nica e la Fotoreattivita, Consiglio Nazionale delle Ricerche (CNR-ISOF) in Bolonia, have developed an efficient method to fabricate a new photonic ma- terial: carbon nanotubes coated with chemicals that are capable of displaying red light. “We take part at the project as a research group specializing in studies on lanthanide compounds. We decided to combine their high luminescent properties with excellent me- chanical and electrical characteristics of nanotubes,” says Prof. Marek Pietraszkiewicz from Warsaw’s Institute of Physical Chemistry of the Polish Acade- my of Sciences (IPC PAS).
Carbon nanotubes can be envisaged as a graphite sheet rolled-up into a seamless cylinder. The surface area of each nanotube is relatively high and allows to attach many other molecules, including those capable to emit light. “Attachment of light-emitting complexes directly to the nanotube is, however, not favourable, because the latter, as a black absorber, would highly quench the luminescence”, explains Valentina Utochnikova, a PhD student at the IPC PAS. To reduce undesired effect of light absorption, the nanotubes are first subject to a thermal reaction at temperature 140-160° C (284° F to 320° F) in a so- lution of ionic liquid modified with a terminal azido function. The reaction yields nanotubes coated with molecules acting as anchors-links. On one side the anchors are attached to the surface of the nanotube, and on the other they can attach molecules capable of displaying visible light. The free terminal of each link bears a positive charge.
So prepared nanotubes are subsequently transferred into another solution containing a negatively charged lanthanide complex – tetrakis-(4,4,4-trifluoro-1-(2- naphtyl-1,3-butanedionato)europium. “Lanthanide
11-06/07 :: June/July 2011
compounds contain elements from the VI group of the periodic table and are very attractive for pho- tonics, as they are characterised by a high lumine- scence quantum yield and a high colour purity of the emitted light,”, stresses Utochnikova.
After dissolving in solution, negatively charged europium complexes are spontaneously caught by positively charged free terminals of anchors attached to nanotubes due to electrostatic interaction. As a result, each nanotube is durably coated with mole- cules capable to emit visible light. Upon completion of the reaction, the modified nanotubes are washed and dried. The final product is a sooty powder. If the powder is, however, exposed to UV irradiation, the lanthanide complexes anchored to nanotubes start immediately to emit red light.
The concept of how to modify the nanotubes and the reagents – ionic liquid and lanthanide complex for carbon nanotube coating – has been developed in Prof. Pietraszkiewicz’s research group at the IPC PAS, whereas the modification of nanotubes and spectral studies have been performed by research groups from the University of Namur, Belgium, and CNR-ISOF from Bolonia, Italy. It is essential that chemical reactions leading to fabrication of new light-emitting nanotubes turned out to be significant- ly simpler than those used so far.
The photonic material received can be used, among others, to detect molecules including those of bio-logical importance. The identification would then take place by analysing of how the luminescence of nanotubes changes upon deposition of molecules under study thereon. Good charge conductivity com- bined with high luminescence properties make new