Fumiaki Ishikawa

TRANSPARENT ELECTRONICS BASED ON TRANSFER PRINTED CARBON NANOTUBES ON RIGID AND FLEXIBLE SUBSTRATES

We report high-performance fully transparent thin-film transistors (TTFTs) on both rigid and flexible substrates with transfer printed aligned nanotubes as the active channel and indium-tin oxide as the source, drain, and gate electrodes. Such transistors have been fabricated through low-temperature processing, which allowed device fabrication even on flexible substrates. Transparent transistors with high effective mobilities (approximately 1300 cm(2) V(-1) s(-1)) were first demonstrated on glass substrates via engineering of the source and drain contacts, and high on/off ratio (3 x 10(4)) was achieved using electrical breakdown. In addition, flexible TTFTs with good transparency were also fabricated and successfully operated under bending up to 120 degrees . All of the devices showed good transparency (approximately 80% on average). The transparent transistors were further utilized to construct a fully transparent and flexible logic inverter on a plastic substrate and also used to control commercial GaN light-emitting diodes (LEDs) with light intensity modulation of 10(3). Our results suggest that aligned nanotubes have great potential to work as building blocks for future transparent electronics.

Real-Time, Label-Free Detection of Biological Entities Using Nanowire-Based FETs

Nanowire (NW)-based FETs are promising devices with potential applications ranging from health monitoring to drug discovery. In fact, these devices have demonstrated the ability to detect a variety of analytes such as particular DNA sequences, cancer biomarkers, and larger entities such as viruses. These sensor devices have also been used to monitor enzymatic activities and study the behavior of potential drug molecules. The detection of the analytes occurs with high specificity and sensitivity in reasonably short time. Here, we review the recent literature produced in the field of NW FET biosensors. We elaborate on the parameters that ultimately influence device performance such as methods of NW production, device dimensionality, and active measurement conditions. Significant progress has been made in this field of technology; however, it is often difficult to compare literature reports due to differences in both measurement conditions and data analysis. The standardization of certain active measurement conditions, such as the ionic strength of the analyte solutions, and manipulation of data are proposed to facilitate comparison between different NW biosensors.

Label-free, electrical detection of the SARS virus N-protein with nanowire biosensors utilizing antibody mimics as capture probes.

Antibody mimic proteins (AMPs) are polypeptides that bind to their target analytes with high affinity and specificity, just like conventional antibodies, but are much smaller in size (2-5 nm, less than 10 kDa). In this report, we describe the first application of AMP in the field of nanobiosensors. In(2)O(3) nanowire based biosensors have been configured with an AMP (Fibronectin, Fn) to detect nucleocapsid (N) protein, a biomarker for severe acute respiratory syndrome (SARS). Using these devices, N protein was detected at subnanomolar concentration in the presence of 44 microM bovine serum albumin as a background. Furthermore, the binding constant of the AMP to Fn was determined from the concentration dependence of the response of our biosensors.

A Calibration Method for Nanowire Biosensors to Suppress Device-to-Device Variation

Nanowire/nanotube biosensors have stimulated significant interest; however, the inevitable device-to-device variation in the biosensor performance remains a great challenge. We have developed an analytical method to calibrate nanowire biosensor responses that can suppress the device-to-device variation in sensing response significantly. The method is based on our discovery of a strong correlation between the biosensor gate dependence (dI(ds)/dV(g)) and the absolute response (absolute change in current, DeltaI). In(2)O(3) nanowire-based biosensors for streptavidin detection were used as the model system. Studying the liquid gate effect and ionic concentration dependence of strepavidin sensing indicates that electrostatic interaction is the dominant mechanism for sensing response. Based on this sensing mechanism and transistor physics, a linear correlation between the absolute sensor response (DeltaI) and the gate dependence (dI(ds)/dV(g)) is predicted and confirmed experimentally. Using this correlation, a calibration method was developed where the absolute response is divided by dI(ds)/dV(g) for each device, and the calibrated responses from different devices behaved almost identically. Compared to the common normalization method (normalization of the conductance/resistance/current by the initial value), this calibration method was proven advantageous using a conventional transistor model. The method presented here substantially suppresses device-to-device variation, allowing the use of nanosensors in large arrays.

Rapid, label-free, electrical whole blood bioassay based on nanobiosensor systems.

Biomarker detection based on nanowire biosensors has attracted a significant amount of research effort in recent years. However, only very limited research work has been directed toward biomarker detection directly from physiological fluids mainly because of challenges caused by the complexity of media. This limitation significantly reduces the practical impact generated by the aforementioned nanobiosensors. In this study, we demonstrate an In(2)O(3) nanowire-based biosensing system that is capable of performing rapid, label-free, electrical detection of cancer biomarkers directly from human whole blood collected by a finger prick. Passivating the nanowire surface successfully blocked the signal induced by nonspecific binding when performing active measurement in whole blood. Passivated devices showed markedly smaller signals induced by nonspecific binding of proteins and other biomaterials in serum and higher sensitivity to target biomarkers than bare devices. The detection limit of passivated sensors for biomarkers in whole blood was similar to the detection limit for the same analyte in purified buffer solutions at the same ionic strength, suggesting minimal decrease in device performance in the complex media. We then demonstrated detection of multiple cancer biomarkers with high reliability at clinically meaningful concentrations from whole blood collected by a finger prick using this sensing system.

Importance of Controlling Nanotube Density for Highly Sensitive and Reliable Biosensors Functional in Physiological Conditions

Biosensors utilizing carbon nanotube field-effect transistors have a tremendous potential to serve as the basis for the next generation of diagnostic systems. While nanotubes have been employed in the fabrication of multiple sensors, little attention has previously been paid to how the nanotube density affects the biosensor performance. We conducted a systematic study of the effect of density on the performance of nanotube biosensors and discovered that this parameter is crucial to achieving consistently high performance. We found that devices with lower density offer higher sensitivity in terms of both detection limit and magnitude of response. The low density nanotube devices resulted in a detection limit of 1 pM in an electrolyte buffer containing high levels of electrolytes (ionic concentration ∼140 mM, matching the ionic strength of serum and plasma). Further investigation suggested that the enhanced sensitivity arises from the semiconductor-like behavior-strong gate dependence and lower capacitance-of the nanotube network at low density. Finally, we used the density-optimized nanotube biosensors to detect the nucleocapsid (N) protein of the SARS virus and demonstrated improved detection limits under physiological conditions. Our results show that it is critical to carefully tune the nanotube density in order to fabricate sensitive and reliable devices.

A Nanoelectronic Enzyme-Linked Immunosorbent Assay for Detection of Proteins in Physiological Solutions

Semiconducting nanowires are promising ultrasensitive, label-free sensors for small molecules, DNA, proteins, and cellular function. Nanowire field-effect transistors (FETs) function by sensing the charge of a bound molecule. However, solutions of physiological ionic strength compromise the detection of specific binding events due to ionic (Debye) screening. A general solution to this limitation with the development of a hybrid nanoelectronic enzyme-linked immunosorbent assay (ne-ELISA) that combines the power of enzymatic conversion of a bound substrate with electronic detection is demonstrated. This novel configuration produces a local enzyme-mediated pH change proportional to the bound ligand concentration. It is shown that nanowire FETs configured as pH sensors can be used for the quantitative detection of interleukin-2 in physiologically buffered solution at concentrations as low as 1.6 pg mL(-1). By successfully bypassing the Debye screening inherent in physiological fluids, the ne-ELISA promises wide applicability for ligand detection in a range of relevant solutions.

Rapid and label-free cell detection by metal-cluster-decorated carbon nanotube biosensors

The article reports the use of a metal-cluster-decorated carbon nanotube biosensors for rapid and label-free algae cell detection. Sensitivities of the devices with or without the metal-cluster coating were subsequently compared using streptavidin (SA) as a model case. The sensing responses of device conductance (G) normalized by initial conductance (G0) plotted versus time for devices without and with metal clusters are shown.

High performance In2O3 nanowire transistors using organic gate nanodielectrics

In this study, we report high performance nanowire transistors using individual in 203 nanowires as channels, a multilayer self-assembled organic nano-dielectric (SAND) as the gate insulator (thickness -15 nm, capacitance -180 nF/cm2, and leakage current density ~1x10-6 A/cm2 up to 2 V). The NWTs use an individually addressable indium zinc oxide (IZO) bottom-gate and Al source/drain electrodes.

pH-sensitive intracellular photoluminescence of carbon nanotube―fluorescein conjugates in human ovarian cancer cells

To add to the understanding of the properties of functionalized carbon nanotubes in biological applications, we report a monotonic pH sensitivity of the intracellular fluorescence emission of single-walled carbon nanotube-fluorescein carbazide (SWCNT-FC) conjugates in human ovarian cancer cells. Light-stimulated intracellular hydrolysis of the amide linkage and localized intracellular pH changes are proposed as mechanisms. SWCNT-FC conjugates may serve as intracellular pH sensors.