Optical and Material Characteristics of MoS2/Cu2O Sensor for Detection of Lung Cancer Cell Types in Hydroplegia

In this study, n-type MoS2 monolayer flakes are grown through chemical vapor deposition (CVD), and a p-type Cu2O thin film is grown via electrochemical deposition. The crystal structure of the grown MoS2 flakes is analyzed through transmission electron microscopy. The monolayer structure of the MoS2 flakes is verified with Raman spectroscopy, multiphoton excitation microscopy, atomic force microscopy, and photoluminescence (PL) measurements. After the preliminary processing of the grown MoS2 flakes, the sample is then transferred onto a Cu2O thin film to complete a p-n heterogeneous structure. Data are confirmed via scanning electron microscopy, SHG, and Raman mapping measurements. The luminous energy gap between the two materials is examined through PL measurements. Results reveal that the thickness of the single-layer MoS2 film is 0.7 nm. PL mapping shows a micro signal generated at the 627 nm wavelength, which belongs to the B2 excitons of MoS2 and tends to increase gradually when it approaches 670 nm. Finally, the biosensor is used to detect lung cancer cell types in hydroplegia significantly reducing the current busy procedures and longer waiting time for detection. The results suggest that the fabricated sensor is highly sensitive to the change in the photocurrent with the number of each cell, the linear regression of the three cell types is as high as 99%. By measuring the slope of the photocurrent, we can identify the type of cells and the number of cells.


Introduction
Molybdenum disulfide (MoS 2 ) is one of the two-dimensional (2D) transition metal chalcogenides that have been studied recently [1][2][3][4][5][6]. Materials related to nanometer-scale electronic and optoelectronic components, such as field-effect transistors, prospective memory components, light-emitting diodes (LED), and sensors, have been manufactured because of the excellent spin-valley coupling and flexural and optoelectronic properties of MoS 2 [7][8][9][10][11][12][13][14][15][16][17][18][19]. However, MoS 2 is a layered structure, which has good lubricity, resistance to pressure, and wear resistance, and is mostly used as solid lubricant [20]. It is used in equipment operating under high-speed, heavy-load, high-temperature, and chemical 2 of 15 corrosive conditions. MoS 2 is a black hexagonal crystal structure with silver-gray luster, its Moiré mass is 160.07 g/mol, density is 5.06 g/cm 3 , and melting point is 1185 • C. MoS 2 itself is insoluble in water, generally insoluble in other acids, alkalis, and organic solvents. However, at 400 • C, oxidation occurs slowly, and molybdenum trioxide (MoO 3 ) gradually forms. PN heterostructures have been widely used in the semiconductor industry; as a typical P-type semiconductor, Cu 2 O is considered to be the most effective material [21]. With these characteristics, Cu 2 O can be effectively used as an absorption layer in the visible wavelength range [22][23][24][25]. In a bulk MoS 2 semiconductor, an indirect energy gap of 1.2 eV between the G-and S-point conduction bands, and a single-layer MoS 2 semiconductor has a direct energy gap of 1.84 eV. Studies on light absorption, light reflection, and light excitation spectroscopy have evolved from bulk materials to single-layer TMDs [26]. In 2014, Yu-Fei Zhao et al. used a simple chemical synthesis method to cover MoS 2 successfully on p-Cu 2 O semiconductors for high-efficiency solar hydrogen production [21]. In 2016, Xinne Zhao et al. assembled Cu 2 O nanoparticles and MoS 2 nanosheets with a 2D planar structure into a 3D MoS 2 /Cu 2 O porous nanocomposite through hydrothermal synthesis [27,28]. In 2017, Linxia Fang et al. successfully decorated Cu 2 O nanoparticles on flower-like MoS 2 and used them for non-enzymatic current detection in glucose. This structure has Cu 2 O nanoparticles dispersed in MoS 2 [29]. Gang Li et al. reported the hydrothermal synthesis of MoS 2 /Cu 2 O nanocomposites with a tunable heterojunction to enhance the photochemical activity and stability of visible light [30]. The lack of energy gap of graphene in two-dimensional materials has shifted the focus on two-dimensional transition metal dichalcogenides (TMDs) with different energy gaps obtained by changing the number of layers. In the review of previous studies on MoS 2 /Cu 2 O, MoS 2 /Cu 2 O has high-performance photocatalysis, excellent electrocatalysis, strong stability, reproducibility, and high selectivity for non-MEI sensors. This structure can be used as a hydrogen oxide biosensor for environmental engineering applications, such as energy storage and water purification. It can also be used as a non-MEI current detection in glucose oxidation. It is better for the signal interference of uric acid, dopamine, and ascorbic acid. Apart from inorganic biosensors, organic materials are also getting attention in the last few years. Some biosensors using poly(methylene blue) (PMB), poly(alizarin yellow R), poly(azure A), poly(azure B), poly(azure C), poly(brilliant cresyl blue), and poly(thionine) has been widely studied and reported [31][32][33][34][35][36][37]. Even though these biosensors have good selectivity, their disadvantages include instability, cost, and a need for a mediator in some cases [38]. On the other hand, in recent years due to the excellent electrochemical properties of Cu 2 O, much research has been conducted towards its application as a sensor [39][40][41][42]. MoS 2 which has a similar structure to graphene due to its conductivity and good electrical and chemical properties has been widely reported to be employed in numerous biosensors [43][44][45].
Hence, in this research, a new MoS 2 /Cu 2 O PN heterojunction structure, can reduce the predominant disadvantage of noise interference to the detection signal through different growth methods, material special characteristics, and the structure itself. This MoS 2 /Cu 2 O structure is applied to a sensor for cancer cells with different canceration levels. In this study, CVD is conducted for single-layer MoS 2 to obtain large-area and uniform MoS 2 films. Several measurement techniques, including Raman mapping, are used to check the number of layers. Atomic force microscopy (AFM) is employed for thickness analysis and scanning electron microscopy (SEM) is utilized to observe surface morphology. X-ray diffraction (XRD; Figure S1 for XRD) is conducted to measure whether a signal appears, and transmission electron microscopy (TM; Figure S4) is performed to determine the direction of the crystal lattice. However, studies on the synthesis of semiconductor materials and nanostructures have also been carried out. During the research, Cu 2 O nanostructures have been successfully fabricated through electrochemical deposition. The major application of this heterostructure is to develop a low-cost and rapid way to detect lung cancer cell types in hydroplegia by fabricating a PEC biosensor that is highly sensitive to lung cancer cells significantly reducing the current busy procedures and longer waiting time for detection.

Materials and Methods
The glass substrate used in the experiment was indium tin oxide (ITO) for Cu 2 O electroplating. The resistance of the ITO surface was 7 Ω, and the thickness of the coating was 200 nm. All the chemicals used were all American Chemical Society (ACS) and guaranteed reagent (GR) to avoid the failure of results because of insufficient purity or residual impurities in the grown crystals, especially low-chlorine Sigma-Aldrich (Burlington, MA, USA) 98+% for sodium hydroxide because the solution of Cu 2 O deposition was susceptible to the influence of chloride ions to produce copper chloride precipitation.  [46,47]. Acetone, methanol, isopropanol, and deionized water were used to reduce the impurities, such as dust, oil stains, or residues, on the electrode surface after ITO plating. The adhesion of impurity contamination also improved the stability of film growth to obtain a higherquality Cu 2 O film (Supplementary Section S3 for the List of substrates, organic solvents, gases, and chemicals used to grow Cu 2 O and MoS 2 ). In the cleaning steps, the electrode surface was first cleaned with acetone in an ultrasonic oscillator for 10 min to dissolve and remove surface oil stains or other organic impurities. After the specimen was shaken and washed with acetone, it was rinsed with methanol in an ultrasonic oscillator for 10 min to dissolve and remove residual copper contamination on the surface. Isopropanol was used for cleaning in an ultrasonic oscillator for 10 min to dissolve and remove surface oil stains or other organic impurities. The glass substrate was soaked in deionized water with a resistivity of 18.2 M W·cm (25 • C), and an ultrasonic oscillator was used for 10 min to remove residual organic solvents. The sample was taken out, and a high-pressure nitrogen gun was used to remove the moisture on the surface of the ITO glass substrate. The ITO glass substrate was baked in an oven at 100 • C for 30 min to remove the residual moisture on the substrate and placed in a clean moisture-proof box for later use. The cut graphite rods with a diameter of 1 cm and a length of 18 cm were subjected to electrochemical Cu 2 O deposition. The main purpose of cleaning the electrodes of the graphite rods was to reduce impurities, such as dust, oil stains, and residual powder, on the electrode surface during graphite rod production. External impurities contaminated Cu 2 O electrolytic deposition and improved film growth stability. For cleaning, the graphite rod was soaked in 1 M sodium hydroxide solution and washed with ultrasonic vibration for 30 min to remove grease and organic pollutants adsorbed on the surface. The surface of the graphite rod was ground with sandpaper and rinsed with deionized water during grinding to remove the powder remaining in the graphite rod manufacturing. The graphite rod was soaked in deionized water and shaken for 30 min with ultrasonic cleaning to reduce the residual impurities on the surface. The graphite rod was baked in an oven at 100 • C for 30 min to remove the surface moisture and placed in a moisture-proof box to complete the pretreatment of the graphite electrode.

Preparation of Cu 2 O Film
A Cu 2 O thin film was prepared for electrochemical deposition to grow a highly uniform thin film on the ITO glass substrate (Supplementary Section S1.2 for Cu 2 O Synthesis). The electrolyte was made of 0.4 M copper sulfate powder (CuSO 4 ), 85% lactic acid, and 5 M sodium hydroxide (NaOH) prepared into a 1000 c.c. aqueous solution. The pH could be adjusted to 10, 11, and 12 through the amount of sodium hydroxide, which was measured in real time by a portable Starter300 pH meter (ST300). A DC power supply (MOTECH LPS505N) was used to power the two-pole electrochemical deposition system equipment in which the positive electrode was a graphite rod, and the negative electrode was con-nected to the ITO glass substrate (Supplementary Section S2.1 for the electrochemical deposition system). The temperature of the growing electrolyte was controlled at 60 • C to ensure that the lactic acid works. The deposition time was fixed at 30 min to achieve high quality and uniformity. At the end of the film, the sample was washed with deionized water, and the surface moisture was removed with a high-pressure nitrogen gun for subsequent experiments.

Cu 2 O Grinding
A plane grinder was widely used for the single-sided grinding and polishing of various materials, such as LED sapphire substrates, optical glass wafers, quartz wafers, silicon wafers, germanium wafers, molds, light guide plates, and optical skewer joints. The Cu 2 O thin film was processed by grinding to smoothen the Cu 2 O surface and facilitate the subsequent transfer of MoS 2 while maintaining the integrity of the morphological characteristics of MoS 2 and reducing the transfer failure rate. The rotation speed of the grinder was controlled at the minimum, and the time was set at 1 h. Considering Cu 2 O was a thin film at the micrometer scale, we chose the micro grade at the scale of a diamond polishing sheet.

Experimental Materials and Drug Specifications of MoS 2
The substrate used to grow MoS 2 was silicon dioxide (SiO 2 ) for chemical vapor deposition (CVD; Supplementary Section S1.2 for obtaining MoS 2 layers). The thickness of the chosen silicon wafer (100) crystal plane was 300 nm. The purity of sulfur powder (S) and molybdenum oxide (MoO 3 ) powder was 99.98% and 99.95%, respectively, to avoid impurities that affect the CVD or the remaining impurities of grown single crystals. The quartz tubes and ceramic crucibles in the tubular thermal tubes were cleaned with aqua regia. The concentrations of nitric acid and hydrochloric acid were 37% and 68-69%, respectively, to avoid the adhesion of impurities from the previous growth during cleaning, which affects the experimental parameters and reproducibility of the next growth of monolayer MoS 2 .

Process and Steps of CVD-Grown MoS 2
The majority of 2D material layer identification studies focus on film synthesis using mechanical stripping [48][49][50]. Most 2D material layer identification studies have focused on film synthesis through mechanical stripping [51]. In the present experiment, a 300 nm SiO 2 silicon substrate was prepared and cleaned with ultrasonic vibration in acetone for 10 min to remove impurities and oil stains on the surface; deionized water was then used to remove the organic solvent acetone (Supplementary Section S5 for CVD-grown MoS 2 ).
After 10 min, the sample was taken out, and the moisture on the surface of the ITO glass substrate was removed with a high-pressure nitrogen gun. The method used to grow MoS 2 was CVD [52][53][54][55][56][57]. The substrate was placed under specific pressure and temperature conditions and one or more precursors were chemically reacted on the surface of the substrate to produce a high-quality large-area thin film. The application of CVD in the preparation of single-layer TMDs starts with MoS 2 growth. The inert gas used to grow MoS 2 through CVD was argon. Afterward, 500 sccm of argon was used to clean the internal cavity and keep it in a clean environment. The heating rate was set to 20 • C per min, the growth temperature was 650 • C, and the temperature was held for 30 min. Once the temperature was dropped to 400 • C, the lid was opened. The MoS 2 structure was obtained when the temperature decreases to room temperature. The experimental process was shown in Figure 1. With molybdenum trioxide (MoO 3 ) and sulfur powder (S) as precursors, a 2 cm × 2 cm silicon dioxide/silicon (SiO 2 /Si) substrate was placed on the crucible and sent into the furnace tube. The vaporization point of MoO 3 was 650 • C [58][59][60]. The vaporization point of S was above 200 • C. Gas-phase MoO 3 undergoes two chemical reactions in high-temperature environments to produce molybdenum oxide (MoO 3 -x) intermediates. The resulting molybdenum oxide intermediates diffuse to the substrate and vaporize to form a MoS 2 film. The distance between the two crucibles was 46 cm. This long distance was to ensure that the vapor concentration gradient of sulfur was negligible on the substrate compared with that of the MoO 3 concentration gradient because the distance between MoO 3 and the substrate was small. CVD can effectively prepare single-and multi-layer MoS 2 . It can grow high-quality single-crystal materials and prepare uniformly distributed thin films on a large area, which was conducive to subsequent optoelectronic component manufacturing. was shown in Figure 1. With molybdenum trioxide (MoO3) and sulfur powder (S) as precursors, a 2 cm × 2 cm silicon dioxide/silicon (SiO2/Si) substrate was placed on the crucible and sent into the furnace tube. The vaporization point of MoO3 was 650 °C [58][59][60]. The vaporization point of S was above 200 °C. Gas-phase MoO3 undergoes two chemical reactions in high-temperature environments to produce molybdenum oxide (MoO3-x) intermediates. The resulting molybdenum oxide intermediates diffuse to the substrate and vaporize to form a MoS2 film. The distance between the two crucibles was 46 cm. This long distance was to ensure that the vapor concentration gradient of sulfur was negligible on the substrate compared with that of the MoO3 concentration gradient because the distance between MoO3 and the substrate was small. CVD can effectively prepare single-and multi-layer MoS2. It can grow high-quality single-crystal materials and prepare uniformly distributed thin films on a large area, which was conducive to subsequent optoelectronic component manufacturing.

MoS2 Transfer Process
A spin coater was used to coat uniformly the photoresistant PMMA A5 on the substrate to transfer MoS2 after CVD growth on the Cu2O surface and left for the PMMA to dry. The substrate was covered with photoresistant liquid into 2 M NaOH as an etching solution for separating the substrate and PMMA A5. The removed photoresist was placed in DI water and blown dry with high-pressure nitrogen to prepare for the subsequent transfer. Then, PMMA A5 was placed on the original SiO2 substrate, which was cut to an appropriate size with a three-axis leveling table and placed on the surface of the Cu2O substrate. The temperature was set to 210° for about 30 s to make the PMMA adhere to the substrate. The sample was soaked in acetone for 30 min, and photoresistance was removed, leaving MoS2 to complete the entire transfer process ( Figure 2).

MoS 2 Transfer Process
A spin coater was used to coat uniformly the photoresistant PMMA A5 on the substrate to transfer MoS 2 after CVD growth on the Cu 2 O surface and left for the PMMA to dry. The substrate was covered with photoresistant liquid into 2 M NaOH as an etching solution for separating the substrate and PMMA A5. The removed photoresist was placed in DI water and blown dry with high-pressure nitrogen to prepare for the subsequent transfer. Then, PMMA A5 was placed on the original SiO 2 substrate, which was cut to an appropriate size with a three-axis leveling table and placed on the surface of the Cu 2 O substrate. The temperature was set to 210 • for about 30 s to make the PMMA adhere to the substrate. The sample was soaked in acetone for 30 min, and photoresistance was removed, leaving MoS 2 to complete the entire transfer process ( Figure 2).

Growth Results of MoS2/Cu2O
Based on the above growth methods and measurement results, the CVD-grown MoS2 was transferred electrochemically to grown Cu2O (Supplementary Section S4 for the

MoS2 Transfer Result on Cu2O
After the Cu2O surface is polished by the abovementioned grinding method, the SEM image result shows that the surface is smoother than the original unpolished surface, and the surface roughness is reduced by 96% in the AFM measurement result (Supplementary Section S2.6 for atomic force microscope). Then CVD-grown MoS2 film is transferred to the Cu2O surface, and SEM is performed ( Figure 5). Although few irregularities are observed on the surface of the Cu2O, SEM analysis shows that MoS2 is transferred successfully, and the morphological characteristics of MoS2 are completely present on the surface of Cu2O.

Analysis of MoS2/Cu2O Materials and Optical Properties
After the transfer of the CVD-grown MoS2 film onto the electrochemically grown Cu2O surface. Its material and optical properties are analyzed by employing OM (Supplementary Section S2.4 for optical microscope), SHG images, SEM, and Raman and PL mapping. A multiphoton image is used to identify the signal display during the MoS2 transfer ( Figure 6). The morphological characteristics of MoS2 grown by CVD are obvious. A core point exists in most triangles, and when this core point corresponds to a multiphoton image, the signal becomes stronger than the color of the triangles other than the core point. Figure 6b shows the multiphoton image of MoS2/Cu2O/ITO after the transfer. Although many blue irregularities in the image, they are not evenly dispersed to make it clear whether they are MoS2 triangles.  (Figure 6). The morphological characteristics of MoS 2 grown by CVD are obvious. A core point exists in most triangles, and when this core point corresponds to a multiphoton image, the signal becomes stronger than the color of the triangles other than the core point. Figure 6b shows the multiphoton image of MoS 2 /Cu 2 O/ITO after the transfer. Although many blue irregularities in the image, they are not evenly dispersed to make it clear whether they are MoS 2 triangles. After the transfer of the CVD-grown MoS2 film onto the electrochemically grown Cu2O surface. Its material and optical properties are analyzed by employing OM (Supplementary Section S2.4 for optical microscope), SHG images, SEM, and Raman and PL mapping. A multiphoton image is used to identify the signal display during the MoS2 transfer ( Figure 6). The morphological characteristics of MoS2 grown by CVD are obvious. A core point exists in most triangles, and when this core point corresponds to a multiphoton image, the signal becomes stronger than the color of the triangles other than the core point. Figure 6b shows the multiphoton image of MoS2/Cu2O/ITO after the transfer. Although many blue irregularities in the image, they are not evenly dispersed to make it clear whether they are MoS2 triangles.  However, the comparison of the obvious bright spots in the image in Figure 6a suggests that a bright spot signal may be the core image after MoS 2 transfer and because Cu 2 O is not completely flat like the SiO 2 surface after being polished. Moreover, this bright spot at the center of MoS 2 is due to the nucleation of MoS 2 . The growth parameters, such as carrier gas, growth temperature, precursors, substrates, and promoters can affect the nucleation and growth modes of MoS 2 [61]. However, Ho Kwon Kim et al. has shown that the pre-exposure of growth substrates to alkali metal halides and the Mo precursor before the growth stage appears can suppress nucleation of MoS 2 [62]. Therefore, the other blue part may be a multi-photon image of a mixture of MoS 2 and Cu 2 O. As shown in Figure 5, the grown MoS 2 has been successfully transferred to the Cu 2 O film. These two data can be combined, proving that MoS 2 has been successfully transferred to the Cu 2 O film by grinding and transfer. Raman mapping, which is the best method to analyze the number of layers, has been used to analyze the image of the transferred MoS 2 /Cu 2 O sample [63][64][65]. In this study, a 532 nm band is used as a laser excitation light source. From Figure 7b we can observe that the blue-green area is the MoS 2 film, and the red area is part of a Cu 2 O film. The 532 nm band was used as the laser excitation light source in this study. As shown in Figure 7a, the Raman shift of MoS 2 has a peak at 384.3 before transfer while the Raman shift had a peak at 402. 8  When the ∆k value of the subtraction of E 12g and A 1g peaks is less than 20 cm −1 , the analyzed MoS 2 is regarded as a single-layer structure. Figure 8a shows the OM image of MoS 2 /Cu 2 O, Figure 8b illustrates the 625-band image selected for the PL mapping image. The MoS 2 spectrum consists of two peaks corresponding to the A1 and B1 excitons at 667 nm (1.86 eV) and 627 nm (1.97 eV), respectively. The measured PL for Cu 2 O grown by the electrochemical method was about 720-900 nm. In the measurement of the PN heterostructure material combination of MoS 2 /Cu 2 O, Raman measurement was first performed on the OM image. After the signal of MoS 2 is found, PL measurement is carried out, and the combined structure is examined through the measurement results to show that the band range covers 625-900 nm. We found that a micro signal is generated at about 627 nm in the orange elliptical dashed circle, and the wavelength of 627 nm belongs to the B2 excitons of MoS 2 . The signal gradually increases when it approaches 670 nm. However, the Cu 2 O signal is not in this part because its light-emitting band starts to generate signals from 720 nm. Therefore, the Cu 2 O film may be too thick, and the single-layer MoS 2 film thickness is only 0.7 nm, so the two heterostructure materials are combined in the wavelength range of 670-700 nm. The entire signal undergoes a red-shifted phenomenon because the Cu 2 O film signal is strong.
the grown MoS2 has been successfully transferred to the Cu2O film. These two data can be combined, proving that MoS2 has been successfully transferred to the Cu2O film by grinding and transfer. Raman mapping, which is the best method to analyze the number of layers, has been used to analyze the image of the transferred MoS2/Cu2O sample [63][64][65]. In this study, a 532 nm band is used as a laser excitation light source. From Figure 7b we can observe that the blue-green area is the MoS2 film, and the red area is part of a Cu2O film. The 532 nm band was used as the laser excitation light source in this study. As shown in Figure 7a, the Raman shift of MoS2 has a peak at 384.3 before transfer while the Raman shift had a peak at 402.8 cm −1 after the transfer. The Raman shift of Cu2O before the transfer has a peak at 221.8 cm −1 . After transferring MoS2 onto the Cu2O film, the Raman shift did reach peaks at 221.8, 384.3, and 402.8 cm −1 . It can be clearly inferred that the Raman peak of Cu2O appearing at 221.8 cm −1 indicates that the surface of Cu2O was partially oxidized. Similarly, the Raman shifts of MoS2 at 384.3 and 402.8 cm −1 are in accordance with E12g and A1g, respectively, and combined vibrations of two different phonons (A1g(M)−LA(M)), respectively, which indicates the existence of MoS2 structures [66][67][68][69]. In the Cu2O/MoS2 composite films, the diffraction peaks of Cu2O and MoS2 are still observed. The result shows that the grown MoS2 has a uniform film in the image and can replace the morphology. E12g and A1g vibration modes are the main signals for the judgment of MoS2. The two vibration modes are highly dependent on the thickness of MoS2. When the Δk value of the subtraction of E12g and A1g peaks is less than 20 cm −1 , the analyzed MoS2 is regarded as a single-layer structure.  Figure 8a shows the OM image of MoS2/Cu2O, Figure 8b illustrates the 625-band image selected for the PL mapping image. The MoS2 spectrum consists of two peaks corresponding to the A1 and B1 excitons at 667 nm (1.86 eV) and 627 nm (1.97 eV), respectively. The measured PL for Cu2O grown by the electrochemical method was about 720-900 nm. In the measurement of the PN heterostructure material combination of MoS2/Cu2O, Raman measurement was first performed on the OM image. After the signal of MoS2 is found, PL measurement is carried out, and the combined structure is examined through the measurement results to show that the band range covers 625-900 nm. We found that a micro signal is generated at about 627 nm in the orange elliptical dashed circle, and the wavelength of 627 nm belongs to the B2 excitons of MoS2. The signal gradually increases when it approaches 670 nm. However, the Cu2O signal is not in this part because its light-emitting band starts to generate signals from 720 nm. Therefore, the Cu2O film may be too thick, and the single-layer MoS2 film thickness is only 0.7 nm, so the two heterostructure materials are combined in the wavelength range of 670-700 nm. The entire signal undergoes a red-shifted phenomenon because the Cu2O film signal is strong.

Photocurrent Response Analysis
This research uses the preparation of biosensors, and the photocurrent measurement system and impedance analysis system set up in the laboratory (see Supplement 1, Section S6 for Analysis of Structural Characteristics of Photoelectrochemical Biosensors). The photocurrent measurement of the three lung cancer cells A549, H460 and H520 was carried out by the carrier transport mechanism of photogenerated charge, and the zigzagstoothed microelectrode was used to perform aggregation and impedance analysis of the cancer cells. In this study, we utilized two characteristic substances, glutathione (GSH) and glutathione disulfide (GSSG), to verify the detection mechanism of the developed biosensor. Generally, GSH in cells exists in two forms: 90% of GSH is present in the cytoplasm as reduced GSH, and 10% is present in the mitochondria. For healthy cells, the ratio of GSH to GSSG is greater than 10:1. However, when cells become cancerous, the ratio of GSH to GSSG decreases [70,71]. Because the majority carriers in N-type PEC are electrons and hence GSSG participating in PEC reaction is more significant. When irradiated, electron-hole pairs are generated, GSSG recombines with photo-generated electrons, and photo-generated holes are detected, forming a system of hole currents. As

Photocurrent Response Analysis
This research uses the preparation of biosensors, and the photocurrent measurement system and impedance analysis system set up in the laboratory (see Supplement 1, Section S6 for Analysis of Structural Characteristics of Photoelectrochemical Biosensors). The photocurrent measurement of the three lung cancer cells A549, H460 and H520 was carried out by the carrier transport mechanism of photogenerated charge, and the zigzagstoothed microelectrode was used to perform aggregation and impedance analysis of the cancer cells. In this study, we utilized two characteristic substances, glutathione (GSH) and glutathione disulfide (GSSG), to verify the detection mechanism of the developed biosensor. Generally, GSH in cells exists in two forms: 90% of GSH is present in the cytoplasm as reduced GSH, and 10% is present in the mitochondria. For healthy cells, the ratio of GSH to GSSG is greater than 10:1. However, when cells become cancerous, the ratio of GSH to GSSG decreases [70,71]. Because the majority carriers in N-type PEC are electrons and hence GSSG participating in PEC reaction is more significant. When irradiated, electron-hole pairs are generated, GSSG recombines with photo-generated electrons, and photo-generated holes are detected, forming a system of hole currents. As the degree of canceration becomes more severe, the concentration of GSSG contained in cancer cells increases, and the measured photocurrent also increases [72,73]. On the contrary, the majority of carriers in the P-type PEC biosensor are holes and hence GSH participates in the PEC reaction more significantly. After being illuminated, GSH recombines with photo-generated holes, photo-generated electrons are detected, and a system of electron flow is formed. The more severe the carcinogenesis of the structure, the smaller the GSH concentration and also a decrease of the photocurrent [74,75]. As shown in Figure 9 the photocurrent decreased with the increase of suspended cell density, which indicated that more GSH was involved in the PEC process. From Figure 10, it can be known that different cells have different amounts of change, there is a highly linear relationship between the photocurrent change and the number of cells, and the linear regression of the three cell types is as high as 99%. The slopes corresponding to the three cell lines A549, NCI-H460 and NCI-H520 were approximately 0.000452, 0.000749 and 0.000174, respectively. By measuring the magnitude of the photocurrent, we can calculate the regression curve equation, thereby identifying which cell of the three cells is. In the growth mechanism of Cu 2 O, as the thickness increases, the stress or dislocation will increase, resulting in a certain optimal thickness of the material. Previous studies also shows that the best growth thickness of Cu 2 O under the environment and parameters of our growth is 2 µm [76]. smaller the GSH concentration and also a decrease of the photocurrent [74,75]. As shown in Figure 9 the photocurrent decreased with the increase of suspended cell density, which indicated that more GSH was involved in the PEC process. From Figure 10, it can be known that different cells have different amounts of change, there is a highly linear relationship between the photocurrent change and the number of cells, and the linear regression of the three cell types is as high as 99%. The slopes corresponding to the three cell lines A549, NCI-H460 and NCI-H520 were approximately 0.000452, 0.000749 and 0.000174, respectively. By measuring the magnitude of the photocurrent, we can calculate the regression curve equation, thereby identifying which cell of the three cells is. In the growth mechanism of Cu2O, as the thickness increases, the stress or dislocation will increase, resulting in a certain optimal thickness of the material. Previous studies also shows that the best growth thickness of Cu2O under the environment and parameters of our growth is 2 μm [76].

Discussion
When the MoS2 film has a single-layer structure, the A1g signal is stronger than that of a multilayer film. If the MoS2 film has a multilayer structure, the E12g signal is stronger than that of a single-layer film. The PL signal of MoS2 also decreases as the number of layers increases. Therefore, we can observe the contrast of the image by selecting the PL mapping images of 625 and 675 nm, corresponding to the color bar value. Single-and multi-layer MoS2 film signals are found in the selected 625-band image compared with much fewer signals in the 675-band image. When we grow the MoS2 film, single-and multi-layer structures form. In the selection of images for mapping, more multi-layer MoS2 films are covered. The PL measurement results in this study use a 532 nm laser as the excitation light source to measure the PL of a single-layer MoS2 film. The measurement results show that a strong signal is generated at the wavelength of 667 nm, and the energy is 1.84 eV after conversion. It is the energy gap of single-layer MoS2. However, from Gang Li et al., the energy band alignment of bulk MoS2 and single layer MoS2 was observed at 1.4 eV and 1.78 eV, respectively [29]. From our study, the single layer MoS2 was observed at 1.84 eV, close to the previous observations. The future scope of this project can be split into two aspects. First, after the two materials are combined, by using them as electric double-layer transistors their electrical properties can be tested. After confirming the positive state of the oxide trap. We can also try to bombard the element with high-energy particles to explore its characteristic influence and figure out if it affects the transformation of the mechanism. If the result of the electron beam bombardment affects the element and causes the characteristics to change, it can be checked again whether the electrical property changes from the positive oxide trap state to the negative interface trap state. Second, CVD can be performed to grow MoS2 in the 2H phase. MoS2 has three crystal structures: 2H (semiconductor characteristics), 1T (metallic characteristics), and 3R (semiconductor characteristics). The intensity difference can be detected in the PL mapping and electrostatic force microscope (EFM) images of 2H-MoS2 and 1T-MoS2. The future scope of this study is to collect a large amount of data to build a large-scale database by measuring the cell impedance and photocurrent in the patient's pleural effusion. After analyzing the data through Artificial Intelligence (AI), a human-machine display interface will be designed which provides the types of cancer cells defined by the system, assisting

Discussion
When the MoS 2 film has a single-layer structure, the A 1g signal is stronger than that of a multilayer film. If the MoS 2 film has a multilayer structure, the E 12g signal is stronger than that of a single-layer film. The PL signal of MoS 2 also decreases as the number of layers increases. Therefore, we can observe the contrast of the image by selecting the PL mapping images of 625 and 675 nm, corresponding to the color bar value. Single-and multi-layer MoS 2 film signals are found in the selected 625-band image compared with much fewer signals in the 675-band image. When we grow the MoS 2 film, single-and multi-layer structures form. In the selection of images for mapping, more multi-layer MoS 2 films are covered. The PL measurement results in this study use a 532 nm laser as the excitation light source to measure the PL of a single-layer MoS 2 film. The measurement results show that a strong signal is generated at the wavelength of 667 nm, and the energy is 1.84 eV after conversion. It is the energy gap of single-layer MoS 2 . However, from Gang Li et al., the energy band alignment of bulk MoS 2 and single layer MoS 2 was observed at 1.4 eV and 1.78 eV, respectively [29]. From our study, the single layer MoS 2 was observed at 1.84 eV, close to the previous observations. The future scope of this project can be split into two aspects. First, after the two materials are combined, by using them as electric double-layer transistors their electrical properties can be tested. After confirming the positive state of the oxide trap. We can also try to bombard the element with high-energy particles to explore its characteristic influence and figure out if it affects the transformation of the mechanism. If the result of the electron beam bombardment affects the element and causes the characteristics to change, it can be checked again whether the electrical property changes from the positive oxide trap state to the negative interface trap state. Second, CVD can be performed to grow MoS 2 in the 2H phase. MoS 2 has three crystal structures: 2H (semiconductor characteristics), 1T (metallic characteristics), and 3R (semiconductor characteristics). The intensity difference can be detected in the PL mapping and electrostatic force microscope (EFM) images of 2H-MoS 2 and 1T-MoS 2 . The future scope of this study is to collect a large amount of data to build a large-scale database by measuring the cell impedance and photocurrent in the patient's pleural effusion. After analyzing the data through Artificial Intelligence (AI), a human-machine display interface will be designed which provides the types of cancer cells defined by the system, assisting physicians to determine the patient's cancer status and provide appropriate treatment methods.

Conclusions
In this study, an electrochemical method was used to grow a Cu 2 O film with a uniform and superior crystal lattice. The micro-roughened surface structure of Cu 2 O was polished by grinding. Then, CVD-grown single crystal MoS 2 was transferred to the Cu 2 O film. OM, SHG, SEM, Raman, and PL mapping measurements were used to analyze the combination of the two materials. Electrochemical methods were successfully used to achieve relatively good electrical properties. TEM showed that Cu 2 O is a single crystal structure, and SEM revealed that the surface should be processed by subsequent grinding methods. After grinding and polishing were performed, AFM and SEM were used to obtain the experimental results. MoS 2 structure was grown through CVD, and E 12g and A 1g peaks were obtained via Raman analysis. The main luminescence peak at 667 nm was determined through PL analysis. The single layer of MoS 2 was transferred to the polished Cu 2 O surface, which was confirmed by SEM and SHG results. Raman and PL mapping image analysis on MoS 2 /Cu 2 O indicated that the grown MoS 2 had a uniform film. The combined structure was measured through PL measurement to verify that the band range covered 625-900 nm. A micro signal was found at the 627 nm wavelength. It belonged to the B2 excitons of MoS 2 and tended to increase gradually as it approached 670 nm. The Cu 2 O film was too thick, whereas the thickness of the single-layer MoS 2 film was only 0.7 nm. As such, the two heterostructure materials combined in the wavelength range of 670-700 nm. The strong signal of the Cu 2 O film itself led to a red shift in the entire signal. This study also successfully fabricated a low-cost PEC biosensor that is highly sensitive to lung cancer cells which is a rapid way to detect lung cancer cell types in hydroplegia. The photocurrent response is measured using the MoS 2 /Cu 2 O biochip material grown by electrochemical deposition. As the number of cancer cells measured increases, the content of oxidized GSSG also increases, and the measured photocurrent decreases accordingly. It is also possible to use dielectrophoresis to gather cancer cells to form a pearl string, measure unlabeled cancer cells, and use the slope of the admittance value to shape lung cancer cells. The linear regression curve is compared with the admittance value and the photocurrent measurement value to distinguish the types of cancer cells in the pleural effusion.

Conflicts of Interest:
The authors declare no conflict of interest.