Due to the Si nanowires have lot of advantages, such as real-time, highly sensitivity, label free and large surface to volume ratio, the application of Si nanowire sensors were developed widely in chemistry and
biological, even based diagnostic tools. In this section, we would introduce some nanowire sensors, which were located importance on general researches, such as pH sensors, DNA sensors, gas sensors, protein sensors, and virus detection.
1.3.1 pH sensor
The first nanowire field effect devices had used to detect analytes in solution occurred in 2001 with the sensing of pH by Leiber’s group [14].
The device structure and chemical response were shown in Fig. 1-8. The SiNW device was transformed into a pH sensor by modifying the silicon oxide surface with and without 3-aminopropyltriethoxysilane (APTES). We could obviously found out that the p-SiNWs devices modified by 3-aminopropyltriethoxysilane exhibited stepwise increases in conductance as the solution pH was increased stepwise from 2 to 9. However, unmodified SiNWs device showed nonlinear response at low pH (2-6) and the same linear response with modified at high pH (6-9). At low pH, the –NH2 group was protonated to –NH3+, which dominated reaction and formed positive charge on the surface. In contrast, at high pH, the –SiOH group was deprotonated to –SiO-, which dominated reaction and formed negative charge on the surface. Without modified 3-aminopropyltriethoxysilane, so it didn’t have the –NH2 group was protonated to –NH3+. Applying the positive charge depletes carriers and reduces the conductance in the p-type nanowire.
In the same way, applying the negative charge accumulates carriers and increase the conductance in the p-type nanowire. This comparison clearly demonstrated that the sening mechanism was successful.
1.3.2 DNA sensor
Gao and co-workers [15] fabricated silicon nanowire arrays for label-free detection of DNA by top-down approach. The nanowire arrays fabrication process shown as Fig. 1-9. The electrical DNA assay described there is ultrasensitive, rapid and label-free, and is able to monitor directly DNA hybridization process in situ and in real time. The high sensitivity of nanowire sensors are prospective because the diameters of these sensors are similar to the size of the DNA molecules being sensed and because important signal changes will be caused by the binding of a small number of DNA molecules on the surface of a single nanowire. The analysis of working principle of the SiNW array biosensor is shown as Fig. 1-10. Simply, they used to determine the amount of conductance change in the binding of DNA with or without on the nanowire surface.
1.3.3 Gas detection
Kim and co-workers [16] used the electro-deposited into anodized alumina template approach of Pd nanowire fabrication to process the nanowire sensor for hydrogen gas sensor. The hydrogen sensor has been widely studied for a long time due to safety reasons. In their study, the hydrogen sensor utilizing a bundle of Pd nanowires is fabricated, which is shown in Fig. 1-11. The bottom gate was on Ti layer and the apparatus of sensor was shown in Fig. 1-12. They used a metal needle as the top contact because there was no other ways to make the excellent contact on the top of the Pd nanowires and to guarantee the hydrogen gas of reaching the nanowire surface. When the hydrogen gas flow presence in apparatus, Pd
metal is subject to swell because the Pd hydride has a larger lattice constant than pure Pd metal. Their groups utilized the characteristic, different sizes of metal have different conductance by measurement. As soon as the hydrogen gas was flowed to the sensing chamber, the electrical resistance of the Pd nanowire sensor stated to decrease suddenly. Overall, the sensor of hydrogen gas would be applied in the feature.
1.3.4 Protein detection
Wang and co-workers [17] demonstrate the sensor of protein molecules by using silicon nanowires fabricated by top-down approach. The Ab1 tyrosine kinase was covalently linked to the p-type SiNWs, which was set as surface modification. The sensing mechanism is similar previous studies.
The observed increases in conductance are consistent with the binding of negatively charged ATP to Abl of nanowire surface. On the other hand, when SiNW is n-type, the conductance decreases as the ATP link to Ab1. The simple diagram of nanowire with or without ATP was shown in Fig. 1-12. If used the characteristic, there will be greater help on medical or drug analysis in the feature.
1.3.5 Virus detection
Lieber and co-workers [18] report rapid, label free, direct and real-time electrical detection of single virus particles with high selectivity by using nanowire field effect transistors. The different antibody receptors were covalently linked to the p-type SiNWs, which were set as surface modification, the diagram was shown in Fig. 1-13. We could find out that
modification of different nanowires within the array with receptors specific for different viruses provided a means for simultaneous detection of multiple viruses. And the Fig. 1-14 showed the conductance (upper) and optical (lower) data that was recored simultaneously. Overall, single viral particle sensitive, and capability of selective multiplexed detection of their approach suggest that this work could lead to useful viral sensing devices. This could provide an indication of mutations in samples as required for robust medical and bioterrorism detection, even drug discovery.