Transport of ions in aqueous phase through single-walled carbon nanotubes

Le transport d’ions et de molécules à l’intérieur de canaux nanométriques diffère du transport à l’échelle micro- ou macroscopique du fait de rapports surface/volume bien plus élevés conduisant à de nouveaux phénomènes de transport. Les nanotubes de carbone avec leurs propriétés uniques apparaissent comme des canaux exceptionnellement intéressants pour mieux comprendre le transport ionique et fluidique à l’échelle nanométrique et pour d’éventuelles applications nanofluidiques. Ce travail est dédié à l’étude et la compréhension des mécanismes de transport des ions en phase aqueuse à l’intérieur de nanotubes de carbone, un sujet particulièrement important pour le développement d’applications dans le domaine du séquençage de l’ADN ou de l’analyse biochimique de petites molécules.Durant ce travail, un protocole a été développé pour la fabrication de dispositifs microfluidiques intégrant des nanotubes de carbone et permettant des mesures à la fois électriques et optiques. Les propriétés de transport à l’intérieur de nanotubes de carbone mono-feuillets ont été étudiées en combinant mesures de courant ionique sous application d’un champ électrique, spectroscopie Raman et modélisation théorique. Les résultats obtenus par cette étude démontrent la forte influence de l’environnement du nanotube sur la densité et la distribution des charges de surface et donc sur les propriétés de transport à l’intérieur de ces nano-canaux dont les parois sont d’épaisseur atomique. Les ordres de grandeur des courants ioniques mesurés expérimentalement sont en bon accord avec les modèles standards de transport ionique dans un nanocanal en considérant des densités de charge de surface et des longueurs de glissement physiquement raisonnables. De manière importante, ce travail a permis de mettre en évidence un transport ionique activé par champ électrique à l’intérieur de nanotubes de carbone, qui peut être expliqué en considérant un modèle de transport plus élaboré intégrant une ou plusieurs barrières d’énergie le long du nanotube. Les résultats de la caractérisation Raman suggèrent que ces barrières d’énergie résultent d’un dopage hétérogène le long du nanotube induit par la matrice polymère.Ionic and molecular transport inside nanometer scale geometries is distinct from micro- and macroscale transport due to the large surface-to-volume ratios which lead to unique transport phenomena. Carbon nanotubes with their peerless properties appear as exceptional channels for understanding fluidic and ionic transport at the nanoscale and for developing nanofluidics-based applications. This work is devoted at studying and understanding the transport mechanisms of ions in aqueous phase through carbon nanotubes, which is especially important for various applications such as DNA sequencing or biochemical analysis of small molecules.During this work, a protocol was developed for the fabrication of carbon nanotubes-based microfluidic devices which are suitable for both electrical and optical measurements. The transport properties through single-walled carbon nanotubes were investigated by combining ion current measurements under an applied voltage, Raman spectroscopy and theoretical modelling. The results obtained from this study highlight the strong influence of the nanotube environment on their surface charge density and distribution and hence on the ionic transport properties through these nanochannels having walls of atomic thickness. The orders of magnitude of the ionic currents experimentally measured are in good agreement with the standard models of ion transport through nanochannels when considering physically reasonable values of surface charge densities and slip lengths. Importantly, this work allowed us to evidence a novel voltage-activated transport of ions through carbon nanotubes which can be accounted for by considering a more elaborate transport model including the presence of one or more energy barriers along the nanotube. Raman characterization results support that these energy barriers result from a heterogeneous doping along the nanotubes induced by the polymer matrix

Evidence of selective cation transport through sub-2 nm single-walled carbon nanotubes

The electrophoretic transport of ions through single wall carbon nanotubes (SWCNTs) of diameters between 1.2 and 1.8 nm was studied for different monovalent chloride salts using microfluidic devices incorporating either a single or a few CNTs in parallel. The ionic conductance was found to be about one order of magnitude higher than expected from a simple electro-migration behavior without any surface effect. Importantly, the ionic conductance measured for different cations did not scale with their bulk electrophoretic mobility thus indicating a probable selective cation transport through these sub-2 nm SWCNTs. The transport of Na+ was notably found to be favored in comparison to that of Li+ and Cs+ or K+ . These results highlight the influence of steric and surface effects induced by the nano-confinement on the transport of ions through sub-2 nm SWCNTs

Study of Ion Transport through One to Several Single-Walled Carbon Nanotubes

International audience

Increased chemical reactivity of single-walled carbon nanotubes on oxide substrates: In situ imaging and effect of electron and laser irradiations

We studied the oxygen etching of individual single-walled carbon nanotubes on silicon oxide substrates using atomic force microscopy and high-temperature environmental scanning electron microscopy. Our in situ observations show that carbon nanotubes are not progressively etched from their ends, as frequently assumed, but disappear segment by segment. Atomic force microscopy, before and after oxidation, reveals that the oxidation of carbon nanotubes on substrates proceeds through a local cutting that is followed by a rapid etching of the disconnected nanotube segment. Unexpectedly, semiconducting nanotubes appear more reactive under these conditions than metallic ones. We also show that exposure to electron and laser beams locally increases the chemical reactivity of carbon nanotubes on such substrates. These results are rationalized by considering the effect of substrate-trapped charges on the nanotube density of states close to the Fermi level, which is impacted by the substrate type and the exposure to electron and laser beams

Voltage-Activated Ion Transport through Single-Walled Carbon Nanotubes

International audience

Transport of ions and molecules inside carbon nanotubes : towards the detection of individual biomolecule

National audienceWe present experimental and theoretical results for the transport of ions and molecules inside carbon nanotubes

Transport of ions in solution through single-walled carbon nanotubes

International audience

Voltage-activated transport of ions through single-walled carbon nanotubes

International audienceIonic transport through single-walled carbon nanotubes (SWCNTs) is promising for many applications but remains both experimentally challenging and highly debated. Here we report ionic current measurements through microfluidic devices containing one or several SWCNTs of diameter of 1.2 to 2 nm unexpectedly showing a linear or a voltage-activated I-V dependence. Transition from an activated to a linear behavior, and stochastic fluctuations between different current levels were notably observed. For linear devices, the high conductance confirmed with different chloride salts indicates that the nanotube/water interface exhibits both a high surface charge density and flow slippage, in agreement with previous reports. In addition, the sublinear dependence of the conductance on the salt concentration points toward a charge-regulation mechanism. Theoretical modelling and computer simulations show that the voltage-activated behavior can be accounted for by the presence of local energy barriers along or at the ends of the nanotube. Raman spectroscopy reveals strain fluctuations along the tubes induced by the polymer matrix but displays insufficient doping or variations of doping to account for the apparent surface charge density and energy barriers revealed by ion transport measurements. Finally, experimental evidence points toward environment-sensitive chemical moieties at the nanotube mouths as being responsible for the energy barriers causing the activated transport of ions through SWCNTs within this diameter range