We report carbon nanotube network field-effect transistors (NTNFETs) that function as selective detectors of DNA immobilization and hybridization. NTNFETs with immobilized synthetic oligonucleotides have been shown to specifically recognize target DNA sequences, including H63D single-nucleotide polymorphism (SNP) discrimination in the HFE gene, responsible for hereditary hemochromatosis. The electronic responses of NTNFETs upon single-stranded DNA immobilization and subsequent DNA hybridization events were confirmed by using fluorescence-labeled oligonucleotides and then were further explored for label-free DNA detection at picomolar to micromolar concentrations. We have also observed a strong effect of DNA counterions on the electronic response, thus suggesting a charge-based mechanism of DNA detection using NTNFET devices. Implementation of label-free electronic detection assays using NTNFETs constitutes an important step toward low-cost, low-complexity, highly sensitive and accurate molecular diagnostics.hemochromatosis ͉ SNP ͉ biosensor T he development of nucleic acids diagnostics has become the subject of intense research, especially in the postgenome era. Current methods have mainly focused on optical detection using fluorescence-labeled oligonucleotides with dyes (1), quantum dots (2), or enhanced absorption of light by oligonucleotidemodified gold nanoparticles (3). On the other hand, label-free electronic methods promise to offer sensitivity, selectivity, and low cost for the detection of DNA hybridization (4). For example, microfabricated silicon field-effect sensors can monitor directly the increase in surface charge when DNA was hybridized on the sensor surface (5). Nanomaterials possess unique properties that are amenable to biosensor applications; they are one-dimensional structures that are extremely sensitive to electronic perturbations, readily functionalized with biorecognition layers, and compatible with many semiconducting manufacturing processes. Thus, one-dimensional silicon nanowires (6-8) and indium oxide nanowires (9) have shown promising performance, because their electronic conductance is more sensitive to DNA-associated charges as a result of their high surface-tovolume ratio. Using smaller nanowires with virtually all atoms on their surface, such as single-walled carbon nanotubes (SWNTs), will provide additional advantages in DNA detection. To date, there are several reports on electrochemical detection of DNA hybridization using multi-walled carbon nanotube electrodes (ref. 10 and references therein, and ref. 11). Whereas electrochemical methods rely on electrochemical behavior of the labels, measurement of direct electron transfer between SWNTs and DNA molecules paves the way for label-free DNA detection. SWNT-based field-effect transistors (12) have excellent operating characteristics (13), and they have already been explored for highly sensitive electronic detection of gases (14, 15) and biomolecules such as antibodies (16,17).Single-stranded DNA (ssDNA) has been recently demons...

We have used nanoscale field effect transistor devices with carbon nanotubes as the conducting channel to detect protein binding. A PEI/PEG polymer coating layer has been employed to avoid nonspecific binding, with attachment of biotin to the layer for specific molecular recognition. Biotin-streptavidin binding has been detected by changes in the device characteristic. Nonspecific binding was observed in devices without the polymer coating, while no binding was found for polymer-coated but not biotinylated devices. Streptavidin, in which the biotin-binding sites were blocked by reaction with excess biotin, produced essentially no change in device characteristic of the biotinylated polymer-coated devices.

Here we demonstrate design, fabrication, and testing of electronic sensor array based on single-walled carbon nanotubes (SWNTs). Multiple sensor elements consisting of isolated networks of SWNTs were integrated into Si chips by chemical vapor deposition (CVD) and photolithography processes. For chemical selectivity, SWNTs were decorated with metal nanoparticles. The differences in catalytic activity of 18 catalytic metals for detection of H(2), CH(4), CO, and H(2)S gases were observed. Furthermore, a sensor array was fabricated by site-selective electroplating of Pd, Pt, Rh, and Au metals on isolated SWNT networks located on a single chip. The resulting electronic sensor array, which was comprised of several functional SWNT network sensors, was exposed to a randomized series of toxic/combustible gases. Electronic responses of all sensor elements were recorded and the sensor array data was analyzed using pattern-recognition analysis tools. Applications of these small-size, low-power, electronic sensor arrays are in the detection and identification of toxic/combustible gases for personal safety and air pollution monitoring.

Aqueous suspensions of two smectite clays, natural bentonite and synthetic laponite B, were prepared at concentrations ranging from 0.01 to 0.1 g/cm 3 . Viewed at rest between crossed polarizers, concentrated gel suspensions (> 0.024 g/cm 3 for bentonite and > 0.020 g/cm 3 for laponite B) show optical birefringence which demonstrates the existence of a liquid-crystalline order of the platelike particles extending over macroscopic distances. The textures of these birefringent gels observed with polarized light microscopy are typical of a nematic phase. At concentrations slightly below that of the phase transition, the suspensions show strong pretransitional effects such as shock or flow birefringence. For both systems, large (≈1 cm 3 ) macroscopically oriented domains of the nematic phase were grown by slowly drying isotropic suspensions. The phase diagrams of these two systems versus clay and NaCl concentrations have been established. Unexpectedly, the nematic phase of bentonite is stabilized with increasing NaCl concentration; that of laponite B is not sensitive to it. Finally, the origin of the liquid-crystalline character of these suspensions and its consequences upon their physical properties are briefly discussed in the light of previous experimental studies published in the literature.