Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Carbon Nanotube Chemical Sensors

Electrochemical biosensors and associated lab-on-a-chip devices are the analytical system of choice when rapid and on-site results are needed in medical diagnostics and food safety, for environmental protection, process control, wastewater treatment, and life sciences discovery research among many others. A premier example is the glucose sensor used by diabetic patients. Current research focuses on developing sensors for specific analytes in these application fields and addresses challenges that need to be solved before viable commercial products can be designed. These challenges typically include the lowering of the limit of detection, the integration of sample preparation into the device and hence analysis directly within a sample matrix, finding strategies for long-term in vivo use, etc. Today, functional nanomaterials are synthesized, investigated, and applied in electrochemical biosensors and lab-on-a-chip devices to assist in this endeavor. This review answers many questions around the nanomaterials...

Functional Nanomaterials and Nanostructures Enhancing Electrochemical Biosensors and Lab-on-a-Chip Performances: Recent Progress, Applications, and Future Perspective.

Reliable determination of binding kinetics and affinity of DNA hybridization and single-base mismatches plays an essential role in systems biology, personalized and precision medicine. The standard tools are optical-based sensors that are difficult to operate in low cost and to miniaturize for high-throughput measurement. Biosensors based on nanowire field-effect transistors have been developed, but reliable and cost-effective fabrication remains a challenge. Here, we demonstrate that a graphene single-crystal domain patterned into multiple channels can measure time- and concentration-dependent DNA hybridization kinetics and affinity reliably and sensitively, with a detection limit of 10 pM for DNA. It can distinguish single-base mutations quantitatively in real time. An analytical model is developed to estimate probe density, efficiency of hybridization and the maximum sensor response. The results suggest a promising future for cost-effective, high-throughput screening of drug candidates, genetic variations and disease biomarkers by using an integrated, miniaturized, all-electrical multiplexed, graphene-based DNA array. Monitoring DNA binding and single-base mismatches accurately in real time is difficult, especially for miniaturized devices. Here the authors report a graphene field-effect transistor array capable of reliably measuring DNA hybridization kinetics and affinity at the picomolar level.

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Real-time reliable determination of binding kinetics of DNA hybridization using a multi-channel graphene biosensor

Electromagnetic wave absorbers have been investigated for many years with the aim of achieving high absorbance and tunability of both the absorption wavelength and the operation mode by geometrical control, small and thin absorber volume, and simple fabrication. There is particular interest in metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) due to their complete fulfillment of these demands. MIM-PMAs consist of top periodic micropatches, a middle dielectric layer, and a bottom reflector layer to generate strong localized surface plasmon resonance at absorption wavelengths. In particular, in the visible and infrared (IR) wavelength regions, a wide range of applications is expected, such as solar cells, refractive index sensors, optical camouflage, cloaking, optical switches, color pixels, thermal IR sensors, IR microscopy and gas sensing. The promising properties of MIM-PMAs are attributed to the simple plasmonic resonance localized at the top micropatch resonators formed by the MIMs. Here, various types of MIM-PMAs are reviewed in terms of their historical background, basic physics, operation mode design, and future challenges to clarify their underlying basic design principles and introduce various applications. The principles presented in this review paper can be applied to other wavelength regions such as the ultraviolet, terahertz, and microwave regions.

Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at Visible and Infrared Wavelengths: A Review

Since the discovery of graphene, its applications to electronic and optoelectronic devices have been intensively and thoroughly researched. Extraordinary and unusual electronic and optical properties make graphene and other two-dimensional (2D) materials promising candidates for infrared and terahertz (THz) photodetectors. Until now, however, 2D material-based performance is lower in comparison with those of infrared and terahertz detectors existing in the global market. This paper gives an overview of emerging 2D material detectors' performance and comparison with the traditionally and commercially available ones in different applications in high operating temperature conditions. The most effective single graphene detectors are THz detectors utilizing the plasma rectification effect in the field-effect transistors. Most of the 2D layered semiconducting material photodetectors operate in the visible and near-infrared regions, and generally, their high sensitivity does not coincide with the fast response time, which limits real detector functions.

Two-dimensional infrared and terahertz detectors: Outlook and status

High responsivity middle-wavelength infrared graphene photodetectors using photo-gating

Metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) generate strong localized surface plasmon resonance (LSPR) on their surfaces. Therefore, MIM-PMAs are expected to enhance the absorption of graphene coated on their surfaces. Graphene-coated MIM-PMAs (GMIM-PMAs) were developed and their optical properties were investigated both experimentally and numerically at infrared wavelengths. Significant modification of the absorption of GMIM-PMAs was achieved only in the main LSPR wavelength region, where the insulator is lossless. The enhancement of the absorption of graphene could be maximized by the optimization of the insulator thickness of the MIM-PMAs. The results obtained here are expected to contribute to the development of high-responsivity graphene-based photodetectors and optoelectronic devices.

Graphene on metal-insulator-metal-based plasmonic metamaterials at infrared wavelengths.

We have developed electrolyte-gated sensors based on a field-effect 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.

Horizontally Aligned Carbon Nanotubes on a Quartz Substrate for Chemical and Biological Sensing

Graphene is a promising new material for photodetectors due to its excellent optical properties and high-speed response. However, graphene-based phototransistors have low responsivity due to the weak light absorption of graphene. We have observed a giant Dirac point shift upon white light illumination in graphene-based phototransistors with n-doped Si substrates, but not those with p-doped substrates. The source-drain current and substrate current were investigated with and without illumination for both p-type and n-type Si substrates. The decay time of the drain-source current indicates that the Si substrate, SiO2 layer, and metal electrode comprise a metal-oxide-semiconductor (MOS) capacitor due to the presence of defects at the interface between the Si substrate and SiO2 layer. The difference in the diffusion time of the intrinsic major carriers (electrons) and the photogenerated electron-hole pairs to the depletion layer delays the application of the gate voltage to the graphene channel. Therefore, th...

Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA–VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino groups that have positive charges at pH 4.1. In contr...

Satoshi Okuda

Papers

High responsivity middle-wavelength infrared graphene photodetectors using photo-gating

Graphene on metal-insulator-metal-based plasmonic metamaterials at infrared wavelengths.

Horizontally Aligned Carbon Nanotubes on a Quartz Substrate for Chemical and Biological Sensing

Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

Graphene Surface Acoustic Wave Sensor for Simultaneous Detection of Charge and Mass