“Smartly” functionalized carbon nanotubes (CNTs) constitute an actively pursued research topic in the fields of nanomaterials and nanotechnology. The development of highly efficient and selective methodologies for dispersing CNTs in the liquid phase has not only made efficient separation and purification of CNTs possible, but also opened the doors to many fascinating material and biological applications. Very recently, the development of CNT hybrid systems with controlled stimuli-responsiveness has achieved significant breakthroughs. This chapter outlines the state of the art within this vibrant research area, and examples from the most recent literature are selected to demonstrate progress in the preparation of CNT composites, the physical properties of which can be readily switched by various external stimuli (e.g., pH, photoirradiation, solvent, temperature, etc.).
- carbon nanotubes
- chemical functionalization
- supramolecular chemistry
Since the first synthesis of carbon nanotubes (CNTs) by Ijima in 1991 , CNT-based materials have attracted tremendous interest from both academia and industry. Indeed, CNTs along with other nano-sized carbon materials (
CNTs are carbon allotropes in a tubular shape with nanoscale diameter (
The synthesis of CNTs can be done in several ways, for example, arc discharge, laser ablation, chemical vapor deposition (CVD), and others . However, all the currently existing processes for CNT production generate mixtures of various CNTs, amorphous carbon, and/or residual metal species. For this reason, post-treatment of as-produced CNT products becomes a critically important and indispensable step toward the application of CNTs , since removal of the unwanted impurities can give rise to considerably improved performances for CNT-based electronic and optoelectronic devices, while the purity of CNTs plays a key role in reducing or averting undesired toxicity and side effects in biological and medicinal applications. Another significant technical challenge encountered in the application of CNTs is the extremely low solubility and hence very poor processability of pristine CNTs in common solvents. The very large hydrophobic π-surface of CNTs makes them prone to aggregation (forming bundles) through π-stacking and van der Waals interactions. Breaking the bundles into individual tubes is an energetically costly process that usually relies on chemical modifications to overcome the strong inter-tube attractions. There are two general approaches for preparation of functionalized CNTs (
The generally employed strategy for non-covalent functionalization of CNTs relies on the use of certain dispersing agents (
In short, the present knowledge and techniques for dispersed CNT systems and subsequent applications have been considerably expanded, thanks to the continued efforts of material chemists on the design and synthesis of novel molecular and macromolecular systems as functional dispersants or agents for CNT modifications. Indeed, the current design of CNT dispersants and development of related processing methods have already surpassed being merely a single-parameter issue of improving dispersity. Rather, the objectives of research have turned out to be more and more diversified and application-oriented. In the following sections, our discussions are firstly focused on a newly emerging topic—reversible dispersion and release of CNTs regulated by rationally designed stimuli-responsive molecular and supramolecular systems—which has attracted rapidly growing attention in recent years. In the second part, the recent progress in polymer-functionalized CNT composites that show controlled responsiveness to one or more environmental stimuli is described.
2. Dispersion and release of CNTs: a microscopic and supramolecular perspective view
Effective dispersion of CNTs requires sufficient energy input (
In the literature, a large array of molecular and macromolecular motifs has been reported to give satisfactory graphenophilic performance. In the category of small molecules, planar structurally rigid polycyclic aromatic hydrocarbons (PAHs) have been known to easily adsorb on the surface of CNTs through a combination of hydrophobic and π-π interactions [28, 29]. Examples of such PAHs include pyrene, naphthalene, and phenathrene, which carry the fused aromatic backbones, mapping out the segmental hexagonal arrangement on the sidewall of CNTs. It is worth noting that since the initial study by Dai and co-workers , pyrene has become a popularly employed anchoring group allowing facile assembly of CNT supermolecular constructs imparted with various optoelectronic properties [31–34] and biologic activities [35–38] By the same token, highly π-extended heterocyclic aromatic compounds, such as phorphyrins, , phthalocyanines , and tetrathiafulvalenes [41, 42], have been found to show strong non-covalent interactions with CNTs, rendering the resulting hybrid systems intriguing photophysical and optoelectronic properties. Besides the small molecule-based systems, π-conjugated polymers have been widely used in CNT dispersion as well . In these conjugated polymers, arene groups are embedded in the repeat units, which collectively engender strong binding to CNTs via π-π stacking. Experimental observations and molecular dynamic (MD) simulations have demonstrated that the polymer backbones with a certain degree of flexibility would favor wrapping around the CNTs in different fashions [43, 44]. DNA or RNA molecules can interact with the CNT sidewall via π-π stacking. In particular, single-strand DNA (ssDNA) oligomers and polymers have been extensively explored as SWNT dispersants and the interactions were found to be sequence-dependent [45–51]. Such properties have enabled fascinating applications including precise sorting of SWNTs, DNA sequencing, and chemical sensing applications . Indeed, for many CNT dispersants developed so far, aromatic functional groups (
With the wide range of CNT dispersants studied over the past decade, fundamental understanding of their non-covalent interactions with CNTs at the molecular and supramolecular levels has been continually developed by state-of-the-art spectroscopic and microscopic analyses as well as high-level theoretical simulations. Knowledge in this field is highly instructive to the design of more sophisticated CNT-dispersing methods that are transferrable to the application of CNT-related materials in science and technology. One topic receiving much attention in the recent research of CNTs is the reversible dispersion and release of CNTs under the control of external stimuli, such as irradiation, chemical reactions, and solvation effects. Basically, to generate stable CNT dispersion in the solution phase, the CNT surface needs to be functionalized or encapsulated with dispersants to a sufficient degree. This in turns makes the dissociation of CNTs from dispersants not an easy task; for instance, many polyaromatics and π-conjugated polymers are known to irreversibly adsorb onto the surface of CNTs, which makes it extremely difficult to remove them from CNTs by physical means (
3. Recent advances in reversible dispersion and release of CNTs with stimuli-responsive dispersants
3.1. By acid-base interactions
Acid-base interactions are probably the most straightforward ways to drastically alter the chemical and physical properties, for instance, changing from neutral to cationic/anionic, from organic solubility to water solubility. For this reason, this approach has become the actively pursued one in this field. Das and co-workers  recently developed a series of cholesterol-based amino acid carboxylates
Based on a similar acid-base exchange concept, Huang and co-workers  recently synthesized a pH-responsive pillararene
In 2015, Bryce and Lambert  synthesized a total of 13 amphiphilic surfactants, the structures of which were made of a pyrene head (hydrophobic) and various hydrophilic tails ending with carboxylate groups. Similar to the design of numerous other CNT dispersants, the pyrene group here was employed to act as a strong graphenophile to irreversibly link the surfactant molecules to the surface of CNTs. The performances of these pyrene-based surfactants in terms of MWNT dispersion in aqueous media were assessed. In particular, the authors examined the pH responsiveness of two of the surfactants
A novel dumbbell-shaped 2-ureido-6[1H]-pyrimidinone (UPy)-based fluorene derivative
In addition to the above-mentioned rationally designed CNT dispersant systems, some other pH-sensitive molecules and biopolymers have also been reported to induce reversible dispersion/precipitation of CNTs in aqueous media. For example, Sun and co-workers  reported that 1-pyreneacetic acid after deprotonation under basic conditions could be non-covalently functionalized on nitric acid-treated as-produced SWNTs to form stable dispersion in water. Upon acidification, 1-pyreneacetic acid was quantitatively removed to yield purified SWNTs. The easy recovery and reuse of dispersants make this method potentially useful for large-scale CNT processing and production. Bhattacharya
Apart from typical protic acids, CO2 has been utilized as a form of acid to trigger reversible release of CNTs from corresponding CNT-dispersant assemblies in solution. In 2010, Zhang and co-workers  reported the use of
3.3. By photoirradiation
Photoirradiation offers an effective way to trigger conformational and bonding changes in molecules, and therefore photo-responsive systems have found wide applications in molecular switches and photochromic devices . In the field of CNT dispersion, photo-responsive dispersant systems have been explored, but not yet to a very large extent.
In 2008, Zhang and co-workers  developed poly(ethylene glycol)
In a very recent report, Feng and co-workers  devised a photo-controlled method to achieve reversible dispersion and re-aggregation of SWNTs by means of photoswitchable guest-host chemistry. In their work, a pyrene-attached cyclodextrin
The easy synthesis and controllable photo-switchability of azobenzene have made it a popular building block in the design of photo-responsive CNT dispersants, but one important issue related to it warrants particular attention. It has been noted in some previous studies that when the azobenzene unit is tightly bound to the surface of CNT, the
3.3. By redox chemistry
There are many redox-active systems known to undergo facile reversible electron transfers under the controls of either chemical or electrochemical means, and often a reversible redox reaction is associated with a dramatic conformational change in the substrate. Such properties can be utilized to exert control over the dispersion and release of CNTs, if the dispersants are rationally designed to carry certain redox-active units. In 2008, Ikeda and co-workers  developed a Cu-based redox-active complex
A class of highly electron-donating organic compounds, namely tetrathiafulvalene vinylogoues (TTFV), has been investigated by Zhao and co-workers as redox-responsive units to be integrated in redox-regulated “smart” molecular and polymer systems . In general, a TTFV unit can undergo a simultaneous two-electron transfer to form a dication in the presence of a chemical oxidant (
3.4. By temperature control
The control over dispersion and release of CNTs can also be achieved using temperature-sensitive dispersants. For example, Wang and Chen in 2007 investigated the dispersion of SWNTs with temperature-responsive poly(
Most recently, a family of pyrene-based non-ionic surfactants was synthesized and studied by Bryce and Lambert . Of these compounds, two surfactants
3.5. By solvent control
Compared with the aforementioned types of stimuli-responsiveness, the tuning of solvent properties is a much easier way as it neither involves multiple steps of addition and separation of chemical species, nor requires significant energy inputs (
In 2013, Mulla and Zhao  reported the synthesis and properties of a series of linear and
Bonifazi and co-workers  in 2011 reported a strategy of solvent-controlled hydrogen bonding interactions to achieve reversible dispersion and release of MWNTs. In their work, H-bonding supramolecular polymers were respectively assembled by complementary H-bonding recognition between di(acetylamino)pyridine-terminated molecules (
4. CNT-polymer composites responsive to single or multiple external stimuli
Stimuli-responsive polymers show the intriguing behavior that their shapes, physical, electrical, and optical properties can be significantly changed in response to small variations of environmental conditions, such as pH, temperature, electrical field, ionic strength, solvent, and so on [86, 87]. As such, stimuli-responsive polymers have been widely used as active building blocks to develop advanced nanomaterials and molecular devices. Investigations on functionalization of CNTs, either covalently or non-convalently, with stimuli-responsive polymers have been actively carried out over the past decade. In 2008, Pan and Hong  contributed a review article outlining the progress in designing
In 2014, Luo
In 2012, Yuan and co-workers prepared a copolymer through free radical polymerization of
In 2016, Mandal and co-workers used RAFT polymerization to synthesize a type of cationic poly(ionic liquid)
Barner-Kowollik and co-workers recently synthesized cyclopentadienyl end-capped poly(
5. Conclusions and perspectives
The literature survey discussed above has demonstrated that stimuli-responsive molecular and macromolecular systems can be successfully applied to attain reversible dispersity of CNTs in solutions as well as sensitive changes in other physical properties. Each type of the external stimuli aforementioned exhibits certain advantages and offers promising opportunities for the preparation of “intelligent” nanohybrids with improved properties and enhanced performance than conventional CNT-based materials. On the other hand, significant challenges are still present, which require continued research efforts to address both the fundamental and practical aspects. For the systems based on chemical or photochemical stimuli, controllability and tenability of the reversibility of the chemical and supramolecular reactions involved are the key issues to investigate. Besides the currently used methods (
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