Franklin Anariba

Mass-Produced Nanogap Sensor Arrays for Ultrasensitive Detection of DNA

In this report, an electrical detection scheme for the quantification of DNA using a nanogap sensor array is detailed. The prime objective is to develop a novel sensing procedure, based on the electronic transduction mechanism, which would mitigate the problems intrinsic to nanostructure-based biosensing devices. Design considerations of the sensor array take into account the feasibility of mass production in a cost-effective way by using standard silicon microfabrication technologies. The sensing mechanism relies on bridging the nanogap upon hybridization of the two termini of a target DNA with two different surface-bound capture probes, followed by a simple metallization step. About 2 orders of magnitude enhancement in conductance, as referred to a clean background (<1.0 pS) observed at a control sensor, was obtained in the presence of as little as 1.0 fM target DNA. This sensitivity is comparable to the best of electrochemical/electrical biosensors. A linear relationship between the conductance and the DNA concentration was obtained from 1.0 fM to 1.0 pM with an exceptional signal intensity of 2.1 x 10(4)% change per unit concentration. This change in conductivity is so large that it can unambiguously detect the concentration of DNA quantitatively and may obviate the need for target amplification used in current DNA tests. Moreover, the sensor array exhibited excellent single-base mismatch discrimination due to its unique vertically aligned nanostructure and the two-probe configuration.

Importance of Oxides in Carbon/Molecule/Metal Molecular Junctions with Titanium and Copper Top Contacts

Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper on monolayers of nitroazobenzene (NAB), biphenyl, and nitrobiphenyl (NBP), and multilayers of NAB and NBP covalently bonded to an sp 2 carbon substrate. The electronic behavior of Ti junctions was extremely dependent on residual gas pressure during E-beam deposition, due to the formation of a disordered Ti oxyhydroxide deposit. The junction resistance decreased with decreasing residual gas pressure, and the hysteresis and rectification observed previously for relatively high deposition pressure was absent for pressures below 5 × 10 - 7 Torr. Deletion of the molecular layer resulted in low-resistance junctions for both high and low deposition pressures. Replacement of the Ti with Al with otherwise identical deposition conditions resulted in insulating junctions with much higher resistance and no rectification. Ti junctions made at low residual gas pressure had resistances and current/voltage characteristics similar to those of junctions with Cu top contacts, with the latter exhibiting high yield and good reproducibility. The current/ voltage characteristics of both the Ti and Cu junctions fabricated with low residual gas pressure were nonlinear and showed a strong dependence on the molecular layer thickness. The hysteresis and rectification previously observed for junctions fabricated at relatively high residual gas pressure depend on the combination of the NAB layer and the semiconducting TiO x film, with the TiO x layer conductivity depending strongly on formation conditions. Rectification and hysteresis in NAB/TiO x junctions may result from either redox reactions of the NAB and TiO x layers, or from electron injection into the conduction band of Ti oxide.

Metal−Molecule Interactions Upon Deposition of Copper Overlayers on Reactively Functionalized Porphyrin Monolayers on Si(100)

The interaction of evaporated Cu deposited on a series of porphyrins in monolayers covalently attached to Si(100) substrates was investigated using cyclic voltammetry and FTIR spectroscopy. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit. The tripod anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. Two of the porphyrins are Zn chelates that possess meso p-cyanophenyl substituentsone, ZnP-CND, contains a single group opposite (distal) to the tripodal surface anchor, whereas the other, ZnP-CNL, contains two groups orthogonal (lateral) to the surface anchor. A third Zn porphyrin, ZnP, containing nonreactive p-tolyl groups at all three nonanchoring meso positions, was examined for comparison. The fourth porphyrin, FbP-HD, is a metal-free species (free base) that contains nonreactive phenyl (distal) and p-tolyl groups (lateral) at the three nonanchoring meso positions. The fifth porphyrin, CuP-HD, is the Cu chelate of FbP-HD, and serves as a reference complex for evaluating the effects of Cu metal deposition onto FbP-HD. The studies indicate that all of the porphyrin monolayers are robust under the conditions of Cu deposition, experiencing no noticeable degradation. In addition, the Cu metal does not penetrate through the monolayer to form electrically conductive filaments. For the ZnP-CND monolayers, the deposited Cu quantitatively reacts/complexes with the distal cyano group. In contrast, for the ZnP-CNL monolayers no reaction/complexation of the lateral cyano groups is observed. For the FbP-HD monolayers, Cu deposition results in quantitative insertion of Cu into the free base porphyrin. Collectively, the studies demonstrate that porphyrin monolayers are amenable to direct deposition of Cu overlayers and that functionalization of the porphyrins can be used to mediate the attributes of the metal-molecule junction.