Fabrication of the MoS 2 nanomesh FET. The general fabrication process of the MoS2 nanomesh consists of organizing the BCP nanopattern on MoS2, etching the MoS2, and subsequently removing the upper BCP layers. Multilayered MoS2 nanosheets were placed on a Si/SiO2 substrate by mechanical exfoliation from bulk MoS2. A 10 nm thick SiO2 layer was deposited onto the MoS2 nanosheets using an electron-beam (e-beam) evaporator. A 1 wt% solution of poly (styrene-r-methyl methacrylate) (PS-r-PMMA) (Mn = 8,500 Mw/Mn = 1.45, styrene content 66%, α-hydroxyl-ω-TEMPO moiety terminated) random copolymer (RCP) in toluene was spin-coated on the MoS2 films at 3,000 rpm, followed by substrate annealing at 250 °C for 2 h under vacuum and finally rinsing with toluene. A solution of 1 wt% poly(styrene-b-methyl methacrylate) (PS-b-PMMA) (PS:PMMA = 55,000 : 22,000, Mw/Mn = 1.09) BCP in toluene was also spin-coated with the same protocol as RCP and annealed at 230 °C. The PMMA portions in the BCP thin film were decomposed by UV irradiation (wavelength of 254 nm) for 30 min and finally dissolved in acetic acid solution. O2 plasma RIE (10 sccm, 10 W, 10 s) was applied to the surface of BCP. SF6 plasma RIE (10 sccm, 200 W, 15 s) was carried out to punch nanoholes on the SiO2 layer. Subsequently, BCl3 plasma RIE (10 sccm, 100 W) was used to make MoS2 nanoholes, with the plasma treatment time varying with the thickness of MoS2. At the end of the process, the substrate was submerged in BOE for 1 s to eliminate the remaining SiO2 and BCP on the MoS2 nanomesh. To construct the FET on the prepared MoS2 nanomesh, the source and drain were prepatterned by photolithography and a lift-off process, followed by deposition of Ti (20 nm)/Au (100 nm) by e-beam evaporation.
Characterizations. Microstructural studies of the MoS2 nanomesh were conducted using STEM (HD-2300A, Hitachi) with an accelerating voltage of 200 kV. For plane-view observation, the MoS2 nanomesh films were transferred onto Cu grids coated with a lacey carbon film. A cross-sectional view was observed by milling the sample using a single-beam focused ion-beam (FB-2100, Hitachi). Low-magnification surface images were obtained by SEM operated at an acceleration voltage of 15 kV and working distance of 8 mm. Raman spectroscopy (ALPHA300, WITec) was used to identify the formation of edge sites on MoS2 nanosheets. A laser beam with a spot diameter of 1 µm and excitation wavelength of 532 nm was used, and the instrument spectral resolution was approximately 1 cm-1. The chemical states of the MoS2 films were explored by XPS (K-Alpha, Thermo Fisher Scientific) using an Al Kα source. All electrical measurements were carried out using a semiconductor characterization analyzer (4200-SCS, Keithley) equipped with a probe station for sample loading and electrode contact.
MoS 2 surface modification and biosensing processes. Specific detection of cortisol was facilitated by functionalizing an aptamer, which is a bioreceptor for cortisol, on the channel surface of the MoS2 nanomesh bio-FET. The aptamer (Bioneer) was dissolved in Tris-HCl (pH 8.0) solution to obtain a concentration of 10− 6 g/mL. The MoS2 bio-FET was first exposed to O2 plasma (10 sccm, 10 W, 15 s) for edge oxidation. The MoS2 was then functionalized with APTES by treating with a solution of APTES in a 19:1 (v/v) mixture of ethanol and deionized (DI) water for 3 h. Subsequently, the device was rinsed in ethanol and dried at 120 °C for 15 min. To convert the amine functional group of APTES to an aldehyde group, the device was immersed in 4.5 mL of glutaraldehyde solution consisting of 0.1 g of NaCNBH4, 1 tablet of phosphate buffered saline (PBS), and 200 mL of DI water, for 2 h, followed by rinsing in DI water. The device was incubated overnight in a 10− 6 g/mL aptamer solution in Tris-HCl (pH 8.0) at 4 °C. To prevent nonspecific binding, 1% (w/v) casein blocker (Thermo Fisher Scientific) was added to the device for 1 h at room temperature. For the detection of cortisol, 16 different concentrations of cortisol solution (Sigma Aldrich) ranging from 10–21 g/mL to 10− 6 g/mL were dissolved in PBS solution (pH 7.4). Each cortisol solution was dropped onto the device for 30 min and subsequently rinsed and dried for electrical measurements.
Detection in saliva and human serum. To understand the sensing behavior in the biological environment of saliva and serum, different concentrations of cortisol were dissolved in artificial saliva and real human serum. Real human serum was purchased from Sigma Aldrich. Artificial saliva was prepared by dissolving 5 mM NaCl, 1 mM CaCl2, 15 mM KCl, 1 mM citric acid, 1.1 mM KSCN, and 4 mM NH4Cl in distilled water.