Congo Red Interactions with Single-Walled Carbon Nanotubes
A new method of dispersion of single-wall carbon nanotubes (SWNT) in aqueous solution using supramolecular compounds is proposed in this chapter. The described system consists of SWNT overlaid by Congo red. SWNT are formed from a rolled layer of graphene, providing a large surface area for binding compounds with planar, aromatic structures (including drugs). Congo red is able to associate with proteins in the form of supramolecular, ribbon-like structures, and may bind various drugs by intercalation.
The study reveals strong interactions between Congo red and the surface of SWNT. The authors’ aim was to explain the mechanism driving this interaction. Spectral analysis of the SWNT-CR complex, effects of sonication on CR binding, microscopic imaging and molecular modelling analyses are all discussed. Results indicate that binding of supramolecular Congo red to the surface of nanotubes is based on face-to-face stacking. Having attached itself to the surface of a nanotube, a dye molecule may attract other similarly oriented molecules, giving rise to a protruding supramolecular appendage. This explains the high affinity of CR for nanotubes and the resulting system’s capability to bind drugs.
Analysis of complexes formed by SWNT-CR with the model drug (DOX) and with other planar compounds (EB, TY) indicates that it may be possible to construct complexes capable of binding multiple compounds simultaneously.
KeywordsSingle-walled carbon nanotubes Supramolecular compounds Bis-azo dye Congo red Chemical container Drug delivery system Shortenng of carbon nanotubes Pi-pi stacking Face-to-face stacking
7.1 Methods of Dispersing Carbon Nanotubes – The Search for Optimal Dispersion Methods
Carbon nanotubes (CNT) are used in electronics , composite materials research [2, 3], catalysis , textiles  and many other areas. A particularly interesting is their use in the field of biomedicine: in biosensors [6, 7], medical diagnostics [8, 9], transplantology [10, 11], tissue engineering  or in pharmacology – including the field of targeted therapies [13, 14, 15, 16].
In studies on the use of CNT in biomedicine it is necessary to obtain their suspension in water media. CNT are most often found not in the form of single fibres, but in the form of bundles stabilized by van der Waals interactions and it is very difficult to evenly distribute them in the liquid phase. The purpose of research is thus to find the compounds that provide effective dispersion of carbon nanotubes and simultaneously bind other compounds, including some drugs. Numerous reports concern both covalent or non-covalent modifications of CNT .
The covalent modification based on the introduction of functional groups into the graphene surface of carbon nanotubes is an effective way to increase CNT dispersion. This may, however, lead to changes in the physicochemical properties of CNT and create defects in their structure [18, 19, 20].
Non-covalent modification is considered a less invasive method. Because of the fact, that there is no heating or acidic environment involved, virtually no damage is done to the structure of nanotubes [8, 21]. Among non-covalent CNT modifications one can distinguish: interactions with amphiphilic molecules – surfactants (e.g. SDS – sodium dodecyl sulphate, SDBS – sodium dodecylbenzenesulphonate , CTAB – cetyltrimethylammonium bromide , DDAB – dimethyldioctadecyl -ammonium bromide, Triton, Pluronic ). The most effective interactions between CNTs and surfactants were observed for surfactants containing a benzene ring in the structure (e.g. SDBS), due to the presence of strong pi-pi stacking interactions between the phenyl ring and the graphene nanotube surface .
Other compounds used for the noncovalent functionalization and dispersing of CNT include: ionic liquids (ILs) – which form bucky gels, polymers – especially those with aromatic rings in their structure , other compounds with an aromatic ring  (for example ammonium salts containing pyrene rings ) and or deoxycholic acid sodium salts .
7.2 The Interaction of Congo Red with Carbon Nanotubes
CR , a bis-azo dye, similarly to the carbon nanotube dispersing compounds mentioned above, has aromatic rings in its structure. It is therefore possible to form a non-covalent pi-pi interaction between CR molecules and the graphene surface of carbon nanotubes. Reports can be found in literature on the interaction of CR with carbon nanotubes [28, 29, 30, 31] and the surface of graphene oxide [32, 33].
Hu C. et al.  describe a complex of single-walled carbon nanotubes (SWNT) with CR (SWNT-CR ), with water solubility of up to 3.5 mg/mL. The described complex was formed by prolonged grinding of SWNT and CR in the agate mortar, followed by drying. In this case CR probably interacts with the surface of nanotubes as single molecules bound to the nanotube surface via pi-pi interactions. The described method enables nanotubes dispersion, however the results may not be reproducible and the CR binding effectiveness is relatively low.
In another paper  authors point to the possibility of using CNT for adsorbing dyes from aqueous solutions – in post-production wastes containing textile dyes. CR was used as a model compound in these studies and the results show an effective binding of CR by carbon nanotubes (with a maximum adsorption of 500 mg/g).
The SWNT-CR complexes were also analysed by molecular modelling, which has shown various possibilities of nanotube-CR interaction depending on nanotube diameter . In the case of narrow nanotubes, CR partially retains the supramolecular ribbon structure, adhering to the outer nanotube surface, while some of the molecules are adsorbed individually on the surface (as shown in Fig. 7.2). In the case of wider nanotubes, individual CR molecules adsorb to the nanotube surface and supramolecular ribbon-like structur e of CR is lost. The geometry of the wide nanotube increases the contact surface between CR molecule and the nanotube which allows to ascribe the dominant role in the creation of the system’s equilibrium structure to the pi-pi stacking . Nanotubes of smaller diameter have a smaller side wall area that is characterized by a greater curvature. For this reason, partial preservation of the supramolecular ribbon structure of CR is preferable. The molecular modelling results show a tendency for CR to group into ribbons, while simultaneously leaving a portion of the nanotube surface exposed, which was also observed in microscopic images. These results were also confirmed by analysis of the radial distribution functions (rdf) between CR and SWNT .
7.3 The Incorporation of Other Compounds by SWNT-CR Complexes – Examples of Possible Biomedical Use
A particularly interesting feature of carbon nanotube-CR complex is its ability to bind other molecules. CNT, as hollow structures, are excellent high volume lightweight containers, in which other molecules can be enclosed while their large surface allows for efficient adsorption of many compounds. CR (and similar compounds) that create ribbon-like supramolecular structures can bind numerous polyaromatic planar molecules that intercalate into supramolecular CR ribbon. Compounds that can be bound this way include e.g. antineoplastic drug doxorubicin, Titan yellow , rhodamine B. The combination SWNT and CR creates a “chemical container” characterized by significantly increased capacity and the ability to bind different molecules simultaneously. It could be used as a drug carrier , which limits the drug’s toxicity and allows for the targeted delivery .
The dispersion of carbon nanotubes based on their interaction with CR is simple, efficient and reproducible. Supramolecular systems (both pure CR and mixed) allow not only to disperse carbon nanotubes, but can also bind other compounds through intercalation. As carriers of different compounds (e.g. fluorescent dyes, drugs, metal ions), these complexes present an interesting alternative to the currently used systems. They can be used in diagnostics as well as targeted delivery system s for drugs or metal ions.
We acknowledge the financial support from the National Science Centre, Poland (grant no. 2016/21/D/NZ1/02763) and from the project Interdisciplinary PhD Studies “Molecular sciences for medicine” (co-financed by the European Social Fund within the Human Capital Operational Programme) and Ministry of Science and Higher Education (grant no. K/DSC/001370).
- 21.Backes C, Hirsch A (2010) Noncovalent functionalization of carbon nanotubes in chemistry of nanocarbons. Wiley, Ltd, Chichester, pp 1–48Google Scholar
- 36.Spólnik P, Król M, Stopa B et al (2011) Influence of the electric field on supramolecular structure and properties of amyloid-specific reagent Congo red Eur. Biophys J 40(10):1187–1196Google Scholar
- 39.Chłopaś K, Jagusiak A, Konieczny L et al (2015) The use of Titan yellow dye as a metal ion binding marker for studies on the formation of specific complexes by supramolecular Congo red. Bio-Algorithms Med-Syst 11(1):9–17Google Scholar
- 41.Stopa B, Piekarska B, Jagusiak A et al (2011) Acta Biochim Pol 58(Suppl. 2):282. Supramolecular Congo red as a potential drug carrier. Properties of Congo red-doxorubicin complexes in Proceedings of the 2nd Congress of Biochemistry and Cell Biology 46th Meeting of the Polish Biochemical Society and 11st Conference of the Polish Cell Biology Society, Kraków, 2011Google Scholar
- 42.Jagusiak A, Piekarska B, Chłopaś K et al (2016) Shortening and dispersion of single-walled carbon nanotubes upon interaction with mixed supramolecular compounds. Bio-Algorithms Med-Syst 12(3):123–132Google Scholar
Open Access This chapter is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The images or other third party material in this book are included in the work's Creative Commons license, unless indicated otherwise in the credit line; if such material is not included in the work's Creative Commons license and the respective action is not permitted by statutory regulation, users will need to obtain permission from the license holder to duplicate, adapt or reproduce the material.