We report a nanofluidic transistor based on a metal-oxide-solution (MOSol) system that is similar to a metal-oxide-semiconductor field-effect transistor (MOSFET). Using a combination of fluorescence and electrical measurements, we demonstrate that gate voltage modulates the concentration of ions and molecules in the channel and controls the ionic conductance. Our results illustrate the efficacy of field-effect control in nanofluidics, which could have broad implications on integrated nanofluidic circuits for manipulation of ions and biomolecules in sub-femtoliter volumes.

Inorganic nanotubes were successfully integrated with microfluidic systems to create nanofluidic devices for single DNA molecule sensing. Inorganic nanotubes are unique in their high aspect ratio and exhibit translocation characteristics in which the DNA is fully stretched. Transient changes of ionic current indicate DNA translocation events. A transition from current decrease to current enhancement during translocation was observed on changing the buffer concentration, suggesting interplay between electrostatic charge and geometric blockage effects. These inorganic nanotube nanofluidic devices represent a new platform for the study of single biomolecule translocation with the potential for integration into nanofluidic circuits.

Hollow inorganic nanotubes are attracting a great deal of attention due to their fundamental significance and potential applications in bioanalysis and catalysis. 1 Among them, silica nanotubes are of special interest because of their hydrophilic nature, easy colloidal suspension formation, and surface functionalization accessibility for both inner and outer walls. These modified silica nanotubes and nanotube membrane have shown potential applications for bioseparation and biocatalysis. 2 Recently, bright visible photoluminescence from sol-gel template synthesized silica nanotubes was observed by Zhang et al. 3 In addition, the study of the physical and chemical nature of molecules or ions confined within the inorganic nanotubes is of great current interest.Silica nanotubes have been synthesized typically within the pores of porous alumina membrane templates using the sol-gel coating technique. 4 Alumina templates can be dissolved to liberate single silica nanotubes. These nanotubes prepared at low temperature [5][6][7] have porous walls and are relatively fragile. Once the templates are removed, the silica nanotubes will generally bundle up and become less oriented. The same applied to the silica nanotubes prepared at low temperature using other templates. [5][6][7] The fabrication of oriented, robust silica nanotube arrays is of interest for their potential use in nanoscale fluidic bioseparation, sensing, and catalysis. Here, we developed a well-controlled process to translate vertical silicon nanowire arrays into silica nanotube arrays through a thermal oxidation-etching approach. The obtained nanotubes perfectly retain the orientation of original silicon nanowire arrays. High-temperature oxidation (800-1000°C) produces relative thick and rigid walls that are made of condensed silica. This method could be useful for fabrication of single nanotube sensors and nanofluidic systems.Silicon nanowire arrays were prepared using chemical vapor deposition (CVD) epitaxial growth employing silicon tetrachloride (SiCl 4 , Aldrich, 99.99%) as the silicon source. Hydrogen (10% balanced by argon) is used to reduce SiCl 4 at high temperature (900-950°C). Gold thin film was coated on Si (111) substrates to initiate the growth of silicon nanowires via the vapor-liquid-solid growth mechanism. This approach was developed recently and has been used for the synthesis of vertical Si/SiGe superlattice nanowire arrays in our lab. 8,9 The silicon nanowire array samples are loaded into a tube furnace and heated at 800-1000°C for 1 h under the continuous flow of pure O 2 . These nanowires are uniformly oxidized to give SiO 2 sheaths with continuous silicon wire cores inside. We then leave the SiO 2 sheath intact and try to remove the thin silicon cores to create SiO 2 nanotubes. The processing details are presented in Figure 1. During the oxidation, the nanowire tips are also oxidized to give an oxide cap on each vertical wire, which could prevent the selective etching of silicon cores. Therefore, the first step after thermal oxidation...