Jeong-O Lee

Aptamers as molecular recognition elements for electrical nanobiosensors

AbstractRecent advances in nanotechnology have enabled the development of nanoscale sensors that outperform conventional biosensors. This review summarizes the nanoscale biosensors that use aptamers as molecular recognition elements. The advantages of aptamers over antibodies as sensors are highlighted. These advantages are especially apparent with electrical sensors such as electrochemical sensors or those using field-effect transistors. FigureFeeling proteins with aptamer-functionalized carbon nanotubes

Formation of low-resistance ohmic contacts between carbon nanotube and metal electrodes by a rapid thermal annealing method

The contact resistance between a carbon nanotube and metal electrodes decreases by several orders of magnitude and becomes long-term stable when the nanotube contacted by Ti-Au electrodes was annealed by a rapid thermal annealing method at 600-800 °C for 30 s. The contact resistances of the annealed samples are in the range 0.5-50 kΩ at room temperature, depending on the electrical properties of the nanotube. The short and relatively low-temperature annealing process enables us to make a surface Ti-nanotube contact suitable for electrical measurements. For the samples with relatively low contact resistances (0.5-5 kΩ) at room temperature, the contact resistance remained constant or decreased slightly as the temperature was lowered. Those with a relatively high contact resistance (5-50 kΩ), on the other hand, showed increasing contact resistance with a lowering of the temperature.

Ultrahigh-density nanotransistors by using selectively grown vertical carbon nanotubes

A type of carbon nanotube transistors, which would be suitable for large-scale integration, has been fabricated from vertically aligned carbon nanotubes. We fabricated highly ordered carbon nanotubes, which are selectively grown on the patterned aluminum oxide nanotemplates. Each device element is formed on a vertical carbon nanotube attached to a bottom (source) and upper (drain) electrodes and a gate electrode, which is electrostatically switchable. The transistors can be integrated in large arrays with the potential for tera-level density (2×1011/cm2). The vertical carbon nanotube transistor shows ON/OFF switching operation at 30 K.

Investigation of the humidity effect on the electrical properties of single-walled carbon nanotube transistors

We investigated the effect of humidity on the electrical transport properties of single-walled carbon nanotube field effect transistors (SWNT-FETs). Water molecules are found to behave as electron donors to the nanotube: Current through the p-type carbon nanotube device is found to decrease under a modest humidity, and starts to increase as the humidity increases over 65%, which is believed to be due to the opening of electron channels. Through first principles calculations based on the density functional theory, we found that water molecules can donate electrons to the carbon nanotube. Moreover, a hydrogen-bonded water monolayer will be formed around the nanotube at fully covered conditions. We suggest that this result could provide a systematic understanding of the humidity effect on SWNT-FETs, which has been believed to be essential in the development of nanotube-based sensors.

Fano resonance in crossed carbon nanotubes

We report the observation of the resonant transport in multiwall carbon nanotubes in a crossed geometry. The resonant transport is manifested by an asymmetric peak in the differential conductance curve. The observed asymmetric conductance peak is well explained by the Fano resonance originating from the scattering at the contact region of the two nanotubes. The conductance peak depends sensitively on the external magnetic field and exhibits Aharonov-Bohm-type oscillation.

The effect of metal cluster coatings on carbon nanotubes

The electrical transport and chemical sensing properties of single-walled carbon nanotube field effect transistors (SWNT-FETs) coated with metal clusters have been investigated. The source–drain current passing through an SWNT-FET coated with Pd nanoparticles showed no change over a range of gate voltages. Nevertheless, the magnitude of the current was still sensitive towards NO2, NH3 and H2 exposure. The Pd nanoparticles coating on the nanotube generated hole carriers, which either became diluted upon NH3 or H2 adsorption, or enhanced upon NO2 adsorption. Unlike the ohmic behaviour demonstrated by SWNT-FETs coated with Pd nanoparticles, the transfer characteristics of SWNT-FETs coated with Al nanoparticles revealed Schottky barrier formations at the metal–nanotube contacts. Here, the conductance through the nanotube decreased, while the device sensitivity towards NO2 and NH3 gases improved greatly. We suggest that coating SWNT-FETs with metal nanoparticles could be exploited for the development of highly sensitive nanotube-based molecular sensors.

Oriented Immobilization of Antibody Fragments on Ni-Decorated Single-Walled Carbon Nanotube Devices

We herein demonstrate that Ni-decorated single-walled carbon nanotube field effect transistors (SWNT-FETs) combined with antibody fragments can be used as effective biosensing platforms. Nanoscales Ni particles 20 to 60 nm in diameter were formed on the sidewalls of SWNT-FETs using an electrochemical method. Carcinoembryonic antigen (CEA)-binding single chain variable fragments (scFvs) with a hexahistidine tag [(his)(6)] were synthesized using genetic engineering, and ordered immobilization of anti-CEA ScFvs on Ni nanoparticles was achieved by exploiting the specific interaction between hexahistidine and Ni. Whereas randomly oriented anti-CEA scFvs did not impart a noticeable change of conductance upon addition of CEA, a clear increase in conductance was observed using Ni-decorated SWNT-FETs functionalized with engineered scFvs.

Detection of tumor markers using single-walled carbon nanotube field effect transistors.

We have developed a biosensor capable of detecting carcinoembryonic antigen (CEA) markers using single-walled carbon nanotube field effect transistors (SWNT-FETs). These SWNT-FETs were fabricated using nanotubes produced by a patterned catalyst growth technique, where the top contact electrodes were generated using conventional photolithography. For biosensor applications, SU-8 negative photoresist patterns were used as an insulation layer. CEA antibodies were employed as recognition elements to specific tumor markers, and were successfully immobilized on the sides of a single-walled carbon nanotube using CDI-Tween 20 linking molecules. The binding of tumor markers to these antibody-functionalized SWNT-FETs was then monitored continuously during exposure to dilute CEA solutions. The observed sharp decrease in conductance demonstrates the possibility of realizing highly sensitive, label-free SWNT-FET-based tumor sensors.

Complete gate control of supercurrent in graphene p–n junctions

In a conventional Josephson junction of graphene, the supercurrent is not turned off even at the charge neutrality point, impeding further development of superconducting quantum information devices based on graphene. Here we fabricate bipolar Josephson junctions of graphene, in which a p-n potential barrier is formed in graphene with two closely spaced superconducting contacts, and realize supercurrent ON/OFF states using electrostatic gating only. The bipolar Josephson junctions of graphene also show fully gate-driven macroscopic quantum tunnelling behaviour of Josephson phase particles in a potential well, where the confinement energy is gate tuneable. We suggest that the supercurrent OFF state is mainly caused by a supercurrent dephasing mechanism due to a random pseudomagnetic field generated by ripples in graphene, in sharp contrast to other nanohybrid Josephson junctions. Our study may pave the way for the development of new gate-tuneable superconducting quantum information devices.

Electrical properties of polyaniline nanofibre synthesized with biocatalyst

Polyaniline (PANI) nanofibres were synthesized using a biocatalyst (recombinant Coprinus cinereus peroxidase) instead of toxic chemical oxidants. Relatively uniform nanofibres with 50?100?nm diameter were easily obtained with this method, and the doping state of the PANI nanofibre could be controlled either with 1N camphorsulfonic acid (CSA) or with 30% NH4OH. Doped (or dedoped) PANI nanofibres were deposited on pre-patterned Au electrodes for electrical characterization. Completely dedoped PANI behaves as an insulator, while a larger current, by more than four?orders of magnitude, was observed from doped PANI nanofibres. A?weak p-type gate effect was observed for PANI nanofibre devices as well. As one could expect from the easy doping nature of PANI, PANI nanofibre devices show high sensitivity toward dedoping (NH3) gases, thereby demonstrating the possibility of using enzyme-synthesized PANI nanofibre devices as sensitive chemical sensors.