Enhanced electrostatic modulation of ionic diffusion through carbon nanotube membranes by diazonium grafting chemistry

Abstract

A membrane structure consisting of an aligned array of open ended carbon nanotubes (~ 7 nm i.d.) spanning across an inert polymer matrix allows the diffusive transport of aqueous ionic species through CNT cores. The plasma oxidation process that opens CNTs tips inherently introduces carboxylic acid groups at the CNT tips, which allows for a limited amount of chemical functional at the CNT pore entrance. However for numerous applications, it is important to increase the density of carboxylic acid groups at the pore entrance for effective separation processes. Aqueous diazonium based electro-chemistry significantly increases the functional density of carboxylic acid groups. pH dependent dye adsorption-desorption and interfacial capacitance measurements indicate ~ 5-6 times increase in functional density. To further control the spatial location of the functional chemistry, a fast flowing inert liquid column inside the CNT core is found to restrict the diazonium grafting to the CNT tips only. This is confirmed by the increased flux of positively charged Ru(bi-py) with anionic functionality. The electrostatic enhancement of ion diffusion is readily screened in 0.1(M) electrolyte solution consistent with the membrane pore geometry and increased functional density.

Towards mimicking natural protein channels with aligned carbon nanotube membranes for active drug delivery

“…The first systematic study (Majumder et al 2005a) to prove that ‘gatekeeper’ chemistry can be formed on CNT membranes was with a series of four chemically distinct gatekeepers: short chained alkane, long chained alkane, long polypeptide, and highly charged dye molecule. In this study, IR data was consistent with the formation of carboimide linking chemistry at the surface which was confirmed by later electrochemical studies (Majumder et al 2008). The simultaneous diffusional transport of ionic dyes through the membrane was measured.…”

Review on carbon nanotubes and carbon nanotube bundles for gas/ion separation and water purification studied by molecular dynamics simulation

“…Since MD simulations predicted the fast water transport through CNTs (Hummer et al 2001), it attracted attentions (Corry 2008;Yu et al 2010), and later it was observed in experimental results (Holt et al 2006;Majumder et al 2005). Recently, in the published research of Majumder et al (2008) it was shown that the rate water flows through multiwalled CNTs was four to five orders faster than the predicted rate by macroscopic hydrodynamics. The reason of flow enhancements was the more ordered and stronger hydrogen bonds between the water molecules onto the pores of CNTs than in bulk water that causes concerted and rapid motion along the tube axis.…”

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications

“…CNTs act as the working electrode and diazonium compounds are dissolved in the KCl and HCl aqueous electrolyte. [296][297][298][299] The functionalized CNTs via electrochemical diazonium grafting enhance the ionic diffusion of the CNT membranes. 298,299 Diazonium salt functionalization favors the electron transfer kinetics across a wide range of potentials.…”

“…[296][297][298][299] The functionalized CNTs via electrochemical diazonium grafting enhance the ionic diffusion of the CNT membranes. 298,299 Diazonium salt functionalization favors the electron transfer kinetics across a wide range of potentials. Through adjusting the electrochemical conditions, applied potential as well as the type of diazonium salt, it is possible to get an electrochemical charge transfer rate over metallic SWNTs 10 4 times higher than that of semiconducting SWNTs.…”