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Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, real-time electrically based sensors for biological and chemical species. Amine-- and oxide-functionalized SiNWs exhibit pH-dependent conductance that was linear over a large dynamic range and could be understood in terms of the change in surface charge during protonation and deprotonation. Biotin-modified SiNWs were used to detect streptavidin down to at least a picomolar concentration range. In addition, antigen-functionalized SiNWs show reversible antibody binding and concentration-dependent detection in real time. Lastly, detection of the reversible binding of the metabolic indicator Ca2+ was demonstrated. The small size and capability of these semiconductor nanowires for sensitive, label-free, real-time detection of a wide range of chemical and biological species could be exploited in array-based screening and in vivo diagnostics.
Planar semiconductors can serve as the basis for chemical and biological sensors in which detection can be monitored electrically and/or optically (1-4). For example, a planar field effect transistor (FET) can be configured as a sensor by modifying the gate oxide (without gate electrode) with molecular receptors or a selective membrane for the analyte of interest; binding of a charged species then results in depletion or accumulation of carriers within the transistor structure (1, 2). An attractive feature of such chemically sensitive FETs is that binding can be monitored by a direct change in conductance or related electrical property, although the sensitivity and potential for integration are limited.
The physical properties limiting sensor devices fabricated in planar semiconductors can be readily overcome by exploiting nanoscale FETs (5-9). First, binding to the surface of a nanowire (NW) or nanotube (NT) can lead to depletion or accumulation of carriers in the "bulk" of the nanometer diameter structure (versus only the surface region of a planar device) and increase sensitivity to the point that single-molecule detection is possible. Second, the small size of NW and NT building blocks and recent advances in assembly (9, 10) suggest that dense arrays of sensors could be prepared. Indeed, NT FETs were shown recently by Dai and co-workers to function as gas sensors (11). Calculations suggested that direct binding of electron-- withdrawing NO^sub 2^ or electron-donating NH^sub 3^ gas molecules to the NT surface chemically gated these devices. However, several properties of NTs could also limit...