Stern07 - Vol 445 | 1 February 2007 |...

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LETTERS Label-free immunodetection with CMOS-compatible semiconducting nanowires Eric Stern 1 , James F. Klemic 2 , David A. Routenberg 2 , Pauline N. Wyrembak 5 , Daniel B. Turner-Evans 2 , Andrew D. Hamilton 5 , David A. LaVan 3 , Tarek M. Fahmy 1 & Mark A. Reed 2,4 Semiconducting nanowires have the potential to function as highly sensitive and selective sensors for the label-free detection of low concentrations of pathogenic microorganisms 1–10 . Suc- cessful solution-phase nanowire sensing has been demonstrated for ions 3 , small molecules 4 , proteins 5,6 , DNA 7 and viruses 8 ; how- ever, ‘bottom-up’ nanowires (or similarly configured carbon nanotubes 11 ) used for these demonstrations require hybrid fab- rication schemes 12,13 , which result in severe integration issues that have hindered widespread application. Alternative ‘top-down’ fabrication methods of nanowire-like devices 9,10,14–17 produce dis- appointing performance because of process-induced material and device degradation. Here we report an approach that uses comple- mentary metal oxide semiconductor (CMOS) field effect transistor compatible technology and hence demonstrate the specific label-free detection of below 100 femtomolar concentrations of antibodies as well as real-time monitoring of the cellular immune response. This approach eliminates the need for hybrid methods and enables system-scale integration of these sensors with signal processing and information systems. Additionally, the ability to monitor antibody binding and sense the cellular immune response in real time with readily available technology should facilitate widespread diagnostic applications. We demonstrate here that the limitations of fabricated nanowire- type devices can be overcome and that nanometre-scale sensors with little mobility degradation from bulk can be achieved. We have used ultrathin silicon-on-insulator wafers 9,10,18,19 , which require only lat- eral (in-plane, two-dimensional) active layer definition to achieve the nanometre dimensions needed for a nanowire-type device. Previous attempts with this approach used reactive-ion etching (RIE) of the active silicon layer, which unacceptably degraded device perform- ance 9,10 . To achieve the nanometre-scale dimensions necessary for sensitivity, we developed a fabrication process using an anisotropic wet etch: specifically, tetramethylammonium hydroxide, TMAH, which etches Si (111) planes about 100 times more slowly than all other planes 20 . This approach allows retention of pattern definition (of a masking oxide layer), and smoothes edge imperfections not aligned to the (111) plane. Previous work on TMAH-defined elec- tronic devices has shown excellent retention of electrical properties 18 , although not in configurations suitable for sensing. We show that ‘nanowire’ devices capable of sensing can be defined by TMAH etch- ing. Our approach uses commercially available (100) silicon-on- insulator wafers that yield trapezoidal cross-section nanowires with dominant Si (111) exposed planes, the preferred surface for selective surface functionalization 21 .
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Stern07 - Vol 445 | 1 February 2007 |...

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