Specific and label-free femtomolar biomarker detection with an electrostatically formed nanowire biosensor

Abstract

We report a specific, label-free and real-time detection of femtomolar protein concentrations with a novel type of nanowire-based biosensor. The biosensor is based on an electrostatically formed nanowire, which is conceptually different from a conventional silicon nanowire in its confinement potential, charge carrier distribution, surface states, dopant distribution, moveable channel and geometrical structure. This new biosensor requires standard integrated-circuit processing with relaxed fabrication requirements. The biosensor is composed of an accumulation-type, planar transistor surrounded by four gates, a backgate, front gate and two lateral gates, and it operates in the all-around-depletion mode. Consequently, adjustment of the four gates defines the dimensions and location of the conducting channel. It is shown that lithographically shaped channels of 400 nm in width are reduced to effective widths of 25 nm upon lateral-gate biasing. Device operation is demonstrated for protein-specific binding, and it is found that sensitive detection signals are recorded once the channel width is comparable with the dimensions of the protein. The device performance is discussed and analyzed with the help of three-dimensional electrostatic simulations. A team of researchers from Israel, led by Yossi Rosenwaks at Tel Aviv University, has devised a facile route towards nanowire-based, label-free biosensors. These sensors have recently garnered interest for applications in biological species such as proteins, DNA or viruses: the presence of the target molecule is detected and converted into an electrical signal. But bottom-up strategies to prepare the nanowires are difficult to scale-up, while top-down approaches involve a relatively complex process. In contrast, the biosensor devised by Rosenwaks and co-workers is organized around a nanowire-like conducting channel that is formed electrostatically after device fabrication. The rectangular device comprises four gates at each extremity and these form depletion regions between which the channel is defined. Biomolecules are then detected with a sensitivity that depends on the size, shape and location of the channel — the presence of proteins was successfully determined at femtomolar concentrations. This combined approach brings together good performance with standard integrated circuit processing and holds great promise for real-time diagnostic applications. Biological sensing with silicon nanowires has drawn much attention due to the enhanced sensitivity of these devices. However, both bottom-up and top-down fabrication techniques are thus far resistant to commercialization, mainly due to the incompatibility of the bottom-up methodology with mass production and the non-standard, top-down process complexity. We report on a specific, label-free, and real-time detection of femtomolar protein concentrations with a novel type of nanowire-based biosensor. The biosensor is based on an electrostatically formed nanowire that requires standard integrated circuit processes with relaxed fabrication requirements.