Hye-Mi So

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

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.

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.

An ultrafast response grating structural ZnO photodetector with back-to-back Schottky barriers produced by hydrothermal growth

An ultrafast response metal–semiconductor–metal type ZnO ultraviolet photodetector was fabricated by ultraviolet nanoimprint lithography (UV-NIL) with hydrothermal synthesis. The extremely fast response time was due to the Schottky barrier formation attributed to the control of the hydrothermal growth time and grating structure of ZnO, produced by a position-controlled pattering method of UV-NIL. With an on/off frequency of ultraviolet light of 2 kHz using an optical chopper, the device exhibits a rising time of 43 μs and a falling time of 54 μs at a low bias voltage (0.5 V) with a responsivity of 22.1 A W−1 in the active area of 5 × 5 μm2. In comparison to other fast response ZnO photodetectors, our device definitely uses easy and low-cost fabrication methods as well as exhibits high performance.

Recovery improvement of graphene-based gas sensors functionalized with nanoscale heterojunctions

We report a development of reduced graphene oxide (rGO)-based gas sensors with a practical recovery by facile functionalization with tin dioxide nanoclusters. Upon the introduction of UV illumination to this nanostructure, the reaction on surfaces of tin dioxide nanoclusters was activated and thereby the nanoscale heterojunction barriers between the rGO sheet and the nanoclusters were developed. This lowered the conductance to quickly recover, which was intensified as the cluster density has reached to the percolation threshold. However, after the formation of the cluster percolating network, the sensor response has totally changed into a deterioration of the sensitivity as well as the recovery.

Vertically aligned carbon nanotube electrodes directly grown on a glassy carbon electrode.

Three-dimensional microelectrodes were fabricated using glassy carbon electrodes combined with vertically aligned carbon nanotubes (VACNTs). VACNTs were grown on various conducting electrode patterns including a carbon electrode fabricated by pyrolysis of a negative photoresist, with plasma-enhanced chemical vapor deposition using a bilayer Fe/Al catalyst. VACNT electrodes grown on the glassy carbon showed excellent electrochemical behavior, whereas VACNT electrodes grown on Pt showed poor electrochemical performance, presumably due to the poor contact between VACNTs and the Pt electrode. Electron microscopy showed that the VACNT layer was strongly bound to the carbon electrode, while that on Pt tended to peel away. The versatility of the all-carbon microelectrodes was also tested by using them for interfacing stem cells. Their superior mechanical properties and the electrical connectivity between the carbon electrode and the VACNTs, along with the simple fabrication process, suggest that glassy carbon may be a good conducting substrate for VACNT electrodes.

Carbon nanotube diode fabricated by contact engineering with self-assembled molecules

The authors report the construction of carbon nanotube Schottky diodes by covering a selectively exposed area of the electrode with self-assembling molecules. Two self-assembling molecules with different polarities, 2-aminoethanethiol and 3-mercaptopropionic acid, were used to modify the Fermi level lineup at the selected contact. The devices showed p-type behavior with symmetric I-V showing clear rectifying behavior after treatment of one contact with 2-aminoethanethiol. Their experiment, in conjunction with the results of ab initio electronic structure calculations, suggests that the diode action stems from the asymmetric Fermi level lineup between the bare and engineered contacts.

Air-stable n-type operation of Gd-contacted carbon nanotube field effect transistors

We report air-stable n-type operations of the single-walled carbon nanotube field effect transistors (SWNT-FETs) fabricated with Gd electrodes. Unlike previously reported n-type SWNT-FETs, our devices maintained their n-type operation characteristics in ambient atmosphere for more than two months. The shallow Gd films with a thickness below 20 nm are corroded by environmental oxygen, whereas the well-contacted Gd-SWNT interfaces underneath the thick Gd layers are protected from contaminations by air molecules. Theoretical studies based on the first-principles electronic structure calculations confirm that Gd layers have an excellent binding affinity to the SWNTs.