Dongjin Lee

Low-cost, transparent, and flexible single-walled carbon nanotube nanocomposite based ion-sensitive field-effect transistors for pH/glucose sensing.

Low-cost, transparent, and flexible ion-sensitive field-effect transistors (ISFETs) are presented as pH and glucose sensors. Single-walled carbon nanotubes (SWCNTs) and poly(diallyldimethyammonium chloride, PDDA) are deposited by layer-by-layer (LbL) self-assembly between two metallic electrodes patterned on a polyethylene terephthalate substrate. The pH ISFETs are characterized based on the fact that the electronic conductance of SWCNTs nanocomposite is determined by molecular protonation/deprotonation of carboxylic groups on SWCNTs and by the external Ag/AgCl reference gate voltage. Glucose is detected by the local pH change in the vicinity of SWCNTs with the aid of glucose oxidase (GOx) enzyme. The glucose sensor shows a sensitivity of 18-45 microA/mM on a linear range of 2-10 mM. The apparent Michaelis-Menten constant is 14.2 mM, indicating a high affinity of LbL assembled GOx to glucose. The LbL self-assembly of nanomaterials and enzymes on the transparent and flexible substrate suggests various chemical and biological sensors suitable for in vivo application.

Bone formation on carbon nanotube composite

The effects of a layer-by-layer assembled carbon nanotube composite (CNT-comp) on osteoblasts in vitro and bone tissue in vivo in rats were studied. The effects of CNT-comp on osteoblasts were compared against the effects by commercially pure titanium (cpTi) and tissue culture dishes. Cell proliferation on the CNT-comp and cpTi were similar. However, cell differentiation, measured by alkaline phosphatase activity and matrix mineralization, was better on the CNT-comp. When implanted in critical-sized rat calvarial defect, the CNT-comp permitted bone formation and bone repair without signs of rejection or inflammation. These data indicate that CNT-comp may be a promising substrate for use as a bone implant or as a scaffold for tissue engineering.

Carbon nanotube electric immunoassay for the detection of swine influenza virus H1N1

Abstract A low-cost, label-free, ultra-sensitive electric immunoassay is developed for the detection of swine influenza virus (SIV) H1N1. The assay is based on the excellent electrical properties of single-walled carbon nanotubes (SWCNTs). Antibody–virus complexes influence the conductance of underlying SWCNT thin film, which has been constructed by facile layer-by-layer self-assembly. The basic steps of conventional immunoassay are performed followed by the electric characterization of immunochips at the last stage. The resistance of immunochips tends to increase upon surface adsorption of macromolecules such as poly-l-lysine, anti-SIV antibodies, and SIVs during the assay. The resistance shift after the binding of SIV with anti-SIV antibody is normalized with the resistances of bare devices. The sensor selectivity tests are performed with non-SIVs, showing the normalized resistance shift of 12% as a background. The detection limit of 180 TCID50/ml of SIV is obtained suggesting a potential application of this assay as point-of-care detection or monitoring system. This facile CNT-based immunoassay also has the potential to be used as a sensing platform for lab-on-a-chip system.

Layer-by-Layer Self-Assembled Single-Walled Carbon Nanotubes Based Ion-Sensitive Conductometric Glucose Biosensors

We present ion-sensitive conductometric glucose sensors prepared by layer-by-layer (LbL) nano self-assembly of single-walled carbon nanotube (SWNT) and enzyme glucose oxidase (GOx). The carboxylated SWNT and GOx are self-assembled alternatively with a positively charged polyelectrolyte, poly(diallyldimethylammonium chloride, PDDA). Quartz crystal microbalance (QCM) study and Fourier transform infrared (FTIR) spectroscopy demonstrate GOx is negatively charged, and it is possible to successively construct multilayer alternatively with PDDA. SWNT multilayer shows pH-dependent conductance at different pH buffer solutions, which decreases exponentially with an increase in pH. The concentration of glucose is electronically characterized based on the fact that hydrogen ions from glucose oxidation with the aid of GOx change the local pH value in the vicinity of SWNT multilayer thin-film, thereby yielding higher conductance. The overall sensitivity is 10.8 muA/mM and the resolution of device fabricated is 1 pM at the bias voltage of 0.6 V. The LbL assembled SWNT multilayer is proven to be versatile for sensing biochemical reactions with many other enzyme systems.

Humidity Sensitivity of Carbon Nanotube and Poly (Dimethyldiallylammonium Chloride) Composite Films

This paper demonstrates a highly sensitive humidity sensor based on carbon nanotube and poly(dimethyldiallylammonium chloride) composite films. The composite film is deposited between interdigitated electrodes on a Si/SiO2 substrate through layer-by-layer self-assembly technique. The resistance stability of the composite film is effectively improved through thermal annealing, and I-V characteristic of the film exhibits a very good linear behavior. The resistance increases exponentially with relative humidity from 20% to 98%, and a much higher sensitivity in comparison with pure carbon nanotube networks is achieved. With temperature increased, the water vapor density versus RH shifts upwards, while the resistance is reduced downwards. The resistance is dependent on temperatures with a negative coefficient. The composite films with multiwalled carbon nanotubes show an adjacent sensitivity, compared with the single-walled carbon nanotube composite films. The experimental results show that the humidity sensors have a fast response and a short recovery time, and their response is reversible. A simple model is proposed to explain the change of composite film resistance with humidity. The carbon nanotubes junctions may play a more important role in the overall resistance change for water molecule absorption.

Layer-by-Layer Self-Assembly of Single-Walled Carbon Nanotubes with Amine-Functionalized Weak Polyelectrolytes for Electrochemically Tunable pH Sensitivity

The layer-by-layer (LbL) assembly of carboxylated single-walled carbon nanotubes (SWCNT) is demonstrated to tune the electrochemical pH sensitivity of thin-film devices. The positively charged amine containing weak polyelectrolyte (wPE) is used as a counter species to control the proximal ions. The LbL assembly process is monitored by the quartz crystal microbalance, which results in the linear growth of a multilayer. The amount adsorbed is strongly dependent on the surface charge of previously deposited species. However, the thickness of the multilayer is determined by both the amount adsorbed and the coiling of polyelectrolyte chains. Indeed, electrical and structural characteristics of the (wPE/SWCNT) multilayer thin film are obtained according to the acid dissociation constants of amino groups in wPE. The electrochemical pH sensitivity in the physiological range demonstrates the effects of both charge carrier doping/trapping and proximal ions on the conductance of the SWCNT multilayer. Although doping/trapping shows the decreasing conductance, the proximal ion effect reveals the increasing conductance with pH in the basic region as a result of the p-type semiconducting nature of SWCNTs and the ability of wPE to capture hydrogen ions. This work sheds light on the applicability of nanostructured and/or engineered functional thin films of SWCNTs as chemical and biological sensors.

Suspended carbon nanotube nanocomposite beams with a high mechanical strength via layer-by-layer nano-self-assembly

The fabrication and characterization of single-walled carbon nanotube (SWCNT) composite thin film micropatterns and suspended beams prepared by lithography-compatible layer-by-layer (LbL) nano-self-assembly are demonstrated. Negatively charged SWCNTs are assembled with a positively charged polydiallyldimethylammonium chloride, and the composite thin film is patterned by oxygen plasma etching with a masking layer of photoresist, resulting in a feature size of 2 µm. Furthermore, the SWCNT nanocomposite stripe pattern with a metal clamp on both ends is released by etching a sacrificial layer of silicon dioxide in the hydrofluoric acid vapor. I-V measurement reveals that the resistance of SWCNT nanocomposite film decreases by 23% upon release, presumably due to the effect of reorientation of CNTs caused by the deflection of about 50 nm. A high Youngs modulus is found in a range of 500-800 GPa based on the characterization of a fixed-fixed beam using nanoindentation. This value is much higher than those of the other CNT-polymer composites reported due to organization of structures by self-assembly and higher loading of CNTs. The stiff CNT-polymer composite thin film micropattern and suspended beam have potential applications to novel physical sensors, nanoelectromechanical switches, other M/NEMS devices, etc.

A pure single-walled carbon nanotube thin film based three-terminal microelectromechanical switch

The electrical and physical properties of pure single-walled carbon nanotube thin films deposited through a layer-by-layer-self-assembly process are discussed. The film thickness was proportional to the number of dipping cycles. The film resistivity was estimated as 2.19×10−3 Ω cm after thermal treatment processes were performed. The estimated specific contact resistance to gold electrodes was 6.33×10−9 Ω m2 from contact chain measurements. The fabricated three-terminal microelectromechanical switch using these films functioned as a beam for multiple switching cycles with a 4.5 V pull-in voltage. This switch is believed to be a promising device for low power digital logic applications.

A Conductometric Indium Oxide Semiconducting Nanoparticle Enzymatic Biosensor Array

We report a conductometric nanoparticle biosensor array to address the significant variation of electrical property in nanomaterial biosensors due to the random network nature of nanoparticle thin-film. Indium oxide and silica nanoparticles (SNP) are assembled selectively on the multi-site channel area of the resistors using layer-by-layer self-assembly. To demonstrate enzymatic biosensing capability, glucose oxidase is immobilized on the SNP layer for glucose detection. The packaged sensor chip onto a ceramic pin grid array is tested using syringe pump driven feed and multi-channel I–V measurement system. It is successfully demonstrated that glucose is detected in many different sensing sites within a chip, leading to concentration dependent currents. The sensitivity has been found to be dependent on the channel length of the resistor, 4–12 nA/mM for channel lengths of 5–20 μm, while the apparent Michaelis-Menten constant is 20 mM. By using sensor array, analytical data could be obtained with a single step of sample solution feeding. This work sheds light on the applicability of the developed nanoparticle microsensor array to multi-analyte sensors, novel bioassay platforms, and sensing components in a lab-on-a-chip.

Suspended carbon nanotube thin film structures with high degree of alignment for NEMS switch applications

We describe microfluidic channel assisted carbon nanotube (CNT) alignment followed by microfabrication and characterization of a suspended CNT thin film. The alignment of CNT is enhanced by heating the CNT dispersion, which is characterized with Raman spectroscopy yielding a high G- to D-band intensity ratio of 22 along the microfluidic flow direction. The sidewall of CNT film pattern, left in a lift-off process, is eliminated by oxygen plasma etching. The resistivity of aligned CNT film is found as 1.45 × 10−3 Ωcm. The aligned CNT film is released by etching a sacrificial layer of amorphous silicon and characterized mechanically demonstrating a nominal high Youngs modulus of 635 GPa and a yield strength of 2.4 GPa through a fixed-end beam deflection test. The lithography compatible fabrication process and the highly conductive film with an excellent mechanical property enable the aligned CNT film to be a potent candidate for nanoelectromechanical device applications.