A nanoscale probe for fluidic and ionic transport.

Surface science and molecular biology are often concerned with systems governed by fluid dynamics at the nanoscale, where different physical behaviour is expected. With advances in nanofabrication techniques, the study of fluid dynamics around a nano-object or in a nano channel is now more accessible experimentally and has become an active field of research. However, developing nanoscale probes that can act as flow sensors and that can be easily integrated remains difficult. Many studies demonstrate that carbon nanotubes (CNTs) have outstanding potential for nanoscale sensing, acting as strain or charge sensors in chemical and biological environments. Although nanotube flow sensors composed of bulk nanotubes have been demonstrated, they are not readily miniaturized to nanoscale dimensions. Here we report that individual carbon nanotube transistors of 2 nm diameter, incorporated into microfluidic channels, locally sense the change in electrostatic potential induced by the flow of an ionic solution. We demonstrate that the nanotube conductance changes in response to the flow rate, functioning as a nanoscale flow sensor. With diminishing system size, surface-related phenomena become more significant. For example, the ions of an aqueous solution interact electrostatically with charged surfaces, thereby coupling fluid dynamic properties and ionic spatial distribution. This produces electrokinetic phenomena such as electro-osmosis or electrophoresis. In particular, this coupling gives access to the fluid dynamic properties through changes in the electrostatic potential or charge distribution. The small size and high electrical conductivity of CNTs makes them an excellent material for nanoscale electrochemical sensors and probes, a topic that has received much attention in recent times. For example, sensors for the chemical potential of solutions containing redox-active molecules have been demonstrated using single-walled nanotubes17, and working electrodes have been realized using both single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). Although a voltage or current has been observed from fluid flow past bulk nanotubes, the change in the conductance of an individual nanotube device in response to fluid dynamic properties has not yet been reported. Here we show that when incorporated into microchannels, electrolytically gated SWNT transistors can sense the local potential generated by a fluidic flow of ionic solutions on charged surfaces, known as the streaming potential. From the linear shift of their counterelectrode voltage characteristics versus flow rate Q, we estimate the solution’s zeta potential (z potential), defined as the electrostatic potential at the no-slip plane of an ionic solution/solid interface. The value of z 20.1 V obtained for dilute NaCl solutions contacting a negatively charged silica/PDMS (polydimethylsiloxane) microfluidic channel is similar to previously measured values for indifferent univalent ions Naþ on silica and Kþ on PDMS (ref. 23). Furthermore, taking advantage of the coupling between electrostatic potential and fluid dynamics, we demonstrate that individual nanotube transistors act as fluidic flow sensors that locally sense potentials with nanometre-scale resolution. This contrasts with recent studies in which a voltage was generated by flow over bulk quantities of nanotubes16 or through glass microchannels and measured to determine flow rate. Also, because of their 2-nm height, nanotube devices can be integrated into microor nanoscale fluidic circuits, making them suitable for use in lab-on-a-chip applications, such as providing active flow sensing and high-resolution flow mapping on surfaces. Another advantage of our approach is that we study the nanotube conductance, using the nanotube transconductance as an amplifier. In particular this ensures that the changes induced by the flow are generated locally from the nanotube only, eliminating potential artefacts that could originate at the electrode level. We estimate for a typical 100 mm 200 mm microfluidic channel that the thermodynamic limit to sensitivity in a 1 Hz bandwidth is 100 nl min. Figure 1a shows a schematic diagram of our experimental geometry where a 2 nm diameter single-walled nanotube transistor is integrated inside a microfluidic channel. Details about the device fabrication and measurements are provided in the Methods section. Each nanotube has a source and drain contact through which we applied voltages and measured the resulting current. We also applied a voltage VCE to the counterelectrode, causing ions to migrate to the nanotube surface to form an ionic double layer, which acts as a gate voltage to modulate the charge density within the nanotube. The double layer capacitance is very large at 4 10 F m (refs 20, 21). This gives our devices a very large transconductance e2/h, as first reported by Krüger et al. on MWNT devices and later by Rosenblatt et al. on SWNT devices. The conductance G of a semiconducting singlewalled carbon nanotube versus VCE is shown in Fig. 1b. The data show an insulating region with G 0 where the carriers are depleted, and an approximately linear rise in G outside the LETTERS