A novel CNT-based hybrid luminescent material was obtained via encapsulation of a C 60 -based Eu(III) complex into single-, double-and multi-walled carbon nanotubes (SWCNTs, DWCNTs and MWCNTs, respectively). Specifically, 10 a luminescent negatively-charged Eu(III) complex, electrostatically bonded to an imidazolium-functionalized fullerene cage, was transported inside CNTs by exploiting the affinity of fullerenes for the inner surface of these carbonaceous containers. The filling was performed under supercritical CO 2 (scCO 2 ) conditions to facilitate the entrapment of the ion-paired assembly. Accurate elemental, spectroscopic and morphological characterization not only demonstrated the efficiency of the filling strategy, but even the occurrence of a nano-ordering of the 15 encapsulated supramolecular luminophores when SWCNTs were employed. Preliminary confocal microscope imaging studies showed that these hybrids could have some potential for diagnostic applications.
IntroductionAlong with an increase of the available methodologies for their manipulation, [1] and the development of atomic-resolution 20 visualization techniques, mainly high-resolution transmission electron microscopy (HRTEM), [2] current efforts in carbon nanotube (CNT) [3] research are directed towards the study and exploitation of their porous properties. Indeed, CNTs can be employed as nano-containers, nano-reactors, or as fillable 25 conductive nano-vessels, able to give rise to advanced hybrid graphitic materials utilizable in electronic devices.[4]The feasibility of the entrance of guest molecules within the inner channel of CNTs relies on the favorable establishment of a range of interactions between the "guest" molecules and the 30 "host" CNTs, [5] amongst which van der Waals forces often appear to be predominant. [6] These are maximized when the host-guest complex occurs between CNTs and related graphitic materials, [7] such as fullerenes, e.g. C 60 and C 70. In fact, the common nature of the two materials, together with the perfect geometrical match of 35 the shape of a fullerene and the inner nanotube surface of singlewalled CNTs (SWCNTs), results in a spontaneous encapsulation.[8]The use of CNTs as templating nano-containers has recently started to draw attention as a possible strategy for the synthesis of 40 novel highly organized photonic materials. It is well established that the emission properties of a chromophore, in terms of color and intensity, are extremely sensitive to the environment. [9] Encapsulation of luminophores within a protective "cage" (i.e. CNTs) might hence grant a sheltering effect from hostile reactive 45 species, as Yanagi et al. demonstrated with the encapsulation of β-carotene within SWCNTs, [12] allowing longer-lasting luminescence performances of the hosted molecules. Furthermore, molecular confinement in such a restrained space might induce a well-ordered mono-dimensional molecular constituents remained preserved.[20]Since then, different approaches to blend CNTs and LnCs have been reported. [21...