３９
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Covered by shadow mask, the alkyne functionalized polymer layer was exposed to UV lights and unreacted polymer was removed by solvent washing of chloroform. Only irradiated polymer region by UV lights is selectively immobilized to BPS layer and then patterned polymer layer was formed, as shown in Figure 3.5. Because the electrodes were deposited in top contact structure, the channel length was designed for 100 μm, which was relatively wide active region, to make it facilitate to align the electrode. But it was confirmed that the pattern of the adhesion layer is possible up to 20 μm and 5 μm.
A finely patterned polymer layer can be obtained without photolithography, through this simple process as shown in Figure 3.6.
Figure 3.5. Image of patterned adhension layer by UV light attachment
Figure 3.6. Optical microscope images of patterned adhension layers
４１
The absorption spectra of a strongly attached polymer film was obtained with little difference in absorption peak height before and after chloroform washing by sonication as shown in Figure 3.7. This is because the polymer layer is chemically attached to the SAM-based substrate and cannot be washed away by solvents. The film thickness of the immobilized P(DPAP-r-PA) polymer film was average 23 nm, which is measured by ellipsometer.
300 400 500
0.0 0.1 0.2 0.3
Absorbance
Wavelength (nm)
P(DPAP-co-PA) before washing P(DPAP-co-PA)after washing
Figure 3.7. Absorbance spectra of P(DPAP-co-PA) films before and after solvent washing
The next step to obtain robust SWNTs films is to chemically react the azide functionalized SWNTs with P(DPAP-r-PA) polymer layer on substrate using Click reaction. The process of immobilizing SWNTs to substrate was carried out as shown in the schematic diagram 6. First, P(DPAP-r-PA) polymer attached substrates were immersed in P(FD-N3)-SWNTs inks and degassed with nitrogen for 5min.
Degassed in nitrogen in same method, copper catalyst and reducing agent are dissolved in D.I water and then injected into immediately SWNT inks. Then Click reaction proceeded between the alkyne of substrate and the azide of the SWNTs by blending the catalyst aqueous solution and sc-SWNTs inks using bath sonication for 5 minutes at 50℃. After the reaction, the remaining catalyst was rinsed in MeOH and DI water by ultrasonication for each 10min and unreacted sc-SWNT was washed with Toluene for 30min, leaving only immobilized SWNTs by Click reaction.
４２
Patterned SWNTs film was formed by the Click reaction with the pre-patterned polymer layer and it was confirmed by Raman spectroscopy shown in Figure 3.8. In ① region where the polymer was patterned, the G-band related to sp2 carbon bonding of SWNTs was observed at 1650 cm-1, whereas the D-band related to defects and impurities of carbon didn’t appear at 1350 cm-1. In ② region where the polymer wasn’t patterned, however, G band and D band absorption peaks of SWNT weren’t observed at all. This Raman spectra mean that undamaged SWNTs were immobilized only on the pre-patterned polymer layer to form a new patterned SWNTs film. This results not only mean that SWNTs film had good solvent stability with the substrate in harsh conditions because of the chemical bonds, but also suggested a method of increasing FETs performance and uniformity by setting accurate channel regions.
Scheme 6. Schematic of azide-alkyne cycloaddition reaction
Figure 3.8. Raman scattering spectras of SWNTs on patterned polymer(pink) and on SiO2(black)
４３
The density change and stability of the patterned SWNTs film depending on the Click reaction time was observed with the scanning electron microscopy (SEM). In Figure 3.9, the SWNT film density on P(DPAP-r-PA) increased with the reaction time even after harsh washing with solvents.
The stability of SWNTs film to substrate has potential in application of SWNTs FETs that could work in aqueous solution as a biosensor. Particularly, as sensors needs to detect a low-concentration target material, sensing with an accurate active area is very important in terms of device reliability.
In SEM image analysis, the linear density of SWNTs is estimated to be approximately 10, 20, 25, 28, 29, 51 tubes/μm in each sample for a click response time of 30 sec, 1 min, 2 min, 3 min, 5 min and 10 min respectively. Based on the results, the SWNT film density change depending on reaction time was plotted as a graph in Figure 3.10.
The sample that achieved the highest linear density had reaction time of 10 minutes. But as SWNTs are stacked with each other, there was a problem that the SWNTs film thickness was increased rather than a thin and dense SWNTs monolayer. With only 5 minutes of simple Click reaction, linear density of 29 tubes/μm was obtained, which was a sufficient density for transport of charge carriers in percolation paths in random SWNTs networks. To obtain similar density SWNTs film through common physiorption method, several times of spin coating or immersion in SNWT ink for a long time are required.
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Figure 3.9. SWNTs film samples of different densities with icreased Click reaction time
Figure 3.10. SWNT density change by Click reaction time
100 200 300 400 500 600 0
10 20 30 40 50
C N T d en si ty ( n u mber /μm )
Reaction Time (s)
４５
To confirm the electrical properties of the fabricated SWNTs films, source and drain electrodes were deposited on the SWNTs networks for structure of top contact bottom gate FETs. In addition, for carrier mobility calculation, the capacitance value of dielectric layer composing with P(DPAP-r-PA) and silicon oxides. 100nm SiO2 has a rather high capacitance per unit of area of 34.5nF/cm2, whereas that of P(DPAP-r-PA) is 26.0nF/cm2. Also the carbon nanotubes FETs exhibit bipolar behavior itself, however, they exhibit p-type characteristics by adsorption of moisture in the atmosphere.
To identify the advantages of active region’s patterning, it was compared with samples which have coated active layers on the entire wafer without patterning. First, in case of the un-patterned sample of which devices weren't individually isolated, the SWNTs of the entire wafers worked as an active region.
Therefore, it was impossible to properly measure the output curve (a) and transfer curve (b) due to the high leakage current and interference between devices as shown in Figure 3.11.
Figure 3.11. Fabricated un-patterned SWNTs FET by Click reaction without isolation
Another comparison of physically isolated SWNTs FETs (a) and chemically patterned SWNTs FETs (b) was represented by schematic diagrams of output curves and transfer curves in Figure 3.12. As shown in the I-V curves, the most obvious advantage of the sample with patterned active regions is that the deviation of the transfer curve is significantly reduced compared to the sample of which devices were isolated physically by cutter. Also, the output curve shows a much improved current value. The table summarizes the electrical carrier mobility (μ) and on/off ratio for each structured transistor.
First, un-patterned samples with isolation process had an average on-state current of 4.23 × 10-6 A and an average on/off ratio of 1.83 × 106 A with a maximum carrier mobility of 3.5313 cm2/V·s. On the other hand, samples with patterned SWNTs layer had a average on-state current of 4.75 × 10-5 A and an average on/off ratio of 1.56 × 107 A with a maximum carrier mobility of 18.0814 cm2/V·s.
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Improved device performances such as carrier mobilities, on/off ratio, and on currents are the result of accurately separated channel regions between the source and drain electrodes. In case of isolated sc- SWNTs film, since it has relatively wide active regions, there are many paths for charge carriers, so the device has poor reliability. But, sc-SWNTs film that are precisely patterned only between source and drain showed improved performance by minimizing leakage currents and interferences between devices.
Figure 3.12. Electrical performance comparsion of patterned FET and unpatterned FET with isolation
４７ 3.3. Applications to biosensors
Scheme 6. Schematic process of fabricating patterned Alzheimer’s biosensor
SWNTs materials have great potentials for biosensor applications due to their small sizes, chemical functionality and excellent electrical properties. In this study, the fabricated SWNTs FETs was studied for application to biosensors. Among various biomarkers, a biosensor technology of detecting Aβ1-42 which can be used for Alzheimer's diagnosing disease, was developed.
For confirming film stability to washing process, SWNTs were immobilized on sensor platform and resistance responses were measured. After that, to determine the effects of PBS buffer on the device performances, the resistance value was measured after bath sonication in PBS solution for 10 minutes.
As shown in Figure 3.13, I-V curves were measured before and after washing with PBS buffer with 20 samples. In the I-V curve, the plot before and after washing was almost the same, and the average resistance value was 0.53 MΩ before washing and 0.59 MΩ after washing, showing no significant difference. This means that SWNTs film wasn’t exfoliated by the PBS buffer due to the chemically bonded SWNTs to the substrate, which means that the reliability problem caused by unstable SWNTs layers in existing bio-platform has been solved.
４８
Figure 3.13. Comparison of I-V curve and resistance change before and after washing with PBS solution to confirm the stability of SWNT films on the sensor. Error bar represents standard devication (n=20)
To immobilize the antibody to the anchored SWNTs sensor platforms, linkers were required between the SWNTs surface and the antibody.49 The hydrophobic pyrenyl groups of 1-PBASE used as linkers were adsorbed to the hydrophobic sidewall of SWNTs through 𝜋-𝜋 stacking interactions irreversibly.
Aβ antibodies used for target antigen detection were immobilized by nucleophilic substitutions of primary and secondary amine groups with succinimide groups in 1-PBASE as a linker.50
The sequence of processes was confirmed by measuring the resistance responses (0.53 MΩ, 0.70 MΩ, 7.65 MΩ) of SWNTs, 1-PBASE adsorbed on SWNTs, and antibodies immobilized to adsorbed 1- PBASE, respectively. As shown in Figure 3.14, the 𝜋-𝜋 interactions of 1-PBSE with SWNTs surface didn’t significantly change the resistance values of SWNTs sensor platforms. Immobilization of antibodies via 1-PBASE significantly increased the resistance values of the biosensor. Compared to 1- PBSE adsorption process, the reason for the significant increase in resistance after Aβ antibodies immobilization is that negative charges are transferred from antibodies to SWNTs with p-type semiconductor properties.51
４９
The Aβ1-42 peptide was finally attached to the antibody immobilized on 1-PBSE in a SWNTs device.
The resistance responses for antibody immobilization and Aβ1-42 attachment were 7.65 MΩ and 31.6 MΩ, respectively. The specific interaction between Aβ1-42 and the antibody interfered with the flow of electric current and greatly increased the resistance value step by step.37 There were no significant differences in resistance values before and after the Aβ1-42 response when 1-PBASE was immobilized on the biosensor platform as shown in Figure 3.15. However, there were significant differences in resistances before and after the Aβ1-42 response after immobilizing antibodies to sensor platforms as shown in Figure 3.14. The resistance value showed a significant change value of 316% only when Aβ1- 42 was detected after antibody immobilization as shown in Table 2. The significant increases in the resistance value is due to the specific interactions between the antibodies and Aβ1-42. These results indicate that the Aβ1-42 peptide doesn’t react with the SWNTs-based sensor platform without antibody immobilizations. This study demonstrated that SWNT-based biosensors could be useful for detecting Alzheimer's proteins such as Aβ1-42.
Figure 3.14. Comparison of I-V curve and resistance change of using the immobilization. Error bar represents standard devication(n=20)
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Figure 3.15. Comparison of I-V curve and resistance change before and after immobilization of Aβ1- 42 to 1-PBASE to confirm the specificity of Aβ1-42 to antibodies. Error bar represents standard devication(n=20)
Table 2. Resistance difference of SWNTs based biosensors to confirm the effect of immobilization steps to Aβ1-42 detection
５１ 4. Conclusion
In summary, a patterned SWNT network with excellent reliability and robustness was fabricated by Click reaction. For this, high-purity sc-SWNTs were selectively dispersed by azide functionalized poly- fluorene. The disturbances of selective dispersion by nitrogen atoms having rich-electrnegativities was overcome by adsorption of metallic SWNTs using silica gel, resulting in increasing purity of sc-SWNTs.
The purity of sc-SWNTs was confirmed by absorption spectra and Raman scattering spectra.
An adhesive layer was coated by synthesis of alkyne-functional acrylic polymers by free radical polymerization for Click reaction with azide. This alkyne functionalized polymer was successfully pre- patterned by UV attachment. The purified sc-SWNTs which has azide functional group are reacted with the alkyne-based adhesive layer by Click reaction. At this time, the SWNTs film selectively patterned only in the active region by the pre-patterned adhesive layer was immobilized to the substrate.
The density of SWNTs film was increased depending on time of Click reaction, and patterned SWNTs-FETs fabricated by chemically anchored SWNT networks exhibited standardized FET performances with dense SWNT networks. In addition, these devices have better uniformity and performances than un-patterned devices fabricated in the same way without patterning.
Afterwards, this platform was used as biosensors. So antibodies were immobilized on SWNTs and Alzheimer's antigens were detected by confirming the change in electrical resistance response. To ensure the stability of the SWNTs films against washing process of biosensors, the resistance before and after the PBS solution washing was measured. As a result, it showed almost same resistance values with little increases in resistances, which means that the immobilized SWNTs films on wafers have high stability for washing. There were significant increases in the resistance values just when the antibodies were immobilized to devices. This means that Aβ1-42 has specificity only for antigen. This results suggested that the patterned SWNTs device fabricated by Click reaction can be successfully used as biosensor platforms.
５２
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