Nanobiosensing Devices

Figure 2 summarizes four types of the most commonly used biological sensing devices based on CNTs and NWs, which covers most studies reported in the literature by now. Depending on the sensing applications, different device architectures and fabrication routes are required to achieve successfully their desired functions. Common to all, CNTs or NWs are critical components of the sensing devices, which are integrated either directly or indirectly during the fabrication routes. A variety of methods, ranging from advanced micro- or

Fig. 2. Schematic of four types of CNT/NW devices for biological sensing: (A) single CNT or NW extended from a microtip to probe a single cell or single molecules; (B) FET device using a single semiconducting CNT or NW; (C) porous CNT/NW film as electrochemical biosensor; (D) nanoelectrode array-based electrochemical biosensor.

Fig. 2. Schematic of four types of CNT/NW devices for biological sensing: (A) single CNT or NW extended from a microtip to probe a single cell or single molecules; (B) FET device using a single semiconducting CNT or NW; (C) porous CNT/NW film as electrochemical biosensor; (D) nanoelectrode array-based electrochemical biosensor.

nanolithographic techniques to handmade processes, have been demonstrated by various researchers to build functional devices. Generally, CNTs/NWs could be the sensing elements whose properties are changed on occurrence of the biological events' or the transducers that transfer the changed signal to the measuring units. The sensing device could use a single CNT/NW or an ensemble of such materials.

The most straightforward biological sensing employs a single CNT or NW to probe the biochemical environment in a single living cell or interrogate a single biomolecule (as shown in Fig. 2A). The CNT/NW probe can be attached to the pulled tip of an optical fiber for optical detection (74) or an electrode for electrical or electrochemical measurements (75). Normally, for such applications, the sidewalls of the nanoprobe have to be shielded or insulated to reduce the background so that the small signal from the very end of the probe can be detected. The CNT/NW probes provide the best spatial resolution as well as ultrahigh sensitivity and short response time. The small size, high aspect ratio, and mechanical robustness of CNTs were also employed as the physical probe for high-resolution scanning probe microscopy (SPM). This technique is powerful for illustrating the structure of single molecules such as DNA and proteins with the resolution down to a few nanometers.

A semiconducting CNT or NW can be used to construct a FET, as shown in Fig. 2B, using nanolithographic techniques. Such a device consists of a semiconducting CNT or NW connected to two contact electrodes (source and drain) on an oxide-covered Si substrate that could serve as the gate electrode (1,13). Conventional FETs fabricated with semiconductors such as Si have been configured as sensors by modifying the gate oxide (without the gate electrode) with molecular receptors or ion-selective membranes for the analytes of interest (76). The binding or adsorption of charged species could produce an electric field that depletes or accumulates carriers within the semiconducting material similar to the gate potential. As a result, the conductance between the source and drain electrodes is dramatically changed. Chemical sensors based on such a mechanism referred to as chemical field-effect-transistors (76) have been used for many applications since the 1980's.

As we mentioned in the Introduction, SWCNTs have well-defined structures. The helicity of the SWCNT determines whether it is metallic or semiconducting. It has been demonstrated that a semiconducting SWCNT (S-SWCNT) FET exhibits p-type transistor characteristics with the conductance tunable over several orders of magnitude by changing the gate voltage applied on the Si substrate (1). Such a phenomenon was attributed to the adsorption of O2 as the electron acceptor in an ambient environment. Since every carbon atom is exposed at the surface, SWCNTs are highly efficient for gas adsorption. In addition, a theoretical work predicts about 0.1 electron transfers per O2 molecule (77). Owing to the small size, this gives a big carrier density in the "bulk" SWCNT that would dramatically change the electronic behavior of the FET. The quantum wire nature of the SWCNT makes the conductance of the tube even more sensitive because any partial point charge could completely deplete the local carrier along the 1D wire. Therefore, the sensitivity of single-molecule adsorption/desorption could, in principle, be achieved with such miniaturized S-SWCNT FETs. Semiconducting NWs (S-NWs) such as Si (3) and In2O3 (4) have also been recently demonstrated in FET sensing devices. The applications of such devices in biological sensing are discussed in Subheading 3.2.

The large surface-to-volume ratio and good electrical conductance along the tube axis make CNTs attractive for enzyme-based electrochemical sensors. CNTs can be cast as a thin film on conventional electrodes (78-80) or used as a 3D porous film (32,36,37) (as shown in Fig. 2C). Such crude CNT ensembles serve both as large immobilization matrices and as mediators to improve the electron-transfer between the enzyme-active site and the electrochemical transducer. Improved electrochemical behavior of NADH (80), neurotransmitters (32), and enzymes (79) has been reported.

The last type of CNT/NW device is also an electrochemical sensor based on an array of vertically aligned CNTs embedded in SiO2 matrices, as schematically shown in Fig. 2D. The electrochemical signal is characteristic of the reduction/oxidation (redox) reaction of the analytes instead of nonspecific charges sensed by FETs, resulting in very good specificity comparable to fluorescence-based optical techniques that are commonly used in today's biology research. In addition, a high degree of miniaturization and multiplex detection can be realized for molecular diagnosis using individually addressed micro-electrode array integrated with microelectronics and microfluidics systems (8184), which is more attractive than optical techniques, particularly for quick and simple diagnostics. However, the sensitivity of electrochemical techniques using traditional macro- and microelectrodes is orders of magnitude lower than those of optical techniques, which limits their use in many applications. Nanoscale sensing elements have been actively pursued to seek solutions for improving the sensitivity of electrochemical techniques.

The performance of electrodes with respect to speed and spatial resolution is known to scale inversely with the electrode radius (85-87). It is of interest for biosensing to reduce the radius of electrodes to 10-100 nm, approaching the size of biomolecules. Li et al. (35) demonstrated that nanoelectrodes, particularly a MWCNT-based nanoelectrode array, with an average diameter of 30 to 100 nm, can be integrated into an electrochemical system for ultrasensitive chemical and DNA detection. The nanoelectrode array is fabricated with a bottom-up scheme resulting in a precisely positioned and well-aligned MWCNT array embedded in a planarized SiO2 matrix (55,88). The vertically aligned MWCNTs were first grown using direct current (DC)-biased hot-filament plasma CVD on a metal film-covered silicon substrate. They were then subjected to tetraethyloxysilicate CVD for gap filling of SiO2. SiO2 dielectrics was found to fill conformally in the gap between individual MWCNTs and hence reinforced the MWCNT array. Excess SiO2 was subsequently removed, allowing exposure of the MWCNT tips via a mechanical polishing step. The open ends of MWCNTs exposed at the dielectric surface act as nanoelectrodes. Figure 3 shows SEM images of a CNT array grown on multiplex microelectrodes with different densities defined by lithographic techniques. The surface of an encapsulated and planarized array shows only the very end of CNTs exposed as well-defined nanoelectrodes. Such a nanoelectrode array showed characteristic electrochemical behavior for redox species both in bulk solution and immobilized at the CNT ends. Dramatic improvements in sensitivity and time constant were observed (35).

CNTs have demonstrated many advantages in nanoelectrode applications. The wide electropotential window, flexible surface chemistry, and bio-compatibility make the MWCNT array an attractive electrode similar to other widely used carbon electrodes. The open end of an MWNT is expected to show a fast electron transfer rate (ETR) similar to that of the graphite edge-plane electrode, whereas the sidewall is inert like the graphite basal plane (89). Fast ETR is demonstrated along the tube axis (75,90). Such an MWCNT electrode could interrogate target species, down to the single-molecule level, at one end and sustain electron transport along the tube axis to the measuring circuit with minimum interference from the environment. Whereas a single

Fig. 3. SEM images of (A) 3 x 3 electrode array; (B) array of MWCNT bundles on one of the microelectrode pads; (C,D) array of MWCNTs at UV-lithography- and e-beam-patterned Ni spots, respectively; (E,F) surface of polished MWCNT array electrodes grown on 2-|im and 100-nm spots, respectively. Panels (A-D) are 45° perspective views and panels (E) and (F) are top views. Bars = 200, 50, 2, 5, 2, and 2 |im, respectively.

Fig. 3. SEM images of (A) 3 x 3 electrode array; (B) array of MWCNT bundles on one of the microelectrode pads; (C,D) array of MWCNTs at UV-lithography- and e-beam-patterned Ni spots, respectively; (E,F) surface of polished MWCNT array electrodes grown on 2-|im and 100-nm spots, respectively. Panels (A-D) are 45° perspective views and panels (E) and (F) are top views. Bars = 200, 50, 2, 5, 2, and 2 |im, respectively.

nanoelectrode can provide the desired temporal and spatial advantages, the nano-electrode array serves the needs for analytical applications requiring reliable statistics and multiplex detection.

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