The method effectively removes PMMA residue while preserving the underlying graphene lattice. The functional device shows consistent results in detecting IgG antibodies in blood serum. Also, the protocol ensures the implementation of CVD graphene in a realtime, label-free biosensing device.
The steps are pretty simple and can be done with minimal training. The device offers high selectivity, high sensitivity, and realtime detection over other biosensing devices Begin by cutting the graphene sheet on a copper substrate in half using a scalpel. Apply heat-resistant tape to fix the four corners of the graphene square on a spinner gasket.
Spin coat the square sheet of the graphene with a thin layer of 100 to 200 nanometers of PMMA 495K A4, spinning at 500 rotations per minute for 10 seconds, and then 2, 000 rotations per minute for 50 seconds. Then, bake the sample at 150 degrees Celsius for five minutes. Remove the backside of the graphene with oxygen plasma at 30 watt using a rate flow of 15 standard cubic centimeters per minute for five minutes.
Cut the plasma-treated graphene square into a dimension of one-centimeter width and two-centimeter height for device fabrication. Cut the pre-cleaned silica substrate into small pieces with an approximate four-centimeter width and two-centimeter height. Etch the copper off using the graphene etchant ferric chloride without dilution.
Float the sample with the copper side down and the PMMA side up on the liquid etchant. After copper etching, lift the graphene film slowly using the plasma-treated substrate. Air dry the transferred graphene film for two hours, then bake on a hot plate.
To remove PMMA, start by warming up the sample with acetone vapor at 70 degrees Celsius by keeping the sample approximately two centimeters above acetone vapor for four minutes with the PMMA side facing down. Then, immerse the sample in acetone for five minutes. Wash the sample with deionized water cautiously.
Finally, gently blow dry the sample with nitrogen. Wash the substrate with the transferred graphene using acetone, isopropyl alcohol, and deionized water. Then, bake the substrate on a hot plate at 75 degrees Celsius for 30 minutes.
Using an electron beam evaporator, deposit nickel and gold of 5 and 45 nanometers thickness, respectively, on the graphene sample. Apply the first photolithography process using mask A for the patterning of the electrodes. Spin AZ 5214E positive photoresist on the sample at 2, 000 rotations per minute for 45 seconds and cure the sample at 120 degrees Celsius for one minute.
Place the sample in the ultraviolet flood exposure system and expose it for approximately 10 seconds under 200 millijoules per square centimeter. Develop the sample with a photoresist developer AZ 300 MIF for approximately two minutes and then rinse with deionized water. Immerse the sample in a gold etchant to etch the gold layer for 10 seconds, rinse with deionized water, and remove the remaining photoresist layer by immersing in acetone for 10 minutes.
Using acetone, isopropyl alcohol, and deionized water, wash the sample followed by baking on a hot plate at 75 degrees Celsius for 30 minutes. Then, apply the second photolithography process using mask B to pattern the graphene channels. Immerse the sample in nickel etchant at 60 degrees Celsius to etch the nickel layer for 10 seconds.
Rinse with deionized water and blow dry using nitrogen. Place the sample in the plasma asher and remove the exposed graphene using oxygen plasma. Later, remove the photoresist layer by immersing in acetone for 10 minutes.
Wash the sample using acetone, IPA, and deionized water and bake on a hot plate at 75 degrees Celsius for 30 minutes. Apply the third photolithography process using mask C to pattern the passivation photoresist layer to protect the underlying graphene on the substrate. Use the same process parameters as previously mentioned, including spinning with positive photoresist, curing the sample, and developing with photoresist developer.
Later, immerse the sample in nickel etchant at 60 degrees Celsius for 10 seconds to remove the remaining nickel layer, and then rinse with deionized water and blow dry using nitrogen. Finally, bake the sample on a hot plate at 120 degrees Celsius for 30 minutes. The representative results show the transferred CVD graphene characterized by Raman and atomic force microscopy.
The G peak and the two-dimensional peaks of the Raman image give comprehensive information regarding the existence and the quality of the transferred monolayer graphene. The figure shows EEG FET biosensor integrated with a standard silver in silver chloride reference electrode and a polydimethyl siloxane well for containing the sample. Further, the enlarged view of the graphene channel demonstrates the connection to the source electrode to the ground, the drain, and the gate electrodes to the source.
PBASE, a widely used functionalization reagent for graphene, can be absorbed on graphene surface through a pi-pi interaction without damaging graphene's electrical properties. A 5-prime amino-modified IgG aptamer is conjugated with PBASE by the amide bond linkages between the reactive and hydroxy-6 cinnamide ester in PBASE, and the amine group on the 5-prime end of the IgG aptamer. Bovine serum albumin incubation was used to block the remaining unconjugated sites after rinsing the device with single-strength PBS The figure shows the IgG detection under different electrolyte conditions.
The quality of the graphene is the key to the best performance of this device. So during plasma etching, it needs to be ensured that the plasma does not harm the useful regions of the graphene. Also, the PMMA residues need to be cleaned altogether to get a clean graphene surface.
The functional device shows consistent results in detecting human IgG antibody, so the procedure can be used as a reference to build devices with other nanomaterials to study interface interactions and biosensing.
