micromachines
ArticleFabrication of a Robust In2O3 Nanolines FET Device as a
Biosensor Platform

Zetao Zhu 1,2, * , Takao Yasui 1,2,3, *, Quanli Liu 1 , Kazuki Nagashima 3,4 , Tsunaki Takahashi 3,4 ,Taisuke Shimada 1 , Takeshi Yanagida 4,5 and Yoshinobu Baba 1,2,6, *
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Citation: Zhu, Z.; Yasui, T.; Liu, Q.;
Nagashima, K.; Takahashi, T.;
Shimada, T.; Yanagida, T.; Baba, Y.
Fabrication of a Robust In2 O3Nanolines FET Device as a Biosensor
Platform. Micromachines 2021, 12, 642.
https://doi.org/10.3390/mi12060642
Academic Editors: Katsuo
Kurabayashi and Pengyu Chen
Received: 12 April 2021
Accepted: 30 May 2021
Published: 31 May 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims inpublished maps and institutional affiliations.

Department of Biomolecular Engineering, Graduate School of Engineering, Nagoya University,
Nagoya 464-8603, Japan; liu.quanli@g.mbox.nagoya-u.ac.jp (Q.L.); shimada@chembio.nagoya-u.ac.jp (T.S.)
Institute of Nano-Life-Systems, Institutes of Innovation for Future Society, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8603, Japan
Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science andTechnology (PRESTO), Kawaguchi, Saitama 332-0012, Japan; kazu-n@g.ecc.u-tokyo.ac.jp (K.N.);
takahashi-t@g.ecc.u-tokyo.ac.jp (T.T.)
Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; yanagida@g.ecc.u-tokyo.ac.jp
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
Institute of Quantum Life Science, National Institutes for Quantum and Radiological Science and Technology,
Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan
Correspondence: zhuzetao@nanobio.nagoya-u.ac.jp (Z.Z.); yasui@chembio.nagoya-u.ac.jp (T.Y.);
babaymtt@chembio.nagoya-u.ac.jp (Y.B.)

Abstract: Field-effect transistors (FETs) are attractive biosensor platforms for rapid and accurate
detection of various analytes through surface immobilization of specific bio-receptors. Since it isdifficult to maintain the electrical stability of semiconductors of sensing channel under physiologicalconditions for long periods, passivation by a stable metal oxide dielectric layer, such as Al2 O3 or HfO2 ,is currently used as a common method to prevent damage. However, protecting the sensing channel
by passivation has the disadvantage that the distance between the target and the conductive channel
increases, and the sensing signal will be degraded by Debye shielding. Even though many efforts use
semiconductor materials directly as channels for biosensors, the electrical stability of semiconductors
in the physiological environments has rarely been studied. In this work, an In2 O3 nanolines FET
device with high robustness in artificial physiological solution of phosphate buffered saline (PBS)
was fabricated and used as a platform for biosensors without employing passivation on the sensing
channel. The FET device demonstrated reproducibility with an average threshold voltage (VTH ) of
5.235 V and a standard deviation (SD) of 0.382 V. We tested the robustness of the In2 O3 nanolines
FET device in PBS solution and found that the device had a long-term electrical stability in PBS withmore than 9 days’ exposure. Finally, we demonstrated its applicability as a biosensor platform bytesting the biosensing performance towards miR-21 targets after immobilizing the phosphonic acidterminated DNA probes. Since the surface immobilization of multiple bioreceptors is feasible, we
demonstrate that the robust In2 O3 FET device can be an excellent biosensor platform for biosensors.
Keywords: In2 O3 nanolines; robust; field effect transistor; biosensor platform

1. Introduction
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms andconditions of the Creative Commons
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Biosensors use biological receptors as their recognition elements in a signal transducing platform that generates a measurable signal in response to changes in the concentrationof a given biomolecule [1–5]. Field-effect transistors (FETs) are attractive biosensor platforms for rapid and accurate detection of various analytes through surface immobilizationof specific bio-receptors [6–9]. However, most semiconductor materials of sensing channels
cannot be maintained under physiological conditions for long periods of time [10,11]. To
solve this problem, surface passivation using a stable metal oxide-based dielectric layer,

Micromachines 2021, 12, 642. https://doi.org/10.3390/mi12060642

https://www.mdpi.com/journal/micromachines

Micromachines 2021, 12, 642

2 of 9such as Al2 O3 or HfO2 , is commonly employed [12,13]. One representative report by
Lieber’s group showed that electronic devices based on Si, Ge/Si and InAs nanowires havelong-term stability of at least 100 days when passivating the nanowires with ultra-thin(10 nm) Al2 O3 shells [14]. However, protecting the sensing channel under physiologicalconditions by passivation is disadvantageous because it increases the distance between
the charged target and the conductive channel. This can lead to a decrease in sensing
sensitivity due to the effect of the Debye shielding length [15,16]. Therefore, robust semiconductor materials for sensing channels that can tolerate long-term use in PBS solution
are highly desired.
Semiconductor metal oxides are one of the key compounds for the development ofsensitive materials for sensors due to a unique set of properties, especially their electricalconductivity and high reactivity of the surface in the interaction with the molecules [17,18].
Our group have also taken an interest and put a lot of effort into improving the electricalstability of semiconductor metal oxides under air and liquid conditions. Yan et al. [19,20]
demonstrated an air/Zn vapor annealing method to obtain a highly conductive Al-doped
ZnO (AZO) nanofilm that can maintain its electrical conductivity in air for a long timeand even in liquids of different pH. However, the long-term stability in liquids is only
increased by 1 to 2 h which is insufficient to meet the needs of biosensor devices, because theimmobilization process is carried out in a liquid for more than 2 h, and the fabricated device
has to be kept in the physiological solution to maintain the stability of the bioreceptors. In
addition, maintaining electrical stability in physiological environments is more challenging
due to the presence of various ions and biomolecules. Therefore, the development of stable
FET-based metal oxide biosensors requires that more stable materials are available for use
as the sensing channels. Although many metal oxide semiconductor-based nanostructures
have been reported for use on biosensors, the details of the electrical stability of these oxide
semiconductors under physiological conditions have rarely been studied [21–23].
In this work, an In2 O3 nanolines FET device with high robustness in artificial physiological solution of PBS was fabricated and used as a platform for biosensors without
passivation on the sensing channel. The fabricated FET device demonstrated good reproducibility with a small standard deviation of threshold voltage. We tested the robustness of
the In2 O3 nanolines FET device in PBS solution and found that the device had a long-term
electrical stability in PBS with more than 9 days’ exposure. Finally, we demonstrated
its applicability as a biosensor platform by testing the biosensing performance towards
cancer-related biomarker of miR-21 after immobilizing the phosphonic acid terminatedDNA probe on the In2 O3 nanolines FET platform.
2. Experimental
2.1. Formation of In2 O3 Nanolines
The fabrication process of the device was shown in the schematic of Figure 1a. In
details, the top surface of a Si wafer (2 ⇥ 2 cm) was covered with an SiO2 layer (100 nm thick)to obtain the Si/SiO2 substrate on which In2 O3 nanolines were deposited. Radio frequency(RF) sputtering and electron beam lithography (EBL) were employed as described in
the literature [24–26]. In brief, the Si/SiO2 substrate was first treated using UV ozone
cleaner for 5 min to remove organic contaminates. Then ZEP-50A electron beam resist
was deposited on the substrate by a spin coating process (3000 rpm for 200 s). EBL was
performed to area-selectively write the In2 O3 line pattern on the substrate using a 30 kVelectron beam. After the pattern area was developed using o-xylene developer (2 min),the patterned substrate was transferred to the chamber of an RF sputtering apparatus.
Then, 10 nm thick In2 O3 was deposited under the following conditions: power, 30 W;
vapor pressure, 1 Pa; O2 /Ar, 1/5; deposition rate, 2.5 nm/min. The resist was removed
in a lift-off process after soaking the substrate in N, N-dimethylformamide for 12 h. The
In2 O3 nanolines were cleaned using isopropanol (IPA), followed by drying in air flow.
Finally, the substrate with the In2 O3 nanolines was annealed at 400 C for 1 h in air. ZnOMicromachines 2021, 12, 642

3 of 9nanolines device was fabricated using the same method. To obtain the conductive device,
the thickness of ZnO nanolines was improved to 100 nm.

Figure 1. (a) Flowchart of the fabrication process of In2 O3 nanolines on Si/SiO2 substrate. The materials used in the
fabrication are colored-coded in the key. (b) Schematic drawings of a cross section and a top view. The signal curve(response) plot of current vs. time when changing the analytes.

2.2. Fabrication of In2 O3 Sensing Device
First, the hexamethyldisilazane tackifier was spin coated (3000 rpm for 8 s) on theannealed substrate with the In2 O3 lines, followed by 60 s baking at 105 C. Then thesubstrate was spin coated with AZ 5200-E photoresist (1000 rpm for 120 s), followed by 120 s
baking at 105 C. The electrode pattern was exposed using photolithography equipment(Model DDB-700). After developing the pattern, Ti/Pt electrodes were deposited on thedevice by DC sputtering. Here, the thicknesses of the Ti and Pt layers were 10 nm and
50 nm, respectively. Next, the resist was removed by a lift-off process after soaking in
acetone for 12 h. The In2 O3 nanolines FET device was obtained after washing with IPA anddrying in N2 flow. The FET device was covered with a SU-8 layer to protect the electrodes
when the device was soaked in phosphate buffered saline (PBS) solution. In this step,a 5 µm SU-8 layer was spin coated on the device (3000 rpm for 120 s). After baking at
95 C for 120 s, the In2 O3 line area was exposed using photolithography equipment. Afterdevelopment, a PDMS well was glued on the sensing device to keep the analytes in the
In2 O3 line area [27].
2.3. Characterizations
The morphologies of the In2 O3 nanolines and other parts of the device such as the Ti
and Pt electrodes were observed using an optical microscope and a field-emission scanningelectron microscope (FESEM; Zeiss, Supra 40 VP) at the accelerating voltage of 15 kV.
Energy-dispersive X-ray spectroscopy (EDS) elemental mapping was done with an FESEM
attachment (JEOL, JSM-7610F).

Micromachines 2021, 12, 642

4 of 92.4. Surface Modifications On Sensing Device
The chemical and biological agents including (3-Aminopropyl)triethoxysilane (APTES),
and phosphonic acid terminated DNA (PO3 H2 -DNA) probe were individually or multiply
modified on the In2 O3 nanolines device. The sequence of DNA of PO3 H2 -DNA probe is
PHO-AAAAATCAACATCAGTCTGAAGCTA. Before surface modification, all the devices
were cleaned by the oxygen plasma for 1 min under 80 W. APTES modified In2 O3 nanolines were obtained by immersing the device in an ethanol solution containing 1% APTES
for one hour. Immobilization of PO3 H2 -DNA probe on In2 O3 surface was performed byimmersing the device in a PBS solution containing the DNA probe (100 µM) and kept it
for 5 h. Co-modification of APTES and DNA probe was also performed by immersing in
APTES for 1 h, followed by immersing in DNA probe contained PBS solution for 5 h.
2.5. Sensing Measurements
The electrical conductance, transfer curve and sensing signal were measured by twoconnected semiconductor parameter analyzers (Keithley, Model 2401). The analyte wasprepared by dispersing miR-21 target in PBS solution (0.01 X). The analyte was dropped
on the sensing device by the pipette. The volume of one droplet was controlled to 3 µL.
The strength of electrical sensing response was expressed as (I I0 )/I0 . I0 and I are theconductance of sensing channel in diluted PBS solution and analyte containing miR-21
targets, respectively.
3. Results and Discussion
Figure 1b shows a cross-sectional view of the In2 O3 nanolines FET sensing device
structure we fabricated. During sensing measurement, analytes were directly dropped into
the PDMS well on the device using a micropipette. When the concentration of the analyte
is changed, the response signal is monitored, as shown by the graph at the bottom right of
Figure 1b.
We demonstrated performance of the FET device (Figure 2a). As seen from the SEM
images (Figure 2b), the two ends of the In2 O3 nanolines were in contact with the Ti/Pt
electrodes as the source and drain, respectively, while the Si substrate was in contact withthe back gated electrode. To optimize the fabrication process of the device, we measured
the transfer curves of In2 O3 nanolines when varying the width of the individual line from
200 nm to 1000 nm. As shown in Figure 2c, the transfer curve shifted to the positive
direction when decreasing the width of nanoline, illustrating that nanolines with small
widths were easily and completely depleted. The optimized width of each In2 O3 nanoline
were set to 800 nm. Furthermore, current modulation when changing the gate voltage
was observed from the output curves of the In2 O3 nanolines FET device (Figure 2d). To
demonstrate the reproducibility of the FET devices, the threshold voltage (VTH ) for 20 unitsof the In2 O3 FET device was measured and plotted in Figure 2e. The VTH was extracted
from the linear transfer curve with drain-source voltage (VDS ) set to 1 V, and the average of
5.235 V and standard deviation of 0.382 V were calculated from the distributed data. Thus,
our In2 O3 nanolines FET device demonstrated good reproducibility.
To demonstrate the robustness of our In2 O3 nanolines FET device in PBS solution, weused ZnO nanolines as a comparison, which is an excellent semiconductor metal oxidematerial for sensing applications. The SEM and EDS mapping images (Figure 3a) showedthat the ZnO nanolines were dissolved in PBS. In contrast, there seemed to be little change
in the In2 O3 nanolines after immersion under the same conditions (Figure 3b). Next, wemeasured the time-dependent conductance of ZnO and In2 O3 nanolines in PBS, as shownin Figure 3c. While the ZnO nanolines were broken after PBS immersion for about 1800 s,
there was a small change in conductance of the In2 O3 nanolines when they were immersed
in PBS. Therefore, we confirmed the robustness of In2 O3 nanolines when immersed in PBS.

Micromachines 2021, 12, 642

Figure 2. The In2 O3 nanolines FET device. (a) Schematic drawing showing the FET device. (b) FESEM images of the In2 O3nanoline area. (c) Transfer curve. The FET devices when varying the width of single line were measured. The drain-source
voltage was 1 V. (d) Output curves. The back-gate voltages were increased from 0 V to 18 V. (e) Measured threshold voltages
for 20 units of the In2 O3 FET sensing device. The width of device is 800 nm.

Figure 3. (a) FESEM images and EDS elemental mapping images of ZnO lines before and after soaking in PBS solution for
30 min. (b) FESEM images of In2 O3 nanolines before and after soaking in PBS solution for 30 min. (c) Time dependenceof electrical conductance of In2 O3 and ZnO nanolines when soaked in PBS solution. (d) Transfer curves of the In2 O3 FET
sensing device for different soaking times in PBS. Drain-source voltage was 1 V.

5 of 9

Micromachines 2021, 12, 642

6 of 9Then, we wanted to know how long the In2 O3 FET device could tolerate the PBS
solution. We demonstrated the long-term stability of the device by showing time dependent
electrical performance when immersed in PBS. The transfer curves of the In2 O3 FET device
were measured when soaked in PBS for 3 days, 6 days and 9 days (Figure 3d). When thesoaking time was increased, the transfer curve and VTH clearly shifted toward the positive
gate voltage. Since In2 O3 might be slowly etched by the ions in the PBS, this degradationin conductive performance was caused by a decrease in the cross-sectional area of the
conductive channel. Even so, the In2 O3 FET device maintained its stability for more than
9 days in the PBS solution. Thus, we demonstrated its long-term electrical stability in PBS.
Finally, we tested the applicability of the robust In2 O3 FET device platform to thebiosensor by immobilizing the DNA probe towards miR-21 target. As shown in Figure 4a,
the FET device was processed into a biosensor device by adding a window formed by aSU-8 protective layer and a PDMS well. Since large distance of the captured targets fromthe channel decreases the intensity of the sensing signal according to the Debye shielding
effect, we chose a phosphonic acid terminated DNA probe in expectation that phosphonicacid group could directly binding to metal oxide surface. [28,29] To demonstrate the effectof DNA probe immobilization on the sensing performance, a common used APTES selfassembled monolayer (SAM) was also modified as a comparison on the In2 O3 sensor
channel by a simple immersion process [30]. The details of experiments can be foundin experimental section. As shown in Figure 4b, a typical current response signal wasobserved when the miR-21 analyte (1 pM/mL in 0.01 X PBS solution) was pipetted on
the window of In2 O3 nanolines sensor device. To evaluate the detection sensitivity ofthe devices with various surface modifications, the normalized response of (I I0 )/I0
was calculated and shown in Figure 4c. The phosphonic acid terminated DNA probe
immobilized immunosensors exhibited the stronger sensing response compared to thepristine In2 O3 nanolines sensors, indicating that DNA probes were immobilized on the
In2 O3 nanolines and could capture the miR-21 targets and change the conductance of In2 O3nanolines channel.
In addition, the sensing response was also higher than the APTES modified sensor,
illustrating a better capturing efficiency than the APTES SAM layer towards miR-21.
However, a large distribution of sensing response for the DNA probe modified sensors
was observed compared to In2 O3 and APTES modified sensors. This might be due to therandom binding orientation of DNA probes and the In2 O3 surface. At last, APTES andDNA probe were successively modified on the In2 O3 sensor device to confirm the direct
immobilization of DNA probes, as APTES would block the direct binding of phosphonicacid group in DNA probes and In2 O3 channel. We found that the response signal of
the sensors was similar in intensity and distribution to the APTES-modified sensors, but
weaker than the DNA probe immobilized sensors. This might be because APTES kept the
DNA probes away from the In2 O3 nanolines surface, reducing the effect of DNA probe on
the sensing response. In other words, the phosphonic acid terminated DNA probe could
be directly immobilized on the surface of In2 O3 nanolines sensor platform and effectively
captured the miR-21 targets. Since the surface immobilization of multiple bio-probes is
feasible, our proposed In2 O3 FET sensing device should be a robust biosensor platform
for biosensors.

Micromachines 2021, 12, 642

7 of 9Figure 4. The In2 O3 nanolines FET sensing device. (a) Schematics of In2 O3 nanolines FET device and several surface
modifications. The information of the APTES and DNA-probe is shown at the lower left. The microscope image of device
is in bottom right. (b) Representative sensing response curve of DNA probe immobilized In2 O3 sensing device towards
miR-21 target. The background solvent is diluted PBS solution (0.01 X). The concentration of miR-21 target is changed.
(c) Sensing response of the sensors after various surface modifications. The concentration of miR-21 target is 500 pM/mL.

4. Conclusions
In this work, an In2 O3 electrically modulated FET sensing device with high robustness
in PBS solution was fabricated and can be used as a platform for biosensors. The fabricated
FET device demonstrated reproducibility with an average VTH of 5.235 V and a SD of
0.382 V. We found it retained high robustness when kept in PBS solution for more than
9 days. We demonstrated the FET device had good performance as a biosensor platform byshowing its sensing response towards miR-21 after directly immobilized the phosphonicacid terminated DNA probes. Since the surface immobilization of multiple bio-probes isMicromachines 2021, 12, 642

8 of 9feasible, we demonstrate that our proposed robust In2 O3 nanolines FET device can be an
excellent biosensor platform.
Author Contributions: Z.Z., T.Y. (Takao Yasui), K.N., T.Y. (Takeshi Yanagida) and Y.B. conceived anddesigned the overall research plan, experiments, and data analysis. Z.Z., T.Y. (Takao Yasui), K.N.,
T.T., Q.L. and T.S. developed the setups and performed experiments. T.Y. (Takao Yasui) and Y.B.
supervised the work. Z.Z. and T.Y. (Takao Yasui) wrote the manuscript with the input of the otherauthors. All authors have read and agreed to the published version of the manuscript.
Funding: This research was supported by the Japan Science and Technology Agency (JST) PRESTO(JPMJPR19H9), the JST SICORP(JPMJSC19E3), AMED Medical Research and Development Program(JP21he2302007), the JSPS Grant-in-Aid for Young Scientists (A) 17H04803, the JSPS Grant-in-Aid for
Scientific Research (S) 18H05243, the JSPS Grant-in-Aid for Exploratory Research 20K21124, the JSPS
Grant-in-Aid for Scientific Research on Innovative Areas “Chemistry for Multimolecular Crowding
Biosystems”, and research grants from each of the following: the Murata Science Foundation,
Advanced Technology Institute Research Grants 2019, Tatematsu Foundation of Public Interest, theNitto Foundation, the G-7 Scholarship Foundation, the Nanotechnology Platform Program (Moleculeand Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology(MEXT), and the Cooperative Research Program of the “Network Joint Research Center for Materials
and Devices”.
Conflicts of Interest: The authors declare no competing interests.

References
1.
2.
3.
4.

5.

6.
7.

8.
9.
10.
11.

12.
13.
14.
15.
16.

Gizeli, E.; Lowe, C.R. Immunosensors. Curr. Opin. Biotechnol. 1996, 7, 66–71. [CrossRef]
Hofstetter, O.; Hofstetter, H.; Wilchek, M.; Schurig, V.; Green, B.S. Chiral discrimination using an immunosensor. Nat. Biotechnol.
1999, 17, 371–374. [CrossRef]
Carneiro, P.; Loureiro, J.; Delerue-Matos, C.; Morais, S.; Pereira, M.D.C. Alzheimer’s disease: Development of a sensitive label-freeelectrochemical immunosensor for detection of amyloid beta peptide. Sens. Actuators B Chem. 2017, 239, 157–165. [CrossRef]
Seshadri, P.; Manoli, K.; Schneiderhan-Marra, N.; Anthes, U.; Wierzchowiec, P.; Bonrad, K.; Di Franco, C.; Torsi, L. Low-picomolar,
label-free procalcitonin analytical detection with an electrolyte-gated organic field-effect transistor based electronic immunosensor.
Biosens. Bioelectron. 2018, 104, 113–119. [CrossRef]
Verma, S.; Singh, A.; Shukla, A.; Kaswan, J.; Arora, K.; Ramirez-Vick, J.; Singh, P.; Singh, P.S. Anti-IL8/AuNPs-rGO/ITO asan Immunosensing Platform for Noninvasive Electrochemical Detection of Oral Cancer. ACS Appl. Mater. Interfaces 2017, 9,27462–27474. [CrossRef] [PubMed]
Ohno, Y.; Maehashi, K.; Matsumoto, K. Label-Free Biosensors Based on Aptamer-Modified Graphene Field-Effect Transistors.
J. Am. Chem. Soc. 2010, 132, 18012–18013. [CrossRef]
Kanai, Y.; Ohmuro-Matsuyama, Y.; Tanioku, M.; Ushiba, S.; Ono, T.; Inoue, K.; Kitaguchi, T.; Kimura, M.; Ueda, H.; Matsumoto, K.
Graphene Field Effect Transistor-Based Immunosensor for Ultrasensitive Noncompetitive Detection of Small Antigens. ACS Sens.
2019, 5, 24–28. [CrossRef]
De Moraes, A.C.M.; Kubota, L.T. Recent trends in field-effect transistors-based immunosensors. Chemosensors 2016, 4, 20.
[CrossRef]
Vu, C.-A.; Hu, W.-P.; Yang, Y.-S.; Chan, H.W.-H.; Chen, W.-Y. Signal Enhancement of Silicon Nanowire Field-Effect Transistor
Immunosensors by RNA Aptamer. ACS Omega 2019, 4, 14765–14771. [CrossRef] [PubMed]
Knopfmacher, O.; Hammock, M.L.; Appleton, A.L.; Schwartz, G.; Mei, J.; Lei, T.; Pei, J.; Bao, Z. Highly stable organic polymer
field-effect transistor sensor for selective detection in the marine environment. Nat. Commun. 2014, 5, 2954. [CrossRef]
Aroonyadet, N.; Wang, X.; Song, Y.; Chen, H.; Cote, R.J.; Thompson, M.E.; Datar, R.H.; Zhou, C. Highly Scalable, Uniform, andSensitive Biosensors Based on Top-Down Indium Oxide Nanoribbons and Electronic Enzyme-Linked Immunosorbent Assay.
Nano Lett. 2015, 15, 1943–1951. [CrossRef]
Daniel, M.G.; Song, J.; Tali, S.A.S.; Dai, X.; Zhou, W. Sub-10 nm Nanolaminated Al2O3/HfO2 Coatings for Long-Term Stability of
Cu Plasmonic Nanodisks in Physiological Environments. ACS Appl. Mater. Interfaces 2020, 12, 31952–31961. [CrossRef] [PubMed]
Correa, G.C.; Bao, B.; Strandwitz, N.C. Chemical Stability of Titania and Alumina Thin Films Formed by Atomic Layer Deposition.
ACS Appl. Mater. Interfaces 2015, 7, 14816–14821. [CrossRef] [PubMed]
Zhou, W.; Dai, X.; Fu, T.-M.; Xie, C.; Liu, J.; Lieber, C.M. Long Term Stability of Nanowire Nanoelectronics in Physiological
Environments. Nano Lett. 2014, 14, 1614–1619. [CrossRef]
Gao, N.; Zhou, W.; Jiang, X.; Hong, G.; Fu, T.-M.; Lieber, C.M. General Strategy for Biodetection in High Ionic Strength Solutions
Using Transistor-Based Nanoelectronic Sensors. Nano Lett. 2015, 15, 2143–2148. [CrossRef]
Kuo, W.-C.; Sarangadharan, I.; Pulikkathodi, A.K.; Chen, P.-H.; Wang, S.-L.; Wu, C.-R.; Wang, Y.-L. Investigation of Electrical
Stability and Sensitivity of Electric Double Layer Gated Field-Effect Transistors (FETs) for miRNA Detection. Sensors 2019, 19,1484. [CrossRef] [PubMed]

Micromachines 2021, 12, 642

17.
18.
19.
20.

21.
22.
23.

24.

25.

26.

27.

28.

29.
30.

9 of 9Liu, H.; Zhang, L.; Li, K.H.H.; Tan, O.K. Microhotplates for Metal Oxide Semiconductor Gas Sensor Applications—Towards the
CMOS-MEMS Monolithic Approach. Micromachines 2018, 9, 557. [CrossRef]
Arya, S.K.; Wong, C.C.; Jeon, Y.J.; Bansal, T.; Park, M.K. Advances in Complementary-Metal–Oxide–Semiconductor-Based
Integrated Biosensor Arrays. Chem Rev. 2015, 115, 5116–5158. [CrossRef]
Yan, R.; Takahashi, T.; Zeng, H.; Hosomi, T.; Kanai, M.; Zhang, G.; Nagashima, K.; Yanagida, T. Enhancement of pH Tolerance in
Conductive Al-Doped ZnO Nanofilms via Sequential Annealing. ACS Appl. Electron. Mater. 2021, 3, 955–962. [CrossRef]
Yan, R.; Takahashi, T.; Kanai, M.; Hosomi, T.; Zhang, G.; Nagashima, K.; Yanagida, T. Unusual Sequential Annealing Effectin Achieving High Thermal Stability of Conductive Al-Doped ZnO Nanofilms. ACS Appl. Electron. Mater. 2020, 2, 2064–2070.
[CrossRef]
Solanki, P.R.; Kaushik, A.; Agrawal, V.V.; Malhotra, B.D. Nanostructured metal oxide-based biosensors. NPG Asia Mater. 2011, 3,
17–24. [CrossRef]
Napi, M.L.M.; Sultan, S.M.; Ismail, R.; How, K.W.; Ahmad, M.K. Electrochemical-Based Biosensors on Different Zinc Oxide
Nanostructures: A Review. Materials 2019, 12, 2985. [CrossRef] [PubMed]
Nakatsuka, N.; Yang, K.-A.; Abendroth, J.M.; Cheung, K.M.; Xu, X.; Yang, H.; Zhao, C.; Zhu, B.; Rim, Y.S.; Yang, Y.; et al.
Aptamer–field-effect transistors overcome Debye length limitations for small-molecule sensing. Science 2018, 362, 319–324.
[CrossRef]
Zhu, Z.; Suzuki, M.; Nagashima, K.; Yoshida, H.; Kanai, M.; Meng, G.; Anzai, H.; Zhuge, F.; He, Y.; Boudot, M.; et al. Rational
Concept for Reducing Growth Temperature in Vapor–Liquid–Solid Process of Metal Oxide Nanowires. Nano Lett. 2016, 16,7495–7502. [CrossRef] [PubMed]
Anzai, H.; Suzuki, M.; Nagashima, K.; Kanai, M.; Guozhu, Z.; He, Y.; Boudot, M.; Zhang, G.; Takahashi, T.; Kanemoto, K.; et al.
True Vapor–Liquid–Solid Process Suppresses Unintentional Carrier Doping of Single Crystalline Metal Oxide Nanowires. Nano
Lett. 2017, 17, 4698–4705. [CrossRef] [PubMed]
Liu, Q.; Yasui, T.; Nagashima, K.; Yanagida, T.; Horiuchi, M.; Zhu, Z.; Takahashi, H.; Shimada, T.; Arima, A.; Baba, Y. Photolithographically Constructed Single ZnO Nanowire Device and Its Ultraviolet Photoresponse. Anal. Sci. 2020, 36, 1125–1129.
[CrossRef]
Ren, Y.; Huang, S.-H.; Mosser, S.; Heuschkel, M.O.; Bertsch, A.; Fraering, P.C.; Chen, J.-J.J.; Renaud, P. A Simple and Reliable
PDMS and SU-8 Irreversible Bonding Method and Its Application on a Microfluidic-MEA Device for Neuroscience Research.
Micromachines 2015, 6, 1923–1934. [CrossRef]
Wang, C.; Hosomi, T.; Nagashima, K.; Takahashi, T.; Zhang, G.; Kanai, M.; Yoshida, H.; Yanagida, T. Phosphonic acid modified
ZnO nanowire sensors: Directing Reaction Pathway of Volatile Carbonyl Compounds. ACS Appl. Mater. Interfaces 2020, 12,44265–44272. [CrossRef]
Zhang, R.; Curreli, M.; Thompson, M.E. Selective, Electrochemically Activated Biofunctionalization of In2O3 Nanowires Using an
Air-Stable Surface Modifier. ACS Appl. Mater. Interfaces 2011, 3, 4765–4769. [CrossRef]
Cheng, Y.; Xiong, P.; Yun, C.S.; Strouse, G.F.; Zheng, J.P.; Yang, R.S.; Wang, Z.L. Mechanism and Optimization of pH Sensing
Using SnO2 Nanobelt Field Effect Transistors. Nano Lett. 2008, 8, 4179–4184. [CrossRef]