Carmen Tîlmaciu

Higher Dispersion Efficacy of Functionalized Carbon Nanotubes in Chemical and Biological Environments

Aqueous dispersions of functionalized carbon nanotubes (CNTs) are now widely used for biomedical applications. Their stability in different in vitro or in vivo environments, however, depends on a wide range of parameters, such as pH and salt concentrations of the surrounding medium, and length, aspect ratio, surface charge, and functionalization of the applied CNTs. Although many of these aspects have been investigated separately, no study is available in the literature to date, which examines these parameters simultaneously. Therefore, we have chosen five types of carbon nanotubes, varying in their dimensions and surface properties, for a multidimensional analysis of dispersion stability in salt solutions of differing pH and concentrations. Furthermore, we examine the dispersion stability of oxidized CNTs in biological fluids, such as cellular growth media and human plasma, and their toxicity toward cancer cells. To enhance dispersibility and biocompatibility, the influence of different functionalization schemes is studied. The results of our investigations indicate that both CNT dimensions and surface functionalization have a significant influence on their dispersion and in vitro behavior. In particular, factors such as a short aspect ratio, presence of oxidation debris and serum proteins, low salt concentration, and an appropriate pH are shown to improve the dispersion stability. Furthermore, covalent surface functionalization with amine-terminated polyethylene glycol (PEG) is demonstrated to stabilize CNT dispersions in various media and to reduce deleterious effects on cultured cells. These findings provide crucial data for the development of biofunctionalization protocols, for example, for future cancer theranostics, and optimizing the stability of functionalized CNTs in varied biological environments.

Design of double-walled carbon nanotubes for biomedical applications

Double-walled carbon nanotubes (DWNTs) prepared by catalytic chemical vapour deposition were functionalized in such a way that they were optimally designed as a nano-vector for the delivery of small interfering RNA (siRNA), which is of great interest for biomedical research and drug development. DWNTs were initially oxidized and coated with a polypeptide (Poly(Lys:Phe)), which was then conjugated to thiol-modified siRNA using a heterobifunctional cross-linker. The obtained oxDWNT-siRNA was characterized by Raman spectroscopy inside and outside a biological environment (mammalian cells). Uptake of the custom-designed nanotubes was not associated with detectable biochemical perturbations in cultured cells, but transfection of cells with DWNTs loaded with siRNA targeting the green fluorescent protein (GFP) gene, serving as a model system, as well as with therapeutic siRNA targeting the survivin gene, led to a significant gene silencing effect, and in the latter case a resulting apoptotic effect in cancer cells.

AFM imaging of functionalized double-walled carbon nanotubes

We present a comparative study of several non-covalent approaches to disperse, debundle and non-covalently functionalize double-walled carbon nanotubes (DWNTs). We investigated the ability of bovine serum albumin (BSA), phospholipids grafted onto amine-terminated polyethylene glycol (PL-PEG(2000)-NH(2)), as well as a combination thereof, to coat purified DWNTs. Topographical imaging with the atomic force microscope (AFM) was used to assess the coating of individual DWNTs and the degree of debundling and dispersion. Topographical images showed that functionalized DWNTs are better separated and less aggregated than pristine DWNTs and that the different coating methods differ in their abilities to successfully debundle and disperse DWNTs. Height profiles indicated an increase in the diameter of DWNTs depending on the functionalization method and revealed adsorption of single molecules onto the nanotubes. Biofunctionalization of the DWNT surface was achieved by coating DWNTs with biotinylated BSA, providing for biospecific binding of streptavidin in a simple incubation step. Finally, biotin-BSA-functionalized DWNTs were immobilized on an avidin layer via the specific avidin-biotin interaction.

Cellular localization, accumulation and trafficking of double-walled carbon nanotubes in human prostate cancer cells

AbstractCarbon nanotubes (CNTs) are at present being considered as potential nanovectors with the ability to deliver therapeutic cargoes into living cells. Previous studies established the ability of CNTs to enter cells and their therapeutic utility, but an appreciation of global intracellular trafficking associated with their cellular distribution has yet to be described. Despite the many aspects of the uptake mechanism of CNTs being studied, only a few studies have investigated internalization and fate of CNTs inside cells in detail. In the present study, intracellular localization and trafficking of RNA-wrapped, oxidized double-walled CNTs (oxDWNT-RNA) is presented. Fixed cells, previously exposed to oxDWNT-RNA, were subjected to immunocytochemical analysis using antibodies specific to proteins implicated in endocytosis; moreover cell compartment markers and pharmacological inhibitory conditions were also employed in this study. Our results revealed that an endocytic pathway is involved in the internalization of oxDWNT-RNA. The nanotubes were found in clathrin-coated vesicles, after which they appear to be sorted in early endosomes, followed by vesicular maturation, become located in lysosomes. Furthermore, we observed co-localization of oxDWNT-RNA with the small GTP-binding protein (Rab 11), involved in their recycling back to the plasma membrane via endosomes from the trans-golgi network.

Filling of Carbon Nanotubes with Compounds in Solution or Melted Phase

Since their discovery, carbon nanotubes (CNT) have been found to exhibit remarkable structural, mechanical and electronic properties. One such property is the ability to encapsulate foreign materials inside their cylindrical cavity, for application in different fields. The procedures to fill CNT may be classified into two main groups: (a) filling in solution, using the wet chemistry route and (b) filling with a melted phase. In both cases, the filling is induced by the capillary forces. It is also possible to fill CNT in the vapour phase, although there are only few very specific examples available in the literature to date. After filling, oxides and metallic particles can be obtained by a subsequent thermal annealing in the required atmosphere. In the wet chemistry route, the nanotubes are usually treated by an oxidizing agent in order to open their tips. The filling is then performed by placing the opened tubes in a solution of the selected compound (or a precursor). When the compound is dissolved in an oxidizing acid such as nitric acid (HNO3), it is possible to combine opening and filling in a single step. Although this method allows the introduction of heat-sensitive species inside carbon nanotubes, the yield varies strongly with the diameter of the carbon nanotubes and is generally rather low in the case of CNT with a small inner diameter. This filling route mainly leads to isolated nanoparticles or short nanowires. Filling with melted compounds is a solvent-free route. The CNT are directly immersed in the melted material and capillary forces drive the compound into the CNT. Although this route is more restrictive in terms of materials, it allows for the continuous filling of CNT with long nanocrystals (up to a few micrometers), with a higher filling yield in the available CNT (up to ca. 70%). This chapter will describe these two different methods for filling CNT and illustrate them with a few selected examples.