Single-wall carbon nanotubes modified with organic dyes: Synthesis, characterization and potential cytotoxic effects

https://doi.org/10.1016/j.jphotochem.2010.01.019Get rights and content

Abstract

This work deals with the covalent functionalization of single-wall carbon nanotubes (SWNTs) with phenosafranine (PS) and Nile Blue (NB) dyes. These dyes can act as photosensitizers in energy and electron transfer reactions, with a potential to be applied in photodynamic therapy. Several changes in the characteristic Raman vibrational features of the dyes suggest that a covalent modification of the nanotubes with the organic dyes occurs. Specifically, the vibrational modes assigned to the NH2 moieties of the dyes are seen to disappear in the SWNT–dye nanocomposites, corroborating the bond formation between amine groups in the dyes and carboxyl groups in the oxidized nanotubes. The X-ray absorption (XANES) data also show, that the intense band at 398.6 eV attributed to 1s  2pπ* transition of the nitrogen of the aromatic PS ring, is shifted due to the bonding with the carbonic structure of the SWNTs. The cytotoxicity data of dyes-modified SWNT composites in the presence and absence of light shows that the SWNT–NB (4 μg/mL) composite presents a good photodynamic effect, namely a low toxicity in the dark, higher toxicity in the presence of light and also a reduced dye photobleaching by auto-oxidation.

Introduction

Single-wall carbon nanotubes (SWNTs) have unique properties, mainly due to their one-dimensional structure; such as exceptionally high tensile strength, high resilience, electronic properties ranging from metallic to semiconducting, high current carrying capacity, and high thermal conductivity [1]. The remarkable physical and chemical properties of carbon nanotubes (CNTs) have stimulated the investigation of new CNT based nanostructures, mainly those formed with polymers, metal particles and organic molecules [2], [3].

Increasing interest is being focused on the rational functionalization of both single- and multiwall CNTs (MWNTs) to fabricate functionalized nanostructures with novel properties [4]. The properties of carbon nanotubes can be modified by chemical interactions, such as electrochemical and/or chemical charge-transfer processes [5], [6]. Combination of both single and multiwall carbon nanotubes to organic molecules can lead to the formation of unique composites, with enhanced chemical and/or physical properties [7]. Chemical functionalization of carbon nanotubes has recently become a very important research area [8], [9], [10].

Assembly of various materials, organic and inorganic, onto the surface of CNTs has been reported. Covalent and non-covalent modifications of nanotubes properties have been explored in order to control the chemistry of CNTs. Electrochemical doping [11], electrostatic [9], covalent [12], and hydrophobic interactions have been investigated and can be used to shape their properties to specific applications. The ability to carry out controlled chemistry on the carbon nanotube surface plays an essential role in the application of these nanostructures, providing a means of purification, solubilization, biocompatibility, and diameter- or chirality-based separation [13], [14].

The challenge is to find a way to reproducibly and reliably chemically alter carbon nanotubes that, like graphite, are fairly unreactive. Nowadays, the SWNTs can be chemically changed through the oxidation of their fullerene-like caps followed by different reactions with the carboxylic groups (–COOH) being formed, to make it possible to attach a large variety of chemical groups in the carbonic structure [15], [16]. Functionalization of carboxylic groups with compounds that have structure like R–NH2 (where the R represents different organic groups) can be made by condensation reactions between amino and carboxylic groups. The reaction is mediated by condensation reagents, such as 1,3-dicyclohexylcarbodiimide (DCC) or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC).

Fullerenes, carbon nanotubes as well as their derivatives modified with light-sensitive molecules, such as dyes, have emerged as a new interdisciplinary research field [17]. The carbonaceous materials can promote the intensification and/or prolongation of the excited states of dyes, owing to charge-transfer interactions. Photosensitizers activated by light can induce the formation of singlet oxygen or other free radicals, which can irreversibly damage the treated tissues [18], [19]. It is generally accepted that singlet oxygen is the primary cytotoxic agent responsible for photobiological activity [20]. This is the background for the photodynamic therapy (PDT), which uses laser, or other light sources, combined with a light-sensitive drug (photosensitizing agent) to destroy cancer cells.

Phenazine dyes and their derivates can be induced to generate long-lived excited states which make them ideal as photosensitizers in energy and electron transfer reactions [21]. Up to now, the characterization of modified carbon nanotubes, modified with photosensitizers phenazine dyes, was scarcely investigated. The self-assembly of phenosafranine (PS dye, see Scheme 1) to acid treated multiwall carbon nanotubes (MWNTs) was investigated [22]. PS has received considerable attention due to its photoredox properties [23], [24]. The authors have shown that the cationic PS dye chemisorbs in the defects along the MWNT surface. UV–vis spectroscopy reveals an electron transfer process from PS to the MWNTs π* band. Nile Blue (NB, see Scheme 1) is a well-investigated oxazine dye and can also be used as a photosensitizing agent when attached to SWNTs in photodynamic therapy studies (PDT) [25].

The main goal of this work is the spectroscopic characterization of the nanocomposites prepared by condensation reactions between SWNTs and PS (phenosafranine) or NB (Nile Blue) dyes. In addition, the in vitro cytotoxic effects of these composites are also investigated.

Section snippets

Chemicals and materials

Organic dyes phenosafranine (PS, Aldrich, 3,7-diamino-5-phenylphenazinium chloride) and Nile Blue (NB, Merck, 5-amino-9-(diethylamino)benzo(a)phenoxazin-7-ium sulfate) were used as received. The closed-ended SWNTs carbon nanotubes (CarboLex AP-grade) are made by Arc Method. According to the producer, these carbon nanotubes have an average diameter 1.4 nm and are found in “ropes” which are typically ∼20 nm in diameter or approximately 50 tubes per rope with lengths of 2–5 μm. Impurities include

SWNT–PS composite

Fig. 1 shows the UV–vis spectra of plain PS (A) and a physical mixture of SWNT + PS (B) and a composite material with the SWNTs (C) in acetonitrile. In the UV–vis spectra of both, the physical mixture and the composite material, the band in the visible region at ca. 503–530 nm is observed, and this band is related to the PS dye absorption. The spectra of solutions of the composites (spectra B and C) have a high background owing to the light scattering of the suspended carbon particles. For the

Conclusions

The reactivity of single-wall carbon nanotubes with organic dyes molecules was investigated. Changes in the characteristic Raman vibrational features of the dyes suggest covalent modification of nanotubes with the organic molecules. Specifically, the vibrational modes assigned to the NH2 moieties of the dyes are seen to disappear in the SWNT–dye nanocomposites, corroborating the bond formation between amine groups in the dyes and carboxyl groups in the SWNTs (ox). The combination of dyes and

Acknowledgments

This work has been supported by the Conselho Nacional de Desenvolvimento Cientifico e tecnológico (CNPq) and Fundação de Amparo a Pesquisa no Estado de São Paulo (FAPESP). The authors would like to thank the National Synchrotron Light Laboratory (LNLS/Brazil) for the use of SGM beamline (XANES N K-edge, Proj. 2169/03). Fellowships from CNPq (G.M. do Nascimento, N.A. Pradie, P. Corio, P.G. Lins, G.R. Martinez and P. Di Mascio) are gratefully acknowledged. The authors are indebted with the LCCA

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