**5. Conclusion**

*Nanowires - Recent Progress*

**No. Reference electrode**

4 Glass

capillary

**Type of sensor**

1 Au Biosensor ZnO

2 ITO Biosensor ZnO

3 Au Biosensor Tetrapod-

5 GCE Biosensor Fork-like

6 Au Biosensor Comb-like

7 Ti Biosensor ZnO/C

8 ITO Biosensor ZnO/

9 GCE Biosensor ZnO/Au

10 Pt Biosensor ZnO/NiO

**Channel material**

nanorod array

nanotube array

like ZnO

nanoflakes

ZnO

ZnO

nanorod array

Cu array matrix

nanorods

nanorods

Biosensor ZnO

**ZnO fabrication process**

Hydrothermal/ chemical

**LOD (**μ**M)**

Hydrothermal 10 <5 [111]

CVD 4 6 [113]

Hydrothermal 0.5 <4 [114]

Annealing 0.3 3 [115]

CVD 20 <10 [116]

Hydrothermal 1 4 [117]

Hydrothermal 40 <6 [118]

Hydrothermal 0.01 <5 [119]

Hydrothermal 2.5 <5 [120]

**Response time (s)**

10 <6 [112]

**Ref.**

**No. Reference** 

**Table 2.**

1 No reference electrode

3 Platinum

wire

**electrode**

**Type of sensor**

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

2 Au Biosensor Si NW Chemical vapor

4 None Biosensor Si NW Reactive-ion

5 None Biosensor Si NW Synthesized by

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

**Channel material**

Biosensor Si NW nanocluster-

**ZnO fabrication process**

mediated vapor–liquid– solid growth method

deposition

etching (RIE)

chemical vapor deposition

Biosensor Si NW SNAP technique 10 <10 [101]

**LOD (pM)** **Response time (s)**

10 <10 [97]

0.002 <10 [98]

0.01 <10 [106]

100 <10 [122]

**Ref.**

**14**

**Table 3.**

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

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

### **Acknowledgements**

N.M.J. Ditshego would like to acknowledge the Botswana International University of Science and Technology (BIUST) for supporting his doctoral studies and the Southampton Nanofabrication Centre for the experimental work. The author would like to acknowledge the EPSRC EP/K502327/1 grant support.
