Simulating the bioavailability of carbon nanotubes

Introduction
Carbon nanotubes (CNTs) are structurally well-defined, chemically inert and electrically conducting or semi-conducting fibers which have extensively been explored for medical applications as active components such as antibacterial agents or electric conductors, as templates for cell growth, drugs and magnetic probes or as sensors. Recently, the enhanced antibacterial activity of dispersed single-wall CNTs was successfully related to the piercing of the bacterial cell membranes by the CNTs [1]. Solution-spun composite fibers from CNTs and DNA have proven their suitability as electrically conducting scaffolds, which target muscle or nerve repair [2]. Regularly aligned CNTs are efficient and inert templates for the growth and proliferation of human osteoblasts [3]. Cancer treatment relies on the ability of CNTs to specifically deliver anti-cancer agents and on the hyperthermal effect induced by magnetically doped CNTs in an oscillating magnetic field [4] CNT-based sensors exhibit an exceptional specificity for measuring electronic properties of small biological structures [5]. Here we focus on the functionalization of CNTs by DNA molecules; we present an analysis of the nanoscale CNT-DNA interactions and the immobilization of such aggregates on oxidic surfaces.

Materials and Methods
Carbon nanotubes embedded in single-stranded DNA (CNT@DNA) were investigated by self-consistent density-functional-based tight-binding calculations (DFTB). A phenomenological model for the stability was derived, which gives the CNT-DNA interaction energy as a function of the nanotube radii and the number of DNA chains [6]. To study anchoring such aggregates on scaffold materials we analyze the interactions which bond the nucleotide to oxidized surfaces by DFTB calculations [7].

Results and Discussion
Single CNTs are readily complexed by DNA, but for CNT bundles an essential energy gain is only obtained, if multiple DNA chains wrap around the tubes. Hence, the destruction of the CNT bundles, e.g. by sonication, can promote the CNT@DNA complex formation. Pyrimidine-based homopolymeric DNA more effectively wraps the CNT, whereas purine-based DNA exhibits a larger radius selectivity. The CNT-DNA interaction is not a genuine van-der-Waals interaction. The electronic structure of most aggregates is close to a superposition of the electronic states of the “free” DNA and CNT fragments. However, within a small structural window there exist several very strongly bonding systems which are characterized by combined electronic states. Hence, magic matching occurs in conjunction with a remarkable charge transfer (Fig. 1).
While the DNA-CNT interaction involves the aromatic part of the DNA immobilization of suchsystems on oxidic surfaces occurs mainly via the phosphate group. The preferred anchoring on titania and alumina [7,8] relies on bonding two oxygen atoms of the phosphate group to the surface by esterification. The resulting structures are stable against solvatation.