Controlled amino-functionalization by electrochemical reduction of bromo and nitro azobenzene layers bound to Si(111) surfaces.

4-Nitrobenzenediazonium (4-NBD) and 4-bromobenzenediazonium (4-BBD) salts were grafted electrochemically onto H-terminated, p-doped silicon (Si) surfaces. Atomic force microscopy (AFM) and ellipsometry experiments clearly showed layer thicknesses of 2-7 nm, which indicate multilayer formation. Decreasing the diazonium salt concentration and the reaction time resulted in a smaller layer thickness, but did not prevent the formation of multilayers. It was demonstrated, mainly by X-ray photoelectron spectroscopy (XPS), that the diazonium salts not only react with the H-terminated Si surface, but also with electrografted phenyl groups via azo-bond formation. These azo bonds can be electrochemically reduced at Ered = -1.5 V, leading to the corresponding amino groups. This reduction resulted in a modest decrease in layer thickness, and did not yield monolayers. This indicates that other coupling reactions, notably a biphenyl coupling, induced by electrochemically produced phenyl radicals, take place as well. In addition to the azo functionalities, the nitro functionalities in electrografted layers of 4-NBD were independently reduced to amino functionalities at a lower potential (Ered = -2.1 V). The presence of amino functionalities on fully reduced layers, both from 4-NBD- and 4-BBD-modified Si, was shown by the presence of fluorine after reaction with trifluoroacetic anhydride (TFAA). This study shows that the electrochemical reduction of azo bonds generates amino functionalities on layers produced by electrografting of aryldiazonium derivatives. In this way multifunctional layers can be formed by employing functional aryldiazonium salts, which is believed to be very practical in the fabrication of sensor platforms, including those made of multi-array silicon nanowires.

Protein detection on biotin-derivatized polyallylamine by optical microring resonators.

Silicon optical microring resonators (MRRs) are sensitive devices that can be used for biosensing. We present a novel biosensing platform based on the application of polyelectrolyte (PE) layers on such MRRs. The top PE layer was covalently labeled with biotin to ensure binding sites for antibodies via a streptavidin-biotin binding scheme. Monitoring the shift in the microring resonance wavelength allows real-time, highly sensitive detection of the biomolecular interaction.

Standard CMOS Fabrication of a Sensitive Fully Depleted Electrolyte-Insulator-Semiconductor Field Effect Transistor for Biosensor Applications.

Microfabricated semiconductor devices are becoming increasingly relevant for detection of biological and chemical components. The integration of active biological materials together with sensitive transducers offers the possibility of generating highly sensitive, specific, selective and reliable biosensors. This paper presents the fabrication of a sensitive, fully depleted (FD), electrolyte-insulator-semiconductor field-effect transistor (EISFET) made with a silicon-on-insulator (SOI) wafer of a thin 10-30 nm active SOI layer. Initial results are presented for device operation in solutions and for bio-sensing. Here we report the first step towards a high volume manufacturing of a CMOS-based biosensor that will enable various types of applications including medical and environmental sensing.

The effect of the number of parallel DNA molecules on electric charge transport through 'standing DNA'.

We present morphological and electrical characterization of double-stranded DNA (dsDNA) molecules covalently bound to two metal electrodes: an underlying gold surface and a gold nanoparticle (GNP). Conductive atomic force microscope (cAFM) with a metallized tip is used to perform current-voltage (I-V) measurements through dsDNA molecules, connected to GNPs of different diameters 5, 10 and 20 nm. The number of DNA molecules coating the GNP is expected to vary with the surface area of the GNP. This number and the portion of the GNP surface area enabling hybridization of the DNA determine the number of DNA molecules connecting the GNP to the gold surface. The larger the diameter of the GNP the higher the expected number of dsDNA molecules connecting it to the gold surface and thus the expected current. Our results show similar currents for all three GNP sizes, indicating that current flows through the same number of molecules regardless of the diameter of the measured GNP. The measured currents, 220 nA at 2 V, are in accordance with our previous reports (Cohen et al 2005 Proc. Natl Acad. Sci. USA 102 11589-93; Cohen et al 2006 Faraday Discuss. 131 367-76) in which we demonstrated the validity of the experimental system. In particular, for the 5 nm GNP, we conclude that the current possibly flows through two to three molecules, likely only one, and that a single short dsDNA molecule can support at least ∼70 nA, and probably 220 nA.