Fluidic and Electronic Transport in Silicon Nanotube Biosensors
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Fluidic and Electronic Transport in Silicon Nanotube Biosensors Nicolas Hibst, Annina M. Steinbach, and Steffen Strehle Institute of Electron Devices and Circuits, Ulm University, Albert-Einstein-Allee 45, D-89081 Ulm, Germany ABSTRACT Silicon nanotubes (SiNTs) represent unique building blocks for future nanoscale biosensor devices merging electronic sensing and nanofluidics. Configured as ion-sensitive field effect transistors (ISFETs), SiNTs have great potential for charge sensing or label-free chemical detection in minute sample volumes flowing through their inner cavity. In the present study, doped SiNTs were synthesized from the gas phase in a bottom-up approach. To study their nanofluidic and electronic transport properties, single SiNTs were functionally integrated as ISFETs and coupled to a microfluidic system. The experimental results for ion diffusion through a SiNT are in full agreement with numerical calculations based on Fick's second law if a diffusion coefficient is assumed approximately one order of magnitude smaller than the bulk value. INTRODUCTION The implementation of silicon nanowires (SiNWs) as ion-sensitive field-effect transistors (ISFETs) has been widely explored with emphasis on sensors allowing charge sensing and labelfree detection of chemical and biochemical species in fluids [1]. Here, the specific or nonspecific binding of certain chemical species to the outer surface of the SiNW alters the surface charge or so-called liquid gate potential. This leads to an accumulation or depletion of charge carriers in the SiNW affecting its conductivity. In contrast to SiNWs, silicon nanotubes (SiNTs) are rarely studied although they offer in principle the same detection capabilities and, furthermore, the option to flow a sample directly through their inner cavity acting as an internal gate (Figure 1a). SiNTs allow therefore to couple micro- or nanofluidics and electronics effectively enabling versatile and novel biosensor concepts as well as nanoscale interfaces to living cells [2]. Additionally, a second gate electrode can be created at the outer SiNT surface readily allowing so-called dual-gate sensing configurations [3]. Similar to SiNWs, SiNTs can be synthesized by established bottom-up gold-catalyzed vapor-liquid-solid (VLS) growth based on germanium nanowires (GeNWs) with a Si shell. The SiNTs are obtained from these radial Gecore/Si-shell heterostructures [4] by selective post-growth removal of the Ge core using hydrogen peroxide (H2O2) chemical etching after device assembly. The inner diameter of the SiNT is therefore precisely controlled by the chosen VLS catalyst diameter [5]. The overall fabrication of single SiNT ISFETs can be done analogous to SiNWs comprising electrical contacts, passivation layers, and a fluidic sample supply (Figure 1a).
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