water at 10 mM, adjusting pH value by buffer titration.
11. Deionized water (DIW)
resistance of water: 18.2 MΩcm ultra-pure water system (Barnstead).
2-2 Instruments
The whole instrument consisted of electrical measurement machine (Dual-channel System Source Meter Instrument Model 2636) (Keithley), probe station with its chamber (EVERBEING(奕葉) ), and programming syringe pump (Kd Scientic) (Figure 4).
2-3 Poly crystalline silicon NWFET fabrication process
First, the fabrication began on Si wafers capped with a 100 nm-thick thermal oxide. The second, a 50nm-thick nitride layer was deposited by low-pressure chemical vapor deposition (LPCVD). After deposition of the nitride layer, following deposing Tetraethyl ortho-silicate (TEOS) 100nm thick sketched with standard photolithographic and etching steps were performed to form the oxide dummy structures. Subsequently, a 100nm-thick amorphous-Si layer was deposited and then annealed at 600°C for 24hr in N2 ambient to transform it into polycrystalline structure.
Afterwards, source/drain (S/D) doping was done with phosphorus ion implantation
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with a dose of 5E15cm2. After the generation of S/D photoresist patterns with a lithographic step, a reactive plasma etching step was performed to form the S/D regions. Because of the anisotropic etching process, two poly-Si NW channels were formed separating by oxide dummy gate simultaneously during the S/D etching step.
By carefully controlling the etching time, the cross-sectional dimensions of poly-Si NW channels can be easily reduced to sub-10 nm scale. Subsequently, all devices were then covered with a 200nm-thick TEOS oxide passivation layer. Finally, remove the oxide layer by 2-step dry/wet etching process to expose the poly-Si NW channels.
2-4 Microfluidic system
The microfluidic channel system will be made with acrylic, PDMS and metal holder. First, the PDMS gel will be covered to the channel patterned glass substrate (channel size: 13 mm X 1 mm X 0.5 mm) at 120oC for 10 minutes and wait for the fluid gel transfer to solid state. The solidification PDMS channel will be separated from the glass substrate and covered to the SNW chip. Then, make the limpid acrylic blanket and drill two holes filling with Teflon tubes (outer diameter: 1.5 mm, inner diameter: 0.5mm) for sample transport. Finally, limpid blanket of acrylic was covered to the PDMS and the chip-PDMS-acrylic sandwich was fastened by a metal holder.
The advantage of the microfluidic channel system is easy alignment, easy observation
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and be reused. The schematic diagram of the microfluidic system is shown in Figure 5.
2-5 Surface modification
We would do some treatments on chip surface before we measure the electrical variation. The immobilization process is based on the microfluidic system. Initially, we aligned the PDMS microfluidic channel covered on nanowire device row and added acrylic blanket on PDMS solid gel. Then, fastened the blanket and PDMS gel on nanowire chip by metal holder like the sandwich structure, tested whether the fluid buffer seep from the sandwich-stacking microfluidic system (Figure 5). After setting up the microfluidic system, it shall be locked an injection tubing connector. Inject APTES in ethanol solution 1 ml through microfluidic channel and react for 17~20 minutes. Then, wash the channel with 95% ethanol. Turn on hot plate and set the temperature about 110℃~120℃, put the whole microfluidic system on the hot plate for 10 minutes. Then, cool the metal holder to room temperature and inject 2.5%
glutaraldehyde reacting for one and half hour in 10 mM pH 7 phosphate buffer.
Nanowire surface functional group would be changed from hydroxide into aldehyde group, we would use DNA sequence modified amino-C6 at 5’end as a DNA probe to bind with surface aldehyde. We Applied Yang’s research[27] to modify a 20-mer
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DNA sequence diluted in 10mM pH 7 phosphate buffer, injecting 1 μM to microfluidic channel for 3 hours or more. Continuously, block unbinding aldehyde group with 50 mM ethanolamine at pH 9.1 phosphate buffer for one and half hour.
Finally, wash the channel with pH 7 phosphate buffer and ready measure. The overall surface modification is shown in Figure 6.
2-6 Sample transport at kinetic equilibrium
In order to maintain the environment ionic strength level, we would keep the microfluidic channel fluent to replace of quiescent state. First, set the microfluidic system connecting a syringe pump at injection end and be locked a liquid gate at the elution end. The waste buffer shunted by T-shaped tubing (Figure 7), sealed the end of metal wire by silicone neutral sealant. Then, fix the flow velocity at 5 ml/h by Syringe Pump (Kd Science) (Figure 8). Sample will be exchanged by changing another syringe tube, a little bubble between two solution samples. The whole transport system is semi-automatic, which fixed flow velocity by machine but changed syringe tubes by people.
2-7 Liquid phase electrical measurement
It has been reported that the electrical properties would be different from air
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to liquid phase environment. We would immobilize chemical compounds and biomaterials on nanowire chip in microfluidic channel for 6 hours, changed surface functional group in liquid phase. After treating the chip for a long time, we measured the device properties by conductance-time method, compared with those relations of the shift of threshold voltage to air condition and its’ stability. We measured these data and calculated mathematically signal process, found the most sensitivity Vg point.
We got a plot for describing the relationship of conductance-Vg (liquid gate) and a voltage which has the most sensitive variation rate. Then, fixed the Vg and measured pH sensing to check correcting variation trend.
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