The overall goal of the following experiment is to demonstrate a new nano electric sensing platform for point of care biomolecular detection in high ionic strength solutions. This is achieved by operating single walled carbon nano tube field effect transistors at high frequency to mitigate the ionic screening effect. First, the nano tube transistors are fabricated and functionalized with receptor molecules.
As a second step, the devices are encapsulated with a microfluidic flow channel, which allows biomolecule sample solutions to be injected for real-time sensing. Next, the single walled carbon nano tube fuel transistors are operated as high frequency mixers. In order to overcome an ionic screening effect at high driving frequency ions and solutions can no longer screen the nano tube sensor effectively allowing the oscillating dipole moments of surface bound biomolecules to be detected.
The results show the detection of strep avid and biotin binding in 100 millimolar Background solution based on monitoring changes in mixing current at 10 megahertz driving frequency. The main advantage of this technique over existing methods like direct current detection, is that it overcomes the device screening effect, which fundamentally impedes dur current detection in high ion strength solutions. In contrast to traditional charge detection based sensors, our method detects the dipole moments of biomolecules by exploring the high frequency response of nano tube transistors.
First place a photo mask with rectangular pits for catalyst onto a photoresist coated silicone wafer and expose it to a UV I radiance of 300 millijoules per centimeter squared for 0.3 seconds. Load the developed wafer into an electron beam evaporation chamber and deposit 0.5 nanometers of iron at a chamber pressure of 10 to the negative six tor. After cutting the catalyst coated silicon wafer from the chamber into smaller dyes, place them on a quartz boat and load the boat into the CVD growth furnace Program the furnace to ramp up the furnace to 800 degrees Celsius at the center of the tube while maintaining a flow of one SLM of argon.
Flow 0.2 SLM of hydrogen for five minutes to reduce the catalyst particles and convert iron oxide to iron. Then 5.5 SCCM of ethylene is introduced for 35 minutes to grow the S wts. A hydrogen flow of 0.2 SLM is maintained throughout the process.
After cooling to room temperature with a small argon flow, remove the dye from the furnace. After defining the area for metal deposition for contacts, place the dye in the evaporation chamber, then deposit 0.5 nanometers of titanium and 50 nanometers of gold as the source and drain contact metals in an electron beam evaporation chamber at tend to the negative six tor. When finished, soak the dye and acetone overnight for metal liftoff.
Then dip an isopropanol for 10 minutes and blow dry with nitrogen. Next place the dye back in the evaporation chamber and deposit 500 nanometers electron beam evaporated silicone dioxide at 10 to the negative six tor evaporate, 50 nanometers of chrome and 50 nanometers of gold as the top gate electrode in the electron beam evaporator. After patterning the photo resist wet, etched the evaporated silicone dioxide using a one to 20 buffered hydrofluoric acid solution for three minutes and 30 seconds.
Now prepare a solution of six millimolar PBSE in Dimethylformamide, incubate the S-W-N-T-F-E-T-D in the PBSE linker molecule solution for one hour at room temperature. Following this, prepare a 20 milligram per milliliter solution of amine. Peg two biotin in deionized water.
Incubate the dye in this solution for 18 hours. Then thoroughly rinse the dye in deionized water. Repeat the rinse eight to 10 times, then blow dry.
Place a new silicon wafer in a Petri dish and pour a DGAs PDMS mixture into the dish until the mixture is five millimeters above the wafer. Next, place the Petri dish in an oven at 70 degrees Celsius for one hour. Remove the Petri dish from the oven and allow the wafer to cool to room temperature.
Use a scalpel to cut out a rectangular piece of PDMS and pull it out using a tweezer. Then place the rectangular dye upside down and punch a hole through the flat side using a three millimeter biopsy punch. After placing the dye under the microscope carefully place the PDMS chamber on top of the dye by aligning it on top of the active area of the fabricated S-W-N-T-F-E-T devices.
Place a silicon wafer with a SU eight mold in a Petri dish and add two to three drops of tri Chloro. 3 3 3 tri fluoro propyl saline. Place the Petri dish in a vacuum chamber for one hour after removing the Petri dish from the vacuum.
Pour the DGAs PDMS mixture onto the wafer and heat it in an oven at 70 degrees Celsius for one hour. When finished, remove the Petri dish from the oven and allow the wafer to cool to room temperature. After cutting a rectangular PDMS stamp, place it upside down and punch a hole at each end of the flow channel using a 0.75 millimeter biopsy punch.
After placing the dye under the microscope carefully place the PDMS flow chamber on top of the dye by aligning it on top of the active area of the fabricated S-W-N-T-F-E-T devices. Next, push a polyethylene tube into each hole and connect one tube to a fluid source syringe barrel and the other tube to a drain syringe. Attach the syringe to a syringe pump to maintain a controlled fluid flow through the channel.
To set up the AM modulated frequency output, connect the ref out signal from the lockin amplifier to the external modulation signal port on the frequency generator. Next, connect the AM modulated RF output and DC voltage to a bias T and connect the output of the bias T to the SWNT source contact. Then connect the gate contact to the voltage port of the DAC card to read the AC current through the nano tube.
Connect the drain contact to a lock-in amplifier. Connect the amplitude and phase ports of the lock-in amplifier to the DAC input ports. Hold the source DC voltage at zero volt and the A signal frequency at 200 kilohertz.
Sweep the gate voltage and measure the current from the drain. Fill the fluid chamber with deionized water using a pipette. Carry out the AC electrical measurement.
Repeat with one millimolar sodium chloride, 10 millimolar sodium chloride, and 100 millimolar sodium chloride. And measure the device response for each solution. After placing the microfluidic flow channel on the device, connect one tube to an empty syringe placed on the syringe pump and connect the other tube to a syringe barrel and fill it with 100 millimolar sodium chloride solution.
Set the syringe pump to withdrawal mode. 100 millimolar sodium chloride solution is pulled into the tube. The current can be monitored on the computer.
Then switch the solution to one milligram per milliliter streptavidin in 100 millimolar sodium chloride, and monitor the real-time current change for streptavidin biotin binding A scanning electron microscope image of S SW NT transistor with a suspended top gate is shown here. The electrode consists of 50 nanometers chromium, and 50 nanometers gold and a thick chrome layer adds strength to the suspended structure. The suspended structure is confirmed by the absence of a leakage current between the top gate and the drain.
To characterize the success of sidewall functionalization, the changes in FET DC transfer curves in air. After each functionalization step were monitored. The transfer curve for pristine nano tube FET shifts to the right after bio tation and strep Aden binding pictured.
Here is the mixed current sensing signal measured as a function of gait voltage for a typical device in 100 millimolar sodium chloride. The graph shows the curves for DC current, mixed current, and theoretical mixed current. The data demonstrates that the mixing current results agree well with a theoretical model.
The representative results of static measurements and real-time flow measurements are shown here. The mixing current versus gait voltage curves represent biotinylated and streptavidin bound SWNT in 100 millimolar sodium chloride. The mixing current changes upon binding of streptavidin in the realtime flow experiment.
Signal change was also measured before and after streptavidin binding in a silicon and oxide ated control device in deionized water at different frequencies, there is no change after binding indicating that the detection mechanism arises due to the SWNT biomolecule interaction at high frequencies. This measurement technique can be applied in general to nano tube nanowire or graphene based electronic biosensors and can be performed directly in high ionic strength background solutions without the need for time consuming steps like desalting.
