Fabrication of Ti₃C₂ MXene Microelectrode Arrays for In Vivo Neural Recording

This synthesis protocol produces high-quality MXene ink with a metallic conductivity that reaches above 10, 000 Siemens per centimeter. The main advantage of this technique is that it enables the precise micro-patterning of MXene films without damaging the film or leaving harmful residues on the electrodes. To prepare a titanium carbide ink, slowly add two grams of titanium aluminum carbide precursor to a 125 milliliter reaction container containing selective etching solution and stir the solution with a Teflon magnetic bar for 24 hours at 35 degrees Celsius at 400 rotations per minute.

At the end of the incubation, add 50 milliliters of deionized water to two 175 milliliter centrifuge tubes, split the etching reaction mixture equally between the tubes, and fill to the 150 milliliter mark. Wash the material by repeated centrifugation, decanting the acidic supernatant into a plastic hazardous waste container and adding fresh water to the tubes between centrifugation steps until the pH of the supernatant solution reaches above pH six. For intercalation of the molecules between multilayer MXene particle to weaken out of plane interactions, add two grams of lithium chloride to 100 milliliters of deionized water and stir the solution at 200 rotations per minute.

When the lithium chloride has dissolved, mix 100 milliliters of the lithium chloride solution with the titanium carbide titanium aluminum carbide sediment. Transfer the solution back to the 125 milliliter plastic container and stir the reaction for 12 hours at 25 degrees Celsius and 200 rotations per minute. For delamination from the bulk multilayer particle into single to few layer titanium carbide MXene, wash the intercalation solution with multiple centrifugations, decanting the clear supernatant until a dark supernatant is observed.

Then centrifuge the dark supernatant for one hour before decanting the diluted green supernatant. Redisperse the swollen sediment with 150 milliliters of deionized water and split the solution equally into three 50 milliliter centrifuge tubes. Centrifuge the samples to separate the remaining MXene sediment from the MXene supernatant and pool the supernatants into a single container.

Then centrifuge the solution for an additional hour for further size selection and optimization of the solution to isolate single to few layer flakes. For titanium carbide microelectrode array fabrication, deposit a four micrometer thick bottom layer of Parylene-C onto a clean silicon wafer. To use photolithography to define metal patterns on the wafer, spin coat photoresist onto the wafer at 3, 000 rotations per minute for 40 seconds before soft baking the wafer on a hot plate for 14.5 minutes at 95 degrees Celsius.

Next, load the wafer and mask one into a mask aligner with the wafer positioned so that the ring on the photo mask overlaps with all of the edges of the wafer. Expose the wafer with 365 nanometer wavelengths of eye line at a 90 millijoule per centimeter squared dose and hard bake the wafer on a hot plate for one minute at 115 degrees Celsius. At the end of the hard bake, immerse the wafer in RD6 developer for two minutes with continuous agitation before rinsing thoroughly with deionized water and blow drying with a nitrogen gun.

Use an electron beam evaporator to deposit 10 nanometers of titanium followed by 100 nanometers of gold onto the wafer. Then immerse the wafer in solvent stripper for about 10 minutes until the photoresist has dissolved and the excess metal has fully lifted off. Once the liftoff appears complete, sonicate the wafer for 30 seconds to remove any remaining traces of unwanted metal and rinse the wafer with fresh solvent stripper and deionized water before drying with a nitrogen gun.

At the end of the liftoff process, gold will be visible in the desired interconnect traces and in the ring around the edge of the wafer. For deposition of the sacrificial Parylene-C layer, first expose the wafer to oxygen plasma for 30 seconds to render the underlying Parylene-C layer hydrophilic before spin coating dilute cleaning solution onto the wafer at 1, 000 rotations per minute for 30 seconds. Allow the wafer to air dry for at least five minutes before depositing three micrometers of Parylene-C onto the wafer as demonstrated.

Following a second round of photolithography using mask two, use oxygen plasma reactive ion etching to etch through the sacrificial Parylene-C layer in the areas not covered by the photoresist to define the MXene electrodes and traces. The etching should partially overlap with the titanium gold interconnects as well as the ring around the edges of the wafer. Then use a profilometer to measure the profile between the exposed titanium and gold interconnects and the bottom Parylene-C layer to confirm a complete etching of the sacrificial Parylene-C layer.

To spin coat the MXene solution onto the wafer, first dispense the solution onto each of the desired MXene patterns before spinning the wafer at 1, 000 rotations per minute for 40 seconds. Dry the wafer on a 120 degree Celsius hot plate for 10 minutes to remove any residual water from the MXene film then use an electron beam evaporator to deposit a protected 50 nanometer silicon dioxide layer onto the wafer. To remove the sacrificial Parylene-C layer, place a small drop of deionized water on the edge of the wafer and use tweezers to peel up the sacrificial Parylene-C layer starting from where the edges of the layer are defined in the ring around the outside of the wafer.

Next, rinse the wafer thoroughly with fresh deionized water to remove any remaining cleaning solution residue and dry the wafer with a nitrogen gun. Place the dried wafer on a 120 degree Celsius hot plate for one hour to remove any residual water from the patterned MXene films before depositing a four micrometer thick layer of Parylene-C onto the wafer as demonstrated. After performing another round of photolithography with mask three, use an electron beam evaporator to deposit 100 nanometer aluminum onto the wafer and immerse the wafer in solvent stripper for 10 minutes until the metal has completely lifted off of the wafer.

After sonication, rinsing, and drying as demonstrated, aluminum can be observed covering the devices with openings for the electrodes and bonding pads. Use oxygen plasma reactive ion etching to etch through the Parylene-C layers surround the devices and through the top Parylene-C layer covering both the MXene electrode contacts and the gold bonding pads. When no Parylene-C residue remains on the wafer between devices, use a wet chemical etch in aluminum etchant type A at 50 degrees Celsius for 10 minutes or until one minute passed when all visual traces of the aluminum have disappeared.

To etch the silicon oxide covering the MXene electrodes, use a wet chemical etch in six-to-one buffered oxide etchant for 30 seconds. Then place a small drop of deionized water at the end of the device. Gently peel up the device as water is wicked underneath the device by capillary action to release the device from the silicon substrate wafer.

Here, sample micro-electrocorticography data recorded on a MXene microelectrode array is shown. The putative cortical DOWN states are based on the trough of the slow oscillation at one to two hertz. Following application of the electrode array onto the cortex, clear physiologic signals were immediately apparent on the recording electrodes with approximately one millivolt amplitude electrocorticography signals appearing on all of the MXene electrodes.

Power spectra of these signals confirmed the presence of two brain rhythms commonly observed in rats under ketamine dexmedetomidine anesthesia, one to two hertz slow oscillations and gamma oscillations at 40 to 70 hertz. Additionally, a signature broadband power attenuation during the DOWN state of the slow oscillation and a selective 15 to 30 hertz beta band and 40 to 120 hertz gamma band power amplification during the UP state of the slow oscillation were observed. Optimization of the synthesis method has allowed expansion of the use of MXenes into other applications such as in optoelectronic devices, smart textiles, and more.

This procedure involves the use of dangerous chemicals including hydrofluoric acid, aluminum etchant, and photoresist developer. Be sure to always use the appropriate PPE when handling these chemicals.