**4.1 Silicon biosensors**

*Nanowires - Recent Progress*

incorporates functionalities that do not necessarily scale according to Moore's law. Under the roadmap, the device falls under the category of sensors and actuators. Other categories include analogue/RF, passives, HV power, and biochips [88, 89]. **Figure 6** shows a typical structure of a biosensor [90–92]. The biomolecules are contained within an analytic solution and attach themselves to immobilized enzymes or immune-agents on the linkers. Linkers in turn are attached to the transducer. The transducer then converts the charge on the analyte into an electrical signal which is then transmitted for data processing. Biosensors can be considered as part of the research field known as "chemical sensors" in that a biological mechanism is used for analyte detection within an analyte solution [93–95]. Quasi-one-dimensional nanostructures have a greater surface-to-volume ratio compared to planar structures

and are therefore expected to be more sensitive than planar sensors [93–95].

*Typical structure of a biosensor. The biomolecules are contained within an analytic solution and attach themselves to immobilized enzymes or immune-agents on the receptors. The transducer then converts the energy* 

*signal produced into an electrical signal which is then transmitted for data processing. [22].*

Nanowires are the same as nanorods. The words can be used interchangeably [80]. These have received enormous attention due to their suitable properties for designing novel nanoscale biosensors. For example, the dimensions of ∼1–100 nm are similar to those of many biological entities, such as nucleic acids, proteins, viruses, and cells [79]. In addition, the high surface-to-volume ratios for nanomaterials allow a large proportion of atoms in the bio-analyte to be located at or close to the surface. Moreover, some nanowire materials have surfaces that can easily be chemically

**12**

**Figure 6.**

Over the past decade, silicon nanowires have been the most researched for application as biochemical sensors [97–108]. Silicon nanowires are of interest for a number of reasons, for example, the material is well known and is compatible with CMOS integrated circuits for the development of sensor systems [97–108]. The nanowire is expected to have high surface-to-volume ratios which give high sensitivity and the electrical sensing will give real-time label-free detection without the use of expensive optical components. Mass manufacturing is also a main advantage for silicon and is critically important for nanowire biosensor applications because of the widespread uptake of biosensors in "point-of-care" settings, the biosensor needs to be disposable [97–108].

A number of fabrication methods are well established for silicon nanowires which utilize both bottom-up and top-down methods (these methods are called hybrids). It still remains that bottom-up techniques have the advantage of simplicity [97–108]. Bottom-up methods are still limited due to the alignment problem. The hybrid methods require further nanowire technologies to achieve alignment, such as electric field or fluid-flow-assisted nanowire positioning to locate the nanowires between lithographically defined source and drain electrodes. The technique is interpreted as a hybrid between bottom-up and top-down. Top-down methods overcome these problems, and several researchers have used advanced lithography techniques to fabricate single-crystal silicon nanowires on silicon-on-insulator (SOI) substrates. SOI wafers are expensive and to overcome the problem some researchers [109] have devised alternatives to SOI. The electrical output characteristics of silicon nanowires are good and they are well suited for biosensing applications. The sensitivity range for most silicon-nanowire based biosensors is between 50 and 400 mV [97–134].

## **4.2 Comparing ZnO nanowire biosensors**

ZnO is investigated as it is expected to be more sensitive than Si due to its wider bandgap [109]. This is observed by comparing **Table 2** with **Table 3**. ZnO devices show results comparable to silicon devices; especially looking at response time and limit of detection. It is required that biosensors should have the liquid reference electrode. There are many different types of ZnO nanostructures being used for sensing application and **Table 2** compares the ZnO nanostructures such as nanotetrapods, nanocombs, and nanorods used for biosensing [110, 121]. Nanotetrapods [123] are like nanorods but with four single crystalline legs. Most of the ZnO devices were synthesized by vapor phase method and then transferred on Au electrode to form a multiterminal network for the sensor receptors. Like all other bottom-up ZnO nanostructures discussed here, they are transferred to a surface of a working electrode to form a thin layer to modify the transducer. The devices have low sensitivity but the nanotetrapods exhibit good detection limit down to ~1.0 nM. The researchers [123] did not explain why the nanostructures possess low sensitivity but its three-dimensional features have the potential for multiterminal communication applications [123].


#### **Table 2.**

*Summary of characteristics for various 1-D ZnO biosensors, adopted from [110].*


#### **Table 3.**

*Summary of characteristics for various 1-D Si biosensors, adopted from [121].*

In nanocombs [116] design, each comb has between 3 and 10 rods connected to one another by a single rod. ZnO nanocombs were used as the channel for sensing glucose [116] and as label-free uric acid biosensor based on uricase [124]. The functionalized

**15**

**Author details**

**Acknowledgements**

Botswana

Nonofo Mathiba Jack Ditshego

Electrical, Computer and Telecommunications Engineering Department, CET, Botswana International University of Science and Technology (BIUST), Palapye,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

of Science and Technology (BIUST) for supporting his doctoral studies and the Southampton Nanofabrication Centre for the experimental work. The author would

N.M.J. Ditshego would like to acknowledge the Botswana International University

\*Address all correspondence to: ditshegon@biust.ac.bw

provided the original work is properly cited.

of lysozyme and bovine serum albumin (BSA) can be achieved.

like to acknowledge the EPSRC EP/K502327/1 grant support.

*ZnO Nanowire Field-Effect Transistor for Biosensing: A Review*

ZnO nanorods showed thermal stability, anti-interference capability, and direct electron transfer (DET) between enzyme electroactive sites and external electrodes. The activity of the enzyme and the sensitivity can be increased by introducing a lipid film between the channel and the enzyme. Another uric acid biosensor [125] example is based on uricase-functionalized ZnO nanoflakes, which was hydrothermally prepared at low temperatures on Au-coated glass. The sensor produced a sensitivity based on subthreshold slope of ~66 mV/decade. Bottom-up ZnO nanorods [126] were also used as lactate oxidase (LOD) biosensor using glutaraldehyde cross-linkers. The device had a subthreshold sensitivity of ~41 mV/decade, with maximum detection of 0.1 μM. To test for cholesterol, porous ZnO mirco-tubes [127] were constructed using 3-D assembled porous flakes. ZnO nanorods [128] were grown on Ag electrode to

Most researchers use bottom-up approaches to fabricate the ZnO biosensors because of the straightforward synthesis process. However, these bottom-up devices have variable electrical performance due to the lack of geometrical dimension control and addressing the nanostructures for sensing application. So far, there is limited research reported on top-down ZnO biosensors, and previous work demonstrated the viability of top-down ZnO NWFET for biosensor applications. In the work, however, there was no passivation layer on the ZnO nanowires, which led to the dissolution of the material. This made the device unstable and the sensing results were not reproducible. There exists a need to develop a passivating layer technology and optimize the fabrication process for biosensor applications. That way, a reliable measurement of sensitivity for the nonspecific and specific sensing

*DOI: http://dx.doi.org/10.5772/intechopen.93707*

make a cholesterol sensor.

**5. Conclusion**

*ZnO Nanowire Field-Effect Transistor for Biosensing: A Review DOI: http://dx.doi.org/10.5772/intechopen.93707*

ZnO nanorods showed thermal stability, anti-interference capability, and direct electron transfer (DET) between enzyme electroactive sites and external electrodes. The activity of the enzyme and the sensitivity can be increased by introducing a lipid film between the channel and the enzyme. Another uric acid biosensor [125] example is based on uricase-functionalized ZnO nanoflakes, which was hydrothermally prepared at low temperatures on Au-coated glass. The sensor produced a sensitivity based on subthreshold slope of ~66 mV/decade. Bottom-up ZnO nanorods [126] were also used as lactate oxidase (LOD) biosensor using glutaraldehyde cross-linkers. The device had a subthreshold sensitivity of ~41 mV/decade, with maximum detection of 0.1 μM. To test for cholesterol, porous ZnO mirco-tubes [127] were constructed using 3-D assembled porous flakes. ZnO nanorods [128] were grown on Ag electrode to make a cholesterol sensor.
