Aqueous cationic, anionic and non-ionic multi-walled carbon nanotubes, functionalised with minimal framework damage, for biomedical application

Abstract

The use of a thermochemical grafting approach provides a versatile means to functionalise as-synthesised, bulk multi-walled carbon nanotubes (MWNTs) without altering their inherent structure. The associated retention of properties is desirable for a wide range of commercial applications, including for drug delivery and medical purposes; it is also pertinent to studies of intrinsic toxicology. A systematic series of water-compatible MWNTs, with diameter around 12 nm have been prepared, to provide structurally-equivalent samples predominantly stabilised by anionic, cationic, or non-ionic groups. The surface charge of MWNTs was controlled by varying the grafting reagents and subsequent post-functionalisation modifications. The degree of grafting was established by thermal analysis (TGA). High resolution transmission electron microscope (HRTEM) and Raman measurements confirmed that the structural framework of the MWNTs was unaffected by the thermochemical treatment, in contrast to a conventional acid-oxidised control which was severely damaged. The effectiveness of the surface modification was demonstrated by significantly improved solubility and stability in both water and cell culture medium, and further quantified by zeta-potential analysis. The grafted MWNTs exhibited relatively low bioreactivity on human immortal alveolar epithelial type 1-like cells (TT1) following 24h exposure as demonstrated by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) and lactate dehydrogenase release (LDH) assays. The exposure of TT1 cells to MWNTs suppressed the release of the inflammatory mediators, interleukin 6 (IL-6) and interleukin 8 (IL-8). TEM cell uptake studies indicated efficient cellular entry of MWNTs into TT1 cells, via a range of mechanisms. Cationic MWNTs showed a more substantial interaction with TT1 cell membranes than anionic MWNTs, demonstrating a surface charge effect on cell uptake.

Adsorption of surfactant protein D from human respiratory secretions by carbon nanotubes and polystyrene nanoparticles depends on nanomaterial surface modification and size

“…In particular, a large positively charged area has been described near the carbohydrate-binding site [23], which may account for its high affinity to AO-MWNTs. Since all the MWNTs were of the same length and diameter, the strong binding of SP-D to AO-MWNTs would not be expected to reflect very large differences in surface area, although our recent comparative work [17] indicates that the topography of the AO-MWNTs is very different from that of the pristine and P(4-PV)-MWNTs, being non-uniform, which may have further enhanced interaction with SP-D.…”

“…CNTs show very poor solubility and a tendency to agglomerate; however, aqueous processing would be advantageous in a wide range of CNT applications, especially in the biomedical field where they are being studied as drug delivery tools [22]. Despite known problems associated with MWNT damage and debris accumulation, oxidation using strong acids is the most common approach to maximize dispersibility, producing good aqueous dispersions with a high degree of functionalization with -COOH groups suitable for further modification [17]. This study shows that SP-D binds more strongly to the negative surface of AO-MWNTs compared with the weakly negative pristine or the weakly positive P(4VP)-MWNTs.…”

“…The MWNTs (diameter 12.1 nm + 3.7 nm, length 700 nm), synthesized by chemical vapour deposition, were obtained from Arkema SA (Lacq-Mourenx, France) and have been thoroughly characterized [17]. The pristine, as-received MWNTs were functionalized with poly(4-vinyl pyridine) (P(4VP)-MWNT) and acid oxidized (AO-MWNT) to provide three surface modifications [17].…”

“…The pristine, as-received MWNTs were functionalized with poly(4-vinyl pyridine) (P(4VP)-MWNT) and acid oxidized (AO-MWNT) to provide three surface modifications [17]. Pristine MWNT are weakly acidic with an isoelectric point of 4.9; the P(4VP)-MWNTs have a nitrogen-containing group, shifting the isoelectric point to 7.0; acid oxidized AO-MWNTs present a generally acidic surface, with a complex mixture of functional groups present [17]. Prior to incubation with human lung secretions, MWNTs were suspended in PBS at a concentration of 1 mg ml 21 then sonicated in a sonicating water bath for 5 min and diluted to a concentration twice the desired final concentration.…”

“…However, in contrast to the MWNTs, the positively charged 100 nm polystyrene NPs also adsorbed SP-D, though not as avidly as the carboxylated NPs, but more than the unmodified polystyrene. This might be due to differences in the morphology of these NMs, or to the higher density of positively charged groups on the amine-modified NPs than in the P(4VP)-MWNTs, which are only weakly positive [17]. A general observation is that SP-D binding to modified NMs is higher than to the unmodified NMs; thus a significant proportion of the SP-D was adsorbed from the cell free secretions, notably by 100 nm functionalized polystyrene NPs.…”