Unusual aggregation of poly(rC)-wrapped carbon nanotubes in aqueous suspension induced by cationic porphyrin

Abstract

Unusual aggregation of biopolymer-wrapped single-walled carbon nanotubes (SWCNTs) induced by cationic porphyrin meso-tetrakis(4-N-methyl-pyridyl)porphine (TMPyP4) in aqueous suspension has been studied by absorption spectroscopy and molecular dynamics simulation. After the addition of a small dose of TMPyP4 (10⁻⁶ M) into a poly(rC)–SWCNTs suspension, the absorption intensity of the nanotubes decreases, SWCNT bands demonstrate a red-shift and visible aggregation of SWCNTs is observed at concentrations above 10⁻⁵ M. However, formed aggregates are stable in suspension without precipitation for a few days. The Soret band of TMPyP4 in the absorption spectra exhibits hypochromism which is indicative of the π–π stacking interaction of the porphyrin core with π-systems of the biopolymer–SWCNT hybrid. The aggregation process of poly(rC)–SWCNT affects the spectral position of the Soret band, which is red-shifted. The spectral shift depends on TMPyP4 concentration: 18 nm at low porphyrin concentrations (10⁻⁷–10⁻⁶ M) and 14 nm at concentrations higher than 10⁻⁶ M. Such behavior of the Soret band is interpreted in terms of different types of porphyrin binding with poly(rC)–SWCNT resulting in effective nanotube aggregation. The molecular dynamics simulation is in good agreement with the experiment and demonstrates that the most stable complex, consisting of two nanotubes, can be formed when TMPyP4 molecules bind nanotubes, both owing to π–π stacking of the porphyrin core with the surface of one nanotube and with the oligonucleotide adsorbed onto another nanotube. The electrostatic interaction between porphyrin positively charged groups and the negative phosphate backbone of the oligonucleotide increases the stability of this complex as well.