The Kinetics of Dye Encapsulation in Single-Walled Carbon Nanotubes Probed By Statistical Raman Imaging

Single-Walled Carbon Nanotubes (SWNTs) are nanosystems with a large aspect ratio that have potential in a wide range of optical applications, such as in biomedical imaging [1]. An important advantage of SWNTs in that context is their capability to support two or more functionalities, for instance detection as well as targeting of molecules of interest. The hollowness of SWNTs interior is therefore particularly interesting since SWNTs can act as a template for the creation of new hybrid nanostructures, while leaving the surface free for further functionalization. It was shown that dyes encapsulated inside carbon nanotubes (CNTs) are protected from degradation and present a strong Raman signature with narrow emission peaks, free of background fluorescence, which could enable their use as Raman optical markers [2]. Further, confinement inside SWNTs was found to largely impact molecular organisation [3], tuning the physical and chemical properties of the encapsulated molecules. However, the kinetics of such a system, as well as the precise mechanism of aggregate formation, is still not completely understood. In this study, we explore the encapsulation mechanism of dye molecules in SWNTs. Sexithiophene (6T) molecules were chosen due to their well-conjugated, rod-like structure and giant Raman signal upon encapsulation, at an excitation of 532 nm. Raman imaging (RIMA, Raman Imaging system, Photon etc.) is used to obtain statistical information from a large number of individual SWNTs (>1000 per data point). The encapsulation is carried out using a liquid-phase protocol, allowing the investigation of the impact of various encapsulation parameters (dye concentration and temperature) on the kinetics of the encapsulation process. Spatially resolved Raman imaging allows the comparison of filling and densification speeds for all encapsulation parameters, showing that, in all studied cases, the filling is faster than densification, which occurs over longer time scales. Raman imaging also enables the tailored analysis of different sub-populations, in order to probe the effect of the length or the diameter of the SWNTs on the encapsulation kinetics. Furthermore, Raman intensity distributions are studied, which provide signatures of single and double aggregation that can be correlated to SWNTs characteristics. Finally, a kinetic model and an encapsulation mechanism are proposed. Based on the intensity distributions and the proposed kinetic model, we predict the apparition of double aggregation for all the encapsulation parameters investigated. These results provide much needed insight on aggregate formation in 1D systems, which is crucial to the development of well-behaved and controlled hybrid materials. [1] Kostarelos, K., Bianco, A., & Prato, M. (2009). Promises, facts and challenges for carbon nanotubes in imaging and therapeutics. Nature nanotechnology, 4(10), 627-633. [2] Gaufrès, E., Tang, N. W., Lapointe, F., Cabana, J., Nadon, M. A., Cottenye, N., ... & Martel, R. (2014). Giant Raman scattering from J-aggregated dyes inside carbon nanotubes for multispectral imaging. Nature Photonics, 8(1), 72-78. [3] Almadori, Y., Alvarez, L., Le Parc, R., Aznar, R., Fossard, F., Loiseau, A., ... & Rahmani, A. (2014). Chromophore ordering by confinement into carbon nanotubes. The Journal of Physical Chemistry C, 118(33), 19462-19468.