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We have developed electrolyte-gated sensors based on a fieldeffect transistor (FET) consisting of horizontally aligned single-walled carbon nanotubes (CNTs) synthesized on single-crystal quartz. Dense well-aligned CNTs serving as device channels enabled high current and large transconductance. Owing to these excellent device properties, the pH resolution was much better in the aligned-channel CNTFETs than in single-channel devices. For immunosensing, selective detection of human immunoglobulin E (IgE) by using aptamer-functionalized CNTFETs was demonstrated in the presence of nontarget proteins at much higher concentration. Moreover, measurements of sensor response versus IgE concentration produced data that fit well to the Langmuir adsorption isotherm. The developed sensors with aligned channels exhibited a drain current of 400-fold that of single-channel devices. Therefore, aligned-channel CNTFETs are useful for highly sensitive and practicable solution sensing.

DNA hybridization was electrically detected by graphene field-effect transistors. Probe DNA was modified on the graphene channel by a pyrene-based linker material. The transfer characteristic was shifted by the negative charges on the probe DNA, and the drain current was changed by the full-complementary DNA while no current change was observed after adding noncomplementary DNA, indicating that the graphene field-effect transistor detected the DNA hybridization. In addition, the number of DNAs was estimated by the simple plate capacitor model. As a result, one probe DNA was attached on the graphene channel per 10×10 nm2, indicating their high density functionalization. We estimated that 30% of probe DNA on the graphene channel was hybridized with 200 nM full-complementary DNA while only 5% of probe DNA was bound to the noncomplementary DNA. These results will help to pave the way for future biosensing applications based on graphene FETs.

To realize the antigen-antibody reaction for specific protein sensing using graphene field-effect transistors (G-FETs), the antigen-binding fragment (Fab), which is a component of conventional antibodies, was functionalized onto the graphene channel surface. Since the height of the Fab is approximately 3 nm, the antigen-antibody reaction is expected to occur inside the electrical double layer in the buffer solution. After functionalization of Fab onto the G-FET, the transfer characteristics shifted in the positive gate-voltage direction, indicating that the Fab was successfully modified onto the graphene surface. Then, the drain current changed after injecting the target proteins, and the dissociation constant was estimated to be 2.3 nM from the concentration dependence. These results indicate that the Fab-modified G-FETs have high potentials as highly sensitive biological sensors fabricated on the basis of the antigen-antibody reaction.

We have successfully fabricated a pH-sensor array based on chemical-vapor-deposition (CVD)-synthesized graphene. As large-scale monolayer graphene is synthesized by this method, the size and the position of graphene can be controlled. Therefore, after transferring graphene onto SiO2/Si substrates, a graphene field-effect transistor (FET) array was produced. The sensing characteristics of the CVD-synthesized graphene-based device were investigated using three buffer solutions with different pH values (pHs 4.0, 6.8, and 9.3). The electrical measurements reveal that for most of the graphene FETs in the array, a similar stepwise increment in drain current was observed upon the introduction of each buffer solution with increasing pH value sequence. This will lead to the realization of the fabrication of multiplex hand-held chemical and biological sensors based on CVD-synthesized graphene.

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We developed a new method for the preparation of connected-spherical gold nanoparticles (GNPs), which exhibited surface plasmon resonance properties different from those of spherical GNPs. In this method, citrate-stabilized GNPs in spherical form were connected to each other by use of cetyltrimethylammonium bromide, and then the progressing connection of GNPs in solution was restrained by the coverage of thin silica layer on them. The connectivity of GNPs was analyzed by transmission electron microscopy images and optical absorption spectra. The modulus of the third-order nonlinear optical susceptibility of the twin-linked GNPs film was estimated as 1.61 × 10−9 esu (the real and imaginary parts were 1.55 × 10−9 and −4.24 × 10−10 esu, respectively).