7. Biosensing

#### 7.1. Biosensors

+/0z+"z 0\$!z/01 %!/z+\* 10! z%\*z 0\$!z,/0z+\*z%\*#(!w.5/0(z%z,.+2% !z!2% !\*!z+"z 0\$!z0¥ tractive bio potentialities of this material and hence suggest similar properties for crystalline SiC. The properties that make this material particularly promising for biosensing applications are: 1) the wide bandgap that, as mentioned before, increases the sensing capabilities of a semiconductor; 2) the chemical inertness that suggests the material's resistance to corrosion in harsh environments such as body fluids (e.g. SiC does not react with any known material at room temperature, the only efficient etch being molten KOH at 400-600 ℃); 3) the high hardness (5.8 GPa), high elastic modulus (424 GPa), and low friction coefficient (0.17) that make it an ideal material for smart-implants and in-vivo biosensors. Studies report the significant finding that SiC surfaces are better substrates for mammalian cell culture than Si in terms of both cell adhesion and proliferation. In the past, the fact that cells could be directly cultured on Si crystalline substrates led to a widespread use of these materials for biosensing applications [25-27].

Singh and Buchanan [28] studied silicon carbide carbon (SiC-C) composite fiber as an electrode material for neuronal activity sensing and for biochemical detection of electroactive neurotransmitters. The SiC-C electrode surface has nanosized pores which significantly increase the real surface area for higher charge densities for a given geometrical area. Neurotransmitters including dopamine and vitamin C were successfully detected using SiC-C composite electrodes. Researchers fabricated impedance and temperature sensors on bulk SiC for a biomedical needle that can be used for open heart surgery monitoring or graft monitoring of organs during transportation and transplantation. According to Godignon [29], other applications can be foreseen, such as DNA polymerase chain reaction (PCR), electrophoresis chips and cell culture micro-arrays. In DNA electrophoresis devices, the high critical electric field and high resistivity of semi-insulating SiC would be beneficial. In DNA PCR, it is the high thermal conductivity which could improve the devices behaviour. In addition, in most of these cases, the transparency of semi-insulating SiC can be used for optical monitoring of biological processes, such as the DNA reaction or the cell culture activity. Ghavami et al. used Field emission scanning electron microscopy (HE-SEM) and transmission electron microscopy (TEM) techniques to examine the structure of the SiCNP/GC modified electrode. The modified electrode shows excellent electrocatalytic activity toward guanine, adenine, thymine and cytosine. Differential pulse voltammetry (DPV) was proposed for simultaneous determination of four DNA bases. The effects of different parameters such as the thickness of the SiC layer, pulse amplitude, scan rate, supporting electrolyte composition and pH were optimized to obtain the best peak potential separation and higher sensitivity. The modified electrode can be used for simultaneous detection of purine and pyrimidine bases without any separation or pretreatment processes and may be used as a DNA biosensor in real samples [30].Caputo et al. [31] reported on biomolecule detection based on a two-color amorphous silicon photosensor. The device design has been optimized in order to maximize the spectral match between the sensor responses and the emission spectra of the fluorochromes. This optimization process has been carried out by means of a numerical device simulator, taking into account the optical and the electrical properties of the amorphous silicon materials. The development of minimally invasive and short-term implantable devices for on-line tissue monitoring is a field of increasing clinical and industrial interest, with applications in areas such as open-heart surgery or insulin control. Multisensory micro-needles have been developed to measure physiologically relevant intratissular parameters during cold transportation of transplantation organs. Si-based minimally invasive probes have already gone beyond the proof-of-concept stage and are currently undergoing Phase-I clinical trials [32]. Gabriel et al. [33] examine the feasibility of using SiC as a substrate for the development of minimally invasive multi-sensor micro-probes in the context of organ monitoring during transplantation. In particular, they make a thorough comparison of Si and SiC material mechanical and electrical properties. As illustrated in Figure 3, Si and semi-insulating SiC micro-needles for impedance and temperature measurement were fabricated using remarkably similar methods.

Figure 3. A) Needle-shaped Si (lett) and SiC (right) probes for impedance and temperature monitoring. (B) Schematic drawing of the technological process for Si (left) and SiC (right) probe production. (C) Encapsulated SiC (left) and Si (right) devices [33].

Their results show that SiC outperforms Si in all respects, with a four times higher modulus of rupture for SiC devices and a 10-fold increase in the frequency range for electrical measurements in SiC-based probes. These results suggest that SiC should be preferably used over Si in all biomedical applications in which device breakage must be avoided or very precise electrical measurements are required [33]. 3C-SiC has a distinct advantage over the other SiC polytypes in that simple micromachining technique can be used to fabricate nanoscale 3C-5iC structures. In fact, the first SiC NEMS were demonstrated in 3C-SiC due to the ability to grow ultrathin 3C-SiC films on Si substrates combined with selective ion etching processes that enable patterning and release of the nanostructures with essentially the same plasma [34]. Other groups have used similar techniques to create 3C-SiC and AIN NEMS structures [35]. Figure 4 shows SEM images of 3C-SiC NEMS microbridges. Naik et al. extended this work toward biosensing applications by demonstrating that 3C-SiC NEMS resonators were able to detect individual protein adsorption events by observing frequency shifts for each exposure event when the resonators were selectively exposed to bovine serum albumin (BSA) and ß-amylase (200 kDa) [36].

Figure 4. Scanning electron micrographs of 3C-SiC NEMS microbridges [35].

Silicon carbide for chemical sensing devices has been demonstrated to be the best candidate for high temperature chemical gas sensors. The wide bandgap, combined with chemical inertness, result in SiC being the best material for gas sensing in harsh environments or at high temperatures. Thin film silicon carbide exhibits thermal conductivity on the same order of single crystalline silicon and has a fast thermal response [13]. MEMS-based microprobes for neural interfacing and biosensing applications are currently the subject of intense research due to the promise of achieving high functionality in a minimally invasive form-factor. A typical planar neural probe, for example, consists of a thin shank that supports multiple, thin film metallic electrodes. A common material for the shank is Si; however, concerns over the electrical performance, mechanical robustness and biological interactions of these structures currently limit their applicability in long-term deployment situations.

For neurostimulation applications, the electrode material must provide charge transfer under low interface impedance to avoid tissue trauma. Surface morphology plays an important role in the electrode's charge carrying capabilities. The real surface area of an electrode can be significantly different from its geometrical area. Higher real surface areas can deliver higher charge densities for a smaller electrode. For a microelectrode, size is a limiting factor, so there should be ways to increase real surface area. Real surface areas can be modified by treating the electrodes electrolytically. The SiC-C electrode surface as seen in the SEM images (Fig. 5) has nanosized pores which significantly increase the real surface area for higher charge densities for a given geometrical area. Thus high real surface area electrodes are highly desirable and were observed in electrolytically modified SiC-C electrode surfaces. Neurotransmitters including dopamine and vitamin C were successfully detected using SiC-C composite electrodes [37].

Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS 365 http://dx.doi.org/10.5772/51811

Figure 5. SEM pictures of (A) silicon carbide carbon composite fiber's etched section, (B) carbon recording tip, (C) low voltage etch (smooth surface), and (D) high voltage etch (high surface area surface) [37]

In many clinical settings, a decrease of the blood supply to body organs or tissues can have fatal consequences if it is not properly addressed promptly (e.g. mesenteric or myocardial ischemia). Sustained ischemia leads to hypoxia, a stressful condition for cells that can induce cell lysis (necrosis) and trigger programmed cell death (apoptosis) and, consequently, lead to organ failure. Aside from ischemic diseases, ischemia underlies other natural and clinically induced conditions, such as tumor growth, cold-preservation of grafts for transplantation or induced heart-arrest during open heart surgery. The ability to monitor ischemia in clinical and experimental settings is becoming increasingly necessary in order to predict its irreversibility (e.g. in the transplantation setting), to develop drugs to prevent and revert its effects, and to develop vascular-targeting drugs for the treatment of growing tumors. To address these issues and to extend the utility of MEMS-based probe technology, a minimally invasive system for the continuous and simultaneous monitoring of tissue impedance has been developed, and experimental results have shown its reliability for early ischemia detection and accurate measurement of ischemic effects. This minimally invasive system consists of a small micro-machined silicon needle with deposited platinum electrodes for impedance measurement that can be inserted in biological tissues with minimal damage. High frequency impedance monitoring, based on both the phase and modulus components of impedance, has been correlated to the combined dielectric properties of the extracellular and intracellular compartments and insulating cell membranes, and can give +),(!)!\*0.5z %\*"+.)0%+\*z +\*z +0\$!.z !""!0/z +"z /1/0%\*! z %/\$!)%^z 
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be attributed to the occurrence of hypoxic edema as the result of cell swelling, which leads to a reduction of extracellular space, an increase in extracellular resistance, and cell-to-cell uncoupling. Upon unclamping of the renal artery (50 min), impedance modulus can be seen 0+z.!01.\*z0+z%0/z/(z2(1!\_zz"0z0\$0z\*z!z00.%10! z%\*z0\$%/z!4,!.%)!\*0(z/!00%\*#z0+zz.!¥ version from a short period of ischemia without substantial structural damage to the tissue. z"((z%\*z%),! \*!z)+ 1(1/z0z(+3z".!-1!\*%!/\_z\$+3!2!.\_z\$/z(/+z!!\*z.!,+.0! z/zz+\*¥ /!-1!\*!z+"z)!).\*!z.!' +3\*z\* z!((z(5/%/z 1!z 0+z/1/0%\*! z%/\$!)%^z 0z%/z%\*z 0\$%/z.!¥ spect that the multifrequency analysis of the phase component of impedance made possible by the use of SiC-based probes conveys useful complementary information [5]. Therefore, according to this set of materials, one can conclude that SiC would be considered a good

Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS

http://dx.doi.org/10.5772/51811

367

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*Task 4.* Detection of quantal released molecules by means of newly designed biosensors.

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can plan the realisation of four activities with the following tasks:

*Task 2.* Monitoring of electrical activity from neuronal networks.

*Task 3.* Resolution of cellular excitability over membrane micro areas.

candidate for biosensing applications.

7.2. Microelectrode arrays

cific pathways.

Figure 6. A) Needle-shaped Si probe for impedance; (B) Needle-shaped SiC probe for impedance; (C) Needle-shaped with packaging [38]

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#### 7.2. Microelectrode arrays

damage. High frequency impedance monitoring, based on both the phase and modulus components of impedance, has been correlated to the combined dielectric properties of the extracellular and intracellular compartments and insulating cell membranes, and can give +),(!)!\*0.5z %\*"+.)0%+\*z +\*z +0\$!.z !""!0/z +"z /1/0%\*! z %/\$!)%^z 
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Figure 6. A) Needle-shaped Si probe for impedance; (B) Needle-shaped SiC probe for impedance; (C) Needle-shaped

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conditions.

366 Physics and Technology of Silicon Carbide Devices

with packaging [38]

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Figure 7. Schematic diagram of the Utah Electrode Array [41]

To demonstrate the feasibility of developing SiC-based devices on large-area SiC substrates, Godignon et al. recently reported the development of 4H-SiC-based microelectrode arrays on semi-insulating 4-in. diameter hexagonal substrates [39]. The microelectrode arrays consisted of a SiO%SigN, passivation layer sandwiched between microfabricated Pt electrodes and the SiC substrate. Carbon nanotubes (CNTs) were easily grown by rapid thermal chemical vapor deposition on the Pt electrodes using CH4 and H2 at 800 °C, conditions for which the SiC substrate is well suited. The detection of biological species using microarrays and lab on-a-chip systems is a powertul diagnostic tool that enables the acquisition of genetic, proteomic, and cellular information. Such approaches allow rapid analysis of disease diagnostics, drug discovery, or food and environmental analysis. In microarray applications, each pixel in the array is functionalised with well-defined probe molecules and a molecular recognition reaction occurs between the probe and the target molecules to be detected [40]. Hsu et al. [41] has recently developed a-SiC as a protective coating for MEMS-based Si penetrating microelectrodes. The technology driver for this research is an integrated, Si-based microneedle electrode array known as the Utah Electrode Array (UEA). Shown schematically in Figure 7, the UEA is a three-dimensional structure consisting of a 10 × 10 array of tapered silicon shanks that are bulk micromachined into a Si substrate. Each shank supports Ti/Pt/Ir electrodes that provide the electrical interface to nerve tissue. The array is integrated with Si-based IC's and packaged using a variety of conventional approaches that are adapted for this particular device. Hsu et al. have developed PECVD-based a-SiC:H coatings to encapsulate components of the UEA.
