Carbon Nanotubes Based Nano PH Sensor Biology Essay


Carbon nanotubes (CNTs) are found to be promising material for nanoelectronics due to its ultra small size and unique properties. Therefore, people have focused on exploring its applications in nanoelectronics, such as replacing the conventional transistors, resisters and sensors, etc. In recent years, monitoring and controlling of pH has become an important aspect of many industrial processes. Micro and nano materials, such as Carbon nanotubes (CNTs), are good candidates to manufacture micro or nano electronic devices. These devices which have a linear relationship of I-V characteristic could make operational amplifying circuits unnecessary. Comparing with other traditional pH sensors, Nano pH sensor with CNTs can provide more benefits due to their unique metallic and high current density properties which are presented in this paper.


After the discovery of CNT in early 1990s, many researchers focus in using carbon nanotube (CNT) to build various nanoelectronic devices. A lot of people have tried to study the properties of CNT recently [1-4]. Advantages of Carbon Nanotube (CNT) electrodes for biosensors include high electrical conductivity, a chemically inert electrode, high mechanical strength of a small probe, the ability to grow the nanotube array on different substrates and in different patterns, and nanoscale size of the electrode with a high aspect ratio [5]. Carbon Nanotubes (CNTs) closely resemble hollow graphite fibers that exist in entangled bundles of tens to hundreds[6]. These come in two different forms: multi walled carbon nanotubes (MWCNT) and single walled carbon nanotubes (SWCNT). SWCNTs and MWCNTs range in diameter from 1-10 nm and 10-50 nm respectively. About 70-80% of SWCNT tend to contain semiconducting properties, whereas 70-80% of MWCNT tend to contain metallic properties [7]. Metallic CNTs can be used as connecting wires for Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS) because of their size and low resistance, while semi-conducting CNTs can be used for nano transistors [8].

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The first type of single pH sensor [11]developed in the 70s years was a pH test stripe based on the absorption of pH indicator covalently immobilized on the cellulose matrix. In 1980 Peterson presented the first optical pH sensor. The sensor made used of the absorbance dye phenol red and was applied for evaluation of blood pH in-vivo and in-vitro. The dye was immobilized into polystyrene microspheres. Lately mostly used absorbance-based pH indicators were phenol red, bromothymol blue and other. In 1982 Saari reported the first fluorescence-based pH sensor where fluoresceinamin was covalently immobilized on cellulose. The most frequently used pH indicators are hydroxypyrene trisulfonic acid sodium salt (HPTS), carbxyfluorescein derivatives (e.g. mono-, dichlorocarboxyfluorescein), seminaphthorhodafluor (SNARF) and hydroxycoumarins [11].

Biomedical engineers have exploited primarily the possibilities of the chip technology to develop silicon-based sensors. Compared to the usually piecewise-assembled sensors, the reproducibility of sensor characteristics should be highly improved due to the replication procedure of silicon technology. Therefore, in biomedical engineering literature, many papers present silicon sensors such as ion sensors. Ion-Selective Field-Effect Transistor (ISFET) pH sensor [9] is one of the most well-known examples. Furthermore, with more and more study on CNTs, as mentioned in [10] CNTs with metallic properties have a huge potential to produce more compact devices for pH measurement. The possibility to store hydroxyl ions (OH-) and hydrogen ions (H+) in carbon nanostructures has been clarified in some papers. In addition, the strong dependence of electronic properties of CNT structure on chemical environment was also reported [10]. Those results led to the applications of CNT as sensing material in pH sensor application. CNT sensor can alter the electrical properties of the CNT by adsorbing the molecules on the surface of the CNT. In this paper, I will discuss the fabrication of CNT based pH sensor and present experiment procedure and show the respond of sensing material surface to pH environment.


1. Introduction to MOSFET

A metal-oxide-semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance between a body electrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide [12]. If dielectrics other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device may be referred to as a metal-insulator-semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping.

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If the MOSFET is an n-channel or nMOS FET, then the source and drain are 'n+' regions and the body is a 'p' region. As described above, with sufficient gate voltage, holes from the body are driven away from the gate, forming an inversion layer or n-channel at the interface between the p region and the oxide. This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between source and drain.

If the MOSFET is a p-channel or pMOS FET, then the source and drain are 'p+' regions and the body is a 'n' region. When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages. When a voltage less negative than the threshold value (a negative voltage for p-channel) is applied between gate and source, the channel disappears and only a very small subthreshold current can flow between the source and the drain.

The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.

The device may comprise a Silicon On Insulator (SOI) device in which a Buried OXide (BOX) is formed below a thin semiconductor layer. If the channel region between the gate dielectric and a Buried Oxide (BOX) region is very thin, the very thin channel region is referred to as an Ultra Thin Channel (UTC) region with the source and drain regions formed on either side thereof in and/or above the thin semiconductor layer. Alternatively, the device may comprise a SEMiconductor On Insulator (SEMOI) device in which semiconductors other than silicon are employed. Many alternative semiconductor materials may be employed.

When the source and drain regions are formed above the channel in whole or in part, they are referred to as Raised Source/Drain (RSD) regions

2. Introduction to ISFET

An ISFET is generally used to measure ion concentrations in solutions. Actually, an ISFET's source and drain are constructed similarly as a Metal-oxide Semiconductor Field-Effect Transistor (MOSFET) [6]. The basic structure of a MOSFET is formed by adding two heavily doped n+ regions to the MOS capacitor on p-type Si as shown in Fig.1. When the gate voltage VG exceeds its threshold voltage, then an inversion layer is formed at the SiO2/Si interface. The n+-source region can supply electrons to the inversion region without depending on the thermal generation rate as required for the MOS capacitor. An n+-drain region should be added so that electrons can flow from source to the drain through the inversion layer when a positive drain voltage VD is applied. This electron flow constitutes the drain current ID. Current into the drain is taken as a positive current and the positive gate voltage controls the number of electrons in the inversion layer and hence controls the drain current.

Fig.1. Schematic diagram of an MOSFET : 1 drain; 2 source; 3 substrate; 4 gate; 5 insulator; 6 metal contacts; 7 inversion layer.[6]

3. Introduction to CNT based ISFET

Although an ISFET is very similar to a MOSFET, there are still some differences. As shown in Fig.2, the metal gate is replaced by the metal of a reference electrode, whilst the target liquid in which this electrode is present makes contact with the bare gate insulator. Both of them have the same equivalent circuit. Then, devices with this structure can be applied to pH measurement [13]. However, the objective of this paper is to enhance the inversion layer with CNTs as NANO wire to conduct electrons between the drain and source, the drain current might be much greater under the same gate voltage.Fig.3 illustrates the potential application of both SWCNTS and MCNTs in an ISFET structure pH measurement device. If this is verified, then we can make these devices compact and cheap. [6]

Fig.2. Schematic diagram of a composite gate, dual dielectric ISFET: 1 drain; 2 source; 3 substrate; 4 insulator; 5 metal contacts; 6 reference electrode; 7 solution; 8 electroactive membrane; 9 encapsulant; 10 inversion layer [6].

Fig.3. CNT based ISFET: 1 N-doped drain; 2 N-doped source; 3 P-type silicone substrate; 4 SWNT as transistor; 5 MWCNT as nano-wire; 6 insulator; 7 metal contacts; 8 refernce electrode; 9 solution; 10 electroactive membrane; 11 encapsulate [6]

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4. Carbon nanotubes

Carbon nanotubes are the most studied class of nanotube/nanowire FETs [14]. A carbon nanotube consists chemically of a sheet of graphite rolled up into a tube (Fig.4). Because the chemical bonds are all satisfied, there are no "dangling" bonds, minimizing surface scattering and leading to high mobility transport. Carbon nanotubes can be single-walled (SWNT) or multi-walled (MWNT). Typical dimensions are 1-3 nm for SWNTs and 20-100 nm for MWNTs. Clearly, for single walled nanotubes, they realize the promise of critical dimensions smaller than any current lithographic technique. [15]

The electronic properties of carbon nanotubes depend on both their diameter and chirality (analogous to the number of turns per inch of a screw). Depending on the chirality, the nanotube can be either metallic or semiconducting. While Raman scattering can determine the chirality, the common test of whether a nanotube is semiconducting or metallic is to test whether the resistance changes with a backgate voltage. For semiconducting nanotubes, the bandgap is approximately 1 eV/d[nm], where d is the diameter in nm. Currently, there is no technique to control chirality of the nanotube during synthesis, and precise control of the diameter is also a challenge. One method of avoiding this problem would be to develop size-sorting and chirality-sorting techniques to isolate a particular nanotube from a heterogeneous mixture.

Fig. 4: Single walled carbon nanotube. [14]

The remarkable electrical properties of SWNTs stem from the unusual electronic structure of the two-dimensional material, grapheme. Graphene-a single atomic layer of graphite-consists of a 2-D honeycomb structure of sp bonded carbon atoms, as seen in Fig. 5(a). Its band structure is quite unusual; it has conducting states at, but only at specific points along certain directions in momentum space at the corners of the first Brillion zone, as is seen in Fig. 5(b). [14] It is called a zero-bandgap semiconductor since it is metallic in some directions and semiconducting in the others. In an SWNT, the momentum of the electrons moving around the circumference of the tube is quantized, reducing the available states to slices through the 2-D band structure, is illustrated in the Fig. 5(b). This quantization results in tubes that are either one-dimensional metals or semiconductors, depending on how the allowed momentum states compare to the preferred directions for conduction. Choosing the tube axis to point in one of the metallic directions results in a tube whose dispersion is a slice through the center of a cone [Fig. 5(c)]. If the axis is chosen differently, the allowed s takes a different conic section, such as the one shown in Fig. 5(d). The result is a 1-D semiconducting band structure, with a gap between the filled hole states and the empty electron states. Nanotubes can, therefore be either metals or semiconductors; depending on how the tube is rolled up. This remarkable theoretical prediction has been verified using a number of measurement techniques. Perhaps the most direct used scanning tunneling microscopy to image the atomic structure of a tube and then to probe its electronic structure [14].

Fig. 5. (a) Lattice structure of graphene, a honeycomb lattice of carbon atoms. (b) Energy of the conducting states as a function of the electron wavevector k. There are no conducting states except along special directions where cones of states exist. (c), (d) Graphene sheets rolled into tubes. This quantizes the allowed ks around the circumferential direction, resulting in 1-D slices through the 2-D band structure in (b). Depending on the way the tube is rolled up, the result can be

either (c) a metal or (d) a semiconductor