Concentration dependent debundling and single tube dispersions of pristine multiwalled carbon nanotubes functionalized with double tail phospholipids

Abstract

Multiwalled carbon nanotubes (MWNTs) exist as aggregates of highly entangled tubes due to large aspect ratios and strong Van der Waals interactions among them in their native states. In order to render them suitable for any application, MWNTs need to be separated and dispersed uniformly in a solvent preferably as individual tubes. In the present work, it is demonstrated that a double tail lipid such as 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) is capable of dispersing MWNTs in ethanol. Ultra-stable suspensions were obtained by optimizing two key parameters: DPPE to MWNT weight ratio (ε) and MWNT concentration (c). Stability of the suspensions increased with the increasing ε value up to an optimum point (ε = 1.8) and then decreased drastically beyond that. CNT dispersions with ε = 1.8 were extremely stable (with a Zeta potential of 108.26 ± 2.15 mV) and could be retained in suspended form up to 3 months. Effect of MWNT concentration on disaggregation was very significant and stable suspensions could be formed for MWNT concentrations only below 0.14 mg ml−1. Above this concentration, no stable dispersions could be obtained even with ε = 1.8. Compression isotherms of Langmuir monolayers of the DPPE functionalized MWNTs spread at the air water interface were highly repeatable, suggesting that the MWNTs in dispersion were present as separate tubes coated with phospholipids. SEM micrographs of the Langmuir–Blodgett (LB) films, deposited at high surface pressures on silicon wafers, show that MWNTs remain as single nanotubes with no signs of reaggregation. TEM micrographs of MWNT suspensions indicated random adsorption of DPPE on MWNTs. Our work makes it possible to explore potential applications of LB films of MWNTs (stabilized by DPPE) in the development of conducting thin films for sensor applications or as supports to immobilize catalysts for heterogenous reactions.