 0.29 wt%, which was relatively higher than those of silica
nanotubes, 0.15 wt% at 77 K under 4.5 MPa, in the absence of
Li
+
.
Naito and co-workers
125
expanded the diversity of metal
nanoparticles incorporated into silica nanotubes to group 8–
10 metals including Pt, Pd, Ru, Rh, Ir and synthesized various
silica nanotubes encapsulating Pt and Pd nanoparticles as well as
Ru-, Rh- and Ir-doped nanocapsules. The unique absorption
behaviour of H
2
and CO over metal nanoparticles inside the
silica wall was investigated. It was concluded that the pore sizes
formed inside of silica nanotubes depended on the nature of the
structure-determining template amine complexes, Pt- and Pd-
doped nanotubes were believed to be preferential for hydrogen
absorption due to their relatively smaller micropores inside the
silica wall, and metal nanoparticles inside the silica nanotubes
were considered as the main absorption sites for H
2
and CO at
room temperature.
4. Summary
This review provides an overview of template-based synthetic
methods to manufacture tubular silica nanostructures covering
a wide range of dimensions. It also illustrates some widespread
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applications of silica nanotubes in different areas. One of the
greatest advantages using template-based synthesis to grow or
prepare silica nanotubes is the precise control and optimization
of the lengths, diameters and the wall thickness of the resultant
nanotubes by rational adjusting the reaction conditions. Another
great advantage of the template-based synthesis of silica nano-
tubes is the possibility of synthesizing specific tubular nano-
structures including multilayered silica nanotubes, chiral or
helical silica nanotubes, doped or hybrid multifunctional silica
nanotubes. However, the literature associated with silica nano-
tubes is expanding very rapidly and the review summarized here
is by no means easy and comprehensive in covering all the rele-
vant literature.
Numerous challenges have to be overcome to provide well-
defined silica nanotubes for many applications, especially for
biological and biomedical applications. First, the large-scale
synthesis of high-quality of silica nanotubes requires a facile and
reproducible process. An unavoidable problem associated with
silica nanotubes is available sources of reliable templates. Only
a small spectrum of materials is suited to act as templates,
although most inorganic templates and biological templates
inspired from nature are thermally and chemically stable with
respect to the specific reaction conditions, their versatility is
limited. The use of organic compounds as templates may
require developed techniques of synthetic chemistry where
multiple-step reaction, harmful organic solvents or toxic
substances are normally involved. Consequently, it is crucial to
develop synthetic strategies and simplify the process in order to
expand the range of effective templates, and to minimize
negative effects of synthetic procedures to the surroundings and
humans. Second, for the purpose of using as tools in potential
applications, the functionalization of silica nanotubes is essen-
tial and complicated which is generally related to sol–gel
chemistry. Although aqueous sol–gel processes have been
investigated for decades, the simultaneous occurrence of
hydrolysis and condensation reactions leads to a wide variety of
different species, which cannot be identified and consequently,
aqueous sol–gel chemistry is not yet fully controllable. Due to
the fact that the sol–gel process is sensitive to reaction condi-
tions, slight changes of experimental parameters such as pH,
concentration, temperature, or nature of the solvent can lead to
substantial modifications of the resulting molecular assemblies.
This may give rise to silica nanotubes with enormous differences
in morphology and structure and, hence, in their properties and
applications. Moreover, different reactivities of precursors make
it difficult to control the composition and the homogeneity of
intermediate silane species as well as resulting silica nanotubes
by the aqueous sol–gel process. Last but not least, for many
inorganic nanomaterials—and silica nanotubes are no excep-
tion—the template-removal process is probably as important as
the synthetic conditions. It is necessary to avoid collapse of the
regular or ordered system during removal of the template.
Heating the templated silica nanotubes too fast during calci-
nation may cause the structure to collapse as gases escape,
heating too high cause the silica to become glassy and lose
structure. Thus, we should investigate the effects of slower
heating and lower temperatures, removal of the template using
other methods such as extraction, and supercritical treatment
would be interesting.
Acknowledgements
This work was financially supported by National Nature Science
Foundation of China (50903040), Natural Science Foundation
of the Jiangsu Higher Education Institutions of China
(10KJB430001) and Scientific Research Foundation for
Advanced Talents, Jiangsu University (10JDG057).
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